gasolina ultra baja en sulfuro

7/30/2019 Gasolina Ultra Baja en Sulfuro.

http://slidepdf.com/reader/full/gasolina-ultra-baja-en-sulfuro 1/7

T6a02 #2

Ultra Low SulfuF> Gasoline:

Overview and Comparison of Gasoline Sulfur Reduction Technologies

Steve McGovern and CK Lee

PetroTech Consultants

3 Chambord Lane, Voorhees, New Jersey 08043

Key Words: Ultra Low Sulfur Gasoline, Gasoline Sulfur Reduc ion Technologi, s

Prepared for presentation at the AIChE Spring National Meet g 2001

Fourth International Conference on Refinery Processing

April 22-26, 2001, Houston, Te

Copyright, Steve McGovern and CK Lee, Petro ech Consultants

January 22, 2001

Unpublished \ ~ AIChE shall not be responsible for statements or opinions containe in the paper or pr ted

publications " \

Abstract

Over the last few years, motor fuel sulfur level has become the most pressing environmental s p e c i ~ a t i o n w i t h ~ n the worldwide refining ~ d u s t r y . Ne,: n v i r o n ~ ~ n t a l regulations have e e l ; ! , s t e a d ~ l y d : c r e a s i ~ th ,

maXImum sulfur level allowed In both gasolIne and dIstIllates. For example, cUI\ent CalIfornIa CARp'\:Y

gasoline requires an average of.30 p p ~ w . s U I ~ r . A similar regulati.on. has been a ' l : 4 e ' recently by EP1 ifrthe rest of the US to be phased In begInnIng In 2004. In Europe, sImIlar e g u l a t l o n ~ ~ ~ e been m a n d ~ t e d toreduce sulfur from the current 150 ppmw to 50 ppmw in 2005. Gennany is c o n s i d ; r i ~ incentives for

refiners to produce 10 ppmw sulfur gasoline starting in 2003. "', -

Refiners have many options to choose from to reduce the sulfur in their gasoline. There are many

competing technologies available in the market, including pretreatment, post-treatment, and FCC catalyst

additives. Some of the technologies are commercially proven and some are stillin

various stages ofdevelopment.

In this tutorial session, an overview of the gasoline sulfur reduction technologies will be presented and

compared. Sometimes, more than one technology may be required to produce the ultra low sulfur gasoline.

However, the optimal technology or combination of technologies for a refinery is very much site-specific.



Introduction

In most parts of the world, sulfur oxide emissions from automobiles are not a serious problem. They are

not a significant contributor to global atmospheric SOX levels. Gasoline sulfur contents are generally lower

than those of most other fuels. The impetus to lower gasoline sulfur content comes from the poisoning

effect that sulfur has on the onboard catalyst systems that are used to control automotive tailpipe emissions

of Carbon Monoxide, Hydrocarbons and Nitrogen Oxides.lhese pollutants contribute to the formation of

atmospheric ozone and haze.

Numerous studies, summarized in a recent US EPA Staff Paper (USEPA 1999), have clearly demonstrated

the deleterious effect of gasoline sulfur content on catalyst performance in automobile emission control

systems. Although the magnitude of the impact is a function of both absolute fuel sulfur level and catalyst

and emission control systems in use, lowering fuel sulfur level, lowered tailpipe emissions in all studies.

The combination of advanced onboard emission control systems and lower fuel sulfur levels results in a

very cost effect method for lowering emissions from new vehicles. Furthermore, lowering fuel sulfur

levels will immediately lower emissions from the existing fleet of vehicles currently on the road.

In light of these results, many government agencies, worldwide, have proposed or implemented regulations

that will lower the maximum allowable sulfur content of automotive gasoline. A few of these regulations

are summarized below:

Table 1 Gasoline Sulfur Regulations

MaXlmum Allowable GasorIne SuIfur Con ten t,ppmw

Currentmplementation

Date

Corporate

Annual Average

30

Per Batch Cap 80

California Ai r Resources Board

2003 2005

15 15

60 30

U.S. Environmental

Protection Aeency (1)

. European

Community

2004- 2006 2005

120 30 50 (2)

300 80

(1) Sulfur specs are phased in over time with full implementation by 2008

(2) Not yet finalized

The regulations noted above are typical of those being considered in other parts of the world as well. Most

currently proposed limits are for sulfur contents of 30-50 ppmw; however, the state of California has

already lowered its limit from 30 to 15 ppnlw. In addition to regulatory limits, some countries such as

Germany are proposing tax incentives for gasoline sulfur contents as low as 10 ppmw and auto emission

studies have shown additional benefits down to as low as 5 ppmw, the practical limit for measuring and

controlling gasoline sulfur contents. This paper will primarily focus on US gasoline regulations,

production, and composition outside of California (OC). The situation in other countries is very similar,

however the large number of FCC units in the US n1akes the problem somewhat more severe.

Current Gasoline Composition

Gasoline is a blend of many different refinery streams that are produced on different process units under

different conditions. These are blended together to produce gasolines with the desired properties. Process

unit operating conditions are varied along with blend compositions to maximize refinery profitability while

meeting the required gasoline specifications. Since all crudes contain some sulfur, which is distributed

throughout the entire crude boiling range, all gasoline components contain some amount of sulfur.

Until the mid 1980's, the sulfur content of gasoline in the US was essentially unregulated. It was limited to

0.5 wt% (as it currently is in some countries), but this limit was almost never reached. The limit was then

reduced to 1000 ppmw, and further reduced as a result of Reformulated Gasoline regulations. In 1998,

total US gasoline production (OC) averaged about 270 ppmw. The sulfur content of individual batches

varied from <20 ppmw up to almost 1000 ppmw, the ASTM limit for pipeline transport. There were three



refineries with 1998 average sulfur contents below 30 ppmw, while 14 refineries had average gasoline

sulfur levels greater than 500 ppmw (USEPA 1999).

This wide variability in gasoline sulfur content is due to: the sulfur contents of the crudes processed, the

type and operating conditions of the refinery process units, and the amount of each blend component used

in the final blends. The typical range of sulfur contents for the common gasoline blend components are

listed below:

T hie 2 G r c eve s

Percent of Pool

<5

<1

20-70

0-10

0-50

0-15

0-15

a aso Ine omponent SuIfur L

Stream

Light Straight Run Naphtha

Light Coker Gasoline

Reformate

Alkylate

FCC Gasoline

Oxygenates

Others (1)

Sulfur Content, ppmw

1-500

1-3000

<5

5-100

50-2000

1-30

1-1000

(1) Includes Butanes, Natural Gas Liquids, Hydrocrackate, Dimate, lsomerate, Poly gasoline and

Petrochemical rej ect streams

In 1998, almost 40 percent of the US gasoline pool was FCC gasoline with an average sulfur content of 650ppmw. The other 60 percent had an average sulfur content of 30 ppmw. Typically, the FCC gasoline

contributes over 90 percent of the total gasoline sulfur. It is clear, that in most refineries, the FCC gasoline

must be desulfurized. In refineries with significant sulfur in their non-FCC streams, they must weigh

desulfurizing the non-FCC streams against over desulfurizing the FCC gasoline. If the non-FCC streams

are sulfur free, then the FCC gasoline can have about 75 ppmw sulfur. If the non-FCC streams contain 30

ppmw sulfur, then the FCC gasoline must be 30 ppmw sulfur or less.

FCC Gasoline Composition

Typical FCC gasoline has a (R+M)/2 octane of about 86-88 and has good volatility properties, thus it is an

excellent gasoline blend stock. The good octane is derived from its olefin, iso-paraffin and aromatic

content. These components, however, are not distributed uniformly throughout the boiling range as shown

below for a typical high sulfur FCC gasoline (Nocca et al. 1995).

Fraction

Boiling Range, F

Boiling Range, C

Vol %

Sulfur, ppmw

Olefins, %

Aromatics, %

(R+M)/2

. aT ble 3 FCC Gaso Ine Coml lOS}'fIon

LCN ICN HCN Full Ran2e

C5 360+

C5

C6-360

182+

24

C6-182

100

50

64 121100 2500

50

3800

26.5 9.3 30

0 30 75 30

88 85.5 866.3

Although all components are distributed throughout the boiling range, the olefins are concentrated in the

lighter fractions, while the aromatics and sulfur increase with boiling point.

There are four major types of sulfur compounds in FCC gasoline: Mercaptans, Sulfides, Thiophenes and

Benzothiophenes. The sulfur in the LeN (Light Cat Naphtha) is mostly mercaptans. There can be small

amounts of carbonyl sulfide and carbon disulfide, but their total is usually less than 5 ppmw. Depending on

the actual cut point and the quality of fractionation, the LCN can also contain some thiophene (b.p.

84C/184F). The dominant sulfur type in the ICN (intermediate Cat Naphtha) is the thiophene homologous

series, wrttTJesser amounts (about 10% of total sulfur) of sulfides (primarily cyclic) and mercaptans. The

sulfur in the HCN (Heavy Cat Naphtha) is predominantly Benzothiophene (b.p. 22 1C/43OF) and ifhigh



enough end point, Methyl benzothiophenes. The total sulfur content ofFCC gasoline is primarily a

function of FCC feed sulfur content and the end point of the FCC gasoline. @Methods for Lowering FCC Gasoline Sulfur

The sulfur content of FCC gasoline can be lowered by one of three methods: 1) removing sulfur from the

FCC feed, 2) modifying FCC operating conditions or catalyst/additives, or 3) post-treating the FCC

gasoline. All three of these methods have been successfully used commercially to lower FCC gasolinesulfur. Since FCC units normally operate for 4-5 years between major maintenance turnarounds, any

process chosen to lower gasoline sulfur should be capable of running for at least 2 years between

shutdowns. Each major type, along with important sub-types, will}Je discussed below.

FCC Feed Pretreatment To produce FCC gasoline with less than 100 ppm sulfur, the FCC feed must

contain less than 0.1 wt% sulfur. The only commercially viable process for lowering the feed sulfur to that

level is Catalytic Feed Hydrotreating (CFHT) (Danzinger 1999 and Shorey 1999). To produce low sulfur

product over the desired 2 year cycle, high pressures (> 1200psi) are normally required. These high

pressures also improve overall FCC feed quality and can have a significant beneficial impact on FCC

yields. Additionally, CFHT lowers the .sulfur content of all other FCC products, including regenerator

SOX emissions. The major disadvantage ofCFHT is the high cost, both capital and operating. In some

special cases, where the FCC feed quality is particularly poor and both diesel sulfur and regenerator SOX

emission reductions are anticipated, the improvement in FCC yields and alternate cost avoidance can justifythe higher costs associated with CFHT.

FCC Modifications The simplest FCC modification to reduce gasoline sulfur is to lower the end point of

the FCC gasoline. The FCC main product fractionator produces gasoline as an overhead product, with

LCO as a side draw product (sometimes a HCN sidedraw is taken between the gasoline and LCO). The

quality of fractionation is usually not high, hence, there can be a significant amount of benzothiophene and

even methylbenzothiophenes in the FCC gasoline. Adjusting the FCC gasoline end point (or increasing

reflux to improve fractionation) can reduce FCC gasoline sulfur content by up to 50%, with minimal loss of

gasoline to LCO. Other FCC operating conditipn changes have minimal impact on FCC gasoline sulfur

levels.

Gasoline sulfur reduction additives have been developed by some of the major FCC catalyst vendors. For

example Grace is offering their GSR additive, which is claimed to lower FCC gasoline sulfur by up to 25°A»

with no other effects on FCC operation (Wormsbecher 1993, AKZO 2000). In some situations, this might

be adequate for meeting the required sulfur levels. The use ofZSM-5 additive in the FCC does not lower

gasoline sulfur, but can change the olefin distribution so that other gasoline desulfurization methods are

more effective (Smith and Evans 1998).

FCC Gasoline Desulfurization By far, the most active area for new process development currently is in

the area of FCC gasoline sulfur removal technology. Removing sulfur from the FCC gasoline (rather than

the FCC feed) gives a much smaller, lower cost unit than treating the FCC feed. Furthermore, the critical

gasoline specification, sulfur, is being controlled at the end of the processing train. USEPA estimates that

over 100 new units will be built in the US over the next 8 years (USEPA 1999). The various technologies

for removing sulfur from FCC gasoline can be divided into two major classes: physical separations and

chemical reactions. By definition, the physical separation processes do not remove the sulfur atoms from

their parent sulfur containing molecules, instead, they concentrate these sulfur compounds in a smaller

stream that must then be treated. As will be discussed later, some of the chemical reaction basedtechnologies are based on the same principal, concentration of the sulfur into a smaller easier to treat

stream.

The most obvious technology for removing sulfur from FCC gasoline is a simple Hydrotreating unit. This

technology has been used extensive for removing sulfur from naphtha reformer feeds and more recently for

removing sulfur from heavy FCC gasoline (HeN).. These units are relatively low cost and long run times

have been obtained at reasonably low pressures and moderate reactor temperatures. The major



shortcoming of this technology for lighter FCC gasoline is that it also saturates the olefins and lowers the

octane of the gasoline by as much as 8 or 10 numbers (Nocca et a1. 1995).

Several improvements over conventional naphtha hydrotreating have been announced over the past several

years. These new technologies will be discussed below.

Optimal Hydrotreating conditions Both ExxonMobil (Greeley et a1. 1999) and IFP (Kasztelan et

a1.1999) currently offer fixed bed, FCC gasoline hydrotreating processes that are based on the concept ofoptimizing process conditions and catalyst choice (Scanfining and Prime G, respectively). Kinetic studies

of naphtha hydrotreating have shown that.at typical conditions thiophene desulfurization and olefin

saturation have similar rate constants, however, olefin saturation has a higher activation energy. Lowering

the reaction temperature and pressure and increasing contact time will favor desulfurization over olefin

saturation. The lower operating temperature limit is set by the equilibrium between mercaptans and H2S

plus olefins. This limit can be circumvented using a multistage process with interstage H2S removal. The

equilibrium limit on the low temperature side and the increased olefin saturation and higher aging rates on

the high temperature side give a rather narrow operating window. Neither process can tolerate significant

amounts of benzothiophene in the feed. Benzothiophene is more difficult to desulfurize and essentially

complete olefin saturation is experienced at the conditions required to remove substantial amounts of

benzothiophene.

Although the Phillips S-Zorb process (Phillips 2000) is marketed as an adsorption process, it is really amoving bed hydrotreating unit with optimized operating conditions and continuous catalyst regeneration.

The continuous catalyst regeneration, allows it to operate at lower pressure and hydrogen circulation rates

, (to minimize olefin saturation), without encountering the catalyst deactivation limitation of a fixed bed unit.

Furthermore, the catalyst contains a second component with high affinity for sulfur (the absorbent) to

reduce the H2S concentration in the reaction zone. This shifts the mercaptan/olefin equilibrium away from

the mercaptan. Although this process should be able to handle a higher level ofbenzothiophene (BT) in the

feed than the fixed bed units, increasing the level ofBT in the feed will increase olefin saturation.

Hydrotreating with Octane Recovery Both ExxonMobil (Tryjankowski et a!. 1999) and UOP/lntevep

(Salazar et a1. 1999) offer processes (Octgain and ISAL, respectively) that use more conventional

hydrotreating conditions, but recover some or all of the octane by isomerizing and cracking the normal

paraffins that are formed from the olefins via hydrotreating. This octane recovery comes at the expense of

some loss in yield. The yield loss is a function of feed quality and target product octane. These processes

are especially suited for refineries that are octane constrained or would like to increase their octane

capabilities. Since the hydrotreating conditions are more severe, these processes can handle higher levels

ofbenzothiophene.

The ultimate process for hydrotreating with octane recovery is to severely hydrotreat the FCC gasoline and

then reform the hydrotreated product. This could be the lowest capital solution for a refinery that has

excess reforming capacity. The hydrotreated ICN is also an above average reformer feed, so this

processing combination can actually result in an increase in total gasoline production at constant pool

octane.

Extraction/ Adsorption GTC Technology Corporation (Gentry and Lee 2000) has recently announced a

technology for extracting the sulfur containing compounds from FCC gasoline. The process is similar to

those used to recover aromatics from reformate, so the aromatics in the FCC gasoline are also extracted

along with the sulfur compounds. This gives a much smaller stream that must be hydrotreated to ultimatelyconvert the sulfur compounds to H2S. Furthermore, the olefms in the FCC gasoline bypass the HDT unit,

eliminating the octane loss that results from their saturation. Favorable economics for this process are

dependent on obtaining high value for the recovered aromatics.

Black and Veach, Pritchard have also announced the IRVAD process (Irvin et a1. 1999). This is a moving

bed adsorption process that is claimed to use a circulating bed of solid adsorbent.to, remove the sulfur

containing molecules from the FCC gasoline. Continuous regeneration of the adsorbent produces a small

high sulfur stream that must be further treated.



Sulfur Concen tation Processes As was discussed earlier, removal of the heaviest portion of the FCCgasoline via Distillation can reduce the sulfur content of the remaining gasoline. Additionally, producing alight cat naphtha via distillation, yields a stream that is high in oletins and contains essentially onlymercaptan type sulfur. The column must be designed and operated to keep thiophene in the bottoms.Higher fractionation efficiency gives higher overhead yield. These mercaptans can easily be extractedusing caustic based extraction processes such as DOP's Merox or Merichem's extractive sweetening to

produce a sulfur free LCN with no yield or octane loss. Both of these extraction processes produce a small,high sulfur reject stream that must be treated in another unit. Since most refiners currently sweeten theirFCC gasoline, the cost of conversion to an extractive sweetening unit is very low.

CDTech's CDHydro process combines distillation and sulfur removal into a single catalytic distillation

column (Rock et al. 1998). The catalyst pronlotes the reaction of mercaptans with diolefins, formingheavier sulfur compounds that leave with the fractionator bottoms. The overhead is eS,sentially sulfur free.Small amounts of hydrogen are also added to the column to saturate any remaining diolefins. Like theconventional distillation column, conditions must be closely controlled to eliminate thiophene from theoverhead product. The bottoms product must be desulfurized using another technology. CDTechadvertises their CDHDS process for this service, however, any other desulfurization process will also work.

Another new process recently announced by BP/Amoco is called the OATS process (BP 2000). This is

essentially a thiophene alkylation process. A proprietary catalyst and process conditions are used to reactsome of the olefins or diolefins with thiophene homologues. These are higher boiling anq can beconcentrated via distillation in the low olefin content heavy naphtha. There is no mention in t h ~ i r literatureabout what happens to the mercaptans and sulfides in the light gasoline.

Comparative Economics

There are two major components to the cost of lowering gasoline sulfur content, fixed costs and variableoperating costs. The fixed costs, which include capital recovery, depreciation, taxes, insurance, manpower,etc. are determined by the type and size of unit that is built. Once the decision is made on the type and sizeof unit, these costs will remain unchanged until the unit.is modified in some way. Even if the unit is notoperating. Variable operating costs include utilities, catalysts and chemicals, yield and quality changes, etc.and are related to the actual amount of feed processed by the unit.

The US EPA has estimated and reported the average cost for the US of producing 30 ppm sulfur gasoline at1.7 cent per gallon (USEPA 1 9 9 9 ) ~ This cost is based on a blend ofproven and unproven technologies withoperating costs accounting for over 75% of the total cost. Several other studies, summarized in the EPAreport, estimate the cost at 2.4-5.7 cents per gallon. These cost differences are due to differences intechnology choice, capital cost estimate and the rate of return assumed for the capital employed. EPAassumed a 7% rate of return, before taxes. The average capital investment per refinery was $44 million.Furthermore, operating costs are dominated by energy cost, ie utilities, hydrogen and yield or octane loss.Energy costs have increased significantly since the report issued.

Some of the new technologies might have hidden costs that have not yet been revealed by long termoperation, while other costs might be loeer than current estimates. California has demonstrated thatsustained production of low sulfur gasoline is feasible, but much greater attention to good operating

practices and process monitoring is required. Furthermore, unscheduled down time makes it much more

difficult to blend finished product.

Summary

Removing sulfur from gasoline is a worldwide problem. While there are several viable technologies foraccomplishing that objective, including some promising technologies that are not yet commercially

demonstrated, each refmery situation is different. At this time, there is no c l e a t : ~ " ~ ' h e s t " te.chnalogy for allrefiners. Sometimes a combination oftechnologies might provide the best solution. The optimaltechnology or combination of technologies for each refinery is very much site specific.



References

AKZO-Nobel, Resolve Technology for FCC Gasoline Sulfur Reduction, www.akzonobel-catalysts.com

2000

BP/Amoco Press Release, BP Announces New Cleaner Gasoline Technology, November 16,2000

Danzinger, F., et al. Revamping OMV's FCC Pretreater to a MAKFining MPHC Hydrocracker for

Maximun Operational Flexibility and Profit, NPRA 1999 Annual Meeting

Gentry, J.C., Lee, F.M., Novel Process for FCC Gasoline Desulfurization and Benzene Reduction to Meet

Clean Fuels Requirements, NPRA 2000 Annual Meeting

Greeley, J.P., Zaczepinski, S., Selective Cat Naphtha Hydrofining with Minimal Octane Loss, NPRA 1999Annual Meeting \

Irvin, Robert L., et aI., IRVAD Process-Low Cost Breakthrough for Low Sulfur Gasoline, NPRA 1999

Annual Meeting

Kasztelan, S., e t aI., Improving Motor Fuel Quality, NPRA 1999 Annual Meeting

Nocca, J.-L., et aI., Sulfur and Olefin Management in the FCC Gasoline, NPRA 1995 Annual Meeting

Phillips Petroleum, S-Zorb Technology, www.fuelstechnology.com/szorbgas.htm

Rock, Kerry L., et al , Improvements in FCC Gasoline Desulfurization via Catalytic Distillation, NPRA

1998 Annual Meeting

Salazar, J.A., et al. The ISAL Process: A Refiner 's Option to Meet RFG Specifications, NPRA 1998

Annual Meeting

Shorey, S.W., e t aI., Exploiting Synergy between FCC and Feed Pretreating Units to Improve Refinery

Margins and Produce Low-Sulfur Fuels, NPRA 1999 Annual Meeting

Smith, G.A., Evans, M., Meeting Changing Gasoline Specifications and Variable Propylene and Butylene

Demand Through the Use ofAdditives, NPRA 1998 Annual Meeting

Tryjankowski, D.A., et aI., Mobil's Octgain Process: FCC Gasoline Desulfurization Reaches a New

Performance Level, NPRA 1999 Annual Meeting

United States Environmental Protection Agency, Regulatory Impact Analysis - Control of Air Pollution

from New Motor Vehicles: Tier 2 Motor Vehicles Emissions Standards and Gasoline Sulfur Control

Requirements, EPA420-R-99-023, www.epa.gov/otag/tr2home.htm1999

Wormsbecher, R.F., et aI., Emerging Technology for the Reduction of Sulfur in FCC Fuels, NPRA 1993

Annual Meeting

gasolina ultra baja en sulfuro

Documents