Tower Optimization Basics

As discussed extensively in Chapter 4, it is important to define major operating parameters or key indicators as they can describe the process and energy performance. A key indicator can be simply an operation parameter like desirable product rate, column overhead reflux ratio, column overflash, column temperature, pressure, and so on. By the name of key indicator, the parameter identified is important and has a significant effect on process and energy performance. 

Optimal reflux rate in operation depends on the operating margin, which is defined as the difference of product sales minus feed cost and energy cost. When energy cost is too high, it could drive the operation toward lower reflux rate and vice versa for the case of lower energy cost. 

2 Overflash Overflash is defined as the ratio of internal reflux at the feed 

Overflash is typically controlled between 2 and 3%. An operation policy focusing on throughput would operate a tower at very low overflash it is not uncommon to observe that a tower is operated at close to 1 % overflash. This low reflux operation could be beneficial if the tower produces intermediate products, which will be processed further downstream. In this case, this operation could lead to energy efficiency as well as high economic margin. 

3 Pressure Lower pressure typically saves energy. This is because the lower the tower pressure the less heat required for liquid to vaporize and thus less energy required. This results in better fractionation as it is easier for vapor to penetrate into liquid on the tray deck. 

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To introduce the way of designing passive compensation systems, we have first sketched the optimization strategy to be applied. A discussion of the basics of compensators follows. Then a simplified model of high and slender structures helps to optimize compensators at high towers. Finally some of the most important results are listed. 

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