Complete Heat Integration

Next we smdy the case where the two columns are fuUy heat integrated and no auxiliary reboiler or condenser is used. The only energy input is the heat-transfer duty in the 

The Design Spec and Vary feature in each of the column blocks in Aspen Plus is used to adjust the bottoms flowrate to achieve the desired product purity in each column. The specifications for the product purities are the same in all cases. The water product from the base 

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With complete heat-integration, the heat removal in the condenser of the high-pressure column Qc2 must be equal to the heat input to the reboiler of the low-pressure column. Therefore, a degree of freedom is lost, and we can only set the reflux ratio on one column, not both as is the case with partial heat integration or no heat integration. 

With the reflux ratio fixed in the low-pressure column, there are three variables being used (Bi, B2, and RR2) to drive the two product compositions to their desired specifications and to make the heat duties equal in magnitude but opposite in sign. 

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We begin with the THF-water system. A comparison of systems with no heat integration, with partial heat integration and with complete heat integration will be presented. The phase equilibria for this system and a nonheat-integrated system have been discussed in Chapter 5. 

These results clearly demonstrate that heat integration can result in very significant reductions in energy consumption. However, as we demonstrate in the next section, the dynamic controllability of the completely heat-integrated system is not as good as that of the nonheat-integrated system. 

The dynamic controllabilities of the three designs considered above are compared in this section. Implementing heat integration in Aspen Dynamics requires the use of Flowsheet Equations. First we look at the case with complete heat integration. 

Pressure-swing systems are very amenable to heat integration because of the inherent temperature differences in the two columns. Complete heat integration is the most economical from a steady-state economic point of view. However, a considerable dynamic controllability penalty can occur, which can be greatly reduced by using partial heat integration. 

Intermediate values for C m can be obtained from a numerical integration of equation (10.158). When all are put together the complete heat capacity curve with the correct limiting values is obtained. As an example, Figure 10.13 compares the experimental Cy, m for diamond with the Debye prediction. Also shown is the prediction from the Einstein equation (shown in Figure 10.12), demonstrating the improved fit of the Debye equation, especially at low temperatures. 

Soft constraints. In Figure 19.2b, there was a situation in which there was some flexibility to change the temperature at which a filtration takes place. These are termed soft constraints. There is not complete freedom to choose the conditions under which the operation takes place, but there is some flexibility to change the conditions. Another example of a soft constraint is product storage temperature. There is sometimes flexibility to choose the temperature at which material is stored. How should such soft constraints be directed to benefit the overall heat integration problem ... 

Constraints (10.1), (10.2), (10.8)-(10.14), in conjunction with the overall plant scheduling constraints, constitute a complete MILP formulation for direct heat integration in batch processes in a situation where the batch size is allowed to vary at different instances along the time horizon of interest. 

Figure 4.11 present the complete flowsheet together with the control structure. The reaction takes place in an adiabatic tubular reactor. To avoid fouling, the temperature of the reactor-outlet stream is reduced by quenching. A feed-effluent heat exchanger (FEHE) recovers part of the reaction heat. For control purposes, a furnace is included in the loop. The heat-integrated reaction system is stabilized... 

In Design 4, the feed was sent to the low-pressure column, which produced a pure low-boiler distillate but a mixed underflow (again, a sloppy separation decreasing the temperature difference across the low-pressure column). The mixed bottoms was then completely separated in the high-pressure column. The condenser of the high-pressure column was the reboiler of the low-pressure column (heat integration in the opposite direction as flow). 

The correct placement of a distillation colunrn with respect to the background process is either above or below the Pinch, (Fig. 11.10). If the column is completely above the Pinch, the process can supply the heating load, while the condenser duty can be rejected back into the process at lower temperature. This operation does not modify the energetic requirements, but the distillation column will operate at zero energy cost The same demonstration is valid when placing a column completely below the Pinch. On the contrary, both hot and cold utility consumption is increased for a distillation column placed across the Pinch. From practical viewpoint a distillation column can be moved in a direction compatible with better heat integration just by shifting the pressure. 

The sorption isotherm will than be the sorption isotherm of amorphous cellulose, intimately connected with its swelling. The total amount of water bound at a given pressure and the total heat given out upon complete wetting (integral heat of sorption), will be proportional to the amount of amorphous fibre substance. 

Evaporator systems are major pieces of process equipment and are often purchased on a total responsibility basis. This is especially true of vapor compression and highly heat integrated systems. Specific design information and fabrication are often proprietary to vendors. Evaporator manufacturers generally are rather specialized. Few offer a complete range of evaporator types some specialize in one type only. 

A novel reactor design called Hot Finger that addresses problems of catalyst deactivation has been patented by Protensive. The reactor allows for some degree of axial heat integration. The unit is illustrated in Figure 5.28, detailed in Figure 5.29 and as a complete unit in Figure 5.30. 

In the following sections the fuUy heat-integrated system is compared with a completely nonheated-integrated system and with a partially heat-integrated system. 

The second column in Table 6.2 gives tuning parameters. The TC2 temperature controller is also retuned because of the somewhat small heat duty in the high-pressure column with partial heat integration. These tuning parameters are very similar to those found in the completely nonheat-integrated system, so we anticipate that control should be good. 

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