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.

batch esterification process

Figure 1. The batch esterification process setup

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.

batch esterification recipe

Figure 2. The batch esterification recipe. The bottom table shows that the batch time is relatively long. There is also a significant consumption of high pressure steam which accounts for a large part of the overall cost for the process.

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.

batch esterification pressure dependency

Figure 3. System pressure dependancy. The throughs in the temperature plot for the reaction are not dependent on a reduction in energy supply to the reactor, but rather on a reduction in pressure. The system is heavily dependant on pressure.

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.

batch esterificationproduct purity

Figure 4. During the esterification stage, there is a near 100% recovery of saturated reactant B refluxed into the reactor. Once the stripping stage commences and the pressure is ramped down (at 20,100 s) we see the contributing amount of reactant B removal that drives the purity of product D.

Recipe optimisation

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

Manual optimisation

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.

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