Kinetic Analysis Of Rate Data For Propylene And Methylacetylene Hydrogenation

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Chemical Engineering and Processing 35 (1996) 203-211

F ng PL*ing

Chemiil

Kinetic analysis of rate data for propylene hydrogenation
Departamento de Ingenieria Quimica-Murcia,

and methylacetylene

J.C. Fajardo, C. Godinez, A.L. Cabanes, G. Villora
Unioersidud de Murcia, Facultad de Quimica, Campus de Espinardo, 30071 Murcia, Spain 19 Received

June 1995: accepted 21September 1995

Abstract

The kinetics of the hydrogenation of propylene and methylacetylene have been individually studied in a plug-flow reactor under similar conditions to the industrial tail-end selective hydrogenation of a steam-cracking C, cut, using 0.05% Pdly-A&O, catalyst. These two reactions represent the two consecutive steps of the actual process in the gas phase. Various homogeneous andheterogeneous models have been derived and compared with the data. The LH-HW and the power-law models have been tested. The power-law models fit the data for both reactions in the operating conditions used. Pseudo-reaction orders of zero for propylene and three for hydrogen for propylene hydrogenation, and orders of zero for methylacetylene and one for hydrogen for methylacetylene hydrogenation,were obtained. Reaction orders obtained for the latter case may also be explained by a LH-HW type model if hydrogen is adsorbed atomically and the hydrogen adsorption is the controlling step of the reaction rate.
Keywords: Selective hydrogenation; Propylene; Methylacetylene; Kinetics; Palladium/alumina

1. Introduction

Steam cracking is a process that takes place at high temperature in thepresence of steam. It is used mainly for producing ethylene, but also for a series of important co-products, such as propylene, 1-butene, butadiene, BTX, etc., from a variety of hydrocarbon fractions ranging from ethane or propane via naphtha to vacuum distillates. The steam cracking of these fractions produces a wide range of olefins in which ethylene predominates, but also other families ofhydrocarbons such as paraffins, diolefins, acetylenics and aromatics, some of which are harmful for later applications. Since the process is rather unselective, a variety of more highly unsaturated compounds have to be removed because of their high reactivity. This means that the raw cuts from [Acetylenicsj Hydrocarbon ----+ Fraction C, fraction (Naphtha) i Olefins ’ H2

the steam cracker are notsuitable for either motor fuel or petrochemical feedstocks, without further purification treatment. For this purpose, refining by catalytic selective hydrogenation has gradually increased in importance [l] because it has a double advantage: it reduces the content of the highly unsaturated compounds (they may even be completely eliminated), and reduces olefin losses in the secondary reactions. Theproportion of olefins may even be increased, thus improving the overall yield. There are other reasons why selective hydrogenation has replaced other refining methods, such as the fact that it is a relatively simple process to implement and is efficient and easy to operate. Nevertheless, the hydrogenation has to be very selective in order to avoid the risk of losing olefins by reaction with hydrogento paraffins, according to the scheme:

H2
Olefins H2 3
Scheme I

Paraffins 2

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J.C. Fajardo et al. / Chemical Engineering and Processing 35 (1996) 203-21 I

The purpose is to promote reaction 1 of Scheme 1 to increase the olefin content, and to avoid reactions 2and 3, which lead to a decrease in olefin yields. Selectivity is possible by using catalysts. Currently, practically all industrial catalysts used in selective hydrogenation are based on palladium supported on an alumina carrier. They have a low palladium content because, on reducing the reaction rate, mass transfer limitations are reduced, thus improving the selectivity and reducing the risk of...