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Final Report: Coking and Activity of Solid Acid Alkylation Catalysts in Supercritical Reaction Media

EPA Grant Number: R824729
Title: Coking and Activity of Solid Acid Alkylation Catalysts in Supercritical Reaction Media
Investigators: Subramaniam, Bala
Institution: University of Kansas
EPA Project Officer: Karn, Barbara
Project Period: October 1, 1995 through September 30, 1998 (Extended to September 30, 1999)
Project Amount: $220,000
RFA: Technology for a Sustainable Environment (1995)
Research Category: Pollution Prevention/Sustainable Development

Description:

Objective:

Alkylation reactions are used industrially to convert light refinery gases (C3-C5) into gasoline range compounds (C7-C9). At present, alkylates constitute roughly 13 percent of the U.S. gasoline pool and play an important role in meeting the reduced emissions gasoline requirements established by the 1990 Clean Air Act. Industrial alkylation processes employ either hydrofluoric acid or sulfuric acid as the catalyst. The cost of acid regeneration as well as the environmental hazards resulting from transportation and disposal of the acid sludge underscore the need for a more economical and environmentally friendlier alkylation technology. The use of solid acids as an environmentally safer alternative to hydrofluoric or sulfuric acids has long been recognized. As reviewed elsewhere (Corma and Martinez, 1993; Rao and VaTcha, 1996), numerous efforts aimed at developing solid acid alkylation catalysts and solid acid-based isobutane-olefin alkylation processes have been reported for more than three decades. To date, however, none of the solid alkylation catalysts has gained acceptance in industry primarily because of rapid catalyst deactivation due to coke formation. In gas-phase media, the heavy coke precursors (such as olefinic oligomers) are poorly soluble while in liquid-phase reaction media, the transport of coke precursors out of the catalyst pores is severely restricted resulting in their readsorption and transformation to consolidated coke.

The proposed technology employs supercritical reaction media, which offer a unique combination of liquid-like density and gas-like transport properties, for the effective removal of the coke precursors. Employing carbon dioxide (Pc = 71.8 bar; Tc = 31.1?C) as an environmentally benign solvent, 1-butene/isobutane alkylation was performed at supercritical conditions resulting in virtually steady alkylate (trimethylpentanes and dimethylhexanes) production in a fixed-bed reactor on solid acid catalysts (such as HY zeolite, sulfated zirconia, and Nafion) for up to 2 days. In sharp contrast, the alkylate production activity falls continuously to negligible amounts within a few hours at subcritical operating conditions. The carbon dioxide-based supercritical process thus eliminates a major technological barrier impeding the application of solid acid catalysts in alkylation practice. The proposed technology falls within the "green chemistry challenge" focus areas as follows: (a) it uses solid acid catalysts (i.e., an alternate catalytic medium), thereby eliminating the use of environmentally hazardous liquid acids; and (b) it employs supercritical (i.e., alternate) reaction conditions employing carbon dioxide as an environmentally-benign solvent to mitigate coke laydown and thereby to obtain extended catalyst activity for alkylate production.

Environmental Benefits of the Technology. The developed technology offers the following environmental benefits:

Industrial Applicability. The demonstrated technology is applicable in a chemical process that produces 15 percent of the U. S. gasoline pool at present. Hydrofluoric acid based alkylation plants are being phased out by refineries. In the absence of a competing technology, existing HF-based alkylation units would be converted to sulfuric acid based plants (considered less hazardous) and future alkylation plants would be sulfuric acid based (Rao and VaTcha, 1996). With the increasing demand for gasoline worldwide, the developed technology offers an environmentally safer alternative to conventional alkylation, which could have a positive environmental impact worldwide.

In addition to eliminating the use of liquid acids (H2SO4 and HF), the carbon dioxide-based fixed-bed alkylation is easier to operate when compared to either moving bed or slurry reactor processes with solid catalysts. Although the addition of carbon dioxide requires operating pressures on the order of 100 bars, such pressures are not uncommon in petrochemical processing (e.g., olefin polymerization). In addition, product separation could be achieved in flash separation units in which the pressure is reduced to separate the carbon dioxide and the C4 reactants from the products (C8's), thereby obviating distillation. The drawbacks include increased costs associated with the high pressure vessels and carbon dioxide recycle. Clearly, pilot-plant scale studies are needed to demonstrate acceptable alkylate quality, catalyst durability, and product separation on a larger scale, and thereby, to evaluate the economics of the carbon dioxide-based process.

The concept of employing inert diluents such as carbon dioxide to realize near-critical reaction mixtures should also be applicable in other reactions that employ liquid acids because of rapid deactivation problems encountered with the use of solid superacids. Examples include n-paraffin isomerization to isoparaffin, alkylated aromatics from aromatic substrates and olefins, and a variety of aromatic acylation processes. Thus, the developed technology has potential application in replacing liquid acids with environmentally friendlier solid acid catalysts in a variety of high-volume chemical processes.

Summary/Accomplishments (Outputs/Outcomes):

Innovative Aspects. "Supercritical" alkylation, performed with excess isobutane (Pc = 36.5 bars; Tc = 135?C) above the critical temperature (Tc) and critical pressure (Pc) of isobutane, has been reported to slow down deactivation of solid acid catalysts (Hussain, 1994; Fan, et al., 1997). The liquid-like densities and enhanced transport properties of supercritical fluids are exploited to extract coke precursors in situ, and thereby, to obtain extended catalyst activity. However, at reaction temperatures exceeding 135?C, undesirable side reactions such as oligomerization and cracking occur, resulting in unacceptable product quality. Although lower temperatures enhance C8 alkylate selectivity, rapid catalyst deactivation due to coke formation is a problem on solid acid catalyst even at temperatures as low as 50?C (Corma and Martinez, 1994).

The use of supercritical reaction media for in situ coke extraction in heterogeneous catalysis relies on operation in the vicinity of the critical temperature (~1.01-1.2 Tc) of the reaction media wherein relatively small pressure changes around the critical pressure (0.9 - 2.5 Pc) yield relatively large changes (from gas-like to liquid-like) in density and transport properties of the reaction medium. For example, at slightly above the critical pressure, while the fluid possesses roughly 70 percent of the liquid density, the diffusivity and viscosity values are more gas-like. The density of the reaction mixture is directly related to its solvating power, and therefore, its ability to remove coke precursors from the catalyst surface. High diffusivities and low viscosities are desirable to transport these coke precursors out of the catalyst pores before they consolidate into unextractable coke compounds. Thus, by pressure-tuning the fluid properties around the critical pressure, optimum combinations of density and transport properties can be realized for maintaining catalyst activity (Ginosar and Subramaniam, 1994; Clark and Subramaniam, 1996; Subramaniam and Ginosar, 1996). An overview of the applications of supercritical reaction media in heterogeneous catalysis may be found elsewhere (Savage et al., 1995).

The innovation behind the developed technology is the use of carbon dioxide (Pc = 71.8 bars; Tc = 31.1?C) as a low Tc diluent to lower the critical temperature of the reaction mixture. This allows supercritical operation at reaction temperatures below the critical temperatures of the reactants (135?C for isobutane and 140?C for 1-butene), while exploiting the liquid-like densities and gas-like transport properties of supercritical fluids. The lower reaction temperatures favor alkylation reactions over oligomerization and therefore, reduce catalyst deactivation. The liquid-like densities of the supercritical reaction favor oligomer (i.e., coke precursor) solubilization, while the gas-like diffusivities and viscosities enhance the removal of butene oligomers from the catalyst before they undergo further consolidation and deactivate the catalyst.

Employing a molar excess of carbon dioxide (Pc = 71.8 bar; Tc = 31.1?C) as a diluent, supercritical 1-butene/isobutane alkylation is performed at temperatures lower than the critical temperature of isobutane (< 135?C), resulting in virtually steady alkylate (trimethylpentanes and dimethylhexanes) production on solid acid catalysts (microporous H-ultrastable Y-zeolite, mesoporous sulfated zirconia and Nafion) for experimental durations of up to nearly 2 days. To the best of our knowledge, we are the first group to demonstrate constant alkylate production activity in a fixed-bed reactor configuration, which is easier to operate when compared to either moving bed or slurry reactor processes with solid catalysts.

Figure 1. Schematic of the Experimental Unit
Figure 1. Schematic of the Experimental Unit

Equipment. A schematic of the experimental unit, capable of operating at pressures up to 400 bars and temperatures from ambient to 600?C, is shown in Figure 1. The feed section consists of an isobutane/1-butene feed cylinder, a carbon dioxide cylinder (Air Products, coolant grade, < 20 ppm oxygen) and two HPLC pumps. The feed pretreatment vessel contains activated alumina and anhydrous sodium sulfate. The activated alumina packing removes any organic peroxides and other trace impurities from the feed. The reactor is approximately 16 mm in inner diameter and 30 cm in length. The reactor is heated by means of two jacket heaters. A back-pressure-regulator (BPR) allows good pressure control (? 0.35 bar) up to nearly 200 bars.

Product analysis section consists of a heated stainless steel line to a valve system and sample loop in the gas chromatograph (GC) for online quantitative analysis. The GC is equipped with a 100 m DB-Petro column (J&W Scientific), which is capable of resolving all C4 isomers, propane, and all reaction products at ambient temperatures. In addition to the online GC analysis, the reactor effluent was collected in a dry-ice/propanol cooled condenser for qualitative offline GC/FID (HP 5890 series II) and GC/MS analysis (Varian Star 3600CX and Saturn System 2000), both GCs operated with the same column and under identical chromatography conditions.

Figure 2. Alkylate production rate histories on a USY catalyst in various reaction phases. (Table 1 summarizes experimental conditions).
Figure 3. Alkylate production rate histories on a SZ catalyst in various reaction phases. (Table 1 summarizes experimental conditions).
Figure 2. Alkylate production rate histories on a USY catalyst in various reaction phases. (Table 1 summarizes experimental conditions).
Figure 3. Alkylate production rate histories on a SZ catalyst in various reaction phases. (Table 1 summarizes experimental conditions).

Table 1. Experimental Conditions
Temperature (?C)
Pressure (bars)
Feed CO2:I:O1 (molar)
Reaction Mixture Phase
Density2(g/cm3)
140
34.5
0:9:1
gas
0.109
50
34.5
0:9:1
liquid
0.522
140
60.7
0:9:1
near-critical
0.377
50
155.1
86:8:1
supercritical
0.645
95
137.9
43:8:1
supercritical
0.271
Olefin Weight Hourly Space Velocity = 0.25 kg/kgcat/h
1I = Isobutane; O = Olefin (1-butene)
2Estimated using Peng-Robinson EOS

Experimental Results. Figure 2 compares the alkylate fraction of the C5+ products versus catalyst age in reaction phases ranging from gas-like to liquid-like physical properties in the bulk fluid phase. At 140?C and 34.2 bars, which is above the critical temperature of isobutane but below the critical pressure of isobutane, the bulk-phase reaction mixture is subcritical and is termed "gas-phase" reaction mixture. At 140?C and 60.3 bars, which is above the critical values for isobutane, the reaction mixture has been termed "supercritical" by both Hussain (1994) and Fan, et al. (1997). Liquid reaction mixture can be realized in the bulk phase when the reaction temperature is less than the critical temperature of isobutane and the pressure is sufficiently high. Accordingly, the run at 50?C and 35 bars is termed the "liquid-phase" run. The alkylate production declines continuously with time in all these runs, the steepest decline occurring at supercritical conditions. It follows from both our results and those of Fan, et al., that supercritical operation without carbon dioxide does not produce stable alkylate production in the time frame studied. In sharp contrast, the alkylate production attains a nearly steady value after a few hours on stream during the supercritical runs employing carbon dioxide. Even though the alkylate fraction in the product is small (around 5 percent), the steady alkylate production is an indication that supercritical operation employing carbon dioxide as a diluent is capable of maintaining the activity of the strong acid sites at a constant level for the duration of the experiments (nearly 30 hours). In other words, the supercritical reaction medium solubilizes and removes the coke precursors from the strongest acid sites preventing the transformation of coke precursors into consolidated coke. Similar results have been observed on the sulfated zirconia catalyst, as shown in Figure 3. Extended runs up to nearly 2 days on the H-USY catalyst continued to show stable alkylate production activity.

The ability of carbon dioxide-based supercritical reaction media to mitigate coke laydown and thereby better maintain pore accessibility in the catalysts also is borne out by the physical appearance and pore volume/surface area measurements of the spent catalysts. Table 2 summarizes the properties of the spent catalysts at the various reactor operating conditions. On both USY and SZ catalysts, the surface area and pore volume losses are lowest with carbon dioxide-based supercritical media at 95?C (CO2:I:O = 43:8:1). The color of the spent catalysts at the end of these runs also is distinctively lighter indicating less coke laydown.

Table 2. Comparison of Fresh and Spent Catalyst Characteristics
Catalyst
Reaction Phase
Feed CO2:I:O:molar ratio
Surface Area (m2/g) ? 3%
Pore Volume (cm3/g) ? 3%
Observed Color
USY
Fresh
-
560
0.33
white
USY
Liquid Phase
0:9:1
350
0.22
yellow-beige
USY
Gas Phase
0:9:1
130
0.09
brown
USY
Near-critical
0:9:1
190
0.13
brown
USY
Supercritical
43:8:1
430
0.25
off-white
USY
Supercritical
86:8:1
420
0.25
beige
SZ
Fresh
-
100
0.07
white
SZ
Liquid Phase
0:9:1
8
0.01
yellow
SZ
Gas
0:9:1
not measured
not measured
brown
SZ
Supercritical
43:8:1
36
0.03
brown
SZ
Supercritical
86:8:1
43
0.06
off-white
I = Isobutane
O = Olefin (1-butene)

The carbon dioxide-based, fixed-bed, supercritical process thus eliminates a major technological barrier impeding the application of solid acid catalysts in alkylation practice and offers an environmentally safer alternative to conventional alkylation that employs liquid acids.


Journal Articles on this Report: 8 Displayed | Download in RIS Format

Other project views: All 20 publications 11 publications in selected types All 8 journal articles

Type Citation Project Document Sources
Journal Article Clark MC, Subramaniam B. 1-hexene isomerization on a Pt/gamma-Al2O3 catalyst: The dramatic effects of feed peroxides on catalyst activity. Chemical Engineering Science 1996;51(10):2369-2377 R824729 (Final)
 not available
Journal Article Clark MC, Subramaniam B. Extended alkylate production activity during fixed-bed supercritical 1-butene/isobutane alkylation on solid acid catalysts using carbon dioxide as diluent. Industrial and Engineering Chemistry Research 1998;37(4):1243-1250. R824729 (Final)
 not available
Journal Article Clark MC, Subramaniam B. Kinetics on a supported catalyst at supercritical, nondeactivating conditions. Aiche Journal. 1999;45(7):1559-1565. R824729 (Final)
R826034 (2001)
 not available
Journal Article Fan L, Nakamura I, Fujimoto K. Supercritical-phase alkylation reaction on solid acid catalysts: mechanistic study and catalyst development. Industrial and Engineering Chemistry Research 1997;36(5):1458-1463. R824729 (Final)
 not available
Journal Article Rao P, VaTcha SR. Preprints, Division of Petroleum Chemistry, American Chemical Society 1996; 212(39); PETR Pt 2. R824729 (Final)
 not available
Journal Article Rao P, Vatcha SR. Solid acid alkylation: Process performance and design criteria and R&D guidelines. Abstracts of Papers of the American Chemical Society 1996;212(PETR Pt 2):39 R824729 (Final)
 not available
Journal Article Savage PE, Gopalan S, Mizan TI, Martino CJ, Brock EE. Reactions at Supercritical Conditions - Applications and Fundamentals. Aiche Journal 1995;41(7):1723-1778 R824729 (Final)
 not available
Journal Article Simpson MF, Wei J, Sundaresan S. Kinetic analysis of isobutane/butene alkylation over ultrastable H-Y zeolite. Industrial & Engineering Chemistry Research 1996;35(11):3861-3873 R824729 (Final)
 not available
Supplemental Keywords:

1-butene alkylation, USY zeolite catalyst using carbon dioxide as a diluent, alkylation catalysts, supercritical reaction media, air pollution. , Sustainable Industry/Business, Scientific Discipline, RFA, Technology for Sustainable Environment, Sustainable Environment, cleaner production/pollution prevention, Environmental Chemistry, Economics and Business, olefin isomerization, cleaner production, alkylation reaction, green chemistry, trace impurities, zeolites, metal oxides, innovative technology, supercritical reaction media, carbon dioxide extraction, coking, pollution prevention

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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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