Automatic pH Control in Water and Industrial Waste Treatment ROBERT T. SHEEN 1300 East d f e r m a i d iice., Philadelphia 18, Pa.
AAutoiutctic ~ I control I is of increaaing Falue i n water t r e a t m e n t systems for p r e i e n t i o n a n d control of corrosion, and for preFention of scale deposits. It is also used in t h e neutralization of excess alkalinit? following h i e - s o d a softening, i n m a i n t a i n i n g o p t i m u m pI1 for proper coagulation, and in a u t o n i a t i c blending of Ii?dropeii and s o d i u m zeolite softened water t o obtain desired residual alkalinit,. In waste disposal sjstenis. a u t o n i a t i c pII control is frequent11 required t o control neutralization of acid antl alLaline wastes a n d t o assure mainteirance of proper pH for satisfactor) coagulation. The element< of design a r e described. T h e buffer action of t h e s1steni is inip o r t a n t . T h e r a t e and degree of c h a n g e of pII of t h e raw liquid n i n s t he eialuated antl t h e s?stcni debigned accordi n p l ? . Retention period, proper mixing of reagent with the liquid. c.1ioic.e of sampling point n i u s t be considered. T h e mechanics of a u t o n i a t i c p l I c*ontrola r e rleicribed: the three elements required being the p11 nieter. the c.011troller, antl t h e chemical feed or regulation de\ice. 6 ‘ here two iariables arc present Luch a s Fariable flow with changing \ nriable composition. correc-tion for eac.11 \ ariaIJe ic desired, a n d t h e control *? z t e m and cheniical feed de\ic e niii*t be designed to conipenwte for the changes i n e i t h e r o r b o ~ hiariables. Seieral typicsal insta1latio11- of autot i l a t i c . 1’11control a r c tlewrihed.
+--Figure 1. S a m p l e Cooling for pll I\Iearurement of Boiler Feed W a t e r COURTESY
L E E D S & Y O R T H R U P COMPANY
T
HE importance of pH control in n-ater treatment has been receiving increasing attention during t’he last ten years. Langeliei,‘s n-ork (1), showing the correlation of pH with calcium ion concentration, alkalinity, temperature, and total ionic activity, was important at the start of this trend, and a number of investigators since have published papers on t’his suhjcct, with furtlicr applications or modifications of the Langelier theory ( 2 , 6, 7 , f f ) . Prevention of corrosion or of scaling is therefore directly cwrrelated t o pH control. Some of the excess allialinity of n-atcr softencd by lime arid soda must be reduced and &o tlie pH valuc must be decreased, for prevention of Lti‘terI.”ecipitation, although certain chemicals now on the market-namely, ,sodium hesaiiictaI)hosphate-liave considerable value for p ipitation, particularly in the cold. 1-arious coagulants, lum, ferric,chloridc, ferric sulfate, and sodium aluiiiiiiate, work be2t xt defiiiite pH ranges, depending on other niaterials presc’rit i n tlie water. Thcrefore pH control for good coagulation is requircd to assure proper results 11-ith a minimum of coagulant and aftc.rr)locipitatioii, follon.ing filtration.
C O U R T E S Y . LEEDS B N O R T H R U P COMPANY
Figure 2.
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pII 3Ieter a n d Controller for Boiler Feed W a t e r
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 39, No. 11
COURTESY
Figure 3.
W R i O H T A E R O N A U T I C A L CORPORATION
Flue Gas Scrubber, Compressor, and Automatic pH Controlled G a s F l o ~F a I b e for Herarbonation of Lime-Soda Softened Water
Wat,ers to be treated by the zeolite process must, as a rule. be neutralized before soft,eningin certain forms of zeolite and can be neither too alkaline nor too acid. Automatic p H control is frequently employed to assure proper preparation of water for this process. Automatic p H cont,rol is increasingly important in industrial waste disposal. Many iiidustrial n-astes are strongly acid or strongly alkaline and must be properly neutralized before they are discharged or in order t,hat they be fit for other treatment processes. ilgain the optimum pH for good coagulation n-ith t h e various coagulant,s is important. ELEMENTS OF DESIGN
Some buffer act,ion in an automatic pH control sgstvni is highl!desirable. It would prove difficult, for example, t o neutralize a solution of strong acid such as sulfuric n-ith a strong base such as caust.ic soda directly and in dilute solution, holding as a control point a neutral p H of 7.0. The neutralization curve is so steep that control on this portion of the curve xould be erratic. For such syst,ems where lit,tle buffer action is present, an increased retention t.iine in the reaction chamber is advisable so that, relatively small amounts of either liquid to be treated or chemical corrective are added at one time, compared to the total volume in the reaction chamber. Where phosphate, borate, carbonare, or other inorganics or organics are present that will act as buffers, the problem of control at, practically any desired p H is facilitated because of the loxyer slope of the neutralization curve, providctl it is possible to reach the desired pH by the addition of corrective acid or alkali. The changes in the pH of the raw liquid, both the rate of change and degree of change, must be known and evaluated. It must be remembered t,hat the term “pH” indicatcs the logarithm of the reciprocal of the hydrogen ion concentration, and that changes in p H are logarithmic and not straight-line. The retent,ion period for the chemical reaction may be as little as 5-10 seconds t o as much as 15 minutes, depending o n t h tem and these various factors. The less t,he slope of the neutral-
C O U R T E S Y , LEEDS 8 NORTWRUP COMPANV
Figure
4.
>lotor-Operated ValTe for Sulfuric Acid Treatment of Boiler Feed Water
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
November 1947
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designed, it is possible to measure pH over a \vide temperature range froin as low as 5' t.o 100' C. K h e n desired, a sample cooling mil can be installed prior t o the electrode flov cell (Figure 1). 31ECHANIC S
I n an automatic 1111 control system a variable is being measured that is the result of t,he react,ion of the chemical n-hose flon is coiit~rolled by tlie meter ir-ith t,he raw liquid being treated. The est;tblislinient of equilibrium in such a system might be likened t o a dog chasing its tail. There is a certaiii aiiiount of lag in such a svstem; thereC O U R T E S Y MILTON R O Y C O M P A V Y fore provision I i i u a t be made 111 tlie meter for opeiation of a contioller that call be adjusted Figure 5 . 1 al\e Design of a Controlled-1 olume PumD bbtli for tlie percentage of full scale range (also knon-n as throttling range) over nhich t,he controller is to operate arid for the rate a t nhich the increase ization curve a t the control point. tht: lon-eI tiit, retention tiiiic or decrease of setting (also kiioir-n as reset rate) of the chemical required for tile individual system. -1neutralizrttion curve for a feed is t o be effected. proposed system is of considerable value in making a recomniendaThis provision is necessary t,o d l o for ~ the factors discussed in tion for a possible application of automatic pH control. t,lie previous section-namely, buffer act ion and slope of the neuBoth the chemical reagent and the incoming raw liquid niust be tralization curve, retention time, r e a d o n time, rate and degree mixed intimately as rapidly as po6sible with all portions of the of change in chemical charact,eristics of the raw liquid, control liquid in the rcaction vessel and in a minimum of time. The point, mixing, sampling, and temperature. Figure 2 shows typisampling point from Jvhich a flow t o the glass electrode is taken cal p1-I meter and coiltroller.. When pFI meters and controllers must be selected t o give a n averagi. sample of the raw liquid ivith ir-ere first placed on the market, antimony-calomel electrode systhe chemical used for treatment. The electrode assembly can be tenis werc popular, Since that t,ime glass electrodes have been arranged pither for direct ininiersioii in the mixing chamber or for dcvc~lopedt,liat are highlj- sensitive and more satisfactory, and placenieiit immediately adjacent to the mixing chamber so that have been employed in all iristallations described in this paper. flow from t h r sampling point to thc elwtrode invol The pH meter in reality measures the electromotive force between possiblclag. It is important to knon- the time required for neutralthe glass electrode and the calomel, which is directly proport,ional ization t o take place. K i t h certain organics, or even with lime, to the hydrogen ion concentration of the solution. The pH consome time is required for the neutralization reaction to be comtroller, taking its impulse from t8hemeter, must funct'ion to act'uat'e pleted and t,o realize t,he full change in pH that n.ill result from the t,lie chemical feed or regular ing device. This regulating device addition of the treating chemical. The true pW of a solution changes with temperature. .in may be an air-actuated valve, a mot,or-operated valve, or the recently introduced controlled-volume pump. An air-achated understanding of the magnitude of the pH differelice hvtween measured and true pII when t,he true pH is a t an elevated temvalrc, for example, may be used to regulate the flon. of flue gas perature is necessary for a proper interpretation of the indicated used for recarbonation of a lime-soda softened water (Figure 3). I t may also be used for regulating larger liquid flows by actuation and recorded dat,a (3, 4,6 ) . With the glass electrode properly II$-VOLT
60- CYCLE A. C. S O U R C E
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METER
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p
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PUSHBU T O N
CONTROLLER
STATION
THYMOTROL
PANEL
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CHEMICAL SOLUTION TANK
MIXIN'C A N D R E A C T I O N T A N K ( C A N BE CLOSED AND IN A CLOSED SYSTEM 1
Figure 6. Automatic pH Control System with Controlled Volume Pumps For Constant .Main Line Flow.
pH controller sets length of pump stroke through motorized adjustment (one aontrol variable o n p u m p capacity). For Varying Main Line Floio. Speed of p u m p is directly proportional t o m a i n line flow through flowmeter by electronic control of motor speed. pH controller sets length of p u m p stroke through motorized adjustment, and thus gives full and independent correction of p u m p capacity for two metered variahles.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 39, No. 11
ated valva for controlling smaller quantities of liquid or the feed of slurries, such as a lime slurry. Controlled-volume pumps are admirably suited to the handling of slurries as well as solutions, since t,hey measure accurately by displacement the quantity of chemical t o be introduced to teni. These pumps can nlt'asure and inject chemicals in quantities from 1 pint per hour to as high as 20 gallons per minute per cylinder, and in the smaller capacities can pump chemicals against pressures as high as 20,000 pounds per square inch. Therefore these pumps makf. possible automatic pH control in a broad nciv field of application. They measure and punip in one operation, and as a result of the pump valve design the volume delivered is practically a constant. regardless of variation in discharge pressure (Figure 5 ) . The delivery may De aut'omatically regulated either through motor sperd by electronic control (9) or through automatic adjustment of the length of stroke of the pump. Through the medium of stroke adjustment, it is possible to obt,ain full range Figure i. T>pical Operation
