Deposit control  is  the  most critical aspect of cooling water treatment because it is  directly  related  to very purpose for a cooling  system  heat  removal. Any deposit present in heat transfer surfaces impairs heat removal and there for reduces the efficiency of entire system.

      Cooling water deposits are of two types - scales and fouling. Scales are hard dense, crystalline  deposits for-med by the precipitation of dissolved materials when their solubility have been exceeded due to change in conditions. Foulants  form  softer  non  crystalline  deposits because suspended materials settle out or adhere to metal surfaces

Bhatt, M M

Patel, L R

Sharma, R K

English

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ON LINE CHEMICAL CLEANING OF CRITICAL HEATEXCHANGERS FOR COOLING WATER DEPOSIT AND THEIRCONTROL TO SUSTAIN HIGH PRODUCTION LEVEL OFMETHANOL. - AN EXPERIENCE AND CASE STUDY AT GNFCDr. R. K. Sharma , Mr. L. It. Patel & Mr. M. M. Bhatt.GNFC, Bharuch, GujratABSTRACTDeposit control is the most critical aspect of cooling water treatment because it isdirectly related to very purpose for a cooling system heat removal. Any deposit present inheat transfer surfaces impairs heat removal and there for reduces the efficiency of entiresystem.Cooling water deposits are of two types - scales and fouling. Scales are hard dense,crystalline deposits formed by the precipitation of dissolved materials when their solubilityhave been exceeded due to change in conditions. Foulants form softer non crystallinedeposits because suspended materials settle out or adhere to metal surfaces.INTRODUCTIONDeposit control is the most critical aspect of cooling water treatment because it isdirectly related to very purpose for a cooling system heat removal. Any deposit present inheat transfer surfaces impairs heat removal and therefor reduces the efficiency of entiresystem. Unless deposition is properly controlled, it can negate the effectiveness of anexpensive and well designed cooling water treatment and its system.This direct effect on cooling efficiency has always required deposits control be anintegrated part of cooling water treatment programs. The potential for deposition isincreased by the trend to conserve water and the need to use more environmentallyacceptable material for controlling corrosion and deposition. To conserve water highercycles of concentration are being used, this increased the levels of scale forming or foulingmaterial in the water. The need to use Non-toxic corrosion inhibitors is also increasing thepotential for deposition. Since many of these less toxic corrosion inhibitors are more effectiveat high pH conditions, scaling potential is increased, while the effectiveness of many microbiocides is decreased. Meeting the need of these conflicting objectives, require a good,understanding of the effects and cause of deposition and their control by adopting newmethodology like on line chemical cleaning and its monitoring , to sustain highproduction level of the plant with out its shut down. This paper deals with thissubject.^Fllrt`l^r^^, I' MII^^I N II', ^^^'ualN^ tlX' IBI^M^^MN #IN I il^ , 70 1EFFECT OF DEPOSITIONDeposits affect cooling system operation both directly/indirectly. Heat transfer canbe reduced directly due to the insulating effect of a deposit as shown in Fig-1. Depositshave lower thermal conductivities than the metal wall of tube and there after act muchlike insulation to impair rapid heat transfer. Deposition can also reduce cooling because itrestricts water flow, across heat exchangers and increase AP. Reduced heat transferincreases production loses due to reduced efficiency.Deposits besides reducing cooling efficiency, can initiate leaks in the system bypromoting localized corrosion. The phenomenon is described as under deposit corrosion.COOLING WATER DEPOSITES AND THEIR CONTROLThe sources of potential deposits are as diverse as the effect of these deposits.Potential deposits can enter the cooling system through the make up water, from the air,through process contamination, as well as being formed within the system throughcorrosion. Make up water may contain both dissolved as well as suspended materials,which can forms deposits. The scrubbing action of a cooling tower can introduce considerablequantities of dissolved gases, dust, dirt and other particulates matter carried by the air.Process contaminants are as diverse as the number of products manufactured and mayenter through leaks and plant upsets etc.COOLING TOWER DEPOSITS ARE TWO. TYPES: SCALES AND FOULING.A. Scales : Scales are hard, dense, crystalline deposits formed by the precipitation ofdissolved materials when their solubilities are exceeded due to condition changes.Calcium Carbonate is the most common scale in cooling water, whose solubility isvery low about 13 ppm. Other common scales of cooling water are Calcium Sulfate, CalciumPhosphate, and Calcium Fluorides etc. Scale occurrence in cooling water system is observedas shown below in table 1.Calcium PhosphateCalcium CarbonateSilicaCalcium SulfateCalcium FluorideTable 11. Calcium Phosphate scale : Calcium Phosphate has become leading form of scale,due to phosphate based corrosion inhibitor. When orthophosphate is present in thewater, calcium phosphate scale is formed, inhibit heat transfer and lead to underdeposit corrosion. The formation of Ca3(PO4)2 scale is governed by the pH, Calciumand Phosphate concentration and temperature of recirculating water.The application of PBTC + HEDP polymer have reduced calcium phosphate deposits.They have performed well with Ca" levels up to 450 ppm as CaCO3 with total phosphate10-18.ppm as P043 The copolymers of poly melates and poly acrylates have exhibitedgood calcium phosphate scale control.u^- 92. Calcium Carbonate: Calcium carbonate is the most common scale. Fig. 2 shows therelation ship of temperature to the solubility of calcium carbonate. The calciumcarbonate deposition is influenced by pH, Calcium and alkalinity level in cooling water.Several new phosphonates are providing exceptional control to calcium carbonate scale,because most of these polymers act as crystal modifier. Outstanding among these isone known as Phosphono- butane-tricarboxylic acid (PBTC). The use of 50:50 PBTC:HEDP, with 3-5 ppm level is found effective under severe scaling conditions. The blendhas enabled our system to tolerate the calcium hardness up to 450 - 500 ppm, withtotal hardness up to 900 - 1000 ppm level, in circulating cooling water in differentplants at GNFC.3. Calcium sulfate: - Specially sulfate ions replaces alkalinity when sulfuric acid is fedto cooling water system to control system pH, but sulfate scale poses limited solubility,exhibit serious problem to cooling water system operation, since its solubilitydecreases as temperature increases *(Fig. 2). Sulfate scale control has been achievedby charging the polymer like poly aoylate /poly acrylamide at 2-3 ppm level in coolingwater containing 500 ppm calcium as calcium carbonate.4. Silicate scales : - High operating cycle of concentration based on silica and the risinguse of reclaimed water are accountable for high silica levels in circulating coolingwater. The formation of silicates can be prevented by limiting silica levels in circulatingwater max. up to 150 ppm as SiO , although precise limit is dependent to make upwater SiO with respect to the oper7ation of cooling tower on high cycle of concentrationbased on Ailica.To prevent magnesium silicate deposition, magnesium and silica concentrations arecontrolled, so that the following solubility product is not exceeded.(Mg, ppm as CaCOs) x (SiO , ppm as SiO ) is < 40,0002 2New treatment (ter polymers) has been developed to increase silica solubilitiesattainable in cooling water system.B. FOULING : - Foullants form softer non-crystalline deposits because of suspendedmaterials settle out or adhere to metal surfaces. Silica besides being scale is also one of thecommon foulants. Fouling with materials high in silica is common because suspendedsolids, mud, slit and dirt are common in most surface water. The extent of fouling fromsediments in circulating water is a result of the time afforded the particle to settle. Sincecooling water basins, water boxes and shell side flow area involve appreciable holdingtimes as a result of their size and the low velocities through them, the settling of sedimentsremains extensive, in above areas.Polymers of high molecular weight (300,000 to 15 million) at doses level of 0.5 to 1.0mg/lit. are commonly used. They attract fine foulant particles on to polymeric chain, forminglarge puffy particles that are easily removed from heat exchangers surfaces. Non-toxic lowfoaming wetting agents such as poly oxyalkylane have provided improved control ongelatinous and oil base foulants.Functional mechanism of organophosphonate / polymer inreduction of scale & fouling in cooling waterOrganophosphonates are one of the best deposit control agents . The Threshold andcrystal distortion property of these compounds interferes with nucleation of hardness ofcrystals causing much higher levels of hardness to stay in solution. When scales are formedthey are distorted that they are non-adherent and form very soft sludge.90 01f NWI ^^" ""117T IllMIl'1110N NCI }^^i^..,.1! • r^ 01"771 r I I '"r I II II rl T- 1 7 -1Phosphonates also provides excellent control of hydrated ferric oxides deposits, whichare formed as a result of corrosion. They adsorb on the particle surfaces and reduce theattraction forces between individual iron particles. The sequestering ability of phosphonateenables it to control heavy metals ( Fe,Cu , Zn) deposits . Phosphonates also help to dispersesuspended particles.Low molecular weight polycarboxylic polymers also exhibit threshold and crystaldistortion property. When used in conjunction with organophosphonate, poly phosphateand zinc, they aid in corrosion inhibition / deflocculation, dispersion and crystal distortion,leading to soft scale.Leading scale control agent and methods are shown in Table 2.Agent SolubilizationCONTROL ACTIONDispersion Crystal modificationPhosphonateHEDP/AMP Primary - -PBTC Primary Moderate -Polvacrvlate CopolymersAcrylate / maleate Slight Moderate PrimaryAcylate/acrylamide Primary Moderate SlightTer PolymerMaleate/Acrylate/sulfonate Slight Slight PrimaryAcrylate/Acrylamide/sulfonate Slight Moderate PrimaryTable 2ONLINE CHEMICAL CLEANING AND ITS GAIN : -On line chemical cleaning of E-405 (Reactor gas condenser) and E 509 (Refiningcolumn condenser) was done first time in GNFC on 15th Sept. 2000 and subsequently doneon 30th Nov. 2000 and 191 Feb. 2001. Sulfamic acid was used with anticorrosive materialand pH of the recirculating water was brought down to 2.8 - 4.0 for about 2 hrs.The acid cleaning was followed by flushing the heat exchangers with low molecularweight polymer along with biodispersent to remove any available loose scale / fouling,either in tube side or shell side.On completion of chemical cleaning the cooling water equipments & piping werepassivated with double dose of treatment chemicals charged.During chemical cleaning the Turbidity of E 405 out let shot up to 77 NTU and E509 out let Shot up to 61 NTU. The E405 & E509 out let pH and Turbidity is shown in Fig.3 & Fig. 4. The turbidity observed high due to removal of scale of Mg, Zn, Fe phosphate etc.The total hardness of circulating water has gone up from 360 ppm to 500 ppm, total solidsfrom 916 ppm to 1620 ppm, Fe from 0.44 ppm to 1.28 ppm. The water analysis before,during chemical cleaning and after passivation is shown in Table-3. The relationship ofnSa.r wva ylZn, Mg & P04 is shown in Fig 5 & 6 for E405 & E509 heat exchangers, which wereresponsible for poor heat transfer and limiting the plant, load, resulting in low productivityof methanol. On their successful cleaning plant production has gone up. The condenserand refining column data before and after chemicals cleaning are placed in Table-4. Thesedata shows the effectiveness of on line chemical cleaning of deposits of scale on shell sidesof E405 and tube sides of E509.ADVANTAGES OF ON LINE CHEMICAL CLEANING1. Plant shut down has been averted.2. Production of methanol has gone up by 5 to 6 MT / day, as shown in Fig. 7.3. The reactor gas condenser (E 405) cooling water out let temperature has gone downfrom 54 to 49°C i.e. 5°C, which enhance gas processing capacity of exchanger.4. E 509 refining column condenser pressure dropped from 0.94 bars to 0.41 bars on30th November cleaning and was hold there. The %valve opening of cooling waterreduced from 100 to 26 %, hence the refining column efficiency has gone up to processmore crude methanol.5. The return header cooling water temperature increased by 1°C, which indicateshigh heat recovery with circulating cooling water rate of 3500M3/Hr.6. Commercial gain, due to in-house technical development & its execution.The details of chemical cleaning, with removal of different type of scales and foulants,with variation in cooling water pH and application of low molecular weight polymer withbiodispersent to ensure the control of scale and biofouling with planned dose of corrosion& scale inhibitor to overcome the corrosion & scale formation and to sustain high productionof methanol, without plant shut down, is over all theme of this paper.ACKNOWLEDGEMENTThe authors are thankful to GNFC management for allowing to carryout on linecleaning of critical heat exchangers like E405 / E509 and MeOH II plant operation groupmembers and M/s. Chembond chemicals personnel for their valuable support in makingon line chemical cleaning successful.REFERENCES1. Sharma R.K., Patel L.R. & Bhatt M.M.:"Monitoring & Evaluation of cooling water of NPComplex plant, based on improved inhouse treatment"- Indian Fertilizer Scene Annual 2000-2001.2. "The role of organophosphates in cooling water treatment."D.J.Banerjee; vol. 4, No.3,1981,corrosion & maintance.3. "Principles of Industrial water treatment." Drew chemical corporation.4. "Scale & Deposits control in cooling water systems."Jeffery R. Townsend, 1978,Drew Chemical Corporation.Analysis report of MeOH II Cooling Water during on line chemical cleaning ofthe E405 / E509 exchangers by sulfamic acid ( HSO3Nh2 ) on 01/02/2001.No Test Unit ResultBefore E -405 01H0112/01) E-509 Basin0112101 15 : 45 16 :00 16 : 35 16 :45 16 :45 11:301 P' 7.0 2 . 9 2.7 3 . 6 2.8 3.1 7.12 Conductivity p. mhos/cm 1118 1519 1642 1467 1660 1380 11643 Turbidity NTU 22 61 61 66 77 61 434 Total alkalinity as CaCOs PPM 46 - - - 565 Total Hardness as CaCOs PPM 360 500 490 500 500 490 4286 Calcium Hardness as PPM 234 260 250 250 260 250 2287 Magnesium Hardness asCaCOsPPM 126 240 240 250 240 240 2008 Iron as Fe PPM 0 . 44 0.91 1 . 17 1.01 1.28 1 . 01 0.639 Chlorides as Cl PPM 188 200 200 200 200 200 15410 Free Chlorine PPM 0 .5 nil nil nil nil nil 0.511 Silica as SiO2 PPM 56 53 55 56 56 57 5712 Ortho Phosphate as P04 PPM 2.7 6.1 7.0 8.3 7.5 7.1 11.813 Poly Phosphate as PC>4 PPM 1.9 3.8 3 . 5 3.9 3.6 3.8 5.714 Organo Phosphate as P04 PPM 3.0 3.8 4.2 7.3 7.1 5.5 8.215 Zinc as Zn PPM 0 . 8 2.14 2. 00 2.43 2 . 29 2.20 2.3216 Anunonia as NHa PPM nt nt 1.0 2.0 2.0 8.117 Sulfate as S04 PPM - 250 225 200 175 188 27518 Total Solids PPM 916 1424 1498 1492 1620 1296 120219 Total dissolved solids PPM - 1360 1434 1420 1540 1232 115820 Total suspended solids PPM - 64 64 72 80 64 44Table 3DATE E 405UL temp. 0/L temp.E-509Press % Valve open27-01 01 103 52 0.41 5228-01 01 103 52 0.41 5029-01 01 103 53 0.41 6230 01-01 103 53 0.51 10031-01-01 102 53 0.51 9401-02-01 103 54 0.46 10002 02-01 102 51 0.41 2903-02-01 102 51 0.41 3004-02-01 102 49 0.41 1905 02-01 101 49 0.41 2506-02 01 102 48 0.41 2407-02-01 102 48 0.40 2708-02 01 102 48 0.42 2409-02-01 104 48 0.41 2510-02-01 102 48 0.41 2611-02-01 103 51 0.41 4312-02-01 102 49 0.41 3113-02-01 101 48 0.41 2614-02-01 104 50 0.41 2515-02-01 102 50 0.41 2616-02-01 102 49 0.42 2617-02-01 102 49 0.42 2418-02-01 103 51 0.41 3919-02-01 104 54 0.42 4720-02-01 104 52 0.42 5021-02-01 103 52 0.41 4122-02 01 102 49 0.40 2723-02-01 100 49 0.42 2224-02 01 102 50 0.42 5925-02-01 102 51 0.42 5926-02 01 102 51 0.42 5727-02-01 101 51 0.42 5328-02-01 100 47 0.42 26Table 49^^ i^ r^ i^ 2001N^^^111i NNW ^ ^^^IIN^^ ^ ^R{pVIpM iP i WHIM p N 7T -7 1^^INSULATION EFFECT OF A DEPOSITFig. 12 400U 7000O 160035 1200^ 800A 400---- - -- - - - - -\ ,^ •Osn Citta'a l e CACO, - -32 GO 104 140 116 212 246 764 320 J5& 39250 Be 122 158 114 730 208 302 3319 314OFig. 2Comparison of solubilities of calcium sulfate and calcium carbonate solubility inequilibrium with normal carbon dioxide content of the system958.07.06.05.0a 4.03.02.01.00.0E 405 OIL pH & turbidity18:0015:35 15:45 15:50 16:00 16:10 16:30 16:50 17:00 18:00 19:30 20:30 20:40Time -a-pH -^-TurbFig. 38.07.06.05.0CL 4.03,02.01.00.0E-509 OIL pH &Turbidity15:35 15:45 15: 50 16:00 16 :10 16: 30 16 :50 17: 00 18 : 00 19 : 30 20 : 30 20:40TimeFig. 4300.0250.0CL 200.0a150.0x131 100.050.0 -0.0 1E-405 OIL Mg-HZn,P0415:35 15:45 16:00 16:35 17:00TimeFig. 596-U-pH -t-Turb20:3090807060 z50403020 H100r 25.0020.00 -E15.00 a10.009C5.00 N4 0.00f Mg-H --*--Zinc -*-T-P04300.0250.0CE 200.0150.0100.050.00.0E-509 OIL Mg-H,Zn,P0415:35 15:45 16:00 16:35Time17:00Fig. 6N 420g 400380z036043 03200wn 300-27- 29- 31- 02- 04- 06- 08- 10- 12- 14Jan Jan Jan Fab Fab Feb Feb Feb Feb FebDATEFig. 7NSCP - 200118:00 20:3025.0020.0015.00 a10.00 05.00 N0.00-*--Mg-H -Zinc -A--T-PO416- 18- 20- 22- 24- 26- 28-Fab Fab Fab Feb Feb Feb Feb97

On Line Chemical Cleaning of Critical Heat Exchangers for Cooling Water Deposit and their Control to Sustain High Production Level of Methanol - An Experience and Case Study at GNFC

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On Line Chemical Cleaning of Critical Heat Exchangers for Cooling Water Deposit and their Control to Sustain High Production Level of Methanol - An Experience and Case Study at GNFC

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