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CHAPTER 2.2UOP SULFOLANE PROCESS

Thomas J. Stoodt and Antoine NegizMarketing Services

UOP LLCDes Plaines, Illinois

INTRODUCTION

The UOP Sulfolane* process is used to recover high-purity aromatics from hydrocarbonmixtures, such as reformed petroleum naphtha (reformate), pyrolysis gasoline (pygas), orcoke-oven light oil.

The Sulfolane process takes its name from the solvent used: tetrahydrothiophene 1,1-dioxide, or Sulfolane. Sulfolane was developed as a solvent by Shell in the early 1960s andis still the most efficient solvent available for the recovery of aromatics. Since 1965, UOPhas been the exclusive licensing agent for the Sulfolane process. Many of the processimprovements incorporated in a modern Sulfolane unit are based on design features andoperating techniques developed by UOP.

The Sulfolane process can be applied as a combination of liquid-liquid extraction(LLE) and extractive distillation (ED) or, with an appropriate feed, ED alone. The choiceis a function of the feedstock and the processing objectives, as explained below.

The Sulfolane process is usually incorporated in an aromatics complex to recover high-purity benzene and toluene products from reformate. In a modern, fully integrated UOParomatics complex (Fig. 2.2.1), the Sulfolane unit is located downstream of the reformatesplitter column. The C6-C7 fraction from the overhead of the reformate splitter is fed to theSulfolane unit. The aromatic extract from the Sulfolane unit is clay-treated to remove traceolefins, and individual benzene and toluene products are recovered by simple fractiona-tion. The paraffinic raffinate from the Sulfolane unit is usually blended into the gasolinepool or used in aliphatic solvents. A complete description of the entire aromatics complexmay be found in Chap. 2.1.

The Sulfolane process can also be an attractive way to reduce the benzene concentra-tion in a refinery’s gasoline pool so that it meets new reformulated gasoline requirements.In a typical benzene-reduction application (Fig. 2.2.2), a portion of the debutanized refor-mate is sent to a reformate splitter column. The amount of reformate sent to the splitter isdetermined by the degree of benzene reduction required. Bypassing some reformatearound the splitter and recombining it with splitter bottoms provide control of the finalbenzene concentration. The benzene-rich splitter overhead is sent to the Sulfolane unit,

2.13

*Trademark and/or service mark of UOP.

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which produces a high-purity benzene product that can be sold to the petrochemical mar-ket. The raffinate from the Sulfolane unit can be blended back into the gasoline pool orupgraded in an isomerization unit.

Improvements in the Sulfolane process have allowed the application of extractive dis-tillation alone to feeds that have traditionally been sent to a combination LLE/ED unit. Forthe same feed rate, an extractive distillation unit is about 80 percent of the installed cost ofa combined LLE/ED unit. The economics of one versus the other is largely a question ofthe utilities required to achieve the high-purity product specifications at satisfactory recov-eries of BTX. The application of extractive distillation is favored when the Sulfolane feedis rich in aromatics. In such a case, there is less raffinate to boil overhead in the extractivedistillation column, which consumes energy.

2.14 BASE AROMATICS PRODUCTION PROCESSES

FIGURE 2.2.1 Integrated UOP aromatics complex.

FIGURE 2.2.2 Benzene-reduction application.

UOP SULFOLANE PROCESS



UOP SULFOLANE PROCESS 2.15

The more economical choice is an economic and engineering decision. Factors to con-sider include:

● New versus revamp equipment● Cost of utilities● Feed composition (boiling range, nonaromatics, impurities)● Product specifications

SOLVENT SELECTION

The suitability of a solvent for aromatics extraction involves the relationship between thecapacity of the solvent to absorb aromatics (solubility) and the ability of the solvent to dif-ferentiate between aromatics and nonaromatics (selectivity). A study of the common polarsolvents used for aromatic extraction reveals the following qualitative similarities:

● When hydrocarbons containing the same number of carbon atoms are compared, solu-bilities decrease in this order: aromatics�naphthenes�olefins�paraffins.

● When hydrocarbons in the same homologous series are compared, solubility decreasesas molecular weight increases.

● The selectivity of a solvent decreases as the hydrocarbon content, or loading, of the sol-vent phase increases.

In spite of these general similarities, various commercial solvents used for aromaticsrecovery have significant quantitative differences. Sulfolane demonstrates better aromaticsolubilities at a given selectivity than any other commercial solvent. The practical conse-quence of these differences is that an extraction unit designed to use Sulfolane solventrequires a lower solvent circulation rate and thus consumes less energy.

In addition to superior solubility and selectivity, Sulfolane solvent has three particular-ly advantageous physical properties that have a significant impact on plant investment andoperating cost:

● High specific gravity (1.26). High specific gravity allows the aromatic capacity ofSulfolane to be fully exploited while maintaining a large density difference between thehydrocarbon and solvent phases in the extractor. This large difference in densities min-imizes the required extractor diameter. The high density of the liquid phase in the extrac-tive distillation section also minimizes the size of the equipment required there.

● Low specific heat—0.4 cal/(g�°C) [0.4 Btu/(lb�°F)]. The low specific heat of Sulfolanesolvent reduces heat loads in the fractionators and minimizes the duty on solvent heatexchangers.

● High boiling point [287°C (549°F)]. The boiling point of Sulfolane is significantly high-er than that of the heaviest aromatic hydrocarbon to be recovered, facilitating the sepa-ration of solvent from the aromatic extract.

PROCESS CONCEPT

The Sulfolane process combines both liquid-liquid extraction and extractive distillation inthe same process unit. This mode of operation has particular advantages for aromaticrecovery:




● In liquid-liquid extraction systems, light nonaromatic components are more soluble inthe solvent than heavy nonaromatics are. Thus, liquid-liquid extraction is more effectivein separating aromatics from the heavy contaminants than from the light ones.

● In extractive distillation, light nonaromatic components are more readily stripped fromthe solvent than heavy nonaromatics. Thus, extractive distillation is more effective inseparating aromatics from the light contaminants than from the heavy ones.

Therefore, liquid-liquid extraction and extractive distillation provide complementary fea-tures. Contaminants that are the most difficult to eliminate in one section are the easiest toremove in the other. This combination of techniques permits effective treatment of feed-stocks with much broader boiling range than would be possible by either technique alone.

The basic process concept is illustrated in Fig. 2.2.3. Lean solvent is introduced at thetop of the main extractor and flows downward. The hydrocarbon feed is introduced at thebottom and flows upward, countercurrent to the solvent phase. As the solvent phase flowsdownward, it is broken up into fine droplets and redispersed into the hydrocarbon phaseby each successive tray. The solvent selectively absorbs the aromatic components from thefeed. However, because the separation is not ideal, some of the nonaromatic impurities arealso absorbed. The bulk of the nonaromatic hydrocarbons remain in the hydrocarbon phaseand are rejected from the main extractor as raffinate.

The solvent phase, which is rich in aromatics, flows downward from the main extrac-tor into the backwash extractor. There the solvent phase is contacted with a stream of lightnonaromatic hydrocarbons from the top of the extractive stripper. The light nonaromaticsdisplace the heavy nonaromatic impurities from the solvent phase. The heavy nonaromat-ics then reenter the hydrocarbon phase and leave the extractor with the raffinate.

The rich solvent from the bottom of the backwash extractor, containing only lightnonaromatic impurities, is then sent to the extractive stripper for final purification of thearomatic product. The light nonaromatic impurities are removed overhead in the extractivestripper and recycled to the backwash extractor. A purified stream of aromatics, or extract,is withdrawn in the solvent phase from the bottom of the extractive stripper. The solventphase is then sent on to the solvent recovery column, where the extract product is separat-ed from the solvent by distillation.

Also shown in Fig. 2.2.3 are the activity coefficients, or K values, for each section ofthe separation. The K value in extraction is analogous to relative volatility in distillation.The Ki value is a measure of the solvent’s ability to repel component i and is defined as themole fraction of component i in the hydrocarbon phase Xi, divided by the mole fraction ofcomponent i in the solvent phase Zi. The lower the value of Ki, the higher the solubility ofcomponent i in the solvent phase.

DESCRIPTION OF THE PROCESS FLOW

Fresh feed enters the extractor and flows upward, countercurrent to a stream of lean sol-vent, as shown in Fig. 2.2.4. As the feed flows through the extractor, aromatics are selec-tively dissolved in the solvent. A raffinate stream, very low in aromatics content, iswithdrawn from the top of the extractor.

The rich solvent, loaded with aromatics, exits the bottom of the extractor and enters thestripper. The nonaromatic components having volatilities higher than that of benzene arecompletely separated from the solvent by extractive distillation and removed overheadalong with a small quantity of aromatics. This overhead stream is recycled to the extrac-tor, where the light nonaromatics displace the heavy nonaromatics from the solvent phaseleaving the bottom of the extractor.





The stripper bottoms stream, which is substantially free of nonaromatic impurities, issent to the recovery column, where the aromatic product is separated from the solvent.Because of the large difference in boiling point between the Sulfolane solvent and theheaviest aromatic component, this separation is accomplished with minimal energy input.To minimize solvent temperatures, the recovery column is operated under vacuum. Leansolvent from the bottom of the recovery column is returned to the extractor. The extract isrecovered overhead and sent on to distillation columns downstream for recovery of theindividual benzene and toluene products.

The raffinate stream exits the top of the extractor and is directed to the raffinate washcolumn. In the wash column, the raffinate is contacted with water to remove dissolved sol-vent. The solvent-rich water is vaporized in the water stripper by exchange with hot circu-lating solvent and then used as stripping steam in the recovery column. Accumulatedsolvent from the bottom of the water stripper is pumped back to the recovery column.

The raffinate product exits the top of the raffinate wash column. The amount ofSulfolane solvent retained in the raffinate is negligible. The raffinate product is common-ly used for gasoline blending or aliphatic solvent applications.

Under normal operating conditions, Sulfolane solvent undergoes only minor oxidativedegradation. A small solvent regenerator is included in the design of the unit as a safeguard


FIGURE 2.2.3 Sulfolane process concept.




FIG

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.2.4

Sul

fola

ne f

low

dia

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against the possibility of air leaking into the unit. During normal operation, a small slip-stream of circulating solvent is directed to the solvent regenerator for removal of oxidizedsolvent.

The extract product from a Sulfolane unit may contain trace amounts of olefins and oth-er impurities that would adversely affect the acid-wash color tests of the final benzene andtoluene products. To eliminate these trace impurities, the extract is clay-treated prior tofractionation. Because clay treating is done at mild conditions, clay consumption is mini-mal.

The treated extract is directed to the aromatics fractionation section, where high-puri-ty benzene, toluene, and sometimes mixed xylenes are recovered. The design of the aro-matics fractionation section varies depending on the particular processing requirements ofthe refiner. The toluene product is often recycled to a UOP Tatoray* unit for conversioninto benzene and xylenes. Mixed xylenes may be routed directly to the xylene recoverysection of the plant for separation into para-xylene, ortho-xylene, and meta-xylene prod-ucts.

Any heavy aromatics in the feed are yielded as a bottoms product from the fractiona-tion section. In most cases, the C9 aromatics are recovered and recycled to a UOP Tatorayunit for the production of additional xylenes. The heavy aromatics may also be blendedback into the refinery gasoline pool or sold as a high-octane blending component.

Figure 2.2.5 shows the process flow of a Sulfolane extractive distillation unit. There aretwo primary columns in the extractive distillation unit: the extractive distillation columnand the solvent recovery column (or solvent stripper column). Aromatic feed is directed tothe ED column. It exchanges heat with the lean solvent and enters a central stage of thetrayed column. The lean solvent is introduced near to the top of the ED column.

Combining solvent and feed alters the relative volatilities of the components to be sep-arated because of the nonideal behavior of the mixture. This is key to the process. Theselectivity of the solvent renders aromatics relatively less volatile than the nonaromatics,as shown in the bottom right chart of Fig. 2.2.3. Good product purity can be achieved ifthere is sufficient separation of K values between the lowest carbon number aromatic andthe higher carbon number nonaromatic species.



RecoveryColumn

ED Column

Raffinate

Feed WaterStripper

SolventRegenerator

Steam

ExtractSteamSteam

FIGURE 2.2.5 Shell Sulfolane process: extractive distillation.




As the hydrocarbon vapor stream flows up the ED column, countercurrent to thedescending solvent, the aromatics are selectively absorbed. The function of the upper sec-tion of the extractive distillation column is to maximize aromatic recovery. The overheadvapor is nonaromatic and is referred to as the raffinate. These vapors are condensed andsent to storage. A portion of the raffinate liquid is used as column reflux to rectifyentrained solvent out of the overhead product. Overhead water is collected in the raffinatereceiver water boot and returned to the unit water circuit. The extractive distillation col-umn is reboiled with steam.

In the lower section of the ED column, the nonaromatics are preferentially stripped outof the liquid and enter the upper portion of the column as a vapor phase due to the solventselectivity, which has made the saturates relatively more volatile than the aromatics. Again,because of finite selectivity, some aromatics, primarily benzene, are stripped into the uppersection of the column where they must be reabsorbed. The lower section of the ED columnserves the function of benzene purification.

The ED column bottoms contain solvent and highly purified aromatics. These materi-als are sent to the solvent recovery column (solvent stripper column). In this column, aro-matics are separated by solvent under vacuum with steam stripping. The overheadaromatic product, depending on the composition (B or BT), is condensed and sent to stor-age or to clay treating prior to product fractionation. A portion of the extract liquid is usedas reflux to remove residual solvent from the extract vapors. The solvent recovery columnis reboiled with steam. Water is collected in the extract receiver boot and is directed to thewater stripper. This small reboiled column (heated by exchange with the solvent stripperbottoms) generates the stripping steam that is returned to the bottom of the solvent recov-ery column via the solvent regenerator. Solvent, as it flows down the recovery column, ispurified of residual hydrocarbons. At the bottom of the recovery column the solvent isessentially pure Sulfolane with a small amount of water. This is then returned to the EDcolumn as lean solvent. A slipstream of lean solvent is directed to a solvent regenerator toremove any degradation products.

FEEDSTOCK CONSIDERATIONS

The feed to a Sulfolane unit is usually a benzene-toluene (BT) cut from a naphtha reform-ing unit. The xylene fraction of the reformate is often already pure enough to sell as mixedxylenes or is sent directly to the para-xylene recovery section of the aromatics complex.In many facilities, the pygas by-product from a nearby ethylene plant is also directed to aSulfolane unit. A few plants also use Sulfolane to recover aromatics from coke-oven lightoil. Before being sent to a Sulfolane unit, the reformate must first be stripped in a debu-tanizer column to remove light ends. Pygas and coke-oven light oils must first behydrotreated to remove dienes, olefins, sulfur, and nitrogen. In general, the feed to aSulfolane unit should meet the specifications outlined in Table 2.2.1.

PROCESS PERFORMANCE

The performance of the UOP Sulfolane process has been well demonstrated in more than100 operating units. The recovery of benzene exceeds 99.9 wt %, and recovery of tolueneis typically 99.8 wt %. The Sulfolane process is also efficient at recovering heavier aro-matics if necessary. Typical recovery of xylenes exceeds 98 wt %, and a recovery of 99 wt% has been demonstrated commercially with rich feedstocks.





UOP Sulfolane units routinely produce a benzene product with a solidification point of5.5°C or better, and many commercial units produce benzene containing less than 100 ppmnonaromatic impurities. The toluene and C8 aromatics products from a Sulfolane unit arealso of extremely high purity and easily exceed nitration-grade specifications. In fact, theultimate purities of all the aromatic products are usually more dependent on the design andproper operation of the downstream fractionation section than on the extraction efficiencyof the Sulfolane unit itself.

The purity and recovery performance of an aromatics extraction unit is largely a func-tion of energy consumption. In general, higher solvent circulation rates result in better per-formance, but at the expense of higher energy consumption. The UOP Sulfolane processdemonstrates the lowest energy consumption of any commercial aromatics extraction tech-nology. A typical UOP Sulfolane unit consumes 275 to 300 kcal of energy per kilogram ofextract produced, even when operating at 99.99 wt % benzene purity and 99.95 wt %recovery. UOP Sulfolane units are also designed to efficiently recover solvent for recyclewithin the unit. Expected solution losses of Sulfolane solvent are less than 5 ppm of thefresh feed rate to the unit.

EQUIPMENT CONSIDERATIONS

The extractor uses rain-deck trays to contact the upward-flowing feed with the downward-flowing solvent. The rain-deck trays act as distributors to maintain an evenly dispersed“rain” of solvent droplets moving down through the extractor to facilitate dissolution of thearomatic components into the solvent phase. A typical Sulfolane extractor column contains94 rain-deck trays.

The raffinate wash column is used to recover residual solvent carried over in the raffi-nate from the extractor. The wash column uses jet-deck trays to provide countercurrentflow between the wash water and raffinate. A typical wash column contains eight jet-decktrays.

The stripper column is used to remove any light nonaromatic hydrocarbons in the richsolvent by extractive distillation. The Sulfolane solvent increases the relative volatilitiesbetween the aromatic and nonaromatic components, thus facilitating the removal of lightnonaromatics in the column overhead. A typical stripper column contains 34 sieve trays.The recovery column separates the aromatic extract from the Sulfolane solvent by vacuumdistillation. A typical recovery column contains 34 valve trays.

The Sulfolane extractive distillation unit has less equipment than a conventional unit.The rain-deck extractor and raffinate wash column are eliminated. Solvent in the raffinate,


TABLE 2.2.1 Sulfolane Feedstock Specifications

Contaminant Effect Limit

Total sulfur Contaminates product 0.2 ppm max.Thiophene Contaminates product 0.2 ppm max.Total chloride Contaminates product, causes corrosion 0.2 ppm max.Bromine number Causes higher solvent circulation, increased 2 max.

utility consumptionDiene index Causes higher solvent circulation, increased 1 max.

utility consumptionDissolved oxygen Causes degradation of solvent 1.0 ppm max.




as described above, is eliminated by the ED column reflux. In the case of a benzene-onlyfeed, all the equipment associated with water circulation and stripping steam can be elim-inated. A Sulfolane unit is approximately 80 percent of the cost of an LLE/ED unit.

The solvent regenerator is a short, vertical drum that is used to remove the polymersand salts formed as a result of the degradation of solvent by oxygen. The regenerator isoperated under vacuum and runs continuously.

The Sulfolane process is highly heat-integrated. Approximately 11 heat exchangers aredesigned into a typical unit.

All the equipment for the Sulfolane unit, with the exception of the solvent regeneratorreboiler, is specified as carbon steel. The solvent regenerator reboiler is constructed ofstainless steel.

CASE STUDY

A summary of the investment cost and utility consumption for a typical Sulfolane unit isshown in Table 2.2.2. The basis for this case is a Sulfolane unit processing 54.5 metric tonsper hour (MT/h) [10,400 barrels per day (BPD)] of a BT reformate cut. This case corre-sponds to the case study for an integrated UOP aromatics complex in Chap. 2.1 of this hand-book. The investment cost is limited to the Sulfolane unit itself and does not includedownstream fractionation. The estimated erected cost for the Sulfolane unit assumes con-struction on a U.S. Gulf Coast site in 2002. The scope of the estimate includes engineering,procurement, erection of equipment on the site, and the initial inventory of Sulfolane solvent.

COMMERCIAL EXPERIENCE

Since the early 1950s, UOP has licensed four different aromatics extraction technologies,including the Udex,* Sulfolane, Tetra,* and Carom* processes. UOP’s experience in aro-matics extraction encompasses more than 200 units, which range in size from 2 to 260MT/h (400 to 50,000 BPD) of feedstock.

In 1952, UOP introduced the first large-scale aromatics extraction technology, theUdex process, which was jointly developed by UOP and Dow Chemical. Although theUdex process uses either diethylene glycol or triethlyene glycol as a solvent, it is similarto the Sulfolane process in that it combines liquid-liquid extraction with extractive distil-lation. Between 1950 and 1965, UOP licensed a total of 82 Udex units.


TABLE 2.2.2 Investment Cost and Utility

Consumption*

Estimated erected cost, million $ U.S. 13.5Utility consumption:

Electric power, kW 390High-pressure steam, MT/h (klb/h) 27.5 (60.6)Cooling water, m3/h (gal/min) 274 (1207)

*Basis: 25.0 MT/h of toluene product, 11.8 MT/h of benzeneproduct, 54.5 MT/h (10,400 BPD) of BT reformate feedstock.

Note: MT/h � metric tons per hour; BPD � barrels per day.





In the years following the commercialization of the Udex process, considerableresearch was done with other solvent systems. In 1962, Shell commercialized the firstSulfolane units at its refineries in England and Italy. The success of these units led to anagreement in 1965 whereby UOP became the exclusive licenser of the Shell Sulfolaneprocess. Many of the process improvements incorporated in modern Sulfolane units arebased on design features and operating techniques developed by UOP. By 1995, UOP hadlicensed a total of 120 Sulfolane units throughout the world.

Meanwhile, in 1968, researchers at Union Carbide discovered that tetraethylene glycolhad a higher capacity for aromatics than the solvents being used in existing Udex units.Union Carbide soon began offering this improved solvent as the Tetra process. UnionCarbide licensed a total of 17 Tetra units for aromatics extraction; 15 of these units wereoriginally UOP Udex units that were revamped to take advantage of the improvementsoffered by the Tetra process.

Union Carbide then commercialized the Carom process in 1986. The Carom flowscheme is similar to that used in the Udex and Tetra processes, but the Carom process takesadvantage of a unique two-component solvent system that nearly equals the performanceof the Sulfolane solvent. In 1988, UOP merged with the CAPS division of Union Carbide.As a result of this merger, UOP now offers both the Sulfolane and Carom processes foraromatics extraction and continues to support the older Udex and Tetra technologies.

The Carom process is ideal for revamping older Udex and Tetra units for higher capac-ity, lower energy consumption, or better product purity. The Carom process can also becompetitive with the Sulfolane process for new-unit applications. By 2002, UOP hadlicensed a total of seven Carom units. Six of these units are conversions of Udex or Tetraunits, and one is a new unit.

BIBLIOGRAPHY

Jeanneret, J. J., P. Fortes, T. L. LaCosse, V. Sreekantham, and T. J. Stoodt: “Sulfolane and CaromProcesses: Options for Aromatics Extraction,” UOP Technology Conferences, various locations,September 1992.





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Documents

oven light oil

increased utility consumption

high specific gravity

operating techniques developed

light nonaromatics displace

light nonaromatic hydrocarbons

light nonaromatic components

light nonaromatic impurities