In Situ Mitigation of Coke Buildup in Porous Catalysts with

Chapter 16

In Situ Mitigation of Coke Buildup in Porous Catalysts with Supercritical Reaction Media Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch016

Effect of Feed Peroxides Bala Subramaniam and Ashraf Jooma Department of Chemical and Petroleum Engineering, University of Kansas, 4006 Learned Hall, Lawrence, KS 66045-2223

For the Pt/γ-Al O catalyzed isomerization of 1-hexene (P = 31.7 bar; T = 231°C), catalyst activity is severalfold higher and deactivation rates are severalfold lower in supercritical reaction media when compared to subcritical media. These improvements are due to the in situ extraction of coke-forming compounds from the catalyst by supercritical reaction media. To fully exploit the benefits of supercritical operation, it is essential to remove organic peroxides from the hexene feed. Otherwise, these peroxides promote the formation of hexene oligomers at reaction conditions. These oligomers, being major coke producers, accelerate catalyst deactivation. Therefore, removal of the peroxides by pretreatment with activated alumina leads to a further severalfold reduction in coke buildup, and concomitant improvements in catalyst activity and longevity. 2

3

c

c

Many industrially significant catalytic reactions such as isomerization, disproportionation of aromatics, and alkylation reactions on acid zeolites are characterized by catalyst deactivation due to coking. The mechanism of coke formation on acid catalysts has been reviewed in detail in several books and papers (1-4). Coking of acid catalysts is typically caused by side reactions that involve mainly acid-catalyzed polymerization and cyclization of olefins that produce highermolecular-weight polynuclear compounds which undergo extensive dehydrogenation, aromatization and further polymerization (7). These products are generally termed as coke and have been characterized as either consolidated carbon deposits (that cannot be dissolved in organic solvents) or mobile deposits which are precursors of the consolidated deposits (2, 3). For example, in the case of reforming catalysts, the mobile deposits are typically polyaromatic hydrocarbons that can be extracted with the help of organic solvents.

0097-6156/95/0608-0246$12.00/0 © 1995 American Chemical Society

Hutchenson and Foster; Innovations in Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

16.

SUBRAMANIAM & JOOMA

Coke Buildup in Porous Catalysts

247

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0608.ch016

Combating Catalyst Coking with Supercritical Reaction Media Conceptually, catalyst deactivation by coking can be mitigated if the mobile coke compounds are desorbed from the catalyst before they can undergo transformation to consolidated carbon. However, the relatively low volatilities of the mobile coke compounds at gas phase (i.e., subcritical) reaction conditions results in the adsorption of these compounds on the catalysts leading to their progressive transformation to consolidated coke. The coked catalyst is typically regenerated by air oxidation of the coke exposing the catalyst to high temperatures (400-500°C), which often cause thermal degradation of the catalyst. The prevention or mitigation of coke formation on catalysts continues to be the focus of active research. Supercritical media offer a unique combination of solvent and transport properties for the in situ extraction of coke-forming compounds from porous catalysts. Within the last decade, our research group at the University of Kansas has experimentally and theoretically investigated the coking and activity of porous catalysts in supercritical reaction mixtures. These studies, summarized elsewhere (5-7), employed 1-hexene isomerization on a Μγ-ΑΙζΟι reforming catalyst as the model reaction system. The isomerization was carried out at 281°C., which is about 1.1 T of 1-hexene. Hence, by varying the reactor pressure from 22 to 350 bars, we were able to investigate the catalytic isomerization in reaction mixtures ranging in densities from subcritical (0.2 p ) to dense supercritical (2.2 pc) values. At a fixed space velocity (135 g hexene/h/g cat.), we found that in gas-like reaction mixtures, the catalyst deactivates rapidly due to accumulation of coke compounds in the catalyst. In near-critical reaction mixtures (1.2 p ), however, the coke laydown decreased significantly due to the in situ extraction of coke-forming compounds, primarily hexene oligomers, from the catalyst pores. While coke laydown continued to decrease in dense supercritical reaction mixtures (1.7-2.2 pç), the isomerization rates also decreased due to pore-diffusion limitations in the liquidlike media. These results are detailed elsewhere (7), and clearly show that nearcritical reaction mixtures offer an optimum combination of solvent and transport properties that are better than either gas-phase or liquid-phase reaction media for maximizing the isomerization rates and for minimizing catalyst deactivation rates. c

c

c

Deleterious Effects of Peroxides in Feed . Besides unreacted 1-hexene and product isomers, hexene oligomers (up to pentamers) were also detected in the reactor effluent (8). We found that these oligomers formed mostly in the fluid phase catalyzed by organic peroxides (about 700 ppm) present in the hexene feed. The oligomer formation steadily increased with pressure with the total amount being roughly 2 wt% of the effluent stream at the highest supercritical pressure investigated. These hexene oligomers are prolific coke producers (7, 8). Thus, the enhanced isomerization rates previously reported (7) at near-critical and supercritical conditions was in spite of increased oligomer formation (and therefore increased coke formation potential) at the higher pressures. In this paper, we show that catalyst performance in near-critical and supercritical reaction mixtures is dramatically improved when the oligomer formation in the fluid phase is curtailed by removing the organic peroxides from the 1-hexene feed. An online alumina trap is used to virtually eliminate organic peroxides from the hexene feed. The resulting decrease in oligomer formation in conjunction with the continued in situ extraction of the coke-forming compounds by supercritical reaction mixtures are shown to significantly improve catalyst activity and longevity. Experimental Continuing our previous investigations (7, 8), the isomerization of 1-hexene over Pt/r-Al 0 catalyst was investigated at 281°C (1.1 T ) and at pressures yielding subcritical to supercritical conditions (0.2-2.2 Table I lists the operating 2

3

c


248

INNOVATIONS IN SUPERCRITICAL FLUIDS

pressures and the corresponding reaction mixture densities estimated using the PengRobinson equation of state. As in the previous studies, a space velocity of roughly 135 g hexene/g cat./h was employed. Table I. Operating pressures and reaction mixture densities: Τ = 281°C (1,1 T )


c

3

3

Pressure (bar)

Estimated Density (kg/m xl O )

Reduced density

21.7 35.5 52.7 70.0 139 222 277 346

0.050 0.101 0.204 0.287 0.412 0.475 0.502 0.528

0.21 0.42 0.85 1.20 1.72 1.98 2.09 2.20

Catalyst. One gram of 1/16" Pt/^Al203 (Engelhard E-302) reforming catalyst extrudates was used. The Pt loading on the catalyst is 0.6 wt%. The catalyst was first pretreated in flowing nitrogen at 100 seem in a pretreatment reactor at 330°C for 18 hours, followed by hydrogen at the same flow rate and temperature for four hours. The pretreated catalyst was found to have a BET surface area of 175 m /g, total pore volume of 0.44 ml/g and an average pore radius of roughly 50 Â. 2

Reactor Unit. Figure 1 shows a schematic of the high pressure experimental unit. This setup is essentially similar to the unit described previously (7), with improved data acquisition and process control instrumentation. The feed section consists of a liquid feed supply bottle connected to an HPLC pump (Waters' Associates #6000A) capable of delivering constant flow rates between 6 and 600 ml/h against pressure heads up to 400 bar. Either hydrogen or nitrogen gas (for catalyst pretreatment or system purging) is admitted to the experimental unit via a three-way solenoid valve (VI). Either gas or liquid feed is selected by opening valve V2 or V3 respectively. The 1-hexene (Ethyl Corporation; Lot# PT060592) feed is pumped through a 30 cm long stainless steel tube (roughly 0.8 cm i.d.) containing 11.6 grams of dry activated neutral alumina to remove the peroxides. The feed is then passed through a high-pressure line filter fitted with a 10 urn frit, before flowing through a safety head equipped with a rupture disc rated to a burst pressure of 400 bar at 20°C. The safety head is directly connected to the building vent with a high-pressure hose. Following the rupture disc, the feed enters the top of a vertically mounted stainless steel tubular reactor (15 ml capacity) and passes over a 3.5 cm long catalyst bed located approximately 10.5 cm from the bottom of the reactor. Thermocouples (J-type) are placed on each end of the catalyst bed to monitor and to provide feedback for PIT) control of the reactor temperature. The reactor pressure is controlled by means of an Autoclave Engineers' 30VRMM micrometering valve (C = 0.04). The micrometering valve is actuated via a microprocessor-controlled stepper motor which is part of the PID control loop for maintaining reactor pressure. A pressure transducer (PT1; 400 ± 1.9 bar) located upstream of the reactor provides feedback for the reactor pressure control loop. The nearly 14,000 steps from the fully-open to the fully-closed valve positions allow fine control of the reactor pressure within transducer precision. Sensitive pressure control is essential along a near-critical isotherm at which small changes in pressure around the critical pressure can lead to relatively large changes in density and transport properties. v

Effluent and Catalyst Analyses. After passing through the micrometering valve, the reactor effluent is cooled in a heat exchanger. A manually controlled three-way valve (V7) is used to either sample the liquid effluent for off-line analysis or collect it Hutchenson and Foster; Innovations in Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

SUBRAMANIAM & JOOMA

249



16.

•a3

ο

1 ο

Oh

J?

1 a ο 00

60

2

X S t£


250



10

T = 281°C WHSV=135ghexene/h

Ο

~ 2 p p m peroxides i n feed

•

- 7 0 0 p p m peroxides i n feed (7)

-O— 0.2

0.7

1.2

1.7

2.2

Reduced Density

Figure 2. Effect of feed peroxides onfluidphase oligomer production rates.

Ρ

Τ = 281°C.; Ρ = 7 0 bars » P

c

= 1.2

Ο

~ 2 p p m peroxides i n feed

•


00-

0

3

6 T i m e (h)

Figure 3. Effect of feed peroxides on isomerization rate histories.


16.

SUBRAMANIAM & JOOMA

251



for safe disposal. Whenever purge gas is used in the setup, a manually controlled three-way valve (V6) is used to direct the gas through a rotameter and to the vent. The reactor effluent was sampled at various times for off-line analysis of 1-hexene, its isomers and oligomers using an HP5890 GC/FID instrument. At the end of a run (typically lasting eight hours), the catalyst was removed and subjected to gravimetric analysis (to determine the amount of coke laydown) and micromeritics analysis (to determine the loss in surface area and in pore volume due to coking) with a Gemini 2000 Pore Volume and Surface Area analyzer. Process Control and Safety Features. All the measurement and control devices in the reactor unit (viz., thermocouples, pressure transducers, stepping motor controls for driving the micrometering valves, solenoid valves and heaters) are interfaced with the Camile 2500 data acquisition and control unit. Programmed sequences developed for reactor startup, operation and shutdown enhance the ability to accurately repeat experimental procedures. Several safety features have been incorporated into the reactor unit are as follows: (a) The reactor unit is shielded with 3/8" thick Plexiglas for physical protection of personnel and equipment; (b) The air space in the Plexiglas enclosure is kept continuously ventilated to the atmosphere through a roof vent equipped with a vacuum pump. Any accidental spills or leakage of reactants and products are therefore vented to the atmosphere; (c) The ability to stop or change flow streams and to shut off power to the heating elements by programmed sequences in the Camile 2500 data acquisition and control system greatly reduces the chance of thermal reactor runaway. Solid state relays have been installed in series with all the heaters to permit automatic shutdown if any of a number of emergency situations occurs. As an example, if reactor temperature exceeds the 'safe' operating temperature, the power to the heating elements is shut off and the feed introduction is continued to cool the reactor. Results and Discussion Peroxide Effect on Oligomer Production Rates. Figure 2 compares hexene oligomer production rates obtained using a hexene feed containing 700 ppm organic peroxides with those obtained using on-line alumina pretreatment of the hexene feed. Analysis by sodium thiosulfate titration following a contacting step with sodium iodide (O'Quinn, Α., Ethyl Corporation, Baton Rouge, LA, personal communication, 1993) indicated that the alumina pretreatment reduced the peroxide content from 700 to 2 ppm, expressed as ppm oxygen. In both cases, oligomer formation was measured in the absence of the Pt/^Al203 catalyst. It is clear from Figure 2 that the peroxide reduction in the hexene feed results in an approximately fivefold decrease in the oligomer production rate at the lowest subcritical density (0.2 pc) and an 18-fold decrease at the highest density (2.2 pc). While the total oligomer production rate increased twofold over this density range in the case of the alumina-pretreated feed, the corresponding increase is fourfold when using untreated 1-hexene containing about 700 ppm peroxide. As shown in Figure 3, reducing the oligomer formation in the fluid phase has a positive effect on catalyst performance. At a supercritical density of 1.20 p (70 bar, 281°C), the initial (10 min.) and end-of-run (8 hours) isomerization rates were about 20% and 250% higher respectively when the peroxide content is lowered. It can also be seen from Figure 3 that the catalyst deactivation rate (i.e., the decline of the isomerization rate with time) was more pronounced in the case of the higher peroxide feed. c

Effect on Coke Laydown. The reduction in coke laydown with on-line alumina pretreatment of the hexene feed was observed in the subcritical as well as in the supercritical runs. Gravimetric measurements of the catalyst at the end of each run revealed that without alumina pretreatment, the coke laydown increased to a Hutchenson and Foster; Innovations in Supercritical Fluids ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

252



T = 281°C Ο

~2 p p m peroxides i n feed

•


-

3-θ

θ \

In Situ Mitigation of Coke Buildup in Porous Catalysts with

Recommend Documents