IGURE 4.7 TWO STAGE V*. CONVENTIONAL CONDENSATION
4.2.6 Heat Pumps
In a conventional distillation column, heat is applied in the bottom reboiler and removed in the top condenser. Commonly, heat is applied by steam in the reboiler and rejected to cooling water or air in the condenser. This represents an energy degradation from the temperature of the heating medium generation system (in case of steam, the combustion temperature) to ambient temperature. This system owes its existence to simplicity, low investment, and cheap energy (7).
The heat pump, on the other hand, takes the energy from the condenser, and uses mechanical compression work to upgrade the energy to a level sufficiently high to be used in the reboiler. The only energy that is totally degraded is the - ->t work required to upgrade the condenser energy, plus the lost ene - from cor.' srting fuel to steam, and then into the mechanical recc ression wcrk. In general, a heat pump has the potential of saving msiderable energy, but at the expense of significant capital and an increase in complexity. Thermodynamically, the heat-pump concept is identical to refrigeration.
The heat pump concept can be implemented in a distillation system by one of many configurations. The open, overhead heat pump scheme (Figure 4.8(a)), is perhaps the most common. Column overhead vapor is compressed to a pressure sufficiently high to enable condensation at a temperature high enough to effect satisfactory boiling in the reboiler. The reboiler performs the dual -function of condensing the overhead vapor while supplying column reboil. The condensed overhead stream is then flashed across a control valve to provide column reflux. Distillate product can be withdrawn either as vapor from the compressor discharge or as liquid downstream of the reboiler-condenser.
AUXILIARIES Figure 4.8b shows the same flow scheme, but with several auxiliary equipment items. These are items which may or may not be included in heat pump circuits to improve energy saving or control. At least some, but rarely all of these items are always included in heat pump schemes. When evaluating a heat pump scheme, it is important to ensure that the appropriate items are included; failure to do this can sometimes lead to erroneous conclusions. Ref 10 presents a case history demonstrating that an analysis reported in a previous paper (11) reached an incorrect conclusion because it did not adequately consider an interreboiler in the heat-pump circuit.
The main auxiliaries sometimes included in a heat pump circuit are:
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