Heat-integrated columns

The last example discussed in this chapter has fairly simple phase equilibrium but has a complex process structure. Two columns are operated at two different pressures so that the condenser for the high-pressure (high-temperature) column can be used as the reboiler in the low-pressure (low-temperature) column. 

To achieve the required temperature differential driving force in the condenser/reboiler, the pressures in the two columns must be appropriately selected. The low-pressure column Cl operates at a pressure of 0.6 atm (vacuum conditions, 456 mmHg) that gives a reflux 

A reasonable differential temperature-driving force is about 20 K. If the AT is too small, the heat-transfer area of the condenser/reboiler heat exchanger becomes quite large. The pressure in the second column is adjusted to give a reflux drum temperature of 367 - - 20 = 387 K. The pressure in C2 is 5 atm. The base temperature in C2 at this pressure is 428 K, which will determine the pressure of the steam used in this reboiler. 

The tenfold increase in energy prices in the 1970s spurred efforts to reduce energy consumption in chemical and petroleum plants. Heat integration was extensively applied to achieve very significant reductions in energy consumption in distillation columns. There are a host of alternative configurations that have been built in industry. We discuss below several of the most widely used process structures and their control schemes. 

This structure is normally used only to separate a binary mixture containing no LLK or HHK components. The presence of these components would lower or raise the temperatures at the two ends of the 

The feed split is approximately 50/50. but slightly more feed goes to the low-pressure column if the separation is easier at lower pressure. The system as pictured runs neat. i.e., all the heat available from condensing the vapor from the high-pressure column is used to reboil the low-pressure column. In some systems auxiliary reboilers and/or condensers are used to balance the heat loads both at steady state and dynamically. 

The system represents a 4 X 4 interacting control problem since there are four product compositions to be controlled at each end of both columns. Reflux flowrrates control the distillate purities in each column. Bottoms purity in the high-pressure column is controlled by manipulating the heat input to the reboiler. Bottoms purity in the low-pressure column is controlled by manipulating the fraction of the feed that is fed into the low pressure column. 

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There are several ways to account for variable pressures. If the total pressure of the column changes but not the pressure drop through the trays (the normal situation in heat-integrated columns, particularly with valve trays whose pressure drops are fairly constant), an approximate variable-pressure model can be used. 

Figure 6.24 Heat-integrated columns, (a i Feed split binary) b) light-split reverse [binary) (e) prefractionator reverse (ternary).

The block of inter-linked columns offers robust simulation of a combination of complex distillation columns, as heat-integrated columns, air separation system, absorber/stripper devices, extractive distillation with solvent recycle, fractionator/quench tower, etc. Because sequential solution of inter-linked columns could arise convergence problems, a more robust solution is obtained by the simultaneous solution of the assembly of modelling equations of different columns. 

By thermal coupling the heat is transferred by direct contact between vapour and liquid flows that connect sections of different columns. This is a major difference with heat integrated columns , where the heat exchange takes place by condenser/reboilers. Hence, thermal coupled columns have a more complex behaviour. Figure 11.21 illustrates two basic arrangements with side-columns. The first is the side-rectifier, derived irom a direct sequence. The second one is the side-stripper that corresponds to... 

The above is a typical example of a spin in a heat-integrated column. Similar principles and corrective actions are frequently used for tackling other heat-integrated spin problems. 

Figure 13.76 illustrates another example of a highly heat-integrated column that was reported to be troublesome at startup (357). The problem occurs when the column is taken out of total reflux and forward feed is started. A rapid rise or fall in product flow rate... 

Figure 13.7 Examples of heat-integrated columns that can experience startup problems, (o) A typical cryogenic gas plant demethanizer that can experience cold spins (b) a heat-integrated column that can experience surging.

TABLE 5.2 Comparison of Single and Heat-Integrated Columns... 

Now, we want to look at the dynamics of this complex flowsheet. There are three major issues that must be addressed in designing a control system for a heat-integrated column process that is operating under neat conditions. Auxiliary reboUers or auxiliary condensers are not used to balance the vapor boUup needed at the base of the low-pressure column with vapor condensation needed at the top of the high-pressure column. 

W. L. Luyben, Heat-integrated columns, in Practical Distillation Control, van Nostrand-Reinhold, 1992, p. 492. 

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