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2001 Progress Report: Polymer-Based Aqueous Biphasic Extraction Technology for Reaction Engineering of the Alkaline Paper Pulping Process
EPA Grant Number: R826732Title: Polymer-Based Aqueous Biphasic Extraction Technology for Reaction Engineering of the Alkaline Paper Pulping Process
Investigators: Rogers, Robin D. , April, Gary C. , Huddleston, Jonathan G. , Wiest, John M.
Institution: University of Alabama - Tuscaloosa
EPA Project Officer: Richards, April
Project Period: October 1, 1998 through September 30, 2001 (Extended to December 30, 2002)
Project Period Covered by this Report: October 1, 2000 through September 30, 2001
Project Amount: $350,139
RFA: Technology for a Sustainable Environment (1998)
Research Category: Pollution Prevention/Sustainable Development
Description:
Objective:The Kraft paper pulping process is a chemical method that produces 70 percent of the world's pulp. Though these processes are effective in the removal of lignin from the cellulose fraction of wood, they have tremendous disadvantages from an environmental perspective. These processes are only approximately 50 percent efficient in delignification of the pulp, requiring the remaining pulp to be bleached. The chemical and energy consumption of these processes is large, and they emit reduced forms of sulfur and aromatic compounds into the environment. The objectives of this project are to utilize wholly aqueous systems, such as polymer-based aqueous biphasic systems, as an environmentally benign extraction media for the separation of reaction products during the pulping process, and thus reducing chemical and energy consumption, eliminating emissions, and increasing the efficiency of the pulping process. Progress Summary:
Our studies on the effectiveness of polymer-based aqueous biphasic systems (ABS) on the delignification of pine and mixed hardwood in conjunction with alkaline pulping solutions have continued (see previous reports for experimental details). Figure 1 shows the total yield of pulp recovered after samples of mixed hardwood were subjected to different pulping conditions expressed as a function of the Kappa number. The Kappa number is a measurement of lignin content remaining in the pulp after the pulping procedure. The industrial standard is to achieve the highest pulp yield and lowest Kappa number possible without compromising the quality of the cellulose fibers. In Figure 1, samples of hardwood were pulped under different conditions of time and temperature, and the results combined in the plots are shown. Pulping solutions either resembled pure Kraft (i.e., no polymer was present) or contained polymer and alkaline pulping solution or an alkaline pulping solution from which the sulfide present in Kraft solutions was omitted. Figure 1 indicates that the yield of pulp is considerably improved compared to conditions that utilize standard Kraft pulping solutions. Comparable if not better reduction in Kappa number also is achieved by the inclusion of polymer, and these conditions rival what can be achieved by the inclusion of both sulfide and polymer. Because sulfide is included to improve the dissolution of lignin during reaction, it may be that the PEG is achieving a similar effect. Similar results are achieved during the alkaline pulping of softwood as may be seen in Figure 2.
Figure 1. Pulping of mixed southern hardwood in the presence of PEG 2000 with and without sulfide compared to standard Kraft conditions.
Figure 2. Alkaline pulping of softwood in the presence of PEG 2000 and in solutions with and without sulfide.
Greater recovery of pulp is achieved at comparable or better Kappa numbers (lignin reduction) when alkaline pulping solutions are compared with and without polymer (PEG 2000). Polymer solutions with and without sulfide included in the Kraft solution behave rather similarly, although it appears that sulfide containing solutions can go to lower Kappa numbers. Also shown in Figure 2, are data for the pulping of softwood in the presence of PEG alone. This is ineffective and demonstrates that it is unlikely that PEG takes any direct part in the reaction. Figure 2 also shows the distribution of literature data based on "Organosolv" processes, which illustrate that the PEG-based reaction is comparable to the best "Organosolv" practice.
For both the hardwood and softwood cases, it was possible to demonstrate a considerable reduction in the consumption of alkali during the polymer-enhanced Kraft pulping processes. An example of this for the softwood polymer-enhanced pulping solution is shown in Figure 3.
Figure 3. Residual alkali fraction following alkaline pulping.
Analysis of the reaction kinetics of alkaline pulping in which the presence of polymers was compared to the normal polymer-free Kraft reaction conditions revealed that the major effect of the addition of polymers occurred during the initial phase of the reaction. This was a somewhat unexpected result. It is generally accepted that the initial phase of pulping reactions is controlled by diffusion limitations, and later stages by reaction kinetics. Our analysis has shown that for both hardwood and softwood, there was little or no effect on reaction kinetics and that the greatest improvement in performance occurred during the initial diffusion limited stages. Analysis of wood samples by SEM appeared to show that the apparent cause of the decrease in diffusional limitations during pulping might be due to the effect of the inclusion of the phase separating polymers causing increased swelling of the wood. These results are shown in Figure 4.
Figure 4. SEM of wood samples following alkaline pulping. Above SEM following Kraft pulping without polymer addition. Below SEM following Kraft pulping in the presence of PEG 2000 but without the inclusion of sulfide.
Examination of Figure 4 appears to show that wood samples following alkaline pulping in the presence of polymer have a more swollen fibrous structure than samples from which polymer was omitted during pulping. Analysis of molecular weight fractions of lignin by Size Exclusion Chromatography derived from the different processes both with and without polymer appeared to show little difference. This seems consistent with the idea that the polymer affects diffusional rather then chemical aspects of the pulping process.
Future Activities:Recently, we have begun to turn our attention to the use of alternative pulping chemistry to alkaline Kraft on which to base a polymeric delignification process. In this context, we have begun to examine the use of alkaline earth catalysts, initially concentrating on lithium and magnesium sulfates in the pulping of mixed southern hardwoods. Initial results with these salts indicated that increased reaction time and temperature resulted in an increased Kappa number. This was found to be due to lignin condensation on the fibers as the pH fell during the course of the reaction (see Figure 5).
When magnesium sulfate is used as a catalyst for the process, pH cannot be controlled. However, reactions may be run at pH greater than 12 using lithium sulfate as catalyst and the problem of lignin condensation on the fibers is thereby controlled. This is illustrated in Figure 6, which shows the delignification selectivity of several different systems. The standard Kraft system is shown (inherently run at pH 14) compared to a magnesium sulfate/PEG pulping system and a lithium sulfate/PEG pulping system. The pH of the magnesium sulfate system is uncontrolled, resulting in lignin condensation as the reaction proceeds and a consequently higher Kappa number. The acidic conditions also lead to more hydrolysis of the pulp. On the other hand, in the lithium sulfate system, higher pulp yields and lower Kappa numbers are attained than in a comparable Kraft process.
This investigation of alternative catalysts to the alkaline Kraft process in the context of reactive extraction using aqueous polymer solutions is continuing. This work will be pursued both in the context of improved fiber production and as a method for the production of chemical feedstocks. Further funding for the continuation of this work also is being pursued.
Figure 5. SEM of residual pulp fiber following MgSO4-PEG ABS pulping 180oC, 90 min.
Figure 6. Comparison of delignification selectivity for different catalyst systems.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
| Other project views: | All 52 publications | 19 publications in selected types | All 19 journal articles |
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Guo Z, Li M, Willauer HD, Huddleston JG, April GC, Rogers RD. Evaluation of polymer-based aqueous biphasic systems as improvement for the hardwood alkaline pulping process. Industrial and Engineering Chemistry Research 2002;41(10):2535-2542. |
R826732 (2001) R826732 (Final) |
not available |
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Guo Z, Li M, Willauer HD, Huddleston JG, Rogers RD, April GC. PEG-based aqueous biphasic systems as improvement for Kraft hardwood pulping process.. Chemical Engineering Communications. 2003;190:1155-1169. |
R826732 (2001) |
not available |
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Guo Z, Huddleston JG, Rogers RD, April GC. Reaction parameter effects on metal-salt-catalyzed aqueous biphasic pulping systems. Industrial & Engineering Chemistry Research 2003;42(2):248-253 |
R826732 (2001) R826732 (Final) |
not available |
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Li MA, Willauer HD, Huddleston JG, Rogers RD. Temperature effects on polymer-based aqueous biphasic extraction technology in the paper pulping process. Separation Science and Technology 2001;36(5-6):835-847 |
R826732 (2001) R826732 (Final) |
not available |
polymer-based aqueous biphasic systems, lignin, cellulose, Kraft pulping, Kappa number, pulp yield, residual alkali, cook, standard pulping solution, engineering, bio-renewables. , Sustainable Industry/Business, Scientific Discipline, Waste, RFA, Technology for Sustainable Environment, Sustainable Environment, Incineration/Combustion, Environmental Engineering, Environmental Chemistry, incineration, chemical reaction systems, SIC = paper pulping, cleaner production, Volatile Organic Compounds (VOCs), alkylation reaction, green chemistry, waste minimization, environmentally conscious manufacturing, hydrolysis, biphasic extraction technology, reduced sulfur from incineration, Alkaline paper pulping process, innovative technology, reaction engineering, aqueous biphasic extraction, pollution prevention
Relevant Websites:
http://bama.ua.edu/~rdrogers 
http://bama.ua.edu/~cgm
Progress and Final Reports:
1999 Progress Report
2000 Progress Report
Original Abstract
Final Report