Brownstock Washing Cost Optimization · to review the cost parameters in brownstock washing optimization for both unbleached and bleached production, explore the sensitivities of

Brownstock Washing Cost Optimization James Cantrell Jacobs Engineering ABSTRACT The objective of brownstock washing is primarily to recover spent pulping chemicals as efficiently as possible. The recovery of pulping chemicals is necessary for the economics of the Kraft pulping process. For bleached mills, a secondary objective is to minimize black liquor solids carryover to the bleach plant to reduce excessive bleaching chemical usage. Aside from specific washer technology and process configuration, the dilution factor is the primary process variable that impacts washing performance. Higher dilution factor results in lower soda loss, but more evaporation load and higher utility steam cost and pumping electrical power cost. The utilities costs depend on the evaporators process configuration, number of effects and the overall evaporator economy. The intent of this paper is to review the cost parameters in brownstock washing optimization for both unbleached and bleached production, explore the sensitivities of the cost parameters and equipment performance, and present a basic method for brownstock washing cost optimization. INTRODUCTION Brownstock washing is the first step of the kraft chemical recovery cycle. The primary objective is to minimize black liquor solids losses with the washed pulp at minimal operating cost. Minimal operating cost occurs at minimal wash water usage. Dilution Factor and Washing Efficiency The dilution factor is the washing parameter used to represent wash water usage. Dilution factor (DF) is the mass of wash water flow added to a washing stage less the mass of liquid leaving with the washed pulp on a per mass of oven-dry pulp basis [1, 2]. The DF is a dimensionless number expressed as lb/lb, kg/kg, or ton/ton, etc. Figure 1 below shows the calculation of DF for a typical washing stage. Also shown are two key performance indicators for washing efficiency – the displacement ratio (DR) and the Norden efficiency normalized to 10% discharge consistency (E10) [3, 4, 5].

Figure 1. Dilution Factor and Washing Efficiency

Brownstock Washer Types Brownstock washing process and performance characteristics are intrinsic to the washer type [6]. Table 1 shows the various washer types and their typical process parameters.

Table 1. Washer Types

The E10 Norden efficiencies are additive for washer stages in series. The total E10 target for good brownstock washing is 12 to 16. Overall Brownstock Washing Flow Diagram Below is a typical three stage vacuum washer flow diagram.

Figure 2. Overall Brownstock Washing Flow Diagram

Figure 2 shows the main balance inputs and outputs around the pulping/washing system. Inputs include:

wood chips

white liquor

steam

wash water (typically evaporator condensate) Outputs include:

washed brown pulp

weak black liquor The relative amounts of balance inputs and outputs depend on the cooking conditions or pulping parameters that include the following:

chemical charge (white liquor)

H-factor (cooking time and temperature)

pulp yield (kappa number target) Pulping Parameters The main difference in pulping balances occurs between bleached and unbleached product grades or rather - low kappa low yield pulp (bleachable grades) versus high kappa high yield pulp (unbleached grades). Table 2 shows the pulping parameters, used in this paper, for both major pulp grades – bleachable and unbleached.

Table 2. Pulping Parameters – Bleachable and Unbleached

This paper will examine washing cost parameters for typical soda loss curves and consider the major cost sensitivities and optimal dilution factors for both bleachable and unbleached pulps. Additional references are listed for those interested in more information on brownstock washing optimization or advanced process controls [7, 8, 9, 10]. DILUTION FACTOR AND WASHER LOSSES For a given washing system, washer sodium loss data can be collected from the washed pulp for various dilution factors. The collected data is then plotted as equivalent saltcake loss (lb/odt pulp) versus dilution factor. Figure 3

Pulping Parameters Bleachable Unbleached

Pulp Yield, % 46.0 56.0

AA charge, % on od chips 18.0 13.0

White Liquor Activity, % 85.0 85.0

White Liquor Sulfidity, % 30.0 30.0

White Liquor Reduction Efficiency, % 95.0 95.0

Black Liquor Solds lb/odt 3,600 2,300

shows washer loss data generated from WinGEMS™ modeling for low and high kappa pulps using a three-stage vacuum washer system plus a final decker. This data is used for demonstration purposes only. As can be seen, it takes more dilution factor for low kappa pulp to achieve the same saltcake loss as the high kappa pulp. This is due to more black liquor solids generated per ton for the low kappa pulp. Therefore, the low kappa pulp requires a higher degree of washing than the higher kappa pulp.

Figure 3. Washer Loss Curves

The above data was generated through a basic WinGEMS™ model of the pulping and washing system using pulping parameters from Table 2. The saltcake loss curves shown in Figure 3 do not represent any actual specific mill. This data has been generated from the model for demonstration purposes to illustrate the cost analysis calculations and techniques discussed in the remainder of this paper. Its important to note that chemical oxygen demand (COD) has become a standard for tracking performance of washing system losses in the brownstock, oxygen delignification, and bleaching areas [11]. Under specific steady process conditions, i.e. production, white liquor chemistry, and pulping parameters, conductivity can be used as a quick measurement to correlate to washing losses [12]. The best approach for measuring actual washer efficiency is to measure black liquor solids because conductivity does not include non-ionic compounds that carryover from the washing process. Black Liquor Solids Two important factors that are needed for washing cost analysis include the ratio of total black liquor solids to equivalent saltcake and the ratio of total organics (fuel) to equivalent saltcake. These factors are calculated from the digester balance and the resultant black liquor solids generated from pulping. Table 3 and figure 4 show the overall digester balance for low kappa pulp and Table 4 and figure 5 show the overall digester balance for high kappa pulp.

Table 3. Digester Balance - Low Kappa Pulp

Figure 4. Black Liquor Solids – Low Kappa Pulp

Table 4. Digester Balance - High Kappa Pulp

Figure 5. Black Liquor Solids – High Kappa Pulp

Total black liquor solids generated is 3,600 lb/odt and 2,300 lb/odt for low kappa and high kappa pulping, respectively. Tables 3 and 4 show the two saltcake ratio factors that are used later for washing costs. WASHING COST ANALYSIS Washing Cost Components Brownstock washing cost includes several components as follows:

Sodium loss as saltcake, Na2SO4 (chemical makeup)

Evaporator steam

Wash water heating steam

Black liquor organics (fuel) loss

Wastewater treatment (BOD load)

Additional bleaching chemicals (for bleachable grades only) All cost calculations for the above cost components are shown in the Appendix. Figures 6 and 7 show typical washing cost distributions for bleached and unbleached pulps.

Figure 6. Low Kappa Bleached Pulp Washing Cost Distribution

Figure 7. High Kappa Unbleached Pulp Washing Cost Distribution

Evaporator steam cost is by far the most significant cost variable, making up 55% and 74% of the total incremental washing cost for bleached and unbleached, respectively. Of course, for bleached pulp, the bleaching chemicals cost is second to evaporator steam at 32% of the total incremental washing cost. Chemicals, Fuels, Utilities Costs The basic cost data needed to determine total washing cost are for chemicals, fuels, and utilities. Table 5 shows the basic cost data used for the calculations later.

Table 5. Basic Cost Data

Chemical Makeup Brownstock washing chemical loss is typically tracked as soda loss or saltcake (Na2SO4) loss. This is because saltcake is the predominate makeup chemical in the recovery cycle and is typically added at the recovery boiler along with precipitator ash which is also predominately sodium sulfate. Other sodium chemicals, i.e. NaOH or NaHS are also added as sodium makeup to the white liquor system depending on the sodium/sulfur balance in the mill. For this cost analysis we use saltcake as the makeup chemical. Saltcake loss data is taken from figure 3 and the saltcake cost is from table 5 above. Below are the low and high kappa saltcake makeup cost curves.

Figure 8. Saltcake Cost vs. Dilution Factor for Low Kappa Pulp

Figure 9. Saltcake Cost vs. Dilution Factor for High Kappa Pulp

ClO2 Cost, $/lb 0.55

NaOH Cost, $/lb 0.25

Na2SO4 Cost, $/ton 100

Steam Cost, $/klbs 3

Fuel Cost, $/MMBTU 4

Electrical Power Cost, $/kWhr 0.07

Evaporation Steam Evaporation steam usage is directly related to the wash water used in brownstock washing and the evaporator process parameters. Assuming the digester as-blown consistency equals the final washing mat consistency, the weak black liquor flow is equal to the wash water flow. The total evaporation required is equal to the weak black liquor feed to the evaporators minus the product liquor flow. The product liquor flow is equal to the black liquor solids generated from cooking (see figures 4 and 5) divided by the product percent dry solids. The steam usage is then calculated from the evaporator economy which is a function of the number of effects and the system configuration.

Table 6. Evaporator Steam Calculation – Process Parameters

Below are figures 10 and 11 showing evaporator steam cost versus dilution factor for low and high kappa pulp.

Figure 10. Evaporator Steam Cost vs. Dilution Factor for Low Kappa Pulp

Figure 11. Evaporator Steam Cost vs. Dilution Factor for High Kappa Pulp

Mat Consistency, % 12

Evaps Product Solids, % 70

Evaps Economy, lb evap/lb steam 4.5

Wash Water Heating Wash water is defined as any process water added to the washing system which is typically the shower water on the final washer stage. The wash water temperature should be as hot as possible without causing excessive flashing in the filtrate downleg of a vacuum washer system, particularly the first stage. The optimal wash water temperature (shower water on the final stage) is typically around 160°F. Often times the process condensate used is already at this temperature and no heating is required. For this analysis, we assume a worst case of using fresh steam to heat wash water from 140°F to 160°F. The wash water heating cost curves are shown below in figures 12 and 13.

Table 7. Wash Water Heating Calculation – Process Parameters

Figure 12. Wash Water Heating Cost vs. Dilution Factor for Low Kappa Pulp

Figure 13. Wash Water Heating Cost vs. Dilution Factor for High Kappa Pulp

Mat Consistency, % 12

Target Temperature, °F 160

Wash Water Inlet Temperature, °F 140

Steam Vaporization Enthalpy, BTU/lb steam 970

Black Liquor Organics Loss Although brownstock washer losses are normally measured as sodium or saltcake loss, it is black liquor solids that is lost in total. A fraction of the black liquor solids lost is organics. The organics contain fuel value that would be used in the recovery boiler to generate steam. Therefore, this heat value can be translated into an incremental fuel cost that is spent to generate incremental steam from a combination or power boiler. Below are the cost curves versus washing dilution factor.

Table 8. Black Liquor Organics Fuel Heat Content and Boilers Efficiencies

Figure 14. Organics Loss (Fuel Cost) vs. Dilution Factor for Low Kappa Pulp

Figure 15. Organics Loss (Fuel Cost) vs. Dilution Factor for High Kappa Pulp

Recovery Boiler Efficiency, % 70.0

Combination Boiler Efficiency, % 75.0

Black Liquor Organics Fuel Value, BTU/lb 10,000

Waste Water Treatment Black liquor solids loss from the brownstock washing system is ultimately lost to either the paper mill or bleach plant effluent which flows to waste water treatment. For this analysis, it is assumed that the waste water treatment system is an aerated stabilization basin. The BOD content of the influent to the treatment system requires oxidation achieved by powered aeration of the waste water. Below are typical values for black liquor solids BOD content and the power requirement to treat BOD per weight.

Table 9. Typical Black Liquor Solids BOD Content and Aeration Power Requirement

Below are the cost curves for low and high kappa pulp.

Figure 16. Waste Treatment Cost vs. Dilution Factor for Low Kappa Pulp

Figure 17. Waste Treatment Cost vs. Dilution Factor for High Kappa Pulp

BOD, lb/lb BLS 0.175

Aeration Power, HPhr/lb BOD 0.5

Bleaching Chemicals For bleachable grade low kappa pulp, black liquor solids carryover to the bleach plant must be consumed in the initial bleaching and extraction stages of the process [13]. Table 10 shows typical factors that can be used for calculating the first stage chlorine dioxide usage and extraction stage caustic usage for washing loss carryover to the bleach plant.

Table 10. Bleaching Chemicals Usage Factors for Washer Carryover

Below is the cost curve versus brownstock washing dilution factor.

Figure 18. Bleach Chemicals Cost vs. Dilution Factor

BROWNSTOCK WASHING COST OPTIMIZATION The total washing cost is a some of the cost components as follows:

The overall washing cost curves for low and high kappa pulp are shown in figures 19 and 20.

D1 Stage ClO2, lb/lb Na2SO4 0.6

E1 Stage NaOH, lb/lb Na2SO4 1.0

Figure 19. Total Cost vs. Dilution Factor for Low Kappa Pulp

Figure 20. Total Cost vs. Dilution Factor for High Kappa Pulp

As can be seen, each of these overall cost curves has a minimum cost. At the minimum cost point, this is the optimum dilution factor. For the analysis conducted in this paper, the optimum dilution factors observed are 5.0 for low kappa pulp and 2.0 for high kappa pulp. Of course, each mill situation will be different with different process configurations and cost parameters, but the cost calculation methodology will be the same or similar. Figures 21 and 22 show the sensitivities of optimum dilution factor (DF) versus steam cost and bleaching chemicals cost for low kappa pulp.

Figure 21. Low Kappa Pulp Optimum DF vs. Steam Cost

Figure 22. Low Kappa Pulp Optimum DF vs. Bleach Chemicals Cost

From figure 21 we see that the optimum DF ranges from 4.4 to 5.4 with a significant variation in steam cost from 25% to 175% of the steam cost shown in table 5. The optimum DF varies more at lower chemicals unit cost as seen in figure 22. Figures 23 and 24 shows the sensitivity of optimum DF versus steam cost and saltcake makeup cost for high kappa unbleached pulp. For high kappa pulp from figure 23, the optimum DF ranges from 1.5 to 2.5 when steam cost is varied from 25% to 175% of the base steam cost from table 5. The unbleached case is much less sensitive to steam unit cost than the bleached pulp. Figure 24 shows the sensitivity of optimum DF to saltcake makeup cost for high kappa unbleached pulp. The optimum dilution factor ranges from 1.7 to 2.2. Hence, not as sensitive since it does not represent a significant portion of the washing cost.

Figure 23. High Kappa Pulp Optimum DF vs. Steam Cost

Figure 24. High Kappa Pulp Optimum DF vs. Saltcake Cost

CONCLUSIONS An overall washing system sodium or solids loss versus dilution factor curve can be generated, from measured field data, that is characteristic for the specific system, regardless of washer technology and configuration. Using the washing loss data and other pulping and washer system process data, washing cost curves can be generated for each of the cost components. Then an overall cost curve can be generated to identify the optimum dilution factor. This is the dilution factor of lowest overall washing cost. By far, the washing cost variable of greatest impact is evaporator steam usage and followed by bleach chemical costs for low kappa bleached pulp. High kappa unbleached pulp has an optimum dilution factor that is typically less than half of the low kappa bleached pulp optimum dilution factor. The method shown in this paper can be used for any washing system by gathering the pertinent washer loss data, process parameters, chemicals unit costs, and utility costs.

ACKNOWLEDGEMENTS The author wishes to thank Jacobs Engineering for supporting the time and resources needed to develop this paper for the 2019 TAPPI PEERS Conference. REFERENCES [1] Compton R., “Economic Analysis of Brown Stock Washing Systems”, TAPPI Pulping Conference, (1997). [2] Henderson M., “Evaluating Brown Stock Washing”, TAPPI PEERS Conference, (2017). [3] Henderson M. and Hjort A., “Addition of New Washers to an Existing Brown Stock Wash Line”, PaperWeek

Canada, (2016). [4] Kopra R., Koivula K. and Dahl O., “Optimization of a Brown Stock Washing Line”, TAPPI Journal, vol. 14,

no. 2, pg. 83-91, (2015). [5] Kopra R., Helttunen J., Tervola P., Tirri T. and Dahl, O., “Refractive Index Measurements for Brown Stock

Washing Loss”, Appita Journal, vol. 63, no. 2, pg. 131-136, (2010). [6] Brunner L., “Brown Stock Washing Experiences – Lessons Learned”, TAPPI PEERS Conference, (2017). [7] Santos R. and Hart P., “Brown Stock Washing – A Review of the Literature”, TAPPI Journal, vol. 13, no. 1,

pg. 9-19, (2014). [8] Hart P., “The Chemical versus Energy Cost Tug of War: A Pulp Mill Perspective”, TAPPI Engineering,

Pulping and Environmental Conference, (2009). [9] Kopra R., Kari E., Harinen M., Tirri T. and Dahl, O., “Optimization of Wash Water in Brown Stock

Washing”, TAPPI Journal, vol. 10, no. 9, pg. 27-33, (2011). [10] Badwe A. and Satini R., “Optimization of Brown Stock Washing Using Advanced Process Control”, TAPPI

Papercon, (2018). [11] Kopra R. and Dahl O., “Determining Wash Loss Levels in the Brown Stock Pulp Washing Line Using

Different Methods”, TAPPI Journal, vol. 16, no. 5, pg. 253-259, (2017). [12] Hart P. and Brown M., “Understanding Conductivity and Soda Loss”, TAPPI Journal, vol. 16, no. 2, pg. 65-

75, (2017). [13] Richardson B., “Monitoring Carryover at the Brown Stock Washers”, Pulp & Paper Canada, vol. 106, no. 6,

pg. 43-47, (2005).

APPENDIX – COST CALCULATIONS Chemical Makeup

$

$

12,000

Evaporation Steam

100

2,000

% 100

$

1,000

$

Wash Water Heating

100

2,000

1.0°

, °

, °

$

1,000

$

Black Liquor Organics Loss

, , %

$

, %

$

Waste Water Treatment

.

.

0.746

$

$

Bleaching Chemicals

2 0.6 2

1.0

2 $

2 2 $ 2

$

$

$

2 $

$

Total Washing Cost

Gateway to the Future

Brownstock Washing Cost Optimization

James Cantrell

Jacobs EngineeringGreenville, South Carolina

October 28, 2019


Introduction

• Brownstock Washing Objectives

• Washing Parameters & Efficiency

• Washer Types

• Pulping & Black Liquor Solids

• Washing Cost Components

• Washing Cost Optimization

• Conclusions


1997 TAPPI Pulping Conference Paper

by R. Compton

“Economic Analysis of Brown Stock Washing Systems”


Brownstock Washing Objective

• Achieve Low Black Liquor Solids Losses with Low Wash Water Flow

BLS Loss With Washed Pulp


Washing Parameters & Efficiency

• Dilution Factor

• Displacement Ratio

• Norden Efficiency



Norden’s efficiency factor (NEF)

• The number (N) of ideal mixing stages in series with

complete mixing of underflow and overflow, required to

achieve the same departing underflow and overflow values

as those of the washing system, when the entering flows of

the mixing stage system are the same as those of the

washing system.

Gateway to the Future Washer Types


3‐Stage Vacuum Drum Washing System



Balance Inputs



Balance Outputs



Other Balance Parameters• Chemical Charge (white liquor)• H‐Factor (cooking time/temperature• Pulp Yield (kappa # target)


Pulping Parameters – Bleached & Unbleached


Dilution Factor And Washer Losses

Low Kappa Pulp

High Kappa Pulp

Note:The saltcake loss curves shown in this slide do not represent any actual specific mill, but this data has been generated from a WinGEMS™ model for demonstration purposes only.

CJ1

Slide 14

CJ1 Cantrell, James, 9/5/2019


Low Kappa – Black Liquor Solids


High Kappa – Black Liquor Solids


Sodium Ratios

Low Kappa Pulp High Kappa Pulp


Sodium Ratios



Washing Cost Components• Sodium loss as saltcake, Na2SO4 (chemical makeup)

• Evaporator steam

• Wash water heating steam

• Black liquor organics (fuel) loss

• Wastewater treatment (BOD load)

• Additional bleaching chemicals (for bleachable grades only)


Basic Cost Data


Low Kappa Bleached Pulp Washing Cost Distribution


High Kappa Bleached Pulp Washing Cost Distribution

Gateway to the Future Chemical Makeup


Gateway to the Future Evaporation Steam


Gateway to the Future Wash Water Heating


Gateway to the Future Black Liquor Organics Loss


Gateway to the Future Waste Water Treatment


Gateway to the Future Bleaching Chemicals

Low Kappa Pulp

Gateway to the Future Washing Cost Optimization


Optimum Dilution Factor

Optimum Dilution Factor

Gateway to the Future Optimum Dilution Factor



Conclusions Washer solids or sodium loss versus dilution factor data can be measured and

curve(s) generated that are characteristic of a specific system.

Washing cost curves can then be generated from washer loss data and pulping parameters.

An overall cost curve can be generated to determine optimum dilution factor.

The largest washing cost item is evaporator steam usage.

For low kappa bleached production, bleach chemical costs is the next largest cost.

High kappa unbleached pulp has an optimum dilution factor that is typically less than half of the low kappa bleached pulp optimum dilution factor.

The washing cost analysis method shown in this paper can be used for any washing system.


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Brownstock Washing Cost Optimization · to review the cost parameters in brownstock washing optimization for both unbleached and bleached production, explore the sensitivities of