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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 
and higher temperatures allow generally minimising the size of the reactor . 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 . 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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