Get the Most Out of Your Batch

The Same Techniques Used to Get the Most Ethanol Out of a Scarce Corn Crop Can Help You Optimize Other Batch Processes

By Greg McMillan

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Cool Solutions for Hot Times

If the cooling tower can't keep up with cooling demand for hot summers and higher yield corn, the higher-than-normal temperature adversely affects the yeast, decreasing the fermentation rate. The peak in the cooling demand can be monitored, and the start of fermenters staggered to space out the peaks and even out the cooling load. If the fermenter temperature is controlled by manipulating the outlet temperature of the heat exchanger in a recirculation line, you can estimate the cooling rate as simply the exchanger inlet temperature minus the outlet temperature multiplied by the recirculation flow. If the recirculation flow does not change, the exact value is not important, since we are looking at when the cooling rate slope reverses sign, indicating a peak. The exchanger inlet temperature is the fermenter temperature, so new sensors are not required. To synchronize the temperatures, the inlet temperature can be sent through a dead time block to simulate the transportation delay through the exchanger.

General Methodology

An off-line or at-line feed analyzer is used to compute the yield of a key raw material. For a continuous front-end or a fed-batch, a process variable is computed that uses the predicted yield to provide an equivalent product flow rate. The production rate controller immediately cuts back on the actual raw material feed rate for a measured increase in predicted yield. An enhanced PID is used to deal with the variable update time from at-line and off-line analyzers. If the batch takes less or more time than normal to reach the endpoint, a portion of the inferred change in yield is used to correct the feed analyzer.

If a periodic analysis is not available during the batch, a cooling rate may provide an inferential measurement of the conversion rate in chemical reactors and in fermenters for alcohol production. An oxygen uptake rate for biological reactors can provide an inferential measurement of cell growth rate for pharmaceutical production.

Continuous on-line and relatively frequent at-line analyzer measurements are inputted to a dead time block to create a continuous train of old measurements. A new measurement minus an old measurement divided by the dead time is the slope of the concentration profile. The dead time is set large enough to provide a good signal-to-noise ratio.

From the slope near the end of the batch, the additional product produced in the dead time interval or the analysis time interval for sampled measurements is computed. The slope (e.g., conversion rate, cell growth rate, product formation rate) near the end of the batch is used to make an economic decision about whether the batch should be terminated for extra capacity or extended for extra yield.

Next, the slope is converted to product mass flow and multiplied by the analysis time interval to get the additional product mass for the given dead time or analyzer time interval. The current product mass in the batch divided by the mass of each key raw material added to the batch gives the yield in terms of product for each key raw material. The additional product mass per batch is divided by this yield and multiplied by the cost per unit mass of each key raw material. The results are summed to arrive at a dollar value of the additional product by extending the batch.

The current product mass in the batch divided by the current batch time in hours offers an estimate of the current batch production rate. Alternately, the production rate can be used from the flow controller based on predicted feed yield. The production rate multiplied by the profit per unit mass, and finally multiplied by the dead time or analysis time interval gives an estimate of the value of additional capacity by terminating the batch.

The analysis time interval should be shortened to be just large enough for a good signal-to-noise ratio near the end of the batch to make the optimization more accurate. If the analysis of the profile of a key composition or product is not available until after the batch has been transferred, the results can be cautiously used for the next batch. 

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