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					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 Washe r
                    2
 CaCO3                                              1

            1                            5                              2

                                                                Washe d CaCO3
                                7
  Flashed Water


  Concentrated                                3
  NaOH                                    6                 3
                            4



                        5              Evaporator

                9               8


FIGURE 1: Process Flow Diagram of Calcium Carbonate Washer Process
                            TABLE 1: Summary of Stream Flows

Case:            Base (0 gpm)               Evaporator Heat Duty    26,500 MMBtu/year
Stream             1        2         3        4        5      6      7      8        9
Temperature, F     185      185        185     185       185   185     185    185     185
Pressure, psia       15       13         13      15       15     13      13     13      13
                                   Flow Rates, metric ton/day
Total             240.0    44.6      195.4      0.0 240.0 195.4       30.7 164.7 164.7
Water             156.0    10.0      146.0      0.0 156.0 146.0       30.7 115.3 115.3
CaCO3              31.2    31.2         0.0     0.0     31.2    0.0     0.0    0.0     0.0
NaOH               52.8     3.4       49.4      0.0     52.8  49.4      0.0  49.4    49.4


Case:            12 gpm                     Evaporator Heat Duty    74,600 MMBtu/year
Stream             1        2         3        4        5      6      7      8        9
Temperature, F      185     185        185      185      185   185     185    185     185
Pressure, psia       15       13         13       15      15     13      13     13      13
                                   Flow Rates, metric ton/day
Total             240.0    44.6      260.8     65.4 305.4 260.8       93.6 167.3 167.3
Water             156.0    10.8      210.6     65.4 221.4 210.6       93.6 117.0 117.0
CaCO3              31.2    31.2         0.0      0.0    31.2    0.0     0.0    0.0     0.0
NaOH               52.8     2.6       50.2       0.0    52.8  50.2      0.0  50.2    50.2


Case:            16 gpm                     Evaporator Heat Duty    90,900 MMBtu/year
Stream             1        2         3        4        5      6      7      8        9
Temperature, F      185     185        185      185      185   185     185    185     185
Pressure, psia       15       13         13       15      15     13      13     13      13
                                   Flow Rates, metric ton/day
Total             240.0    44.6      282.6     87.2 327.2 282.6 114.6 168.0 168.0
Water             156.0    11.0      232.2     87.2 243.2 232.2 114.6 117.6 117.6
CaCO3              31.2    31.2         0.0      0.0    31.2    0.0     0.0    0.0     0.0
NaOH               52.8     2.4       50.4       0.0    52.8  50.4      0.0  50.4    50.4


Case:            23 gpm                     Evaporator Heat Duty    119,000 MMBtu/year
Stream             1        2         3        4        5      6      7      8       9
Temperature, F      185     185        185     185       185   185    185     185    185
Pressure, psia       15       13         13      15       15     13     13      13     13
                                   Flow Rates, metric ton/day
Total             240.0    44.6      320.8 125.3 365.3 320.8 151.9 168.9 168.9
Water             156.0    11.3      270.1 125.3 281.3 270.1 151.9 118.2 118.2
CaCO3              31.2    31.2         0.0     0.0     31.2    0.0    0.0     0.0    0.0
NaOH               52.8     2.1       50.7      0.0     52.8  50.7     0.0   50.7   50.7
                             120,000
                                                                                         $2.00/MMBtu

 Annual Savings, $/year
                             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


                             25                                                                   120,000
Optimal Dilution Flow, gpm




                                                                                                  100,000




                                                                                                            Annual Savings, $/year
                             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
ChemCAD Model Input
Lime Mud Washer (FLTR)
      Calculation Mode:           1
      Type:                       Rotary Drum Filter
      Cake Formation Angle:       15 degrees
      Drum RPM:                   2
      Cake Specific Resistance:   1 ft/lb
      Cake Moisture Fraction:     0.3

Heat Exchanger for Evaporator (HTXR)
       Heat Duty:                 Varied to achieve 30% NaOH after flash separator

Evaporator Flash Tank (FLAS)
      Flash Mode:                 5
      Pressure:                   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)

				
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