Indian Journal of Engineer ing & Materia ls Sciences Vol. 9. October 2002. pp. 330-334

A novel double distilled water plant: Fully automatic without automation

R Prasad

Department of Chemica l Engineering & Technology, Institute of Technology, Banaras Hindu Un iversity . Varanasi 22 1005, India

Received 14 Morch 2002; occep!ed 5 Allglls! 2002

A novel double distilled water plan t has been designed and fabri cated w ith the main obj ecti ves to overcome the draw­backs of ex isting conventional plants and to save energy. T he deta il ed schematic drawing of the plant is given; its operating proced ure is al so discussed. Its work ing is based on a non-convention al method of vapori zing water. A ll the aspects of the experimental observa ti ons and results arc discussed thoroughly . The rate of distill ation is observed to increase w i th ri se in the cumulati ve vo lume of Ihe di still ate. The proposed plant has got versalile appl ications. Single di still ed waler can be pro­duced from Ihe same unit. Good quality py rogen free distilled waler is obtained.

Double di still ed watcr is used for prec ise analysis in educational, medical , biochemi ca l, environmental and industrial laboratories. It is al so used in large quanti­ti es for infusion of drugs in the body of pati en ts in hospitals. In conventi onal di stillcd water plants, heat ing fi laments embedded in either metalli c or ce­rami c cas ings are used which offer res istance to heat transfer from the hot filament to water. Further, heat transfer is reduced due to the insulatin g effect of scales formed over the coveri ng of the fi lament s dur­ing use. These factors result in hi gher energy con­sumption and the effi ciency of the plant also goes down. A more serious drawback in addition to the wastage of energy is the over-heati ng of the filaments caused by one or the other reasons, e.g., dry heating of the filam ent s in case of the Lli lure of water suppl y, hi gher vo ltage supply and excess ive scaling on the coverin g of the fil aments whi ch reduce the life of the fi laments. Very often, rep lacement of the filam ents is requ ired. Further, tv\"o separate heaters are used in the conventi onal units for the prod ucti on of double di s­till ed water. Failure of either one of the heaters stops production of doub le di stilled water. Automati c di s­ti li ed water plant needs ex tra cost of automation. Heaters burn-out is frequent ly observed even in case of automati c di sti lIel s.

The present di stillati on dev ice not only overcomes

these problems and deficiencies of the conventional uni ts but also possesses a number of other exceptional features, as di sc Ll ssed in the paper. The present work is an ex tension of the author's earlier work Ion a novel

sin gle di still ed water plant. The literature records no

such other work. The quartz double di sti lieI' , patented by MIS Bhanu Scienti fic I nstrument Company us i ng heating filament as a source of heat , is quite different than the one used in the present plant.

Experimental Procedure Experill/ ental set-lip - The dctailed scilcmatic

diagram of the di still ati on plant fab ricated from corning glass is shown in Fi g. I. The plant consists of

C OO l i n g

o

ill Double dis t ill e d

~ wo ter

--=*--~ Fig. I - Schematic drawing or thc nove l double distilled \\,atc:r plant : A (ammcte' ), B (teflon cork), C (condenser) . E (clec trodc:s). F ( fceding funnel), H ( insulation or tilerrnocl lc), I (dra inage), P ( fuse cut-out). S (sti ll ), V (vo ltmeter), W (wattmeter)

PRASAD: A NOVEL DOUBLE DISTILLED WATER PLANT: FULLY AUTOMATIC WITHOUT AUTOMATION 33 1

a single pi ece of two coax ial cy lindrical still s of 4 L (SI) and 3 L (S2) of net holding capaciti es and a flanged lid with ground joints for fitting a top con­denser (C I) and a pair of verti cal electrodes (E ,). Two parallel hori zontal electrodes (E2) are placed at the bottom of the outer still (S2)' For visuali zati on they are shown on the same hori zontal axis. The electrodes are made-up of stainl ess steel sieve and fitted to the stills through tefl on corks (B). Both pairs of elec­trodes are connected in parall el to the mains via a voltmeter (V), an ammeter (A) and a '.vattmeter (W). A fu se cut-out is provided for safety. A glass tube fitt ed with a thermometer (T) is prov ided to measure the temperature of the first di stillate running down from the condenser (C I) to the still (S2) ' Thi s tu be is fixed to the condenser and the still with the help of ball and cup joints. Prov isions fo r feed water with overfl ow (F) and for co ndensing doubl e di stillate in a side condenser (C2) are made as shown in the figure. Water level in the still can be adjusted to any des ired pos iti on by adj usting the verti cal placement of the feeding funn el connected with po lythene tubing to the dra inage pipe (I). The change in the water leve l in the sti II causes change in the acti ve area of the electrodes and hence, the rate of di stillation can be varied eas ily to the desired level. Proper insulation (H) of thermo­cole and coaxial arrangement of the still s in a single closed system prevent heat losses, and increases the thermal effic iency of the device thereby. Drainage plug (I) is provided at the bottom for parti al cleaning of the sti II (S I)'

Method - The plant works on the principl e of the heat generated due to the res istance offered by water through whi ch the elec tri c curren t fl ows between the electrodes. A few minutes after the power is sw itched on, water in the still s starts boiling and first di stillate starts coming out from the top condenser fl ow ing down to the still (S2) co ntaining already a di st ill ed water so lution of potass ium permanganate (250 mg/L). Addition of potass ium permanganate so lution in the beginning is essential to make first di still ate conducting. No further additi on of potassium perman­ganate is req uired. The vapors produced in the still (S2) condense in the condenser (C l ) to give double distilled water. Water equi valent to the di stillate is fed automatically in the still (SI) through feeding funnel connected to the condensers in series. The excess amount of water flows out of the over-now port of the feed ing funnel and goes to sink, and thus, the water level in the still (S I) remai ns constant.

The concentrati on of salts in water in the still (SI) increases due to continuous di still ati on and conse­quently, the elec tri cal current increases th rough the electrodes (E I) resulting in a ri se in the rate of fi rst distill ation. The variation in the rate of first di sti llate which enters still (S2), causes change in the level of the water and the acti ve area of the electrodes (E2),

conseq uentl y the rate of double disti ll ate also varies. Hence, a true steady state condition can' t be achi eved before saturation of sa lts in the still (SI)' But, ror all pract ical purposes, readi ngs recorded in very short duration of about 5 min can be taken as under pse udo­steady-state conditi ons (PSSC). The water level in the still (SI) is adjusted and the readings regardi ng the rate of di stillation , cumul ati ve vo lume of distillate co llec ted, energy consumed, vo ltage appli ed, current pass ing and the time elapsed are recorded under PSSc. Data are presented in Fig. 2. Constant voltage is maintained with the help of an autotransformer.

For better heat recovery, the temperature of the first di sti ll ate enterin g the still (S2) should be above 90°C and that of the feed water entering still (SI) should be above 70°C. Thi s can be achi eved by pro­viding suffi cient condensing area and by contro lling the fl ow rate of coo ling water. To avoid excessive deposition of solids on the electrodes as their conce n­tration increases wi th disti lI ation in sti II (S I) and be on the safer side (as the curren t al so increases), it is ad­visable to flush-out a part the con tents of the still after

Vo lume of distillate , L

'5r------ °r---__ 5r-____ 'T° _____ 'T5 _____ 2T°-,5 Heat ing--l-------- Distillation -------;--:--1

220 v

12 4

~ 9 :;;

.:.: -~ c '-

'" '" '- 3 '- / 0 :::J 0. u 6 I 2

I I

I I

I

3 I

O~-----OL. 0-----0~.5-----1~.0-----'~.5-----2~.0~0

Rate of dist illa tion, Llh

Fig. 2 - Characteri sti c curves for the nove l double distiller

332 INDIAN J. ENG. MATER. SCI.. OCTOI3ER 2002

every 15 L o/" di sti lla ti on or when the current reaches 14 A , by lowering the feeding funnel to sink. The funnel should be brought back to it s original positi on 10 allow tap water to enter the still and in thi s way disti llation cycle can be repeated several times.

For overni ght operation , th e initial rate is adjusted by manipulaling the level of water in the still (SI) such that about 15 L of water is distilled during the night. Next morni ng the distillation cyc le, as men­ti oned, can be repeated, if needed. By opening drain­age plug (I) , bigger so lid particl es collected at the bottom can be removed with the help o/" a brush when the unit is shut-ofT. In thi s way, the device can be made capable of working continuously without any interrupti on in the production o/" di stilled water for many months. After about every 1000 L of distilla­tion , the lid should be opened anel the wall of the still be cleaned off the sca ling. The hard scaling on the electrodes can be removed with the help o f a knife. There is pract icall y no depositi on of so lids on the electrodes (Ec). Hence, cleaning o /" still (Sc) is not re­quired. The life of the electrocks is very long as in the present case, no need is felt to change these even after the production of more than 5000 L o/" the double dis­t i lI ate. The electrodes arc eas i ly replaceable.

Even in the event of the failure o f wa ter supply, the water in the still s v<'lporizes and the electrodes remain intact and safe. On resumpti on of water supply , the plant starts worki ng automat icall y. A so lenoid val ve (X) can be used to automati ca lly regulate the supply o/" wa ter used for cooling in the condensers and boil­ing in the still s so that the wastage of the water can be avoided in the case of th e failure of power supply. The same plant can be used for the production o f sin­gle di stilled water by collecting the first di stillate /"rom the condenser (C I). The still (S2) can be used as a re-boiler for separation o f liquid mi xture kept in the stil l (S I) with boili ng point o/" one of the components

less than IOOcC at atmospheric pressure.

Results and Discussion When the plant is sw itched on , temperature of the

sti II (S I) increases f rom the room temperature to the boiling point of water w ithin a few minutes. Current al so increases to til e maximum value and then drops sharpl y to a lower va lue on commencement of boil­ing. Further, a slow drop to the minimum value 1'01-100ved by gradual increase in the current with con­tinuous di stillation is observed (Fig. 2) within the safe operati onal limit. This behavior can be explained as

follows: Tap water contains a number of di sso l ved salts most ly comprising of carbonates, bicarbonates, sulfal es and ch lorides o f calcium, magnesium. sodium and potassium, and shows conductance due to the presence o /" the ions o f these salts. The mobility of the ions (conducti. vity) increases as the temperature of the water increases causing ri se in the current in i tially . When water starts boi ling, most of the bicarbonates responsible fo r temporary hardness of water are de­composed to less so luble carbonates dec reas ing the concentration of ions in the water. Al so, some portion of the elec trodes is covered by the issuing steam bub­bles decreasi ng the acti ve area of the elec t rode. : con­sequentl y, a steep fall in the current is observed on commencement of boiling. On continuous boiling, remaining bi arbon ate. in the still decompose as be­ing not an instantaneous process, causing a further low rate of decrease in the current to reach the mini ­mum value. To substantiate the sta tements, an ex­periment was performed to observe the variation of conductivity and temperature of water with respect to the time. The results presented in Fig. 3 are in perfect agreement wi th the gi ven reason i ng.

Water containing impuriti es is fed continuousl y into the still and distilled water in the form o f steam is removed and thus, the impuriti es are le ft behind. A s a result, the concentration o f ions causing permanent hardness such as sulfates and chlorides o f calcium and magnesium along with the other so luble salts in­creases towards their respecti ve sa turati on va lues. Therefore, a linear increase in the cu rrent with in­creas ing cumul ative volume of disti l late prod uced is observed (Fig. 2), within the ex perimen tal limits. The prec ipitation of dissolved salts causing perrnanent hardness begins beyond their respect ive sa turat ion va lues and i t results in the depos iti on in addi tion to the decomposed sa lts causing temporary hJrdness 0 11

0 · 7 .-----~------~-------~

120

100

vop ori z ing - --------1,0

0+----,-----,-----,-----,-----,----,---_,----_,-_ .-1 a 10 20 )0 i.e !iO 60 70 &0 90

Timt' ,min

Fig. :1 - Varia tion of conducti vi ty of tap water- and temperature as a functi on of ti me during initial heating and boiling

PRASAD: A OVEL DO BL E D ISTILLED W ATE R PLA T: FULLY AUTOMATI W ITHOUT AUTOM AT ION 333

the elec trodes (E I ) and the wall of the st i II (S I) ' The solubility data" of some of the salts dissolved in Ihe tap water are gi ven in Table I . It can be seen from the tabl e that so lubility of the salts increases with tem­perature except sulfate of ca lcium and sod ium. It has never been tried to reach sa turation becau se of the safety limitati on of the current. Therefore, scaling is mostl y caused by the decompos ition of salts causing temporary hardness along with few salts responsible fo r permanent hardness whose so lubility is very low at boiling point of wate r. Graduall y more and more salt s arc depos ited prod ucing a thi ck layer of scale .

It is observed that the layer or sca le on the elec­trodes is very hard whi le that on the wall of the sti ll is qui te so ft. The salts deposited on the electrodes are conducti ng and hence, th e elec tric current passes through scaled elec trodes easi Iy. Therefore, care has been taken during des igning of the plant to keep suf­fici ent gaps between the elec trodes as we ll as elec­trodes and wall of the still . A lso, the electrodes (EI) are positioned verti ca ll y to avo id short-circuiting due to bridg ing of the elec trodes or touching the scaled wa ll of the still . Verti cal pos itioning of elec trodes have other benefits like, changing the rate of distilla­tion by changing water level in the still and easy cleaning of the still .

A s the cu rrent increases with continuous di still a­ti on, the rate of distill ation al so increases. It can be . een from the characteri stic curves fo r the double dis­tilled water plant (Fig. 2) that at 220 V, an increase in the current from 3.6 to 14.0 A, increases the rate o f distillati on from 0.5 to 2.0 Llh and the corresponding power requ irement increases from O.S to 3.0 kW. Fig. 2 al so shows th at I S L o f the distillate is pro-

Tab le I - So lu bi lit y o r some o r the sa it s d isso lved in the tap wa te r

Compounds So lubility

O°C 100°C

Ca lc iulll carbonate 0.00 14 0 .002

Magnesiulll carbonat'~ 0.0 11

Ca lc iu m chlo ride 59.5 347.0

Magnes iulTI chlo ride 52 .8 73.0

Ca lc iu lll sulfate 0.298 0. 162

Magnesium sulfate 26.9 6!U

Sodium chlo ride 35 .7 39.S

Potass ium chl oride 27.6 56.7

Sodi ulll sulfate 48.8 42.5

Potass ium sul fa te 7.4 24 .1

duced starting w ith the initi al minimum current o f 2.5 A at room temperature which increases to 7.0 A at boiling point and then drops suddenly to a minimum va lue of 4.0 A on commencement of boi l ing. Further, gradual increase in current reaches to 14.0 A in abou t 14 h o f continuous operat ion.

A comparative study shows that the present plant fo r producing double di still ed water is more effi cient than a con ventional type having two separate heat ing filaments. The convent ional plant consumes 2.0 kWh energy/L for the producti on o f double di still ed water whereas; thi s value is 1.5 kWh for the nove l plant. T he di still ed water produced by the present model in the laboratory (prototype) is o f appropriate qualit y as shown in Table 2.

The quality o f distill ed water is pyrogen free and free of heavy metal s. Comparing the va lues of p H, conducti vity. and di ssolved so lids correspondi ng to the single dist ill ate, double distillate and tap water, it becomes obv ious that the performance of the device is very sati sfactory.

The performance or such a di stillati on plant de­pends on the concentrati on of dissolved so lids in the tap water. The concentration of di sso lved so l ids in underground water supplied in our uni ve rsity varies according to the seasons. It has been found maximum (5 10 mg/L) at the end of summer and minimum (3S0 mg/L) at the end of rainy season .

I t has been observed that i f the power fai Is just af­ter commencement o f boiling of water and the unit remain closed for sufficient long time so that the wa­ter in the still gets coo led, then on resumption of power, the ti me taken for boiling in the still (SI) is more than tw ice the ti me taken to boi I fresh tap water in the beginning. Thi s i because the boiled water contains lesser amount of ions (due to decompos ition of bicarbonates) than fresh tap water. The conducti v­

ity of fresh tap water at 30°C is 2.7x I0-4 S/cm, while

Table 2 - Specific parame le rs regarding the novel dcv ice and til e d istillate produ ced

Paramete r Tap wate r Si ng le di s- Double di s-till ate tillate

Capac ity. (Llh) 0.5- 3.0 0 .5- 2. 0

Power req uired , (kW) 0.4- 2.2 D.H-3.0

p H 7.76 6.79 6 .9-1

Cond uc ti vity at 30°e. 2.7x 10-1 4 .3x 10-8 I.-l ~ IO-S

(S/c m)

Disso lved solid s, (mg/L) 400 Zcro Zero

334 INDI AN J. ENG. MATER. SCI. , OCTOBER 2002

16

14

12 I , , 10

<l

C ~

I I II'

C"O, "0'"0"/ ~ ~ u

nov

o +----.-----.----.---~----~----~--~ o 8 10 12

Volume at distillate. L

Fig. -l - Conlinuous distill ati on with illtcrillillcnt partial drainage

it is 1.6x 10-l S/CI11 of boil ed, filt ered and cooled water.

Fi g. 4 shows the vari ati on of current during con­tin uous di stillati on with intermittent partial drainage of the content of still (SI) . About 6.5 L of doub le di s­till ed water is produced starting with boiling current of 9.0 A reaching 14.0 A during a cycle of 4 h operati on.

Parti al drainage is sugges ted because of the fol ­lowing obj ecti ves: ( I) To a vo id excessi ve deposilioll

(~r solids -- As sca ling is mainl y due to Ihe deco m­posed bi carbonate salts, partial drainage foll owed by feed of tap water in the still means presence of lesser amount of bicarbo:lates in comparison to total re­placement of still water by tap water; (2) To relllaill

II 'illiill Ill e safe lillli/ of clIrrelll -- Ri se in current is caused by increase in the concentrati on of so luble salts of sulfates and chlorides. Solubiliti es of these salt s are very hi gh and current corresponding to the saturated solution at boiling point may be beyond the limit of safety. Partial replacement of still water by lap water decreases the co ncentration of these salts, thereby lowering the current to remain within the safe limit ; (3) To lIIake dislillalioll a llllOsl COWill/IOIIS -- It takes so me rime with total fresh feed of tap water to start boiling. Partial drainage followed by feed of tap water starts boiling immedi ately as the remaining content is hot itself ;l nd it takes some time to reach the constant level in th r~ still ; and (4) To dislill 01 Ilig lz er

,We -- Foll ow ing the procedure, the variation of co n­centrati on of so luble salts occurs within a narrow range of hi gher values resulting in hi gher average rate of di stillation .

Conclusions The proposed new doubl e distill ed water pl ant is

free from the drawbacks of conventi onal plants and is more energy effi cient. Some of the exceptional fea­tures of the new device are: Full y automatic but with­out any automation contri vance; Free from costly heating fila ments and their frequent repl acement ; Free from any ri sk of dry heatin g of filament s on fail ure of water supp ly; Fl ex ible in use and conve ll ient to oper­ate; Capable of producing single as >ve ll as double di stilled water; Capable of working sati sfactoril y even under wide range of voltage fluctuati ons from 50-300 V ac suppl y without any ri sk of damage; Capable or producing di still ed water unattended day and ni ght; Contro lling the rate of di still ed water producti on within the specified range of 2 Llh: Prac ti call y free from maintenance; Easy-to-fabri cate locall y with the help of a glass blower; No ri sk of contaminati on of the di stillate at the transfer from the first to the second stage because of the simull aneous and continuous working of the two di stillati on stages ; Thermal effi­ci ency (94%) based on theoreti cal power requirement; Great benefi ts to the pharmaceuti cal laboratori es and industri es. The plant can be fruitfull y utili zed in re­search , educational , pathol ogical, environmental, and any other such laboratori es. Al so, the stainl ess stee l di still er l after fittin g with safety val ve. pressure gauge and del i very val ve could be used as a boi lerlre-bo i IeI' for vari ous uses such as steam di still ati on of essential oil s.

The performance of the device is quile sati sfactory. It has been catering to the needs of di stilled water to all the laboratories of our department.

References Prasad R, I lldiw/ J Ellg Marer Sci, 3 ( 1995) l OS.

2 Lil ey P E & Gambil l W R in Chelllical 1:·lIgill eers , Halld­boo/.:, edited by Perry R H & Chil lO l~ C H, Fifth edn (McGraw- Hili Kogakusha, Ltd ., Tokyo), 1973,3-6.