Control of alcohol distillation

Control of multiple distillation columns for producing anhydrous alcohol suitable for blending with gasoline to produce gasohol. The distillation process involves production of a first-stage distillate containing a predetermined amount of water, followed by azeotropic distillation in the presence of a hydrocarbon entrainer to strip the distillate of its water content, leaving anhydrous alcohol as a bottom product. Tight controls are present during first-stage distillate production to hold its proof at an optimum value derived at through material balance calculations to minimize energy consumption for the overall system. Control over the dehydrating stage is accomplished by a combination of ratio control to regulate and maintain the proper proportion of the entrainer and temperature control to regulate within the column the actual inventory of entrainer. This latter control loop includes adjusting the rate of boiling so as to be sufficient to keep the hydrocarbon out of the alcohol product and yet not high enough to drive alcohol overhead out of the column.

FIELD OF THE INVENTION 
This invention relates to the control of distillation columns in general, 
and particularly to the control of such columns for producing anhydrous 
alcohol, especially pure ethanol suitable for blending with motor fuel. 
BACKGROUND OF THE INVENTION 
The commercial production of alcohol by distillation has been in widespread 
operation for many years. Control systems for assuring product quality 
within reasonable efficiency limits have paralleled the growth of this 
industry. In the past, most of the alcohol distilled was for beverage 
purposes, and accordingly, there was no crucial requirement for a 
dehydrated end product, thereby alleviating to some extent both the energy 
required to distill the alcohol and the need for tight controls over the 
process. However, the rising cost of energy has focused attention on the 
need for better product optimization of energy intensive (endothermic) 
processes such as distillation through the application of dynamic control 
strategies. 
Attempts to alleviate energy dependence on petroleum based fuels have been 
directed to the use of renewable energy source. One such technique 
involves the production of ethanol from grain for blending with gasoline 
to form the motor fuel "gasohol". To be effective as an alternative energy 
source, the process by which the ethanol is produced must minimize energy 
consumption so as to achieve "a net energy gain," and moreover final stage 
ethanol should be essentially anhydrous. 
The net effect of these circumstancees is to place an increased emphasis on 
the systems controlling the operation of the distillation unit so that the 
desired end products can be produced with minimum energy drain. In the 
specific example of pure ethanol production, the problems are compouned 
because, as is well known, ethanol and water form an azeotrope whose water 
percentage is unacceptable for use in making gasohol. To separate the 
components of the azeotrope, further process steps over and above 
conventional distillation are required involving the expenditure of more 
energy and rather expensive entrainer substances. All of this adds 
detrimentally to the overall cost of making the finished product to 
desired specifications. 
Although a variety of techniques have been applied in the past for the 
control of alcohol distillation columns, these methods usually rely on 
simple temperature measurements in conjunction with single-loop flow and 
level controllers. Generally these loops respond to conditions occurring 
external of the columns which are used to predict internal column 
activity; however, such control loops are slow responding and, most often, 
not truly representative of the dynamics within the column. Such 
techniques have thus had difficulties in maintaining effective control 
over the process, most notably during upset process conditions. This 
results in particularly significant deficiencies in bringing about 
unacceptable alcohol losses, high energy consumption, contaminated end 
product or combinations of the above. Therefore, at present there is a 
pressing need for an improved process for controlling the production of 
anhydrous alcohol through a multi-stage process involving a combination of 
conventional distillation and azeotropic distillation. 
SUMMARY OF THE INVENTION 
The present invention is directed to the control of multiple distillation 
columns for the production of anhydrous alcohol. The control system is 
readily implemented with either pneumatic or electronic instrumentation 
and provides a dehydrated, uncontaminated end product with minimal overall 
energy consumption and minimal loss of alcohol. 
In a preferred embodiment of the invention to be described below, 
production of anhydrous ethanol from grain is accomplished in three 
stages, the first involving stripping the alcohol from the solid grain, 
the second including passing the just derived alcohol vapor to a 
conventional distillation rectifying column to produce a distillate of 
predetermined product proof, with the third stage being a dehydrating 
column (dehydrator). This final column involves introduction under 
controlled conditions of an azeotropic entrainer, such as pentane--a 
volatile hydrocarbon. As is known, it is highly desirous to precisely 
match the proper molecular proportions of entrainer to the amount of water 
entering the dehydrator. 
Control over the stripping and rectifying stages of the process is achieved 
by proper regulation of steam supplied to the first stage column (beer 
still) which drives alcohol vapor overhead and leaves solids and water at 
the bottom of the column. Rectifying column control is more or less 
conventional, but the proof of the distillate is closely regulated 
because, as will be explained in detail subsequently, product proof at 
this stage is an important parameter in minimizing energy consumption. 
Control over the dehydrating stage is accomplished by a combination of 
ratio control to regulate and maintain the proper proportion of the 
entrainer and temperature control to regulate within the column the actual 
inventory of entrainer. This latter control loop includes adjusting the 
rate of boiling so as to be sufficient to keep the hydrocarbon out of the 
alcohol product and yet not high enough to drive alcohol overhead out of 
the column. 
Considering the control system with more particularity, near the top of the 
rectifying column vapor pressure is compared to that of azeotropic alcohol 
at about 190 proof. Although feasible to produce at this stage in the 
process a drier product, the effect of having a greater amount of water 
entering the dehydrator is predetermined such that the boilup rates of the 
beer still and dehydrator combined are lower than would have resulted from 
the dehydrator having to accommodate a higher proof feed. Thus an 
important part of the overall control of the azeotropic production and 
dehydration stages involves considering the two in concert so that the 
overall energy consumption for the system is minimized. 
Within the dehydrator, the control system is constructed so as to maintain 
the dynamic inventory of hydrocarbon entrainer in the top part of the 
column. This is accomplished by sensing the temperature near the top and 
the middle of the column. A large differential temperature indicates 
insufficient hydrocarbon in this region and results in the generation of a 
control signal which raises the amount of entrainer entering as reflux. A 
differential-vapor-pressure sensor at the bottom of the column detects 
excess hydrocarbon and through a controller increases the rate of boilup 
to the column driving the hydrocarbon upward. 
By performing material balance calculations for each column of a given 
production plant, the optimum proof of alcohol that is supplied to the 
dehydrating column can be found and controllers adjusted accordingly to 
minimize energy consumption of the distillation system as a whole. 
Other aspects and advantages of the present invention will become apparent 
in light of the following description read in conjunction with the 
associated drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Process Overview 
FIG. 1 is a schematic diagram of an azeotropic distillation process plant 
for the production of anhydrous alcohol from fermented grain, otherwise 
known as beer. For ease in explanation, the plant as shown is divided into 
three major areas, each with its own column, namely, a beer still 10 where 
solid mash is stripped of its alcohol; a rectifying column 30 for 
producing azeotropic alcohol of 190 proof; and a dehydrating column 
(dehydrator) 50 from which anhydrous ethanol (200 proof) is separated as a 
bottom product. In actuality, the rectifying column is placed in close 
thermal proximity, most often on top of the beer still, to enhance the 
heat transfer characteristics involved in both stripping alcohol from the 
beer and volatilizing the remaining liquid to produce the desired proof 
azeotrope. There is a feed preheater-heat exchanger 40 associated with the 
beer still. It is reboiled with open steam, whereas the dehydrator has a 
separate reboiler 52. The columns are heated via steam lines 12, 54. 
Regulation of heat energy supplied is accomplished by appropriate 
manipulation of individual steam valves 14, 56 respectively located in the 
lines 12, 54. 
The remainder of the process plant (aside from the arrangement of the 
control system) includes conventional apparatus such as pumps, condensers, 
accumulators, valves, and the like whose operation and use is well 
understood by those of skill in the art. Specific comments concerning 
various functional details of such items as they relate to the overall 
control strategy will be deferred until later. For enhanced 
representation, a graphic distinction is made between the conduits for 
transferring product from one column to another (indicated by the 
double-thick lines) and the measurement and control lines (shown with 
narrow width lines). The various symbols and nomenclature used on the 
drawing are conventional and well known to those in the art, but 
nevertheless are included in the chart of FIG. 1A. 
Turning now to an overview description of the process, the feed entering 
the beer still 10 is, as previously noted, a mixture of fermented grain 
containing about 10 percent ethanol. The beer is preheated in the heat 
exchanger 40 against stripped stillage (i.e., the bottom product) before 
being transported to the top of the still by a conduit 18. The 
construction of sieve trays 16 for such a still is well known to those 
skilled in the art, primarily involving perforations sufficiently large so 
as not to plug with grain. However, as will be explained, this 
construction limits to a narrow range the amount of permisible vapor flow. 
Steam is introduced at the bottom of the column to boil the beer still 
through the valve 14. 
After sufficient heating, a mixture of ethanol and water vapor richer than 
10 percent ethanol by weight is carried out of the top of the beer still 
10 to the bottom of the rectifying column 30 by means of a conduit 22. The 
rectifying column is similarly of conventional structure with the 
low-ethanol content vapor traveling upward in contact with a downward flow 
of liquid from an accumulator 20 that refluxes the beer still. The effect 
of this distillation is to produce an ethanol-rich vapor at the top of the 
column. 
Also occurring during the rectifying stage is a buildup of undesired 
azeotropes called fusel oils whose boiling point is between ethanol and 
water. To prevent undesirable accumulation and hence boilover to the 
overhead product, a conduit 28 near the bottom of the column withdraws 
periodically or continuously a sidestream from which fusel oil is 
transported to an accumulator (not shown) for recovery. 
The overhead vapors of the desired proof ethanol are passed out of the 
rectifying column 30 through a conduit 34 to a condenser 36 where they are 
cooled and collected in the accumulator 20. No phase separation between 
ethanol and water occurs in this condensed state. The liquid in the 
accumulator is withdrawn by a conduit 42 where it intersects a main 
conduit 44 linking the rectifier 30 and the dehydrator 50 at a "T" branch 
46. Here, depending on the positioning of a pair of control valves 45, 47, 
part of the ethanol-water mixture is returned to the rectifier as reflux 
and the remainder is fed to the dehydrator along the conduit 44. 
Within the dehydrator 50, a predetermined amount of entrainer initially 
stored within an accumulator 58 is introduced as reflux through a control 
valve 59 by means of a conduit 60. This forms a ternary azeotrope whose 
boiling point is below ethanol and which contains a water-to-ethanol ratio 
higher than the feed, thereby resulting in the preferential distillation 
of water (together with entrainer) overhead, with anhydrous ethanol left 
as bottom product. After passing through a condenser 62 connected by a 
conduit 64 to the top of the dehydrator, the subcooled liquid separates 
into two layers within the accumulator 58. The light, top layer, of 
predominantly entrainer is returned as reflux via the conduit 60, whereas 
the heavy, bottom layer of predominantly ethanol and water is recycled 
along a conduit 66 to the rectifying column 30. 
Control Systems 
An inspection of FIG. 1 immediately highlights the control instrumentation 
used in the production process, with the thin-lined measurement and 
control signal paths emanating from and/or terminating at the circled 
representations of the various portions of the instrumentation package. 
The larger circles whose identifiers end with the letter "C" depict 
controllers, while the smaller circles represent transmitters. At this 
point it may be helpful to review the definitions found in FIG. 1A. It 
should be noted that the control system for this installation is 
implemented with pneumatic transmitting instruments and controllers of the 
type readily commercially available from a variety of manufacturers, as, 
for example, The Foxboro Company, as listed in its General Catalog #577. 
However, the nature of the control instrumentation is irrelevant to the 
working of the present invention, as it may just as easily be implemented 
with analog or digital electronic controls. 
The primary control over the beer still 10 rests with the amount of steam 
supplied to boil the beer resulting from a control loop 9 contained within 
the dashed lines. To assure that sufficient steam is being added to strip 
the ethanol from the grain to thus keep the ethanol out of the bottom, a 
differential-vapor-pressure transmitter 11 with a water-filled temperature 
bulb 13 is placed near the bottom of the still. Such transmitters are 
often used in distillation control and are commercially available from The 
Foxboro Company as Model No. 13VA. This unit is essentially a 
differential-pressure measuring device whose low-pressure measurement is 
attached to a temperature bulb filled with a sealed reference fluid of 
known composition containing the substance and its vapor, about which 
control is to be effectuated. A high pressure connection indicated at 
point 15 is maintained at the same elevation as the bulb 13, thus allowing 
a proportional pressure difference to be transmitted along a control line 
17 to a differential-vapor-pressure controller 19 whose set point is 
maintained slightly above zero differential vapor pressure. Hence, in this 
specific instance, if alcohol being drifting down the column, the vapor 
pressure at that point in the column will exceed that of the water in the 
bulb, a signal will be sent to the differential-vapor-pressure controller 
producing an error signal which results in the controller generating an 
output signal to appropriately increase the level of steam entering the 
column. 
As shown, high and low limits are set into the differential-vapor-pressure 
controller 19 to regulate the maximum and minimum amount of steam which 
can flow through the steam valve 14 into the still. Such limits would, of 
course, vary according to the internal structure of the particular still. 
However, generally such limits are required to prevent too much steam flow 
which would result in solids being carried up and out into the rectifying 
column 30. On the other hand, if too little steam is supplied, this would 
result in too little a vapor flow along the trays causing the feed to weep 
through the perforations. The concomitant reduction in liquid level on the 
trays would allow ethanol to drop to the bottom and escape unrecovered 
with the stillage. In either event, once the internal design of the still 
is known, the desired limits can be set by observing the pressure drop 
across the trays. 
After the ethanol-enriched vapor enters the bottom of the rectifying column 
30, it encounters a downward flow of reflux of condensed ethanol/water 
vapor stored in the accumulator 20 after having been withdrawn from the 
top of the column 30. Reflux is controlled by the level of liquid in the 
accumulator 20 as determined by a differential pressure transmitter 23 
which regulates the positioning of the control valve 45 to adjust the 
amount of reflux required in relation to the withdrawn product. Meanwhile 
the concentration of ethanol in relation to the amount of water in the 
vapor withdrawn is controlled by the rate of its withdrawal. The controls 
required are included within the dashed lines as signified by the 
reference numeral 25. The composition being withdrawn is sensed by a 
second differential-vapor-pressure transmitter 27 and its associated 
controller 29. It is possible to approach an azeotropic composition of 
95.6 percent ethanol by weight but is more practical to produce at this 
stage 90 percent ethanol as sufficient water will be necessary to form a 
separate phase in the accumulator 58 of the dehydrating column 50. This 
desired proof is controlled by having the temperature bulb 31 of the 
differential-vapor-pressure transmitter 27 filled with 95 percent ethanol. 
The exact concentration of ethanol in the bulb is not crucial, because 
vapor pressure in the range of 95 to 100 percent is fairly constant. 
If the amount of ethanol is being withdrawn too rapidly, this will be 
sensed by a decrease in vapor pressure at the top of the column. The 
corresponding controller output signal is cascaded as the set-point input 
to a flow-indicating controller 33 whose measurement input is derived from 
the combination of the differential pressure transmitter 23 and an orifice 
plate 35. The output of the flow indicating controller closes the control 
valve 47 to reduce the amount of withdrawal. Just the opposite effects and 
corresponding actions occur if too little ethanol is withdrawn. 
The setpoint of the differential-vapor-pressure controller 29, which is the 
desired proof of the ethanol-water mixture, is chosen to take into account 
the energy consumption needs of the beer still and that of the dehydrator. 
Although allowing more water to be contained in the overhead vapor at this 
point requires more steam in the dehydrating stage, concurrently less 
steam will be required in the beer still. The optimum point may be chosen 
for each individual process plant, bearing in mind the number of trays in 
the particular column, their efficiency and the type of hydrocarbon 
entrainer utilized. 
Although simultaneous control of product composition at remote ends of a 
distillation column (in this case the stripping of alcohol from beer at 
the bottom of the beer still 10 by control of heat input and the 
production of relatively impure ethanol at the top of the rectifier 30 by 
control of withdrawal rate) does result in a degree of interaction between 
the two control loops, it is possible using the procedures outlined by 
Shinskey in his book entitled "Distillation Control for Productivity and 
Energy Conservation" to predict the severity of the interaction. In the 
embodiment being described, the relative gain between the two loops was 
calculated to be in the range of 0.9 to 1.0. Relative gains approaching 
unity indicate very little interaction is present between the two loops. 
In fact, the control of ethanol content in the top product may be 
controlled to any desired concentration over a wide operating range. 
The feed is admitted near the top of the dehydrator column by the conduit 
44. A signal representing this feed rate, indicated schematically by the 
letter "F," is shown entering a multiplier unit 67 whose operation will be 
more fully explained presently. Azeotropic distillation by addition of 
pentane or other suitable hydrocarbon as an entrainer has been the subject 
of recent study. For example, a paper by C. Black, entitled, "Distillation 
Modeling of Ethanol Recovery and Dehydration Process for Ethanol and 
Gasohol" in the September 1980 issue of Chemical Engineering Progress 
discusses in detail how to calculate the amount of hydrocarbon required to 
effectuate separation. According to Black, for an incoming feed to a 
stripper column (i.e., dehydrator) containing about 7.5% water and 
utilizing a pentane entrainer, a nineteen-tray dehydrator would produce 
the desired separation if an entrainer-to-ethanol ratio of approximately 
3.0 (mole basis) or 4.69 (mass basis) were maintained. To assure success 
in the event of upset conditions, past control strategies involving 
azeotropic distillation have been geared around adding excess amount of 
hydrocarbon as reflux to bring about the desired separation. However, even 
this is uncertain during periods of upset, and as previously mentioned 
above, energy inefficient, as all the entrainer must be heated to the 
boiling point and vaporized. 
An important aspect of the control system of the present invention is that 
the amount of entrainer introduced as reflux in the dehydrator column 50 
is adjusted depending on the concentration of hydrocarbon (e.g., pentane) 
within the column itself. At initial conditions, with all compositions 
assumed constant, the reflux-to-feed ratio needed for water removal is set 
as calculated according to the water content of the feed. This is 
accomplished by introducing the feed flow signal into the multiplier unit 
67. However, such a setting is approximate being contingent on accuracies 
of flowmeters and constant compositions. To bring about the desired 
"on-line" control, a feedback loop centered around a pair of temperature 
bulbs 69, 71 located respectively near the top and at mid-column point of 
the dehydrator 50 is added. Composition profiles, as indicated in the 
above-mentioned Black article, have shown that the concentration of 
pentane, and therefore column temperature, varies little from the top of 
the column to past the mid-column point (see FIG. 2). Hence, any 
significant temperature difference sensed by the bulbs 69, 71 will cause a 
differential-temperature controller 75 to send an appropriate feedback 
signal to the multiplier unit to increase the rate of reflux from the 
accumulator 58. This is done by a reflux flow loop 57 consisting of a flow 
indicating controller 73 receiving inputs from both the multiplier unit 
and a differential pressure transmitter 77 in concert with an orifice 
plate 79. The output of the controller 73 manipulates the control valve 59 
to adjust the rate of reflux. 
Within the accumulator 58, a liquid phase separation occurs between the 
entrainer (the top layer) and the water-rich ethanol on the bottom as 
shown by the reference numeral 81. The level of the aqueous layer in the 
accumulator 58 is controlled by manipulation of its rate of withdrawal, 
with the interface 81 being sensed by the buoyant force acting on a float 
83 suspended therein. The location of the interface is sensed by a level 
transmitter 85 which acting through an associated controller 87 regulates 
the amount of water/ethanol mixture withdrawn through a control valve 89 
by the conduit 66 for recycling in the rectifying column 30. The present 
arrangement can thus be seen to include enough water in the feed to 
produce a phase separation in the accumulator 58 relatively soon after 
startup, but to prohibit excess buildup of the aqueous layer to prevent 
overfill and subsequent flooding of the dehydrator. Although not 
specifically controlled, the level of entrainer is free to move as its 
inventory shifts between the dehydrator column and the accumulator. 
Therefore, periodic losses of hydrocarbon solvent will have to be made up 
by introduction of new solvent in the decanter. 
In order to keep the entrainer out of the bottom product, i.e., the 
anhydrous alcohol, a third differential-vapor-pressure transmitter 91 is 
located near the bottom of the column 50. The bulb 93 of this transmitter 
is filled with pure ethanol, and hence a rising vapor pressure at this 
position will indicate the presence of entrainer. A signal is then sent to 
a differential-vapor-pressure controller 95 whose output is in turn fed to 
the steam valve 56 to adjust the rate of steam passing through the 
reboiler 52 for entry to the dehydrator column. The effect of this control 
loop is to drive the entrainer upward. It should be remembered that the 
differential-vapor-pressure transmitter 91 is insensitive to the presence 
of water at this level in the column; therefore, two composition control 
loops are required, with water removal accomplished by manipulation of 
reflux, as described above. 
Anydrous alcohol is withdrawn from the bottom of the dehydrator 50 through 
a conduit 100 while the water/ethanol mixture, withdrawn as the lower 
layer from the accumulator 58, can either be recycled into the rectifying 
column or drawn off as an end product for use in another application. 
Although a preferred embodiment has been set forth in detail above, this is 
solely for the purpose of illustration. Modifications will become apparent 
to those skilled in the art without departing from the scope of the 
present invention as defined in the accompanying claims.