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(iv) Trim condenser or trim reboiler are often used because the condensing and boiling- duties of- the column are net perfectly matched. These devices are also very helpful for startup. A trim condenser is more common when the product is withdrawn as liquid, while a trim reboiler is more common when the product is withdrawn as vapor, but other factors such as presence of a discharge cooler and the feed conditions are also important in determining which one of these (if any) is needed.

(v) Discharge cooler. When the condensation heat duty exceeds the reboiling heat duty (as is often the case when the distillate is withdrawn as liquid), the trim condenser may have quite a large duty. If the duty is supplied by a refrigerant, there is an incentive to minimize it. Depending on the compressor discharge temperature, an air, water or lower grade refrigerant can be used to desuperheat the vapor, thus reducing the refrigeration demand at the trim condenser. An innovative variation of the discharge cooler scheme which improves flexibility, and reduces potential fouling and pressure drop in the vapor line has been proposed by Quadri (17).

(vi) A subcooler is sometimes used when the temperature of liquid leaving the reboiler can be further reduced by a close-approach cooling using a readily available cooling medium. This reduces the amount of flash-vapor generated, and therefore, the compressor power consumption. The reduction is usually relatively small.

(vii) A flash separator is often included to improve the stability and controllability of the system by providing some surge capacity for reflux flow. A pump may be included, but is usually not needed.

(viii) A product condenser is sometimes included when a liquid product is desired, and withdrawal from upstream of the reboiler is more attractive than from downstream of the reboiler for heat economy or heat balance reasons.

CONFIGURATIONS The common heat pump configurations are as follows:

(i) Open, overhead ("vapor recompression") heat pump

(Figure 4.8(a)). This configuration has already been described above. One disadvantage of this system is the possibility of contamination from leaking compressor oil. Oil leakage can contaminate both top and bottom products, and in low-temperature services can also cause flow restrictions in the column. In most cases, this does not constitute a major problem (10).

(il) Open, bottom ("reboiler flashing") heat pump

(Figure 4.8(c)). In this arrangement, liquid from the botter. cf the column is expanded to a low enough pressure sc that it chills to 2 temperature sufficiently low to condense the column overheads. The condensed overheads stream provides reflux to the column, while the vaporized bottom stream is compressed to provide reboil vapor.

The energy savings achievable with the open bottom heat pump are of the same order as those achievable with the open overhead heat pump. The potential for contamination is lower with the open bottom heat pump, because any leaking oil is likely to only affect the bottom system. In general, it has been suggested (8) that the open bottom heat pump is the favored arrangement at high pressures while the open overhead heat pump is the favored arrangement at low pressures.

(iii) Closed-loop ("Indirect") heat pump (Figure 4.8 (d)). In this configuration, the fluid used for reboiling and condensing is circulated in a closed loop, separate from the column fluids. Thermodynamically, this system is less efficient compared to the open heat pumps, because the heat pump fluid must be compressed from a pressure low enough to provide sufficient condensation £T in the condenser to a pressure high enough to provide sufficient boiling AT in the reboiler. Thus, the heat pump must overcome two temperature approaches, compared to only one in each of the open schemes. On the other hand, this configuration dees not suffer from the potential contamination problems th=-may affect the open schemes. Concern over such contamination is the main reason why this system is used.

(iv) Intermediate vapor recompression. .This configuration is similar to (i) above except that the compressed overhead vapor is used for interreboiling instead of reboiling. Although this system was shown to be attractive in some circumstances (8,22,40), and even superior to the system in (i) above (40), it is rarely applied in industrial practice. One study in which compressed overhead vapor was used to preheat the feed was also reported (36) , but for the system considered the configuration was unattractive (36).

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Reformer General Arrangement

bottom Product

FIGURE 4.8(e) GENERAL ARRANGEMENT

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