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Sep
23

Batch vs. Continuous Control and Optimization – Part 3

Reactors are the pivotal unit operation that sets plant capacity and efficiency besides product quality. Reactors provide good examples of the common and distinctive opportunities for the PID control of batch, fed-batch, and continuous unit operations. Here we examine pressure, temperature, and endpoint control.

The pressure control of batch, fed-batch, and continuous reactors may have a similar vent system. All gas pressure loops on vessels tend to have an integrating response unless the pressure drop across the vent valve is extraordinarily high providing some self-regulation. Even then the response may be near-integrating. In some cases there may be a split range transition between the vent and nitrogen valve at the beginning or end of the batch or the startup or shutdown of a continuous plant.

The PID pressure controller for batch and fed-batch reactors is faced with a decreasing vapor space volume as the level rises making the integrating gain faster. The vapor load is also quite variable increasing at high temperatures and high reaction rates for gas reactants demanding more corrective action.

Similarly, the temperature control of batch, fed-batch, and continuous reactors may have the same jacket or coil cooling and heating system but the batch reactor poses additional challenges in terms of varying phase volume and reaction rate. Batch and fed-batch temperature have an integrating response whereas continuous temperature has a near integrating response, unless an exothermic heat release with temperature exceeds the cooling system capability in which case all types of reactors can develop a runaway response.    

Adaptive tuning of both pressure and temperature as a function of level (liquid and gas volume) and temperature (reaction rate) may be helpful.

There are more setpoint changes with batch and fed-batch reactors whereas setpoint changes in continuous reactors are generally relegated to startup, shutdown, and grade changes. There could be a batch, fed-batch, or continuous startup, shutdown, and grade change profiles. A highly exothermic reaction rate with a propensity of undesirable side reactions was carefully optimized by temperature profile control as described in pages 47-54 of the Online eBook  A Funny Thing Happened on the Way to the Control Room 

Fed-batch and continuous reactor production rate can be maximized by valve position controller (VPC) monitoring each key coolant and vent valve position that may be limiting reactant feed rate. In order for increases in fed-batch feed rate to translate the increases in production rate the batch must be permitted to end sooner. The setup of PID loops for basic control and optimization with VPC are shown in slides 3-6 in the attached excerpt Batch-and-Continuous-Reactor-Control-and-Optimization from my tutorial on reactor control at ISA Automation Week 2011.

There is also an opportunity for end point control for fed-batch and continuous reactors when one of the reactants is a gas and the product is in the liquid phase as shown in slide 7 of the excerpt from my tutorial. The pressure controller manipulates the gas reactant flow automatically matching gas reactant feed with demand preventing the accumulation of excess reactant in the vapor space. A purge from the vapor space gets rid of inerts. Here the term “endpoint control” describes continual matching of reactant feeds to reaction end point. In batch and fed-batch operation, the term “endpoint” denotes a process variable (typically composition) reaching its final desired value signifying the batch is done.

The time available in a continuous reactor for reaction can be controlled by a residence time controller that is a level controller whose setpoint increases with total reactant feed rate to maintain a specific residence time (ratio of volume to total feed rate). For batch and fed-batch reactors, the reaction time is set by a cycle time since there is no level control and no discharge liquid flow until the batch is done.

In the final part 4 of this series we will see that heat transfer and composition control can be quite different for batch and fed-batch versus continuous reactors.