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Disproportionation of Toluene*

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Disproportionation of Toluene* Disproportionationof Toluene* by Takao Iwamura**,Seiya Olam*** and Masaki Sato** プ 解 Summary: Toray has developedthe "TATORAY" process which is applicable to produce benzeneand xylenefrom tolueneand C9 aromatics by disproportionationand transalkylationtech- nique. The first commercialplant was built in the Kawasaki plant of Toray in summer 1969 and the plant has been successfullyon-stream for over oneyear. Distinguishingfeatures of "TATORAY" process proved in commercialoperation are as follows: (1) Product molar ratio of benzeneand xylene is adjustable in accordancewith demand of nylon and polyesterfibers. (2) The operatingcondition is much milderthan a conventionalhydrodealkylation process. Over all aromatic yield can be maintained over 97% in "TATORAY". (3) For productionof p- and o-xylenes,low contentof ethylbenzene(normally less than 2mol%) and high content of p- and o-xylenes (more than 20mol%) are of great advantage becauseof higher separation efficiencyand yield. (4) Consumptionof hydrogenduring the reaction is very low since the "TATORAY" reac- tion is carried out without appreciableside reactions. (5) The catalyst is totally regenerableand a single on-streamperiod can be extendedover several months, as coke formation on the catalyst is substantiallyprevented. This makes a singlefixed bed reactor systempossible. 1 Introduction has been successfully on-stream since the begin- Recently the demand for aromatics, particu- ning of October, 1969 at the Kawasaki plant larly benzene and xylenes, has been considerably of Toray Industries, Inc.. increased in Japan. A large and growing amount 2 Disproportionation of Toluene of benzene is being used as a starting material 2.1 Explanation of the Reaction in manufacturing nylon, polystyrene and other Disproportionation of toluene is normally car- synthetic materials, while rapid expansion in the ried out in vapor phase with a solid acid catalyst. production of polyester fiber and film has en- Principal reactions: hanced the position of xylenes. The aromatics have mainly come from the catalytic reforming of naphtha, pyrolysis gasoline and gas oil. However, these processes also give less valuable aromatics, such as toluene and C9 aromatics, as well as valuable benzene and If trimethylbenzene is present in the feed, xylene. the following transalkylation reaction also takes There are several routes to treat toluene. For place. example, toluene can be converted to benzene, caprolactam, terephthalic acid and toluene di- isocyanate. Recently Toray has developed new catalyst and process to convert toluene not only Typical side reactions are disproportionation to benzene but also to xylene, and it was called of xylene and hydrodealkylation of toluene. "TATORAY" . It can more favourably treat C9 aromatics as a feed stock to produce more xylene. The first commercial plant, a unit of 70,000 metric tons of fresh toluene feed per year * Received January 12, 1971. ** Chemicals Research & Development Laboratories, Other side reactions are aromatic ring rupture Toray Industries, Inc. (1,111, Tebiro, Kamakura, Kanagawa, Japan) and formation of heavier materials, such as fluo- *** Kawasaki Plant, Toray Industries, Inc. rene, naphthalene and its derivatives. To pre- Bulletin of The Japan Petroleum Institute Iwamura, Otani and Sato: Disproportionation of Toluene 117 vent side reactions and coke formation on a cata- temperature was measured by thermocouples lyst, the presence of hydrogen is effective, through installed at several points in the reactor. For no hydrogen is required in the formula33),44). normal experiments, T-81 catalyst pellets having 2.2 Catalyst 5mm in diameter and 4mm in length were Much has been reported in previous publi- packed in the reactor. Gas chromatography was cations regarding disproportionation catalysts. used to analyse feeds and effluents. Basically, there are the following kinds of cata- 2.3.2 Chemical Equilibrium lyst systems; Friedel-Crafts1)～10), silica-alu- Chemical equilibrium data on the toluene mina11)～31) and zeolite type32)～65). disproportionation reaction was reported an For an efficient commercial catalyst, there are the system of the methylbenzenes71)～73). How- several requirements. One of the most impor- ever, the equilibrium with the feed which con- tant is to prevent fast fouling in catalytic activi- tains ethylbenzene and ethyltoluene has not been ty due to coke formation on catalyst surface. reported thus far. Hence the experimental runs If the amount of coke deposite on catalyst can were conducted using the apparatus shown in be decreased, regeneration of catalyst would not Fig. 1 and the results summarized in Table 1. be needed as frequently, and a single fixed-bed The results shows fairly good agreement with reactor can be achieved. The Toray effort had the equilibrium concentrations calculated by been devoted to develop such catalyst systems Egan. Only a slight different was observed in and over five hundred kinds of catalyst were ex- the C8 aromatic compositions, which was appa- plored. Finally an excellent new catalyst rently caused by disproportionation reactions of system (T-81) was developed, which shows high Table 1 Equilibrium Concentration Unit: mol selectivity, high conversion per pass and good stability over a period of several months without any regeneration66)～70). As well as firm development of catalyst, cor- responding suitable operating conditions have been established. Final reactor zone consists of a single adiabatic fixed bed reactor. 2.3 Kinetics of the Reaction 2.3.1 Experimental Apparatus8) Fig. 1 shows an experimental apparatus for investigating the reaction at high pressure. The reactors inner diameter is 28mm and it is made of stainless steel (SUS 32). Heating of the reactor was done by surrounding electric heater and Fig. 2 Equilibrium Composition of Methylbenzenes Fig. 1 Experimental Apparatus Calculated at 750°K by API Data84) Volume 13, No, 1, May 1971 118 Iwamura, Otani and Sato: ethylbenzene and ethyltoluene. vity as the reaction proceeds. Fig. 2 shows equilibrium composition on me- However, T-81 catalyst is completely regene- thylbenzenes and it indicates that the maximum rable by combustion of coke on the catalyst. conversion level of toluene per pass would be The ignition point of coke on the T-81 catalyst restricted to less than 58 mol%. was measured by a differential thermal analyser 2.3.3 Kinetics of the Reaction (Shimazu DT-2A) and it was found to be 430℃. Since the catalytic reaction with T-81 catalyst Also, the authors measured the combustion rate was carried out in a quite stable manner, the of coke on the T-81 catalyst under atmospheric study on the details of the reaction kinetics was pressure. The results are expressed by the fol- possible by employing a long test period with lowing formula. the same catalyst. rc ∞exp(-13,300/RT)nc3Ρ02 First of all, the external film diffusion around where the pellet was examined. Conversion was mea- nc: amount of coke on catalyst<2.0×10-3 sured with different amount of packed catalyst (mol-C/g-cal) with the other conditions remaining the same. T: Temperature= 450～530℃ The result showed that the film diffusion resistance P02: partial pressure of oxygen (atm) is negligibly small. This is also supported by For regeneration, oxygen less than two mol% independent calculation on film resistance. in nitrogen gas can be used. For combustion As the next step, internal pore diffusion within of coke without any damage on the catalyst ac- the pellet was studied. Pellets of different size tivity. including catalyst powder were made and con- 3 "TATORAY" Process version was measured. The result indicated that pore diffusion resistance is again negligible 3.1 Outline of the Process under operating conditions and consequently The flowscheme of the "TATORAY" pro- effectiveness factor of the pellet is approximately cess is shown in Fig. 378). Feed toluene, carried unity and calculation using Thiele modulus74) with makeup and recycle hydrogen, is heated, agreed with the result, thus the intrinsic rate first in an exchanger, then in a furnace. The for the catalyst could be measured. mixture then passes to a single adiabatic fixed The study on the effect of total pressure on the bed reactor at a moderate temperature and rate indicated that the surface reaction appears pressure. The reactor effluent is sent to the to be the rate controlling step. The initial re- separator and separated gas is recycled to the lative rate curve, strongly effected by partial feed. The liquid is sent first to a stabilizer and pressure of toluene, indicated that it will be then to the clay treater and to benzene, toluene, suitable to summarize the results by the Lang- xylene and trimethylbenzenes recovery columns. muir-Hinshelwood equation17). Recovered toluene and trimethylbenzenes are re- Temperature dependency of initial relative cycled to the feed. Suitable operating ranges are: rate were correlated by Arrhenius plot. The Total pressure 10～50atm activation energy for the case is calculated to be Temperature 350～530℃ 22.6kcal/mol-toluene. As a summary of kine- Mole ratio of H2/feed 5～20mol/mol tics study, initial rate of toluene disproportio- H2 concentration in recycle gas over 70 nation on T-81 catalyst, can be described by mole% rc ∞ KPT2/(1+KTPT)2 Process performances such as conversion, aro- where matic ring loss, over-all yield and on-stream periods are enterdependent. Higher pressure or higher temperature increases conversion per pass, but give larger ring loss, resulting in decrease in over- all yield. Lower ratio of H2/feed gives more For scaling up of a reactor, those rate data coke formation on catalyst; it is desirable to obtained by experiments can be directly adopted. maintain this ratio above 5. All these con- For other catalysts, several papers reported on ditions were well optimized in the plants67). kinetic study of toluene disproportionation75)～77). 3.2 Feature of the Process 2.4 Regeneration of T-81 Catalyst 3.2.1 Comparison with a Hydrodealkylation Disproportionation catalyst is decrease in acti- Process Bulletin of The Japan Petroleum Institute bisproportionation of Toluene 119 Fig.
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