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In reality, the latent heat in the LP column will be greater than in the HP column, but the reflux ratio will be lower. | These effects need to be taken into account when determining

| the relative quantities of feed processed by each column.

| APPLICATION GUIDELINES The following guidelines have been | recommended for the application of multi-effect distillation:

| (i) Double-effect distillation substantially increases (with the 1 split-feed arrangements it roughly more than doubles) the temperature difference for the separation. Increasing the | temperature difference is constrained by several factors.

| The maximum temperature (at the bottom of the HP column) may

| be restricted by the psuedo-critical temperature of the j mixture, excessive product degradation, temperature of the available heat sources, the possible need to switch to a higher-grade heating medium (eg, MP or HP steam instead of LP steam). The minimum.temperature (at the top of the LP column) may be restricted by the condensing medium (eg, | cooling water) temperature, or the need to switch to a

| higher grade cooling medium (lower temperature refrigerant).

j These constraints imply that more than two effects are !

| rarely justified (this would further increase the ;

! temperature difference), and that double-effect distillation j is mainly applicable to columns where the temperature j difference between the bottom and top is not large. These comments apply to split-feed double effect distillation, and to a lesser extent also to the forward and backward-feed double-effect distillation. I

(ii) Double-effect distillation represents a significant increase in capital costs, and in order to be economic, it needs to effect large energy savings. This can only be achieved when the reboiler duty is large; further, the greater the rebciler/condenser duties, the more attractive double-effect distillation is likely to become. It has been suggested that a reboiler duty of at least about 3 0 MM Btu/hr in conventional distillation is needed for double-effect distillation to become economical (8,34). This technique is therefore most likely to be applicable where column feed rate, reflux ratio and latent heat of vaporization are high (8,34).

(iii) Guidelines (i) and (ii) above single out superfractionatcrs as the main type of service for which double-effect distillation is likely to be attractive.

(iv) Double-effect distillation competes with heat pumps. In many situations, both show comparable economics (7,34,38), but in others (33,34,36) one is at a distinct advantage. Compared to double-effect distillation, the heat-pump has important simplicity advantages, and is often preferred. On the other hand, double-effect distillation has the advantage of being able to eliminate rotating machinery with the maintenance problems and potential oil leakage problems it creates, and also offers the advantage of easier integration of other energy saving techniques such as preheating.

(v) Due to the large investment involved, the economics of double-effect distillation is very sensitive to the cost of energy and the payout period.

(iv) Controlling double-effect distillation is difficult because of the heat integration between the columns, with disturbances transmitted from one column to the other and sometimes back and forth. Often, auxiliary boilers and auxiliary condensers are used to overcome this problem (42). The control problem is discussed in detail by Tyreus and Luyben (42).

! 4.2.8 Thermally Coupled Distillation

'A conventional distillation system for separating 3 components A, B, C (Figure 4.ila,b) separates one component first, and then the remaining two. In thermally coupled systems (Figure 4.11c,d), the initial separation is between components A and C, which are easiest to separate, forming an AB and a BC cuts, which are then separated downstream in a binary manner. These binary separations may be carried out in single column (Figure 4.11c,d), which is essentially a : double-effect ternary column which uses direct contact heat transfer instead of an intermediate condenser-reboiler. A distillation system |contains direct coupling when (46)

j (i) A heat flux is utilized for more than one fractionation, anc

(ii) The heat transfer between fractionation sections occurs by direct contact of vapor and liquid.

Brugma (47) invented the thermally coupled scheme in Figure 4.11c. The variation shown in Figure 4. lid was proposed by Petîyuk et al (48), and studied in detail by Stupin and Lockhart (46,47). This scheme offers a greater degree of thermal coupling and uses only one reboiler and condenser. Other variations were reported (23,47,48) for 3- and 4-component separations.

jAn analytical model of arrangement 4.lid is shown in ¡Figure 4.12 (47). Components A and B are separated in the upper 2 ¡sections of the second column. The separation is essentially binary, since all the C component is in the lower sections of the second column. Similarly, B is separated from C in the lower sections of

'the second column in a binary manner, with no interference from A.

: The overall reboiler heat required with the conventional scheme is ; that required to separate A from BC plus that required to separate B | from C (alternatively, that required to separate AB from C plus that ; required to separate A from B) . In the thermally coupled scheme, the separation of A from B is carried out by reused heat, sometimes at the expense of the heat needed to perform the easier separation of ABC into the AB and BC cuts. A design procedure is available in ; Reference 47. Overall, the heat requirement is generally lower for 'the thermally coupled scheme compared to conventional distillation, 1 ; and energy savings of the order of 20 percent, and up to 40 percent |were reported in some applications (46-48) without increasing the j

: total number of stages (46,47). An analysis by Cahn and DiMiceli !

j reported in Reference 47 showed both energy and capital savings for a j |thermally coupled scheme. Thermally coupled distillation can j

|therefore be regarded as a lower-cost alternative to the conventional | ¡system (23,46-48). It has also been shown (46,47) that flexibility i in terms of vapor and liquid flows for handling variations in feed I compositions can be incorporated into the design of thermally ccuplec | systems.

figure 4.11(a) conventional scheme í

figure 4.11(a) conventional scheme í

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