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Chapter 5Whisky Distillation by Heat Pump
Sing first that green rem ote CockaigneWhere whiskey rivers ru
n,A nd every gorgeous number m ayBe laid by any one;WH Auden
5.1 Whisky distillationTh e empirical art of whisky distillation
is underlain (49) by the followingchemical sequence:
H , 0 H ? 0(C6H1005),> > > C,,H,,O,, > > >
2(C6H,,0,) > > > 8CH,CH,OH+ 4C0,s t a r c h m a 1 t s u c
r o s e y e a s t g l u c o s e z y m a s e e t h y l c a r b o
n
e n z y m e e n z y m e a l c o h o l d i o x i d eThe starch in
malted grain is hydrolyzed (mashed) to a sucrosesolution called
wort. Fermentation of the wort with yeast leads viaglucose to a
water a lcohol wash which is double distilled, first in a
washstill, then in a sp irit still. This is done for purification
an d to increase theconcentration by volume of alcohol from 7
percent (wash) to 22 percent(low wines) and an eventual 70 percent
(spirit).There is doubt about the M in the formula for starch, and
itsthermodynamic properties are not tabulated in (42), although
thesubsequent sugars and the alcohol are so tabulated. The grain
sprou tingor malting process generates the enzyme (organic
catalyst) for thehydrolysis process, and the yeast supplies the
enzymes for fermen tation.The distillation requires steam,
traditionally from a boiler. In this case,however, the alternative
is used: a simple steam-based prototype heatpump to supply the wash
still steam. Electricity with a hydroelectric
component is fairly stable in price, but fuel oil for the boiler
fluctuates
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102 Power and P rocess - an Exergy Approachfour to one in price,
so that at minimum oil price the boiler iscompetitive.The pure
exergy in the electricity supply is grafted on to a lowtemperature
energy flow, with some exergy losses in the simple
heatPump.Alternatively, the fuel exergy is degraded in the boiler
to steam atabout 2 bar, Tsat 33.5 "C. Before the practical losses
begin, the Carno tefficiency is only 108.5/406.65=26.7 percent. Yet
still the boiler cancompete!
5.2 Steam heat pumpsSince the steam power plant regenerative
cycle is very highly evolved an ddeveloped, an inversion of that
cycle suggests itself as a n efficient heatpum p. The inverse of a
bled steam turbine is a fed steam compressor. Theinverse of a feed
heater train is a multi-stage flash cooler adapted from
thedesalination equipment of Chapter 2 . The requirement is to
producesaturated steam for a quasi-continuous batch process, so
that thereheaters an d superheaters of the power plant are
irrelevant. A saturatedsteam cycle with an infinite number of feed
heaters has equality ofefficiency with a Carnot cycle working
between the same temperaturelimits. An infinite number of cooling
stages should confer perfection(reversibility) on the hypothetical
heat pump. The temperature entropydiagram and flow sheet of a
notional but practical high-performancefour-stage regenerative
steam heat pump are shown in Fig. 5.1.Starting with the ideal
infinite version, the cycle is represented by thecontinuous path
DCBA. The heat source is a large flow of river or seawater, on the
tube-side of a heat exchanger. Water sprayed on the shellside
generates an atmosphere of dry saturated steam at the
compressorintake. The line A D is equal in length to BC so that the
cycle generatesno entropy. A D does not imply that the intake steam
is wet, but that itsflow magnitude is such that AD =BC. At each
compression stage thesteam is spray desuperheated. The finite
process is isentropiccompression XY, followed by desuperheat YZ.
The infinite process ha sto be imagined! The practical process has
five desuperheat sprays whichcool mixed compressor an d flash
cooler steam feeds. If the fifth spray isomitted, the end product
will be slightly superheated dry steam .Note that the specific
volume of steam at ambient temperature is veryhigh, there being a
factor of 19 between 20C and 80C, so thecompressor would be large
and expensive (as are the low pressure ends ofpower plant turbines)
if the source temperature is chosen low. Hence in
