JL.
An entirely new evaporation technique which can eliminate scaleformation, generate high pressure heating steam, and operate at large capacities a direct heat transfer medium for A desalinatingbedseaaswater is feasible and meets a number fluidized
"
of requirements necessary to lower costs of the distillation method (3). It represents an entirely new evaporation technique which possibly can eliminate the major problem of scale formation, provide high pressure heating steam, and operate at large capacities. Operation of the proposed process is similar to that for the fluidized catalytic cracking process used in the petroleum industry. In one vessel solid salt particles are heated and fluidized by hot gases obtainable from several possible sources. This hot fluidized bed of salt particles is then transferred in a conventional manner to a second vessel into which sea water is sprayed. The sea water is vaporized by sensible heat from the particles, and its dissolved salts are deposited on the particles. When thus cooled, the particles are recycled to the first vessel for reheating. This is a continuous process which requires a side stream of salt particles to maintain inventory. Removal of entrained solids by cyclones can yield a stream of high purity. The high temperature steam produced in the reactor must be recovered in an economical manner. It could be used to generate electricity and then condensed and used for normal consumption. Or it could be used to produce additional water by a more conventional process such as multieffect evaporators. The only major problem seems to be a tendency of the bed to agglomerate from being wetted by the feed soh34
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion. However, in the experimental work, size of the reactor was a factor, because an effective spray system could not be designed for handling very small flows. In a larger commercial unit, this problem could be eliminated by properly designed feed injection nozzles. The proposed method could produce fresh water at about $2.85 per 1000 gallons. Although this is high compared to currently reported conversion costs, the experimental reactor is only a first approximation to a commercial facility. With refinements in design, cost can be reduced. Also the design reflects costs for a number of locations, and even in its present state compares favorably with some existing plants. Ex~rime*'
The fluid bed reactor system (Figures 1 and 2) is a modification of one previously used in the study of fluidized calcination of nuclear waste material by Jackson and others ( 7 , Z ) . Although sea water contains a great variety of elements (35,000 p.p.m. of dissolved solids), the major constituent is sodium chloride. In this work the saline feed solution was prepared by dissolving 560 grams of sodium chloride in water to make 16 liters of solution. At the beginning of each run, approximately 1000 grams of salt (-42 to +80 mesh) was charged to the reactor. The heating furnace was then brought up to operating temperature with the preheated fluidizing air running through the bed. After steady operating temperature had been achieved, salt solution was fed to the
,...
JAMES KANYOK
ROBERT
s.
'
BRODKEY
reactor by a calibrated Lapp W e e d e r . To disperse the feed into as tine a spray as pmible, a stream of atomizing air was used. After pansing through a cyclone and glass-fiber filter, the overhead water vapor was condensed and analyzed for chlorides. Upon completion of a run, the reactor was disassembled and the salt separated from the copper shot, screened, and weighed. Large agglomerates were separated by hand while the smaller ones were determined by difference. Carryover to the filter was determined by washing the filter with a measured amount of distilled water and titrating with silver nitrate solution. This method was also used to detamine residual salt in the cyclone. A cooling jacket was required on the feed tube to prevent its plugging by premature boiling in the tube. However, this led to the tendency of the bed to agglomerate by being wetted from the feed solution. The problem was partially solved by using a small outlet on the feed tube, to get better atomization, and regulating the cooling water, to allow the feed to enter as hot as possible without actually boiling in the tube itself. This allowed runs of about 45 minutes for the low feed rates and 25 minutes for the higher rates before agglomeration k a m e significant enough to impair the fluid qualities of the
TABLE 1.
DATA AT VARYING SPACE VELOCITY AND CONSTANT FEED RATE
Run7
Run2
Run 3
6.02
5.54
6.47 3.53
Feed rate, ml./min.
% bed agglomerated yobed to cyclone % bed to filter
Efficiency of cyclone Purity of water, p.p.m.
TABLE II. DATA AT VARYING FEED R I T E AND CONSTANT SPACE VELOCITY
Run4
Run2 space docity, it./.%% Feed rate, ml./min. Temperature, F. yobed agglomerated yobed to cyclone yobed to filter JBiciency of cyclone h & V of Water. D.D.m.
2.42 2.24 6.54 11.29 785 765 22.6 5.54 1.50 2.20 0.0388 0.0628 0.971 0.972 70 269
Run5 2.28 9.46 755 7.79 1.84 0,0597 0.967 212
bed. A series of five runs was made to give three runs at various space velocities with a constant feed rate and three runs at various feed rates with a constant space velocity (Tables I and 11). In each case the salt balance was excellent (2).
AUTHORS James Kanyok is with t h Monsmlo Chemical Co:, Texas City, Ta.,and Rob& S.Brodkey is Associate Rofesmr in thc Chemical Enginem.ng Depmmrcnt, The Ohio Statc
Univcrsity, Columbus, Ohio. VOL 56
N0.4
A P U I I 1964
35
Discussion of Exp.rim.nbl Rerulh
As the feed rate is increased, salt content in the fresh water increases. This suggests that an increasing amount of very fine salt particles, not separated by the cyclone or filter, is produced as the feed rate is increased. This problem could be solved by designing more efficient cyclones, or by operating at a feed rate where formation of the fine material is kept at a minimum. Actually, the purity of water desired would dictate the conditions here. Bed agglomeration was nearly constant for a given feed rate and increased as the feed rate was increased, owing to increased wetting of fluid bed. It should be noted here that even though specially designed nozzles would tend to decrease agglomeration, a point will be reached at high feed rates where agglomeration cannot be controlled. That an increasing amount of salt carry-over to the cyclone at increasing space velocities occurs is to be expected. The main controlling factors in the design of a commercial unit are the purity of water desired, feed rate, and space velocity. All of these factors are interrelated and even though wide limits of operation may be set by proper design of equipment, the effect of changing one on the others should be considered.
A central aim of the design is to prevent excessive scale build-up on heat transfer surfaces. This is a problem that has plagued most processes now in operation that utilize heat transfer surfaces. One method of alleviating this problem is to employ as dilute solutions as possible. In the proposed design, the maximum concentration.of dissolved solids is 5% in any stream of the heat recovery system. Another method of reducing scale formation is to keep the temperature of the streams reasonably low. This has been adhered to as closely as economic heat recovery will allow. In addition, the various scale prevention methods now employed by some of the processes can be utilized. These simply involve the injection of chemically treated scale particles into the stream, causing the incoming scale to deposit on the suspended particles. In the reactor itself there is no scale problem because the saline feed is injected directly into the fluidized bed of salt particles with the resulting new salt particles becoming part of the bed itself. Although details of the bed design havenot beenemphasized, it is clear that for a large commercial unit, a great deal of care will have to be exercised and further study will be necessary in order to avoid agglomeration, handle fines, and eliminate possible corrosive action of the high temperature streams.
Design of a Convenion Piow
In the suggested commercial plant (Figure 3), hot flue gases are used to heat and fluidize salt particles in a salt heater. The sensible heat of the particles is then transferred to sea water in a fluid bed reactor where water is vaporized, leaving the dissolved solids to crystallize and become part of the bed. For economical heat recovery, the steam produced in the reactor is used to produce additional fresh water through the utilization of conventional multiple effect evaporation. Additional heat recovery is realized by using sensible heat from the stack gases in a waste-heat boiler and in preheating fresh sea water feed. The alternate procedure where the steam can first be utilized to generate electricity before condensing into fresh water is not considered here.
-S Figure 1. 36
Thefluid bed reactor systm
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Figure 2.
-DISCHARGE INLEI
Thefluid bed readw unit
Economics d the Plant
,
To compare cost of producing fresh water by the various processes, the Office of Saline Water has established a procedure whereby all estimates are placed on a comparable basis (4). The economics of the proposed plant presented here were estimated according to this standard procedure and, although the cost estimates are only preliminary, they do represent the order of magnitude for the cost of the water under the assumed conditions. Also, they permit valid comparison of various processes. The estimated cost of producing fresh water by the proposed plant is $2.85 per 1000 gallons based upon I-million-gallon-per-day plant. Although this cost may seem high when compared to the reported cost of approximately $1.40 for the demonstration plants of the Office of Saline Water, some specific factors are worth mentioning. The proposed process compares favorably to plants operating from 1950 to 1955. During thii period the cost range was from $2.00 to 14.00 per 1000 gallons. However, the proposed design is only a first approximation (the only one tried) to a commercial plant, and therefore the cost should be much lower for a plant in the final design stage. Other items to be considered in comparing costs are salinity of feed water used, quality of fresh water, availability of energy, and location of the plant. Most of the reported costs for existing plants are for their particular locations and do not necessarily represent the cost at another location. For example, the demonstration plant a t Freeport, Tex., utilizes waste steam from The Dow Chemical Co. a t approximately 30
cents per 1000 pounds. Since energy costs are at a significant factor in the over-all cost figure, the reported cost of $1.50 per thousand gallons would be considerably higher a t a location where cheap steam would not be available. In the design presented here, an attempt has been made to present an independent plant that would reflect costs independent of location outside of those without any source of fuel. The design itself does not depend on the use of natural gas as such, but may employ other types of fuel as well. Also a major source of electricity is in itself not a prime prerequisite. In a large plant the steam produced could be utilized in generating the electrical requirements. The Office of Saline Water has made a preliminary evaluation of a similar system (5) proposed by the Battelle Memorial Institute. It also concludes that the costs would be higher, owing mainly to the large size of fluidized bed necessary for adequate heat transfer. However, in the Battelle design the salt bed is to be heated by condensing steam in an exchanger; this transfer is relatively inefficient and requires a large surface area. In the design suggested here, this disadvantage is removed by using a balanced two-bed system with direct gas to particle heat transfer. REFERENCES (1) Jscksoo, 1. D.,So mfi, H. A., Wilmx,
G. A., Bmdkcy, R. S., Iwo. &a. Cnay. 52, 75%8 (198). (2) Kan ok J M S tbab in Chcmical En#heeriw. The Ohio State Uoivmity,
mnmk, oh, ii96z). (3) Muller, J.G.,Ckn. Ev%.h~r,59,No.12,538(1963). (4) U.S. Dcpt. of& Interior, OmccofSar? W i t a “Standardized pmccdurefa bli-tiq Cau of salioc Waf= Convmm,” Ma;ch 1956. (5) U. S. Dcpt. of the Interior, Offvc of Saline Water, “Saline Wafer Convcmion Report for 1962,” January 1963. VOL 56
NO. 4
APRIL 1964
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