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CHAPTER 3.2DEEP CATALYTIC CRACKING,

THE NEW LIGHT OLEFIN GENERATOR

Warren S. LetzschDCC Program Manager

Stone & Webster Inc.Houston, Texas

BASIS

The fluid catalytic cracking (FCC) unit is the most important and widely used heavy oilconversion process in the modern refinery. Historically, the FCC unit has operated in max-imum gasoline and maximum distillate modes, depending on seasonal product demandsand refinery locale. Recently, with the advent of reformulated gasoline requirements, theFCC unit has been increasingly required to operate in the maximum olefin mode. Lightisoolefins, isobutylene and isoamylene, from the FCC unit are necessary feedstocks formethyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) oxygenatedreformulated gasoline blending components. Increased alkylate demand to meet reformu-lated gasoline requirements also necessitates an increase in light olefins.

At the same time as these changes are occurring in the refining industry, the petro-chemical industry is experiencing increased demands for propylene for the manufacture ofpolypropylene products. Nearly one-half of the propylene used by the chemical industry isobtained from refineries, and the remainder comes from steam cracking (SC).1 As a result,the demand for propylene from both FCC units and SC units is rising. Since SC units pro-duce ethylene as the primary product, a catalytic process is more suitable for makingpropylenes and butylenes.

The demand for propylene, both as an alkylation feed and for polypropylene produc-tion, is expected to continue growing well into the 21st century. More isoolefins are alsoneeded for those locations where MTBE and TAME can be used in the gasoline pool. Thisplaces a considerable strain on the FCC unit and SC unit in order to meet the demand.Obviously, a need for an economical light olefin generating process is required to meetthese demands for light olefins (C3 through C5).

To this end, Stone & Webster has entered into an agreement with the Research Instituteof Petroleum Processing (RIPP) and Sinopec International, both located in the People’sRepublic of China, to exclusively license RIPP’s Deep Catalytic Cracking (DCC) technol-ogy outside China. DCC is a fully commercialized process, similar to FCC, for producing

3.35

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light olefins (C3 to C5) from heavy feedstocks such as gas oils and paraffinic residuals.Stone & Webster’s proven position in FCC technology and steam cracking is a naturalcomplement to DCC technology.

Numerous DCC units have been put in commercial service. Table 3.2.1 is a list of allDCC units operating at present. Figure 3.2.1 shows the unit built in Thailand currentlyoperating at about 18,000 B/D and producing about 150,000 MTA of propylene.

PROCESS DESCRIPTION

DCC is a fluidized catalytic process for selectively cracking a variety of feedstocks to lightolefins. A traditional reactor/regenerator unit design is employed with a catalyst havingphysical properties much like those of FCC catalyst. The DCC unit may be operated in oneof two modes: maximum propylene (type I) and maximum isoolefins (type II). Each oper-ational mode employs a unique catalyst and operating conditions. DCC reaction productsare light olefins, high-octane gasoline, light cycle oil, dry gas, and coke. A small amountof slurry oil may also be produced.

DCC maximum propylene operation (type I) employs both riser and bed cracking atsevere reactor conditions. Maximum isoolefin operation (type II) utilizes riser cracking, asdoes a modern FCC unit, at slightly milder conditions than a type I operation. Figure 3.2.2,a process flow diagram of a type I DCC process, serves as a basis for the process descrip-tion. (Note that the only difference between the type I and type II designs is an extendedriser with a riser termination device above the reactor bed level.)

Fresh feed is finely atomized by steam and injected into the riser through Stone &Webster proprietary FCC feed injection nozzles over a dense phase of catalyst. The atom-ized oil intimately mixes with the catalyst and begins to crack into lighter, more valuableproducts. A good feed injection system is required for DCC, just as for FCC operations, toensure rapid oil vaporization and selective catalytic cracking reactions.

Riser steam is injected just above the feed injection point to supplement feed disper-sion and stripping steam in order to achieve optimal hydrocarbon partial pressure for theDCC operation. Simple steam injection nozzles are employed for riser steam injection.(Steam requirements for DCC type II operation are considerably less and may not needadditional steam injection nozzles.)

3.36 CATALYTIC CRACKING

TABLE 3.2.1 DCC Commencement Status

Feed,Location MTA* Start-up DCC type

Jinan, China 60,000 1990 IJinan expansion 150,000 1994 I and IIAnqing, China 400,000 1995 IDaqing, China 120,000 1995 IJinmen, China 800,000 1997 IITPI, Thailand† 900,000 1997 IShenyang, China 400,000 1998 IIJinzhou, China 300,000 1999 IUrumchi, China 800,000 1999 II

*MTA � metric tons per year.†Design by Stone & Webster Engineering Corporation.

DEEP CATALYTIC CRACKING, THE NEW LIGHT OLEFIN GENERATOR



DEEP CATALYTIC CRACKING, THE NEW LIGHT OLEFIN GENERATOR 3.37

Slurry recycle is injected, if required, just above the riser steam nozzles. This recyclestream is not required to increase overall conversion but rather to optimize the unit heatbalance, as a large slurry reaction product is coke.

At the top of the riser, catalyst, steam, and hydrocarbon pass through a riser terminatorlocated below the reactor bed. Conversion of the DCC feedstock can be regulated byadjusting the catalyst bed height (hydrocarbon weight hourly space velocity) above the ris-er distributor, the catalyst circulation rate, and/or the reactor temperature. Two-stage high-efficiency reactor cyclones remove entrained catalyst from the reactor vapors. Products,inerts, steam, and a small amount of catalyst flow from the reactor into the bottom of themain fractionator to begin product separation.

The regenerated catalyst slide valve controls the reactor bed temperature by regulatingthe amount of hot regenerated catalyst entering the riser. Nominal reactor temperatures andpressures are listed in Table 3.2.2.

FIGURE 3.2.1 DCC unit at TPI refinery, Thailand.




The stripper portion of the reactor vessel uses baffles to create multiple stages. Steamfrom the main steam ring fluidizes the catalyst bed, displaces the entrained hydrocarbons,and strips the adsorbed hydrocarbons from the catalyst before it enters the regenerationsystem. A steam fluffing ring, located in the bottom head of the stripper, keeps the catalystproperly fluidized and ensures smooth catalyst flow into the spent catalyst standpipe. Analternative to the baffled stripper is the use of packing to create the staging.

Spent catalyst leaves the stripper through a slanted standpipe. Aeration taps, locatedstepwise down the standpipe, serve to keep the catalyst aerated and replace the gas volume


FIGURE 3.2.2 Maximum propylene DCC unit (type I) process flow diagram.




lost by compression. The spent catalyst slide valve, located near the point where the stand-pipe enters the regenerator, maintains proper bed level in the reactor/stripper. Reactor bedlevel is optimized with respect to conversion and unit operability.

Spent catalyst is dispersed inside the regenerator by a catalyst distributor just above thecombustion air rings. Combustion air rings provide even air distribution across the regen-erator bed, resulting in proper fluidization and combustion. The regenerator operates in afull combustion mode with approximately 2 vol % excess oxygen. Regenerator flue gases





exit through two-stage high-efficiency regenerator cyclones which remove entrained cata-lyst from the flue gas. Typical regenerator temperature is near 700°C. Regeneration/reac-tor differential pressure is controlled by a flue gas slide valve.

Hot regenerated catalyst is withdrawn from the regenerator, just below the regeneratorbed level, into a catalyst withdrawal well. The withdrawal well allows the catalyst to deaer-ate properly to standpipe density before entering the vertical regenerated catalyst stand-pipe. A small air ring located in the withdrawal well serves to maintain proper catalystfluidization. Aeration taps, located stepwise down the standpipe, replace gas volume lostby compression. Catalyst passes through the regenerated catalyst slide valve, which con-trols the reactor temperature by regulating the amount of hot catalyst entering theriser/reactor section. A straight vertical section below the feed nozzles stabilizes the cata-lyst flow and serves as a reverse seal, preventing oil reversals into the regenerator.

The DCC gas recovery section employs a low-pressure-drop main fractionator designwith warm reflux overhead condensers to condense the large amounts of steam used in theconverter. A large wet gas compressor is required, relative to FCC operation, because ofthe high amounts of dry gas and liquefied petroleum gas (LPG). The absorber and strippercolumns, downstream of the wet gas compressor, are specifically designed for enhancedC3 recovery at relatively low gasoline rates. Following the traditional debutanizer anddepropanizer for contaminant removal, a deethanizer and C3 splitter are required to pro-duce polymer-grade propylene. For DCC units in or near a petrochemical process, a cryo-genic ethylene recovery unit utilizing Stone & Webster’s Advanced Recovery System(ARS) technology may be of interest for ethylene recovery and essentially completepropylene recovery. For a grassroots petrochemical plant, the gas recovery system can beoptimized using Stone & Webster’s maximum olefin recovery (MOR) technology, savingconsiderable investment capital.

The flue gas handling system, downstream of the DCC regenerator, requires consider-ations no different from those of an FCC system. It consists of a flue gas slide valve to con-trol the differential pressure between the reactor and regenerator followed by an orificechamber. Heat is recovered by a flue gas cooler in the form of high-pressure superheatedsteam. Depending on local particulate emission specifications, the system may contain athird-stage cyclone separator upstream of the flue gas slide valve or an electrostatic pre-cipitator (ESP) upstream of the stack. SOx or NOx emission requirements may necessitatea flue gas scrubber or SOx-capturing catalyst additive to reduce SOx emissions and/or aselective catalytic reduction (SCR) process for NOx removal.

CATALYST

The most critical part of the DCC process is the catalyst. RIPP’s research and developmentefforts have resulted in the development of several proprietary catalysts, each with uniquezeolites. All catalysts have physical properties similar to those of FCC catalysts.

The catalyst designated CRP-1 was developed for use in the DCC maximum propyleneoperation (type I). CRP has a relatively low activity to ensure high olefin selectivity andlow hydrogen-transfer reactions. The catalyst also exhibits a high degree of hydrothermalstability and low coke selectivity.

CS-1 and CZ-1 were developed to produce high isobutylene and isoamylene selectivi-ty as well as propylene selectivity. Again, these catalysts are low hydrogen-transfer cata-lysts with good hydrothermal and coke-selective properties.

All three types of catalyst are currently manufactured by Qilu PetrochemicalCompany’s catalyst facility in China. Stone & Webster has qualified suppliers outside ofChina.





FEEDSTOCKS

The DCC process is applicable to various heavy feedstocks for propylene and isoolefinproduction. Feedstocks include wax, naphtha, thermally cracked gas oils, vacuum gas oils,hydrotreated feeds, and residual oils. Paraffinic feedstocks are preferred; however, suc-cessful pilot-plant trials have also been performed with naphthenic and aromatic feeds,although the olefin yields are significantly lower due to their lower hydrogen contents.

OPERATING CONDITIONS

A range of typical operating conditions for both type I (maximum propylene) and type II(maximum isoolefins) are shown in Table 3.2.2. Also indicated are typical FCC and SCoperating conditions for comparison. A more severe reactor temperature is required for theDCC process than for FCC. Type II DCC reactor temperature is less severe than type 1, toincrease isoolefin selectivity, but still more than FCC. Steam usage for DCC operations ishigher than for FCC, but considerably less than for SC. DCC catalyst circulation rates arehigher than FCC operations, while regenerator temperatures are similar or lower.

DCC PRODUCT YIELDS

DCC Maximum Propylene (Type I)

A typical DCC maximum propylene yield slate for a Daqing (paraffinic) VGO is shown inTable 3.2.3. For comparison purposes, FCC and SC maximum olefin yields for the samefeedstock are also shown in Table 3.2.3.

Propylene is abundant in the DCC LPG stream and considerably higher than that forFCC. DCC LPG also contains a large amount of butylenes where the isobutylene fractionof the total butylenes is higher than that for FCC (38 to 42 wt % versus 17 to 33 wt %).2

Subsequent MTBE production is enhanced over FCC operations because of the addition-al available isobutylene. These high olefin yields are achieved by selectively overcrackingnaphtha.

Large amounts of dry gas are produced by the DCC type I process because of the severereactor temperature. DCC dry gas is rich in ethylene, which can be recovered for petro-


TABLE 3.2.2 DCC, FCC, and SC Operating Conditions

DCC type I DCC type IImax, C3 max., isoolefins FCC SC

Temperatures:Reactor, °C 550–565 525–550 510–550 760–870Regenerator, °C 670–700 670–700 670–730 —

Reactor pressure, kg/cm2 gage 0.7–1.0 1.0–1.4 1.4–2.1 1.0Reaction times, s * 2 (riser) 2 (riser) 0.1–0.2Catalyst/oil, wt/wt 9–15 7–11 5–8 —Steam injection, wt % feed 20–30 10–15 2–7 30–80

*Riser residence time approximately 2 s plus 2–20 weight hourly space velocity (WHSV) in reactor bed.




chemical sales. Nonetheless, the DCC operation produces considerably less dry gas andmore LPG than steam cracking does. The primary DCC product is propylene, whereas eth-ylene is the major SC component. (Steam cracking is a thermal reaction whereas DCC ispredominantly catalytic.)

Because of high conversion, the DCC C5� liquid products are all highly aromatic.Consequently octane values of the DCC naphtha are very high. For this yield slate, an 84.7motor octane number, clear (MONC) and 99.3 research octane number, clear (RONC)were measured.3 DCC C5� naphtha has greater than 25 wt % benzene, toluene, and xylene(BTX) content and is a good BTX extraction candidate. Because of high diolefin content,selective hydrotreating is usually required. Selective hydrotreating can be achieved with-out losing octane. The coke make is somewhat higher than that in FCC operation. Thehigher heat of reaction required for the conversion of the feed to DCC products and thehigh reactor temperature add to the coke yield.

The sensitivity of olefin yield for three VGO types is shown in Table 3.2.4. DaqingVGO is highly paraffinic. Arabian light is moderately aromatic, while Iranian is highlyaromatic. Propylene and butylene yields are very high for paraffinic feedstocks anddecrease for the most aromatic feeds. The data were generated in RIPP’s 2 barrel per day(BPD) DCC pilot unit but have been commercially verified.

DCC Maximum Isoolefin (Type II)

DCC type II yields are shown in Table 3.2.5. Large olefin yields are produced by over-cracking naphtha at less severe conditions than for type I. The high olefin selectivity isindicative of very low hydrogen transfer rates. Butylene and amylene isomer breakdownsare shown in Table 3.2.6. Note that the isoolefins in the DCC type II operation approachtheir respective thermodynamic equilibrium. As a result, isobutylene and isoamyleneyields are very large, each over 6.0 wt % of feed.


TABLE 3.2.3 Yields for DCC Type I versus FCC and SteamCracking

Wt % of feed

Component DCC (type I) FCC SC

H2 0.3 0.1 0.6Dry gas (C1-C2) 12.6 3.8 44.0LPG (C3-C4) 42.3 27.5 25.7Naphtha (C5-205°C) 20.2 47.9 19.3Light cycle oil (205–330°C) 7.9 8.7 4.7Slurry oil (330°C�) 7.3 5.9 5.7Coke 9.4 6.1 —Light olefins:

C2 5.7 0.9 28.2C3 20.4 8.2 15.0C4 15.7 13.1 4.1

Source: Lark Chapin and Warren Letzsch, “Deep Catalytic Cracking,Maximum Olefin Production,” NPRA Annual Meeting, AM-94-43, Mar.20–22, 1994.




DCC INTEGRATION

It is possible to incorporate a DCC process in either a petrochemical or a refining facility.Idled FCC units in operating facilities are particularly attractive for DCC implementation.A few possible processing scenarios are discussed.

One possible scenario is utilization of a DCC unit to increase propylene production inan ethylene facility. DCC naphtha, ethane, propane, and butane could be sent to the SC unitfor additional ethylene yield. It may be possible to debottleneck the existing product split-ter to accommodate the DCC gaseous stream. A petrochemical facility can be designed totake whole crude oil as the feed where the naphtha goes to a steam cracker and the heav-ier components go to a DCC unit.


TABLE 3.2.4 DCC Type 1 Olefin Yields for Various VGO Feedstocks

Daqing Arabian Light* Iranian*

Specific gravity 0.84 0.88 0.91UOP K factor 12.4 11.9 11.7Olefin yield, wt % feed:

C2 6.1 4.3 3.5C3 21.1 16.7 13.6C4 14.3 12.7 10.1

*Hydrotreated vacuum gas oil.

TABLE 3.2.5 DCC Maximum IsoolefinYields (Type II)

Component Yield, wt % of feed

C2� 5.59C3-C4 34.49C5� naphtha 39.00Light cycle oil 9.77Heavy cycle oil 5.84Coke 4.31Loss 1.00Light olefins:

C2 2.26C3 14.29C4 14.65i-C4 6.13C5 9.77i-C5 6.77

Source: Z. T. Li, W. Y. Shi, N. Pan, and F. K.Jaing, “DCC Flexibility for Isoolefins Production,”Advances in Fluid Catalytic Cracking, ACS, vol.38, no. 3, pp. 581–583.




A DCC unit could be incorporated into a refining facility for polypropylene and styreneproduction. An example of such a processing scheme is shown in Fig. 3.2.3.

Another example of DCC integration is for supporting reformulated gasoline produc-tion, as shown in Fig. 3.2.4. An ethylene recovery unit using Stone & Webster’s ARS tech-nology could be incorporated into this scheme for polymer ethylene and propylene sales.

REFERENCES

1. Lark Chapin and Warren Letzsch, “Deep Catalytic Cracking Maximize Olefin Production,” NPRAAnnual Meeting, AM-94-43, Mar. 20–22, 1994.

2. C. Xie, W. Shi, F. Jiang, Z. Li, Y. Fan, Q. Tang, and R. Li, “Research and Development of DeepCatalytic Cracking (DCC Type II) for Isobutylene and Isomylene Production,” PetroleumProcessing and Petrochemicals, no. 5, 1995.

3. L. Zaiting, J. Fakang, and M. Enze, “DCC—A New Propylene Production Process from VacuumGas Oil,” NPRA Annual Meeting, AM-90-40, Mar. 25–27, 1990.

4. Lark Chapin, W. S. Letzsch, and T. E. Swaty, “Petrochemical Options from Deep CatalyticCracking and the FCCU,” NPRA Annual Meeting, AM 98-44.

5. Wang Yamin, Li Caiying, Chen Zubi, and Zhong Xiaoxiang, “Recent Advances of FCCTechnology and Catalyst in RIPP,” Proceedings of 6th Annual Workshop on Catalysts in PetroleumRefining and Petrochemicals, December 1996. KFUPM, Dhahran, Saudi Arabia.

6. Andrew Fu, D. Hunt, J. A. Bonilla, and A. Batachari, “Deep Catalytic Cracking Plant ProducesPropylene in Thailand,” Oil & Gas Journal, Jan. 12, 1998.

7. Zaiting Li, Jiang Fukang, Xie Chaogang, and Xu Youhao, “DCC Technology and ItsCommercial Experience,” China Petroleum Processing and Petrochemical Technology, no. 4,December 2000.


TABLE 3.2.6 Olefin Isomer Distribution DCC Type II Operation

Component, wt % Equilibrium value DCC max. isoolefin

Butylene isomers:1-butene 14.7 12.8t-2-butene 24.5 26.7c-2-butene 16.7 18.6Isobutylene 44.1 41.9

Amylene isomers:1-pentene 5.2 5.2t-2-pentene 12.2 17.6c-2-pentene 12.0 7.9Isoamylene 70.6 69.3

Source: Z. T. Li, W. Y. Shi, N. Pan, and F. K. Jaing, “DCC Flexibility forIsoolefins Production,” Advances in Fluid Catalytic Cracking, ACS, vol. 38, no.3, pp. 581–583.





FIGURE 3.2.3 Polypropylene and styrene production scheme (EXT = aromatics extraction, HDA =hydrodealkylation, SHP 5 selective hydrogenation).

FIGURE 3.2.4 Reformulated gasoline production scheme.




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