UMIST Polymer Engineering Report

S F Bush

Introduction

Earlier works (Ref 1, Appendix A) on the cracking of hydrocarbons and chlorination of hydrocarbons has provided a general mechanism and explicit rate expressions for the formation of carbon which have been found to be consistent with observations in a number of practical cases. The purpose of this note is to consider how the treatment might be extended to combustions where oxygen rather than chlorine is the oxidant and where the temperatures involved are likely to cause cracking.

References

Ref 1: S F Bush, Mechanism of Cracking and Carbon Formation, Norwegian Institute of Technology, 16th August 1977.

Report to Petrocarbon Developments Ltd.

S F Bush

Objective

Overall, this is to summarise the likely behaviour of the existing reactor and feed systems and the scope for improvement.

A preliminary report was tendered on 21st April and this report confirms and extends its conclusions.

See also the section on Applications to Existing Products & Processes.

Paper A7 to the Institution of Mathematics and its Applications Symposium on Partial Differential Equations and distributed Parameter Control Systems, University of Warwick, England, 13th-16th July 1971.

S F Bush with P Sinclair.

Introduction

By far the most general technical problem in the operation of an industrial-scale chemical process is the optimisation of performance by selecting appropriate steady-state values of the manipulable variables. This is thus more truly a design than a control problem. An exception to this generalisation is provided by the class of vapour-phase hydrocarbon oxidative processes of which combustion and chlorination systems are notable sub-divisions. Here there are often genuine problems of (a) moving between steady-states without entering regions in which explosions or excessive degradation of product can occur, and (b) determining steady-states which are sufficiently stable in some sense.

A feature of these systems is that the reactions are exothermic and carried out in combustion chambers or reactors designed to permit or induce an internal feedback of material by means of which hot reacted gas is mixed with cool feed gas, diluting it and bringing it rapidly to reaction temperature. In the system referred to here, chlorine and hydrocarbon are premixed and enter the reactor in an entraining jet which induces the recirculation and which is also allowed to impinge on the walls of the reactor.

Group II Technical Note, ICI Central Instrument Research Lab.

S F Bush

Summary

A number of principles to be followed in the design or modification of chlorinators is given. Design is intended to facilitate both optimum start-up and steady running.

Group II Research Note, ICI Central Instrument Research Lab.

S F Bush with P A Sinclair

Summary

An important method of manufacture of 1.1.1.-trichloroethane proposed by Mond Division is the vapour-phase chlorination of 1.1.-dichloroethane made from liquid-phase hydrochlorination of VC,

CH₂ = CHCl + HCl → ^fast → CH₃.CHCl₂.

For this process the organic feed is conveniently brought to a preferred reaction temperature of 400-410 ^oC by mixing with reacted product, the whole to run autothermally. Factors limiting design on the 25,000 tons per year scale are (a) the stability of the reaction to external disturbances, particularly flow disturbances and (b) the difficulty of ensuring that the preferred steady reaction temperature can be sustained even with no flow disturbances.

The theory and models worked out for the chloromethanes process have been applied to this proposed new process with a view to (a) predicting if a process based on a chloremethanes-type reactor would be satisfactory on the scale proposed, and (b) if so, what size should the semi-technical reactor be to give adequate experimental proof of the design and the control scheme. The intricate programming and many attendant calculations have been carried out by P A Sinclair.

This note gives an interim account of the conclusions reached in this study, which is continuing, and which has involved several recomputations from changed kinetic data.

See also the section on Development of new Products and Processes.

Group II report, ICI Central Instrument Research Lab

S F Bush

Summary

Sudden cessation of reaction has been observed in reactors in which fresh feed gas is brought to reaction temperature by the entrainment of hotter, partially reacted gas. Observations have been made on the plant-scale and pilot-scale versions of these reactors. On the pilot-scale, the instability was induced by small measurable disturbances in the inlet flows. A mathematical analysis of the dynamics of the reacting system is used to predict the range of conditions for which steady reaction is possible, the magnitude of the disturbances necessary to extinguish steady reaction once established, and the influence of geometrical factors on the stability of the reaction system. Within the limits of the analysis, the predictions are in reasonable agreement with experimental results on both the plant-scale and the pilot-scale reactors. The results of the study have been used to modify the internal design of the plant reactors in order to obtain greater stability of reaction. So far as is known the occurrence of unexplained temperature collapses has been largely, if not completely, eliminated.

Group II Research Note, ICI Central Instrument Research Lab.

S F Bush with D Grant

Summary

Results of experiments carried out in small silica and pyrex vessels are presented, continuing the work reported in Part I of the series. The results are correlated roughly by means of a radical-molecule mechanism. A model based on this mechanism allows a prediction of the results on the full-scale to be carried out.

The effect of increasing surface-age on the reaction has been determined with reasonable reproducibility. A large number of by-products have been detected and chromatograph assignments made. In general, the by-products represent a small part of the organics conversion to carbon tetrachloride.

Group II paper, ICI Central Instruments Lab (CIRL), for CIRL/Mond Division Symposium, Runcorn Heath, paper 5.

S F Bush

Summary

This paper contains a review of a number of aspects of gas-phase reactor design which have been encountered in the course of determining, and subsequently employing, the kinetics of methane and methyl chloride chlorination. Kinetic studies based on reaction in a paddle-stirred reactor are summarised together with results obtained from a mathematical model of the reactor. This model has led to the design of a jet-stirred reactor and preliminary results obtained from this device are also given. Finally, the problems surrounding the design of tubular chlorinators of the Rocksavage type are outlined and partial solutions described.

Group II report, ICI Central Instrument Lab.

S F Bush

Summary

Two trains of methyl chloride chlorinators are currently in operation at Rocksavage Works. The chlorinators in one train (stream A) have internal diameters of 26 inches; the chlorinators in the other train (stream B have internal diameters of 32 inches. The second train works well, but the first train produces unacceptable quantities of soot and hexachloroethane. Experiments carried out on 1/9 linear scale models indicate that the different geometry is one cause, at least, of the different performances of the models and this conclusion holds over a large range of throughputs. The evidence for this view is summarised in this report.

Preliminary report from Group II, ICI Central Instrument Research Lab.

S F Bush

Summary

The possible used of stirred reactors at Rocksavage depends on their performance relative to that of the existing plant and proposed extensions. The conclusions of this report are therefore based on estimates made by the author and by Mond Division of the performance of a train of chlorinators and on estimates made by the author of the performance of stirred reactor trains. These estimates are made with respect to conversion and capacity, and suggest a simple model of the chlorinators which will be refined and presented, together with more general information, in a later report. It must be emphasised that the estimates made here depend on somewhat scanty information about the plant performance and on kinetics derived from a small stainless steel stirred vessel. However, whenever possible the least favourable, but still realistic, data have been used in estimating. Thus it is thought, and has been assumed here, that the steel walls of the stirred reactor would yield a lower overall rate of reaction than the brick lined walls of the chlorinators, and that kinetics so based would thus yield a pessimistic estimate of plant capacity.