Reboiler design considerations

Column process design specifies the separation, and sets column pressure, reflux, stages, and feed point. These in turn yield internal flows and reboiler and condenser duties. This chapter addresses the main column process design considerations. The column is optimized during the process design, and many times later during operation. Computer control continuously optimizes the column on-line. Both design and on-line optimization are also addressed in this chapter. 

Both the reboiling and condensing processes normally take place over a range of temperature. Practical considerations, however, usually dictate that the heat to the reboiler must be supplied at a temperature above the dew point of the vapor leaving the reboiler and that the heat removed in the condenser must be removed at a temperature lower than the bubble point of the liquid. Hence, in preliminary design at least, both reboiling and condensing can be assumed to take place at constant temperatures. ... 

Orrell, W. H., Physical Considerations in Designing Vertical Thermosyphon Reboilers, Chemical Engineering, September 17, 1973. 

Eollowing are two examples (16.1 and 16.2) of a distillation column that demonstrate the effect of applying different pairing strategies. In both examples the control loops for the column pressure and the liquid levels in the condenser accumulator and the column bottom are determined independently based on practical considerations. Thus, the column pressure is controlled by various techniques that may involve the condenser coolant rate, and the liquid levels are controlled by the product flow rates. What remains to be decided is how to pair the distillate and bottoms compositions with the reflux rate and the reboiler heat duty. The same distillation column is used in both examples, having a total condenser and a reboiler, one feed and two products. The column is designed to separate a benzene-toluene mixture into benzene and toluene products with specified purities. 

Column 2 is a sinple distillation that can be designed by the methods discussed in Chapter 4. Column 1 is considerably more cortplex, but the bubble-point matrix method discussed in Chapter 6 can often be adapted. Since the system is nonideal and K values depend on the solvent concentration, a concentration loop is required in the flowchart shown in Figure 6-1. Fortunately, a good first guess of solvent concentrations can be made. Solvent concentration will be almost constant in the middle section and also in the bottom section except for the reboiler. In the top section of the column, the solvent concentration will very rapidly decrease to zero. These solvent concentrations will be relatively unaffected by the tenperatures and flow rates. The K values can be calculated from Eq. (2-35) with the activity coefficients determined from the appropriate VLE correlation. Process simulators are the easiest way to do these calculations (see appendix to Chapter 81. 

After the process heat recovery is done, the next design step is to determine the utility supply in terms of heating and cooling, and power based on the needs and characteristics of process energy demand. In this step, the means of heat supply for the reaction and separation system will be addressed. For example, a choice for the reboiling mechanism must be made for a separation column between a fired heater and steam heater. Similarly, a choice of process driver between steam turbine and motor will be determined. Selection is made based on operation considerations, reliability and safety limits, and capital cost. Selection of process utility supply defines the basis for the design of a steam and power system. 

These results are perhaps surprising because the side-reboiler for the diabatie design introduces strong cross-coupling effects causing considerable oscillations within the column. However the improved controlled response is due to the removal of the sharp nonlinearity in the temperature profile (Fig 7). Shifts in the internal temperature, used as a measured variable in the control scheme, are less drastic producing a more even elosed loop response. 

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