Commercial Application of the RAR Sulfur Recovery and Tail Gas Treating Process

ReviewsCommercial Application of the RAR SulfurRecovery and tail gas treating ProcessGuo Hong; Zhang Songping(Refinery of Jinling Petrochemical Company, Nanjing 210033)Abstract: The 40kt/a sulfur recovery unit for tail gas treating applying the reduction-absorption-recycling(RAR)technology is aimed at regeneration of the rich amine solution and recovery of sulfur to operate intandem with the 1. 2Mt/a diesel hydrofining unit. The process unit calibration data have revealed that therecovery of total sulfur reaches 99.86%, which is 6.65 percentage points higher than that before applicationof the RAR technology. The so, content in vented tail gas is 0. 27 t/d, which is much less than the latestemission standard prescribed by the State. The factors that can affect the unit operation have been analyzedand corresponding measures have been suggested including the necessity to improve the control over thereaction temperature in the tail gas hydrogenation unit.Key words: sulfur recovery unit; tail gas; commercial scale; Claus sulfur recovery1 Introduction2 Process scheme of the unitThe 40kv/a sulfur recovery unit(SRU) at the refinery of Jinling 2.1 Process scheme of claus unitPetrochemical Company is built to meet the environmentalprotection purpose during processing of imported sour crude. The feedstock to Claus unit is composed of two streams. TheThis unit in tandem with operation of the 1. 2Mt/a diesel first stream is the acid gas from all units at the refinery, thehydrofining unit can regenerate the rich amine solution used second stream is the recycle acid gas from the tail gas treatin the sulfur removal process to provide the lean amine ment section. A part of the recycle acid gas after mixing withsolution, while treating the acid gas from hydrofining pro- refinery acid gas is routed into the main combustor and iscess to recover sulfur. The acid gas generated in the second- sent into the first zone of the thermal reactor with the remain-ary processing units and wastewater stripping unit is also der gas routed into the second zone of the thermal reactorrouted into SRU for further treatingAcid gas combines with the combustion air in the maincombustor, while the air flow entering the main combustor isThis SRU includes two parts consisting of the sulfur recov- adjusted properly to fully oxidize all hydrocarbons and am-ery section and the tail gas treatment section, the basic de- monia contained in the acid gas and to maintain the volumetsign of which is provided by KTI of Italy using the Claus ric ratio of H, S versus SO, in the tail gas exiting the Claus unitpartial combustion process and the RAR (reduction/absorp- at 2: Ition/recycling)tail gas treating technology. The design ca-pacity for sulfur recovery is 120 t/d with a sulfur recovery rate The process gas after exiting the thermal reactor enters theof 99.9%. This unit can normally operate at a range between waste-heat boiler where the heat of process gas is recovered25%--115% of the design capacity. This unit was put on to generate low-pressure steam, which after being superheatedstream in December 2000. The RAR section came officially in the tail gas incinerator is delivered into the low-pressureon stream in May 2001 and was subjected to calibration teste grid. The waste heat boiler consists of twon june 2001中国煤化工 ocess gas exiting theCNMHGChina Petroleum Processing and Petrochemical TechnologyNo 4 December 2003upper part of the first tube pass is about 650C and the gas is obtained in the second-stage sulfur condenser is dischargedused to control the temperature of process gas entering the through the sulfur trap into sulfur pit. The process gas exitfirst-stage Claus reactor. The liquid sulfur condensed at the ing the second Claus reactor enters the third-stage sulfuroutlet of the first tube pass is discharged from the bottom condenser, where the sulfur is condensed and dischargedthrough a sulfur trap into the sulfur pit. The process gas exit- through the sulfur trap into sulfur pit.nd tube pass after being cooled to 290Cthe first-stage sulfur condenser, where sulfur contained in The flow scheme of the Claus unit is shown in Figure 1. Thegas is condensed inside the tube and is then discharged amount of sulfur recovered in this unit accounts for 95% ofthe total sulfur contentThe process gas exiting the first sulfur condenser is mixed 2.2 Flow scheme of the RAr unitwith the hot gas leaving the outlet of the first tube pass ofwaste heat boiler and is heated to 220C prior to entering the The Claus tail gas upon entering the rar section is pre-first Claus rector, where H,S and SO, continuously enter into heated in the gas/gas heat exchanger and is then mergedreaction until an equilibrium is reached. Gas exiting the first with a hydrogen-enriched gas in the hydrogen mixer prior toClaus reactor passes through the gas/gas heat exchanger, being routed into the reactor. The reactor is filled with thewhere the process gas from the second-stage sulfur condenser dedicated CoMoX catalyst, which can convert SO, COS, CSis preheated prior to entering the second Claus reactor. Sulfur and vaporized sulfur into H, S. During startup of the unit orSteamRecycle10Tail gas to RARRefingasLiquid sulfur exiting unitFigure I Flow diagram of 40 kt/a claus SRU at refinery of Jinling Petrochemical Company1- Recycle acid gas scrubber; 2- Knockout drum for refinery acid gas; 3--Recycle acid gas preheaterAir preheater; 5--Main combustor: 6--Thermal re7..+- heat boiler8--lst and 2nd stage sulfur condensers 9--Electric heater12-Gas/gas heat exchanger; 13--Liquid sulfur pit; 14-LiEhS中国煤化二d Claus reactorCNMHGfur condenserReviewsin case of abnormal operation of Claus unit an electric heater column, where most H,S is absorbed by amine to form richan be used to increase the gas temperature at rector inlet. amine solution. The rich amine solution exiting the absorberbottom is sent by the rich amine pump into the regeneratorThe reduction hot gas exiting the hydrogenation reactor after column and the recycle acid gas obtained in the regeneratorhaving been heat-exchanged with the reactor inlet gas in the column is routed back to the Claus unitgas/gas heat exchanger passes through the quench tower,where the gas is cooled to below 40C in order to be sepa- The desulfurized gas leaving the absorber overhead still con-rated from the saturated water content in the gas phase, and tains remnants of hydrogen sulfide and is separated from thethen enters the quench gas knockout drum to remove all sub- carryover substances in the knockout drum prior to beingstances carried over by gas prior to entering the absorber burned in the incinerator. The flue gas exiting the incineratorcolumnis used to preheat the low-pressure saturated steam from thewaste heat boiler to a superheated state, and the flue gasThe gas contacts with the lean amine solution in the absorber after heat exchange is then discharged through the stack intoTail gas to incineratorRich amine solution to regeneratorSour water to stripper13Tail gasDemineralized. dre 2 Flow diagram of the RAR reduction/absorption section1--Gas/gas heat exchanger; 2-Hydrogenation reactor; 3---Hydrogen mixer; 4--Electrical heater: 5--Quench tower:6-Knockout drum; 7-Hydrogen analyzer: 8--pH-meter; 9--Filter: 10-Air cooler; 11-Water cooler; 12--Absorbercolumn; 13--Tail gas knockout drum; 14--Recycle中国煤化16--Rich amine solution pumpCNMHGChina petroleum Processing and Petrochemical TechnologyNo 4 December 2003the atmospherereduction of hydrogen-enriched gas consumption for the SRUtail gas hydrogenation reaction(even without consumptionDuring the start-up of presulfidation phase of the hydroge- of this gas), whereas the RAR process must use the H, -ennation catalyst and shutdown of the RAR unit the nitrogen riched gas in hydrogenation reactioncirculation is maintained by means of the operation of recyclegas blower. The flow scheme of the reduction-absorption 3. 2 Operation of thermal reactor (acid gassection of the RAR unit is presented in Figure 2combustion furnace) with different zonesAmine regeneration in the RAR section is an independent The temperature of thermal reactor should be controlled atsystem, which can treat at the same time the rich amine solu- over 1350C in order to decompose ammonia contained intion from tail gas treatment or the rich amine solution from acid gas. However, the volume fraction of acid gas recovereddiesel HDS unit. The lean solvent obtained after regenera- after hydrogenation of tail gas is equal to around 55%, whichtion of rich amine solution is recycled for use in the system. is not able to raise the temperature of acid gas reaction toThe regeneration process is similar to the conventional one. over 1350C. In order to attain this required temperature, thisoperation mode with separate zones is applied. The ammo-3 Analysis on the Characteristics of the nia-containing acid gas in conjunction with other high-con-RAR Processcentration acid gas streams from the refinery are sent into thefirst zone of thermal reactor and the whole air flow is als3.1 Absence of on-line combustion furnacerouted into the first zone of thermal reactor. a part of the lowconcentration recycle acid gas stream is sent into the firstThe striking feature of the RAR process is the absence of fuel zone of thermal reactor with the remainder routed into thegas online combustion furnace in the reactor inlet, because second zone of thermal reactor. The flow of recycle acid gasthe inlet process gas is preheated by the gas/gas heat ex- sent into the first zone of thermal reactor is adjusted based onchanger using the hot gas stream exiting the reactorthe temperature in the first zone of thermal reactor. If thereaction temperature is not high enough, all recycle acid gasThe advantage of this option is the simplicity and high reli- should be sent into the second zone of thermal reactor. If theability of operation. The operation of online combustion fur- ammonia content in recycle acid gas is too high, the acid ganace requires precise air feed to maintain the stoichiometric scrubber should be used to remove the ammonia containedcombustion of fuel gas, excess or insufficient burning air can in recycle gas. In order to guarantee the reaction temperaturehave negative impact on hydrogenation catalyst, which can in the first zone of thermal reactor, preheating of recycle acidin some critical cases cause overheating or poisoning of gas and the associated air feed should be applied when neccatalyst. The gas/gas heat exchanger is provided with air essary(Refer to Figure 1)feeding and fuel gas flow regulator, and the amount of heatexchange is controlled through adjusting the opening of valve 3.3 Shared amine regeneration systemstalled in the heat exchanger bypass to control the gastemperature at reactor inletThe rich amine solution generated in the rar process can becombined with rich amine solution obtained in other H S reThe limitation for using the gas/gas heat exchanger is as moval processes and should be subjected to centralizedfollows: (1)During startup or in case of abnormal operation regeneration. The amine regeneration system of the 40kw/athe temperature at reactor outlet is lower due to smaller amount SRU at the refinery of Jinling Petrochemical Company needsof heat released from the reactor, and an auxiliary heating to process the rich amine solution obtained in the diesel hDssystem is needed to secure the required temperature at reac- unittreatment section using the same regenera-tor inlet; (2) The online combustion furnace process can uti- tion中国煤化工 advantage of this con-lize the hydrogen resource in fuel gas, which can allow for figurCN MH Quipment and save the4area occupied by the unit in order to solve the problem asso- hydrogenation catalyst. Before presulfidation takes place,ciated with low pressure of feed gas at the inlet of SRU caused hydrogenation reactor and quench system must be purgedby high pressure drop resulted from long-distance transmis- with nitrogen to keep the oxygen content in the system to besion of acid gas. This measure can stabilize the quality of acid less than 10 ppm. The circulation blower is used to set up thegas. However, the size of the regeneration column is quite nitrogen circulation in the hydrogenation reactor-quenchlarge and its operating cost is higher, as a minimum solvent tower-hydrogenation reactor loop, and an auxiliary heatingcirculation rate must be maintained in case of shutdown of a system is used to preheat the recycle gas. When the tempercertain sulfur removal unit. Hence it is necessary to ensure a ture at the reactor inlet reaches above 230C, the sulfidinghigh load of the regeneration unit as far as possible to avoid medium and a small amount of hydrogen are introduced whilewaste of resources. Furthermore, there are a lot of solvent keeping the mole fraction of H, in recycle gas at around 2%0irculation loops in the shared regeneration system, and the The sulfur compound can displace the oxygen contained insolvent circulation rate in every loop should be stabilized. the catalyst in the course of gas circulation. Although theThere are a variety of causes leading to leakage and loss of presulfidation reaction is an exthothermic one, the heat resolvent, the management of which needs more attentionleased is not significant because of the low concentration ofsulfidizing medium used4 Operation of the UnitWhen the acid gas is used as the presulfiding medium, am-4.1 Presulfidation of hydrogenation catalystmonia must be removed from this gas, and water scrubbingmay be applied if necessary. If the SrU tail gas is used for4.1.1 Selection of presulfidation mediumpresulfidation, the mole fraction of H, S in Claus tail gas shouldnot exceed 2% with SO, content in gas kept at less than 300Acid gas or SRU tail gas is used for presulfidation of the ppm, which would require adjustment on air feeding in theAcid gas to Claus unitFlash gas to incineratorRich amine solutionfrom diesel HDSSteamLean amine solution todiesel hDsRich amine solutionCondensatefrom RFigure 3 Flow diagram of solvent regeneration system of 40kt/a srU at the refinery of JinlingPetrochemical company1-Flash drum; 2-Regeneration column; 3- Air cooler for acid7-Condensate tank; 8-Lean/rich amine solution heat exchanger;cooler; I I-Rich amine solution pump; 12-Lean amineYH中国煤化工drum;6— Reboiler;Lean amine solutionCNMH Gux pumpChina petroleum Processing and Petrochemical TechnologyNo 4 December 200Claus unit to meet the demandnation heat released. Thus it is necessary to reduce the ratiobetween H, S and SO,, and even to control their volume ratio4.1.2 Control over presulfidation temperaturewithin a range of 0.5-1.0. Although this operating modecan waste more hydrogen, this measure is necessary for mainAt the initial stage of presulfidation the exothermic reaction taining the required reaction temperature. The hydrogenaof hydrogenation catalyst is rather weak, and the gas tem- tion reactor besides conducting hydrogenation reaction alsoperature at the reactor outlet is lower than the gas inlet carries out hydrolysis of organic sulfur compounds, and atemperature. In this case the recycle gas exiting the reactor temperature of above 310C is the optimal temperature fodoes not need to pass through the heat exchanger at reactor hydrolysis reactionoutlet and should pass along the bypass before entering thequench tower to avoid heat carry-off from incoming gas by It is especially necessary to emphasize that a very low H, S/e exiting cold gas at the reactor outlet. After the gas tem- sO, ratio should be absolutely prohibited. Extremely higlperature at reactor outlet exceeds the reactor inlet temperature, sO, content in tail gas can not only result in over-tempera-he recycle gas exiting the reactor should pass through the ture inside the hydrogenation reactor, and the entry ofgas/gas heat exchanger(as shown in Figure 2). In order to unhydrogenated so, into quench tower can lead to sulfursecure the heat exchange efficiency and minimize the heat blocking of the system. Entry of so, carrying tail gas into theloss, the pipeline and equipment of this system should be solvent circulation system is detrimental to the amine solventprovided with good heat insulation and heat tracing4.3 Selection of valves for the rar tail gasThe impact of circulating airflow on presulfidation tempera- pipelinture cannot be neglected, Avoidance of excess heat carryover from reactor into the quench tower can raise the The SRU tail gas always carries over a certain amount ofpresulfidation temperature. Furthermore, an excessively high sulfur, which is apt to condensate upon insufficient heat trac-nitrogen replenishment rate can also lead to presulfidation ing and can cause malfunction of valves in the tail gas pipelineheat loss. The operation should be carried out to minimize Hence it is better to select regulating valves equipped withrecycle gas leakage so that the nitrogen replenishment rate steam jacket. The sealing ability of tail gas inlet valves mustcan be reduced as far as possible. The design of this unit be good enough, and these valves should operate automatishould envisage the selection of tightly sealed valves, and cally during interlocked trip. In general these pipelines arefugitive emission of recycle gas into incinerator should be not provided with auxiliary lines and shut-off valves, andavoided during operation of process unit.great troubles would occur in case of emergency leaks. Ifleakage takes place in valves on pipeline for transmitting Claus4.2 Requirement for sru tail gas composition tail gas to the incinerator to bar the entry of some amount oftail gas into the RAR treatment unit, the tail gas emissionsClaus reaction demands the volumetric ratio of Hs/so to be from the sru as a whole would not meet the environmentalcontrolled at 2: 1 which is also appropriate for the RAr pro- protection requirementscess at normal operating conditions. In particular during ahigh-load operation of unit the ratio of 2: 1 or even a slightly 4.4 Application of catalysthigher ratio is permissible, which can save the amount ofhydrogen introduced, albeit under sufficient reaction During the first cycle following startup of the SRU unit theimported Claus catalyst and tail gas hydrogenation catalystwere applied, Deactivation of the Claus catalyst caused byIf this demand is followed during a low-load operation, the twice shocks of hydrocarbon carryover in acid gas waseactor bed temperature usually cannot be maintained above dete中国煤化工 talyst was discarded310C because of less total sO, in tail gas and less hydroge- duriCNMHGof imported catalystReviewsbeing replaced with the LS-971 guard catalyst for sulfur pro. The operating indicators of the hydrogenation reactor defi-duction developed by Qilu Petrochemical Research Institutelll. nitely lagged behind the design values mainly owing to theCurrently this guard catalyst operates well. The status of following two causescatalyst fill in different reactors is presented in Table ITable i Status of catalyst fillType1"stage SRU reactor2 stage SRU reactor Tail gas hydrogenation reactorCR-3SLS-97178C924Carrier5 Effect of Exploitation ofTable 2 Main operating regime of process unit duringthe rar Unitcalibration①Value Design valueThe 40kt/a SRU at the refinery of Jinling Pet- Thermal reactorrochemical Company after successful first- Refinery acid gas flow rate a ,m/h2521attempt startup in December 2000 has been Recycle acid gas flow rate@,mh420③running well. The maximum sulfur output af- Air feed@,mh10110ter operation of SRU had reached 145Inlet pressure, kPa3137and the maximum throughput of unit reached Hydrogenation reactoover 4000 Nm /h. The Claus section was sub-Inlet temperature,℃jected to calibration in February 2001 and the Bed temperature,t340-350whole process scheme was subjected to cali- Outlet temperature, Cbration in June 2001Inlet pressure, kPaower5. 1 Main operating regime ofOverhead pressure, kPaprocess unitOverhead temperature, C33Quench water circulation rateThe main operating regime of process unit pH-value of queduring calibration is shown in Table 2. It cannetric fraction of hydrogen at outlet, %:4.01be seen from Table 2 that the total pressureAbsorber columndrop in the Claus section was 8 kPa, which Overhead temperature, crevealed its significant operating flexibilityOverhead pressure, kPaThe load of amine regeneration tower acLean amine entering absorber, th202256counted for 75.31 design target, because Regeneration columnthe sulfur removal section of dieselRich amine entering regenerator,thhydrofining unit did not run at full load. As Overhead temperature, cthe h,S content in rich amine solution was Bottom temperature, Crather low, the regeneration steam consump- Overhead pressure, kPation was equal to 45.09% design consumpSteam consumption, t/h1245tion norm, and the regeneration tower worked Notes: Sampling time: 6 am on June 8, 2000.quite well. The operating indicators of the(2) Referred to as data collected under standard conditionsRaR quench tower and absorber column ba-③ln中国煤化工 ng unit released fromsically complied with design requirements. rich amine solutionCNMHG7China Petroleum Processing and Petrochemical TechnologyNo 4, December 2003(1) The design temperature at hydrogenation reactor inlet is The main cause might be partial plugging in the tail gas inlet312C, while this temperature only reached 301 C during op- pipeline, The inclination angle of the pipeline at hydrogenaeration and the temperature of catalyst bed was also lower tion reactor inlet deviated to some extent from the design sothan the design value. The auxiliary electrical heater had to that the liquid sulfur could not completely flow back into thework at 75.31% production load of the unit. The main cause 3n stage Claus sulfur condensermight be the deviation of actual heat exchange efficiency ofthe rar gas/gas heat exchanger from the theoretical calcula- 5. 2 Major gas compositions in process unittion data, along with high heat loss due to non-ideal effect ofinsulation and heat tracing for equipment and pipeline of The results of analysis on major gas compositions in thehydrogenation reaction systemprocess unit are shown in Table 3. It can be seen from Table 3that the hydrogenation reaction in the rar process could(2)The pressure at hydrogenation reactor inlet was 23 kPa, proceed relatively completely, with the H, s content at thewhich was slightly higher than the design value of 20 kPa. quench tower inlet increased by 0.55 percentage points com-pared to that before hydrogenation. The content of othersulfur compounds was very low. The hydrogenation reactorTable 3 Analysis of major gas compositions o had reached its reaction efficiency despite the lower thandesign value temperature in the reactor. The H,Sbsorber outlet was 80 ppm, which was far lower than theTail gas leaving Clausunitdesign value of 254 pp080435.3 Material balance before and after start-up00of rar unitGas at hydrogenation reactor inlet065 The material balance before and after start-up of the RARunit is presented in Table 4. It can be seen from Table 4 thatCOS010 after startup of the rar process the total sulfur recoveryHwas increased to 99,86%, which was 6.65 percentage pointsGas at quenchhigher than that before startup. The amount of so, in the27)vented tail gas was 0. 27 t/d(corresponding to 1 1. 25 kg/h),which was far less than the new national standard for gasCOs(170kg/h)Gas at quench tower outlet6 Conclusions60(16453The RaR process for sulfur recovery and tail gas treatmentGas at absorbing column outletby means of the reduction/absorption/recycling technologyH,s,ppm80(254)can recover the sulfur compounds contained in the SRU tailgas to meet the new national environmental standard for emisRecycle acid gassion of so, in vented tail gas. However, in order to bring thiprocess into full play, the reaction temperature control sys-Hydrocarbonstem for tail gas hydrogenation should be improved so thatO Sampling time: 8 am on June 8, 2000. Data in parentheses therefer to design dataYH中国煤化工9it could be secured toCNMHGReviTable 4 Material balance before and after start-up of rAr unitAfter RAR startupBefore RAR startupItem(une 8-9)(February 8-9)feedstockAcid gas109.14815HcH9044air( in thermal rea2517182392Hydroge052Fuel gasAir(in thermal oxidizer)952448321219529Total47969productsSulfurFlue gas o39469生19724SO③N2298.79143302404318729HO47.1947969Total sulfur recovery rate,9986① The temperature of vented flue gas is over450℃2 The data before RAR startup refer to the total sulfur amount at the last stage of sulfur condenser, and data afterRaR startup refer to the total So, vented from stack of this unitReference971 Deoxygenating Guard Catalyst for Sulfur Recovery.J Tang Zhaozheng et al, Commercial Application of the LS- troleum Processing and Petrochemicals. 2001,32(9): 35中国煤化工CNMHG