alkenes in sulfuric acid: applications to alkylation

Applied Catalysis A: General 255 (2003) 231–237

Solubility measurements of isobutane/alkenes in sulfuric acid: applications to alkylation Wen-Shing Chen∗ Department of Chemical Engineering, National Yunlin University of Science & Technology, Yunlin 640, Taiwan ROC Received 4 March 2003; received in revised form 20 June 2003; accepted 9 July 2003

Abstract An easy experimental method to measure the solubility of isobutane/alkenes in sulfuric acid is established. By addition of minor amounts of anionic, cationic or ampholytic surfactants, the solubility value of isobutane/alkenes changes; that also influences the yield pattern of sulfuric-acid catalyzed alkylation reaction. According to solubility parameters, some additives are evaluated to enhance the solubility ratios of isobutane/alkenes. This gives rise to an improvement of the higher selectivity to trimethylpentanes; the research octane number is between 100 and 110 and the material is an excellent component of gasoline. These additives are applied to isopentane/alkenes alkylation reaction simultaneously. It was found that the quality of alkylate gets better, due to the obvious decrease of the yield of heavy end materials (C10+ ). These facts reveal the industrially practical of solubility parameter and additives. Meanwhile, the yield of trimethylpentane is related linearly with the solubility ratio of isobutane/alkenes. © 2003 Elsevier B.V. All rights reserved. Keywords: Alkylation; Sulfuric acid; Isobutane/alkenes solubility ratio

1. Introduction The alkylation unit which translates C3 –C5 alkenes to high quality gasolines is becoming more important in petroleum refineries, owing to limitation on the blending amounts of some components of gasolines, e.g. benzene, alkenes and aromatics. For gasoline, the value of octane number consists mainly of those gradients which are derived from reforming or fluidizing catalytic cracking units. Furthermore, blending MTBE into the gasoline may be prohibited in future. This gives a big challenge for the octane number demand in gasoline blending stocks. The octane number of alkylate is fortunately between 92 and 97 and the vapor ∗ Fax: +886-5-531-2071. E-mail address: [email protected] (W.-S. Chen).

pressure is only 1.4 × 104 to 2.8 × 104 Pa. Since the major components are alkanes, alkylate is an excellent and environmentally clean gasoline. Nowadays, sulfuric acid or hydrofluoric acid has been used as catalyst in the commercial alkylation process which synthesizes alkylate (C5 –C9+ alkanes) from isobutane/(C3 –C5 alkenes). Although the installation cost of hydrofluoric acid plant is lower than that of sulfuric acid, the environmental problems derived from leaking accidents are obviously serious for the former. Due to safety considerations, most planning and constructing of plants have selected the latter. Until now, the alkylation reaction has gained much attention. To obtain higher operating efficiency and reduction of sulfuric acid consumption, planners have kept feedstocks of different reactors in separate streams according to alkene gradients and acid strength of

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00594-5

232

W.-S. Chen / Applied Catalysis A: General 255 (2003) 231–237

sulfuric acid [1,2]. Those major operating parameters create obvious differences between each reactor, e.g. isobutane/alkene ratio and reaction temperature, owing to the existence of chemical reactivities between alkenes and sulfuric acid [3]. Several investigators have tried to increase the interface between the sulfuric acid and hydrocarbons by impregnating the former solution over solid support to obtain higher selectivity of trimethylpentane [4–7]. In another way, some additives have been developed to enhance solubility of hydrocarbons in sulfuric acid solution. Many oil companies have registered their research in patents and applied these industrially. For example, Cities Service Oil Co. proposed dodecylbenzene sulfonic acid and p-phenylenediamine, respectively [8,9], Texaco Co. found N,N ,N -tris(n-alkyl)phosphoric triamide [10,11] and sulfoamide [12], and trifluoromethane sulfonic acid has also been suggested as an additive [13]. In our previous patents, N,N -dimethyl-1,4-phenylenediamine and naphthalenesulfonic acid have been proved to be better additives [14,15]. Nevertheless, the promoting effect of additives has rarely been discussed [16,17]. According to studies of Albright and coworkers [18–21], the compositions of heavy end of alkylate and the selectivity of trimethylpentane are under the influence of alkenes in feedstocks. The sulfuric acid/alkenes volume ratio is responsible for the product distribution of the intermediate. Nonetheless, higher concentration of alkenes induces a polymerization reaction which leads to production of heavy end materials. Furthermore, the alkylation process must be operated in low temperature (5–10 ◦ C) to prevent side reactions, e.g. polymerization, disporprotination and cracking, from happening. Another report has been issued in the past on the use of propylene, butene or pentene as feedstocks [22]. A detailed investigation of the consumption of sulfuric acid and the yield pattern of alkylate has been reported. Recently, Kranz and Millard [23] studied the sulfuric acid-catalyzed alkylation of isopentane/alkenes. The vapor pressure of gasoline would be reduced intensively by diminishing the content of isopentane. In contrast, the quality of alkylate is worse than that of isobutane/alkenes on decreasing of octane number. The consumption amount of sulfuric acid was enhanced simultaneously. In this research, the solubility value of isobutane/alkenes in sulfuric acid changed by means of

addition of surfactants. An experimental solubility parameter was established to give a guide-line for selecting a proper surfactant to be additive in alkylation process. The sulfuric acid-catalyzed alkylations of isobutane/alkenes and isopentane/alkenes were conducted, respectively, to elucidate the relationships of solubility parameter and yield pattern. Moreover, the alkylation reaction under lower ratio of isopentane/alkenes was undertaken to assess the role of surfactant. Finally, the increment of selectivity of trimethylpentane was correlated with the solubility ratio of isobutane/alkenes. 2. Experimental 2.1. Solubility measurement of isobutane/alkenes The solubility tests were carried out in a semi-batch system under atmospheric pressure at 282 K. Sulfuric acid (Merck GR grade 95–97%) was situated in the autoclave (Autoclave Engineers, Erie PA 16512) in advance under nitrogen circumstances. The proportionate isobutane/alkenes (mole ratio 6/1) whose gradients were the same as that of activity testing were supplied into the autoclave by a syringe pump (ISCO SERIES D Syringe Pump 500 D Model), in a liquid flow rate of 1 ml/min, until the compositions of effluent remain constant. Effluent was periodically injected into a gas chromatograph (Carlo Erba GC 6000), equipped with a capillary column (CP-Al2 O3 /KCl fused silica PLOT, 50 m × 0.53 mm) and a flame ionization detector. The flow rate was measured by the wet-type gas meter (Ritter TG 1 Model) and the used amount of isobutane/alkenes was doubly checked by the weight difference of hydrocarbon feedstock in a balance (METTLER TOLEDO ID1 KB60.2 Type). By means of material balance of isobutane and alkenes between influent and effluent, the solubility value of isobutane/alkenes was evaluated. 2.2. Alkylation testing Activity tests were conducted in a autoclave reaction system (Fig. 1) under 6.53 × 105 Pa at 282 K. Prior to alkylation reaction, a proportionate amount of sulfuric acid was situated in the autoclave under nitrogen stream. Feedstocks of isobutane/alkenes (pu-

W.-S. Chen / Applied Catalysis A: General 255 (2003) 231–237

233

Fig. 1. Schematic diagram of the sulfuric acid-catalyzed alkylation system.

rified by passing through a trap of molecular sieve 5 Å) were supplied by a liquid metering pump (LDC Analytical Consta Metric 3200 Model), equipped with a cooling jacket. The flow rate of hydrocarbons was calibrated by the weight counts in a balance simultaneously. The reactor was made of stainless steel 316, it was equipped with a cooling circulating bath, heating jacket and the stirring propeller of which the rotating speed is controllable. One thermocouple was inserted into the reaction zone for reading and controlling the temperature. The feed mixture was composed of isobutane:n-butane:trans-2-butene:cis-2butene:1-butene:isobutene = 85.10:1.18:3.47:2.09:0. 49:7.67. During the course of reaction, the volume ratio of hydrocarbons/sulfuric-acid was adjusted to be 1/1. The amount of additive was decided preliminarily to be one weight percent. The gas effluent from the reactor was analyzed by a gas chromotagraph (Hewlett Packard 6890 SERIES). A capillary column (CP-Al2 O3 /KCl fused silica PLOT, 50 m × 0.53 mm) operated from 323 to 473 K was used to identify the gas product compositions. Furthermore, the liquid product of alkylation reaction decanted from reactor was neutralized by mixing with sodium hydroxide solution (1 M) several times. After that, the liquid sam-

ple was injected into a gas chromotagraph, equipped with a capillary column (SPB-1 SULFUR fused silica, 30 m × 0.32 mm), and a flame ionization detector. For the sulfuric acid-catalyzed alkyaltion of isopentane/alkenes, the operating procedures were similar to those of isobutane/alkenes except for some differences existing between the compositions of hydrocarbon feedstocks. One of the feed mixtures was composed of isopentane:n-butane:trans-2-butene:cis-2-butene:1butene:isobutene = 75.20:12.86:3.77:3.14:2.27:2.76. The other feed components were listed as following: isopentane:n-butane:trans-2-butene:cis-2-butene:1-butene:isobutene = 70.11:14.31:4.36:4.29:3.50:3.43. 3. Results and discussion Very little isobutane will dissolve in sulfuric acid. Owing to that, the molecular ratio of isobutane/alkenes had to be over 5/1 to obtain better quality of alkylate in industrial practice. In the past, surfactants have been used as additives to enrich the solubility of isobutane, e.g. N,N ,N -tris(n-alkyl)phosphoric triamide [10,11] and sulfoamide [12]. The choice of proper surfactant depends strongly upon details of laboratory alkylation

234

W.-S. Chen / Applied Catalysis A: General 255 (2003) 231–237

Table 1 Surfactants selected can be divided into three categories Anionic surfactant (A)

Cationic surfactant (C)

Ampholytic surfactant (AM)

A1. Vinylsulfonic acid A2. Amino hydroxynaphthalene disulfonic acid A3. Aminobenzoic acid

C1. Benzoyl phenyl hydroxylamine C2. Dibenzylamine C3. Dimethylbenzylamine

A4. A5. A6. A7.

C4. C5. C6. C7.

AM1. Naphthylamine sulfonic acid AM2. Aminoethanesulfonic acid AM3. Dimethyldodecylammonio propane sulfonate AM4. Aminobenzoic acid AM5. Amino naphthol sulfonic acid AM6. Aminomethane sulfonic acid AM7. Naphthylamine sulfonic acid

Ammonium sulfate Naphthalenesulfonic acid Sodium naphthyl phosphate Tributyl phosphate

Dimethyl naphthylamine Dimethyl toluidine Dimethylaminododecane Dimethyldecylamine

reaction experiments. In this work, an easy solubility measurement method was established, wherein 21 kinds of chemicals were chosen according to anionic, cationic or ampholytic surfactants analysis to verify the application of this method (shown in Table 1). In alkylation reaction process, the hydrocarbon was dispersed phase and sulfuric acid was continuous phase. It means that surfactants with higher value of Hydrophile Lipophile Balance (HLB) were suitable for alkylation [24]. That was also the basis for selecting the chemicals in this research. Table 2 lists the results of consumption of isobutane/alkenes in feedstock and solubility ratio of isobutane/alkenes during solubility testing. On addition of minor amounts of anionic surfactant into sulfuric acid, the used amounts of isobutane/alkenes and solubility ratio of isobutane/alkenes were enhanced. In contrast, the effects of cationic and ampholytic surfactants were not coincident. That is, the solubility ratio of isobutane/alkenes for some surfactants was higher than that of fresh sulfuric acid, while for others it was lower. The promoting effect of anionic surfactants on the solubility of isobutane may be interpreted with higher value of HLB in hydrophilic group [24]. That is, the anionic surfactant dissolves more easily in sulfuric acid-phase. It is worth noting that the trend of solubility ratio of isobutane/alkenes was similar to that of consumption percentage of isobutane/alkenes. That reveals that the higher solubility of isobutane resulted from participation of surfactants. To understand the relationship between the solubility of isobutane/alkenes and the yield pattern of alkylation, we chose five surfactants (A2, A5, C3, C5 and C7) to undertake the activity tests. Table 3 presents the product distributions, especially for trimethylpentane (TMP) in alkylation reaction. The conversion of

alkenes reaches a level as high as 100%. The heavy end (C9+ ) of fresh sulfuric acid-catalyzed material was 43.59%, this reduced to 25.46–32.48% owing to the existence of surfactants. This phenomenon can be interpreted as being due to the increasing solubility of isobutane, which inhibits the polymerization of carbenium ion intermediate with alkenes in alkylation [21]. Moreover, the selectivity of trimethylpentane has increased from 37.57 to 44.31%, which is also ascribed to the increasing solubility of isobutane, that induces the trimethylpentane by hydrogen transfer with Table 2 Solubility ratio of isobutane/alkenes and consumption amount in solubility measurement testings Surfactant

Consumption of isobutane/alkenes

Solubility ratio of isobutane/alkenes

None A1 A2 A3 A4 A5 A6 A7 C1 C2 C3 C4 C5 C6 C7 AM1 AM2 AM3 AM4 AM5 AM6 AM7

0.26 0.28 0.27 0.27 0.27 0.28 0.28 0.27 0.26 0.24 0.27 0.23 0.28 0.24 0.27 0.27 0.27 0.24 0.26 0.25 0.26 0.25

1.07 1.19 1.14 1.16 1.12 1.21 1.17 1.13 1.08 0.83 1.12 0.81 1.23 0.91 1.16 1.15 1.10 0.87 1.08 1.02 1.05 0.97

W.-S. Chen / Applied Catalysis A: General 255 (2003) 231–237

235

Table 3 The product distributions of sulfuric acid-catalyzed isobutane/alkenes alkylation reaction in the presence of surfactants Surfactant item

None

A2

A5

C3

C5

C7

Catalyst Surfactant (wt.%) Isobutane/alkenes Hydrocarbon/acid (v/v) Conversion of alkenes (%)

H2 SO4 0.0 6.2 1.0 100

H2 SO4 1.0 6.2 1.0 100

H2 SO4 1.0 6.2 1.0 100

H2 SO4 1.0 6.2 1.0 100

H2 SO4 1.0 6.2 1.0 100

H2 SO4 1.0 6.2 1.0 100

0.00 1.63 4.50 50.28 43.59 37.57

4.88 7.18 7.14 51.78 29.02 38.52

0.28 2.22 5.32 59.70 32.48 44.31

1.17 3.53 5.10 54.87 35.33 40.87

2.95 6.69 6.88 57.69 25.79 43.21

3.72 6.68 7.10 57.04 25.46 42.57

Compositions C5 C6 C7 C8 C9+ TMPa a

TMP: trimethylpentane.

carbenium ion. The fact suggests that production of better quality of alkylate with higher octane number can be achieved by means of surfactants. The main component of C5 group is isopentane, of which the carbenium ions proceed hydrogen transfer with isobutane. The increasing yield of isopentane gives another proof of the higher solubility of isobutane. A similar promoting effect was also observed for the C6 group, composed of dimethylbutane and methylpentane, of which yields increased simultaneously as compared with that of fresh sulfuric acid. In order to verify the influence of surfactants on solubility enhancement of isoalkane in alkylation re-

action, the isopentane/alkenes alkylation reactions in the presence of some surfactants were performed. The results are shown in Table 4. The conversion of alkenes reached a level near 100%. As expected, a similar catalytic behavior was obtained, which decreased the yield of heavy end (C10+ ) on addition of surfactants. It has been clearly described in a preceding paragraph that tertiary carbenium ions of pentanes are responsible for the alkylation reaction. By means of hydrogen transfer with isopentane, the alkanes of C5 –C9 are formed. Further proofs of the effect of solubility enrichment of isobutane or isopentane on the alkylation are the higher yields of trimethylpentane

Table 4 The product distributions of sulfuric acid-catalyzed isopentane/alkenes alkylation reaction in the presence of surfactants Surfactant item

None

A5

A5

None

A5

A5

Catalyst Surfactant (wt.%) Isopentane/alkenes Hydrocarbon/acid (v/v) Conversion of alkenes (%)

H2 SO4 0.0 6.3 1.0 100

H2 SO4 0.1 6.3 1.0 100

H2 SO4 0.2 6.3 1.0 100

H2 SO4 0.0 4.5 1.0 100

H2 SO4 0.15 4.5 1.0 100

H2 SO4 0.30 4.5 1.0 100

3.75 17.37 5.77 15.29 35.93 21.89 8.92 15.57

4.39 20.31 7.95 18.07 37.08 12.20 11.53 18.92

4.45 17.91 6.73 17.93 37.92 15.06 11.08 18.99

1.96 15.64 5.76 15.87 35.75 25.02 9.25 15.87

3.74 18.00 9.88 18.69 36.79 12.90 12.05 18.21

3.13 17.68 7.21 18.04 37.46 16.48 11.85 18.57

Compositions C5 C6 C7 C8 C9 C10+ TMPa TMHb a b

TMP: gtrimethylpentane. TMH: gtrimethylhexane.

236

W.-S. Chen / Applied Catalysis A: General 255 (2003) 231–237

and trimethylhexane. Additionally, the identical phenomenon was observed also for both methylpentane and dimethylpentane, the major components of C6 and C7 groups, respectively. Thus, one may deduce that the improvement of the quality of alkylate can be achieved by solubility increment of isopentane induced from surfactants. The reaction data of lower feed ratio of isopentane/alkenes (see Table 4) illustrate the analogous yield pattern of fresh sulfuric acid. Nevertheless, the product distributions of the lower ratio of isopentane/alkenes were shifted to higher molecular weight, e.g. heavy end (C10+ ) being 25.02% versus 21.89%. It is worth noting that selectivity of heavy end drops sharply to only 12.90% owing to the existence of surfactants. In addition, the yields of trimethylpentane and trimethylhexane of the latter were higher in comparison with those of fresh sulfuric acid. Under these operating conditions, the effect of surfactants on the improvement of yield pattern was more obvious. This implies that the solubility of isopentane may play an important role in the alkylation reaction. The results were in agreement with those described in the previous paragraph. In order to elucidate the role played by surfactants, we performed the alkylation reaction in a series of concentrations of surfactant. As shown in Table 5, the conversion of alkenes reached nearly 100%, as described previously. Similar catalytic behavior was observed between the sulfuric acid-containing surfactants, e.g. the selectivity of heavy end decreased intenTable 5 Effect of concentration of surfactants on the product distributions in the alkylation reaction Surfactant item

None

A5

A5

A5

Catalyst Surfactant (wt.%) Isobutane/alkenes Hydrocarbon/acid (v/v) Conversion of alkenes (%)

H2 SO4 0.0 6.2 1.0 100

H2 SO4 0.5 6.2 1.0 100

H2 SO4 1.0 6.2 1.0 100

H2 SO4 2.0 6.2 1.0 100

0.00 1.63 4.50 50.28 43.59 37.57

2.10 5.76 6.64 58.35 27.15 43.42

0.28 2.22 5.32 59.70 32.48 44.31

4.40 7.94 7.50 56.01 24.15 41.53

Compositions C5 C6 C7 C8 C9+ TMPa a

TMP: trimethylpentane.

Fig. 2. Effect of solubility ratio of isobutane/alkenes on the yield of trimethylpentane in sulfuric acid-catalyzed alkylation reaction. Operating conditions: pressure = 6.53×105 Pa; temperature = 282 K; hydrocarbon/sulfuric acid = 1.0 (v/v).

sively and the selectivity of trimethylpentane increased obviously. With regard to the yield of trimethylpentane, it seems that one weight percent of surfactant was more suitable during these testings. This fact gives another piece of evidence for the influence of solubility of isobutane on the yield pattern of alkylation reaction. Fig. 2 demonstrates the correlation between the yields of trimethylpentane and solubility ratio of isobutane/alkenes by adding different kinds of surfactant into sulfuric acid. Apparently, there exists a linear relationship indicating the increasing trends of yield of trimethylpentane and solubility ratio of isobutane/alkenes. In other words, the octane number of alkylate was under the influence of the solubility ratio of isobutane/alkenes. Thus, our results suggested strongly the suitability of solubility measurement method of isobutane/alkenes for alkylation process. On the basis of the above discussion, it appears that the solubility ratio of isobutane/alkenes plays an important role in the sulfuric acid-catalyzed alkylation reaction. By the addition of proper surfactants into sulfuric acid, the solubility ratio of isobutane/alkenes has increased obviously. That leads to a higher yield of trimethylpentane being the dominant in products. Therefore, we can conclude that this simple solubility measurement method is a more promising tool for

W.-S. Chen / Applied Catalysis A: General 255 (2003) 231–237

choosing proportionate surfactant in applications to the alkylation process.

Acknowledgements The financial support of the Chinese Petroleum Corporation is gratefully acknowledged.

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

References [1] S.A. Imhoff, D.C. Graves, NPRA AM-95-29, 1995. [2] D.C. Graves, K.E. Kranz, J.K. Millard, L.F. Albright, US Patent 5 841 014 (1998). [3] Worldwild Refinery Processing Review, Hydrocarbon Publishing Company, Southeaster, 3rd Quarter, 1998, p. 5. [4] E. Benazzi, J.F. Joly, C. Marcilly, US Patent 5 731 256 (1998). [5] J.F. Joly, C. Marcilly, E. Benazzi, F. Chaigne, J.Y. Bernhard, US Patent 5 489 730 (1996). [6] J.F. Joly, N. Ferrer, J.Y. Bernhard, E. Benazzi, US Patent 5 475 184 (1995). [7] J.F. Joly, C. Marcilly, E. Benazzi, US Patent 5 420 093 (1995).

[18] [19] [20] [21] [22] [23] [24]

237

M.S. Rakow, W.H. Lockwood, US Patent 3 655 807 (1972). M.S. Rakow, W.H. Lockwood, US Patent 3 689 590 (1972). F.C. McCoy, E.L. Cole, US Patent 3 865 896 (1975). F.C. McCoy, E.L. Cole, US Patent 3 951 857 (1976). F.C. McCoy, E.L. Cole, US Patent 3 926 839 (1975). J.W. Brockington, R.H. Bennett, US Patent 3 970 721 (1976). W.S. Chen, K.Y. Tsai, ROC Patent 097 269 (1998). W.S. Chen, C.T. Hong, K.Y. Tsai, ROC Patent 128 177 (2001). M.P. Nicholson, R.F. Miller, R.L. Knickerbocker, Caridad Go, Oil Gas J. 84 (39) (1986) 47. V.G. Ryabov, A.L. Sviridenko, A.V. Kudinov, S.A. Stepanov, Khim. Tekhnol. Topl. Masel. 10 (1992) 2. L.F. Albright, K.V. Wood, Ind. Eng. Chem. Res. 36 (1997) 2110. L.F. Albright, M.A. Spalding, J.A. Nowinski, R.M. Ybarra, R.E. Eckert, Ind. Eng. Chem. Res. 27 (1988) 381. L.F. Albright, M.A. Spalding, C.G. Kopser, R.E. Eckert, Ind. Eng. Chem. Res. 27 (1988) 386. L.F. Albright, M.A. Spalding, J. Faunce, R.E. Eckert, Ind. Eng. Chem. Res. 27 (1988) 391. E.J. Chang, SRI International, February 1993. K.E. Kranz, J.K. Millard, US Patent 5 583 275 (1996). D. Attwood, A.T. Florence, Surfactant Systems, Chapman & Hall, London, 1983, p. 471.