0071455914_ar011

CHAPTER 2.4BP-UOP CYCLAR PROCESS

Lubo ZhouUOP LLC

Des Plaines, Illinois

INTRODUCTION

In recent years, light hydrocarbons have become increasingly attractive as fuels and petro-chemical feedstocks, and much effort has been devoted to improving the recovery, pro-cessing, and transportation of liquefied petroleum gas (LPG) and natural gas. Becauseproduction areas are often located in remote areas that are far removed from establishedprocessing plants or consumers, elaborate product transport infrastructures are required.Although natural gas can be moved economically through pipelines, condensation prob-lems limit the amount of LPG that can be transported in this way. Thus, most LPG is trans-ported by such relatively expensive means as special-purpose tankers or railcars. The highcost of transporting LPG can often depress its value at the production site. This statementis especially true for propane, which is used much less than butane for gasoline blendingand petrochemical applications.

British Petroleum (BP) recognized the problem with transporting LPG and in 1975began research on a process to convert LPG to higher-value liquid products that could beshipped more economically. This effort led to the development of a catalyst that was capa-ble of converting LPG to petrochemical-grade benzene, toluene, and xylenes (BTX) in asingle step. However, BP soon realized that the catalyst had to be regenerated often in thisapplication and turned to UOP* for its well-proven CCR* technology, which continuous-ly regenerates the catalyst. UOP developed a high-strength formulation of the BP catalystthat would work in CCR service and also applied the radial-flow, stacked-reactor designoriginally developed for the UOP Platforming* process. The result of this outstandingtechnical collaboration is the BP-UOP Cyclar* process.

PROCESS CHEMISTRY

The Cyclar process converts LPG directly to a liquid aromatics product in a single opera-tion. The reaction is best described as dehydrocyclodimerization and is thermodynamical-ly favored at temperatures above 425°C (800°F). The dehydrogenation of light paraffins

2.29

*Trademark and/or service mark of UOP.

Source: HANDBOOK OF PETROLEUM REFINING PROCESSES

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(propane and butanes) to olefins is the rate-limiting step (Fig. 2.4.1). Once formed, thehighly reactive olefins oligomerize to form larger intermediates, which then rapidly cyclizeto naphthenes. These reactions—dehydrogenation, oligomerization, and cyclization—areall acid-catalyzed. The shape selectivity of the zeolite component of the catalyst also pro-motes the cyclization reaction and limits the size of the rings formed. The final reactionstep is the dehydrogenation of the naphthenes to their corresponding aromatics. This reac-tion is highly favored at Cyclar operating conditions, and the result is virtually completeconversion of the naphthenes.

The reaction intermediates can also undergo a hydrocracking side reaction to formmethane and ethane. This side reaction results in a loss of yield because methane andethane are inert at Cyclar operating conditions.

Because olefins are a key reaction intermediate, they can of course be included in thefeed to the Cyclar unit. Heavier paraffins, such as pentanes, can also be included in thefeed. Olefins and pentanes are almost completely converted in the Cyclar unit, but the unitmust be designed to handle them because they result in a higher catalyst-coking rate thanpure butane and propane feedstocks.

Although the reaction sequence involves some exothermic steps, the preponderance ofdehydrogenation reactions results in a highly endothermic overall reaction. Five moles ofhydrogen are produced for every mole of aromatic components formed from propane orbutane.

Because propane and butanes are relatively unreactive, the Cyclar process requires acatalyst with high activity. At the same time, the production of methane and ethane fromunwanted hydrocracking side reactions must be minimized. An extensive joint effort by BPand UOP has resulted are a catalyst that combines several important features to ensure effi-cient commercial operation:

● At the conditions necessary for high selectivity to aromatics, the conversion perfor-mance of the catalyst declines slowly.

● The selectivity to aromatics is nearly constant over the normal range of conversion,resulting in stable product yield and quality. Thus, economic process performance canbe maintained despite normal fluctuations in unit operation.

● At normal process conditions, the rate of carbon deposition on the catalyst is slow andsteady, amounting to less than 0.02 wt % of the feed processed. Because the carbon lev-els on spent catalyst are low, regeneration requirements are relatively mild. Mild regen-eration conditions extend the life of the catalyst and make it insensitive to process upsetsand changes in feedstock composition.

2.30 BASE AROMATICS PRODUCTION PROCESSES

Paraffins Olefins Oligomers Naphthenes Aromatics

Hydrogen

By-Products

FIGURE 2.4.1 Cyclar reaction mechanism.

BP-UOP CYCLAR PROCESS



BP-UOP CYCLAR PROCESS 2.31

● The catalyst exhibits high thermal stability and is relatively insensitive to common feed-stock contaminants. Regeneration fully restores the activity and selectivity of the cata-lyst to the performance seen with fresh catalyst.

● High mechanical strength and low attrition characteristics make the catalyst well suitedfor continuous catalyst regeneration.

DESCRIPTION OF THE PROCESS FLOW

A Cyclar unit is divided into three major sections. The reactor section includes the radial-flow reactor stack, combined feed exchanger, charge heater, and interheaters. The regen-erator section includes the regenerator stack and catalyst transfer system. The productrecovery section includes the product separators, compressors, stripper, and gas recoveryequipment.

The flow scheme is similar to the UOP CCR Platforming process, which is used wide-ly throughout the world for reforming petroleum naphtha. A simplified block flow diagramis shown in Fig. 2.4.2. Fresh feed and recycle are combined and heat exchanged againstreactor effluent. The combined feed is then raised to reaction temperature in the chargeheater and sent to the reactor section. Four adiabatic, radial-flow reactors are arranged ina vertical stack. Catalyst flows vertically by gravity down the stack, and the charge flowsradially across the annular catalyst beds. Between each reactor, the vaporized charge isreheated to reaction temperature in an interheater.

The effluent from the last reactor is split into vapor and liquid products in a productseparator. The liquid is sent to a stripper, where light saturates are removed from the C6�aromatic product. Vapor from the product separator is compressed and sent to a gas recov-ery section, typically a cryogenic unit, for separation into a 95 percent pure hydrogen prod-uct stream, a fuel gas stream of light saturates, and a recycle stream of unconverted LPG.Hydrogen is not recycled.

Because coke builds up on the Cyclar catalyst over time at reaction conditions, partial-ly deactivated catalyst is continually withdrawn from the bottom of the reactor stack forregeneration. Figure 2.4.3 shows additional details of the catalyst regeneration section. Adiscrete amount of spent catalyst flows into a lock hopper, where it is purged with nitro-gen. The purged catalyst is then lifted with nitrogen to the disengaging hopper at the topof the regenerator. The catalyst flows down through the regenerator, where the accumulat-ed carbon is burned off. Regenerated catalyst flows down into the second lock hopper,where it is purged with hydrogen and then lifted with hydrogen to the top of the reactorstack. Because the reactor and regenerator sections are separate, each operates at its ownoptimal conditions. In addition, the regeneration section can be temporarily shut down formaintenance without affecting the operation of the reactor and product recovery sections.

FEEDSTOCK CONSIDERATIONS

Propane and butanes should be the major components in the feedstock to a Cyclar unit. TheC1 and C2 saturates should be minimized because these components act as inert diluents.Olefins should be limited to less than 10 percent of the feed. Higher concentrations ofolefins require hydrogenation of the feed. The C5 and C6 components increase the rate ofcoke formation in the process and should be limited to less than 20 and 2 wt %, respec-tively, in Cyclar units designed for LPG service. Cyclar units can be designed to processsignificantly higher amounts of C5 and C6 materials if necessary. In general, the feed to aCyclar unit should meet the specifications outlined in Table 2.4.1.




FIG

UR

E 2

.4.2

Cyc

lar

flow

dia

gram

.

2.32




PROCESS PERFORMANCE

The major liquid products from a Cyclar process unit are BTX and C9� aromatics. Theseproducts may be separated from one another by conventional fractionation downstream ofthe Cyclar stripper column.

In general, aromatics yield increases with the carbon number of the feedstock. In a low-pressure operation, the overall aromatics yield increases from 62 wt % of fresh feed withan all-propane feedstock to 66 percent with an all-butane feed. With this yield increasecomes a corresponding decrease in fuel gas production. These yield figures can be inter-polated linearly for mixed propane and butane feedstocks. The distribution of butane iso-mers in the feed has no effect on yields.

The distribution of aromatic components in the liquid product is also affected by feed-stock composition. Butane feedstocks produce a product that is leaner in benzene and rich-er in xylenes than that produced from propane (Fig. 2.4.4). With either propane or butanefeeds, the liquid product contains about 91 percent BTX and 9 percent heavier aromatics.

The Cyclar unit produces aromatic products with nonaromatic impurities limited to1500 ppm or less. Thus, marketable, high-quality, petrochemical-grade BTX can beobtained by fractionation alone, without the need for subsequent extraction.

The by-product light ends contain substantial amounts of hydrogen, which may berecovered in several different ways, depending on the purity desired:

● An absorber-stripper system produces a 65 mol % hydrogen product stream.● A cold box produces 95 mol % hydrogen.● An absorber-stripper system combined with a pressure-swing absorption (PSA) unit pro-

duces 99 mol % hydrogen.


FIGURE 2.4.3 Catalyst regeneration section.




● A cold box combined with a PSA unit is usually more attractive if large quantities of99� mol % hydrogen are desired.

EQUIPMENT CONSIDERATIONS

The principal Cyclar operating variables are feedstock composition, pressure, space veloc-ity, and temperature. The temperature must be high enough to ensure nearly complete con-version of reaction intermediates to produce a liquid product that is essentially free ofnonaromatic impurities but low enough to minimize nonselective thermal reactions. Spacevelocity is optimized against conversion within this temperature range to obtain high prod-uct yields with minimum operating costs.

Reaction pressure has a big impact on process performance. Higher pressure increasesreaction rates, thus reducing catalyst requirements. However, some of this higher reactivi-ty is due to increased hydrocracking, which reduces aromatic product yield. UOP current-ly offers two alternative Cyclar process designs. The low-pressure design is recommendedwhen maximum aromatics yield is desired. The high-pressure design requires only one-half the catalyst and is attractive when minimum investment and operating costs are theoverriding considerations (Fig. 2.4.5).

Various equipment configurations are possible depending on whether a gas turbine,steam turbine, or electric compressor drive is specified; whether air-cooling or water-cool-ing equipment is preferred; and whether steam generation is desirable.


TABLE 2.4.1 Feedstock Specifications

Contaminant Limit

Sulfur �20 mol ppmWater No free waterOxygenates �10 wt ppmBasic nitrogen �1 wt ppmFluorides �0.3 wt ppmMetals �50 wt ppb

FIGURE 2.4.4 Cyclar aromatic product distribution.




CASE STUDY

The overall material balance, investment cost, and utility consumption for a representativeCyclar unit are shown in Table 2.4.2. The basis for this case is a low-pressure Cyclar unitprocessing 54 metric tons per hour (MT/h) [15,000 barrels per day (BPD)] of a feed con-sisting of 50 wt % propane and 50 wt % butanes. The investment cost is limited to theCyclar unit and stripper column and does not include further downstream product frac-tionation. The estimated erected cost for the Cyclar unit assumes construction on a U.S.Gulf Coast site in 1995. The scope of the estimate includes engineering, procurement,erection of equipment on-site, and the initial load of Cyclar catalyst.

Economic Comparison of Cyclar and Naphtha Reforming

Figure 2.4.6 shows the economic comparison of an aromatic complex based on a Cyclarand naphtha reforming unit. The feed to Cyclar was assumed at 50 percent C3 and 50 per-cent C4 by weight. The same para-xylene production rate was assumed for both cases inthe study. From 1995 to 1999, the aromatic complex based on a Cyclar and a reformingunit had similar gross profit. The price for the feed and major products used in the studyis listed in Table 2.4.3, and the price for by-products is listed in Table 2.4.4.

COMMERCIAL EXPERIENCE

The UOP CCR technology, first commercialized in 1971 for the Platforming process, hasbeen applied to the Oleflex* and Cyclar process technologies. More than 100 CCR unitsare currently operating throughout the world. The combination of a radial-flow, stackedreactor and a continuous catalyst regenerator has proved to be extremely reliable. On-stream efficiencies of more than 95 percent are routinely achieved in commercial CCRPlatforming units.


FIGURE 2.4.5 Effect of Cyclar operation pressure.

*Trademark and/or service mark of UOP.





Gro

ss P

rofi

t $M

M

Constant p-xylene production

1995

350

300

250

200

150

100

50

0

Reforming

Cyclar

1996 1997 1998 1999 2000

FIGURE 2.4.6 Economic comparison of gross profit for 1995–1999 values.

TABLE 2.4.2 Material Balance and Investment Cost*

Overall material balance MTA

LPG feedstock 430,000Products:

Benzene 66,700Toluene 118,800Mixed xylenes 64,000C9� aromatics 24,600Hydrogen (95 mol %) 29,400Fuel gas 126,500

Estimated erected cost, million $ U.S. 79.0

Utility consumption:

Electric power, kW 5500High-pressure steam, MT/h 27 (credit)Low-pressure steam, MT/h 7Boiler feedwater, MT/h 33Cooling water, m3/h 640Fuel fired, million kcal/h 70

*Basis: 54 ton/h (15,000 BPD) of LPG feedstock. Feed com-position: 50 wt % propane, 50 wt % butanes.

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




BP commissioned the first commercial-scale Cyclar unit at its refinery inGrangemouth, Scotland, in January 1990. This demonstration unit was designed to process30,000 metric tons per annum (MTA) of propane or butane feedstock at either high or lowpressure over a wide range of operating conditions. The demonstration effort was a com-plete success because it proved all aspects of the Cyclar process on a commercial scale andsupplied sufficient data to confidently design and guarantee future commercial units. TheCyclar unit at Grangemouth demonstration unit was dismantled in 1992 after completionof the development program.

In 1995, UOP licensed the first Cyclar-based aromatics complex in the Middle East.This Cyclar unit is a low-pressure design that is capable of converting 1.3 million MTA ofLPG to aromatics. The associated aromatics complex is designed to produce 350,000 MTAof benzene, 300,000 MTA of para-xylene, and 80,000 MTA of ortho-xylene. This newaromatics complex started in August 1999 and continues to operate at present.

BIBLIOGRAPHY

Doolan, P. C.: “Cyclar: LPG to Valuable Aromatics,” DeWitt Petrochemical Review, Houston, March1989.

Doolan, P. C., and P. R. Pujado: “Make Aromatics from LPG,” Hydrocarbon Processing, September1989.

Gosling, C. D., G. L. Gray, and J. J. Jeanneret: “Produce BTX from LPG with Cyclar,” CMAI WorldPetrochemical Conference, Houston, March 1995.

Gosling, C. D., F. P. Wilcher, and P. R. Pujado: “LPG Conversion to Aromatics,” Gas ProcessorsAssociation 69th Annual Convention, Phoenix, March 1990.

Gosling, C. D., F. P. Wilcher, L. Sullivan, and R. A. Mountford: “Process LPG to BTX Products,”Hydrocarbon Processing, December 1991.

Martindale, D. C., P. J. Kuchar, and R. K. Olson: “Cyclar: Aromatics from LPG,” UOP TechnologyConferences, various locations, September 1988.


TABLE 2.4.3 Price Used for Feed andMajor Products, $/MT

Year Naphtha LPG Bz p-X

1995 139 120 300 9601996 168 130 305 5701997 171 145 310 4401998 115 85 225 3801999 150 125 210 375

TABLE 2.4.4 Price for By-products

● Raffinate Same as naphtha

● Hydrogen $105/MT (fuel)● Fuel gas $35/MT● Isomar liquid 0.5 � benzene value● Naphthalenes $100/MT● A10� $35/MT







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