Sulfuric Acid Plant Cooling Tower Improvements

Transcription

1 Technical Paper Sulfuric Acid Plant Cooling Tower Improvements Ryan Barbour, Herb Quinones and Tom Refi GE Water & Process Technologies 4636 Somerton Road Trevose, PA Find a contact near you by visiting and clicking on Contact Us. * Trademark of General Electric Company; may be registered in one or more countries. 2012, General Electric Company. All rights reserved. TechnicalPaper_ AICHE_Clearwater_CH (4).doc Apr-12

2 This is the summary of improvements made at a central Florida concentrated phosphates complex sulfuric acid plant cooling tower system. Large amounts of steam are produced during the manufacture of sulfuric acid, which is used to produce electricity via turbine generators. A surface condenser condenses the steam, and heat is removed from the system via a counter-flow recirculating cooling tower. Table 1. Well Water Makeup Chemistry Parameter Average of 3 samples The cooling system makeup is well water from a local spring with consistent chemistry summarized below. Specific Conductance at 25C, mmhos Alkalinity, M as CaCO3, ppm Chloride, as Cl ppm Calcium Hardness as CaCO3, ppm Magnesium Hardness as Ca- CO3, ppm Sodium as Na, ppm Phosphate Ortho as PO4, ppm Silica as SiO2, ppm Page 2 Copyright 2012 General Electric Company Technical Paper

3 Cooling tower operating data basis of design and plant operating conditions are summarized below. Table 2. Cooling Tower Basis of Design Capacity, gallons 100,000 Operating Volume, gallons 75,000 Recirculation Rate, gpm 30,000 Hot Water Temp, degrees F 110 Cold Water Temp, degrees F 92 Delta Temp, degrees F 18 Load, MM BTU/hr 150 Average Load, % 50 Metallurgy Copper/Nickel Condenser, Mild Steel Piping GE Power & Water was given an opportunity to benchmark the results of the existing chemical water treatment program for the cooling system. Operating conditions were below neutral at ph creating a corrosive environment that resulted in elevated mild steel and copper corrosion rates. Mild steel corrosion rates measured were in excess of 25 mils/yr, while copper corrosion was documented at greater than 1 mpy, both well above industry standards. High concentrations of iron and copper in the bulk water also confirmed the corrosive environment. Lack of control or automation of the chemical feeds also contributed to the corrosive conditions. Sodium hypochlorite (bleach) was used as the primary oxidizing biocide, being fed via offline wet testing of free chlorine residual with manual adjustments. High free chlorine residuals in the tower were consuming the copper corrosion inhibitor (tolytriazole or TTA) which is not stable in high concentrations of halogens. Low cycles of concentration due to limits of chemical scale inhibitors used increased blowdown and flushing of the cooling system, requiring elevated fresh water usage. The foundation of effective cooling water treatment includes management of the interrelationship between corrosion, scale/deposition, and microbiological activity. The local GE team, with support from GE Application Engineers and Plant Operating Engineers, designed a treatment program utilizing GE s patented Stress Tolerant Polymer (STP) and Halogen Resistant Azole (HRA) technology. The program was monitored and controlled by GE s TrueSense* automation package. The STP offers truly superior protection when compared to any other chemistry available today, effective in both neutral and alkaline scenarios. STP is a very forgiving chemistry with excellent inhibition of calcium phosphate, calcium pyrophosphate, zinc phosphate, and aluminum phosphate. It also is oustanding at dispersion of high iron and silt levels and handling high exchanger skin temperatures and ph excursions. HRA is an improved azole that provides unequaled copper alloy corrosion protection. HRA is halogen stable and capable of maintaining effective corrosion inhibition even in the presence of chlorine and Technical Paper Page 3

4 bromine-based biocides. Conventional azole inhibitors, such as tolyltriazole and benzotriazole, are readily halogenated in the cooling water, preventing the establishment and repair of a protective film on the metal surface. HRA maintains its activity both in the water and on the metal surface, ensuring continuous protection for copper alloys. HRA also reduces copper levels in the cooling water, minimizing copper discharge and effectively controlling destructive galvanic pitting on steel surfaces. TrueSense Online technology directly measures the functional chemistry of all three key elements of a treatment program, including: Orthophosphate for steel corrosion control, Polymers for the prevention of deposits from mineral scales and dispersion of suspended solids, and Free halogen for the costeffective control of microbiological growth. GE set the optimum target points for available polymer concentration, soluble orthophosphate, and free halogen, based on the specific conditions of the cooling system. The targeted available polymer concentration is always maintained, despite fluctuations in demand caused by system variations or upset conditions that exert stress. Maintaining adequate soluble orthophosphate in the system is critical to ensure effective steel corrosion inhibition. GE s definition of soluble phosphate is its ability to pass through a 0.22 micron filter. TrueSense Online technology measures both unfiltered and filtered (soluble) orthophosphate, and automatically monitors the difference between them. Polymer addition is automatically adjusted to maintain the difference under a target maximum. As a result, the corrosion inhibition system remains intact and phosphate is not lost to the formation of insulating deposits on heat transfer surfaces. All of this allowed the cooling water ph target to be increased and cycles of concentration to be increased based on total dissolved solids. The cycled conditions were evaluated on solubility limits for given chemistry. All bulk water sampling mass balances showed scaling compounds were accounted for and often cycles increased indicating cleanup of existing conditions, indicating no signs of deposition fouling or loss of heat transfer in critical heat exchangers. This also helped in making the water conditions less corrosive than the previous environment. All of the chemical feed was automated via the online measurement and control technology improving reliability. Page 4 Copyright 2012 General Electric Company Technical Paper

5 The following benefits were shown: Reduced fresh water makeup over 18 million gallons per year by increasing cycles of concentration, representing a 50% reduction in cooling tower blowdown. Graph 1. Cooling Tower Conductivity Reduced copper corrosion rate greater than 90%, nearly doubling copper/nickel condenser life expectancy Graph 2. Cooling Tower Copper Corrosion Rate Technical Paper Page 5

6 Reduced mild steel corrosion rate 75%, increasing piping and condenser shell life expectancy Graph 3. Cooling Tower Mild Steel Corrosion Rate Reduced iron and copper in the bulk water by 60% and 35% respectively even with double the cycles of concentration. The bulk water metals concentrations shown below are on 5 cycles of concentration basis. This helped confirm no active corrosion was taking place in the cooling tower system. Graph 4. Cooling Tower Iron Concentration Page 6 Copyright 2012 General Electric Company Technical Paper

7 Graph 5. Cooling Tower Copper Concentration Showed clean-up effect of previous corrosion by-products. Showed lower levels of metals in the bulk water at higher cycles of concentration. Lower total program costs while updating chemistry and monitoring/control to best available technology. Reduced chemical usage via the online control of chemical feeds versus manual feed. Graph 6. Cooling Tower Parameters Controlled On-line The protocol set forth for this study considered current system conditions, industry standards, and Key Performance Indicators established by the parties involved. The bulk water sampling analyses were performed by a certified laboratory analysis. Corrosion coupon analyses were performed by a certified laboratory per industry standards. All online measurements were validated with off-line field wet analytical testing procedures. Technical Paper Page 7

8 In conclusion, the goals to reduce freshwater usage by increasing cycles of concentration, minimize corrosion without sacrificing heat exchanger performance, and improve monitoring and control of chemical feed were all achieved. Advanced chemistry and online measurement and control of all critical system parameters were the keys for this success. Page 8 Copyright 2012 General Electric Company Technical Paper