Substituting the above in equation (4-1) gives w T , /\ _ = _c ln[_R j
: Equation (4-2) shows that the ratio W/QR can be expressed I the log of the pressure ratio.
.n terms of iEither the temperature difference, or the pressure ratio, can be used | to measure the potential of applying the heat pump technique. ¡Compared to the relationship in equation 4-1, the relationship of ! equation 4-2 is more complex and more difficult to express in terms I of readily available parameters such as column top and bottom temperatures; for this reason, the temperature difference provides ?. better indicator of the potential economics of applying the heat pump technique.
APPLICATION GUIDELINES The following guidelines have been recommended for the application of. heat, pumps in distillation:
(i) Equation (4-1) indicates that a high temperature difference between the top and bottom of the column is likely to lead tc a high W/QR ratio, and therefore, to a high compression work requirement for achieving the reboiler duty. Heat pur.p application is therefore most likely to be economical when this temperature difference is low.
It has been stated (8,23) that heat pumping is best suited to columns whose bottom to top temperature difference does not exceed about 65°F. This temperature difference has been identified as the prime factor for determining heat pump economics (8,16). Results from an extensive economic study by Null (13) support these findings, but show that temperature differences greater than 65°F can be economical for new refrigerated columns, while the temperature difference needs to be smaller than 50°F for heat pumps ir. new, water-cooled columns to be economical.
(ii) The greater the reboiler/condenser duties, the more attractive the heat pump becomes. This- can be expected, since the smaller the quantity of heat that can be saved, the less likely it is to achieve attractive payouts on the capital required for a compressor system. These findings are also supported by results from Null's economic analysis (13) and comments by others (16,25,32). Some workers (21,23) propose that generally, heat pumps are unlikely tc be economical when the reboiler duty is lower than 5-7 mm Btu/hr; however, Null's study (13) shows that this may not always be the case.
(iii) Heat pumps are most attractive in services where chilled water or refrigeration are used for condensation (13,14).
(iv) A heat pump can be an economic alternative to a steam-heated column which is condensed by cooling water or air (7,13,14,16,17,29,32), but is unlikely to be an economic alternative when a water (or air) cooled column is reboiled by a waste-heat medium (7,17,29).
(v) Heat pump application is far more economically attractive for new plants as compared to retrofits (8,13,14).
(vi) Low column pressure drop, low exchangers and line pressure drop along the vapor path, and low temperature difference across the reboiler or condenser, greatly enhance the attractiveness the heat-pump system. Even small reductions in these can bring about major reductions in compressor power consumption. This can be inferred from equation (4-1), keeping in mind that the greater the pressure crop, the higher is the bottom to top temperature difference. The application of devices designed to enhance heat transfer such as hi-flux tubing (16,17), or use of low pressure drop exchangers and column internals, can be greatly beneficial.
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(vii) Other factors which may also improve the attractiveness cf heat pumps are shortage- cf steam ar.d cooling water (15,32) .
(viii) Zxergy analysis is sometimes used to determine the attractiveness of a heat pump system (11,19). This a-p-zrozc'r. however, often fails to take economics into account, anc rr.ay give misleading results (19).
(ix) For steam-heated columns, heat pump economics is more favorable in plants where steam is used mainly for heating, as compared to plants in which steam is used both for heating and for driving turbines (13,14,32). "his is because when steam is used mainly for heating, low pressure steam is almost as expensive as high-pressure steam, while if steam is used also for driving turbines, low pressure steam is cheaper.
(x) Application of heat pumps is more difficult in vacuum than in superatmospheric services for two reasons: First, -he pressure drop inside the column has a large effect on the temperature difference between column bottom anc top. Second, air and compression seal gas leakage causes inerts to be present in the reboiler-condenser,. and these must be allowed for by recycling more vapor through the compressor. Both these increase compressor energy consumption and unfavorably affect the economics of the heat pump. Use of low pressure drop column internals, as well as paying attention to the compressor sealing design can help improve the economics. Pilot studies (21,37) showed successful application of heat pumps under vacuum, and studies (21,3") shcv-rd good payouts can be achieved upon scale-up.
In industrial practice, the most common application of heat pumps is in new designs for superfractionators. These columns usually perform close separations at high reflux ratios, and therefore typically have a small bottom to top temperature difference, and high reflux and reboil ratios, which suits guidelines (i) and (ii) above.
This technique originates from evaporator technology, where multi-effect arrangements are common. The technique is based on the principle that thermodynamic efficiency can be enhanced by a repeated use cf a giver, quantity of energy.
Each scheme in Figure 4.10 separates a feed stream into two product streams. In each of these schemes, energy is introduced into the reboiler of the high-pressure (HP) column, and the overhead streair. of the HP column reboils the low-pressure (LP) column, thus re-using this -energy. Theoretically each arrangement in Figure 4.10 can be made up of several effects; in practice, it is rare to use more than two effects. Compared to a conventional (single-effect) column, the double-effect arrangement can save roughly half the heat input, but !it also uses roughly twice as many trays and auxiliaries and is a more complex arrangement.
I Multi-effect distillation is best suited for new plants, but has beer. I successfully implemented in revamps (33). For maximum effectiveness, this technique is often coupled with some of the previous techniques such as feed preheaters. Several configurations of the double-effect technique are available; the most common types are discussed below for a two-product separation. Theoretically, multi-effect distillation can also be applied in a 3-product separation system, but it is uncommon. The more common technique used for this type of system is heat integration (4.2.9).
! CONFIGURATIONS Figure 4.10 a-f illustrates configurations commonly !used for double-effect distillation. The simple split feed scheme j (Figure 4.10a) splits the feed into two roughly equal streams, with i each stream feeding one column. Each column produces about half of the total distillate and half of the total bottom. The columns operate at different pressures, with the high-pressure column overhead stream reboiling the low-pressure column. Compared to a conventional (single-effect) column, each column contains about the same number of trays, but is smaller in diameter because it only receives half the conventional column throughput. The energy I consumption of this double-effect arrangement is roughly half that of a conventional column. This arrangement was studied in detail by Tyreus and Luyben (34).
A heat-pumped variation of the split-feed arrangement (Figure 4.10b) compresses the LP tower overhead to provide reboil for the HP tower. Compared to a heat-pumped single-effect tower, the heat-pump compressor in arrangement 4.10b has to compress the vapor over about twice as large a pressure difference, but it only needs to compress half the flow. A case study using this arrangement was described by Patterson and Wells (7).
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