Distilasi Atmosfer

PROSES DISTILASI

Proses distilasi disebut juga penyulingan adalah proses pemisahan berdasarkan

tingkat penguapan atau titik didih suatu campuran. Pada penyulingan minyak bumi yang

berupa multi komponen pemisahannya di dasarkan pada trayek didih atau fraksi-

fraksinya sehingga prosesnya sering disebut juga proses fraksinasi. Proses distilasi secara

dibedakan menjadi 3 yaitu :

1. Distikasi Atmosfer

2. Distilasi Hampa ( Vaccum )

3. Distilasi Bertekanan

Pada Crude Oil / Minyak mentah distilasi yang dipakai adalah Distilasi Atmosfer atau distilasi pada tekanan sedikit diatas tekanan atmosfer

Uraian proses dari penglohan crude oil menjadi produk BBM diatas adalah

sebagai berikut : 1. Crude oil dipanaskan tahap awal dalam pipa pada peralatan yang

biasa disebut dengan alat penukar panas ( Heat Exchanger / HE ). Crude oil dipanaskan

dengan media pemanas produk yang akan di dinginkan atau memanfaatkan panas yang

akan di buang. Dari pemanfaatan panas suhu yang dapat di capai ± 219 OC 2. Selanjutnya

Crude oil dipanaskan lebih lanjut dalam tungku/dapur/tanur atau furnace dengan

menggunakan bahan bakar, sampai suhu 330 OC – 345 OC. Crude oil dimasukkan dalam

kolom fraksinasi, minyak yang mudah menguap ( berubah bentuk gas) akan menguap dan

naik ke kolom bagian atas. Semakin mudah menguap semakin naik. Dalam perjalanan

naik uap minyak memerlukan energi, sehingga minyak seperti solar dan minyak tanah

akan mengembun lebih dulu tertampung diatas tray.

Uap minyak yang naik keatas akan dipertemukan dengan cairan yang sudah

terbentuk menggunakan suatu alat yang disebut alat kontak. Suhu dalam kolom fraksinasi

semakin keatas semakin rendah. Suhu dasar ( bottom ) tidak jauh beda dengan suhu

masuk kolom, sedangkan suhu puncak kolom ( top ) ± 100 OC. 3. Cairan yang terbentuk

dikeluarkan dari tray yang telah ditentukan sebagai produk. Produk yang keluar panasnya

dimanfaatkan terlebih dulu dengan menggunakan HE untuk memanaskan bahan baku

(Crude oil). Setelah panasnya terserap maksimal produk ini didinginkan dalam Pendingin

atau Cooler dan kemudian disimpan dalam tanki. 4. Untuk produk top kolom pada suhu

100OC masih berbentuk uap, maka di embunkan atau di kondensasikan dengan

kondensor, suhu keluar dari kondesor ± 37OC. Hasil kondensasi ditampung pada

Accumulator dan antara uap yang terkondensasi yang merupakan komponen bensin dan

yang tidak terkondensasi merupakan produk LPG. Gambaran proses dan diagram diatas

adalah untuk menjelaskan proses pada masyarakat awam. Jika kita mau belajar lebih

dalam akan kita temukan bahwa proses CDU lebih komplek dari yang saya tulis diatas.

Produk Crude Distilling Unit

PRODUK PENGOLAHAN CRUDE DISTILLING UNIT

Sebagaimana yang dapat dilihat dari gambar proses yang terdahulu produk yang

dihasilkan kilang yang diketahui awam adalah : gas, bensin, minyak tanah ( kerosene ),

solar, dan residue. Gambar proses yang terdahulu hanya untuk menggambarkan secara

awam bagaiman produk kilang yang lebih dikenal dengan BBM itu dibuat, perlu

diketahui bahwa sebenarnya produk-produk tersebut keluar dari proses belum merupakan

produk jadi, tapi merupakan bentuk komponen-komponen yang harus di proses lagi

dengan cara pencampuran (Blending) sehingga didapat produk yang dikehendaki.

Adapun produk stream atau produk yang langsung keluar dari proses kilang adalah

sebagai berikut : LPG, Light Naphta, Heavy Naphta, Kerosine ( minyak tanah ), LGO,

HGO, dan Residue. Untuk lebih jelasnya akan kita bahas satu persatu mulai keluar kolom

dari top sampai bottom hingga keluar produk tersebut jadi.

Yang pertama Produk Light End merupakan produk top kolom, yang meliputi LPG, Light Naphta, dan Heavy Naphta. Pemisahan produk ini dapat di lihat pada diagram di atas.

Urainan Proses:

Uap minyak dari top kolom fraksinasi dikondensasikan, hasil kondensasi berupa

naphta yang tidak stabil karena banyak mengandung gas. Gas yang terkandung dalam

naphta dipisahkan di stabilizer, yang kemudian dipisahkan lagi antara gas yang tidak

terkondensasi berupa C1-C2 yang keluar sebagai Off Gas dan gas terkondensasi C3-C4

sebagai LPG. Komponen terbesar LPG adalah C4 ( butan ).

Produk bottom stabilizer berupa naphta dipisahkan menjadi lihgt naphta dan

heavy naphta di splitter. Produk light naphta digunakan untuk komponen blending bensin

atau digunakan sebagai pelarut semisal thinner. Heavy naphta disamping digunakan

untuk komponen blending juga digunakan untuk feed/umpan proses Naphta Hydro

Treater ( NHDT ) yang produknya disebut Sweet Naphta dan digunakan feed proses

Platforming untuk menghasilkan Reformat yang merupakan Hight Okta number Mogas

Component atau digunakan sebagai umpan proses selanjutnya.

Bensin yang kita kenal adalah merupakan hasil pencampuran (Blending) HOMC,

L. Naphta dan Heavy Naphta. HOMC ( Hight Octane number Mogas Component ) atau

komponen bensin dengan octane number tinggi, dihasilkan dari proses sukunder. Produk

selanjutnya adalah produk midle distillate yang akan saya sampaikan nanti.

Hal yang perlu diperhatikan pada proses pengolahan minyak (CDU)

HAL-HAL YANG PERLU DIPERHATIKAN PROSES PENGOLAHAN MINYAK BUMI / CDU ( DISTILLASI ATMOSFERIK )

Pada umumnya tujuan suatu proses produksi adalah ditujukan untuk memenuhi

kebutuhan akan sesuatu, sehingga ada syarat atau baku mutu yang diharapkan. Semisal

kita pingin punya sepeda motor maka kita akan berpikir berapa jumlah, warna, cc, jenis

yang kita inginkan. Demikian juga pada pengolahan di kilang yang perlu kita perhatikan

adalah hasil produk yang kita inginkan baik itu jumlah maupun baku mutunya. Penetapan

baku mutu produk dalam hal ini BBM ditentukan oleh dirjen migas, berdasarkan

kebutuhan mesin dan standar lingkungan yang harus dipenuhi.

Untuk mendapatkan hasil distilasi yang sesuai dengan harapan, variable operasi

yang perlu diperhatikan adalah suhu dan tekanan proses. Pada proses distilasi yang

sederhana letak keluarannya (draw off) produk sudah diatur sedemikian rupa sehingga

pengaturan suhu dialakukan pada masukan (inlet) kolom distilasi dan topnya, sedang

pada draw off masing-masing produk akan teratur dengan sendirinya. Tekanan top kolom

dijaga tetap kurang lebih 0,3 kg/cm2. untuk suhu inlet kolom berkisar antara 329 – 345

oC, pemanasan tidak boleh lebih dari 370OC karena pada suhu tersebut minyak bumi akan

mengalami cracking ( patah rantainya ).

sedangkan suhu top kolom dijaga ±100oC. Pada design kilang yang lebih modern

tiap-tiap keluaran produk atau draw off memiliki beberapa fasilitas pangaturan seperti

pump around dan reflux, sehingga pengaturan untuk mencapai baku mutu produk lebih

flexible.

Kondisi yang lain yang perlu diperhatikan kebanyakan berhubungan dengan keselamatan ( safety ), dan design masing-masing peralatan proses.

PROSES PENGOLAHAN MINYAK

Ditinjau dari sifat proses pengolahan crude oil menjadi berbagai produk dikelompokkan menjadi 2 yaitu :

1. Proses fisis

Proses fisis adalah proses pengolahan atau pemisahan minyak bumi dan turunannya, tanpa merubah struktur kimia dari komponen-komponen minyak bumi tersebut. Atau proses yang terjadi adalah prose fisika saja. Proses-proses tersebut diantaranya adalah :

- Proses Distilasi

- Proses ekstraksi

- Proses kristalisasi

- Absorpsi

2. Proses kimia

Proses kemis adalah proses pengolahan minyak bumi dan turunannya dengan merubah struktur kimia dari komponen minyak bumi tersebut. Perubahan tersebut dapat berupa pemisahan rantai ( Cracking ), peruraian, atau penggabungan komponen minyak bumi.Proses-proses tersebut diantaranya adalah :

- Cracking

- Reforming

- Hydrotreating

- Alkilasi

- Polimerisasi

- Dan lain-lain

PROSES DISTILASI

Proses distilasi disebut juga penyulingan adalah proses pemisahan berdasarkan

tingkat penguapan atau titik didih suatu campuran. Pada penyulingan minyak bumi yang berupa multi komponen pemisahannya di dasarkan pada trayek didih atau fraksi-fraksinya sehingga prosesnya sering disebut juga proses fraksinasi. Proses distilasi secara umum dibedakan menjadi 3 yaitu :

Distikasi Atmosfer

Distilasi Hampa ( Vaccum )

Distilasi Bertekanan

Proses diatas dipilih berdasarkan bahan

baku yang diolah.

Distillation

Introduction

Distillation separates chemicals by the difference in how easily they vaporize. The

two major types of classical distillation include continuous distillation and batch

distillation.Continuous distillation, as the name says, continuously takes a feed and

separates it into two or more products. Batch distillation takes on lot (or batch) at a time

of feed and splits it into products by selectively removing the more volatile fractions over

time.

Other ways to categorize distillation are by the equipment type (trays, packing),

process configuration (distillation, absorption, stripping, azeotropic, extractive, complex),

or process type (refining, petrochemical, chemical, gas treating).

In all cases, what must be kept in mind is that distillation involves both equipment

and theory. Sound analysis with basic principles underlies any successful distillation

process. However, putting basics into practice requires real equipment. Process design

tells us what equipment needs to accomplish to meet our plant goals. Equipment limits

set what a specific unit can achieve. Putting successful distillation units in place requires

combining both the theoretical knowledge of the distillation fundamentals along with

equipment understanding. The Distillation Group puts both of these areas together to

work in your process. We approach troubleshooting, equipment design, process analysis,

and revamps by combining knowledge of fundamentals and of how equipment really

works. This gives reliable results and effective (and profitable) plant operation.

The following information gives a short background to the rest of the distillation information in this site.

Historical Background

Distillation has been around for a long time. Earliest references are to Maria the

Jewess who invented many types of stills and reflux condensers.Common Middle Ages

and Renaissance uses of distillation included the manufacture of brandy and other spirits

from wine. Another early use was the manufacture of perfumes and essences. Other early

users of distillation include the Alchemists. Of course, the history of distillation does not

end there. Today we use it for more than just spirits.

Many industries use distillation for critical separations in making useful products.

These industries include petroleum refining, beverages, chemical processing,

petrochemicals, and natural gas processing.

The beverage industy is the one of the oldest users of distillation. Distillation of

ethanol for both consumption and other uses was one of the first major industries ever

developed. Ethanol has often been considered as a fuel. At times, this has even been

done. F. B. Wright published a major work on production and distillation of fuel ethanol

in 1906. A copy of the second edition (1907) from the DGI collection of historical

material can be viewed or downloaded. This is a large document (293 pages, 2.43M). A

smaller version using low resolution graphics is also available (1.47M).

Natural gas processing started using distillation in the early 1900's. An interesting

historical document, Condensation of Gasoline from Natural Gas, documents some early

steps in this industry.

Recent developments energy shortages have re-focused attention on major

industrial energy users. Distillation is a major energy consumer. During the energy 'crisis'

of the 1970s much effort was put into making distillation more efficient. A good example

of this work is summarized in the Distillation Operations Manual from the Texas

Industrial Commission.

Distillation Today

Distillation Categories

Distillation services can be sorted out into many different categories. Here are some basic definitions:

System composition

System refers to the chemical components present in the mixture being distilled. The two main groups are binary distillation and multicomponent distillation.

Binary distillation is a separation of only two chemicals. A good example is separating ethyl alcohol (ethanol) from water. Most of the basic distillation teaching and a lot of theoretical work starts with looking at binary distillation; it's a lot simpler.

Multicomponent distillation is the separation of a mixture of chemicals. A good example is petroleum refining. Crude oil is a very complex mixture of hydrocarbons with literally thousands of different molecules. Nearly all commercial distillation is multicomponent distillation. The theory and practice of multicomponent distillation can be very complex.

Processing Mode

Processing mode refers to the way in which feed and product are introduced and withdrawn from the process. Distillation occurs in two modes, continuous distillation and batch distillation.

Continuous distillation is feed is sent to the still all the time and product is drawn out at the same time. The idea in continuous distillation is that the amount going into the still and the amount leaving the still should always equal each other at any given point in time.

Batch distillation is when the amount going into the still and the amount going out of the still is not supposed to be the same all the time. The easiest example to use is like old fashioned spirit making. The distiller fills a container at the start, then heats it, as time goes by the vapors are condensed to make the alcoholic drink. When the proper quantity of overhead (drink) is made, the distiller stops the still and empties it out ready for a new batch. This is only a simple case, in industrial usage what goes on gets very complex.

Both continuous and batch distillation are very important to industry. Continuous distillation is most often used with big volume products like jet fuel, benzene, plastic monomers. Batch distillation is most often used with smaller volume products and in

plants that make lots of different things and use the same still for many products (in different batches).

Processing sequence

Fractionation systems have different objectives. The major processing objectives set the system type and the equipment configuration needed. The common objectives include removing a light component from a heavy product, removing a heavy component from a light product, making two products, or making more than two products. We will call these major categories are called stripping, rectification, fractionation, and complex fractionation.

This terminology may be a little confusing because we also use the terms stripping and fractionation when we discuss heat flow options through the unit. This confusion results from historical use of the terms and you just need to keep the context in mind when reading or discussing the material. With a little practice you will find that the reason for using the same terms is that many of the systems called stripping or fractionation systems have the same characteristics regardless of using a processing sequence or heat flow analysis of the unit.

Stripping systems remove light material from a heavy product.

Rectification systems remove heavy material from a light product.

Fractionation systems remove a light material from a heavy product and a heavy material from a light product at the same time.

Complex fractionation makes multiple products from either a single tower or a complex of towers combined with recycle streams between them. A good example of a multiple product tower is a refinery crude distillation tower making rough cuts of naphtha (gasoline), kerosene (jet fuel), and diesel from the same tower. A good example of a complex tower with internal recycle streams is a Petlyck (baffle) tower making three on-specifications products from the same tower.

System type

The behavior of the chemicals in the system also determines the system configuration for the objectives. The three major problems that limit distillation processes are close-boilers, distributed keys, and azeotropes. Other problems that may require using special system configurations include heat sensitive materials.

Close boiler systems include chemicals that boil at temperatures very close to each other. So many stages of distillation or so much reflux may be required that the chemicals cannot be separated economically. A good example is separation of nitro-chloro-benzenes. Up to 600 theoretical separation stages with high reflux may be required to separate different isomers.

Distributed keys are systems where some chemicals that we do not want in either the heavy or the light product boil at a temperature between the heavy and the light product.

Azeotropic systems are those where the vapor and the liquid reach the same composition at some point in the distillation. No further separation can occur. Ethanol-water is a perfect example. Once ethanol composition reaches 95% (at atmospheric pressure), no further ethanol purification is possible.

Close boilers and distributed keys are economic problems. The compounds can be separated, but it costs a lot. Azeotropic systems are fundamental thermodynamic problems. At the distillation conditions, the products can only be distilled to a certain point, no further.

Different ways to get around these problems include using other techniques (membranes, crystallization, adsorption, adduction, extraction, precipitation), using complex distillation configurations, changing system conditions, or adding extra chemicals to the process. Adding extra chemicals includes azeotropic distillation, extractive distillation, or salt distillation.

System type

Azeotropic and extractive distillation use the addition of a mass separating agent (MSA) to modify the thermodynamic behavior of the system. Many different azeotropic and extractice distillation configurations are in use.

Azeotropic distillation uses a MSA that forms a minimum boiling azeotrope with some of the feed components is used. The azeotrope is taken overhead and the MSA rich phase decanted and returned to the column as reflux.

Extractive distillation uses a MSA that increases the volatility difference between the compounds to be separated. A good example is sulfolane to increase the relative volatility difference between similar molecular weight aromatic and paraffinic hydrocarbons. The sulfolane unit combines liquid-liquid extraction, extractive distillation, and solvent stripping in one process.

Salt distillation adds a salt to the system to modify the thermodynamic behavior of the system. The salt is normally added to the liquid supply of a batch distillation system.

All of these types of systems are normally considered complex systems. Other equipment is needed to separate and reuse the added MSA. Very complex configurations can result. Good understanding of system thermodynamics is required to predict behavior.

Heat flow

Energy transfer is required to make separations work. Heat flow refers to the arrangement of the distillation column to its heat source and heat sink. The major categories are fractionation (distillation), absorption, stripping, and contacting.

This terminology may be a little confusing because we also use the terms stripping and fractionation when we discuss processing sequence options in distillation. This confusion results from historical use of the terms and you just need to keep the context in mind when reading or discussing the material. With a little practice you will find that the reason for using the same terms is that many of the systems called stripping or fractionation systems have the same characteristics regardless of using a processing sequence or heat flow analysis of the unit.

Fractionation refers to units that have both a reboiler and a condenser. Something is attached to the bottom of the tower to put heat into the tower and something attached to the top of the tower to take heat out of the tower.This is what is normally called distillation

Absorption is a unit that has no method at the top of the tower to take heat out. An external stream is supplied from outside the system to absorb material from the vapor.

Stripping is a unit that has no method at the bottom of the tower to put heat in. An external stream is supplied from outside the system to strip material from the liquid.

Contacting is a unit that has neither a method at the top of the tower to remove heat nor a method at the bottom of the tower to put heat in. Two streams run countercurrent to each other. Both streams are generated outside the mass-transfer system.

What can make things unclear is that these terms have both other meanings and can be used imprecisely. Also, towers can have intermediate heat input and heat removal equipment in the middle. This confuses the picture. But we will use the strict definitions above. An absorber is a tower without a condenser. A stripper is a tower without a reboiler. A contactor has neither and a fractionator has both.

Reaction

Reactive distillation uses a reaction in the distillation equipment to help the separation. The reaction may or may not use a catalyst. DMT manufacture uses reactive distillation without a catalyst. One process to make methy-tert-butyl-ether uses a catalyst inside the distillation tower. The reaction changes the composition, allowing the distillation to work better.

Equipment Type

Distillation equipment includes two major categories, trays and packing.

Trays force a rising vapor to bubble through a pool of descending liquid.

Packing creates a surface for liquid to spread on. The thin liquid film has a high surface area for mass-transfer between the liquid and vapor.

Photograph Gallery

Distillation units come in many different configurations. The configuration depends upon the service, capital versus operating cost optimization, and technology available when the unit is constructed. To see a selection of different units visit our photo gallery.

Distillation

Laboratory display of distillation: 1: A heating device 2: Still pot 3: Still head 4: Thermometer/Boiling point temperature 5: Condenser 6: Cooling water in 7: Cooling water out 8: Distillate/receiving flask 9: Vacuum/gas inlet 10: Still receiver 11: Heat control 12: Stirrer speed control 13: Stirrer/heat plate 14: Heating (Oil/sand) bath 15: Stirring means e.g.(shown), anti-bumping granules or mechanical stirrer 16: Cooling bath.

Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction.

Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from seawater. Air is distilled to separate its components—notably oxygen, nitrogen, and argon—for industrial use. Distillation of fermented solutions has been used since ancient times to produce distilled beverages with a higher alcohol content. The premises where distillation is carried out, especially distillation of alcohol, are known as a distillery.

History

Distillation apparatus of Zosimus, from Marcelin Berthelot, Collection des anciens alchimistes grecs (3 vol., Paris, 1887-1888).

Early types of distillation were known to the Babylonians in Mesopotamia (in what is now Iraq) from at least the 2nd millennium BC.[2] Archaeological excavations in northwest Pakistan have yielded evidence that the distillation of alcohol was known in the Indian subcontinent since 500 BC,[3] but only became common between 150 BC - 350 AD.[3] Primitive tribes of India used a method of distillation for producing Mahuda liquor. This crude and ancient method is not very effective.[4]

Distillation was later known to Hellenistic alchemists from the 1st century AD,[5][6][7] and the later development of large-scale distillation apparatus occurred in response to demands for spirits.[5] According to K. B. Hoffmann the earliest mention of "destillatio per descensum" occurs in the writings of Aetius, a Greek physician from the 5th century.[8] Hypatia of Alexandria is credited with having invented an early distillation apparatus,[9]

and the first clear description of early apparatus for distillation is given by Zosimos of Panopolis in the fourth century.[7]

The invention of highly effective "pure distillation" is credited to Arabic and Persian chemists in the Middle East from the 8th century. They produced distillation processes to isolate and purify chemical substances for industrial purposes such as isolating natural esters (perfumes) and producing pure alcohol.[10] The first among them was Jabir ibn Hayyan (Geber), in the 8th century, who is credited with the invention of numerous chemical apparatus and processes that are still in use today. In particular, his alembic was the first still with retorts which could fully purify chemicals, a precursor to the pot still, and its design has served as inspiration for modern micro-scale distillation apparatus such as the Hickman stillhead.[11] The isolation of ethanol (alcohol) as a pure compound through distillation was first achieved by the Arab chemist Al-Kindi (Alkindus).[12]

Petroleum was first distilled by the Persian alchemist Muhammad ibn Zakarīya Rāzi (Rhazes) in the 9th century, for producing kerosene,[13] while steam distillation was invented by Avicenna in the early 11th century, for producing essential oils.[14]

As the works of Middle Eastern scribes made their way to India and became a part of Indian alchemy, several texts dedicated to distillation made their way to Indian libraries.[15] Among these was a treatise written by a scholar from Bagdad in 1034 titled Ainu-s-Sana'ah wa' Auna-s-Sana'ah.[15] Scholar Al-Jawbari travelled to India.[16] By the time of the writing of the Ain-e-Akbari, the process of distillation was well known in India.[17]

Distillation was introduced to medieval Europe through Latin translations of Arabic chemical treatises in the 12th century.[18] In 1500, German alchemist Hieronymus Braunschweig published Liber de arte destillandi (The Book of the Art of Distillation)[19]

the first book solely dedicated to the subject of distillation, followed in 1512 by a much expanded version. In 1651, John French published The Art of Distillation the first major English compendium of practice, though it has been claimed[20] that much of it derives from Braunschweig's work. This includes diagrams with people in them showing the industrial rather than bench scale of the operation.

Distillation

As alchemy evolved into the science of chemistry, vessels called retorts became used for distillations. Both alembics and retorts are forms of glassware with long necks pointing to the side at a downward angle which acted as air-cooled condensers to condense the distillate and let it drip downward for collection. Later, copper alembics were invented. Riveted joints were often kept tight by using various mixtures, for instance a dough made of rye flour.[21] These alembics often featured a cooling system around the beak, using cold water for instance, which made the condensation of alcohol more efficient. These were called pot stills. Today, the retorts and pot stills have been largely supplanted by more efficient distillation methods in most industrial processes. However, the pot still is still widely used for the elaboration of some fine alcohols such as cognac, Scotch whisky, tequila and some vodkas. Pot stills made of various materials (wood, clay, stainless steel) are also used by bootleggers in various countries. Small pot stills are also sold for the domestic production[22] of flower water or essential oils.

Early forms of distillation were batch processes using one vaporization and one condensation. Purity was improved by further distillation of the condensate. Greater volumes were processed by simply repeating the distillation. Chemists were reported to carry out as many as 500 to 600 distillations in order to obtain a pure compound[23].

In the early 19th century the basics of modern techniques including pre-heating and reflux were developed, particularly by the French[23], then in 1830 a British Patent was issued to Aeneas Coffey for a whiskey distillation column[24], which worked continuously and may be regarded as the archetype of modern petrochemical units. In 1877, Ernest Solvay was granted a U.S. Patent for a tray column for ammonia distillation[25] and the same and subsequent years saw developments of this theme for oil and spirits.

With the emergence of chemical engineering as a discipline at the end of the 19th century, scientific rather than empirical methods could be applied. The developing petroleum industry in the early 20th century provided the impetus for the development of accurate design methods such as the McCabe-Thiele method and the Fenske equation. The availability of powerful computers has also allowed direct computer simulation of distillation columns.

Applications of distillation

The application of distillation can roughly be divided in four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinals (herbal distillate), and food processing. The latter two are distinctively different from the former two in that in the processing of beverages, the distillation is not used as a true purification method but more to transfer all volatiles from the source materials to the distillate.

The main difference between laboratory scale distillation and industrial distillation is that laboratory scale distillation is often performed batch-wise, whereas industrial distillation often occurs continuously. In batch distillation, the composition of the source material, the vapors of the distilling compounds and the distillate change during the distillation. In batch distillation, a still is charged (supplied) with a batch of feed mixture, which is then separated into its component fractions which are collected sequentially from most volatile to less volatile, with the bottoms (remaining least or non-volatile fraction) removed at the end. The still can then be recharged and the process repeated.

In continuous distillation, the source materials, vapors, and distillate are kept at a constant composition by carefully replenishing the source material and removing fractions from both vapor and liquid in the system. This results in a better control of the separation process.

Idealized distillation model

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the pressure in the liquid, enabling bubbles to form without being crushed. A special case is the normal boiling point, where the vapor pressure of the liquid equals the ambient atmospheric pressure.

It is a common misconception that in a liquid mixture at a given pressure, each component boils at the boiling point corresponding to the given pressure and the vapors of each component will collect separately and purely. This, however, does not occur even

in an idealized system. Idealized models of distillation are essentially governed by Raoult's law and Dalton's law, and assume that vapor-liquid equilibria are attained.

Raoult's law assumes that a component contributes to the total vapor pressure of the mixture in proportion to its percentage of the mixture and its vapor pressure when pure, or succinctly: partial pressure equals mole fraction multiplied by vapor pressure when pure. If one component changes another component's vapor pressure, or if the volatility of a component is dependent on its percentage in the mixture, the law will fail.

Dalton's law states that the total vapor pressure is the sum of the vapor pressures of each individual component in the mixture. When a multi-component liquid is heated, the vapor pressure of each component will rise, thus causing the total vapor pressure to rise. When the total vapor pressure reaches the pressure surrounding the liquid, boiling occurs and liquid turns to gas throughout the bulk of the liquid. Note that a mixture with a given composition has one boiling point at a given pressure, when the components are mutually soluble.

An implication of one boiling point is that lighter components never cleanly "boil first". At boiling point, all volatile components boil, but for a component, its percentage in the vapor is the same as its percentage of the total vapor pressure. Lighter components have a higher partial pressure and thus are concentrated in the vapor, but heavier volatile components also have a (smaller) partial pressure and necessarily evaporate also, albeit being less concentrated in the vapor. Indeed, batch distillation and fractionation succeed by varying the composition of the mixture. In batch distillation, the batch evaporates, which changes its composition; in fractionation, liquid higher in the fractionation column contains more lights and boils at lower temperatures.

The idealized model is accurate in the case of chemically similar liquids, such as benzene and toluene. In other cases, severe deviations from Raoult's law and Dalton's law are observed, most famously in the mixture of ethanol and water. These compounds, when heated together, form an azeotrope, which is a composition with a boiling point higher or lower than the boiling point of each separate liquid. Virtually all liquids, when mixed and heated, will display azeotropic behaviour. Although there are computational methods that can be used to estimate the behavior of a mixture of arbitrary components, the only way to obtain accurate vapor-liquid equilibrium data is by measurement.

It is not possible to completely purify a mixture of components by distillation, as this would require each component in the mixture to have a zero partial pressure. If ultra-pure products are the goal, then further chemical separation must be applied. When a binary mixture is evaporated and the other component, e.g. a salt, has zero partial pressure for practical purposes, the process is simpler and is called evaporation in engineering.

Batch distillation

A batch still showing the separation of A and B.

Heating an ideal mixture of two volatile substances A and B (with A having the higher volatility, or lower boiling point) in a batch distillation setup (such as in an apparatus depicted in the opening figure) until the mixture is boiling results in a vapor above the liquid which contains a mixture of A and B. The ratio between A and B in the vapor will be different from the ratio in the liquid: the ratio in the liquid will be determined by how the original mixture was prepared, while the ratio in the vapor will be enriched in the more volatile compound, A (due to Raoult's Law, see above). The vapor goes through the condenser and is removed from the system. This in turn means that the ratio of compounds in the remaining liquid is now different from the initial ratio (i.e. more enriched in B than the starting liquid).

The result is that the ratio in the liquid mixture is changing, becoming richer in component B. This causes the boiling point of the mixture to rise, which in turn results in a rise in the temperature in the vapor, which results in a changing ratio of A : B in the gas phase (as distillation continues, there is an increasing proportion of B in the gas phase). This results in a slowly changing ratio A : B in the distillate.

If the difference in vapor pressure between the two components A and B is large (generally expressed as the difference in boiling points), the mixture in the beginning of the distillation is highly enriched in component A, and when component A has distilled off, the boiling liquid is enriched in component B.

Continuous distillation

Main article: Continuous distillation

Continuous distillation is an ongoing distillation in which a liquid mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams as time passes during the operation. Continuous distillation produces at least two output fractions, including at least one volatile distillate fraction, which has boiled and been separately captured as a vapor condensed to a liquid. There is always a bottoms (or residue) fraction, which is the least volatile residue that has not been separately captured as a condensed vapor.

Continuous distillation differs from batch distillation in the respect that concentrations should not change over time. Continuous distillation can be run at a steady state for an arbitrary amount of time. Given a feed of in a specified composition, the main variables that affect the purity of products in continuous distillation are the reflux ratio and the number of theoretical equilibrium stages (practically, the number of trays or the height of packing). Reflux is a flow from the condenser back to the column, which generates a recycle that allows a better separation with a given number of trays. Equilibrium stages are ideal steps where compositions achieve vapor-liquid equilibrium, repeating the separation process and allowing better separation given a reflux ratio. A column with a high reflux ratio may have fewer stages, but it refluxes a large amount of liquid, giving a wide column with a large holdup. Conversely, a column with a low reflux ratio must have a large number of stages, thus requiring a taller column.

Continuous distillation requires building and configuring dedicated equipment. The resulting high investment cost restricts its use to the large scale.

General improvements

Both batch and continuous distillations can be improved by making use of a fractionating column on top of the distillation flask. The column improves separation by providing a larger surface area for the vapor and condensate to come into contact. This helps it remain at equilibrium for as long as possible. The column can even consist of small subsystems ('trays' or 'dishes') which all contain an enriched, boiling liquid mixture, all with their own vapor-liquid equilibrium.

There are differences between laboratory-scale and industrial-scale fractionating columns, but the principles are the same. Examples of laboratory-scale fractionating columns (in increasing efficiency) include:

Air condenser Vigreux column (usually laboratory scale only) Packed column (packed with glass beads, metal pieces, or other chemically inert

material) Spinning band distillation system.

Laboratory scale distillation

Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as a still, consists at a minimum of a reboiler or pot in which the source material is heated, a condenser in which the heated vapour is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid, called the distillate, is collected. Several laboratory scale techniques for distillation exist (see also distillation types).

Simple distillation

In simple distillation, all the hot vapors produced are immediately channeled into a condenser that cools and condenses the vapors. Therefore, the distillate will not be pure - its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law.

As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C),[26] or to separate liquids from involatile solids or oils. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be neglected due to the insignificant contribution of the less volatile component. In this case, the distillate may be sufficiently pure for its intended purpose.

Fractional distillation

For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column. This separation, by successive distillations, is also referred to as rectification [27].

As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component.[28] In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description.

More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.[29]

Steam distillation

Main article: Steam distillation

Like vacuum distillation, steam distillation is a method for distilling compounds which are heat-sensitive.[30] This process involves using bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water.

Steam distillation of various aromatic herbs and flowers can result in two products; an essential oil as well as a watery herbal distillate. The essential oils are often used in perfumery and aromatherapy while the watery distillates have many applications in aromatherapy, food processing and skin care.

Dimethyl sulfoxide usually boils at 189 °C. Under a vacuum, it distills off into the receiver at only 70 °C.

Perkin triangle distillation setup

1: Stirrer bar/anti-bumping granules 2: Still pot 3: Fractionating column 4: Thermometer/Boiling point temperature 5: Teflon tap 1 6: Cold finger 7: Cooling water out 8: Cooling water in 9: Teflon tap 2 10: Vacuum/gas inlet 11: Teflon tap 3 12: Still receiver

Vacuum distillation

Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator.

This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure.

Molecular distillation is vacuum distillation below the pressure of 0.01 torr.[31] 0.01 torr is one order of magnitude above high vacuum, where fluids are in the free molecular flow regime, i.e. the mean free path of molecules is comparable to the size of the equipment. The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure. That is, because the continuum assumptions of fluid dynamics no longer apply, mass transport is governed by molecular dynamics rather than fluid dynamics. Thus, a short path between the hot surface and the cold surface is necessary, typically by suspending a hot plate covered with a film of feed next to a cold plate with a clear line of sight in between. Molecular distillation is used industrially for purification of oils.

Air-sensitive vacuum distillation

Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a "pig" adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used.

The Perkin triangle, has means via a series of glass or Teflon taps to allows fractions to be isolated from the rest of the still, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of reflux. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such as nitrogen or argon) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected.

Short path distillation

Short path vacuum distillation apparatus with vertical condenser (cold finger), to minimize the distillation path; 1: Still pot with stirrer bar/anti-bumping granules 2: Cold finger - bent to direct condensate 3: Cooling water out 4: cooling water in 5: Vacuum/gas inlet 6: Distillate flask/distillate.

Short path distillation is a distillation technique that involves the distillate travelling a short distance, often only a few centimeters, and is normally done at reduced pressure.[32] A classic example would be a distillation involving the distillate travelling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures or to purify small amounts of compound. The advantage is that the heating temperature can be considerably lower (at reduced pressure) than the boiling point of the liquid at standard pressure, and the distillate only has to travel a short distance before condensing. A short path ensures that little compound is lost on the sides of the apparatus. The Kugelrohr is a kind of a short path distillation apparatus which often contain multiple chambers to collect distillate fractions.

Other types

The process of reactive distillation involves using the reaction vessel as the still. In this process, the product is usually significantly lower-boiling than its reactants. As the product is formed from the reactants, it is vaporized and removed from the reaction mixture. This technique is an example of a continuous vs. a batch process; advantages include less downtime to charge the reaction vessel with starting material, and less workup.

Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous membrane.

Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture.

Flash evaporation (or partial evaporation) is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations, being equivalent to a distillation with only one equilibrium stage.

Codistillation is distillation which is performed on mixtures in which the two compounds are not miscible.

The unit process of evaporation may also be called "distillation":

In rotary evaporation a vacuum distillation apparatus is used to remove bulk solvents from a sample. Typically the vacuum is generated by a water aspirator or a membrane pump.

In a kugelrohr a short path distillation apparatus is typically used (generally in combination with a (high) vacuum) to distill high boiling (> 300 °C) compounds. The apparatus consists of an oven in which the compound to be distilled is placed, a receiving portion which is outside of the oven, and a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump.

Other uses:

Dry distillation or destructive distillation, despite the name, is not truly distillation, but rather a chemical reaction known as pyrolysis in which solid substances are heated in an inert or reducing atmosphere and any volatile fractions, containing high-boiling liquids and products of pyrolysis, are collected. The destructive distillation of wood to give methanol is the root of its common name - wood alcohol.

Freeze distillation is an analogous method of purification using freezing instead of evaporation. It is not truly distillation, but a recrystallization where the product is the mother liquor, and does not produce products equivalent to distillation. This process is used in the production of ice beer and ice wine to increase ethanol and sugar content, respectively. It is also used to produce applejack. Unlike distillation, freeze distillation concentrates poisonous congeners rather than removing them.

Azeotropic distillation

Main article: Azeotropic distillation

Interactions between the components of the solution create properties unique to the solution, as most processes entail nonideal mixtures, where Raoult's law does not hold. Such interactions can result in a constant-boiling azeotrope which behaves as if it were a pure compound (i.e., boils at a single temperature instead of a range). At an azeotrope, the solution contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95.6% at 78.1 °C.

If the azeotrope is not considered sufficiently pure for use, there exist some techniques to break the azeotrope to give a pure distillate. This set of techniques are known as azeotropic distillation. Some techniques achieve this by "jumping" over the azeotropic composition (by adding an additional component to create a new azeotrope, or by varying the pressure). Others work by chemically or physically removing or sequestering the impurity. For example, to purify ethanol beyond 95%, a drying agent or a (desiccant such

as potassium carbonate) can be added to convert the soluble water into insoluble water of crystallization. Molecular sieves are often used for this purpose as well.

Immiscible liquids, such as water and toluene, easily form azeotropes. Commonly, these azeotropes are referred to as a low boiling azeotrope because the boiling point of the azeotrope is lower than the boiling point of either pure component. The temperature and composition of the azeotrope is easily predicted from the vapor pressure of the pure components, without use of Raoult's law. The azeotrope is easily broken in a distillation set-up by using a liquid-liquid separator ( a decanter ) to separate the two liquid layers that are condensed overhead. Only one of the two liquid layers is refluxed to the distillation set-up.

High boiling azeotropes, such as a 20 weight percent mixture of hydrochloric acid in water, also exist. As implied by the name, the boiling point of the azeotrope is greater than the boiling point of either pure component.

To break azeotropic distillations and cross distillation boundaries, such as in the DeRosier Problem, it is necessary to increase the composition of the light key in the distillate.

Breaking an azeotrope with unidirectional pressure manipulation

The boiling points of components in an azeotrope overlap to form a band. By exposing an azeotrope to a vacuum or positive pressure, it's possible to bias the boiling point of one component away from the other by exploiting the differing vapour pressure curves of each; the curves may overlap at the azeotropic point, but are unlikely to be remain identical further along the pressure axis either side of the azeotropic point. When the bias is great enough, the two boiling points no longer overlap and so the azeotropic band disappears.

This method can remove the need to add other chemicals to a distillation, but it has two potential drawbacks.

Under negative pressure, power for a vacuum source is needed and the reduced boiling points of the distillates requires that the condenser be run cooler to prevent distillate vapours being lost to the vacuum source. Increased cooling demands will often require additional energy and possibly new equipment or a change of coolant.

Alternatively, if positive pressures are required, standard glassware can not be used, energy must be used for pressurization and there is a higher chance of side reactions occurring in the distillation, such as decomposition, due to the higher temperatures required to effect boiling.

A unidirectional distillation will rely on a pressure change in one direction, either positive or negative.

Pressure-swing distillation

Further information: Pressure-Swing Distillation (section on the main Azeotrope page)

Pressure-swing distillation is essentially the same as the unidirectional distillation used to break azeotropic mixtures, but here both positive and negative pressures may be employed.[clarification needed]

This has an important impact on the selectivity of the distillation and allows a chemist[citation needed] to optimize a process such that fewer extremes of pressure and temperature are required and less energy is consumed. This is particularly important in commercial applications.

Pressure-swing distillation is employed during the industrial purification of ethyl acetate after its catalytic synthesis from ethanol.

Industrial distillation

Typical industrial distillation towersMain article: Continuous distillation

Large scale industrial distillation applications include both batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical and chemical plants and natural gas processing plants.

Industrial distillation[27][33] is typically performed in large, vertical cylindrical columns known as distillation towers or distillation columns with diameters ranging from about 65 centimeters to 16 meters and heights ranging from about 6 meters to 90 meters or

more. When the process feed has a diverse composition, as in distilling crude oil, liquid outlets at intervals up the column allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column and are often called the bottoms.

(Diagram of a typical industrial distillation tower)

Large-scale industrial towers use reflux to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large-scale industrial distillation tower. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficacy of the distillation tower. The more reflux that is provided for a given number of theoretical plates, the better the tower's separation of lower boiling materials from higher

boiling materials. Alternatively, the more reflux that is provided for a given desired separation, the fewer the number of theoretical plates required.

Such industrial fractionating towers are also used in air separation, producing liquid oxygen, liquid nitrogen, and high purity argon. Distillation of chlorosilanes also enables the production of high-purity silicon for use as a semiconductor.

Section of an industrial distillation tower showing detail of trays with bubble caps

Design and operation of a distillation tower depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method [27] [34] or the Fenske equation [27] can be used. For a multi-component feed, simulation models are used both for design and operation. Moreover, the efficiencies of the vapor-liquid contact devices (referred to as "plates" or "trays") used in distillation towers are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation tower needs more trays than the number of theoretical vapor-liquid equilibrium stages.

In industrial uses, sometimes a packing material is used in the column instead of trays, especially when low pressure drops across the column are required, as when operating under vacuum.

Large-scale, industrial vacuum distillation column[35]

This packing material can either be random dumped packing (1-3" wide) such as Raschig rings or structured sheet metal. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Unlike conventional tray distillation in which every tray represents a separate point of vapor-liquid equilibrium, the vapor-liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns, it is useful to compute a number of "theoretical stages" to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.

Another factor in addition to the packing shape and surface area that affects the performance of random or structured packing is the liquid and vapor distribution entering the packed bed. The number of theoretical stages required to make a given separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio will not be correct in the packed bed and the required separation will not be

achieved. The packing will appear to not be working properly. The height equivalent of a theoretical plate (HETP) will be greater than expected. The problem is not the packing itself but the mal-distribution of the fluids entering the packed bed. Liquid mal-distribution is more frequently the problem than vapor. The design of the liquid distributors used to introduce the feed and reflux to a packed bed is critical to making the packing perform to it maximum efficiency. Methods of evaluating the effectiveness of a liquid distributor to evenly distribute the liquid entering a packed bed can be found in references.[36][37] Considerable work as been done on this topic by Fractionation Research, Inc. (commonly known as FRI).[38]

Distillation in food processing

Distilled beverages

Main article: Distilled beverage

Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Components other than ethanol, including water, esters, and other alcohols, are collected in the condensate, which account for the flavor of the beverage.

Distillation : Basic Theory Part 01

Distilasi adalah suatu proses yang melibatkan campuran liquid atau uap yang terdiri dari dua atau lebih komponen dipisahkan menjadi fraksi komponen yang diinginkan, dengan memasukan dan mengeluarkan panas. Pemisahan komponen dari campuran liquid dengan distilasi tergantung pada titik didih masing-masing komponen. Dan juga tergantung pada konsentrasi, karena masing-masing mempunyai karakteristik titik didih. Sehingga proses distilasi tergantung pada karakteristik tekanan uap campuran liquid

Dalam kolom distilasi akan terdapat transfer panas atau energi yang tentu akan menaikan tekanan uap, di mana tekanan uap berhubungan dengan titik didih. Liquid akan mendidih pada saat tekanan uapnya sama dengan lingkungannya. Kemudahan liquid untuk mendidih tergantung pada jumlah komponen volatile yang ada pada liquid. Liquid dengan tekanan uap tinggi (high volatility) akan menguap pada temperatur yang lebih rendah. Distilasi terjadi karena adanya perbedaan komponen volatility pada campuran liquid.

Perpindahan massa pada kolom distilasi terjadi pada suatu stage dengan memanfaatkan kesetimbangan fasa uap-cair dari suatu komponen. Tekanan uap liquid pada temperatur tertentu terjadi kesetimbangan antara molekul meninggalkan atau masuk permukaan liquid. Cairan dan uap yang tidak berada dalam kondisi setimbang akan dikontakkan hingga terjadi perpindahan massa dan produk dalam stage tersebut akan mendekati kondisi kesetimbangan. Komponen-komponen volatile diharapkan akan banyak berada pada uap yang meninggalkan stage dibandingkan dengan uap yang memasuki stage, sebaliknya diharapkan cairan yang meninggalkan stage akan memiliki

komponen-komponen volatile. Bila proses ini dilakukan berulang-ulang diharapkan akan di dapatkan derajat pemisahan yang tinggi. Distilasi secara umum dapat dibedakan menjadi:

1. Distilasi Atmosferik

• Dilakukan pada tekanan sedikit diatas tekanan atmosfir

• Minyak dipanaskan sampai temperatur tertentu sebelum terjadi perengkahan.

• Aplikasi : Crude Distillation Unit

2. Distilasi Vakum

• Untuk minyak berat bertitik didih tinggi yang jika dipanaskan lebih lanjut pada

tekanan atmosfir akan terjadi perengkahan.

• Dilakukan pada tekanan dbawah satu atmosfir (vakum).

• Aplikasi : Vacuum Unit

3. Distilasi Bertekanan

• Untuk minyak yang sudah menguap pada temperatur kamar.

• Aplikasi : Light End Unit (Debutanizer, Depropanizer, naptha splitter).

Sistem kompleks adalah adalah sistem yang terdiri dari banyak sekali komponen sehingga tidak layak untuk menentukan komposisi campuran tersebut dinyatakan dalam komponen-komponen murninya. Contohnya adalah campuran petroleum. Pada umumya analisa penentuan titik didih pada campuran petroleum menggunakan dua cara, yaitu distilasi ASTM (American Society for Testing and Material) dan distilasi TBP (True Boiling Point).

1. Distilasi TBP

• Disebut distilasi 15/5, kolom eqivalent dengan 15 tahap (plate) & perbandingan

refluks 5/1.

• Derajat kemurnian relatif tinggi, setiap komponen terpisahkan dengan baik (dari

komponen ringan sampai dengan komponen berat).

• Kondisi operasi, tekanan atmospferik & temperatur sampai dengan 316 oC (600 oF), kemudian dilanjutkan dengan tekanan vacum dengan tujuan mencegah

perengkahan fraksi minyak yang berat.

• Volume minyak mentah 1000-5000 cc sehingga volume distilate setiap fraksi

banyak dan cukup untuk analisa kualitas fraksi.

2. Distilasi ASTM atau distilasi Engler

• Derajat kemurnian relatif rendah (tidak ada kolom & refluks).

• Hasil distilasi ASTM dapat digunakan untuk menganalisa minyak mentah.

• Analisa cepat.

• Banyak digunakan untuk mengontrol operasi.

• Untuk minyak mentah dan produk – produk minyak mentah.

• Volume 100 cc.

• Tekanan atmosferik.

• Pemanasan diatur sedemikian rupa pada 5 – 10 menit diperoleh tetesan pertama,

hasil dikumpulkan dengan kecepatan 4 – 5 cc per menit.

• Temperature uap tetesan pertama disebut IBP (Initial Boiling Point).

• Temperature selanjutnya dicatat setelah hasil distillate terkumpul 5 ml, 10 ml

dan setiap mendapat 10 ml distilate berikutnya.

• Temperature uap maksimum pada tetesan terakhir disebut (End Point)

Campuran dalam fasa cair yang dipanaskan dalam suatu kolom (bejana) akan mengalami keseimbangan fase uap dan fase cair yang berlangsung singkat, peristiwa ini disebut Equilibrium Flash Vaporization (EFV). Kurva Equlibrium Flash Vaporization dibuat berdasarkan data Distilasi TBP atau Distilasi ASTM dengan bantuan grafik. Kurva EFV bermanfaat untuk menetukan performance dari unit Flash Distillation pada campuran kompleks.

Variabel Operasi Variabel-variabel yang mempengaruhi operasi kolom stripper adalah sebagai berikut:

1. Temperatur umpan masuk kolom

Temperatur umpan mempengaruhi jumlah komponen yang teruapkan pada flash zone,

bila temperatur terlalu rendah, maka akan banyak fraksi ringan yang jatuh ke produk

bawah dan sebaliknya bila terlalu tinggi fraksi berat akan terikut ke atas

2. Tekanan kolom

Tekanan kolom akan berpengaruh terhadap temperatur penguapan cairan, bila

tekanan kolom rendah maka temperatur yang dibutuhkan juga rendah.

3. Sifat fisik umpan

Semakin banyak fraksi berat pada umpan, maka dibutuhkan energi yang lebih besar

untuk memisahkannya.

4. Refluks

Refluks berfungsi untuk menurunkan beban pendinginan pada kondensor, dengan

pendinginan ini secara tidak langsung refluks mempengaruhi perolehan produk. Bila

laju refluks terlalu tinggi dkhawatirkan fraksi ringan akan terikut pada fraksi di

bawahnya dan begitu juga sebaliknya.

Pengertian Distilasi

Distilasi adalah suatu cara pemisahan larutan dengan menggunakan panas sebagai pemisah atau “separating agent”. Jika larutan yang terdiri dari dua buah komponen yang cukup mudah menguap, misalnya larutan benzena-toluena, larutan n-Heptan dan n-Heksan dan larutan lain yang sejenis didihkan, maka fase uap yang terbentuk akan

mengandung komponen yang lebih menguap dalam jumlah yang relatif lebih banyak dibandingkan dengan fase cair.

Jadi ada perbedaan komposisi antara fase cair dan fase uap, dan hal ini merupakan syarat utama supaya pemisahan dengan distilasi dapat dilakukan. Kalau komposisi fase uap sama dengan komposisi fase cair, maka pemisahan dengan jalan distilasi tidak dapat dilakukan.

Proses distilasi dalam kilang minyak bumi merupakan proses pengolahan secara fisika yang primer yang mengawali semua proses-proses yang diperlukan untuk memproduksi BBM dan Non-BBM. Proses distilasi ini dapat menggunakan satu kolom atau lebih menara distilasi, misalnya residu dari menara distilasi dialirkan ke menara distilasi hampa atau ke menara distilasi bertekanan.

Secara fundamental semua proses-proses distilasi dalam kilang minyak bumi adalah sama. Semua proses distilasi memerlukan beberapa peralatan yang penting seperti :

- Kondensor dan Cooler

- Menara Fraksionasi

- Kolom Stripping

Proses pemisahan secara distilasi dengan mudah dapat dilakukan terhadap campuran, dimana antara komponen satu dengan komponen yang lain terdapat dalam campuran :

a. Dalam keadaan standar berupa cairan, saling melarutkan menjadi campuran homogen.

b. Mempunyai sifat penguapan relatif (α) cukup besar.

c. Tidak membentuk cairan azeotrop.

Pada proses pemisahan secara distilasi, fase uap akan segera terbentuk setelah sejumlah cairan dipanaskan. Uap dipertahankan kontak dengan sisa cairannya (dalam waktu relatif cukup) dengan harapan pada suhu dan tekanan tertentu, antara uap dan sisa cairan akan berada dalam keseimbangan, sebelum campuran dipisahkan menjadi distilat dan residu.

Fase uap yang mengandung lebih banyak komponen yang lebih mudah menguap relatif terhadap fase cair, berarti menunjukkan adanya suatu pemisahan. Sehingga kalau uap yang terbentuk selanjutnya diembunkan dan dipanaskan secara berulang-ulang, maka

akhirnya akan diperoleh komponen-komponen dalam keadaan yang relatif murni.

Keseimbangan Uap –Cair

Untuk dapat menyelesaikan soal-soal distilasi harus tersedia data-data keseimbangan uap-cair sistim yang dikenakan distilasi. Data keseimbangan uap-cair dapat berupa tabel atau diagram. Tiga macam diagram keseimbangan yang akan dibicarakan, yaitu :

· Diagram Titik didih

Diagram titik didih adalah diagram yang menyatakan hubungn antara temperatur atau titik didih dengan komposisi uap dan cairan yang berkeseimbangan. Di dalam diagram titik didih tersebut terdapat dua buah kurva, yaitu kurva cair jenuh dan uap jenuh. Kedua kurva ini membagi daerah didalam diagram menjadi 3 bagian, yaitu :

1. Daerah satu fase yaitu daerah cairan yang terletak dibawah kurva cair jenuh.

2. Daerah satu fase yaitu daerah yang terletak datas kurva uap jenuh.

3. Daerah dua fase yaitu daerah uap jenuh dan cair jenuh yang terletak di antara kurva cair jenuh dan kurva uap jenuh

· Diagram Keseimbangan uap-cair

Diagram keseimbangan uap-cair adalah diagram yang menyatakan hubungan keseimbangan antara komposisi uap dengan komposisi cairan. Diagram keseimbangan uap-cair dengan mudah dapat digambar, jika tersedia titik didihnya.

· Diagram Entapi-komposisi

Diagram entalpi-komposisi adalah diagram yang menyatakan hubungan antara entalpi dengan komposisi sesuatu sistim pada tekanan tertentu. Didalam diagram tersebut terdapat dua buah kurva yaitu kurva cair jenuh dan kurva uap jenuh. Setiap titik pada kurva cair jenuh dihubungkan dengan gari hubung “tie line” dengan titik tertentu pada kurva uap jenuh, dimana titik-titik tersebut dalam keadaan keseimbangan. Dengan adanya kedua kurva tersebut, daerah didalam diagram terbagi menjadi 3 daerah, yaitu

1. Daerah cairan yang terletak dibawah kurva cair jenuh.

2. Daerah uap yang terletak diatas kurva uap jenuh.

3. Daerah cair dan uap yang terletak diantara kurva cair jenuh dengan kurva uap jenuh

Dibawah kurva cair jenuh terdapat isoterm-isoterm yang menunjukkan entalpi cairan pada berbagai macam komposisi pada berbagai temperatur.

2.2 Macam-macam Distilasi

Distilasi berdasarkan prosesnya terbagi menjadi dua, yaitu :

1. Distilasi kontinyu2. Distilasi batch

Berdasarkan basis tekanan operasinya terbagi menajdi tiga, yaitu :

1. Distilasi atmosferis (0,4-5,5 atm mutlak)2. Distilasi vakum (≤ 300 mmHg pada bagian atas kolom)3. Distilasi tekanan (≥ 80 psia pada bagian atas kolom)

Berdasarkan komponen penyusunnya :

1. Distilasi sistem biner2. Distilasi sitem multi komponen

Berdasarkan sistem operasinya terbagi dua, yaitu :

1. Single-stage Distillation2. Multi stage Distillation

Distilasi Vakum

Distilasi vakum adalah distilasi yang tekanan operasinya 0,4 atm (300 mmHg absolut). Distilasi yang dilakukan dalam tekanan operasi ini biasanya karena beberapa alasan yaitu:

a. Sifat penguapan relatif antar komponen biasanya meningkat seiring dengan menurunnya boiling temperature. Sifat penguapan relatif yang meningkat memudahkan

terjadinya proses separasi sehingga jumlah stage teoritis yang dibutuhkan berkurang. Jika jumlah stage teoritis konstan, rasio refluks yang diperlukan untuk proses separasi yang sama dapat dikurangi. Jika kedua variabel di atas konstan maka kemurnian produk yang dihasilkan akan meningkat.

b. Distilasi pada temperatur rendah dilakukan ketika mengolah produk yang sensitif terhadap variabel temperatur. Temperatur bagian bawah yang rendah menghasilkan beberapa reaksi yang tidak diinginkan seperti dekomposisi produk, polimerisasi, dan penghilangan warna.

c. Proses pemisahan dapat dilakukan terhadap komponen dengan tekanan uap yang sangat rendah atau komponen dengan ikatan yang dapat terputus pada titik didihnya.

d. Reboiler dengan temperatur yang rendah yang menggunakan sumber energi dengan harga yang lebih murah seperti steam dengan tekanan rendah atau air panas.

Distilasi Multikomponen

Perhitungan distilasi multikomponen lebih rumit dibandingkan dengan perhitungan distilasi biner karena tidak adapat digunakan secara grafis. Dasar perhitungannya adalah penyelesaian persamaan-persamaan neraca massa, neraca energi dan kesetimbangan secara simultan. Bila distilasi melibatkan C komponen dengan N buah tahap kesetimbangan maka jumlah persamaan yang terlibat dalam perhitungan adalah N × C persamaan neraca massa, N × C relasi kesetimbangan dan N persamaan neraca energi.

Perhitungan distilasi multikomponen dilakukan dengan 2 tahap :

1. Perhitungan awal, dilakukan dengan metode pintas (Shortcut Calculation)

Perhitungan awal digunakan untuk analisis kualitatif dari suatu kolom distilasi atau perhitungan awal rancangan dengan tujuan :

1.* Memperkirakan komposisi produk atas dan bawah* Tekanan sistem* Jumlah tahap kesetimbangan* Lokasi umpan masuk2. Perhitungan tahap demi tahap dilakukan dengan metode eksak yang merupakan penyelesaian banyak persamaan aljabar :* Metode sederhana dengan kalkulator* Metode MESH dengan program komputer

Single-stage Distillation

Single-stage Distillation biasa juga disebut dengan flash vaporization atau equilibrium distillation, dimana campuran cairan diuapkan secara parsial. Pada keadaan setimbang, uap yang dihasilkan bercampur dengan cairan yang tersisa, namun pada akhirnya uap tersebut akan dipisahkan dari kolom seperti juga fase cair yang tersisa. Distilasi jenis ini dapat dilakukan dalam kondisi batch maupun kontinyu.

2.3 Tray Tower

Tray tower merupakan bejana vertikal dimana cairan dan gas dikontakkan melalui plate-plate yang disebut sebagai tray. Fungsi dari penggunaan tray adalah untuk memperbesar kontak antara cairan dan gas sehingga komponen dapat dipisahkan sesuai dengan rapat jenisnya, dalam bentuk gas atau cairan. Jumlah tahapan atau tray dalam suatu kolom tergantung pada tingginya kesulitan pemisahan zat yang akan dilakukan dan juga ditentukan berdasarkan perhitungan neraca massa dan kesetimbangan. Efisiensi tray dan jumlah tray yang sebenarnya ditentukan oleh desain yang digunakan dan kondisi operasi, sedangkan diameter kolom bergantung pada jumlah gas dan cairan yang melewati kolom per unit waktu.

Untuk mendapatkan produk yang baik diperlukan alat kontak antara uap dengan cairan. Beberapa jenis alat kontak antara uap dengan cairan adalah bubble cap tray, grid tray, sieve tray dan valve tray.

Sieve Tray

Sieve tray merupakan jenis tray yang paling sederhana dibandingkan jenis tray yang lain dan lebih murah daripada jenis bubble cap. Pada Sieve tray uap naik ke atas melalui lubang-lubang pada plate dan terdispersi dalam cairan sepanjang plate. Cairan mengalir turun ke plate di bawahnya melalui down comer dan weir.

Meskipun sive tray mempunyai kapasitas yang lebih besar pada kondisi operasi yang sama dibandingkan dengan bubble cap, namun sieve tray mempunyai satu kekurangan yang cukup serius pada kecepatan uap yang relatif lebih rendah dibandingkan pada kondisi operasi normal. Pada sieve tray, aliran uap berfungsi mencegah cairan mengalir bebas ke bawah melalui lubang-lubang, tiap plate di desain mempunyai kecepatan uap minimum yang mencegah terjadinya peristiwa “dumps” atau “shower” yaitu suatu peristiwa dimana cairan mengalir bebas mengalir ke bawah melalui lubang-lubang pada plate.

Kecepatan uap minimum ini yang harus amat sangat diperhatikan dalam mendesain sieve tray dan menjadi kesulitan tersendiri dalam kondisi operasi sesungguhnya.Efisiensi sieve tray sama besarnya dengan bubble cap pada kondisi desain yang sama, namun menurun jika kapasitasnya berkurang di bawah 60% dari desain.

Sectional construction

Seksi plate dipasang pada cincin yang dilas di sekeliling dinding kolom bagian dalam dan pada balok-balok penyangga. Lebar balok penyangga dan cincin sekitar 50 mm, dengan jarak antar satu balok dengan yang lainnya sekitar 0.6 m. Balok penyangga dipasang horizontal sebagai penyangga plate, biasanya di bentuk dari lembaran yang dilipat atau dibentuk. Satu bagian dari plate di desain bisa di pindahkan yang berfungsi sebagai manway. Hal ini bertujuan untuk mengurangi jumlah manway yang dapat mengurangi biaya konstruksi.

Downcomers

Downcomer terdapat pada semua equilibrium-stage trays, bertujuan sebagai media cairan untuk mengalir dari tray atas ke tray di bawahnya. Downcomer di desain untuk menyediakan kapasitas penanganan cairan yang cukup untuk kolom distilasi dan pada waktu yang sama untuk memenuhi luas minimum dari area cross-sectional, sehingga area aktif dari pada tray akan maksimum. Jenis-jenis downcomer dapat dilihat pada gambar di bawah ini.Merupakan jenis yang paling sederhana dan murah dalam konstruksi dan paling memuaskan untuk berbagai macam tujuan. Channel downcomer dibentuk dari plat rata yang kemudian disebut apron yang dipasang dengan posisi ke bawah dari outlet weir. Apron biasanya vertikal, namun bisa juga agak miring untuk meningkatkan area plate untuk perforation.

Flooding

Flooding terjadi jika busa pada plate berakumulasi melebihi penyangga downcomer. Downcomer kemudian mengandung campuran yang mempunyai densitas yang lebih rendah dari cairan murni, kapasitasnya berkurang, level cairan meningkat pada downcomer sampai akhirnya mencapai tray di atasnya dan selanjutnya akan mencapai keadaan dimana cairan memenuhi kolom

Weep Point.

Weep point bisa diartikan sebagai kecepatan minimum uap yang dapat memberikan kestabilan kondisi operasi.

Tray spacing

Tray spacing merupakan jarak antara satu tray dengan tray yang lainnya. Biasanya sekitar 6 inci lebih pendek dari bubble cap tray. Sieve tray beroperasi pada spacing sekitar 9 inci sampai 3 inci. Yang biasa digunakan adalah sekitar 12-16 inci.

Hole Size, arrangement and Spacing

Diameter lubang dan pengaturannya bervariasi tergantung kebutuhan dan keinginan dari yang mendesain. Yang biasa dipakai untuk kegiatan komersil yaitu diameter ¾ dan 1 inci. Diameter lubang direkomendasikan untuk self cleaning yaitu 3/16 inci. Diameter ½ inci bisa digunakan untuk berbagai macam kebutuhan termasuk yang melibatkan fouling dan cairan yang mengandung solid tanpa kehilangan efisiensi. Diameter 1/8 inci sering digunakan untuk kondisi vakum

Pengaturan posisi lubang atau arrangement bisa berupa triangular pitch (segitiga) atau square pitch (segiempat), lebih jelasnya bisa dilihat pada gambar di bawah ini.Jika jarak antar lubang dua kali diameter maka cenderung akan mengalami “unstable operation”. Jarak lubang yang direkomendasikan adalah 2.5 do sampai 5 do, dan yang paling direkomendasikan 3.8 do.

Active Hole Area

Ialah luasan total pada plate termasuk di dalamnya ialah perforated area dan calming zone.

Perforated Area

Perforated area atau hole area ialah area pada plate dimana masih terdapat lubang-lubang tempat kontaknya cairan dan uap.

Calming Zone

Ialah area pada plate yang tidak terdapat lubang-lubang.

Height of Liquid Over Outlet Weir, how

Batas minimum tinggi weir adalah 0.5 inci, dengan 1-3 inci yang paling direkomendasikan. Untuk lebih jelasnya biasa dilihat pada gambar di bawah ini.

Untuk menentukan jumlah tahap yang dibutuhkan pada distilasi multi komponene diperlukan dua kunci, yaitu Light Key Component (LK) dan Heavy Key Component (HK) komponen. Light Key Component adalah komponen fraksi ringan pada produk bawah dalam jumlah kecil tapi tidak dapat diabaikan. Heavy Key Component adalah komponen fraksi berat pada produk atas dalam jumlah kecil yang tidak dapat diabaikan. LK dan HK diperlukan untuk mengetahui distribusi komponen lain. Jumlah tahap yang diperlukan untuk pemisahan juga tergantung pada rasio refluks (perbandingan refluks)

yang digunakan.

R=

Dengan menaikkan reflux akan menurunkan jumlah tahap yang dibutuhkan dan menurunkan capital cost tetapi hal ini akan menaikkan kebutuhan steam serta operating cost. Sehingga diperlukan nilai rasio optimum yang memberikan biaya operasi yang rendah. Untuk mendapatkan beberapa sistem nilai rasio optimum antara 1,2 sampai 1,5 kali refluks minimum.

Efisiensi Tray

Efisiensi tray adalah pendekatan fraksional terhadap kondisi kesetimbangan yang dihasilkan oleh tray aktual. Untuk itu dibutuhkan pengukuran terhadap kesetimbangan seluruh uap dan cairan yang berasal dari tray, namun karena kondisi dari beberapa lokasi pada tray berbeda antara tray sartu dengan yang lain, digunakan pendekatan titik efisiensi akibat perpindahan massa tray

Untuk menghitung efisiensi dari pemisahan umpan menjadi produk atas dan produk bawah digunakan tahapan-tahapan sebagai berikut:

1. Menentukan jumlah plate minimum dengan metode Fenske.

2. Menetukan jumlah refluk minimum dengan metode Underwood.

3. Menentukan jumlah plate teoritis dengan metode:

a. Grafik Gilliland

gambar menara destilasi