An assessment of vapor compression distillation

AN ASSESSbf EN-I- OF VAPOR COMPRESSION BADAWI L~nivcrsity

W. TLEl!klAT. of Culifornia,

Sea

EVERE-IT Itkter

D. HOWE

Conwrsion

ASD

Laboratory.

DISTILLATION* ALAN

D. K. LACRD

Richmond. Calif. (U.S.A.)

The tuo p;1rt~ of this paper deal with the sa\ing of energy which may be accomplished through staging of CGOor more vapor comprcskn plants and with an evaluation of the forced-circulation arrangement for the prwcss. Eltpcrimcnls using a smJll vapor compressor are presented IO exemplify [hoc two modes of operation. Algehnic exprtvsions for compressor work arc also presented and the ichults of calculations arc compaxd \\ith experimental information.

a,

-

activity,

c c,

-

spxific

h

-

J In P

_-

PO R s I T IYK Re Pr ;

--

IV

-

dimensionless heat of brine.

Btu :Ib’F

specific hear of wafr‘r vapor, Btu/lb’F en?GvY. i3tul;b convcrsitin factor, ft Ib’Btu ‘circulation ratio. (Ib of b&x/lb of proriuct) prrssurc, lbfsq in. vapor pressure, ibjsq in. gas constant, BtuJb ‘R entropy, Btu/lb “R temperature, ‘F absolute tempcraturc, “R work, Btu/lb of product Reynolds number, dimensionless Prandtl number. dimensionless latent heat of condensation. Btu_!lb difference sea water feed. (Ib of sea water/lb of product) INTRODUCTIOX

Vapor compression distillation has many attractive features and has been used extenGvely for small portable plants. However, for large plants the only process mentioned recently is thermal distillation, in spite of the superior thermodynamic l Paper presented at the Second European Symposium on Fresh Water from the Sea, May 9-12. 1%7, Athens, Greece. European Federation of Chemical Enginaring.

287 Desalination, 2 (1967) 287-298

288

BADAWl W. NMAT,

EVERETT D. HOWI

AND ALA!!

D. K. LAlRD

efficiency of the vapor compression cycle. In an attempt to assess the probkms and merits of vapor ompression, the University of Cahfornia Sea Water Conversion Laboratory has conducted small-scale experiments to explore certain features of the proo%. This paper deals with two of these experiments.

The basic equipment for the vapor compression process, shown xhcmaticaily in Fig. l,issimpk and requires a net ideal energy input, given by Tribus (1) and others, equal to

Under the tcmpcrature conditions generally encountered. thr water vapor obeys the perfect ga: law and u, = pa+,, so that the work of the cycle is equal to the work of reversibic isothermal compression from the evaporating side OCthe heat en&anger to the condensing side. If, in addition to overcoming the vapor pressure lowering due to brine concentration, the compression work is increased to provide a temperature difference, AT,, for heat transfer, as indicated in Fig. 2, the isentropic work involved is given by Chambers and Larsen (2) as follows:

(2) The work computed from this equation is shown graphically i.1 Fig. 3. Values for u, for sea water were taken from Tribus (I). It should be noted that, if no heat energy is to bc used, the incoming saline water must be preheated to the evaporation temperature, a condition closely approximated by countercurrent interchange of heat between the incoming water and the outgoing streams of brine and distillate. Desahdon,

2 ( 1967)287-298

AN ASSESSML’NT OF VAPOR

COMPRESSION

DISTILLATION

289

T t

I

I

r

-,_A_------,

Er:“g(i’l

?C -

5’G.

b

@a;,

S

Fig. 2. The vapor cumprrsston process with lxntropic compression. (Comprusion ad o~~~omcs vapor prcssurc lowering. Comprasion d-b provldn heat transferdifTercntia!.)

guTLET

Operating

SLL!PJ!TY

iz : %l IO’ldT!

Fig. 3. Compression work for vapor comprcsslon still. on 3.5 percent sea water. with evaporation at 212.F, as predicted from Eq. (24 and lower curve of minimum work as prdicted from Eq. (I).

Desalinadon, 2 (1%7)_287-298

290

E.QDAWI

W.

TLEIMAT,

EVERJZIT

SERIES

OPFIRATION

D. HOWE OF

RRISE

AND

ALAN

D.

K. LAIRD

SYSTEMS

will bc observed that, in the simple arrangement of equipment shown in Fig. 1, the incoming sea water is mixed with the brine in the evaporator, SO that the vapor formed has a pressure corresponding to the salinity of the outgoing brine The work of compressing this vapor to the condensing temperature is appreciably greater than wouId bc required for io\+cr salinities. lt was thercforc proposed that two or mot-c vaporcompression plants bc arranged in series. For a given outlet brine conccntration,at lcast half of the vapor would then be produced at a pressure above that corresponding to the outlet brine concentration. and the work required for compression would lx appreciably reduced. To tect the above hypothesis, Chambers and Larsen (2) set up and ran a simple vapor-compression distiller, arranged as in Fig. 1. and varied the brine salinity from 3 to 17 percent. During these tejts. mcasurcmcnts wcrc made of the poucr used, at the flew rates of the various fluid stream!-, and the pressure and tcmpcnture various points in the system. The results of these tests, shown in Fig. 4, demonstrate it

I

-’

/

I-

I-

iINITIAL

a

a

3% INITIAL SEA WATER &FTER CHdMREAS dNO

,.

OUTLET

BRINE

IO CONCENTRATION.

I

15 C2 (%

solids)

isentropic work of compsesion. AT! were plotted from Eq. (2) for I, = 217’F. u~iog sza water data. Desalination, 2 (1967) 287-298

4. Experimental

Reference lines at constant

mm

SoLUTICl?.l-628r0m LdRSEN

I

I

5 Fig.

SEA WATER SOLUTION - 725

0

AS

the

very

strong

ASSESSMENT OF VAPOR

dependence

COMPRESSION

of the compression

work

DISTILLATION

291

on the salinity

of the

brine.

The lines of constmt. AT,. on this figure were computed from Eq. (2) and the points were bawd on measured state points of liquid and vapor. Applying these data to series of stagzd

hypothetical

systems, Chambers

and

Larsen

showed

that

power

savings of at leazt IO percent couid be nchievcd by placing t\vo plants in skes. FORCE!) CIRCL!:‘I.,\TIOS SYS JCblS

The Iargcst items of capi:al cost arc those for :he cvapora:or and the compressor. It had been proposed (S) that the cost of the evaporator be reduced by improving the cffcctivcncss of heat transfer through the use of forced convection. The rrrangemcnt of equipment ncedcd fo accompli!;h this is shoun in Fig. 50. from which it ~111 be

SEA

--

WATLO

Fig. Sa. Forccucirculation vaporcomoreszion still. (z$xnatic flow diag-

.

Tar,

-I-

Fig. 5b. Temperature profile inside condenser. (Letters and numbers correspond to Figs. Sa and 6.) seen that the circulation

of the brine through

ditional input of mechanical energy. given by the following expression: IYK,

The =

the flashing orifice will require

minimum

amount

of additional

MAP*

an ad-

energy is (3)

J

The work of compression is also increased, as shown in Fig. 6, since the vapor must condense at a temperature above that of the brine before flashing. The new expression for the ideal minimum work of compression is then: (4) Dedinoiion

,2 (I 967) 287-298

_

EAVAWIw. -rI.EmAT, EVEJEIT

292 values

AND

NAN

D. K.

LAIRD

have been mmputed for a ck Uktting of 8.3 percent, and the results have been plotted in Fig. 7.

for the work indicated

brine salinity

D. HOWE

by this expression

1

AT,-AT2”

EVAPOi+ATOR

tm-

tw

TEMPER

e

L ENTROPY

- SW/lb%

S

Fig. 6. The foradcirculation vapor compression proaS with islntfoPiC COmPrCSim Compression a-d ovcrcotnes vapor prc~urc lowctin~ Compnzssioo d-b provides heat transfer diffcrentiaI_ 1-2-3-t-l brine circulation qck corresponds to Figs. Sa and b.

;IRCULAiION Fig.

7. Expaimattal

RATIO

-

-tropic

l of Br~ncle

ofm,On

work of compression. Rcfcrrws kcs w-em plot&i from Eq. (4) for the indicated paramctcrs and salinity of 83 pacent. Drs&ta&n. 2 (1967) 287-298

AN AS!XSME?rT

OF VAPOR

COMPRESUON

DISTILLATION

293

By inspection of Eq. (4). it is seen that the work of compression decreases as nz increases and becomes asymptotic to vaIues from Eq. (2) as IPI becomes very large. Thus, the work of isentropic compression is greater for forced circulation than for the natural sircuiation evaporator for the same value of AT, as shown in Fig. 5b.

The cost of the increased energy must be balanced against the net change in the capital amortization char= for the equipment. The size and cost of the evaporator and condenser will be strongly affcyted by the overall heat-transfer coefficient. It appeared

from the literature that this coefficient might vary as much as tweto fourfold, due to the speed of circulation, but precise values would have to be determined by test. Thus, it was decided to undertake a test of forced circulation to determine both the energy required and the heat-transfer factors possible with forced circulation. An important detail of the flashing process is the tendency for drops of liquid to be carried over into the compressor and the condenser. These saline drops not only contaminate the product water but also may interfere with the operation of the compressor. as was reported for the compressor at the Roswell plant (S, 5). The use of mist eliminators in a vapor-compression system is undesirable since it imposes an additional energy input on the system, so that a vapor release device with minimum carryover of liquid drops was desired. Previous tests of small devices of tbis chracter-

istic had been carried out in this laboratory by Ewoldsen et al. (6). The device found most effective in these tests is shown in Fig. 8 and was chosen for inclusion in the test equipment. -.

‘NATEP FLOU

Fi

8.

LTD five-inchfalling film iokt. Desahwiim,

2 (1967) 2874W8

294

BADAWI

W. TLEIMAT,

EVERElT

D.

HOWE

AXD

ALAN

D. Ii. LAIRD

,-..

-.

: i

..-._r__-

c

;

i

Lksah-man.

2 (1967) 287~t98

As

ASSESSMENT

OF VAPOR

COMPRESSlOX

DISTILLA-IXON

29s

A forc~d-circulation vapor-compression distiller was assembled from the major components listed in Table I, arranged as indicated in Fig. 9 and shown in the photo; gnph of Fig. 10. By changing vapor temperature and compressor speed, the capacity of the plant could be varied from 150 to 600 gallons per day. Instrumentation of the

Fig. IO. Vapor compression pilot plant Note compressor and motor drive at lower left. product tank and vapor separator at right center. and ion c.xchangc prctrcatment plant in rear. TABLE I EQUIPMESF

Conipnssor

SPECIFICATIOSS

Sutorbilt. Model 6H. rotary, two-lobe, posithc Displacement: 0.22 cubic foot per rwolution

displacement

type.

Spced:Oto IOOOrpm Connections: 2.5-inch threaded pipe inlet and outlet. Djnamometcf

Condenser

Flash chamber Pressure release nozzk Circo!ating pump

Termina! heat exchangers

General motor.

Electric.

Model

AHl,

cradkd

synchronous

dynamometer

1to/240 Volts, 3-ph;Lsc. *clc A. C. 7.5-horsepower at 1800 rpm. Cast-iron headers and steel shell. sin& pxss. JO tubes. 0.375 inch o.d. >. 0.305 itxh i.d. x 54 inches long. Outside surface area: 17.7 square feet, inside surface area: 14.4 squase feet, mean arca: 16 square feet. Tube material: -10 Cupro nickel alloy. Steel chamber. 15.5 inches i-d. s 36 inches long with Z-S-inch flanged vapor outlet. 5-inch diameter falling-film. as shown in Fig. 8. Jacuni. Series D. 35-gpm. 50-foot head. 1750-rpm. directly coupled to a 1.5-horsepower 240-Volt AC. induction motor. Two shop fabricated concentric tubular hea? exchangers. 2.54 square feet each, made of brass tubes of 0.25 inch 0-d. x 0.206 inch i.d. inside tube and 0.50 inch o-d. x 0.430 inch i-d. outside tube x 18 inches long. arranged in series to txoduce countercurrent flow. Desalination, 2 (1967) 287-298

_

296

BADAWlw. TLEMAT, EERElT

D. HOWE

AND

ALAN

D. K. LAIRD

plant is indicated on the flow diagram and was arranged to show steadiness of conditions as well as to measure various quantities with the necessary precision. The “sea water” used in the tests was obtained from the San Francisco Ray at Richmond. it consisted of a mixture of ocean water and river water, the exact damposition varying with the season of the year, the phase of the tide, and the level of the river upstream. The relative proportions of the important ions in solution are not too diflerent from those in sea water by itself, so that this water was regarded as quite satisfactory fol the tests. The forced-circulation distiller was tested under a number of different conditions. It was possible to vary the brine temperature from 160” to 210°F and the brine velocity from 3.65 to 13 feet per second, and to run the compressor at speeds of 608,725 and 935 rpm. Combinations of the above variables resulted in values of the circulation ratio ranging from 100 to 400 and values of AT, varying from 3.6’ to 19.4’F. The experimental results are shown in Fig. 7, 11 and 12. The points plotted on Fig. 7 were computed from steam tables using the experimental pressure and temperature of the vapor before compression and the condenser pressure. Agreement between the individual points and the theoretical curves were within 5 percent for val-

Fig. 11. Computed condenser overall beat transkr cocfkimt. (Y’korctical line’*from Dittus-Bgllpax&n for brine and Nuuelt ---_ emutinn _____

AN WT 6

OF VAPORcoMPREssloN mnLLmxorc 1

I

I

7-

t,n-

At

-

0 . D 4

Al AT &T ttt

I

I

I

I 2OG

I 210

297

1,

2-4'F I-6.F 6-6-F a-IO’F

0 0

6-

0 .

0 I50

I 160

I 170 VAPOR

I I I90 I00 TEMPERATURE

220

-‘F

Fig 12 Resure drop of brine across Bash no$e to spcc~fial ftiuhing ranLines indicate vapor pressure diffcc~_yryonding

vues of m between 150 and 400 and for values of AT2 less than 12°F. This agreement indicates that Eq. (4) properly relates the isentropic work to the circulation ratio. The overall compressor efficiency, blsed on dynamometer data, was low and ranged from 28 to 42 percent, with the higher values pertaining to the higher speeds. These low values arc assumed to be due to the small size of the compressor, since Gieringer (5) reported that the compressor at Roswell operated with an efficiency of 95 percent. Heat transfer results of the tests are given in Fig. 11. in which the overall heat transfer coefficient of the condenser is plotted as a fuction of the product of Reynolds number and Prandtl number. The solid line was determined by calculation, using the Dittus-Boelter equation for the liquid side and the Nusselt equation for the condensing side. There is seen to be good agreement between experimental points and the computed curve for low values of flow and high values of compressor speed. The considerable and systematic deviation betwen experimental points and the computed curve at low compressor speeds is thought to he due to excessive heat loss from the condenser under conditions of low vapor flow and high pressure. The computed curve and the experimental points which lie close to it confirm the premise that considerable improvement in heat transfer coefficients results from forced circulation. It Desalinolion,2 (1967) 287-298

29%

BADAWI

W. TLEIMAT,

R’EREIT

D. HOWE

AND

ALAN

D. K. LAIRD

should bc noted that the ion exchange softening system appeared to be very effkztive in preventing the formation of scale on the heat transfer surfaces. The resuks of the tests of the vapor release device are shown in Fig. 12. in which the pressure drop across the nozzle is plotted against the temperature of the brine after flashing. The curves represent the vapor pressure differences across the nozzle corresponding to the flashing temperature ditfirenccs, AT. Experimental points above their corresponding lines rcprescnt conditions in which the pressure of the brine upstream from the nozzle is greater than the vapor pressure, while points below the line would indicate that the pressure of the brine upstream from the nozzle is less than the vapor presssure. Tchis latter condition would result in flashing ahcad of the nozzle and consequent fog formation. The fact that all of the cxpcrimental points iie above the curves means that the nozzle was conservatively designed, and presented excessive resistance a: low values oft,. At the same time, measurements of carryover, based on salinity of the distillate. showed very saisfactory operation without the use of dcmisters. The measured values of salinity lay bctwccn 2 ppm and 20 ppm, with the bulk of the points below IO ppm.

The two test programs described above suggest ways in which the very considerable advantages of the vapor compression cycle may be exploited. For example. wherever more than one compressor is used in a vapor compression distiller plant, serious consideration should be given to placing the brine systems in series. In any case, e\zry effort should be made to evaporate water from as low a salinity as possible. The use of forced circulation in any particular case should be examined critically, since the gain in cost amortization on the condenser may be more than offset by the added cost of compressor power. The effects of salinity and other factors on compressor power may be estimated reliably using Eq.(2) and (4).

REFERENCES 1. 2. 3.

4.

S. 6,

M. TRIBUS.et al., Thamodynamic and Economic Considerations in the Preparation of Fmsh Water from the Sea. Department of Engineering, University of California, Los Angeles, Report 59-34 (September 1960). J. T. CIUMBERS AND P. S. LARXS, Series Staging of Vapor Compression Distillation. Sea Water Conwrsion Program. University of California, Berkeley, Series 75, Issue 20, (May l!%O). B. F. D~LICE ASD A. M. ESHAYA, Economic Evaluation Study of Distillation of Saline Water by Means of Forxd-Circulation Vapor Compression Equipment, USDI, Offi of hlinc Water, Ruearch and Dcvclopmcnt Report (No. 21, 1958). P. L. GEIRIXCERet al., First Annual Report, Brackish Water Conversion Demonstration Plant No. 4. Roswe:l, New Mexico, USDI. Offii of Saline Water, Research and Development Progress Report No. 169 (May, 1%3-June. 1964). GEIRI~~~ et aL. Second Annual Report. Brackish Water Conversion Demonstration Plant No. 4, Roswtll. New Mexico, USDI, Office of Saline Water, Rescarc h and Dcvelopmcnt Progress Report No. 170 (May 1966). E. I. Ewomsm ct aL, Sea Water Entrainment in Low Temperature Flash Evaporators. Sea Water Conversion Laboratory Report No. 64-3, Uniwrsity of California, Bcrkeky, Water Resources Center Contribution No. 90 (December 1961). Desalination.2 (1967) 287-298