Study on Technology for Quenching Catalytic Pyrolysis Gas

China Petroleum Processing and Petrochemical TechnologyProcess Research2011,Vol. 13, No.1, pp 58-63March 30, 2011Study on Technology for Quenching Catalytic Pyrolysis GasLu Weimin(Research Institute of Petroleum Processing, Bejing 100083)Abstract: This article describes the application of technology for quenching catalytic pyrolysis gas at the Daqingcommercial CPP test unit and the Shenyang commercial CPP production unit. On the basis of results for application of theShenyang CPP unit this paper puts forward an improved process flow scheme for quenching the pyrolysis gas and madecalculations using the process flowsheet software. Case 1 of the process flow scheme, which is designed for full circulationof slury, intends to use the pyrolysis light oil and fresh feed oil as the quenching media with the product slurry oil and freshfeedstock being discharged from the quench cooler bottom and routed directly to the reactor so that the fresh feed oil can bepreheated prior to pyrolysis. Case II of the process flow scheme intends to adopt recycle oil as the quenching medium withthe product slury and recycle oil being discharged fom the quench cooler bottom to the fractionator, which then deliversthe slurry from the bottom. These two cases for improving the process flow diagram can all efectively control the densityand viscosity of the quenching medium to secure the smooth operation of quench cooler.Key words: catalytic pyrolysis process; quenching medium; CPP gas1 Introductioncalculations performed by means of the ASPEN processflowsheet simulation software in an attempt to draw outThe catalytic pyrolysis process (CPP) is a patented tech-measures for process improvements.nology developed by the Research Institute of PetroleumProcessing for manufacturing ethylene and propylene.2 Technology for Quenching PyrolysisGas Adopted at Steam Cracking UnitThis process developed on the basis of traditional fluidcatalytic cracking technology intends to produce ethylene' The technology for quenching pyrolysis gas is extensivelyind propylene by using heavy oil as the feedstock in theadopted at the steam cracking unit. Since the reactionpresence of novel designated catalyst.temperature of the pyrolysis fumace at the steam crackingThe catalytic pyrolysis process adopts a reaction tem- unit is as high as 800- _900 C, the pyrolysis gas exitingperature in the range of 600- 640 C, which is higherfrom the fumace would continue its pyrolysis reaction atthan that adoped by the FCC process. In order to avoid high temperature. The longer the residence time is, the .coking resulted from extended residence of pyrolysis gasseverer the secondary reaction would be, resulting in anduring the reaction, an oil/gas quench cooler is used toincreased loss of target products. Thereby it is required toprovide direct contact of cooling medium with the py-quench the high-temperature pyrolysis gas at the furmacerolyis gas discharged from the disengager, so that the py outlet as quickly as pssible to terminate the crackingrolysis gas is cooled to below 400 C prior to entering thereaction. There are two kinds of quenching methods, i. e.:fractionator through the oilgas manifold. This measure direet cooling or indirct colig".is one of the key techniques that can guarantee the long-The direct cooling method makes the cooling medium incycle operation of the CPP unit.direct contact with the pyrolysis gas to absorb the heat byThis article intends t0 introduce the aplication of the the coline mediup in order to auickly lower the tem.technology for quenching the pyrolysis gas at the Daq-中国煤化工ing commercial CPP test unit and at the Shenyang com- CorrreYHCN MHG Tlephone +8610-mercial CPP production unit, with the technological 82368724; c-mal: uwemr.ppsinopec.com.58●Lu Weimin. Study on Technology for Quenching Catalytic Pyrolysis Gasperature of pyrolysis gas. The direct cooling method uses high heat recovery rate and capability of generatingwater or oil as direct quenching medium.high-pressure steam. However, the heavy componentsThe direct water cooling method sprays misty water into contained in the pyrolysis gas may condense in the wastethe quench cooler, in which water upon direct contact heat boiler and may be subjected to coking, this indirectwith the pyrolysis gas is preheated and vaporized to ab-quenching method is feasible provided that the tempera-sorb the heat released by the pyrolysis gas to quickly re- ture of pyrolysis gas exiting from the waste heat boiler isduce the temperature of pyrolysis gas. Since direct water higher than the dew point of pyrolysis gas. If the tempera-cooling needs a large amount of water, water recycling must ture of pyrolysis gas exiting the waste heat boiler is equalbe adopted in order to avoid formation of colossal oily wa- to or lower than the dew point of pyrolysis gas, a largeter, which could form oil- water emulsions, making oil/water amount of liquid would condense on the tube walls ofseparation a tough task that requires improved eficiency for quench heat-exchanger and could be further subjected toseparating oil from water. The direct water cooling method coking to block heat-exchanger tubes to afect the normalcannot recover the high heat energy of pyrolysis gas along operation of the quench heat- exchangers.with the formation of a large amount of oily water.The direct oil cooling method sprays the quenching oil3 Technology for Quenching PyrolysisGas Adopted by CPP Unitinto quench cooler, in which the oil upon direct contactwith the pyrolysis gas is preheated and vaporized or The catalytic pyrolysis process uses atmospheric residueis subjected to partial cracking reaction to absorb the (AR) as the feedstock, and the CPP gas contains a smallheat released by the pyrolysis gas to quickly reduce the amount of unconverted residuum, the pyrolysis product -temperature of pyrolysis gas. Since quenching oil in the slurry, and catalyst fines. The residuum and slurry are li-course of recycling is prone to polymerization, viscosity able to condensation to form coke, while catalyst fines areincrease, and coking in the quench cooler, it is necessary prone to sedimentation to foster the formation of coke.to maintain a stable viscosity of quenching oil. The diret Therefore the CPP gas cannot be cooled down by theoil cooling method can recover the heat of pyrolysis gas indirect method. Direct cooling of the pyrolysis gas byby the recycle quenching oil to generate low-pressure and water is not recommended, because it may lead to emul-medium-pressure steam.sion formation that can cause difficulty for separatingThe direct cooling method features high cooling eficien- water from oil, resulting in formation of large amount ofcy and simple process flow diagram, but it cannot recoveroily water. Hence at the initial stage of CPP developmenthigh heat energy of pyrolysis gas with low economic it was decided to adopt direct oil cooling of CPP gas.benefits. This method is only used to treat the coking sus- The Daqing commercial CPP test unit is provided witha py-ceptible feedstocks, including crude oil, VGO and other rolysis gas quench cooler, which uses whole VR and recycleheavy hydrocarbon feedstocks.slury as the quenching medium that is in direct contact withThe indirect cooling method adopts waste heat boil-pyrolysis gas exiting from the disengager for heat exchangeers or quench boilers to cool through the boiler wall the to reduce the temperature of pyrolysis gas and also washhigh-temperature pyrolysis gas, while generating high- away catalyst fines entained by pyrolysis gas. Over morepressure steam in tandem with recovering the heat from than 2 months during the CPP commercial tests the quenchthe high-temperature pyrolysis gas. Generally, the high- cooler operated normally. Upon inspection after shut downtemperature pyrolysis gas passes through inside the tubes of the CPP unit it was realized that no coking was detectedof quench heat-exchanger, and the high-pressure water at the disengager overhead, in cyclones, quench cooler andpasses outside the tube walls. The water outside the oil/gas manifold and at the fractionator bottom.heat-exchanger tube walls generate high-pressure steam The Shenvang commercial CPP production unit is alsothrough absorbing the heat of gas to quickly coo down provid中国煤化工coler, which usesthe temperature of pyrolysis gas passing inside the heat fresh fYHCNMHGoilasthequench-exchanger tubes. The indirect quenching method features ing medium that is injected into the top of quench cooler59●China Petroleum Processing and Petrochemical Technology2011,13(1):58-63to be in direct contact with pyrolysis gas exiting from the Table 1 that the pyrolysis light oil and recycle oil containdisengager for heat exchange to reduce the temperature of less fractions boiling over 350 C and they are suitable topyrolysis gas, also to wash away catalyst fines entrained be used as the quenching media for CPP gas based on thein pyrolysis gas, and to raise the temperature of AR feed.operating practices of the steam cracking unit.Till now the Shenyang commercial CPP production unitTable 1 Main properties of quenching mediahas been operating for more than one year.PyrolysisIt was disclosed during the operation of the ShenyangItemsAR| Slurry Recycle oillight oil .commercial CPP production unit that upon using AR as Density (at 20 C),g/cm’0.89591.1521 1.0600.9883the quenching medium the light fraction of AR was va-Viscosity (at 80 C), mm's| 37.02| 141.7 | 4.29 3.98 (20 C)porized and routed to the fractionator along with oil vVa- Viscosity (at 100 C), mm'/s I 20.92 45.98 2.77 72.03 (50 C) .pors of reactants and after separation during fractionation ASTM dilltioa，twas further mixed with the pyrolysis light oil, leading toIB27249171a reduced density of pyrolysis light oil. Since the light59| 354| 369336195fraction of AR directly made up a part of the product after10%377| 381345204vaporization without taking part in the pyrolysis reaction,30%441| 41153235the utilization rate of fresh feed oil was decreased. How-50%| 515| 452 .358259ever, if only slurry oil was used as the quenching medium70%65286without routing fresh feed (AR) to the quench cooler,90%77329the density and viscosity of slurry would increase, which95%384343could, jeopardize the smooth operation of quench cooler.EBP3494 Quenching Medium and Quench FlowBased on the actual conditions of the Shenyang commer-Diagramcial CPP production unit two cases of fow diagrams forThe steam cracking unit generally applies a fraction ofquenching the CPP gas are proposed. Case I of processpyrolysis products that is heavier than gasoline as the flow diagram for quenching the CPP gas is shown in Figquenching medium with its viscosity equating to less than ure I, and Case II of process flow diagram for quenching450 mm2/s (at 50 "C)P!.the CPP gas is shown in Figure 2.The heavier the fraction is, the higher the viscosity would Case I of flow diagram for quenching the CPP gas, whichbe!". The fraction of quenching medium biling over 350 Cuses un-preheated fresh feed and cooled pyrolysis light oilhas a higher viscosity, whereas the fraction boiling be-manufactured by the CPP unit itself as the quenching me-low 350 C has a lower viscosty. The viscosity of quenching dia, is shown in Figure 1. The fresh feed oil and pyrolysismedium is mainly dependent upon the content of the fraction light oil enter the upper part of quench cooler and contactboiling over 350 C, therefore it is very important to maintain countercurrently with the rising upward gas that enters thea proper ratio of fraction boiling below 350 C in the com-quench cooler from the bottom, and the quenching me-position of the quenching medium, which is the key factor dium can take away a part of heat of the gas and washedfor contolling the viscosity of quenching medium. Thereby away a small amount of catalyst fines entrained in the gas,it is necessary to adopt a lower temperature in the gasoline resulting in the reduction of gas temperature. The pyroly-fractionator kettle at the steam cracking unit to guarantee sis light oil in the quenching medium is vaporized and isan appropriate viscosity of the kettle bottoms used as the routed with pyrolysis gas through the oil/gas manifold toquenching medium.the bottom of fractionator, whereas the fresh feed afterThe fractions of CPP products having density that is being preheated through heat exchange with gas coupledheavier than gasoline include: fresh feed (AR), slurry, with t中国煤化Iyvrolysis gas is dis-recycle oil, and pyrolysis light oil, with their properties charge:MHCNMHGI cooler and routedpresented in Table I. It can be seen from the data listed in to the nountcavwn.60●Lu Weimin. Study on Technology for Quenching Catalytic Pyrolysis Gas+ Pyrolysis gas toping is discharged from the stripper bottom and is deliveredwater cooler. Reflux of coldafter cooling from the unit as the pyrolysis light oil product.pyrolysis naphthaThe middle of the fractionator is provided with pumpar-feesh-ound for heat removal, and the upper part of fractionatorPyrolysis ligis equipped with an overhead reflux for heat removal,oil stripperwhile the fractionator overhead is provided with coldSteamPyrolysis lightnaphtha refuxing. The pyrolysis gas vented from the frac-Pyrolysis. oil efluenttionator overhead is sent to the water cooling tower for心Pyrolysis heavy .further temperature reduction and separation.Fresh fee and slurry to reactoroi1 efluentThe advantage of this process flow diagram is the once-dthrough passing of the product slurry in the quench coolerFigureI Case Iofprocess flow diagram forto avoid involvement in cooling of pyrolysis gas, withoutquenching CPP gasparticipating in circulation and without being in contactwith high-temperature pyrolysis gas, so that there would. Refux of coldbe no changes in density and viscosity of slurry. Duringthe quenching operation the recycle oil amount entering?|the riser reactor can be regulated by changing the amountof pyrolysis light oil entering the quench cooler, changingPyrolysis light oilthe temperature at the quench cooler overhead, or chang-ing the liquid amount at the bottom of quench cooler.。 SteamScamesk. Prolysis lightLikewise, the density and viscosity of pyrolysis light oil"oil effluentPyrolysis gas* Slurrycan be regulated through changing the amount of pyroly-efluentsis heavy oil discharged from the bottom of fractionator.The product slurry produced by this process flow diagramFigure2 Case I1 of process Jlow diagram forcan be completely mixed with fresh feed, and is wellsuited to the working conditions for full recycle of slurry.Pyrolysis gas after entering fractionator is countercur- Since the pyrolysis light oil is susceptible to vaporizationrently in contact with the cooled down recycle pyroly-in the course of pyrolysis gas quenching and is routedsis heavy oil at the lower part of fractionator to further along with pyrolysis gas to the fractionator via the oil/wash away catalyst fines entrained by the pyrolysis gas. gas manifold, the pressure drop in the manifold should beThe pyrolysis heavy oil after being discharged from the taken into consideration.fractionator bottom is used to generate steam, and after Case II of process flow diagram for quenching CPP gas,temperature reduction it is then retumed through the up-which uses the self- made at the process unit recycle oilper and lower goverming valves respectively to the top of as the quenching medium, is presented in Figure 2. Thechevron trays and the fractionator bottom, from which the recycle oil is extracted from the fractionator bottom, andpyrolysis heavy oil product is discharged and after cool- after heat exchange for steam generation and cooling foring is delivered as fuel oil from the process unit.temperature reduction is returned to the upper part ofThe pyrolysis light oil after being extracted from thequench cooler. Recycle oil inside the quench cooler is inmiddle of fractionator is divided into two streams, one of contact countercurrently with the pyrolysis gas, which en-which is injected after cooling into the quench cooler as ters the quench cooler from the lower part of quench coolerthe quenching medium. Another stream is routed to the and moves upward.and a Dart of beat contained in pyroly-upper part of the pyrolysis light oil stipper and is in contact sis gas中国煤化工nt of calyst finescounter currently with superheated steam fed to the bottomentrain!YHC N M H G. The pyrolysis gasof stripper for stripping. The pyrolysis light oil after strip- after being cooled leaves the top of quench cooler and en-61●China Petroleum Processing and Petrochemical Technology2011,13():58-63ters the lower part of fractionator via the oilgas manifold, calculations has two theoretical plates, while the fractionatorwhereas the recycle oil after temperature rise through heat has 15 theoretical plates. The temperature of pyrolysis gasexchange leaves the bottom of quench cooler and then en- at the quench cooler outlet can be changed through varyingters the lower part of fractionator by gravity flow.the amount of quenching medium, while the temperatureThe pyrolysis gas upon entering fractionator is in contact at fractionator overhead can be changed via varying thecountercurrently with the circulating slury at the lower amount of overhead reflux and the amount of cold reflux.part of fractionator to further wash away catalyst fines en-The simulated calculation results for Case I of processtrained by the pyrolysis gas. The slurry discharged from flow diagram for quenching CPP gas are presented inthe fractionator bottom is used to generate steam and after Table 3. The quenching medium一pyrolysis light oil- istemperature reduction via heat exchange with fresh feed fully vaporized in the quench cooler to drastically afectis divided into two streams, one of which is then returmed the temperature reduction of pyrolysis gas, while the Va-through the upper and lower goveming valves respective-porized pyrolysis light oil is routed along with pyrolysisly to the top of chevron trays and the fractionator bottom, gas through the oil/gas manifold to the fractionator. Thewith the another stream being sent for further cooling fresh feed after being preheated is discharged with the con-prior to being delivered outside the process unit.densed product slurry from the bottom of quench cooler.The pyrolysis light oil leaves the middle of fractionator The simulated calculation results for Case II of processand flows by gravity into the upper part of pyrolysis lightflow diagram for quenching CPP gas are presented in Ta-oil stripper and is countercurrently in contact with super- ble 4. The quenching medium - recycle oil - is partiallyheated steam routed into the bottom of fractionator. The py- vaporized in quench cooler, and the vaporized recyclerolysis light oil after stripping is extracted from the stripper oil is routed along with pyrolysis gas through the oilgasbottom and is delivered from the process unit after cooling.manifold to the fractionator. A part of recycle oil and theThe middle of the fractionator is provided with pumpar- condensed product slurry are discharged from the bottomound for heat removal and the upper part of fractionator of quench cooler and routed to the fractionator.is equipped with an overhead reflux for heat removal, Case I of process flow diagram for quenching CPP gaswhile the top of fractionator is provided with cold naph- would cause an increased amount of oil and gas, result-tha refluxing. The pyrolysis gas leaving the fractionator ing in an increase in pressure drop in the oi/gas manifold,overhead is sent to the water cooling tower for further which should be calculated according to actual conditions.temperature reduction and separation.Case II of process flow diagram for quenching CPP gas hasThe heat removal ratio in fractionator can be cbanged ifrelatively small impact on the operation of oi/gas manifoldnecessary by diverting a part of pyrolysis light oil into theand is suitable for retfiting the existing process unit.recycle oil stream or diverting a part of recycle oil into theTable2 Composition of pyrolysis gas entering theslury stream in order to regulate the viscosity of recycle oil.quench coolerThe advantage of this process flow diagram is to avoidfresh feed and slurry from involvement in quenching ofTemperature, c610pyrolysis gas, coupled with the possibility of effectivelyPressure (g), MPa0.08changing the viscosity of quenching medium through ad-Flow rate, kgh94832justing the operating conditions.Molecular weight31Composition, m%5 Simulated Calculation of Quench CoolerVater24.28Non-hydrocarbons1.54The simulated calculations for the proposed two cases ofH2- C, hydrocarbons49.97pyrolysis gas quench flow diagrams were carried out us-ing the Aspen Plus 12.1 software.中国煤化工6.41The composition of pyrolysis gas entering the quenchCNMHG 1.46cooler is presented in Table 2. The quench cooler adopted in.Tola100.00Lu Weimin. Study on Technology for Quenching Catalytic Pyrolysis GasTable 3 Simulated calculation results for Case ITable 4 Simulated calculation results for Case IIHeat- Pyrolysis gasFreshRecycleexchanged| at quenchRecycle| Oil at quenchPyrolysis gas at| pyrolysisItemsquench coolerfeedlight oilfresh feed+cooleroicooler bottomoverbeadslurryoverheadTemperature, C808390251Temperature, c30127Pressure (g), MPa0.080.070.5Flow rate. kgh| 62500 79000[ 63534173 298Flow rate, kgh200 000 I132 075.162 811Molecular weight467218467023325848Composition, m%Water13.58Non- hydrocarbons0.85Non-hydrocartbons0.91H- C, byrocarbons |27.35H2 - C, hydrocarbons29.14C58.22C,+55.49Total100.00ASTM dslilation, CASTM ditltion, CIBP2771723072012465%3541935610%77203789%2442722638330%4123544250%51525951330070%2862873220%32934995933990%43647495%4915186 Conclusionsdium, with the product slurry and recycle oil dischargedfrom quench cooler bottom to the fractionator, from theThis article relates to the analysis and resuts on the aP- bottom of which the sury is discharged. For both of theplication of technology for quenching pyrolysis gas fromtwo improved cases for quenching the CPP gas the slurycatalytic pyrolysis process at the Daqing commercial CPP is not involved in the quenching of pyrolysis gas in or-test unit and the Shenyang commercial CPP production der to efctively control the density and viscosity of theunit.quenching medium to ensure smooth operation of quenchBased on the actual conditions of Shenyang commercial cooler.CPP production unit, the proper quenching media andtwo improved process flow schemes were proposed andReferencesrelevant calculations were performed by means of the[1] Wang Songhan. Technology of Steam Cracking Units[M].process flow scheme simulation software. Case I adoptsBeijing: China Petrochemical Press, 1994 (in Chinese)pyrolysis light oil and fresh feed oil as the quenching me- [2] Gou Hongxia, Zhu Yongle. Simulated calculations anddia, and the product slurry and fresh feed are dischargedsystematic analysis on primary fractionation system for eth-from the quench cooler bottom and are routed directly toylene unit[J]. Chemical Engineering Design, 2003, 13 (2):the reactor with the fresh fee being preheated. Case I is8-13, 15 (in Chinese). .suitable for the process conditions requiring full recycle [3] Liang Wenjie. Chemistry of Petroleum[M]. Beijing: Chinaof slurry. Case II uses recycle oil as the quenching me-University of Petroleum Press, 1995 (in Chinese).中国煤化工YHCNMHG63●