Operational Improvements and Process Control

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A beneficial source for saving ideas is the improvement of the operating parameters in a plant. In many cases the potential is high and the necessary investment rather low or even zero. Some examples for operating parameters are reflux ratios in columns, recycle and purge ratios, pressure levels and temperatures or solvent concentrations as well as reduction of reactant excess.

The best tool for the improvement of these parameters is the analysis of a process model (see Section 4.3.5). It is not so laborious to optimize the parameters in a model by a sensitivity analysis in comparison with plant or lab experiments. An experienced engineer is able to find improvement ideas (not only for energy savings) performing numerical experiments in the model and assessing the results.

An example: We modeled a complete polymerization process with reaction and processing. The reactor was cooled by boiling off the reaction mixture, and the gas stream from the reactor was condensed and recycled to the reactor. Our model showed a high concentration of by-products in the loop and in the reactor, until then unknown to the plant staff. This by- product came into the plant with one of the monomers. With the help of the model we could find out the optimal localization of a purge stream and its recommended size. The existing purge was modified according to our suggestion. As a consequence the by-product concentration decreased as predicted by the model. This measure reduced the energy consumption and led to a higher capacity of the plant.

Process control is also an important method for improved energy efficiency. The first step in our projects is to check all process control loops. In some cases we see that distillation columns are operated at high, constant reflux flows and the process control is switched off. Here we have to convince the operators to switch on the installed control loop. The development of energy awareness and an energy conscious mindset in the team (see Chapter 3 of this book) is therefore a crucial part of energy saving projects.

In the second step our process control experts identify potential savings which are possible by implementing additional loops.

An example: The reflux ratio determines the energy consumption of a column. We suggested an advanced process control loop in order to come to a more exact control of the product quality. The implementation was successful. The variations of the product concentration decreased considerably. The effect is shown in Figure 4.9.

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