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Reaction: a novel simulation package for batch chemical reactors.

Reaction: A Novel Simulation Package for Batch Chemical Reactors

Chemical manufacture by batch production forms a major and highly profitable sector of the chemicals industry. With the present trend towards batch production of high-added-value products (pharmaceuticals, fine and speciality chemicals etc.) this sector of the chemical industry as a whole is rapidly expanding.

The majority of these processes are carried out batchwise or semi-batchwise in large steam-heated, water-cooled stirred pots. A multi-purpose plant usually has a number of such reactors producing a variety of established and development products at any one time. Many companies face the problem of scaling up a process from bench to full scale production, often with limited time available for pilot-scale process development and optimisation. The application of computer-aided design (CAD) as an aid to solving these problems is an obvious step.

Simulation can be of considerable use at all process development and design stages. A process model enables a large number of alternatives to be examined quickly. Increasingly stringent safety legislation will also require the manufacturer to carry out more detailed safety studies. The cost of a successful simulation experiment is the same as that for a failure, and the limits of safe operation can be determined without any risk to equipment or personnel.

A number of computer-aided design packages are available for various aspects of plant design, flowsheeting, piping, instrumentation and layout. These packages are mainly intended for engineers concerned with the simulation of steady-state operation of continuous plant. They are of limited use in batch processing where processes are characterised by non-steady state operation, often involving complex chemistry, carried out with a combination of reagent addition sequencing and temperature profiling.

Even if the benefits of simulation are recognized most engineers and chemists with no formal training in process modelling, reaction engineering or programming would find the development of a dynamic process simulation incorporating feed sequencing and temperature control a daunting, if not impossible, task. The time required to derive the algebraic and differential equations describing the process chemistry and heat transfer in and out of the reaction vessel, and then to produce and test a computer program to solve these equations can amount to several weeks work. Simulation languages do not offer an immediate solution. To model a process all the mathematical equations required to describe the system must be developed, and then a computer program written. With many simulation packages intended for process simulation a Fortran module must be written by the user if a rigorous or complex model of process chemistry or operating strategy is required.

To meet the specialist needs of the batch processing industry, an English process engineering software company, BatchCAD, has developed a general reactor modelling package which offers a novel solution to the problem of batch-process simulation. The package, Reaction, interprets the users specification of process chemistry and operation, and creates from this all the underlying mathematics. This innovative approach therefore gives any company access to the benefits of process simulation, not just those companies who have the resources to employ specialists trained in modelling.

Dynamic simulation package Reaction (REACtor simulaTION) has been developed with the capability of modelling the reaction kinetics and dynamic heat and mass balances of almost any jacketed batch, fed batch or continuous (CSTR) reactor system with the option of simultaneous multicomponent distillation for virtually any set of chemical reactions. The package uses an intelligent syntax analyser to lead the user through the model construction stages by prompting for all the necessary information. The input data required for a complete process model is reduced to: . a specification of the reaction scheme, . process data, . selection of operating conditions and associated control


The system allows any size reaction vessel to be simulated, ranging from laboratory glassware to full scale production plant. A complete batch sequencing and temperature-control system is included, allowing the simulation of batch, semi-batch or continuous reactor operation with open- or closed-loop temperature control for a variety of heat transfer fluids. Any process parameters may be edited at any time during the simulation allowing all aspects of dynamic process design, operation and safety to be explored.

Control system Complex sequences of feed additions and offtakes to the reactor can be specified as a series of step flow changes. Thus the reactor can be operated in batch, semi-batch or CSTR mode. The reflux ratio on the condenser can similarly be changed.

The reaction temperature is controlled by feedback regulation of steam or coolant supply or by controlled addition of reagents. To accommodate the wide range of methods used in industry for heating and cooling, the control system and hence control strategy may be quickly configured and changed. The package has been developed with four basic heating/cooling options. These are: . alternate steam and water flow through the jacket with

an option to use deadbands on the controllers to stagger

the switchover point. . heat transfer fluid flows through the jacket at a constant

rate. Fluid temperature can be varied. . fluid at a constant supply temperature passes through

the jacket at a controlled rate. . the addition rate of a feed stream is regulated, with a

constant flow of a coolant through the jacket.

Three term digital PID control algorithms are used throughout. The controllers may also be used in open-loop, enabling manual operation of the plant to be simulated. The supply temperature, pressure and characteristics of the heat transfer fluids can be defined and thus these basic options can be configured to simulate a range of other industrial control techniques, such as hot oil jackets, freons and direct injection of liquid nitrogen into the reaction vessel.

Graphical output The package provides full graphical output of all process variables together with a comprehensive process report. Graphs can be rescaled and colour highlighting is used throughout to give a clear understanding of many aspects of the process. In addition all the simulation results can be written to files suitable for post processing with spreadsheets. Some of the available graphs are listed below: . concentration profiles of all species of reactor, vapour and

condensate, . yield expressed as selectivity or conversion, . temperature profiles for reaction mixture, jacket,

condenser film, condenser coolant, . rate of heat generation by reaction and rate of heat

transfer to jacket (thermal stability), . condenser cooling duty, . total pressure and partial pressures of each species, . mole fractions, . services including valve actions, steam and coolant

consumption, . heat transfer coefficients, viscosities etc.

Changes to process development Perhaps the most significant aspect of this software is the speed at which a sophisticated and accurate process model can be produced by untrained personnel. The simulation described in the reference was reported as taking six-man weeks to develop the mathematics and test the software. The same exercise can be carried out using Reaction in under ten minutes. In another instance the research director of a large British chemical company stated that during a sales demonstration of Reaction, in which the company provided limited experimental data for one of their processes, the equivalent of over 18 months experimental development effort was achieved in about two hours.

The implications of this technique for batch process development are very significant. Simulation can now be applied to virtually any process with potential for enormous savings in development costs. One European chemical manufacturer has estimated this saving to be in excess of 25% of the overall costs.

The software has been marketed throughout Europe and is being used successfully by both large and small companies involved in the development and manufacture of fine chemicals, pharmaceuticals, pharmaceutical intermediates, agrochemicals, flavours and fragrances. Reaction is used at all stages of process development, from laboratory experimentation through scale up and safety studies to optimisation of the full production process. The software is now being marketed in North America and Canada through Logiciels BatchCAD d'Amerique du Nord.
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Author:Bramfitt, Victoria
Publication:Canadian Chemical News
Date:May 1, 1989
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