Study On Technology Of Corrosion Inhibition Of Cycle Cooling Water From Petrochemical Industry

Owing to the shortage of water resources, most of the domestic petrochemicalindustries use the waste water as cycle cooling water. The complicated compositionand corrosion problems of cycle cooling water caused serious corrosion of equipmentsin recycling process. At present the petrochemical industries mainly use staticmonitoring and inhibitors as corrosion inhibition measures, which is lack of real-timestudy and systematic designs. This article focused on the phenomenon ofpetrochemical waste water corrosion while recycling. The mechanism and technologyof inhibiting corrosion were systematically discussed and investigated by staticweight-loss method coupled with dynamic simulation device, which is helpful toprolong the service life of materials and equipment as well as improve the economicbenefit of enterprise. From this paper,it was concluded as follows:（1） The corrosion processes of carbon steel in refinery reused water werestatically monitored by RCC-Ⅲ, and several affecting factors and mechanism ofcorrosion were discussed. It was showed that COD, ammonia nitrogen, temperature,turbidity, oil, acidity play a mild role in promoting corrosion, while acidity andtemperature were the most important. The corrosion processes of carbon steel insimulated distilled water were also discussed. It was indicated that the corrosion raterose sharply with the increase of the concentration of ions when Cl-<1200mg/L, SO4^2-<1350mg/L, HCO3-<1450mg/L, NO3-<650mg/L, and Cl-, SO4^2-promoted thecorrosion most obviously, the corrosion rate increased slowly while the concentrationwas out of the range. The impact of S^2-on corrosion rate was related to pH. Thecorrosion rate decreased with the pH increase when5.5<pH <8, the corrosion of S^2-on carbon steel was very weak when pH>8. NO^2-,PO43-inhibited the pitting ofCl-,especially when the concentration ratio of NO^2-to Cl-was greater than0.8, or agreat quantity of Ca²⁺was coexisting with PO43-. The influence of SO4^2-on pitting of Cl-was related to concentration ratio, the higher the concentration of SO4^2-was,thelower the pitting of Cl-would be.When Ca²⁺,Cl-coexisted with one or several kindsof oxyacid, the trend of corrosion was dependent on hardness. The corrosion rate ofwater of low hardness（calcium hardness <100mg/L） rose sharply with the increase ofhardness due to under-deposit corrosion.Concentration of oxyacid decided corrosionrate in water of high hardness（calcium hardness>450mg/L）,high concentration ofoxyacid resulted in increasing resistance of the corrosion reaction which in turn inhibitcorrosion.（2） A prediction model of carbon steel in cycle cooling water was set up on a basicof BP neural network theory in order to predict average corrosion rate and to analyzethe problems of corrosion that caused by simulated cycle cooling water. The modelused stepwise regression to select six major factors: pH, Cl-, Ca²⁺, SO4^2-, HCO3-,temperature. These factors were used as input parameters while the average corrosionrate as the output parameter.It was showed that the model has good forecastingprecision, which could reflect the corrosion factors effectively.（3） Optimal concentration and property of several corrosion inhibitors werestudied. It was showed that PBTCA is better in inhibition rate,scale inhibition rate andoptimal concentration than the others. PBTCA has synergistic effect with HEDP,ATMP, RP-34, RP-34（1）. The best formula turned out to be when the concentrationratio of PBTCA to HEDP was1:2,which is the best formula for inhibiting scale, thescale inhibition rate was85.58%. The formula of four factors which made byorthogonal experiment was:15mg/L PBTCA+20mg/L HEDP+20mg/L ATMP+3mg/L zinc sulfate. The corrosion inhibition rate was86.18%of refinery reused water,86.44%of ethylene plant, the scale inhibition rate was87.42%.Corrosion rate ofrefinery and ethylene plant reused water reduced to0.0168mm/a and0.0091mm/a,which meet the need of corrosion standard of reused cycle waste water. The corrosioninhibition rate of RP-98was89.54%of refinery reused water,90.34%of ethylene plant,scale inhibition rate was90.34%. RP-98was better than the mixed inhibitor ofcorrosion and scale, but the mixed inhibitor had a smaller dosage.（4） Reused cycle water from refineries and ethylene plant were analyzed and thecorrosion mechanism of them were discussed. NJHL-C integral dynamic device wasused to simulated the process of cooling water circulation in oil refinery and ethyleneplant by real-time monitoring.It was indicated that the scale of reused water fromethylene plant was mainly composed of calcium carbonate and calcium sulfate.Composite inhibitor RP-98could control the fouling thermal resistance of ethylene plant waste water at very low level, even when100%of the ethylene plant reused waterwas in low stable range. The scale thickness was smaller than0.3mm. When reusedwater from ethylene plant recycle to cooling water system, it’s better blend with tapwater and the proportion of reused water was less than75%. Corrosion of refinerieswaste water was due to a great quantity of microbial growth which lead to serious dirt.Composite inhibitor RP-98could also control the fouling thermal resistance in a lowstable level. Scale thickness was smaller than0.5mm.When reused water from refineryrecycle to cooling water system, it’s better blend with tap water and the proportion ofreused water was less than50%to avoid forming scale seriously.