IMPULSE DRYING SLUDGE

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War. Res. Vol. 32, No. 1, pp. 258-260, 1998 @) 1998 Elsevier ScienceLtd.Allrightsreserved Printedin Great Britain 0043-1354/98 $19.00 + 0.00

RESEARCH NOTE IMPULSEDRYING SLUDGE SUJIT BANERJEE*, TALAT MAHMOOD, PAUL M. PHELAN and RUSSELL W. FOULKE Institute of Paper Science& Technology,500 Tenth Street, NW, Atlanta, GA 30318-5794,U.S.A. (Received October 1996; accepted in revised form April 1997)

Abstract—In a new approach to energy-efficientsludge dewatering, sludge is contracted briefly under pressure by a hot surface. The steam generated at the interface betweenthe sludgeand the surface forces out some of the water in liquid form. Laboratory demonstrationswith belt-pressedprimary sludgefrom a paper mill removedmore than 20 additional percentage points of water. Correspondingvahtes from municipal sludge and from a mixed primary/secondary industrial sludgewere 10 and 15 additional percentagepoints,respectively. G 1998ElsevierScienceLtd.All rightsreserved Key words—heat,pulse, dewatering,primary, secondary, municipal

Impulse drying was developed for drying paper (Wahren, 1982;Lavery, 1988).The wet paper sheet contacts a hot roll, and the steam pressuregenerated at the roll-sheetinterfaceexpelssome of the water in liquid form, thereby conserving the energy that otherwise would be required for evaporation. A difficulty is that pressure differentials developed within the sheet may cause delamination (Orloff, 1994;Phelan er al., 1995;Orloff and Phelan, 1993, 1994;Crouse et al., 1989),and special roll surfaces have been developed to control heat transfer to the sheet (Orloff, 1992;Orloff and Lindsay, 1992).Since delamination is not an issuewith sludge,a laboratory simulation with an electrohydraulic press was conducted to determine whether the impulse drying concept could be applied to sludge. For paper, the results of the laboratory press correspond to those achieved with a pilot impulse dryer. The press used (illustrated in Fig. 1) was designed to simulate the transient mechanical and thermal conditions, and the pressure profile that a sample would experiencein a commercialimpulsedryer. The thermal profilewas simulatedby usinga platen of the same composition as the surface of the roll press maintained at the operating temperature of the process. As the dominant direction of flow is out-of-plane, the electrohydraulicpress provides an excellent simulation of the impulse drying process (Orloff et al., 1992). A blotter is placed between the sample and the felt to impede solids movement to the felt. The

sample, blotter, and felt are weighed before placement in the nip. The hydraulic system is activated to give a haversine pressure pulse of 14&1500-msduration and a peak pressure between 400 and 1200psi. The sample, blotter, and felt are reweighedafter the impulse, and the water removal calculated. For paper, the water loss compares well with values obtained from the IPST’Spilot impulse dryer. Primary sludge from Riverwood International’s Macon GA facility was belt-pressed at the mill to 30V0solids.The millproducesliner and coated board and uses 50°/0 recycle. The sludge was sent to Ashbrook Corp., Houston TX, where it was further dewatered to 39~0solids using their state-of-the-art high solids (14 roll) Winklepress. We were able to impulse-drythe same sludge with a contact time of 0.7s to more than so~o solids, as shown in Fig. 2. This figure is only one example of multiple measurementsmade at differenttemperatures and nip residencetimes. The lower line reflectsa run at room temperature where the cold roll essentiallyacts as a press. Clearly, pressing alone leads to minimal additional dewatering. The upper line shows that impulse drying at 350”C takes the solids level to almost 60’Yo. A small part of the water lost is released as steam. If the yardstick is weight-gained by the blotter instead of weight lost by the sludge, then the solidslevelattained by liquid water loss is 52Y0,with the remainder evaporating as steam. However, the loss as steam is overstated because the blotter does not capture and hold all the water released from the sludge. Some of the water passes right through *Author to whom correspondence should be addressed the blotter, and some visibly evaporates while the blotter is removed and weighed. [Fax: +1 404894 4778]. 258

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Research Note 32

360C

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Fig. 1. Schematicof the electrohydraulicpress.

800 1200 pressure(psi)

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Fig. 3. Impulse drying of waste activated sludge from the city of Houston.

If the two lines in Fig. 2 are extrapolated to zero pressure, they meet at the ingoing solids level. This suggeststhat that little evaporation occurs when the hot surface is lightly placed on the sludge. While increasing pressure will generate steam, the amount ofsteam developedmust also be linear with pressure. Thedwell time of 0.7s is optimum for this sludge; shorter dwell times lead to reduced dewatering, whereas longer periods increase evaporative loss. In order to determine the effect of initial water content, the Macon sludge pressed to 39°/0 solids at Ashbrook as described above was impulse-dried.A solids level of 5t)0/owas reached, a value lower than that obtained from the original 30% solids sludge. Furthermore, only 25~0of the water releasedwas lost as liquid, with the rest escaping as steam—a much poorer performance than the Fig. 2 results. This probably occurs because a lower steam pressure is generated with the higher solids,which suggeststhat 60

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impulse drying is most efficient when there is sufficientwater available to build up steam pressure at the interface. Our preliminary work suggeststhis optimum water to be at about qOO/o solids for the sludgeused, whichis typicalof the performanceof an inexpensive belt press. Hence, these experiments demonstrate the potential of retrofitting existingbelt presses with add-on impulse dryers. Impulse drying also applies to biological sludge, although dewatering occurs to a lower extent. Figure 3 illustrates our results with municipal waste activated sludgefrom the city of Houston, TX. This material was first belt-pressedto 16Y0 solids at the facility and then impulse dried in the laboratory at 3500c with a contact time of 0.7s. The gain in solids is approximately 10 percentage points. Decreasing the contact time to 0.5s lowered the solids gain by about 2 percentage points across all pressures. 44 40

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Fig.2. Impulsedryingof primarysludgefromRiverwood’s Fig.4. Impulsedryingof mixedsludgefromBigIsland. Maconmill.

Research Note

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feed from belt press\

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Fig. 5. Schematicof a sludge impulse dryer.

Results from impulse drying an approximately equal mixture of primary and secondarysludgefrom the Georgia-PacificBig Island mill are illustrated in Fig. 4. The contact time used was 0,5s, and a 15 percentage point improvementin solids was realized in the best case. Hence, the performance of impulse with this mixedsludgeis intermediatebetweenthat of pure primary and pure secondarymaterial. The effect of increasing temperature seems to be most pronounced up to about 200”C, beyond which, the gain is relatively small. In summary, we have demonstrated the potential of impulse drying to sludge. The additional removal of water in liquid form has a major cost reduction potential for drying, burning, or landfillingprimary or secondarysludge.We envisagethat a commercial impulse unit will follow a belt press as illustrated in Fig. 5. Resultsfrom preliminarypilot runs with a l-m impulse dryer with sludge from Riverwood Macon

correspond well to values obtained with the electrohydraulicpress. Acknowledgement—This study was funded by the Georgia Consortium for Pulp and Paper. REFERENCES Crouse J. W., Woo Y. D., and Sprague C. H. (1989) Delamination—astumblingblock to implementimpulse drying technologyfor linerboard. Tappi J. 72, 211. Lavery H. P. (1988) High-intensity drying processes— impulsedrying,US Department of EnergyReport No. 3, DOE/CE/40738-T3. OrloffD. L (1992)Impulsedryingof linerboard: Control of delamination. .1. Pulp Paper Sci. 18, J23-J32. Orloff D. I. (1994) Impulse drying of recycled multi-ply Iinerboard: laboratory scale studies. Tappi J. 77, 169. OrloffD. L, Jones G. L. and Phelan P. M. (1992)Effkctsof heating mode on roll durability and efficiencyof impulse drying, HTD-VO1193,Fundamentals of Heat Tramfer in Porous Media, pp. 141-150,ASME, August 1992. Odoff D. I. and LindsayJ. D. (1992)The influenceof yield, refining and ingoing solids on the impulse drying performance of a ceramic coated press roll. Proc. 1992 TAPPI Papermaker’s Conference, pp. 85-94, Nashville, TN, APril 1992. Orloff D. I. and Phelan P. M. (1993)Heat transfer during impulse drying: the influence of press impulse and pressure profile. In HTD-VO1. 238, Heat and Mass Transfer in Pulp and Paper Processing, pp. 17-23,ASME, August 1993. Orloff D. L and Phelan P. M. (1994) Impulse drying of multi-ply Iinerboard: laboratory-scale studies. U.S. Department of Energy Report No. DOE/CE/40738-T8. Phelan P. M., OrloffD. I. and Kerschner C. M. (1995)The effect of thermal mass and peak pressure on impulse drying energy. Proc. 1995 TAPPZ Engineering Conference, pp. 651-658,TAPPI Press, September 1995. Wahren D. (1982)Methods and Apparatus for the Rapid Consolidation of Moist Porous Webs, US patent 4,424,613.