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Ind. Eng. Chem. Res. 1997, 36, 4516-4522
Heats of Catalytic Cracking. Determination in a Riser Simulator Reactor Ahmet Pekediz,† Dan Kraemer,‡ Alberto Blasetti,§ and Hugo de Lasa* Chemical Reactor Engineering Centre, The University of Western Ontario, London, Ontario, Canada N6A 5B9
This study shows the ability of the riser simulator, a novel internal fluidized bed, to determine the heats of gas oil cracking. With this goal, a gas oil feedstock is cracked using both Octacat and GX-30 catalysts. The experimental conditions studied include a catalyst-to-oil ratio of 4.85, typical reaction times of 3, 5, and 7 s, and temperatures in the 500-550 °C range. The heats of cracking are calculated from stoichiometric considerations and following a methodology consistent with classical thermodynamics. Stoichiometric coefficients for the product spectrum are estimated from the GC-FID chromatogram of the cracked products. The significance of estimations of the heats of cracking is demonstrated, in the present study, using a model for the FCC plant. These calculations are based on overall enthalpy balances applicable to the riser cracker and to the catalyst regenerator. 1. Introduction Determination of the heats of cracking is essential for the FCC units heat balancing. In fact, the overall enthalpy balance of these units is affected by various process steps, and this, in turn, impacts both the operation and the design of industrial-scale FCCs. The following are the main contributors to the overall enthalpy balance in the FCC plant: (a) the enthalpies of cracking and coke combustion (∆Hcrack and ∆Hcomb), (b) the enthalpy of vaporization of the gas oil feedstock (∆Hvap), (c) the enthalpy content of various process streams (air, combustion gases, catalyst, steam, vaporized feedstock, cracked products). In FCC units the gas oil feedstock is first vaporized while being fed to the FCC riser unit. To achieve this, the enthalpy of vaporization is furnished to the gas oil feedstock by a stream of hot catalyst and steam. Following vaporization, hydrocarbons are cracked in the riser. Given the overall endothermicity of cracking reactions, an additional enthalpy has to be provided to the hydrocarbon reacting mixture. At the riser outlet, the products are separated from the catalyst in a cyclonic device, with the catalyst being stripped of adsorbed hydrocarbons with steam. The spent catalyst is recycled to the fluidized-bed regenerator where “coke-on-catalyst” is combusted with air. Coke combustion yields CO and CO2 and a significant amount of heat and this given the high exothermicity of combustion reactions. As a consequence, catalyst regeneration increases the catalyst temperature and a hot and essentially “free-of-coke” catalyst is returned to the bottom of the riser, ready to start a new cracking cycle. Careful balancing of the heat evolved in the regenerator and the heats required for gas oil vaporization and gas oil cracking is always a major challenge for modern refineries. This difficulty is even more acute nowadays given the increased need for processing heavier feed* Author to whom correspondence should be addressed. † Current address: Exxon Corporate Research Laboratories, Exxon Research and Engineering Co., Annandale, NJ 088010998. ‡ Current address: Imperial Oil Research Laboratories, Sarnia, Ontario, Canada N7T 7M1. § Current address: Departamento de Procesos, Facultad de Ingenieria, Universidad de La Patagonia, Comodoro Rivadavia, Chubut, Argentina. S0888-5885(97)00122-X CCC: $14.00
stocks. Heavier feedstocks yield, while being cracked, more coke, generate more heat in the regenerator, and may lead to severe upsets of the thermal balance of a refinery. As well, other added complexities are the result of more strict environmental regulations concerning CO emissions. These regulations complicate the operation of FCC regenerators, under the partial combustion mode, with relatively high levels of CO in the regenerator outlet gases (Peters et al., 1992) and the requirement of highly efficient CO boilers (Rheaume and Ritter, 1976). In this respect skillfully designed catalyst coolers are essential for the implementation of viable solutions to remove the excess heat in the FCC plants (King, 1992). All the mentioned matters complicate nowadays the engineering of the FCC units and call for the use of accurate models and simulators providing various relevant parameters (e.g., kinetic constants, adsorption constants, thermodynamic parameters). Regarding the various enthalpies involved, it has to be mentioned that the enthalpies of coke combustion can reasonably be well estimated using sound approximations (de Lasa et al., 1981). The enthalpy of cracking, however, is a major unknown, and this is particularly true given the strong dependence of ∆Hcrack on the type of catalyst and on the specific slate of products produced in the gasoline range. For example, for typical conditions for FCC operations, the recorded heat of cracking corresponds to approximately 15-40% of the heat transferred to the riser via the hot catalyst. In this respect, it is important to refer to Mauleon and Courcelle (1985), who questioned current methods for calculating both the heat of cracking and the heat of vaporization. Given the above-mentioned facts, the aim of the present study is to determine the heats of cracking using a riser simulator, a novel laboratory scale unit, developed at CREC-UWO. With this goal, a typical feedstock and two catalysts, Octacat and GX-30, were selected for the study. By doing so the goal was also to demonstrate the ability of this unit to assist FCC unit operation providing periodic monitoring of the heats of cracking and as a consequence effective and frequent (e.g., on a daily basis) heat balancing of the overall FCC process. With respect to the novel riser simulator, it has to be mentioned that this unit was successfully applied in © 1997 American Chemical Society
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Figure 1. CREC riser simulator components: (a) riser simulator reactor, (b) vacuum box, (c) glass chamber, (d) six-port valve, (e) four-port valve.
recent studies for catalytic cracking both in the context of kinetic modeling (Kraemer and de Lasa, 1988) and for the evaluation of adsorption constants of various pseudospecies (lumps) (Pruski et al., 1996). In addition to this, the present study demonstrates the riser simulator is adequate for assessing heats of cracking and consequently has the ability to provide data for proper and rigorous simulation of FCC units. 2. Experimentation in the Riser Simulator In order to simulate catalytic cracking reactions similar to those taking place in a commercial riser reactor, a CREC riser simulator employing a minifluidized-bed of catalyst is employed. The incentive for this novel unit (de Lasa, 1992; Pekediz and de Lasa, 1994) is to provide intense mixing of gas and solids, preventing formation of coke profiles and gas channelling. Coke profiles and gas channelling during cracking are plausible events in a fixed bed of fine solids (60 µm particle size) case of MAT test (Micro-Activity-Test) units. In this respect, the CREC riser simulator, which can be classified as an internal recycle fluidized-batch reactor, provides a novel approach for studying catalytic cracking reactions in a reaction regime close to the one of riser or downer reactors. In the context of the present study, experimental runs were performed in a 45 mL riser simulator reactor. The reactor was connected to a 455 mL vacuum system by means of a four-port valve, whereby the cracked products were removed from the riser simulator at the end of the reaction period (Figure 1). A four-port valve was controlled by a timer/actuator assembly linked to the gas oil injection system. The vacuum system was also connected to a manually operated six-port sampling valve which allowed for sample injections into the gas chromatograph. Both the reactor and the vacuum system were equipped with two pressure transducers which permitted continuous pressure monitoring during the reaction and postreaction evacuation periods. The riser simulator, the vacuum system, the connecting lines and valves were all well insulated. The gas oil injector system included a 1 mL glass syringe connected to the injection needle and to a 50 mL gas oil reservoir by means of a two-way valve (sample/inject). It was also equipped with electrically actuated switches which controlled the timer/actuator assembly on the four-port valve as well as the data acquisition system. The data acquisition system al-
lowed for collecting the pressure profiles in the reactor and vacuum system as a function of time during the reaction and postreaction evacuation periods. The operating reaction conditions employed closely resembled those present in commercial FCC installations. Several runs at 3, 5, and 7 s reaction time, 500550 °C, and 4.85 catalyst-to-oil ratios were performed (Kraemer, 1991). Each run involved loading the catalyst basket, located inside the riser simulator, with a predetermined amount of catalyst (about 1 g), sealing the system, and heating the reactor (50 mL) to the desired temperature. The vacuum system (550 mL) along with all its associated valves and lines was also heated to 250-350 °C in order to prevent hydrocarbon condensation. The heating process was carried out under a continuous flow of argon. When equilibrium was attained, the flow of argon was cut off and the reactor at 15 psia was sealed off from the vacuum system. The pressure inside the vacuum system was subsequently reduced to 2 psia. The reaction was initiated by the injection of a predetermined amount of gas oil into the reactor. After the selected reaction time, the reactor and the vacuum system were again connected. Because of the large pressure and volume differences between the two systems, all the contents of the reactor was effectively removed from the reactor into the vacuum system, thus terminating the reaction and preventing the possibility of overcracking. After achieving pressure equilibrium between the two systems, the reactor was again sealed off and a sample of the gaseous products held within the vacuum system was sent to the gas chromatograph equipped with an FID detector for analysis. Chromatographic separation was achieved at optimized oven-temperature conditions. A temperature ramp of -30 to 320 °C was found the best for gasoline range peak detection. 3. Heats of Cracking A typical analysis for the feedstock used is reported in Table 1a (Blasetti, 1994). This includes, among other relevant characterisics, simulated distillation data, Conradson carbon, and low-resolution mass spectrometric analysis. Feedstock components were classified, first, by boiling point range using a simulated distillation (Table 1a). Moreover, light and heavy fractions of the gas oil were assigned using the method described by Kraemer (1991). Fractions with a boiling range between 220 and 345 °C were classed as the light fraction of the gas oil. Fractions with a boiling range between 345 and 527 °C were classed as the heavy fraction of the feedstock. In each of these fractions, light and heavy, component subfractions were defined, grouping hydrocarbons in paraffinic, naphthenic, and aromatic. Paraffinic and naphthenic hydrocarbons were identified with combined capillary column GC and MS analysis. The remaining fraction was assigned to the aromatic hydrocarbons. With the above-mentioned parameters, a single pseudocomponent model of the gas oil feedstock was implemented using a commercial Hysim flowsheeting package by Hyprotech. Using Hysim the pseudocomponent model was divided into several cuts (autocut function), with a density and a distillation curve matching the ones of the feedstock (Table 1a). As a result of this, each cut was represented by API gravity, molecular weight, and boiling point. Using the API gravity and the boiling point information, the carbon/hydrogen ratio (r) was determined (API, 1966). This ratio, r, allows
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pressure change on enthalpy, the riser simulator reactor was modeled as a “stoichiometric reactor”:
Table 1 a. Characteristics of the Gas Oil Used test
method
value
1. metals in oil
AA PE-5000 V
