1、Henry Becker is V.P. of Technology, CTO, H-O-H Water Technology, Inc. Palatine, IL. Sean Parmelee is a PhD candidate at the University of Illinois Chicago (UIC), Chicago, IL. Demonstration of Chemical and Non-Chemical Cooling Water Treatment Principles and Performance Henry Becker Sean Parmelee Memb
2、er ASHRAE Graduate Student ABSTRACT A major concern in the operation of cooling tower systems is the prevention of scale on heat transfer and evaporative surfaces. The process by which scale forms is not often clear or well explained, and the means by which scaling may be controlled is commonly not
3、explained to any significant degree. This paper attempts to outline the mechanisms of scale formation, discuss chemically and non-chemically based strategies for scale control, provide real-time instrumental data illustrating the effectiveness of both chemical and non-chemical control and then discu
4、ss the basic requirements for successful implementation of each strategy given the many chemical, physical and operational variables generally associated with cooling tower operation. Each strategy has its advantages and disadvantages and no one strategy is well suited to all circumstances. It is ho
5、ped that this paper will help clear up some of the misconceptions concerning chemical and non-chemical cooling water treatment and promote more meaningful and informed discussion during the treatment strategy selection process. INTRODUCTION The principal and most common mineral deposit formed in eva
6、porative cooling water systems is calcium carbonate (CaCO3). Municipal and other sources of make-up water to cooling tower systems contains what is termed water hardness consisting primarily of dissolved calcium (Ca2+) and magnesium (Mg2+) in conjunction with bicarbonate (HCO3-) and possibly a low l
7、evel of carbonate (CO32-). Bicarbonate and is best suited for large industrial or power-generation cooling water systems. c. Reverse osmosis (RO) to remove over 90% of all species dissolved in water.RO produces excellent water that allows for significant reduction of tower bleedoff, but with high in
8、itial capital and subsequent operating costs. RO is usually not a practical option for cooling tower make-up water conditioning unless special conditions are encountered. 2. CHEMICAL TREATMENT OF COOLING TOWER WATERa. A strong acid such as sulfuric acid (H2SO4) may be added directly to the recircula
9、ting cooling water to convertbicarbonate and carbonate to carbonic acid, which dissociates into carbon dioxide and water. Carbon dioxide is then easily discharged to the atmosphere as water flows down through a cooling tower. 2HCO3- + H2SO4 H2CO3 +SO42- CO2 +H2O + SO42- (4) Using sulfuric acid to re
10、duce tower water alkalinity is a well-established, economically practical strategy for controlling and even eliminating calcium carbonate formation. This strategy, though, requires tight control of acid introduction to avoid lowering pH to the point of increasing general corrosion. There is also ris
11、k of inducing microbiologically influenced corrosion (MIC) associated with substantial elevation of sulfate. If all or parts of a cooling water system are inactivated long enough for water to lose its dissolved oxygen content, bacteria such as sulfate reducing bacteria (SRBs) can become active and p
12、roduce significant pitting corrosion. Sulfuric acid use also incurs the risks associated with of storage and handling of a highly corrosive chemical. b. Anti-precipitants a.k.a. scale inhibitors act to significantly slow the kinetics of CaCO3 formation; polymericdispersives act to minimize CaCO3 par
13、ticulate agglomeration and surface adhesion. There is a class of organic phosphorous compounds termed phosphonates that have the ability to significantly slow the formation of calcium carbonate both before and after the point of nucleation. The two most commonly used phosphonates are 1-hydroxyethyli
14、dene,-1, 1-diphosphonic acid (HEDP), C2H8O7P2 and 2-phosphonobutane-1, 2.4-tricarboxylic acid (PBTC), C7H11O9P. In the absence of treatment, the nucleation time of CaCO3 from newly introduced make-up water is on the order of minutes, hours or a few days. Phosphonates have the ability to extend nucle
15、ation times to hours, days or weeks. Dispersive agents are typically used in conjunction with phosphonates to retard agglomeration and stickiness of CaCO3 just after nucleation while average particle size is still relatively small. Low molecular weight (5,000) polymers such as polyacrylate and polym
16、aleate, along with a number of others, act to impart a uniform negative electrical charge over the surface of very small particles of CaCO3 and dust they act to significantly slow formation of adherent forms of CaCO3. 3. Combination of acid for alkalinity control & scale inhibitors and dispersives f
17、orparticulate control. a. The performance of scale inhibitor programs is limited by a maximum calcium tolerance based on make-uphardness, treatment inhibition capability, and permissible water concentration (i.e. cycles-of-concentration (CC) and holding time (HT) of water within a system). It is imp
18、erative that the calcium tolerance not be exceeded to avoid rapid destabilization of quasi-soluble calcium and rapid formation of fully crystallized CaCO3 that can form sediment or adhere to cooling loop surfaces. Using acid to reduce alkalinity effectively raises the calcium tolerance level so that
19、 higher CCs may be allowed and/or the negative effect of increased HT on hardness stabilization can be overcome as cooling load diminishes seasonally. b. The amount of acid used in combination with scale inhibitors is typically considerably less than if scalecontrol were to be achieved using acid al
20、one. This is important because reliance on corrosion inhibition is reduced, risk of acid overfeeding and unacceptably low pH is lessened, and control of acid introduction need not require as costly instrumentation. c. The ratio of acid to scale inhibitors can be adjusted to accommodate a wide range
21、of make-up waterproperties and cooling system design parameters. Figure 1 This figure illustrates the trend in free calcium ion concentration and pH over time as hard water is destabilized. Initially this “seed water” contained 200 ppm of Ca2+ and 25, 50 & 100 ppm of sulfate stabilized by CO2 at con
22、stant temperature to a pH of 6.2, “Seed Water”. A fourth run illustrates the effect of adding 1-ppm HEDP anti-precipitant Air is bubbled thru the water to expel CO2 and slowly allow pH to rise. As pH increases, the concentration of bicarbonate decreases more rapidly than the concentration of carbona
23、te increases. When the nucleation threshold is reached, free calcium ion is rapidly taken up in the formation of crystalline CaCO3 and pH is seen to stop rising and even decline at least initially. The three runs at increasing sulfate concentrations illustrate that other common species found in wate
24、r can slightly influence the precipitation of CaCO3. When 1-ppm of a scale inhibitor is introduced, it is seen that pH can rise to much higher level before the inevitable “crash” associated with nucleation. The area between the dashed lines labeled “Growth” & “Nucleation” define a zone of meta-stabi
25、lity where nucleation could potentially occur as time progresses. The effect of HEDP is to extend meta-stability, but it cannot prevent a crash as pH rises in response to increasing formation of carbonate ion.4. Non-Chemical Conditioning of Cooling Tower Watera. Electrostatic capture of CaCO3.Fine s
26、uspended particles of CaCO3 do not have uniform electric properties but have distinct areas of positive and negative charge. If a constant electrostatic charge can be established on a plate exposed to cooling water, it is theoretically possible to establish sufficient attractive force to secure capt
27、ure of micro-crystals of calcium carbonate. For this principle to be used successfully, the amount of CaCO3 captured per unit of time must match the rate of CaCO3 formed in that same time. Sufficient active electrostatic surface along with factors such as flow & back-mixing of tower water relative t
28、o the capture surface are necessary to insure sufficient particulate flux, removal of captured material from the cooling system and others are among considerations. b. Electrochemical precipitation and capture of CaCO3CaCO3 can be readily precipitated and extracted from cooling water electrochemical
29、ly employing basic water splitting principles. If a DC current is made to flow through two electrically conductive surfaces not in intimate contact from one another and immersed in water containing sufficient ionic species to support current flow, positive & negative poles are established, i.e. anod
30、e & cathode respectively. Hydrogen ion (H+) & oxygen gas (O2) will form at the anode and hydrogen gas (H2) & hydroxyl ion (OH-) will form at the cathode. Hydroxyl ion formed at the cathode surface reacts directly with any bicarbonate in close proximity to the cathode to form carbonate ion which then
31、 accumulates along the cathode surface at potentially very high concentrations. The presence of concentrated carbonate ion induces rapid reaction with any calcium present and significant amounts of CaCO3 can be induced to form on the cathode. Since the amount of 01020304050607080901001101206 .0 6 .5
32、 7 .0 7 .5 8 .0 8 .5 9 .0Calciumactivity (ppm)pHS ee d w a t er s t a b i l i t y d i a g r a mS t r a i g h t s eed w a t er 5 0 p p m ex c es s s u l f a t e1 0 0 p p m ex c es s s u l f a t e 1 p p m HE D Pcurrent that may be applied is limited only by the capacity of the power source, design of
33、the electrodes and the conductivity of the electrolyte, it is theoretically possible to extract and isolate considerably more CaCO3 than is produced and not transported to bleedoff. As with electrostatic capture, extraction design and engineering considerations are specific to each individual coolin
34、g system. Electrochemical extraction, if properly applied, has the result of extracting quasi-soluble CaCO3 and thus securing the removal of a fraction of the particles that contribute to nucleation. Since the dynamics of extremely small entities such as ion clusters and sub-nucleated nanoparticles
35、reciprocally confine particle size and number to narrow ranges, restricting the population of sub-nucleated nanoparticles retards size development toward nucleation. Figure 2 The data displayed to the left shows accumulation in micrograms of calcium carbonate in micrograms per square centimeter on a
36、n electrochemically active quartz crystal microbalance (QCM) with fixed cathodic current input in competition with a secondary electrochemical cell where current was adjusted between 0.00 to 2.52 microamps per square inch. The test media is “Seed Water” as described above. With no current applied to
37、 the secondary electrochemical cell, the QCM accumulated 90 micrograms of CaCO3 in 4 hours. With 0.25 microamps applied to the secondary cell, it is seen that QCM capture stabilized 9 micrograms for the remainder of the study period. Higher applied current to the secondary cell resulted in total rem
38、oval of material initially deposited. c. Hydro-cavitation induced precipitation of CaCO3.Hydro-cavitation technology involves creating extremely high shear stress in a limited volume of water so that intense energy focused in a constrained volume of water creates extremely short lived micro-bubbles
39、of steam and gasses. The bubbles created form and collapse so quickly that the energy of collapse can generate intense heat and even pulses of light. This effect can be created unintentionally at the leeside of a pump impeller that is rotated much too fast for the volume of water being pumped, or in
40、tentionally by the direct collision of very high velocity jets of water, or by application of intense ultrasonic energy to a constrained volume of water. Intense focused heating can force the formation of carbonate ion and its reaction with calcium to form calcium carbonate and thereby, to a degree,
41、 accelerate the formation of CaCO3 in a portion of the cooling tower volume. It then becomes a necessity to remove this CaCO3 from the recirculating cooling water usually by filtration- to prevent adherence or settling somewhere in the cooling water system. As with electrochemical induction and capt
42、ure of CaCO3, several engineering variables must be taken into account to insure that rates of induction, capture and removal of CaCO3 from the tower system match the rate at which CaCO3 is produced naturally at any given time. 0123456789100 2 4 6 8 10 12 14Massdeposited(g/cm2)T i m e e l a p s e d
43、( h o u r s )E f f ect o f a co m p et i n g elect r o ch em i ca l cell o n Q C M d epo s i t i o n d u r i n g b u l k d es t a b i l i z a t i o n2 .5 2 m A / s q .i n . 1 .2 6 m A / s q .i n . 0 .2 5 m A / s q .i n . N o c u r r en td. Ultra-filtration.Various manufacturers of hollow-fiber ultra
44、filtration media indicate filtration capabilities ranging from 50down to 5 nanometers. This range is somewhat past the nucleation point of calcium carbonate, but well suited for capturing the smallest of nucleated crystals perhaps prior to significant agglomeration and growth in size. Ultrafiltratio
45、n is also highly effective in limiting bacterial populations in cooling water and removing particulate matter and possibly large hydrocarbon molecules captured from the atmosphere. Large scale ultrafiltration of the order needed for cooling water conditioning may be economically challenging but the
46、technology is quite promising. Hollow fiber media requires regular attention and periodic cleaning. This adds to operating costs that may be significant. In numerous respects Ultrafiltration is similar to RO. CONCLUSION Calcium carbonate deposition can result from sedimentation, but is primarily the
47、 result of attractive activity found on cooling system surfaces. The solubility of CaCO3 is reduced by elevated temperature very near to heat transfer surfaces. Warming also tends to accelerate crystal growth and rearrangement. These influences make heat exchange surfaces a primary target for CaCO3
48、deposition. There are, however, several other conditions that produce effective attractive activity for micro-crystalline CaCO3. Surface biofilm on particles of CaCO3 can act as an excellent binding agent to promote deposition on cooling tower fill and within piping. “Hyper-saturation” at wet/dry bo
49、undaries of cooling towers leads to disproportionate deposit formation and mimics natural and induced attractive activity. Filming of oil and grease anywhere in a cooling water loop can lead to capture and building carbonate rich sludge. Debris such as cottonwood fiber, insect fragments, fly ash and paper fragments can all accumulate CaCO3 sludge if such debris is captured anywhere within a cooling water system. The key to minimizing attractive activities certain to exist somewhere throughout cooling tower systems is to eliminate or minimize nucleation of CaCO3 in the first place
