Tars are less deteriorating to the catalytic activity

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Unformatted text preview: ivation can result from both the tars and the carbon formed. Tars are less deteriorating to the catalytic activity, because they tend not to form a continuous layer over the surface, however neither of these forms is desirable. Control of coking always depends on a balance between coke formation and coke removal. As radical polymerisations are favoured by high temperatures and highpressure the coke formation can be minimised by keeping the pressure and the temperature as low as possible. However, this is not always favourable for the whole 1. Introduction 31 ytrtrghrhrhhrhrhhffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff process. For example the power requirements for the methanol synthesis from syngas can be almost halved by increasing the pressure of the syngas from 10 to 80 bar [10] and higher temperatures allow generally minimising the size of the reactor [72]. If the temperature and the pressure cannot be minimised, the residence time in the gas phase should be minimised by careful engineering of the design of the whole process equipment. As coking by gas phase reactions is a radical polymerisation reaction, coke formation can generally also be minimised by keeping the radicals apart using diluents. Inert diluents such as N2 and He as well as reactive ones such as H2, O2, CO2 or steam may be used [18, 71, 72]. However, also the use of diluents depends on the pressure requirements, considering the costs of compression of the diluent [10]. Reactive diluents minimise the coke formation not only by dilution of the feed but also by gasification of coke or coke intermediates by reactions such as C + 0.5 O2 → CO ; C + H2O → CO + H2 ; C + CO2 → 2 CO ; C + 2 H2 → CH4. Gases like hydrogen may also act to trap radicals, R⋅ + H2 → RH + H⋅. If the radicals hit the surface, the coking sequence may be interrupted before coke is formed, H⋅ + H⋅ + Surface → H2 + Surface. As a result it is an incentive t...
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