DISTILLATION TURNDOWN

Chemical plants developed in the 1900s traditionally operated at fairly fixed capacities. Large tanks for feed material and products were used to handle swings in supply and 

Distillation Design and Control Using As/ en Simulation, Second Edition. William L. Luyben. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

If the chemical plant must operate over a wide range of throughputs, the distillation columns in the plant must be able to turn down to low throughputs. Hydrauhc limitations will be unavoidably encountered, so the control structure must be able to avoid these limitations. In this chapter, we assume that the minimum vapor rate is the limitation. 

The inverse problem was studied by Kanodi and Kaistha where the objective was to maximize throughput. In this situation, the limitation is a maximum vapor flow rate. Several strategies were explored for adjusting the feed flow rate to the column so as to operate at the maximum vapor limitation. 

The objective in this chapter is to avoid minimum vapor flow rate limitations. If feed rates to a distillation column are reduced due to a reduction in plant throughput or a diversion of the feed stream to a more economical product (i.e., hydrogen gas to combustion turbine during peak power demand instead of methanol), the distillation control structure needs to be robust enough to handle these disturbances while maintaining product specifications. Three alternative control structures are explored to obtain this objective. 

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At low vapor rates, valve trays will weep. Bubble cap trays cannot weep (unless they are damaged). For this reason, it is generally assumed that bubble cap trays have nearly an infinite turndown ratio. This is true in absorption processes (e.g., glycol dehydration), in which it is more important to contact the vapor with liquid than the liquid with vapor. However, this is not true of distillation processes (e.g., stabilization), in which it is more important to contact the liquid with the vapor. 

For distillation columns Add sufficient trays to account for disturbances and anticipated expansion of production. The expected turndown ratios affect the choice of internals in a distillation column with a large turndown ratio, suggesting the use of bubble caps a low turndown ratio may point to structured packings. At the same time, safety and hazard analysis indicates that we want a minimum of liquid holdup in the system. 

Sieve trays have wide applications due to lower capital cost and less maintenance required. However, valve trays can promote froth regime and thus achieve distillation efficiency up to 20% higher than sieve trays (Anderdson et ah, 1976). Furthermore, valve trays can achieve 3 1 turndown compared to sieve trays in 2 1. However, the disadvantages of valve trays are higher pressure drop and cost. 

Tube plugging, 348 Tuning instruments, 510 Turndown problems (distillation tower), 387—388... 

In addition to entrainment flooding, we need to be concerned with the turndown ratio, the maximum vapor flow divided by the minimum vapor flow. This ratio is important because it determines the flexibility of our distillation column. In an ideal world, we would want to operate our separation at full capacity all the time. In a real world, we will not always have a high, steady demand for our product, or we may have a fluctuating feedstock. In this real world, we may want to run at less than full capacity without the possibility of problems like weeping. 

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