Riser Simulator: Testing of Adsorption Effects - American Chemical

Chapter 23

Riser Simulator:

1

Testing of Adsorption Effects 2

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: June 6, 1996 | doi: 10.1021/bk-1996-0634.ch023

Jacek Pruski , Ahmet Pekediz , and Hugo de Lasa Faculty of Engineering Science, Chemical Reactor Engineering Center, University of Western Ontario, London, Ontario N6A 5B9, Canada

Investigation of the adsorption phenomena present during fluidized catalytic cracking of hydrocarbons was conducted utilizing the Riser Simulator, a novel unit developed at CREC-UWO. A series of cracking experiments were carried out using commercial gas oils and cracking catalysts which resembled the ones used in industrial FCC units. Determination of the adsorption coefficients of the various hydrocarbon lumps was possible based on mass balance considerations. Based on the experimental results and subsequent analysis, the adsorption coefficients were related mainly to molecular weight and reaction temperature with other factors, such as catalyst/oil being of less importance.

Modelling catalytic cracking of hydrocarbons in industrial FCC units requires a thorough understanding of combined kinetic and adsorption phenomena. Cracking reactions considered in the technical literature are interpreted in terms of kinetic modelling as a pseudo-homogeneous reaction process. Two recent contributions emphasize the importance of this approach in a pulse microreactor (2) and in a continuous riser unit (3). These studies also highlight the critical interest of adsorption coefficients for adequate simulation of cracking reactions. To this end, the Riser Simulator, a novel unit developed at CREC-UWO (1) was adapted and employed in the joint determination of these parameters. The use of accurate pressure monitoring devices allowed for good mass balances closures which in turn were crucial to the reliable determination of the other experimental parameters. 1

Current address: SACDA, Inc., 343 Dundas Street, Ontario N6B 1V5, Canada Permanent address: Chemical Engineering Department, Gazi University, Ankara, Turkey

0097-6156/96/0634-0312$15.00/0 © 1996 American Chemical Society

In Deactivation and Testing of Hydrocarbon-Processing Catalysts; O'Connor, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

23. PRUSKI ET AL.

Riser Simulator: Testing of Adsorption Effects

313


Experimental Set-up Experimental runs were performed in a 45mL Riser Simulator reactor in operation at CREC-UWO (Figure la). The reactor was connected to a 455mL 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 lb). 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 for continuous pressure monitoring during the reaction and post-reaction evacuation periods. The Riser Simulator, the vacuum system as well as the connecting lines and valves were well .insulated. The gas oil injector system included a 1000/xL glass syringe connected to the injection needle and to a m 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 allowed for collecting the pressure profiles in die reactor and vacuum system as a function of time during the reaction and post-reaction evacuation periods. A typical pressure profile obtained from the two transducers is presented in the Figure 2. Curve I along with points A, B and C illustrates the characteristic pressure profile observed during the operation of the reactor. Meanwhile, curve II depicts the pressure profile inside the vacuum chamber. Point A of curve I indicates the pressure condition inside the Riser Simulator just prior to the hydrocarbon injection. Point B gives the Riser Simulator pressure at the end of the reaction period (just before evacuation commences) and Point C represents the equilibrium pressure once the pressures between the vacuum chamber and the Riser Simulator have stabilized. Experimental Procedure. FCC catalysts employing the submicron zeolite structure were used in this study. These catalysts were synthesized, spray-dried into 60 fim pellets and impregnated with the metals by the incipient wetness technique at CREC-UWO. The operating reaction conditions employed during this study closely resembled those present in commercial FCC installations. Several runs at various residence times (5-10s), reaction temperatures (500-550°C) and catalyst-to-oil ratios of 4 and 6 were performed. Each run involved loading the catalyst basket located inside the Riser Simulator with a pre-determined amount of catalyst, sealing the system and heating the reactor to the desired temperature. The vacuum system along with all its associated valves and lines were also heated to 250350°C in order to prevent hydrocarbon condensation. The heating process was carried out under 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



314

DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS

Figure la. Schematic of the Riser Simulator with a general view of the unit

ARGON/AIR REACTOR

El

H*l 4PV/

22

HEATED V, VACUUM g

4, 4 —

CARRIER GAS 6PV

TO GC • VENT/VACUUM

GLASS CHAMBER

Figure lb. Riser Simulator Components: reactor, vacuum box, glass chamber



23. PRUSKI ET AL.


Reaction Coordinate (seconds) Figure 2. Curve I: Riser Simulator Pressure. (A) Prior to Injection, (B) Before Evacuation, (C) Equilibrium Pressure. Curve II: Vacuum Chamber Pressure.


315


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DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS

the injection of a pre-determined amount of gas oil into the reactor. After the selected residencetime,the reactor and the vacuum system were again connected. Because of the large pressure and volume difference between the two systems, all the contents of the reactor were effectively moved 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 for analysis. The gas oil used was a typical hydrotreated fraction with a high content of aromatics. The detailed properties of the feedstock can be examined in Table I (5). Gas oil condensation in the sampling lines was found negligible from experimental observation. In addition, dew point calculations at temperatures and pressures prevalent, after sample evacuation, in the vacuum box confirmed the negligible condensation of hydrocarbons in the sampling system. Modelling, Results and Discussion Modelling catalytic cracking reactions involves a through understanding of kinetics and adsorption phenomena taking place simultaneously on catalyst surface while catalytic cracking reactions are progressing. Catalytic cracking reactions are heterogenous processes with significant adsorption of both reactants and products taking place on catalyst surface. This phenomenon has significant repercussions not only on the kinetic parameters but also on the modelling of continuous riser units. As a result of this adsorption phenomenon, riser volumetric flows and consequently, fluid dynamics can be severely affected. Therefore, if these facts are not properly taken into account, significant miscalculations in the fluid dynamics may occur. In this study, we adopted a model with one balanced algebraic equation which incorporated various lumps (gas oil, cycle oil, gasoline, light gases) in both gas and solid phase. Hydrocarbons lumps are distributed and coexist between the two phases at alltimesduring the reaction period (before product evacuation): M

=M

+M

+M

hc,inj hc,gas hc,cat coke

(

1

)

At the same time, the total pressure in the vacuum chamber during pressure equalization as well as in the Riser Simulator during reaction is the sum of the partial pressure contributions of the various lumps: ^total^go+Pcy^ga+Plg

®

Furthermore, for each of the hydrocarbons lumps, given the relatively low pressures employed, the ideal gas law gives:


23. PRUSKI ET AL.


Table I. Feedstock properties


Metals in oil

Anilin Point Bromine Number Conradson Carbon Residue Density @15°C, densitometer Nitrogen, chemiluminescence Sulphur, Leco SC32 NMR Aromaticity C H Viscosity ©40° C Low resolution mass spectrometry Paraffins Cycloparaffms Monoaromatics Diaromatics Triaromatics Tetraaromatics Pentaaromatics Aromatic sulphur Polar compounds Simulated distillation IBP 5 wt% 10 wt% 30 wt% 50 wt% 70 wt% 90 wt% 95 wt% FBP

V

Riser Simulator: Testing of Adsorption Effects - American Chemical

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