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Washer control to save chemical costs

Control dilution factor in brown stock washing with DLT-5500

The primary purpose in washing pulp is to recover and recycle costly chemicals. To recover these chemicals, the water used for the washing must be evaporated. Without proper washing, these impurities in the pulp mat would interfere with both bleaching and paper making. Where bleaching is to follow, a tightly controlled washing process will reduce the amount of bleaching chemicals needed and allow a more stable bleaching process. Therefore, the overall objective of washer control on pulp lines is to obtain the maximum cleaning with the minimum loss of chemicals, and do this without any excursions in the process.

Many existing rotary washers are poorly automated.  That is, they depend heavily on manual intervention from the operator and often do not have adequate control of the washing process itself.  Conventional approaches to brown stock washing controls use measurement inputs that have no direct relationship to the actual control parameters or have high measurement errors. Some of these poor approaches use manual measurement of the solids content in the liquor leaving the first-stage of washing, stock consistency, filtrate conductivity and other unsatisfactory approaches.

BTG’s brown stock washing solution provides a breakthrough approach. The DLT-5500, Dissolved Lignin Transmitter, directly measures washer losses in the pulp stream continuously. Combined with proper feed consistency measurement and control, and a recommended solids refractometer on the first stage filtrate going to the evaporaotors, an optimized washing solution can be implemented.

Highlights

  • Reduce variation in Soda carryover to the bleach plant
  • Control BOD (Biological Oxygen Demand) load
  • Lower evaporator steam consumption
  • Lower use of defoamers
  • Increased production if mill is evaporator, recovery, or BOD limited
  • Reduced soda loss
  • Provide optimum washing with minimal shower waster

Soda losses are a direct function of dilution factor in the washing operation.

In bleach mills the cost of soda loss is magnified by the extra bleaching cost due to carryover liquor.  These costs are directly related to the dilution factor and can be quantified in BTG’s brown stock washing solution where the dilution factor is measured and controlled.

Pollution control and waste treatment are important items and are related to the dilution factor used and are taken into consideration by the controls.  In mills where the dilution factor is not measured and controlled the safe side limits often result in significant cost to the mill.

Mills are often limited in production by the pulp washing operation due to overloading of the evaporators, excessive stream pollution, or even machine run ability. When they exist these productions related factors can easily override all other costs in pulp washing.

Simply measuring soda loss or conductivity does not take into account the total effect of these relationships. BTG’s brown stock washing solution closes the loop on the impact of poor washing.

The objective of brown stock washing operation is to remove the maximum amount of black liquor solids (lignin and inorganics), from the pulp while using as little fresh water input as possible. The pulp washer system also has an important role as a pollutant control device. The dissolved solids that are left in the pulp will not only interfere with the bleaching and papermaking operation, but also may end up in the effluent stream leaving the mill. The loss of black liquor solids due to solids left in the pulp also means that less heat will be recovered by the recovery boiler since these solids are not burned. Makeup chemicals must also be added to replace the inorganics that are lost. Poor washing can add unnecessary costs to mill operations. This often leads to a bottleneck in the mill. 

BTG’s Brown Stock Washing Solution Benefits

  • Increase production with reduced evaporator load
  • Reduce bleach plant chemical consumption
  • Reduce defoamer consumption
  • Increase operator effectiveness
  • More uniformly washed pulp
  • Reduced soda carryover with better control
  • More uniform delivery of solids to the evaporator process

Early methods of pulp washing depended on the operator observing the washer operation and adjusting the control parameters based on knowledge and past experience. However, the process is comprised of many interacting variables. This often meant that the operator had to pay attention to a few key variables while many other important variables were basically ignored.

Control of dilution factor is extremely difficult when other variables involved in the calculation are not precisely known.  Instruments applied in mills to measure these variables are inaccurate for many reasons. In conventional brown stock washing controls, these inaccuracies are then magnified by the calculations since the dilution factor is the amount of water that goes all the way through the pulp mat. But, the values used in conventional brown stock washing calculations start from the weight of the dry pulp in the mat and the estimated amount of water associated with this pulp mat that must be totally displaced before one drop actually goes all the way through the mat. Also, small errors in consistency result in large errors in dilution factor control. Measurement of the conductivity of the filtrate of the last washer has been used in some conventional approaches. This method is unsatisfactory due to an imprecise relation between the conductivity and the liquor solids as conductivity is influenced by the nature of the solids. The DLT-5500 can be calibrated to read COD directly and this continuous value is then used to control the shower flows and ultimately the dilution factor.