Black Liquor Disposal Treatment
Background
Specialty grade pulp mills face increased demand on their process requirements, if they are to be a responsible corporate citizen in today's environmentally sensitive industrial atmosphere. Driven to reduce production costs and become more efficient to achieve respectable returns on assets, plants applying new technologies and processes to enhance cost reductions are finding appropriate justification for such change. A review of current practices and methods for disposing of the pulping liquors generally results in an active interest to consider system cost reductions.
The ability to reduce treatment costs of extract liquors from the pulping system in specialty mills requires application of advanced filtration technologies along with new methods and considerations for disposal of the spent liquor solids. The conventional method of discharging these liquors to effluent or municipal wastewater treatment systems is becoming extremely costly. Discharge limits are getting more stringent in many areas, providing incentive to consider process changes.
Objective
Because many specialty pulp mills spend a good portion of their processing costs on spent liquor disposal and treatment, a new simple and economical process to concentrate the liquors along with providing alternative methods for solids disposal for final treatment is needed. To meet these operational requirements economically, the concentration of extract liquor streams allows a mill to reduce the load on existing disposal systems, or can be combined with other solids disposal methods. Justification for these changes results from energy costs and/or final solids disposal costs along with possible recovery and use of cooking liquor.
Solution
Technological advances in membrane filtration systems have created an opportunity for specialty pulp mills to treat black liquor streams in order to meet stricter environmental constraints and to improve the mill energy efficiency and/or waste disposal costs, thus reducing overall operating expenses. The "Vibratory Shear Enhanced Processing" or VSEP, developed by New Logic Research, Inc., makes it possible to filter black liquor streams without the fouling problems which would be exhibited by conventional membrane systems.
The VSEP treatment system uses ultra or nanofiltration membrane modules to treat the black liquor in order to concentrate solids (suspended and/or dissolved solids) prior to the final treatment or discharge. At the same time, the VSEP generates a permeate stream that will meet discharge criteria or may be reused for pulp cooking in the process. The VSEP membrane system also reduces BOD, COD, TDS, and TSS from pulp mill effluent streams. Reverse osmosis filtration can also be employed in the VSEP system if removal of color or more of the dissolved solids is required. In summary, the VSEP treatment system can be used to supplement or replace an existing system to increase the mills energy efficiency and/or the final waste disposal volume, as well as to possibly recycle a significant portion of the permeate/caustic stream.
Process Conditions
When the cooking extractives are washed/pressed from the pulp following the digester, the result is a filtrate of spent liquor, at 6 to 10% by weight total solids (TS). This liquor is typically treated chemically to prepare it to be discharged to a wastewater treatment system, if the mill does not have evaporation or other processes in place for concentrating the solids. The addition of a VSEP system is used ahead of other treatment options for concentrating the liquor such as:
- Concentration of solids to be used as supplemental fuel for boilers on-site or off-site.
- Solids can be further concentrated in an existing plant evaporator/drying system.
- The concentrate can be treated with stabilization compounds for:
- Erosion control materials
- Land fill
- Used for land application as agricultural supplementation
The VSEP system will reduce the load on current discharge treatment systems significantly, possibly debottlenecking the overall process and allowing for additional processing capacity. The permeate can be reused as recovered caustic or treated for final disposal.
The VSEP produces a concentrated liquor stream at a flow rate of ~14 gpm which is then sent for further treatment for solids disposal. The concentrated stream contains approximately 16% by weight of total solids (TS). The VSEP treatment system also generates a permeate stream of about 56 gpm which is recycled to the process. The permeate, a caustic solution with a solids concentration less than 5 mg/L of TSS, can be recycled back to the process or discharged.
Using a nanofiltration module in the VSEP system is a commercially viable option for treatment of black liquor streams. Nearly 80 to 90% of the feed black liquor is recovered, suitable for reuse or discharge. Less than 10 to 20% of the feed is a concentrate, thereby reducing the solids load to be handled. Membrane selection is based on material compatibility; flux rates (capacity), permeate quality and concentration requirements. The high pH of the liquor stream makes membrane selection an important criterion in terms of pH tolerance. In this example, the COD reduction is over 70% while the rejects are concentrated from 8% to 16% by weight which would allow the mill to reduce disposal costs significantly. The permeate quality from the VSEP can be controlled though laboratory selection of membrane materials available to fit the application parameters.
A higher concentrate solids level can be achieved with a two stage VSEP treatment system. The two stage system would include a second VSEP stage to further increase the solids from the concentrate of the first stage units, i.e. from 16% to 30+% solids. The final concentration level required is determined by the most economical option for final disposal of the concentrate.
The permeate flux rate in the VSEP can range from 20 to over 70 gallons per day per square feet (GFD), depending on process parameters such as temperatures, membrane selection, and required concentration or BOD/COD/TDS removal. The concentration level out of the VSEP unit is controlled by an automatic timed control valve. This valve is set such that the concentration of the solids is held at the desired level. A multi-stage feed pump supplies the VSEP unit at a pressure suitable for the membrane used. A variable frequency electronic drive is used to set feed pressure through P.I.D. (Proportional-Intregral-Derivitaive) control loop. This kind of drive acts to control the rotational speed of the pump, thus controlling the flow rate.
Economic Value
New Logic's VSEP system provides an alternative approach for black liquor concentration and final treatment options, far more economical than discharging to a local public utility. VSEP will provide solids concentration in an energy efficient compact package. The justification for the use of VSEP treatment system for black liquor concentration is determined through analysis of the system costs and benefits. The VSEP system may:
- Provide expanded capacity in an economical manner.
- Recover spent liquor for concentration.
- Improve energy efficiency of the process and reduces the volume of waste generated.
- Provide high quality caustic permeate for recycle, resulting in pulping chemical savings.
- Reduce BOD/COD/TDS and color levels of effluent that is discharged from the pulp mill.
Summary
New Logic International has supplied VSEP separation technology successfully into many industrial processes. The pulp and paper industries effort to meet environmental regulations and to lower process costs will be enhanced with the utilization of membrane filtration techniques combined with "Vibratory Shear Enhanced Processing". Successful pilot tests have been conducted by New Logic for treatment of black liquor. A VSEP system for black liquor treatment is currently under construction. Our development program for pulp and paper applications, along with the availability of new membrane materials and VSEP technology make it possible to treat the more difficult streams with very successful, economic results.
Contact a New Logic representative to develop an economic analysis and justification for the VSEP in your system. For additional information and potential application of this technology to your process, visit New Logic's Website @ http:/www.vsep.com or contact New Logic, 1295 Sixty Seventh Street, Emeryville, CA 94608, Phone: 510-655-7305, Fax: 510-655-7307, E-mail: info@vsep.com.
References
Smook, G. A., Handbook for Pulp & Paper Technologists, Second Edition, Angus Wilde Publications, Vancouver B.C., Canada, pp. 98-132.
Dexter, R. J., "Industry's Efforts at Effluent Closure Must Focus on Competitive Innovation," Pulp and Paper, February, 1996, pp. 2-4.