0071455914_ar012

CHAPTER 2.5UOP ISOMAR PROCESS

Patrick J. SiladyUOP LLC

Des Plaines, Illinois

INTRODUCTION

The UOP Isomar* process is used to maximize the recovery of a particular xylene isomerfrom a mixture of C8 aromatic isomers. The Isomar process is most often applied to para-xylene recovery, but it can be used to maximize the recovery of ortho-xylene or meta-xylene. The term mixed xylenes is used to describe a mixture of C8 aromatic isomerscontaining a near-equilibrium distribution of para-xylene, ortho-xylene, meta-xylene, andethylbenzene (EB). In the case of para-xylene recovery, a mixed-xylenes feed is chargedto a UOP Parex* unit where the para-xylene isomer is preferentially extracted at 99.9 wt% purity and 97 wt % recovery per pass. The Parex raffinate is almost entirely depleted ofpara-xylene and is then sent to the Isomar unit (Fig. 2.5.1). The Isomar unit reestablishesa near-equilibrium distribution of xylene isomers, essentially creating additional para-xylene from the remaining ortho and meta isomers. Effluent from the Isomar unit is thenrecycled to the Parex unit for recovery of additional para-xylene. In this way, the ortho andmeta isomers and EB are recycled to extinction. A complete description of the entire aro-matics complex may be found in Chap. 2.1.

PROCESS CHEMISTRY

The two main categories of xylene isomerization catalysts are EB dealkylation catalystsand EB isomerization catalysts. The primary function of both catalyst types is to reestab-lish an equilibrium mixture of xylene isomers; however, they differ in how they handle theEB in the feed. An EB dealkylation catalyst converts EB to a valuable benzene coproduct.An EB isomerization catalyst converts EB to additional xylenes.

UOP offers both EB isomerization catalysts I-9,* I-210,* and I-400 and EB dealkyla-tion catalysts I-300* and I-330.* Both types are bifunctional catalysts that have a balanceof catalytic sites between zeolitic (acid) and metal functions. The acid function on eachcatalyst serves the same function: isomerization of xylenes.

The EB isomerization catalyst systems I-9 and I-210 isomerize EB to xylenes througha naphthene intermediate (Fig. 2.5.2). The metal function first saturates the EB to ethyl-

2.39

*Trademark and/or service mark of UOP LLC.

Source: HANDBOOK OF PETROLEUM REFINING PROCESSES

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)Copyright © 2004 The McGraw-Hill Companies. All rights reserved.

Any use is subject to the Terms of Use as given at the website.

cyclohexane, then the acid function isomerizes it to dimethylcyclohexane, and finally themetal function dehydrogenates the naphthene to xylene. Because the isomerization of EBis an equilibrium-limited reaction, the conversion of EB is usually limited to about 30 to35 wt % per pass. In a typical aromatics complex using the I-9 catalyst, naphthenes arerecycled to the Isomar unit through the xylene column and Parex unit to suppress the for-mation of naphthenes in the Isomar unit and thereby increase the yield of para-xylenefrom the complex.

UOP will be introducing I-400, a new EB isomerization catalyst, in 2003. I-400 willprovide enhanced EB conversion, increased xylene yield over I-9 and I-210, while allow-ing longer processing cycles between regenerations.

The EB dealkylation catalyst systems I-300 and I-330 use an EB dealkylation mecha-nism in which the ethyl group is cleaved from the aromatic ring by the acid function of thecatalyst (Fig. 2.5.3). This reaction is not equilibrium-limited, thereby allowing EB con-version of up to 70 wt % or greater per pass. Because this reaction does not involve a naph-thene intermediate, C8 naphthenes need not be recycled through the Parex-Isomar loop.

All xylene isomerization catalysts exhibit some by-product formation across the reac-tor. A large portion of the total feed to the Parex-Isomar loop is recycled from the Isomarunit. A typical Parex-Isomar loop is designed with a recycle/feed ratio of 2.5 : 3.5. By-product formation across the isomerization process is magnified accordingly. Therefore, asmall reduction in the by-product formation across the Isomar reactor translates to a large,overall yield advantage. In the Isomar process, the precise level of expected by-productformation varies with catalyst type and operating severity, but it is normally in the rangeof 1.0 to 4.0 wt % per pass of the feed. The lower end of the range is representative of oper-ation with the later-generation catalysts I-100, I-210, and I-300. The upper end of the rangeis representative of operation with the I-9 catalyst. By-products are predominantly aro-matic, such that overall ring retention is greater than 99 percent. The proper selection ofthe isomerization catalyst type depends on the configuration of the aromatics complex, thecomposition of the feedstocks, and the desired product slate. The choice of isomerizationcatalyst must be based on an economic analysis of the entire aromatic complex. The C8fraction of the reformate from a typical petroleum naphtha contains approximately 15 to17 wt % EB, but up to 30 wt % EB may be in a similar pyrolysis gasoline (pygas) fraction.Using an EB isomerization catalyst maximizes the yield of para-xylene from an aromat-ics complex by converting EB to xylenes. An EB isomerization catalyst is usually chosen

2.40 BASE AROMATICS PRODUCTION PROCESSES

ClayTreater

MixedXylenes

ClayTreater

Light EndsToluene

para-Xylene

OXColumn

XyleneSplitter

Dehept.Column

ortho-Xylene

H2

A+

Parex Isomar

FIGURE 2.5.1 Typical Parex-Isomar loop.

UOP ISOMAR PROCESS



UOP ISOMAR PROCESS 2.41

when the primary goal of the complex is to maximize the production of para-xylene froma limited supply of feedstock. The EB isomerization catalyst system will also minimize thequantity of benzene by-product produced.

Alternatively, an EB dealkylation catalyst can be used to debottleneck an existing Parexunit or crystallizer by converting more EB per pass through the isomerization unit andeliminating the requirement for naphthene intermediate circulating around the Parex-Isomar loop. For a new aromatics complex design, using an EB dealkylation catalyst min-imizes the size of the xylene column and Parex and Isomar units required to produce agiven amount of para-xylene. However, this reduction in size of the Parex-Isomar loopcomes at the expense of lower para-xylene yields, because all the EB in the feed is beingconverted to benzene rather than to additional para-xylene. Lower para-xylene yieldmeans that more feedstock will be required, which increases the size of the CCR*Platforming,* Sulfolane,* and Tatoray units in the front end of the complex, as well asmost of the fractionators.

CH3

CH3

CH3CH3

CH3

CH3

CH3 CH3

CH3CH3

Xylene Isomerization

Acid Acid

EB Isomerization

C2H5C2H5

Metal Acid Metal

FIGURE 2.5.2 EB isomerization chemistry.

Xylene Isomerization

Acid Acid

EB Dealkylation

MetalAcid

CH3

CH3

H2

CH3 CH3CH3

CH3

C2H5

+ C2H4 + C2H5

FIGURE 2.5.3 EB dealkylation chemistry.

*Trademark and/or service mark of UOP LLC.

UOP ISOMAR PROCESS



The EB dealkylation catalysts I-300 and I-330 are high-activity catalysts. As such, theycan operate at higher space velocity, allowing a reduced catalyst loading for a given pro-cessing rate. Compared to its predecessor I-100, roughly one-half the amount of catalyst isneeded. Unlike some EB dealkylation catalysts, I-300 and I-330 do not require continuousaddition of ammonia to achieve desired activity and selectivity. Since 1999, I-300 catalystshave been loaded into a dozen units. I-300 exhibits highly stable performance with ongo-ing cycle lengths expected to reach 4 to 5 years without regeneration. I-330 providesenhanced benzene selectivity over a wide range of space velocities.

DESCRIPTION OF THE PROCESS FLOW

An Isomar unit is always combined with a recovery unit for one or more xylene isomers.Most often, the Isomar process is combined with the UOP Parex process for para-xylenerecovery (Fig. 2.5.1). Fresh mixed-xylenes feed to the Parex-Isomar loop is sent to a xylenecolumn, which can be designed either to recover ortho-xylene in the bottoms or to simplyreject C9� aromatic components to meet feed specifications for the Parex unit. The xylenecolumn overhead is then directed to the Parex unit where 99.9 wt % para-xylene is pro-duced at 97 wt % recovery per pass. The Parex raffinate from the Parex unit, which con-tains less than 1 wt % para-xylene, is sent to the Isomar unit.

The feed to the Isomar unit is first combined with hydrogen-rich recycle gas and make-up gas to replace the small amount of hydrogen consumed in the Isomar reactor (Fig. 2.5.4).The combined feed is then preheated by exchange with the reactor effluent, vaporized in afired heater, and raised to reactor operating temperature. The hot feed vapor is then sent tothe reactor where it is passed radially through a fixed bed of catalyst. The reactor effluent iscooled by exchange with the combined feed and then sent to the product separator.Hydrogen-rich gas is taken off the top of the product separator and recycled to the reactor.A small portion of the recycle gas is purged to remove accumulated light ends from therecycle gas loop. Liquid from the bottom of the product separator is charged to the dehep-tanizer column. The C7� overhead from the deheptanizer is cooled and separated into gasand liquid products. The deheptanizer overhead gas is exported to the fuel gas system. Theoverhead liquid is recycled to the Platforming unit so that any benzene in this stream may


ChargeHeater

Reactor

ProductSeparator

ClayTreater

Deheptanizer

DeheptanizerBottoms

OverheadLiquid

LightEnds

XyleneSplitter

Overhead

Purge Gas

ParexRaffinate

MakeupHydrogen

FIGURE 2.5.4 Isomar flow diagram.

UOP ISOMAR PROCESS



be recovered in the Sulfolane. The C8� fraction from the bottom of the deheptanizer is clay-treated, combined with fresh mixed-xylenes feed, and recycled to the xylene column.

FEEDSTOCK CONSIDERATIONS

The feedstock to an Isomar unit usually consists of raffinate from a Parex unit. At times,charging the fresh mixed-xylenes feed directly to the Isomar unit may be desirable; or theIsomar unit may be used in conjunction with fractionation to produce only ortho-xylene. Inany case, the feed to an Isomar unit should meet the specifications outlined in Table 2.5.1.

Nonaromatic compounds in the feed to the Isomar unit are primarily cracked to light endsand removed from the Parex-Isomar loop. This ability to crack nonaromatic impurities elim-inates the need for extracting the mixed xylenes, and consequently the size of the Sulfolaneunit can be greatly reduced. In a UOP aromatics complex, the reformate from the CCRPlatforming unit is split into C7� and C8� fractions. The C7� fraction is sent to theSulfolane unit for recovery of high-purity benzene and toluene. The EB dealkylation cata-lysts I-300 and I-330 allow recovery of high-purity benzene by fractionation alone. Becausemodern, low-pressure CCR Platforming units operate at extremely high severity for aromat-ics production, the C8� fraction that is produced contains essentially no nonaromatic impu-rities and thus can be sent directly to the xylene recovery section of the complex.

PROCESS PERFORMANCE

The performance of the xylene isomerization catalysts can be measured in several specif-ic ways, including the approach to equilibrium in the xylene isomerization reaction itself,the conversion of EB per pass, and the ring loss per pass. Approach to equilibrium is ameasure of operating severity for an EB isomerization catalyst, and EB conversion is ameasure of operating severity for an EB alkylation catalyst. For both catalyst types, ringloss increases with operating severity. In a para-xylene application, for example, high EBconversion in the Isomar unit is beneficial for the Parex unit but is accompanied by high-er ring loss and thus lower overall yield of para-xylene from the complex.

Perhaps the best way to compare xylene isomerization catalyst is to measure the over-all para-xylene yield from the Parex-Isomar loop. Figure 2.5.5 compares the para-xyleneyield, based on fresh mixed-xylenes feed to the Parex-Isomar loop, for the I-9, I-300, andI-210 systems. The basis for the comparison is the flow scheme shown in Fig. 2.5.1. Thecomposition of the mixed-xylenes feed is 17 wt % EB, 18 wt % para-xylene, 40 wt %meta-xylene, and 25 wt % ortho-xylene. The operating severity for the I-9 and I-210 cata-lysts is 22.1 wt % para-xylene in the total xylenes from the Isomar unit. The operating


TABLE 2.5.1 Isomar Feedstock Specifications

Contaminant Effect Limit

Water Promotes corrosion, deactivates catalyst, irreversible 200 ppm, max.Total chloride Increases acid function, increases cracking, reversible 2 ppm, max.Total nitrogen Neutralizes acid sites, deactivates catalyst, irreversible 1 ppm, max.Total sulfur Attenuates metal activity, increases cracking, reversible 1 ppm, max.Lead Poisons acid and metal sites, irreversible 20 ppb, max.Copper Poisons acid and metal sites, irreversible 20 ppb, max.Arsenic Poisons acid and metal sites, irreversible 2 ppb, max.

UOP ISOMAR PROCESS



severity for the I-300 and I-330 catalysts is 65 wt % conversion of EB per pass. With theI-9 catalyst, the overall yield of para-xylene is 84 wt % of the fresh mixed-xylenes feed.Because they have lower ring loss per pass, the I-300 and I-330 catalysts exhibit a higheroverall yield of benzene plus para-xylene, but the yield of para-xylene is only 76.5 wt %.Thus, more mixed xylenes are required to produce a target amount of para-xylene with theI-300 and I-330 catalysts.

Figure 2.5.5 also shows the yields for the UOP EB isomerization catalyst called I-210.The I-210 catalyst relies on the same reaction chemistry as I-9 but is more selective andexhibits lower by-product formation. The by-product formation of the I-210 catalyst isonly about 1.5 wt % compared to 4 wt % for I-9. With the I-210 catalyst, the overall yieldof para-xylene is 91 wt % of fresh mixed-xylenes feed, a yield improvement of 7 wt %over that of the I-9 catalyst.

EQUIPMENT CONSIDERATIONS

The charge heater is normally a radiant convection-type heater. The process stream is heat-ed in the radiant section, and the convection section is used for a hot-oil system or steamgeneration. The heater can be designed to operate on either fuel gas or fuel oil, and eachburner is equipped with a fuel gas pilot. A temperature controller at the heater outlet reg-ulates the flow of fuel to the burners. Radiant-section tubes are constructed of 1.25% Cr-0.5% Mo. Tubes in the convection section are carbon steel.

The Isomar process normally uses a radial-flow reactor. The vapor from the chargeheater enters the top of the reactor and is directed to the sidewall. The vapors then travelradially through a set of scallops, through the fixed bed, and into a center pipe. The reac-tor effluent then flows down through the center pipe to the reactor outlet. The advantageof the radial-flow reactor is low pressure drop, which is important in the Isomar processbecause the reaction rates are sensitive to pressure. Low pressure drop also reduces thepower consumption of the recycle gas compressor. For I-300 and I-330 the operating pres-sure is directionally higher, and a downflow reactor configuration is more readily accom-modated. The reactor is constructed of 1.25% chrome (Cr)-0.5% molybdenum (Mo) alloy.


Pro

duct

Yie

ld, w

t-%

of

Fee

d

100

I-9 I-300 I-210

90

80

70

60

50

para-Xylene

Benzene

84.5

84.0

87.3

76.5

91.1

91.0

FIGURE 2.5.5 Parex-Isomar yields.

UOP ISOMAR PROCESS



The purpose of the product separator is to split the condensed reactor effluent into liq-uid product and hydrogen-rich recycle gas. The pressure in the product separator deter-mines the pressure in the reactor. Separator pressure is regulated by controlling the rate ofhydrogen makeup flow. Hydrogen purity in the recycle gas is monitored by a hydrogenanalyzer at the recycle-gas compressor suction. When hydrogen purity gets too low, asmall purge is taken from the recycle gas. The product separator is constructed of killedcarbon steel.

The recycle gas compressor is usually of the centrifugal type and may be driven by anelectric motor or a steam turbine. The compressor is provided with both seal oil and tubeoil circuits and an automatic shutdown system to protect the machine against damage.

The purpose of the deheptanizer column is to remove light by-products from the reac-tor effluent. The deheptanizer usually contains 40 trays and incorporates a thermosiphonreboiler. Heat is usually supplied by the overhead vapor from the xylene column locatedupstream of the Parex unit. The deheptanizer column is constructed of carbon steel.

The combined feed-effluent exchanger is constructed of 1.25% Cr-0.5% Mo. Otherheat exchangers in the Isomar unit are constructed of carbon steel.

CASE STUDY

A summary of the investment cost and the utility consumption for a typical Isomar unit isshown in Table 2.5.2. The basis for this case is an Isomar unit processing 5600 MT/day(40,000 BPD) of raffinate from a Parex unit. This case corresponds to the case study foran integrated UOP aromatics complex presented in Chap. 2.1. The investment cost is lim-ited to the Isomar unit, deheptanizer column, and downstream clay treater. The estimatederected cost for the Isomar unit assumes construction on a U.S. Gulf Coast site in 2002.The scope of the estimate includes engineering, procurement, erection of equipment onsite, and the initial load of catalyst.

COMMERCIAL EXPERIENCE

The first UOP Isomar unit went on-stream in 1967. Since that time, UOP has licensed atotal of 54 Isomar units throughout the world. Fifty-two UOP Isomar units have been com-missioned, and another two are in various stages of design and construction. UOP hasoffered both EB isomerization and EB dealkylation catalysts longer than any other licen-


TABLE 2.5.2 Investment Cost and UtilityConsumption*

Estimated ISBL million cost $ U.S. 29.3(including initial catalyst inventory)

Utility consumptionElectric power, kW 918High-pressure steam, MT/h 16.9Cooling water, m3/h 236Fuel fired, million kcal/h 20.8

*Basis: 5600 MT/h (40,000 BPD) Parex raffi-nate.

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

UOP ISOMAR PROCESS



sor of xylene isomerization technology. This choice of catalyst coupled with the relatedoperational experience and know-how gives UOP increased flexibility to design an aro-matics complex to meet any customer’s desired product distribution.

BIBLIOGRAPHY

Ebner, T. E.: “Improve Profitability with Advanced Aromatics Catalysts,” UOP Seminar, EPTCConference, Budapest, June 2002.

Ebner, T. E., K. M. O’Neil, and P. J. Silady: “UOP’s New Isomerization Catalysts (I-300 Series),”American Institute of Chemical Engineers Spring Meeting, New Orleans, March 2002.

Jeanneret, J. J.: “Development in p-Xylene Technology,” DeWitt Petrochemical Review, Houston,March 1993.

Jeanneret, J. J.: “para-Xylene Production in the 1990’s,” UOP Technology Conferences, various loca-tions, March 1995.

Jeanneret, J. J., C. D. Low, and J. Swift: “Process for Maximum BTX Complex Profitability,” DeWittPetrochemical Review, Houston, September 1992.


UOP ISOMAR PROCESS



Documents

0071455914_ar012