cong nghe butamer

8/14/2019 Cong Nghe Butamer

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Department of Chemical Engineering

Title: The Hydroisomerization of Butane to Isobutane

Class: CHE 594: Refining of Oil and Synthetic Liquids

Report Written by: Ryan Lee Robles, 1100884

Submitted to: Dr. Arno de Klerk

Date Submitted: December 8, 2010

Signature of Report Writer: _______________________________________________


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Summary

The renewed interest in the hydroisomerization of butane has become an emergent process in

the wake of increased environmental concerns. The phase out and reduction of octane boosters,

such as MTBE and aromatic compounds has left a niche void in the fuel industry. Isomerization

serves as a viable precursor for the production of high performance gasoline, possessing the

ability to meet more stringent environmental regulations and standards. Specifically, iso-butane

is used as feed to an alkylation unit, responsible for the production of high octane fuel additives.

The hydroisomerization of butane to isobutane was overviewed.

The dominant hydroisomerization technology in industry is the UOP Butamer process,

capable of adaptive flow schemes dependent on process requirements.

The process is a fixed-bed, catalytic process occurring in the vapour phase. Optimal feed

will contain relatively high amounts of normal butane. However, the process is equilibrium

limited, typically peaking at 60% yield. Operation using the classic bifunctional catalyst-

platinum impregnated aluminum- occurs under moderate conditions due to the high selectivity.

Acidity is maintained through continuous injection of chlorinated compound, potentially

introducing corrosion to both process and downstream equipment.

The reaction proceeds through an alkene intermediate formed by dehydrogenation on the

metal sites and completes with hydrogenation of an iso-alkene to isobutane.

Extra precaution must be taken during feed pre-treatment as the catalyst is highly sensitive to

feed poisons, especially water and sulphur. A suggested alternative catalyst uses a zeolitic

structure more resistant to contaminants, but requires harsher operating conditions.

Process economics will always have a significant role in many factors of the overall

process. Thus, it is crucial to pay close attention to the significant operating parameters

including temperature, pressure and the hydrogen-to-hydrocarbon ratio of the feed. Alternative

schemes and consideration of advanced process control techniques can lead to optimal operating

conditions, making hydroisomerization a practical process in meeting continually changing fuel

specifications.


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Table of Contents

Summary i

Table of Contents ii

List of Figures iii

List of Tables iii

Introduction 1

1 The Process

Feed

Products

Process Performance

Process Unit: DriersProcess Unit: Reactors

Process Unit: Stabilizer

Utilities

2

5

5

5

56

6

7

2 Reaction Chemistry

Temperature

Pressure

Hydrogen-to-Hydrocarbon Ratio (H2/HC)

7

8

8

9

3 Catalysis

New Generation Catalysts

9

10

4 Process Engineering

Mass and Energy Balance

Concerns

11

11

13

5 Supplement

Process Control

Process Economics

13

13

14

Conclusion 15

References 16


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List of Figures

Figure 1.1 Hydroisomerization Process with Complete Recycle 3

Figure 1.2 Basic Process Flow Diagram for the UOP HOT Butamer Process 4

List of Tables

Table 4.1 Estimated Operating Requirements 11

Table 4.2 Composition of Field Butane Feedstock 12

Table 4.3 Estimated Product Yield Based on the Field Butane Feedstock 12


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Introduction

Based on biogenic theory, oil is composed of compressed hydrocarbons formed ages ago

in a process that began when ancient biomass and other fossilized organic material, buried under

millions of years of sediment, were subjected to extreme pressures and temperatures. The oil

industry began to develop over five thousand years ago19. Oil seeping from the ground was

utilized in the Middle East for waterproofing boats and baskets. It was also proposed the

Neolithic period pioneered the oil movement wherein high commodity whale oil was used as

lamp oil. It was not until the advent of kerosene production through simple atmospheric

distillation that the modern era began. From then, two major developments changed the face of

the oil industry: Edisons economical light bulb virtually eliminated the use of kerosene as a light

source and the emergence of the internal combustion engine created demand for both diesel fuel

and motor gasoline fractions. New techniques were required in order to meet the growing

demand for these specific petroleum fractions. For instance, alkylation, catalytic cracking (the

hammer) and catalytic reforming (including hydroisomerization) are all processes used to

maximize the volume and overall quality of fuels.

In the modern era, environmental consciousness has become equally important as

meeting demands. The push to become more green has led to more stringent environmental

conditions. Concern over the usage of fuel and oil has resulted in drastic changes in regulations,

having an impact on gasoline, jet fuels and lubricating oils. Regulations are calling for more

efficient and cleaner burning fuels. To overcome these new challenges, there is a continual need

for both the progressive improvement of current technologies and the development of practical,

innovative ideas.

For instance, the hydroisomerization (isomerization) process was greatly affected by the

advent of World War II. With the increasing demand for high octane aviation fuels, came the

need for improved refinery technologies. Aliphatic alkylation processes based on the Friedel-

Crafts reaction were developed in the 1930s as a collaborative effort of several American

companies to fuel Allied warplanes17. During alkylation, an alkene is essentially bonded with

isobutane (iC4) to form a high octane gasoline additive. Despite being adequate for the times,

there were still several issues with the process. Excessive corrosion, sludge formation and high

catalyst consumption meant alkylation units were plagued with costly maintenance and operating

expenses. As well, the supply of iC4 from straight-run sources was limited. Renewed interest in


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the hydroisomerization process began to emerge with the initial phase out of tetra ethyl lead in

the 1970s. Following amendments to the Clean Air Act25 which led to the phase out of leaded

gasoline in Europe and the US, oxygenates (such as MTBE, ETBE) became primary gasoline

additives as octane boosters. However, oxygenates have recently garnered environmental

attention due to their discovery as a surface and groundwater contaminant2, suspected of emitting

toxic formaldehyde or peroxyacetyl nitrate. Further, the European Program on Emission, Fuel

and Engine Technologies has suggested the environmental sustainability of gasoline may be

improved through the reduction of alkene, aromatic, oxygen and sulphur contents12. Aromatics

and alkenes react with nitrogen oxides to form ozone, contributing to smog formation. Despite,

the reduction of aromatic content in motor gasoline will have negative effects on the fuel quality.

An alternative process must be found to replace the void left by these components.

Light branched alkanes are a viable replacement for the production of environmentally

sustainable fuels. However, as mentioned before, the straight-run fraction of i-alkanes in crude

oil is minimal. This shortcoming stimulated the original development of the bifunctional catalyst

and its use in the hydroisomerization process of converting normal butane (nC4) to isobutane.

1 THE PROCESS

The hydroisomerization of alkanes is of substantial importance in petroleum refining.

The transformation of a normal straight-chain alkane into an isomerised branched alkane results

in a high octane additive, meaning a more efficient and environmentally cleaner burning fuel. In

general, the degree of branching is directly related to the octane number. There are two main

isomerization processes: the isomerization of lower n-alkanes (nC5-nC7) for the production of

high octane blending components; and the nC4 conversion as feed for the production of alkylate.

Of interest is the hydroisomerization of butane to isobutane, which is a result of skeletal

rearrangement in the presence of hydrogen. The end products are simply isomerized

hydrocarbons containing the same carbon number distribution as the feed.

The iC4 is used primarily as feed to an alkylation unit. Alkylate blending has seen a

significant increase in demand due to its high octane, low vapour pressure blending

characteristics and the gradual shift away from certain oxygenates. For instance, the reduction of

MTBE as a gasoline additive must be compensated with other environmentally sound fuel

quality boosters. Motor fuel alkylate can effectively fill this void, resulting in reduced carbon


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monoxide (CO) and hydrocarbon (HC) emissions, crucial to meeting current environmental

regulations.

Figure 1.1 outlines the main hydroisomerization process with a complete recycle stream

for unconverted hydrogen and normal alkanes.

Reactor(s)

Stabilizer

nC4 Feed

Makeup Hydrogen

Gas

Isomerate

Desorption

Recycle of Uncoverted H2 and nC4

Adsorption

LPG

Figure 1.1 Hydroisomerization Process with Complete Recycle5

Hydroisomerization is a fixed bed process occurring in the vapour phase which employs

the use of a platinum impregnated chloride-alumina catalyst. Due to the high activity of the

catalyst, the process is able to operate under moderate conditions (180-220C, 1.5-3MPa and

space velocity of 2h-1)18. However, a continuous injection of a chlorinated organic compound

(carbon tetrachloride) is necessary to maintain the acidity of the support which will inherently

produce a corrosive by-product. A make-up stream of hydrogen is also introduced and serves

two main purposes. First, in conjunction with platinum, there is prevention of coke deposits.

Second, the hydrogen acts as a suppressant to the polymerization of alkene intermediates formed

during the reaction.

The normal butane is separated from the isobutane through fractionation. Since the

reaction is equilibrium limited, any unconverted feed material and hydrogen is recovered for

recycling to the isomerization reactors.


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The first hydroisomerization unit was commissioned in 1953 by UOP17, followed by

British Petroleums C4 Isomerization unit and Shells HYSOMER unit in 1970. The Butamer

process currently dominates industry as the pioneer technology for the hydroisomerization of

butane. Depending on the specific application, the overall process will vary slightly for each

hydroisomerization unit. A simplified process flow diagram of the hydrogen-once-through

(HOT) Butamer process is shown in Figure 1.2.

Dryer

Dryer

Reactor(s)

Stabilizer

Separator

nC4 Feed

Makeup Hydrogen

Gas to Scrubb ing

and Fuel

Isomer ised

Product

Figure 1.2 Basic Process Flow Diagram for the UOP HOT Butamer Process23

Many C4 isomerization units (and specifically the Butamer Process) have been

commissioned. Facilities currently employing this technology include Amoco (Texas City,

Texas), Chevron (Salt Lake City, Utah), Marathon (Garyville, LA) and the Paso Refinery (El

Paso, Texas)24.


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Feed

The catalyst used in the process is highly susceptible to heteroatom and water poisoning.

Before reaching the isomerization unit, the feed is commonly pretreated through hydrotreating

processes such as hydrodesulphurization (HDS) and hydrodenitrogenation (HDN).

According to UOP (2007), the Butamer unit is able to process natural gas liquids (NGL)

from one of their other proprietary processes, the UOP NGL recovery unit. Otherwise, the

optimal feedstock into the hydroisomerization unit will contain high amounts of normal butane

with lower concentrations of isobutane, pentane (nC5) and other heavier components. For feed

streams with considerable amounts of iC4 or nC5 (approximately 30% or more17), an isostripper

column or deisobutanizer (DIB) can be used to enrich the stream with nC4. Feeds already rich in

nC4 do not require any pre-treatment and are fed directly to the reactor section. The initial

feedstock is combined with make-up hydrogen, preheated and fed to the reactor. The hydrogen

to carbon ratio does not significantly influence the reaction, but more importantly prevents

deactivation of the catalyst through decreasing the coke deposits. The presence of hydrogen also

suppresses the polymerization of alkene intermediates in the reaction. A recycle gas stream and

product separator may be necessary for excess hydrogen and unconverted feed material recovery.

Products

The process produces iC4and hydrogen streams. Heavier by-products and light gas may

also be created. Typical yields, regardless of feed content are approximately 60 % by volume17.

Typical compositions will be outlined in section 4.

Process Performance10

Naturally, the isobutane ratio is the main indicator of performance for a butane

isomerization unit, indicating the amount of desired product.

(1)

Process Unit: Driers13

A molecular sieve is commonly used to remove water from both the hydrogen and

hydrocarbon feeds. As an extra precaution to prevent catalyst deactivation, certain molecular


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sieves are also capable of removing sulphur compounds. Driers may be operated in parallel to

allow for continuous operation during off-line water desorption in the saturated molecular sieve.

Process Unit: Reactors17

There is immediate catalyst deactivation at the inlet of the reactor which progressively

deteriorates moving down the catalytic bed. A two reactor swing system can be employed to

counteract this loss of on-stream efficiency introduced by this deactivation profile. Once the

catalyst in one reactor is killed, the swing operation to the second reactor will allow

simultaneously, the replacement of the spent catalyst in reactor one and the continuous

isomerization of feed in the current reactor. In addition, the first reactor may be operated at high

temperatures in order to achieve the faster reaction rates, while the second reactor can be

operated at moderate temperatures to increase the selectivity through a more favourable

thermodynamic equilibrium. The equilibrium concentration of iC4 is 60% at 180C, but only

40% at 300C6.

There are two main advantages with the dual reactor operation. First, the cycle length

will only be contingent on regular shutdown and maintenance allowing for continued operation

during catalyst replacement. The spent catalyst is usually vacuum- or gravity dumped from the

reactor. Prior to replacement, residual hydrocarbons may be removed from the catalyst through a

nitrogen and oxygen sweep

24

. The removal and replacement of spent catalyst may becomparatively lengthy and will require, at minimum, shut down of the reactor.

The second advantage is a reduction in catalyst consumption. When operating with a

single reactor, the effectiveness of the catalyst will be insufficient far before the catalyst is

completely spent. This will result in considerable amounts of capable catalyst being discarded.

A double reactor system will allow for the complete deactivation of catalyst, virtually resulting

in complete catalyst utilization. However, process economics will always be employed in

refinery design. Regardless of the apparent advantages, the expense of installing two reactors

will factor in whether or not a refinery will choose to run a single or dual reactor system.

Process Unit: Stabilizer

The reactor effluent is cooled and then flows to the stabilizer unit where any light gas

coproduct is removed. The isomerate is sent downstream to a deisobutanizer for separation.


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Utilities

Aside from the regular utilities such as power, the process will include various grades of

steam for heating. As well, the reactor effluent is cooled prior to entering the stabilizer

suggesting a cooling medium, such as cooling water, is also necessary. Section 4 will outline the

operating requirements of the Butamer process.

2 REACTION CHEMISTRY

Bifunctional catalysts comprised of a noble metal and acidic function are favourable for

alkane isomerization. The Butamer process uses a fixed-bed reactor containing a highly-

selective, chloride promoted catalyst to perform the desired conversion of isomerizing the nC4 to

iC4. The reaction as specified for butane is17

(2)

The primary reaction pathway is proposed to initiate through an alkene intermediate

formed through dehydrogenation on the platinum site18

(3)

Hydrogenation (equilibrium to the left) is favoured as the reaction is operated under high

hydrogen pressure. Despite the unfavourable conditions, alkene formation is promoted as a

sufficient amount of alkenes created are converted to carbonium (carbenium7) ions (driving the

equilibrium to the right). Carbonium ions are formed from the reaction of neutral hydrocarbon

molecules with the acid sites. Protonation of the alkene on the acid site to form ions is given by

(4)

Skeletal isomerization reaction is proposed to follow the rearrangement of the carbonium

ion through a cyclo-alkyl intermediate4

(5)

The isomerized carbonium ion loses a proton to the catalyst site and is converted to an

isomerized alkene in a process analogous to reaction (3)

(6)

The iso-alkene intermediate is hydrogenated on the metal site to isobutane

(7)


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Without termination, chain propagation would continue indefinitely given a sufficient

feed supply. However, acid consumption and catalyst deactivation can be attributed to chain

termination reactions. As proposed by Sie (2008), chain termination occurs when highly

unsaturated molecules with strong proton binds are created. For instance, the hydride transfer

between an alkene and carbonium ion will form an alkane and unsaturated carbonium ion. These

molecules are no longer capable of participating in the reaction recycle and are responsible for

both a loss in catalyst activity and effectiveness of the acid sites.

As shown in the reaction steps, the hydroisomerization process is a reversible reaction

and it is equilibrium conversion limited8. Since there are several transformations taking place -

namely isomerization, hydrocracking and coking- the influence of equilibrium on each individual

reaction step must be considered. The following operating parameters affect the degree of

isomerization.

Temperature

The operating temperature is the dominant influence on the equilibrium of the process.

The dehydrogenation on the metal sites is favoured by increased temperatures; while only the

reaction rate is affected on the acid sites. In spite of the initiation step being favoured, higher

temperature will also favour hydrocracking of the feed to propane and other lighter components.

Further, as reaction on the sites is accelerated, increased alkene concentrations on the catalyst

surface will favour coking. As a result, the conversion equilibrium to the iso structure is highly

reliant on temperature and can be increased with lower conditions. However, the inherent caveat

of lowering the operating temperature is a decreased reaction rate- requiring the selection of a

highly active catalyst.

Pressure

As the process is a rearrangement, there is no change in the number of moles meaning the

equilibrium is not significantly affected by pressure. Ultimately, an increase in pressure will

only cause a decrease in the rate of conversion to branched isomers. This minor influence on

conversion occurs as there is an inhibition of the initiation step due to the increase in the partial

pressure of hydrogen. However, one main advantage involves the reduced rate of the

hydrocracking and coking reactions.


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Hydrogen-to-Hydrocarbon Ratio (H2/HC)

The conversion can be increased by operating at lower hydrogen to feed ratios. However,

the hydrogen used in the process serves a more valuable application in preventing the

deactivation of the catalyst by reducing coke formation. It is also a dominant variable in

determining the process economics. Operating at certain H2/HC ratios will affect the size and

ultimately the needs for the hydrogen recycle. Typically the H2/HC ratio is about 0.5-2.06.

Selection of optimal operating conditions is contingent on obtaining a practical hydrogen

partial pressure through balancing the operating pressure and the H2/HC ratio which will allow

for an extended catalyst life. The ability to operate at lower temperatures has become obtainable

through the advancement of initial catalysts developments.

3 CATALYSIS

The aim of catalyst development throughout history was to obtain a high activity catalyst

which could be practically implemented. In the 1960s, early aluminum chloride catalysts were

plagued with high corrosivity, high consumption and high processing costs7. The main

advantage, however, was the low operating temperatures (< 200C) due to the high acidity.

Catalysts which incorporated platinum on an acid support of Al2O3were later developed, but

were less active and required harsh operating temperatures (~320-450C).

The third generation of catalysts consists of chlorinated alumina combined with platinumand is predominantly used in industry today. Similar catalysts are commonly used in the

isomerization of other light alkanes (C5/C6), suggesting the catalysts is not highly sensitive to

the carbon number in the feed; provided adequate nC4 amounts are present in the feed.

Examples of this type of catalysts are the commercially available I-12 and I-20 catalysts readily

used in UOPs Butamer process.

The balance between the acidic and metallic sites plays a role significant role in the

performance of the process. It implicitly determines the probability of acid catalyzed side-

reactions taking place by determining the alkene partial pressure6. Production of the catalyst is

performed by treating the platinum alumina matrix with carbon tetrachloride at high

temperatures without affecting the pore structures. It is crucial to control acidity so as not to

favour undesired hydrocracking reactions. As well, the amount of platinum will depend on the

molecular mass of the feed- heavier feeds are more likely to have coke formation. The alumina


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matrix will consist of about 8-15 wt%Cl2and 0.3-0.5 wt%Pt8. Continuous injection of a

chlorination agent maintains the high acidity and allows for lower operating temperatures.

The advantage of a bifunctional catalyst is the opportunity for stable isomerization under

adequately high hydrogen pressure. From the chain termination reactions, formation of the

unwanted, unsaturated molecules can be reduced by ensuring alkene intermediates are kept

saturated with hydrogen. In addition, the presence of platinum prevents coke deposition.

The catalyst is highly sensitive to poisons and pre-purification of the feed is a necessity.

The catalyst is especially susceptible to water. Molecular sieve drying systems are effectively

used to remove any water present in the hydrocarbon or hydrogen feeds. Other heteroatoms,

such as sulphur, must also be removed before being fed into the hydroisomerization unit.

Typical processes would involve various hydrotreating reactions (HDN, HDS, HDO) and caustic

extraction to remove sulphur contents. Fluoride which may be introduced from coupling with an

alkylation unit will degrade the catalyst and can be removed by passing the feed over a hot bed

of alumina. The molecular sieves may also have the capacity to remove heteroatoms.

The main disadvantage of the catalysis is hydrogen chloride (HCl) formation directly

caused by a combination of the hydrogen pressure and continued injection of chloride. Elution

of chlorine from the catalysts will affect all downstream components due to the highly corrosive

nature of HCl. A caustic scrubber is necessary to neutralize any HCl present in the off-gas.

New Generation Catalysts

A fourth generation of catalysts has recently been developed. It is composed of a

predominantly platinum impregnated zeolite. The high resistance to contaminants avoids the

need for feed pre-treatment. In addition, the zeolitic process does not require expensive drying

facilities or chloride promotion, avoiding the need for off-gas treatment. The catalysts are also

regenerable. However, since the activity is lower, operation temperatures are generally higher

resulting in lower concentration of branched isomers. The higher temperatures suggest the

necessity of a fired heater which may be financially cumbersome. Industrially used zeolites are

platinum-containing, modified synthetic mordernite17. Examples include, UOPs HS10 and

HYSOPAR from Sd-Chemie. Platinum loaded sulphated zirconia was recently commercialized

for the n-butane isomerization, but possesses shorter catalyst lifetime. Ultimately, the

development of new catalysts should naturally seek to incorporate the advantages of the zeolitic


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structure, but improve on activity to become more economically feasible and competitive with

the dominant process.

4 PROCESS ENGINEERINGMass and Energy Balance

Table 4.1 outlines an estimated utility requirement based on figures for the UOP Butamer

process. The basis feed rate is 115 000MTA(3800BPSD). A simple estimated yield based on a

given feed was also extracted from Meyers (2003) and is summarized in Table 4.2 and Table 4.3.

The make-up hydrogen added in this case is about 65.6m3/h.

While many stochastic optimization procedures seek to predict and optimize utility costs,

it should be noted utility costs are inherently volatile in nature and expenses can never truly be

predicted or known (unless a contract is signed for fixed rates)20, 26. For instance, the average

electricity rate for the general service large" customer class varied in increments from 2005 to

201015. The cost basis in the tables are as follows: electric power, $0.05/kWh; MPS, $3.50/klb;

LPS, $2.50/klb.

Table 4.1 Estimated Operating Requirements

Utility Requirement Deisobutanizer Butamer Unit $US/SD

Power, kW 200 300 600

Medium Pressure Steam

14.1 kg/cm3, 1000 kg/h

200 lb/in2gage, 1000 lb/h

5.0

11.1 936

Low Pressure Steam

3.5 kg/cm3, 1000 kg/h

50 lb/in2gage, 1000 lb/h

16.3

35.9 2153

Cooling Water, m /h 35 89 77

Catalyst and chemical consumption

$US/SD 523 523

Total 4289


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Table 4.2 Composition of Field Butane Feedstock

Feed MTA wt %

Propane 978 0.85

Isobutane 29 325 25.50

n-butane 82 282 71.55

Isopentane 1 805 1.57

n-pentane 610 0.53

Total 115 000 100

MTA = metric tons per annum

Table 4.3 Estimated Product Yield Based on the Field Butane Feedstock

Product MTA wt %

Isobutane

Propane

Isobutane

n-butane

978

104 190

3 922

0.85

90.60

3.41

Heavy-end by-product

Isobutane

n-butane

Isopentane

n-pentane

69

2 702

1 058

978

0.06

2.35

0.92

0.85

Light gas

Methane

Ethane

Propane

252

357

541

0.22

0.31

0.47

Total 115 000 100


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Concerns

The environmental footprint of this process is increased with the continual need for

chlorination. There is concern downstream regarding the hydrochloric acid (HCl) inherently

produced from this process. Regardless, carbon steel equipment is still successfully used due to

the dry conditions. However, caustic scrubbers are still needed to neutralize the acid in the off-

gas formed to prevent corrosion of downstream equipment.

A significant cost which should be addressed along with utilities is that of the catalyst

and chemical consumption. Since the process is highly reliant on a continuous injection of a

chlorinated compound, readily available amounts of compound must be maintained.

According to a study published in 199624, most refineries place spent catalyst directly

into closed containers (e.g. 55 gallon drums, flow-bins, 1 cubic yard supersacks). The

frequency of generation occurs between 2 and 10 years. Waste reduction methods may include

sending the exhausted catalyst to reclaim platinum which is a precious metal. Despite no oxygen

is present during operation, the presence of chlorine in the process may cause the spent catalyst

to contain toxic dioxins formed during the unit turnaround and catalyst replacement.

Commercial solutions to dioxin removal are readily available (e.g. ADIOX, MercOx).

5 SUPPLEMENT

Process Control

A study by Lukec et al (2007) proposes the implementation of advanced process control

techniques, namely model predictive control (MPC), on the pentane hydroisomerization process

at the Rijeka Refinery in Croatia. MPC theories have been applied in industry to allow for

optimal control for instance, in terms of minimizing energy consumption, maximizing plant

capacity and maintaining quality product. Proposals outlined in the paper can be feasibly applied

to butane isomerization. The basics of soft sensor design include statistical determination of

significant inputs and outputs. A mathematical model can then be developed to enable on-line

estimation of the desired properties, such as iC4 yields, and any highly correlated inputs can be

adjusted accordingly using the MPC control scheme. Through the initial investigation, it is

known the temperature and the H2/HC ratio are highly influential on the yield. Soft-sensor

design can both build on and reinforce this supposition. Although, the proposal is not without its

caveats. The practical implementation of this type of control holds various barriers. If historian


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data of the parameters is not readily available, sensors need to be installed in order to collect

data. In addition, it is unlikely online estimation of the product composition is performed. This

may mean disparity between the lab analysis and online data could exist, introducing

unnecessary data processing work. Internal obstacles to overcome are red tape measures upon

unproven/new techniques and the requirements of other processes in the refinery- optimizing the

hydroisomerization unit will affect both up and downstream processes. Further, excitation of the

system may be necessary for proper process identification. It is highly unlikely a company will

allow costly experimental changes to an operating process. Regardless, proper application of

control techniques can lead to reduction in utility costs, improvement in catalystslifetime and

ultimately, more efficient operation.

Process Economics

It should be recognized additional costs need to be factored in to the overall economics of

the hydroisomerization process. For instance, labour, repair and in this case, royalties and

patents must be included when considering this process.

Alternative means of saving capital costs can be found in a slightly altered process. It

suffices to mention, UOPs hydrogen-once-through flow scheme (Figure 1.2) eliminates the need

for a high pressure separator and recycling compressor for hydrogen.

A common practice in industry is the integration of a hydroisomerization unit with an

alkylation unit23. Unconverted nC4 can be removed from the alkylation process isostripper and

fed back into the isomerization unit. With the increased iC4 concentrations, reduction in size of

the isostripper will ultimately lead to reduction in utilities. As refinery economics is paramount

in industry, synergy of the hydroisomerization and alkylation processes can lead to favourable

decreases in both capital and operating costs.


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Conclusion

The hydroisomerization of butane to isobutane has become a commonplace technology in

meeting the demand for high quality gasoline. Light branched alkanes, in this case, iso-butane is

used as feedstock to an alkylation unit, responsible for the production of high octane additives.

The technological overview of the process can be summarized as follows:

Hydroisomerization is a fixed-bed, catalytic process occurring in the vapour phase UOPsButamer process is the dominant technology in industry Operating conditions are typically: 180-220C, 1.5-3MPa, space velocity of 2h-1and

H2/HC ratio of 0.5-2

Equilibrium limitations yield about 60% iso-butane per pass; recycling ofunconverted material can result in virtually 100% conversion

Acidity of the chlorinated-Pt/Al2O3catalyst is maintained by continuous injection ofchlorinated compound

The catalysts is highly sensitive to feed poisons, including water and sulphur and pre-treatment of the feed is a necessity

The main issue is corrosion of process and downstream equipment. Thus, a causticscrubber is needed to remove any traces of HCl in the off-gas

Process economics may be improved through direct combination with an alkylationunit; improved catalysts and process flow schemes; and the implementation of

advanced process control techniques

Ultimately, the hydroisomerization process of butane to isobutane can be a very practical

process to implement in meeting the perpetually changing demands in both environmental and

fuel regulations and standards.


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[2] Albright, L.F., Present and Future Alkylation Processes in Refineries, Ind. Eng. Chem.Res., 48, pp. 1409-1413 (2009).

[3] Allain, J.F., P. Magnoux, Ph. Schulz and M. Guisnet, Hydroisomerization of n-hexaneover platinum mazzite and platinum mordenite catalysts kinetics and mechanism ,Applied Catalysis A: General, 152, pp. 221-235 (1997).

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