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Energy efficient drying, evaporation and similar processes
Heat Recovery Systems Vol 5, No 6, pp 551-559, 1985 Pnnted m G r e a t B n t a m
ENERGY
0198-7593/85 $3 O0 + 0 00 P e r g a m o n Press Lid
EFFICIENT DRYING, EVAPORATION AND SIMILAR PROCESSES* G.
ETSU,
J.
B r a i d i n g 156, A E R E
NEWBERT
Harwell, Oxfordshire
OXll
0RA,
UK
Abstract--Wtth the asmstancc o f examples taken from the U K Energy Effioency Demonstratton Scheme, this p a p e r d e s c r i b e s energy efficient technologies which can benefit drying, evaporation and dtsttllatton processes
INTRODUCTION Drying, evaporation and similar processes are energy intensive and widely used by British Industry. These processes, which are usually to remove water from substances, use significant quantities of energy both nationally and for individual companies they are a worthwhile target for improvements in energy efficiency. The national energy used for drying, evaporation and distribution in industrial activities where they are used most Is estimated in Table I. The energy used for dying, evaporation and distillation in these industries add up to about 200 x l06 GJ/yr, the equivalent of 7.5 million tonnes of coal.
T a b l e 1 N a u o n a l energy
consumption for d r y m 8, e v a p o r a t m n and thsullation Energy use 106GJ/yr D~stillation
Evaporatmn
Drying
--3 12 2 30 0 55 --------
-24 ---n.a" I 66 0 14 28 10 3 ---
5 36 ----n.a * 2 2 ---4 5 34
--
--
89
-I 57 -n a* -59 3
-3 08 ---14 I
3 1 9 3 6 3 n.a ° 38 6 18 I
6684
3448
9976
Malt Beer M a l t whisky G r m n whisky
Other spirits W l a s k y byproducts M d k powde r s
Condensed/evaporated milk S u p r (from cane) Suga r (from beet) Beet sugar byproducts
-
Pottery Bncks Timber
Texules Laundries D r y c le a mng P a p e r and board Cho'mcals
*n a means d a t a not avmlable_
A range of energy efficient technologies can reduce this consumption. The major efficient technologies are listed here:
Drying Heat recovery using heat exchangers and heat pumps Improved instrumentation and control Optimised dryer design and operation
*Paper
first presented a t M E C C A
'85, Organised
by
Energy Systems Trade AssocmUon, London, F e b r u a r y 551
1985.
552
G. J NEWBERT Improved mechanical dewatering Radio frequency drying Multistage drying
Evaporation Heat recovery using heat exchangers and heat pumps Thermal vapour recompresslon Mechanical vapour recompression Reverse osmosis
Distillation Heat recovery using heat exchangers and heat pumps. To highlight some of the more general energy efficient technologies this paper looks at the process to make one particular product that uses dry|ng, evaporation and distillation at some stage in its manufacture. This route examines some practical methods of improwng energy efficiency in these acUvities calling on the evidence of projects supported under the Energy Efficiency Office's Energy Effic|ency Demonstration Scheme. Whilst these projects are aimed at particular industrial sectors they also show energy efficient schemes helpful more generally in other industries. M E T H O D S OF R E M O V I N G W A T E R Available methods of removing water are as follows. This is an incomplete list but it does show some of the more successful methods used by industry: (1) (2) (3) (4)
Deposition of moisture as ice or water Absorption Mechanical separanon Vaporization.
Deposition Moisture can be removed from a gas by condensing vapour onto a cold surface. This process is called dehumidification and dehumidifiers for operations of this character are now in general use. Timber drying and swimming pools are examples of applications of this idea. Water may be removed from liquids by converting it into ice and in this solid form separating it from the portion having a lower freezing point. The alcoholic content of a liquid can be readily increased in this way.
Absorption Absorption is a process in which moisture is removed by the capillary action of porous bodies. In this way a cream of clay and water used for casting pottery is deprived of its water by placing it in molds of plaster of paris. The capillary actton in the mold draws the water from the liquid clay and as a result a layer of solid clay, the thickness of whmh ts controlled by the time of standing, deposits itself on the mold. Certain types of sweets are dried in a similar way by contact with the starch mold in which they are cast. The drying effect of towels is due to the same capdlary action. Some substances have the wonderful ability to absorb moisture and this makes possible their appllcatton to drying gases and to some extent liquids. Silica gel is often seen in the packaging of some products to ensure it keeps dry over long periods of storage.
Mechanical dewatermg Some materials are of a spongy nature and hold moisture by capillarity. Large quantities of moisture may be expelled from this type of material by pressure alone. When this is possible it is desirable to get rid of as much water as posmble before using more expensive methods. Thts ts usually the case with textiles and paper at some stage in their drying. The opportunity for this depends very much on the product to be dried and examples given earher e g. pottery, textiles and paper use non-thermal methods where they can. Water which is
Energy efficient drying, evaporation and stmdar processes
553
not chemically bound to materials are readily amenable to mechanical dewatenng techmques. In textiles the most common extraction device ~s the stmple pad or squeeze roller. The energy required is that which drives the motor and turns the mangle. Assuming on entering fabric is soaking wet at about 150~ moisture content and exits at 75~ moisture content, the energy required per kg of water removed Is 50 kJ. Compare this with the latent heat of water of 2260 kJ/kg which would have to be used by simple thermal methods to dry to the same extent. While squeezing methods are energy efficient they are not always applicable. Some substances can be destroyed by squeezing. Other efficient mechanical extraction methods used in textdes are functioned on pressure processes that draw or blow the water through the fibres.
Thermal processes The most important and the most widely used process for removing water from liquids and solids depend on vaporization. This is turning the water or, in the case of distillation, the other substance with lower boiling point into vapour winch can then be readily separated from the liquid or solid substance it was once part of. When water is placed in an open vessel and heated the molecules increase their energy and a temperature is reached when the vapour and the water is driven off as steam pressure overcomes atmospheric pressure. We call this temperature the boiling temperature and the energy necessary to convert a kilogram of water to vapour is the latent heat of vaporisation. One might expect the energy required in removing water in this way is only that needed to raise the temperature of water to its boiling point and the latent heat of vaporisation. This is not always the case since substantial energy is often needed to get the product to the required quality. In air drying energy is used to heat the air which is discharged from the drying chamber with lower and lower humidities in the a~m of achieving the required moisture content in the product. This is an extremely expensive process in terms of the energy needed to remove a quantity of water. The energy costs of air drying and other means of removing water can be much reduced through using energy efficient technology, but the benefits change with the application, i.e. whether in drying, evaporation and distillation and the product requirement. The examination of the manufacture of one product will show some of the opportunities for energy efficiency.
THE MAKING OF MALT WHISKY The making of malt whisky, including the production of the malted barley is a process that uses drying, evaporation and distillation at some stage. Since 1979 the fuel consumption to produce malted barley has gradually reduced from around 5250 MJ per tonne of malt to about 2100 MJ per tonne today at two of the major malt producers in the UK, through the application of energy efficient techniques to drying. At present the fuel consumption in the malt whisky industry is 40 MJ/i. ETSU estimates through the application of current demonstrated technology could reduce energy consumption to 24 MJ/I and further novel ideas still unproven could reduce consumption still further. With 14 kg used per litre of pure alcohol the application of known technology reduces the energy consumption of producing the malt and the malt whisky from 113 to 53 MJ/I of alcohol, a saving of over 50~o.
Maitings Malt is a germinated barley m which natural enzymes in the gram are allowed to develop and these are subsequently used by the brewer or whisky distiller. The first operation carried out m a typical maltings is steeping. This consists of standing the grain m water so that its moisture content increases from about 10 to 50%. The grain is then taken from the steep tanks and allowed to germinate. L~ttle rootlets sprout from the grain. At this stage germination is stopped by kilning. This is achieved in a malt kiln in which a stream of warm air is passed through a bed of malt. Most of the energy consumed by a maltings is at the kilning stage and is usually supplied by natural gas to heat the air passed through the malt bed. To appreciate the considerable scope for saving energy the full drying cycle must be examined.
554
G J NEWnERT
Constant drying rate The first stage of the drying process is the removal of free water and is called the constant drying rate period. During this period the surface of the product being dried is saturated with water. Whilst saturated the rate of water removal is constant for a given air flow, air temperature and air humidity and the air leaving the drying chamber becomes saturated with water. Eventually a point is reached when there is no free water and drying within the substance begins.
Falling drying rate After the removal of the free water the drying rate steadily falls. The point of change over is called "the humidity break" or more simply "the break" The second stage of the drying process is called either the falling rate period or the post break period. In the post break period a dry outer layer builds up on the substance being dried which acts as a barrier to heat transfer into the substance and water removal from the substance and this reduces the drying rate. The more dry the substance becomes the more the drying rate falls. In the falling drying rate period the air leaving the drying chamber is no longer saturated as the drying proceeds the air beccomes less humid and the temperature of the leaving air will gradually increase. When items are dried to very low moisture contents this period predominates in determining the overall drying time and the energy consumption as air is being heated to remove ever decreasing amounts of water.
Final airing When the malt is hand-dry the malt is aired by allowing air at 70-100°C to pass through the bed to develop particular product characteristics.
AREAS FOR E N E R G Y S A V I N G S Malt kilns which operate without specific conservation measures require a fuel input of 40-50 therms/tonne of malt. Energy saving schemes can be implemented in order of: (a) improved operating procedure; (b) air flow control; (c) heat recovery and (d) heat pumps.
Improved operatmg procedures The elimination of air leaks, good insulation and e~cmnt operauon save energy. With malt kilning the treatment of the malt to reach product of the malt follows a menu or a succession of varying air flow rates and air temperatures flowing onto the bed and kiln microprocessor controller can provide automatic control of kiln air temperatures and flows with benefits in both product quality and energy consumption.
Air flow control The first main area of energy recovery is air flow control. By equipping a fan with a variable speed motor or fitting dampers or guide varies the flow through the drying chamber. In malting in the part break phase of drying, when there is a lower rate of moisture removal, the air flow rate can be significantly reduced. In malting this saves 8 therms/tonne. In other industries drying chambers are set to evacuate a constant volume of make-up mr. Certain operating conditions such as a low moisture content in the incoming air allow drying at lower flowrates. It can be advantageous to monitor the humidity of the exhaust air and from the reading vary the flowrate through the drier.
Reclrculation The next stage for energy savings is air rectrculation. The warm unsaturated air leaving the malt kiln in the post break phase is recirculated to the inlet of the malt kiln in a prebreak phase, reducing the need to heat as much air for the prebreak drying stage. This expedient saves 5 therms/tonne. The aim of air flow control and recirculation is to take maximum advantage of the air that is heated for drying by ensuring it carnes away as much water as possible by it leaving the drying chambers with a high humidity as is possible with the products being dried.
Energy eflicmntdrymg, evaporatmnand similarprocesses
555
Heat recovery
Heat recovery from the large quantities of warm humid air exhausted from the kiln is the next step for reducing energy used. Recovery of the heat is carried out by air-to-air heat exchangers such as run around coil which can yield savings of about 7 therms/tonne of malt. The effectiveness of heat recovery in this way is limited by the fact that the incoming air can only be heated up to the temperature somewhat below the temperature of the exhaust air and as a result there is still more heat that can be recovered. The application therefore becomes suitable for a heat pump to extract more exhaust heat and upgrade it in temperature to the level required by the air entenng the kiln. The demonstratton of a waste heat recovery scheme on two kilns at A B M ' s Louth Maltings
Two kilns at ABM's Louth Maltings were fitted with a waste heat recovery system incorporating a run-around coil and a gas engine driven heat pump. At the same time a microprocessor controller was installed to automatically adjust kiln air flows and temperatures. The waste heat recovery system includes a heat exchanger in the exhaust duct from the kiln which removes heat from the warm humid kiln exhaust air and warms the water/glycol mixture bemg circulated through the exhaust air heat exchanger. The cooled exhaust air is discharged to atmosphere and the heated water/glycol is first passed through the run-around coil heat exchanger in the fresh air duct, preheating the cold fresh air. The partially cooled water/glycol then flows through the evaporator of the heat pump where it is cooled still further before it Is recirculated to the heat exchanger in the exhaust duct to recover more heat. The heat taken in at the temperature is upgraded in the heat pump to a high temperature and heats the incoming air still further with the condenser coil in the fresh air duct. Gas engines are used to drive the compressors and heat recovered from the engine cooling system and exhaust gases provides a third stage of heating via fresh air that flows into the kiln (Fig. l). The demonstration at ABM Louth was monitored by British Gas Midlands Research Station and the results are described in detail in the final monitoring report. Gas consumption figures revealed that average gas consumption dropped from about 38 therms/tonne to about 17 therms/tonnes for a gram with a moisture content of 5~ to 23 therms/tonne for a grain with a moisture content of 2.5~o. ABM have been very pleased with the results and have a 3½year payback on their investment in the plant. Since the plant became fully operational some problems were encountered with shaft seals or compressors and the failure of oil separator baffles. The original plastic coated heat exchanger also failed but it has now been replaced by a stainless steel heat exchanger which is now operating satisfactorily. Summary of methods of saving energy
A summary of potential energy savings in malt kilning are: Description Control and operation Air flow control Recirculation Run-around coil Heat pump
Potential Savings 4 8 5 7 6
therms/tonne therms/tonne therms/tonne therms/tonne therms/tonne
which together have brought the average energy consumption down from around 50 therms/tonne to about 20 therms/tonne. Malt whisky
Malt whisky production is a batch process carried out by 97 small to medium distilleries, the processes are fairly uniform between distilleries. The main processes are mashing, fermentation and distillation.
556
G J. NEWBERT Heat pump system
or
II
Compressor
c°°°'°'rl Woter c l r c u l l inlet
o~r
Fig 1.
Malt is ground and mixed ruth hot water in the mash tun. The hot water causes the enzymes m the malt to convert soluble starch to fermentable sugars. Once mashing is complete the solution--known as wort--is decanted and cooled Yeast ~s added and wort ferments. After 2 days the resulting liquor--known as wash--contains about 7 ~ alcohol by volume. In most distilleries the wash undergoes a two stage distillation. The first stage is carried out in the wash still and the distillation--called low wines from the first still ~s condensed in a water cooled condenser. The residue left behind in the wash still is pot ale. The low wines are subjected to a second distillation m the spirit still. The first vapours from the sprat still contain impurities, they are condensed, collected and recycled. The next vapours driven off are the main spirit run. After some time the strength of the distillate weakens and the final run of condensate are again recycled. The residue in the spirit still is the spent lees which is usually thrown away after appropriate effluent treatment. The spirit is mixed with water to a strength of about 68% alcohol by volume and stored in oak casks for 3-25 years, the final product being malt whisky. A R E A S FOR E N E R G Y S A V I N G The basic whtsky process with no heat recovery and a combustion efficiency of 70% uses 40 MJ/litre of pure alcohol produced. Energy Schemes can be implemented through: (1) Automatic control (2) Heat recovery (3) Heat pumps.
Automatic control An examination of the energy use in a distillery shows that it is not the alcohol that demands most heat ~t is the water as it is repeatedly heated and evaporated as it travels around the distillery before it Is discarded. Energy will be saved if less water Is used m the process. The cntical stages are the stills because of the large amount of heat necessary to boil water and the amount of dlstdlate that is recycled for redistfllation. The reduction in the quantity o f water distilled over with the alcohol will have the most significant effect on energy consumption.
The demonstranon of the automatic control of malt whisky pot still distdlat:on at Bell's Dufftown Distillery The demonstrauon project at Bell's Dufftown-Glenhvet Distillery, Dufftown is about the automatic control of the operation of wash and spirit stills to stop the steam supply to the stdl when the value of the alcohol produced ~s equal to the cost of steam required to produce it. If the stills were not shut off at the economic end point but allowed to run on then not only is extra steam used in distilhng water but the extra runnings are returned to the spirit stills and the number of still charges will increase to cope w~th the extra volume. The economic end point for the distillation of whisky is when the distdlation holds about 1~o whisky (Fig. 2).
Energy efficient drying, evaporation and similar processes
St~
Condenser
Con,,oL vo,,e
/ ~(.n ~FI\ )
i
~
l
-
.
557
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Dens|tytronsducer
T ~ SI'~01"COnverter "- = I L l SequenUcecont rotLerComputeT Fsg.2
Pot ale evaporation A typical distillery will produce about 8 kg of pot ale for each litre of alcohol produced and the distillery may get rid of this byproduct in one of three ways. It may be discharged directly as an effluent but ~t is a very fortunate distillery that can get rid of its pot ale as simply as that. Most distilleries get rid of their pot ale by road tanker either to be spread on the land or driven to a byproducts plant where it is concentrated into syrup which can be sold as animal feed. Pot ale contains about 4% solids which are rich in protein. Conventionally pot ale was evaporated in four effective evaporators but the rise in energy prices has forced some byproducts plant to close causing distilleries to revert to other means of disposing of their pot ale. The answer to rising energy costs in these circumstances is to use the waste heat at a distillery.
The demonstratton of a pot ale waste heat evaporator at Bell's lnchgower Distillery The malt whisky process produces far more waste heat than can be used directly in the mashing and distillation process streams. Waste heat in large quantities is available for export to adjacent process and activities. At the Inchgower Distillery, Arthur Bell & Sons Ltd have installed a double effect evaporator using waste heat to process pot ale. Whereas the pot ale was previously discharged to the sea, the concentrated pot ale is now sold as animal feed. The evaporator installation consists of a two-stage falling film unit with a flask tank and horizontal shell and tube condenser. The design reqmred that an average of 37,200 kg/h of water should be supplied from the existing coridenser system. This hot water is fed directly from the condensers to the flask tank. The resultant steam released from hot water flasks down at 56°C and is fed to the first stage of the evaporator where it condenses against the pot ale which is evaporating at 47°C. The concentration process in the evaporator is counter current. The vapour arising from the pot ale at 47°C is condensed in the second stage of the evaporator to evaporate the incoming pot ale. Vapour from the second stage vessel at 40°C passes to a condenser where ~t heats incoming cold water which with the hot water from the flask tank is recycled to the distillery for process heating (Fig. 3). The installation at Inchgower has demonstrated the practicabihty and economic effect o f waste heat evaporators. At Inchgower the main benefit was not energy saving, since previously energy was not used for concentrating pot ale. The economic benefit was from the sale of syrup. The financial benefit during the monitored period gave a rumple payback of just over 4 years. The benefit for a distillery that currently uses energy to process its pot ale and sells the concentrated syrup is the energy cost saving and the financial benefit to a distillery in this position is less than 3 years.
The demonstration of the use of thermocompression to recycle condenser heat m indirectly fired stills at Stanley P. Morrzson's Bowmore Distillery The s~mplest form of heat pump is a steam ejector or thermocompressor. This .s a nozzle which discharges a high pressure steam jet to entrain vapour at lower pressure to produce mixed steam at a useful pressure.
558
G J NEWBERT 2.5
m
m
_
bar From boiler
w
Vacuum pump To air
b-~_Sg._~j--[.__ OF aT
L_ Over/ fLow wat.er tank S~rlt cooler
__I
I
I
. /
Subcooler
TO dram (sewer)
To drain F~g 3
The project at the Bowmore distillery involves the recovery and use of the heat rejected at the condensers of four steam fired whisky stills. Steam is produced at less than atmospheric pressure in modified condensers and thus is compressed using medium pressure boiler steam m a thermocompressor. The resulting steam maintains the distillation. Remaining hot water from the condensers is used from process heating in the distillery and associated maltings (Fig. 4). The vitally important point of the thermocompressor is its performance, a criteria for this is the ratio of the amount of the steam output to the amount of driving steam. A practical performance of 1.3 is anticipated which will give a saving of 25~ of the present energy costs for distillation with a further 20~ savings attributable to heat recovery from condenser water for elsewhere in the process. The expected payback is 2.6 years. Ejectors are easy to operate and they require very httle maintenance. They have long lives, low initial capital cost and low maintainance cost. As a heat pump their performance is low but other major attractions can outweigh this. The relatively low thermal performance will not allow full condenser heat recovery through the thermocompressor but this is not a handicap if the remaining condenser heat can be used elsewhere as at the Bowmore Distillery.
Mechanical compression Full heat recovery of condenser heat back into the still requires another form of heat pump technology. This is mechanical compression Steam at 90°C produced in the condenser would be L. P-" steam
I
H R steam
I
Thermocompressor ~--~Sub- c o o t e r ~ M-R steam
JHeatlnt
s~fo~J Hot
~woter
storage Fig 4
i water riser
Hot water storage
Energy el~clent drying, evaporation and slrmlar processes
559
mechanically compressed to a higher pressure and temperature and be sufficient to completely maintain the distillation process. The important point of mechanical compression is its performance ratio. Five units of heat can be delivered for every unit of electricity to drive the compressor. As electricity is more expensive per unit than oil or gas the 5:1 ratio will not be fully reflected in the financial or primary energy savings. But it is anticipated that energy costs for heating the still will be halved with mechanical compression
Summary of methods of saving energy A summary of the potential savings in malt whisky production are:
Description
Potential Savings
Heat recovery in process Thermocompressor heat pump Mechanical compression: on one still on two stills
10 MJ/lltre alcohol 6 MJ/litre alcohol 12 MJ/litre alcohol 23 MJ/litre alcohol
FINALE The making of malt and its use in the making of malt whisky are industries that use drying, evaporating and distillation processes and have found great scope for saving energy. Through good housekeeping, improved operating procedures, air flow control, heat recovery and the use of heat pumps the energy use for making the malt and the whisky is being more than halved with existing established or demonstrated technologies and the possibility of greater savings as advanced technologies are developed. The processes in the two industrial sectors covered in this paper are similar to processes in other industries and a similar range of savings could be expected. Of course the most suitable form of eqmpment for removing water by drying evaporation and distillation depends on the characteristics of the product being dried and the lessons learnt from applying a technology in the industry cannot be bhndly adopted by another industry without thought to the specific requirements of the product being treated, e.g. temperature sensitivity or to alternative technologies that are only appropriate m some circumstances, e.g. mechanical dewatering. However the major methods of improving energy efficiency in malting and whisky production are of general interest and should have wider application than these industries alone and the review of generic energy saving measures in these two particular industries shows the practical case for these ideas. None of the projects described have run without problems. Commissioning difficulties, component failures, corrosion, operation at less than design performance and operation at off design conditions, air leaks in drying systems and less than anticipated usage have all happened. In general there has been close and expert engineering supervision that have identified and made the necessary changes so that the plant operates as it should. Some of the projects are still continuing and plant is still being brought up to scratch. These projects have not always led to the s~mple life for the hosts of equipment but they all have lead to savings in energy and money.
ENERGY
0198-7593/85 $3 O0 + 0 00 P e r g a m o n Press Lid
EFFICIENT DRYING, EVAPORATION AND SIMILAR PROCESSES* G.
ETSU,
J.
B r a i d i n g 156, A E R E
NEWBERT
Harwell, Oxfordshire
OXll
0RA,
UK
Abstract--Wtth the asmstancc o f examples taken from the U K Energy Effioency Demonstratton Scheme, this p a p e r d e s c r i b e s energy efficient technologies which can benefit drying, evaporation and dtsttllatton processes
INTRODUCTION Drying, evaporation and similar processes are energy intensive and widely used by British Industry. These processes, which are usually to remove water from substances, use significant quantities of energy both nationally and for individual companies they are a worthwhile target for improvements in energy efficiency. The national energy used for drying, evaporation and distribution in industrial activities where they are used most Is estimated in Table I. The energy used for dying, evaporation and distillation in these industries add up to about 200 x l06 GJ/yr, the equivalent of 7.5 million tonnes of coal.
T a b l e 1 N a u o n a l energy
consumption for d r y m 8, e v a p o r a t m n and thsullation Energy use 106GJ/yr D~stillation
Evaporatmn
Drying
--3 12 2 30 0 55 --------
-24 ---n.a" I 66 0 14 28 10 3 ---
5 36 ----n.a * 2 2 ---4 5 34
--
--
89
-I 57 -n a* -59 3
-3 08 ---14 I
3 1 9 3 6 3 n.a ° 38 6 18 I
6684
3448
9976
Malt Beer M a l t whisky G r m n whisky
Other spirits W l a s k y byproducts M d k powde r s
Condensed/evaporated milk S u p r (from cane) Suga r (from beet) Beet sugar byproducts
-
Pottery Bncks Timber
Texules Laundries D r y c le a mng P a p e r and board Cho'mcals
*n a means d a t a not avmlable_
A range of energy efficient technologies can reduce this consumption. The major efficient technologies are listed here:
Drying Heat recovery using heat exchangers and heat pumps Improved instrumentation and control Optimised dryer design and operation
*Paper
first presented a t M E C C A
'85, Organised
by
Energy Systems Trade AssocmUon, London, F e b r u a r y 551
1985.
552
G. J NEWBERT Improved mechanical dewatering Radio frequency drying Multistage drying
Evaporation Heat recovery using heat exchangers and heat pumps Thermal vapour recompresslon Mechanical vapour recompression Reverse osmosis
Distillation Heat recovery using heat exchangers and heat pumps. To highlight some of the more general energy efficient technologies this paper looks at the process to make one particular product that uses dry|ng, evaporation and distillation at some stage in its manufacture. This route examines some practical methods of improwng energy efficiency in these acUvities calling on the evidence of projects supported under the Energy Efficiency Office's Energy Effic|ency Demonstration Scheme. Whilst these projects are aimed at particular industrial sectors they also show energy efficient schemes helpful more generally in other industries. M E T H O D S OF R E M O V I N G W A T E R Available methods of removing water are as follows. This is an incomplete list but it does show some of the more successful methods used by industry: (1) (2) (3) (4)
Deposition of moisture as ice or water Absorption Mechanical separanon Vaporization.
Deposition Moisture can be removed from a gas by condensing vapour onto a cold surface. This process is called dehumidification and dehumidifiers for operations of this character are now in general use. Timber drying and swimming pools are examples of applications of this idea. Water may be removed from liquids by converting it into ice and in this solid form separating it from the portion having a lower freezing point. The alcoholic content of a liquid can be readily increased in this way.
Absorption Absorption is a process in which moisture is removed by the capillary action of porous bodies. In this way a cream of clay and water used for casting pottery is deprived of its water by placing it in molds of plaster of paris. The capillary actton in the mold draws the water from the liquid clay and as a result a layer of solid clay, the thickness of whmh ts controlled by the time of standing, deposits itself on the mold. Certain types of sweets are dried in a similar way by contact with the starch mold in which they are cast. The drying effect of towels is due to the same capdlary action. Some substances have the wonderful ability to absorb moisture and this makes possible their appllcatton to drying gases and to some extent liquids. Silica gel is often seen in the packaging of some products to ensure it keeps dry over long periods of storage.
Mechanical dewatermg Some materials are of a spongy nature and hold moisture by capillarity. Large quantities of moisture may be expelled from this type of material by pressure alone. When this is possible it is desirable to get rid of as much water as posmble before using more expensive methods. Thts ts usually the case with textiles and paper at some stage in their drying. The opportunity for this depends very much on the product to be dried and examples given earher e g. pottery, textiles and paper use non-thermal methods where they can. Water which is
Energy efficient drying, evaporation and stmdar processes
553
not chemically bound to materials are readily amenable to mechanical dewatenng techmques. In textiles the most common extraction device ~s the stmple pad or squeeze roller. The energy required is that which drives the motor and turns the mangle. Assuming on entering fabric is soaking wet at about 150~ moisture content and exits at 75~ moisture content, the energy required per kg of water removed Is 50 kJ. Compare this with the latent heat of water of 2260 kJ/kg which would have to be used by simple thermal methods to dry to the same extent. While squeezing methods are energy efficient they are not always applicable. Some substances can be destroyed by squeezing. Other efficient mechanical extraction methods used in textdes are functioned on pressure processes that draw or blow the water through the fibres.
Thermal processes The most important and the most widely used process for removing water from liquids and solids depend on vaporization. This is turning the water or, in the case of distillation, the other substance with lower boiling point into vapour winch can then be readily separated from the liquid or solid substance it was once part of. When water is placed in an open vessel and heated the molecules increase their energy and a temperature is reached when the vapour and the water is driven off as steam pressure overcomes atmospheric pressure. We call this temperature the boiling temperature and the energy necessary to convert a kilogram of water to vapour is the latent heat of vaporisation. One might expect the energy required in removing water in this way is only that needed to raise the temperature of water to its boiling point and the latent heat of vaporisation. This is not always the case since substantial energy is often needed to get the product to the required quality. In air drying energy is used to heat the air which is discharged from the drying chamber with lower and lower humidities in the a~m of achieving the required moisture content in the product. This is an extremely expensive process in terms of the energy needed to remove a quantity of water. The energy costs of air drying and other means of removing water can be much reduced through using energy efficient technology, but the benefits change with the application, i.e. whether in drying, evaporation and distillation and the product requirement. The examination of the manufacture of one product will show some of the opportunities for energy efficiency.
THE MAKING OF MALT WHISKY The making of malt whisky, including the production of the malted barley is a process that uses drying, evaporation and distillation at some stage. Since 1979 the fuel consumption to produce malted barley has gradually reduced from around 5250 MJ per tonne of malt to about 2100 MJ per tonne today at two of the major malt producers in the UK, through the application of energy efficient techniques to drying. At present the fuel consumption in the malt whisky industry is 40 MJ/i. ETSU estimates through the application of current demonstrated technology could reduce energy consumption to 24 MJ/I and further novel ideas still unproven could reduce consumption still further. With 14 kg used per litre of pure alcohol the application of known technology reduces the energy consumption of producing the malt and the malt whisky from 113 to 53 MJ/I of alcohol, a saving of over 50~o.
Maitings Malt is a germinated barley m which natural enzymes in the gram are allowed to develop and these are subsequently used by the brewer or whisky distiller. The first operation carried out m a typical maltings is steeping. This consists of standing the grain m water so that its moisture content increases from about 10 to 50%. The grain is then taken from the steep tanks and allowed to germinate. L~ttle rootlets sprout from the grain. At this stage germination is stopped by kilning. This is achieved in a malt kiln in which a stream of warm air is passed through a bed of malt. Most of the energy consumed by a maltings is at the kilning stage and is usually supplied by natural gas to heat the air passed through the malt bed. To appreciate the considerable scope for saving energy the full drying cycle must be examined.
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Constant drying rate The first stage of the drying process is the removal of free water and is called the constant drying rate period. During this period the surface of the product being dried is saturated with water. Whilst saturated the rate of water removal is constant for a given air flow, air temperature and air humidity and the air leaving the drying chamber becomes saturated with water. Eventually a point is reached when there is no free water and drying within the substance begins.
Falling drying rate After the removal of the free water the drying rate steadily falls. The point of change over is called "the humidity break" or more simply "the break" The second stage of the drying process is called either the falling rate period or the post break period. In the post break period a dry outer layer builds up on the substance being dried which acts as a barrier to heat transfer into the substance and water removal from the substance and this reduces the drying rate. The more dry the substance becomes the more the drying rate falls. In the falling drying rate period the air leaving the drying chamber is no longer saturated as the drying proceeds the air beccomes less humid and the temperature of the leaving air will gradually increase. When items are dried to very low moisture contents this period predominates in determining the overall drying time and the energy consumption as air is being heated to remove ever decreasing amounts of water.
Final airing When the malt is hand-dry the malt is aired by allowing air at 70-100°C to pass through the bed to develop particular product characteristics.
AREAS FOR E N E R G Y S A V I N G S Malt kilns which operate without specific conservation measures require a fuel input of 40-50 therms/tonne of malt. Energy saving schemes can be implemented in order of: (a) improved operating procedure; (b) air flow control; (c) heat recovery and (d) heat pumps.
Improved operatmg procedures The elimination of air leaks, good insulation and e~cmnt operauon save energy. With malt kilning the treatment of the malt to reach product of the malt follows a menu or a succession of varying air flow rates and air temperatures flowing onto the bed and kiln microprocessor controller can provide automatic control of kiln air temperatures and flows with benefits in both product quality and energy consumption.
Air flow control The first main area of energy recovery is air flow control. By equipping a fan with a variable speed motor or fitting dampers or guide varies the flow through the drying chamber. In malting in the part break phase of drying, when there is a lower rate of moisture removal, the air flow rate can be significantly reduced. In malting this saves 8 therms/tonne. In other industries drying chambers are set to evacuate a constant volume of make-up mr. Certain operating conditions such as a low moisture content in the incoming air allow drying at lower flowrates. It can be advantageous to monitor the humidity of the exhaust air and from the reading vary the flowrate through the drier.
Reclrculation The next stage for energy savings is air rectrculation. The warm unsaturated air leaving the malt kiln in the post break phase is recirculated to the inlet of the malt kiln in a prebreak phase, reducing the need to heat as much air for the prebreak drying stage. This expedient saves 5 therms/tonne. The aim of air flow control and recirculation is to take maximum advantage of the air that is heated for drying by ensuring it carnes away as much water as possible by it leaving the drying chambers with a high humidity as is possible with the products being dried.
Energy eflicmntdrymg, evaporatmnand similarprocesses
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Heat recovery
Heat recovery from the large quantities of warm humid air exhausted from the kiln is the next step for reducing energy used. Recovery of the heat is carried out by air-to-air heat exchangers such as run around coil which can yield savings of about 7 therms/tonne of malt. The effectiveness of heat recovery in this way is limited by the fact that the incoming air can only be heated up to the temperature somewhat below the temperature of the exhaust air and as a result there is still more heat that can be recovered. The application therefore becomes suitable for a heat pump to extract more exhaust heat and upgrade it in temperature to the level required by the air entenng the kiln. The demonstratton of a waste heat recovery scheme on two kilns at A B M ' s Louth Maltings
Two kilns at ABM's Louth Maltings were fitted with a waste heat recovery system incorporating a run-around coil and a gas engine driven heat pump. At the same time a microprocessor controller was installed to automatically adjust kiln air flows and temperatures. The waste heat recovery system includes a heat exchanger in the exhaust duct from the kiln which removes heat from the warm humid kiln exhaust air and warms the water/glycol mixture bemg circulated through the exhaust air heat exchanger. The cooled exhaust air is discharged to atmosphere and the heated water/glycol is first passed through the run-around coil heat exchanger in the fresh air duct, preheating the cold fresh air. The partially cooled water/glycol then flows through the evaporator of the heat pump where it is cooled still further before it Is recirculated to the heat exchanger in the exhaust duct to recover more heat. The heat taken in at the temperature is upgraded in the heat pump to a high temperature and heats the incoming air still further with the condenser coil in the fresh air duct. Gas engines are used to drive the compressors and heat recovered from the engine cooling system and exhaust gases provides a third stage of heating via fresh air that flows into the kiln (Fig. l). The demonstration at ABM Louth was monitored by British Gas Midlands Research Station and the results are described in detail in the final monitoring report. Gas consumption figures revealed that average gas consumption dropped from about 38 therms/tonne to about 17 therms/tonnes for a gram with a moisture content of 5~ to 23 therms/tonne for a grain with a moisture content of 2.5~o. ABM have been very pleased with the results and have a 3½year payback on their investment in the plant. Since the plant became fully operational some problems were encountered with shaft seals or compressors and the failure of oil separator baffles. The original plastic coated heat exchanger also failed but it has now been replaced by a stainless steel heat exchanger which is now operating satisfactorily. Summary of methods of saving energy
A summary of potential energy savings in malt kilning are: Description Control and operation Air flow control Recirculation Run-around coil Heat pump
Potential Savings 4 8 5 7 6
therms/tonne therms/tonne therms/tonne therms/tonne therms/tonne
which together have brought the average energy consumption down from around 50 therms/tonne to about 20 therms/tonne. Malt whisky
Malt whisky production is a batch process carried out by 97 small to medium distilleries, the processes are fairly uniform between distilleries. The main processes are mashing, fermentation and distillation.
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Malt is ground and mixed ruth hot water in the mash tun. The hot water causes the enzymes m the malt to convert soluble starch to fermentable sugars. Once mashing is complete the solution--known as wort--is decanted and cooled Yeast ~s added and wort ferments. After 2 days the resulting liquor--known as wash--contains about 7 ~ alcohol by volume. In most distilleries the wash undergoes a two stage distillation. The first stage is carried out in the wash still and the distillation--called low wines from the first still ~s condensed in a water cooled condenser. The residue left behind in the wash still is pot ale. The low wines are subjected to a second distillation m the spirit still. The first vapours from the sprat still contain impurities, they are condensed, collected and recycled. The next vapours driven off are the main spirit run. After some time the strength of the distillate weakens and the final run of condensate are again recycled. The residue in the spirit still is the spent lees which is usually thrown away after appropriate effluent treatment. The spirit is mixed with water to a strength of about 68% alcohol by volume and stored in oak casks for 3-25 years, the final product being malt whisky. A R E A S FOR E N E R G Y S A V I N G The basic whtsky process with no heat recovery and a combustion efficiency of 70% uses 40 MJ/litre of pure alcohol produced. Energy Schemes can be implemented through: (1) Automatic control (2) Heat recovery (3) Heat pumps.
Automatic control An examination of the energy use in a distillery shows that it is not the alcohol that demands most heat ~t is the water as it is repeatedly heated and evaporated as it travels around the distillery before it Is discarded. Energy will be saved if less water Is used m the process. The cntical stages are the stills because of the large amount of heat necessary to boil water and the amount of dlstdlate that is recycled for redistfllation. The reduction in the quantity o f water distilled over with the alcohol will have the most significant effect on energy consumption.
The demonstranon of the automatic control of malt whisky pot still distdlat:on at Bell's Dufftown Distillery The demonstrauon project at Bell's Dufftown-Glenhvet Distillery, Dufftown is about the automatic control of the operation of wash and spirit stills to stop the steam supply to the stdl when the value of the alcohol produced ~s equal to the cost of steam required to produce it. If the stills were not shut off at the economic end point but allowed to run on then not only is extra steam used in distilhng water but the extra runnings are returned to the spirit stills and the number of still charges will increase to cope w~th the extra volume. The economic end point for the distillation of whisky is when the distdlation holds about 1~o whisky (Fig. 2).
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Pot ale evaporation A typical distillery will produce about 8 kg of pot ale for each litre of alcohol produced and the distillery may get rid of this byproduct in one of three ways. It may be discharged directly as an effluent but ~t is a very fortunate distillery that can get rid of its pot ale as simply as that. Most distilleries get rid of their pot ale by road tanker either to be spread on the land or driven to a byproducts plant where it is concentrated into syrup which can be sold as animal feed. Pot ale contains about 4% solids which are rich in protein. Conventionally pot ale was evaporated in four effective evaporators but the rise in energy prices has forced some byproducts plant to close causing distilleries to revert to other means of disposing of their pot ale. The answer to rising energy costs in these circumstances is to use the waste heat at a distillery.
The demonstratton of a pot ale waste heat evaporator at Bell's lnchgower Distillery The malt whisky process produces far more waste heat than can be used directly in the mashing and distillation process streams. Waste heat in large quantities is available for export to adjacent process and activities. At the Inchgower Distillery, Arthur Bell & Sons Ltd have installed a double effect evaporator using waste heat to process pot ale. Whereas the pot ale was previously discharged to the sea, the concentrated pot ale is now sold as animal feed. The evaporator installation consists of a two-stage falling film unit with a flask tank and horizontal shell and tube condenser. The design reqmred that an average of 37,200 kg/h of water should be supplied from the existing coridenser system. This hot water is fed directly from the condensers to the flask tank. The resultant steam released from hot water flasks down at 56°C and is fed to the first stage of the evaporator where it condenses against the pot ale which is evaporating at 47°C. The concentration process in the evaporator is counter current. The vapour arising from the pot ale at 47°C is condensed in the second stage of the evaporator to evaporate the incoming pot ale. Vapour from the second stage vessel at 40°C passes to a condenser where ~t heats incoming cold water which with the hot water from the flask tank is recycled to the distillery for process heating (Fig. 3). The installation at Inchgower has demonstrated the practicabihty and economic effect o f waste heat evaporators. At Inchgower the main benefit was not energy saving, since previously energy was not used for concentrating pot ale. The economic benefit was from the sale of syrup. The financial benefit during the monitored period gave a rumple payback of just over 4 years. The benefit for a distillery that currently uses energy to process its pot ale and sells the concentrated syrup is the energy cost saving and the financial benefit to a distillery in this position is less than 3 years.
The demonstration of the use of thermocompression to recycle condenser heat m indirectly fired stills at Stanley P. Morrzson's Bowmore Distillery The s~mplest form of heat pump is a steam ejector or thermocompressor. This .s a nozzle which discharges a high pressure steam jet to entrain vapour at lower pressure to produce mixed steam at a useful pressure.
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The project at the Bowmore distillery involves the recovery and use of the heat rejected at the condensers of four steam fired whisky stills. Steam is produced at less than atmospheric pressure in modified condensers and thus is compressed using medium pressure boiler steam m a thermocompressor. The resulting steam maintains the distillation. Remaining hot water from the condensers is used from process heating in the distillery and associated maltings (Fig. 4). The vitally important point of the thermocompressor is its performance, a criteria for this is the ratio of the amount of the steam output to the amount of driving steam. A practical performance of 1.3 is anticipated which will give a saving of 25~ of the present energy costs for distillation with a further 20~ savings attributable to heat recovery from condenser water for elsewhere in the process. The expected payback is 2.6 years. Ejectors are easy to operate and they require very httle maintenance. They have long lives, low initial capital cost and low maintainance cost. As a heat pump their performance is low but other major attractions can outweigh this. The relatively low thermal performance will not allow full condenser heat recovery through the thermocompressor but this is not a handicap if the remaining condenser heat can be used elsewhere as at the Bowmore Distillery.
Mechanical compression Full heat recovery of condenser heat back into the still requires another form of heat pump technology. This is mechanical compression Steam at 90°C produced in the condenser would be L. P-" steam
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Energy el~clent drying, evaporation and slrmlar processes
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mechanically compressed to a higher pressure and temperature and be sufficient to completely maintain the distillation process. The important point of mechanical compression is its performance ratio. Five units of heat can be delivered for every unit of electricity to drive the compressor. As electricity is more expensive per unit than oil or gas the 5:1 ratio will not be fully reflected in the financial or primary energy savings. But it is anticipated that energy costs for heating the still will be halved with mechanical compression
Summary of methods of saving energy A summary of the potential savings in malt whisky production are:
Description
Potential Savings
Heat recovery in process Thermocompressor heat pump Mechanical compression: on one still on two stills
10 MJ/lltre alcohol 6 MJ/litre alcohol 12 MJ/litre alcohol 23 MJ/litre alcohol
FINALE The making of malt and its use in the making of malt whisky are industries that use drying, evaporating and distillation processes and have found great scope for saving energy. Through good housekeeping, improved operating procedures, air flow control, heat recovery and the use of heat pumps the energy use for making the malt and the whisky is being more than halved with existing established or demonstrated technologies and the possibility of greater savings as advanced technologies are developed. The processes in the two industrial sectors covered in this paper are similar to processes in other industries and a similar range of savings could be expected. Of course the most suitable form of eqmpment for removing water by drying evaporation and distillation depends on the characteristics of the product being dried and the lessons learnt from applying a technology in the industry cannot be bhndly adopted by another industry without thought to the specific requirements of the product being treated, e.g. temperature sensitivity or to alternative technologies that are only appropriate m some circumstances, e.g. mechanical dewatering. However the major methods of improving energy efficiency in malting and whisky production are of general interest and should have wider application than these industries alone and the review of generic energy saving measures in these two particular industries shows the practical case for these ideas. None of the projects described have run without problems. Commissioning difficulties, component failures, corrosion, operation at less than design performance and operation at off design conditions, air leaks in drying systems and less than anticipated usage have all happened. In general there has been close and expert engineering supervision that have identified and made the necessary changes so that the plant operates as it should. Some of the projects are still continuing and plant is still being brought up to scratch. These projects have not always led to the s~mple life for the hosts of equipment but they all have lead to savings in energy and money.