Reduction of Caustic Losses During Calcium Carbonate Washing Marge Inovera (Statistics and Process Modeling) September 21, 2000 Reduction of Caustic Losses During Calcium Carbonate Washing Summary The addition of a calcium carbonate (lime mud) dilution system prior to the washing step will save $60,000 to $105,000 per year by reducing the amount of caustic carryover with the washed lime mud. Actual savings are strongly influenced by the cost of energy due to the higher load of water to the recovered caustic evaporator system. Optimal dilution ranged from 12 gallons per minute (gpm) at $3.00 per MMBtu energy cost to 23 gpm at $2.00 per MMBtu. Due to the low investment for installing the dilution system, it is recommended that the project proceed to field trials for validation of the model results used in this study. Introduction Currently, 6.4% of the caustic entering the lime mud washer is lost as carryover with the lime mud fed to the lime kiln. The cost of replacing this caustic is over $710,000 per year at the current price of $661.50 per metric ton and 350 actual operating days per year. The mud washer is running at design levels (70% solids leaving the filter) and improvements would likely bring only marginal returns. The addition of hot condensate to the mud slurry prior to the washer will allow a greater recovery of caustic. Due to the constraint that the filtrate from the filter must be concentrated to 30% caustic, the additional burden of the condensate will translate into more energy demand for the evaporator. This study investigates whether the trade off between enhanced recovery and higher steam demand is economically feasible. Modeling Study The process flow diagram for the washer and evaporator system is depicted in Figure 1. The current design has no dilution water (Stream 4). The slurry composition used in this modeling study was taken from the operator’s log sheets and averaged over the past six months. The slurry is concentrated to 70% mud solids and the filtrate is evaporated to 30% caustic concentration. Three energy costs were used in this study: (1) $3.00 per MMBtu, (2) $2.50 per MMBtu, and (3) $2.00 per MMBtu. These costs cover the normal annual range that the plant pays for steam. Since the plant currently sewers the hot condensate, no cost was taken for this utility. The system was modeled using the ChemCad simulation package, version 5.0, and all results and conclusions are based on the model study. The strategy used in the model was to vary the dilution water flow and determine the amount of caustic remaining with the washed lime mud and the energy requirements of the evaporator system. The model assumptions were that the washer performance is unchanged at the lower slurry solids concentration when dilution water is added. Prior experiences with other washers showed that the solids content from the washer should actually improve by feeding slurry with lower solids content. This will have to be evaluated in a field trial. Analysis Table 1 lists the stream compositions and flows at four dilution water flowrates: 0, 12, 16, and 23 gpm. The base case is at 0 gpm and caustic cost savings and energy demand were based on this case. The optimum dilution flow was determined for the three energy costs and the results are shown in Figure 2. The impact of energy cost on the optimum dilution flow is shown in Figure 3. Using this figure as a guide, the proper dilution flow can be set based on the current energy cost for maximum savings. The maximum annual savings possible is $790,000 if all caustic is recovered and a source of steam is found for the evaporators at no cost. Alternatively, should the post-evaporator caustic concentration constraint be relaxed, additional savings can be achieved. Recommendations Field trials should be implemented at the earliest possible date to verify the model results. Caustic losses, lime mud solids after the washer, and the evaporator steam demand should be monitored for each dilution flow rate tested to determine if the assumptions used in the model study are valid. Caustic savings should be further verified by monitoring makeup caustic purchases during the trial phase to ensure buy-in from all business units within the plant. Other manufacturing facilities within the company can benefit from this study and should conduct a similar analysis to determine the potential impact of the process modification. Dilution Water 4 CaCO3 Washer 2 CaCO3 1 5 1 2 Washed CaCO3 7 Flashed Water 3 Concentrated NaOH 6 4 5 9 3 Evaporator 8 FIGURE 1: Process Flow Diagram of Calcium Carbonate Washer Process TABLE 1: Summary of Stream Flows Case: Stream Temperature, F Pressure, psia Total Water CaCO3 NaOH Case: Stream Temperature, F Pressure, psia Total Water CaCO3 NaOH Case: Stream Temperature, F Pressure, psia Total Water CaCO3 NaOH Case: Stream Temperature, F Pressure, psia Total Water CaCO3 NaOH Base (0 gpm) 1 2 185 185 15 13 240.0 156.0 31.2 52.8 44.6 10.0 31.2 3.4 12 gpm 1 185 15 2 185 13 240.0 156.0 31.2 52.8 44.6 10.8 31.2 2.6 16 gpm 1 185 15 2 185 13 240.0 156.0 31.2 52.8 44.6 11.0 31.2 2.4 23 gpm 1 185 15 2 185 13 240.0 156.0 31.2 52.8 44.6 11.3 31.2 2.1 Evaporator Heat Duty 26,500 MMBtu/year 3 4 5 6 7 8 9 185 185 185 185 185 185 185 13 15 15 13 13 13 13 Flow Rates, metric ton/day 195.4 0.0 240.0 195.4 30.7 164.7 164.7 146.0 0.0 156.0 146.0 30.7 115.3 115.3 0.0 0.0 31.2 0.0 0.0 0.0 0.0 49.4 0.0 52.8 49.4 0.0 49.4 49.4 Evaporator Heat Duty 74,600 MMBtu/year 3 4 5 6 7 8 9 185 185 185 185 185 185 185 13 15 15 13 13 13 13 Flow Rates, metric ton/day 260.8 65.4 305.4 260.8 93.6 167.3 167.3 210.6 65.4 221.4 210.6 93.6 117.0 117.0 0.0 0.0 31.2 0.0 0.0 0.0 0.0 50.2 0.0 52.8 50.2 0.0 50.2 50.2 Evaporator Heat Duty 90,900 MMBtu/year 3 4 5 6 7 8 9 185 185 185 185 185 185 185 13 15 15 13 13 13 13 Flow Rates, metric ton/day 282.6 87.2 327.2 282.6 114.6 168.0 168.0 232.2 87.2 243.2 232.2 114.6 117.6 117.6 0.0 0.0 31.2 0.0 0.0 0.0 0.0 50.4 0.0 52.8 50.4 0.0 50.4 50.4 Evaporator Heat Duty 119,000 MMBtu/year 3 4 5 6 7 8 9 185 185 185 185 185 185 185 13 15 15 13 13 13 13 Flow Rates, metric ton/day 320.8 125.3 365.3 320.8 151.9 168.9 168.9 270.1 125.3 281.3 270.1 151.9 118.2 118.2 0.0 0.0 31.2 0.0 0.0 0.0 0.0 50.7 0.0 52.8 50.7 0.0 50.7 50.7 Annual Savings, $/year 120,000 $2.00/MMBtu 100,000 80,000 60,000 $2.50/MMBtu 40,000 20,000 $3.00/MMBtu 0 0 10 20 30 40 Dilution Flow, gpm FIGURE 2: Optimization of Dilution Flow 120,000 100,000 20 80,000 15 60,000 10 40,000 5 Savings Optimal Dilution 20,000 0 0 2 2.2 2.4 2.6 2.8 3 Cost of Energy, $/MMBtu FIGURE 3: Determination of Optimum Dilution Annual Savings, $/year Optimal Dilution Flow, gpm 25 ChemCAD Model Input Lime Mud Washer (FLTR) Calculation Mode: Type: Cake Formation Angle: Drum RPM: Cake Specific Resistance: Cake Moisture Fraction: 1 Rotary Drum Filter 15 degrees 2 1 ft/lb 0.3 Heat Exchanger for Evaporator (HTXR) Heat Duty: Varied to achieve 30% NaOH after flash separator Evaporator Flash Tank (FLAS) Flash Mode: Pressure: 5 12.7 psia Controller for Heat Exchanger (CONT) Controller Mode: Feed Backward Adjustable Parameter: Heat Exchanger Heat Duty Target Parameter: 0.30 (NaOH Concentration in Stream 8)