In the process optimization blog category, most posts highlight ways to optimize continuous processes. Many of these processes are energy intensive and energy efficiency is often the objective of optimization efforts.
But what about batch processes? I saw an email exchange on this subject and wanted to share some key points that Emerson’s John Dolenc made. You may recall John from several posts about system modernization and project justification methods. From John’s experience, most objectives for batch process optimization center on capacity improvements.
A major opportunity area is to eliminate non-productive time normally caused by waiting for the operator to perform an action. Plant engineers can identify opportunities to automate the batch sequences and to automate the manual activities. These activities can include:
- Motors starts through the control system
- All valves are opened and closed using air actuated valves automatically activated through the control system
- Use of automatic control loops versus manual valve manipulation based on operator observations
A second source of optimization that improves batch cycle time is batch reaction process optimization. One way to accomplish this is through better control of raw material additions and control of the reaction conditions. This can include running the reaction at maximum constraint conditions, and normally includes additional instrumentation and control loops over that originally used.
While optimization may increase the reaction rate, a larger increase of capacity can be obtained by eliminating batch adjustments done at the end of the batch to meet product specifications. In studies John and the Consulting Services team have performed, they have found that a high number of batches require adjustments at the end. Many process manufacturers even build in one adjustment into the batch procedure.
Eliminating adjustments is accomplished by correctly performing the batch sequences, especially those affecting the reaction, such as high accuracy raw material additions and tight temperature / pressure control.
Energy efficiency-related objectives have not been common in the projects John has seen. He notes that most energy reductions are produced by eliminating (or reducing) the number of batch adjustments that need to be done at the end of a batch. This elimination / reduction reduces the time the batch contents needs to be held at elevated temperatures, or the times the batch needs to be cooled to add additional raw material and re-heated to achieve reaction temperature.
Surprisingly, many batch processes still use manual methods to add heat to batch reactions. Many times this can lead to overheating of the contents and in some cases actually send raw material into the overhead system. Energy can be saved by using automated temperature control that can be based upon temperature, heating media flow, or overhead pressure measurements. John and the consulting team have developed batch strategies that use different temperature control methods dependent upon the current phase of the reaction process.
However, the real benefit is from producing an on-specification product, thus eliminating the need for adjustments at the end of the batch–not from energy savings.