Simulation and optimisation of a batch esterification process
This case shows how simulation and optimisation can be used to gain a deeper understanding of a batch esterification process and how its recipe can be optimised to maximise profitability. It highlights the advantages of dynamic simulation over more conventional trial-and-error/manual optimisation techniques. The batch time and utility costs are reduced while still maintaining product purity standards.
Business implications and objectives
Specialty chemical processes deliver high purity products to a range of different customers. To be able to change over from one product to another in a cost effective manner, processes need to be agile and efficient from the start. However, reducing production costs while increasing throughput and maintaining product purity is not a trivial task. The aim of this case is to simulate the process to gain futher understanding of key parameters so that it can be successfully optimised with the following aims:
- minimise batch time
- maximise throughput
- minimise cost of raw materials
- maximise profitability
The batch esterification process
The batch esterification process is shown in Figure 1. The reaction is a two-step homogeneous reaction that takes place in water. The first step, producing reactant C from A and B, is very fast while the second step, to produce the product D, is slower and follows a catalytic route.
The recipe for the process is complex, involving a rangne of process steps for reaction, reflux and storage, recovery, stripping, cooling and steam sparging as show in Figure 2.
Simulation of the batch esterification process - key findings
To gain better understanding of how key process parameters affect process performance a process a simulation in gPROMS ProcessBuilder was set up. During the steam sparging step a couple of throughs in the temperature plot were identified. These could not be explained by a reduction in energy supplied to the reactor by high and low pressure steam. In fact they were the result of pressure drops in the reactor - the system is heavily dependant on pressure as shown in Figure 3.
It was also found the temperature alone is not a sufficient driving force for purity. Purity plots of the intermediate C and product D highlight that although all of intermediate C is converted to product D by the end of the esterification stage, the process benefits greatly in pressure reduction during the stripping of reactant B and water from the reactor and driving the purity of product D as shown in Figure 4.
Based on the findings of the pocess simulation two appoaches for optimisation were used:
- Manual optimisation using a trial-and-error approach
- Dynamic optimisation using the optimiser in gPROMS ProcessBuilder
For both approaches the constraints of the processe considered.
- Operating parameters:
- the reactor pressure should remain between 0.013 - 1.07 bar at all times
- the reactor temperature: < 220 °C
- reactor holdup total liquid: < 85 m3
- reactor holdup total liquid for stable stirrer operation: 45 m3
- End of batch parameters:
- Desired product purity in the product tank of + 97 wt.%
- Minimising the amount of intermediate C
- Ensure a minimum amount of product is generated, i.e. 57 tonnes
Based on the findings of the simulation a manual optimisation of the process was carried out.
The upper limit of the temperature envelope for esterification is 220 °C before there are any adverse effects on product quality. The base case operates at a temperature far lower. Thus, performing simulations with incremental increases in the flowrate of high-pressure steam circulating the reactor jacket until the desired temperature is reached would naturally be a good option.
The manual optimisation resulted in an improved product puity. However, the consumption of expensive high pressure steam and the duration of the process also increased resulting in a batch time that was not acceptable to the operator.