EIW T R E N D S IN E V‘APO R A T IO N Adz)anceS zn
SZiugle
pro(dU.5 ,t 2‘mPr.eve,merit
TS
euapoYCchon equzjjment Yield marked
aitrh boniU S o f increased capacity
W I L L I A M G. D E D E R T
J A M E S G. M O O R E
vaporation continues to receive from researchers, equipment manufacturers, and the chemical, food, and pharmaceutical producers throughout the world. This review covering the years 1961 and 1962 presents only that literature which, in the authors’ opinions, represents significant major trends. There is no intent to question the importance of work well reported in the past or specialized advancements which have been omitted.
value obviously is only an average due to the short-circuiting potential of most of the common recirculating evaporators. Now there is a n understanding that a more exact measure of retention time must be determined if evaporation equipment is to realize its full benefit from the application of the time-temperature relationship. This has been demonstrated mathematically and in the laboratory for several types of evaporators ( 7 3 ) . As a basis for analysis, the true time for replacement of 97% of a feed stream by actual discharge from the system has been determined. For any of the recirculating classes of evaporators, this 97y0 replacement actually is achieved in 3.2 times the normally calculated average retention time. T h e marked discrepancy becomes an apparent obstacle in handling heat-sensitive materials in recirculating evaporators. The paper stresses the importance of using single pass evaporators for heat-sensitive materials since only in this case is the true retention time actually equal to the average retention time. Figure 1 from the reference paper shows the pertinent data relative to several types of laboratory evaporators including test data for both single pass as well as recirculating types. T h e actual holding time of any evaporator can be calculated mathematically using Taylor’s Series Expansion: % removed = 1 - e-(‘’’) where t = time in minutes and r = ratio of holding volume to discharge
E attention
Retenlion Time Concept
T h e major significant trend is the acceptance of the time us. temperature relationship as applied to concentration of heat-sensitive materials. Although there always has been general agreement regarding these factors, the availability of suitable equipment has not been so general. A commercial example illustrating this relationship in the manufacture of protein foods is found in the literature (18). This particular industry is aware of the detrimental effect and reduced appeal of its food products if the timetemperature relationship is not considered during processing. Fundamental to the spread of this trend is the public understanding of the retention time concept. For many years there has been an indefinite usage of this parameter, but it has involved the application of what should properly be termed “average holding time.” Average holding time in an evaporation process is calculated by dividing the operating volume of the system by the discharge rate. The resulting
Typical mechanically induced film evaporator with external bearings, stufing box, and external vapor-liquid separator
in consistent units. Several years ago, emphasis was placed upon reduction in temperature as a means of protecting heatsensitive materials. This resulted in new techniques and equipment giving improved product characteristics but at a considerable increase in cost. Today the suggested increased emphasis on control of retention time is based upon experience indicating that time is the more important of the time-teinperature variables. Even on viable materials with a maximum critical temperature limit, the practical operating temperature range normall\ results in the time factor being the more important. The orange juice industry furnishes an excellent example (12). Historically, recirculating. falling film evaporators operating at a maximum temperature of approximately 75' F. have been used. Now multiple effect evaporators, based on high velocity, single pass principles. are producing a superior product at 130" F. This increase in operating temperature level results in lo\Ler equipment investment, reduced operating costs, and a new improvement in product quality. Almost all references in the literature to the low retention concept are mainly concerned with product quality improvement. The corresponding reduction in evaporation costs has not been so well reported but should be of interest to those engaged in this field. As compared to recirculation, single pass operation provides a lower average concentration of liquor in contdct with the total heating surface, resulting in
7
EXTERNAL UPPER BEARING
I
/
7
FEED INLET
p AIR EJECTOR
CONDENSER
-
PUMP
Singiepnss f a l l i n g j l m ecapoiatoi using tube l m g t h s up to 40 feet. hiph uelocit) a n d retention time is low
lower average viscosity- and lower average boiling point elevation. Both are important factors in establishing the required heat transfer surface which controls the final equipment cost. .Also, the power cost as well as initial equipment cost for recirculation is eliminated. Both research and commercial applications of the single pass units can
The film has
be divided into two categories, namely, mechanically induced films and vapor induced films. The literature indicates a stronger inclination in Europe to apply the mechanically induced film systems. I n the United States, the trend seems clearly to be toward the use of the vapor induced film equipment in those applications where it can be demonstrated that
O P E R A T I N G D A T A FOR S I N G L E PASS AND R E C I R C U L A T I N G EVAPORATORS Disch Ratr, G.P.M.
Fppd Rate.
G.P.itl.
Evaporator Tifie
Calandria Forced circulation and falling films V.R.C. natural circulation
I
0.9 2.8 5.2
1 ~
4.23 3.34
1
0.2560 0.3600
I I
1
0.0186
0.0207 0.0366
0.1024
0.1440
Holding T i m e . M i n u t e s Dasch. Hale/ S9. Ft.
~
0.0277
~
1
Calculated aLi.a
53.8
i::?
Single Pass Agitated film Rotovakb R.F.C., single pass (with flash)b a
58
~
;::
0.33 0.26
1
0.4550 0.7000
1.8 0.2800
Calculated h o l d i q timejigures found v.b dividing holding volume bv aoerage dischar,ge. INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
,
Calculated A clual
173
'3:
1
~ .. . .. . I
170
'i: 4.50 4.00
Operated as true single pass, with no recirculation of liquid.
the mechanically induced film is not absolutely necessary. Mechanically Induced Films
T h e interest in mechanically induced film evaporators continues to spread. T h e literature does not describe any basically new ideas. T h e various types of vapor separation systems, bearing and stuffing box arrangements, and the conventional fixed, centrifugally loaded, and spring loaded blade systems are all available. T h e extension of the use of the mechanically agitated film evaporator foIIows the normal application areas. These involve high viscosity, reduction of fouling, and the handling of foamy liquids, together with a n increased emphasis on its abilities as a short retention time unit. Low holding volume, short retention time, and maintenance of all liquid as a film on the heating surface is emphasized for the rapid concentration of materials which are either heat sensitive or viscous, or both (10). A series of tests were conducted varying the temperature difference, rotor speed, and inlet liquor distribution with systems employing both fixed and centrifugally loaded blades. Special work on the concentration of various fruit purees also was reported ( 9 ) . Viscosity was readily correlated with the overall heat transfer coefficient, and performance was found to be superior for non-Newtonian fluids as compared to Newtonians of equivalent consistencies. T h e use of a typical mechanically induced film evaporator in a rising film configuration as compared to the usual falling film arrangement is reported ( 8 ) . A patent describes a mechanically induced film evaporator designed to permit concentration to dryness ( 1 4 ) . T h e final dried film is continually scraped from the heated walls and is obtained as a finely divided dried product. i(Continued on page 60)
W i l l i a m G. Dedert is Cenlral Regional Sales Manager and James G. Moore i s Dirertor of Research, Bujovak Equipment Div., Blaw-Knox Co., B u f a l o , N . Y .
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AMPCO METAL, INC.
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W e s t c o a s t D I V . : H u n t i n g t o n P a r k , C a l . . Southwest D I V . : Garland lDallasCounty),Tex.
SAND
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pn.24
C i r c l e NO. 8 on Readers' S e r v i c e C a r d
VOL. 5 5
NO. 6
JUNE 1963
59
Vapor Induced Films
Pressure a n d vacuum gauges in five case styles and five Bourdon tube materials
Dial indicating thermometers in four case sizes with four actuoting media, vapor tension, mercury, liquid and gas
e WRITE FOR
INSTRUMENTS
INDICATING, RECORDING CONTROLLING INSTRUMENTS FOR 1EMPIRATURE PRESSURE P HUMIDITY
ITERATURE ABOUT
liquid-in-Glass industrial thermometers in four case sizes a n d f o u r types with rigid stem, adjust-angle, a d . just-all, and dual scale styles
Recording instruments in four case sizes with up to four pens
;ea Water Evaporation
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60
Bi-metal d i a l thermometers in three case sizes, with fhree stem styles. Straight and angle forms
INDUSTRIAL AND
ENGINEERING CHEMISTRY
The conventional vapor induced evaporator systems now have been augmented by improved short retention time evaporators. Improvements in technique have extended the use of the conventional falling film evaporator as a high velocity single pass unit. Exceedingly long tube, single pass, downflow systems using tube lengths up to 40 feet with very high film velocities and low retention times are in use (see figure on page 58). Evaporators using plate type heat exchangers provide similar advantages. Articles have appeared (4, ? I ) which describe some advantages and performance characteristics of these evaporators when handling heat-sensitive materials. They are single pass, except that plate-type heat exchangers rather than tubular surfaces are used. A risingfalling single pass evaporator was applied to the continuous concentration of 43” Baume corn sirup (6). Use of this evaporator takes advantage of reduced average viszosity as well as the desirable low retention time. Continuous operation now replaces batch operation ,vith reduced costs and improved xoduct quality. A design procedure has been mblished that predicts liquid side ‘alling film heat transfer coefficients Test results were based upon ;ingle pass operation confined to ;ea water feed materials. A feature )f this work is pressure drop data ibtained due to changing vapor low through 24-foot-long tubes 1 ind 2 inches in diameter. T h e mportance of this pressure drop is iometimes considerable as it results n a necessary rise in liquor temlerature a t the top of the tube t o iffect boiling.
Sea water evaporation and re:ompression evaporation continue o be well covered by the literature, )ut coverage is confined mainly to xfinements of existing concepts and rends. The Office of Saline Water ias two 1 million gallon-per-day
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demonstration plants in operation a multiple effect evaporator in Free port, Tex., and a multiple stage flasl evaporator a t Point Loma in Sal Diego, Calif. They are operated ti provide cost data for comparisoi with fresh water systems such a freeze concentration and ion ex change (77,79). Scale prevention in multiple effec evaporators has been studied (77) The schemes involve pH control wit1 acid, and maintenance of suspendec seed crystals in the evaporatinl liquid. Use of seed suspensions wa considered more economical becausi the salt is reusable and cost of acic avoided. A similar calcium sulfate seedin! technique was applied to a forcec circulation evaporator pilot plan for handling bradtish water (8) Based on energy requirements, i method is given (75)for comparinl capital costs us. operating costs and for selecting a suitable system. Recompression evaporation still has not become important but isolated cases where energy requirements make this technique preferable continue to occur (3). Recompression evaporators should be evaluated for economic reasons when the optimum number of effects is desired. Steam jet thermal recompression continues to be the most widely used. If power is adequate, mechanical recompression may be worth considering for moderate operating pressures, mildly corrosive vapor, small elevations of the liquor boiling point, and small temperature differences across the heat exchanger. The direct mechanical vapor recompression evaporator (6)may be economical for moderate capacities where boiler plant facilities. are lacking or where fuel is expensive. Evaporators are tending to become larger, even though flexibility is sacrificed, particularly for large tonnage applications such as potash, pulp and paper liquors, and alumina liquon. Where extremely large water removal systems are used, this loss of flexibility is accepted in order to gain the advantage of low initial and operating costa which a single large unit entails. This is especially true for equipment made of mild
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steel where extreme diameters involving thicker plate sections do not offset other savings. For the first time, a comprehensive procedure for evaluating performance has been published (2). I t should be of considerable value because it can be applied to all types of steam heated equipment and, with proper allowances, also to energy sources other than steam. However, the procedure does not apply to auxiliary equipment. Undoubtedly, the trend toward single pass evaporators will continue. At first, it developed slowly because the fundamentals involved, not only for the process but also for equipment, were not well understood. Today, these aspects are better understood and commercial value of the approach has been proved.
Size Requirements Getting Tougher? Sturtevant Air Separators Increase 40 to 400 Mesh Output as Much as 300%
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62
REFERENCES (1) Am. Chem. SOC.“Saline Water Conversion,” A d m n . Chem. Ser. 27, 105, 115, 128, 147, 166 (1962). (2) Am. Inst. Chem. Engrs., Evaporators Subcommittee, Equipment Testing Procedures Committee, 1961. (3) Beagle, M. J., Chem. Eng. Progr. 58, No. 10, 78 (October 1962). (4) Dummett, G. A , , Dechemn Monograph 33, 177 (1959). (5) Food Eng. 33, 46 (Decembei 1961). (6) Zbid., 35, 68 (February 1963). (7) Gainey, R. J., Thorp, C. A,, Cadwallader, E. A., ISD. EXG. CHEM.55, 39 (March 1963). (8) Hadley, G. F., Thomas A. L., Zbid., 52, 71 (January 1960). (9) Harper, J. C., Food Technol. 14, 557 (November 1960). (10) Jones, H . H. M., Znd. Chem. 36, No. 430, 599 (December 1960). (11) Lawler, F. K . , Food Eng. 60 (March 1960). (12) Milleville, H. P., Food Processing 23, 59 (October 1962). (13) Moore, J. G., Hesler, W. E.; Chem. Eng. Progr. 59, No. 2, 87 (February 1963). (14) Samesreuther, R., others (to Farbenfabriken Bayer A.G.), U. S. Patent 2,974,725 (March 14>1961). (15) Silver, R. S., Wier, W.K. J., J . Insi. Fuel 34, No. 251, 514 (December 1961). (16) Sinek, J. R., Young, E. H., Chem. Eng. Progr. 58, No. 12, 74 (December 1962). (17) Standiford, F. C., Sinek, J. R., Chern. Eng. Progr. 57, No. 1, 58 (January 1961). (18) Stark, W.: Food Process. Packaging 29, 323 (1960). (19) U. S. Department of the Interior, Saline Water Conversion Repts. 1960, 1961.
on Readers’ Service Card
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Closed-circuit air separation is of proved advantage in reduction processes. Result is a better, more uniform product. Grinding mills perform at top efficiency, output frequently increases as much as 300%, power costs drop as much as 50%. Precise separation of all dry powdered materials. Sturtevants currently classify sulfur, soybeans, phosphate, chocolate, feldspar, sand and aggregates, pigments, limestone fillers, flour, abrasives, plastics, gypsum, ceramics, cement and other products. Improve screening - Sturtevant Air Separators prevent blinding by removing undesirable tailings or fines from screen feed loads. Works Like Winnowing Done in a Whirlwind Sturtevant Air Separators do a mechanical job of winnowing. Precise control of whirlwind air currents and centrifugal force results in the desired size being lifted into fines cone, oversize falling into tailings cone. A 16 ft. Sturtevant, for example, has taken a feed rate of 800 tph, containing only a small percentage of desired fines, and delivered 30 tph 90% 200 mesh, reEirculating the oversize through the grinding circuit. Send for Bulletitz No. 087. I
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Crushers Grinders Micron-Grinders Separators Granulators Conveyors Elevators Blenders C i r c l e No. 47 on Readers’ Service Card
