Energy & Fuels 1994,8, 953-959
953
Dual Polymer System in Peat Dewatering Lena Ringqvist' and Par Igsell Centre for Peat Research, University of Agricultural Science, Box 4097, S-904 03 Umeii, Sweden Received December 21,1993. Revised Manuscript Received April 20, 1994'
T o make mechanical dewatering of peat economically competitive to conventional peat harvesting, a DS content of about 50% must be achieved after the pressing stage. A pretreatment of the peat involving the addition of chemicals is necessary to reach a high dewatering capacity. In the papermaking industry, systems with two different polymers are used as retention agents. Paper pulp is similar to peat in several aspects. The aim of this study was to investigate the effect of a dual polymer system using two oppositely charged polymers on flocculation and dewatering of peat. In the investigation, an anionic polymer and a cationic polymer, earlier used as single polymer system in peat dewatering, were used in combination. In the peat slurry the dual system gave larger aggregates than the anionic polymer alone and smaller aggregates compared to the cationic polymer alone. From this study it can be concluded that the cationic polymer has a stronger influence on the dewatering properties than the anionic polymer. The anionic polymer was also more dependent of the surface load than the cationic polymer. In low-pressure filtration and during the initial phase of a highpressure dewatering, an additive effect between the two polymers could be seen. The final DS content after completion of the high-pressure dewatering stage was, however, almost completely dependent on the concentration of cationic polymer.
Introduction Peat contains in its natural state about 90 wt 7% water. This amount of water must be reduced to faciliate the use of peat as a fuel or for other applications. Traditional peat dehydration involves sun and wind drying. The peat is dried to a dry solids (DS) content of 40-60%. An alternative method is mechanical dewatering. This method requires a pretreatment, due to a large amount of very small, mainly electrostatically stabilized particles in the peat. Chemical additives, heating or freezing, can be used before the dewatering operation to enhance water removal. An economic and technical evaluation of different peat production methods, completed by the Swedish State Power Board, shows that a DS content of about 50% must be achieved in mechanical peat dewatering systems to make this method competitive to traditional peat harvesting.' The presented cost calculation for a system with mechanical peat dewatering was based on pretreatment with lowering of pH and addition of a cationic polymer. A DS content of 37% was reached in a high-pressure chamber filter (Rittershaus and Blecher). The DS load was 5-6 kg DS/m2. The pressure (60 bar) was applied during 50 min. The peat used was Sphagnum peat with a decomposition degree 5-6 according to the von Post scale. Mechanical dewatering studies using a carex peat type and a weakly anionic polymer have shown that a DS content of 35 7% can be reached with a good capacity and at reasonable ~ o s t s . Recent ~ * ~ furnace developments have Abstract published in Advance ACS Abstracts, June 1,1994. (1)Wilde, J.; Henfridsson, U.; Liljekvist, J-0. Torvbrikettproduktion, Alternativa metoder. Swedish State Electric Board, Report no. VU-V92:s (in Swedish). (2) Pirkonen, P. Dewatering of Peat Slurry Produced by Wet Milling Technique. Filtration Separation 1993,30 (4),327-330. (3)Aho, M.; Pirkonen, P. Efficiency and enviromental effects of dewatering by mechanical pressing. Fuel 1993, 72 (2), 239-243.
0887-0624/94/2508-0953$04.50/0
shown that a DS content of 35-4076 is sufficient for combustion in a pressurized combustion power plant.49s The most promising pretreatment method so far in peat dewatering is based on the use of polyelectrolytes. Addition of polyelectrolytes to a peat slurry leads to flocculation of the mainly negatively charged fine particle fraction in the peat. The optimum dosage of polymer can be predicted from the surface charge of the peat.6 Highly charged and high molecular weight cationic polymers are very effective. The degree of cationic charge of the polymer is more important than the molecular eight.^ This type of polymer gives flocs that form an open and rigid pore structure.8 The flocs also re-form themselves after being ruptured. Lowering the pH in the peat slurry to 3 decreases the polymer demandae Low charge anionic polymers in combination with lowering of pH flocculate peat suspensions by bridging the distances between the particles.8 Reflocculation is incomplete since the bridging polymers will generally rearrange their conformation after rupture.8 Addition of an anionic polymer in combination with lowering of pH gives the same dewatering capacity in filtration experiments (1bar), a t a surface load of 0.5-1.0 kg DS/m2, as a cationic polymerlo (also in combination with lowering of (4) Flyktman, M. New Power Plant Connected to ADEWA Process. h o c . VTTSymp. 134,ArtificialDewatering of Peat, Jyvirskylii, Finland, 15-16 Oct. 1991. ( 5 ) Aij&M,M.; Hulkkonen, S.; Raiko,M. New Process Alternatives for Utilisation of Wet Peat. h o c . V T T Symp. 134, Artificial Dewatering of Peat, Jyvtiskylii, Finland, 15-16 Oct. 1991. (6) Ringqvist, L.; Igsell, P.; Bergner, K.; Lind, E-L. Optimization of Polymer Dosage for Peat Dewatering. Energy Fuels 1992, 6, 578-580. (7) Heeley, G.; Richards, S. R. Use of flocculanta to aid the dewatering of peat. Fuel 1990,69,53-59. (8) Forsberg, S.; Alden, L. Dewatering of peat. Fuel 1989,68,446-455. (9) Jbnsson, B.; Peterason, E.; Lindman, B. Mechanical dewatering of peat. Fuel 1987,66,785-793. (10) Ringqvist, L.; Igsell,P. Four pretreatment methods for mechanical dewatering of peat. 9th Int Peat Congr. Uppsala Sweden, 22-27 June 1992.
0 1994 American Chemical Society
954 Energy & Fuels, Vol. 8, No. 4, 1994
pH). However, pressing of thick cakes (DS load more than 7 kgDS/m2)was not possible when the peat was pretreated with an anionic po1ymer;ll the dewatering capacity attained was too low. The cost for highly charged cationic polymers is twice that of anionic polymers. Paper pulp is similar to peat in several respects, and the combination of two oppositely charged polymers (dual polymer systems) has proved to give higher retention and drainage than single polymer systems.12-16 The dual polymer system formed a n extraordinary strong floc compared to single polymers in paper pulp.14 The sequence of addition was important. A lower amount of polymer was required if addition of a cationic polymer was followed by addition of an anionic polymer.16 Simultaneously added polyelectrolytes gave poor retention.12 The influence of the molecular weight of the cationic polymer on the retention was small. However, highly branched high charge density cationic polymers produced higher retention than linear cationic polymers in the dual polymer system. The agitation level and control of floc formation after the cationic dosage were very important in the cationiclanionic polymer system.15 Objective. The aim was to investigate if using a dual polymer system instead of a single polymer system could prove to be useful in a peat dewatering perspective. A peat slurry with a pH value of 3 was used since a lower amount of polymer is needed at this p H compared to the natural pH of peat. The sequence of addition, as well as the amount of polymer, was varied. The polymers used in the investigation have earlier been used in peat dewatering. Measurements of floc size, settling velocity, floc strength, and dewatering characteristics, both in a low-pressure and a high-pressure dewatering stage, were made for different treatments.
Experimental Section Samples. The peat used throughout this paper is a highly humified (6-7von Post) carex peat with a DS content of 12.8% and a calorific value of 25.0 MJ/kg ash-free dry peat. The peat is taken from a mire in the north of Sweden and is thoroughly described in ref 17 (there referred to as code b). Chemicals. The anionicpolymer used is a low charged density, high molecular weight copolymer of sodium acrylate and acrylamide from Allied Colloids, England. The polymer is diluted to a 0.3% working solution with deionized water. The cationic polymer used is a high charged density, branched copolymer of (dimethy1amino)ethyl acrylate and acrylamide with a high molecular weight (12million) from Allied Colloids. The polymer is diluted to a 0.5% working solution with deionized water. Levels of Polymer Additions. Throughout this paper if not stated otherwisethe following levels of polymer addition are used. For the anionic polymer 0.06 wt % of DS and for the cationic polymer 0.05 wt % of DS. In the case of a dual polymer treatment the two optimum dosages were chosen. These levels of polymer ~~
(11) Pirkonen,P.EvaluationoftheADEWAProcess.Proc. VTTSymp. 134, Artificial Dewatering of Peat, Jyo&skyld,Finland,15-16 Oct. 1991.
(12)Britt, K. W. Retentionof AdditivesDuring SheetFormation.Tappi 1973,56, 83-86.
(13)Britt, K.W.Mechanisms of Retention During Paper Formation. Tappi 1973,56 (lo),46-50. (14) Britt, K. W; Unbehend, J. E. New methods monitoring retention. Tappi 1976,59 (2),67-70. (15) Petiji, T. Fundamental Mechanisms of Retention Agents. Part 11. Dual Polymer Systems. Kemia-Kemi 1980, 7 (5), 261-263. (16)Onabe, F.;Yamazaki, A,; Usuda, M.; Kadoya, T. Effect of the Polyion-Complex Formation on the Drainage Behavior of Pulp Suspen1983, 29 (l),60-67. sions. J. Jpn. Wood Res. SOC. (17)Bohlin, E.; HZundlHinen, M.; SundBn, T. Botanical and Chemical Characterization of Peat Using Multivariate Methods. Soil Sci. 1989,147 (4), 252-263.
Ringqvist and Igsell additions have earlier in conjunction with filtration experiments and peat chargemeasurements3proven to be the optimum dosage as filtration times are concerned. All slurries are adjusted to pH = 3.0 prior to polymer addition. Equipment. The dewatering studies were performed at room temperature with laboratory pressure filter equipment and laboratory pressing equipment. The filter cell consists of a Plexiglas cylinder of i.d. 140 mm with stainless steel top and bottom plates. The bottom plate supports a perforated steel plate with a filter paper. The top plate is fitted with inlets for compressedair, a pressure gauge,and a pressure relief valve. The bottom plate is fitted with an outlet of i.d. 9 mm. The pressing equipment is a hydraulic piston press with dewatering in two directions, capable of producing pressures of up to 100 bar. The press chamber has an i.d. of 150 mm, and the top and bottom of the chamber support a gauze. The press is controlled and monitored by an IBM compatible computer, fitted with a 1/0 board. The data collected are time, pressure, piston position, and weight of reject water collected on a scale. The macroscopic equipment used for floc studies is a Wild Macroscope M420 fitted with an Apozoom 5.8-35 and equipped with an Ikegami CCD camera that through computer software produces tiff images. The turbidity of the reject water was measured in a Hach ratio turbidimeter. Settling of the Flocculated Particles. A 40-g sample of wet peat was weighed in, corresponding to approximately 5 g of DS. Deionized water was added up to 450 g. The slurry was stirred for 60 s. The pH of the slurry was adjusted to pH = 3.0 by addition of 1.0 M HCl during a total stirring time of 30 s. Polymer additions were carried out according to the following protocol: addition of the first polymer with a following stirring time of 60 s; addition of the second polymer with a following stirring time of 30 s. The weight of the slurry was corrected to 500 g by addition of deionized water. The slurry was transferred to a 500-mL measuring cylinder, which before the start of the experiment was turned upside down a couple of times. The volume of the settled peat was measured over a period of 21/2 h. A last measurement was done after 2 days, in order to determine the final volume of settled peat. All the stirring was done using a paddle, set at 250 rpm. Sizeand Shapeof FlocculatedPeat Particles. The relative sizes and shapes of the differently flocculated peat particles were determined using the macroscopicequipment. Flocculated peat slurry was spread on slides, and using several samplings, a representative image of the type of flocculation of a particular treatment was achieved. Predewatering Low-Pressure Filtration. The earlier described filter cell was used. Peat slurries were prepared in the same manner as in the settling experiments, with these alterations: 400 g of raw peat was weighed in corresponding to approximately 50 g of DS peat, deionized water was added to 900 g, and the slurry’s final weight corrected to 950 g. After completion of the slurry preparation the slurries were filtered on a MunktellOO3filter paper at 1 bar pressure. The filtration was stopped when air penetrated the filter cake and caused a loss of pressure. The filtration time, amount of filtrate, turbidity of the filtrate, wet weight of peat cake, and the filtrate weight as a function of time were noted. The peat cake was dried so that the DS weight was obtained. Strength of the Flocculated Particles. Four identical slurries (prepared as described above) for each type of treatment and 5 min. The deterioration of the were stirred an extra 0,1,2, filterability, as the slurries were stirred, was used as a measurement for the floc strength. All the stirring was done using a paddle set at 250 rpm. High-pressure Dewatering. Peat slurries were prepared in the same manner as in the settling experiments, with these alterations: 800 g of raw peat was weighed in corresponding to approximately 100 g of DS peat, deionized water was added to 2000 g, and the slurry’s final weight corrected to 2100 g. The slurry were predewatered in the laboratory filter equipment so that the weight of the predewatered filter cake was 950 g. This
Dual Polymer System in Peat Dewatering
Energy & Fuels, Vol. 8, No. 4, 1994 955
Figure 1. Macroscopic images of the flocculation achieved using different types of flocculants: (a, top left) no polymer; (b, top right) anionic polymer; (c, center left) cationic polymer; (d, center right) dual anionic/cationic; (e, bottom) dual cationic/anionic. predewateringwas done in order to enable loadingin the pressing was conducted. The levels of polymer addition were set to below equipment. The filtration time and the turbidity of the filtrate the previously determined optimum dosage of the individual were also measured in connection with these filtrations. The polymers. The slurries were prepared in the same manner as in filtercake was then loaded in the pressing equipmentand pressed. the high-pressure dewatering experiment, using the same preA linear pressure increase of approximately 10 bar/min with a dewatering and pressing conditions. The amounts of anionic final pressure of 90 bar was used. The filter used .was Novatek and cationic polymer were varied in accordance with the 25300AN from NovatekAB Sweden, a polypropylenegauze with experimentaldesign. The results, i.e., capacityin the low-pressure a wire mesh of 10 pm. During the pressing the amount of reject filtration, capacity to increase the DS content from the initial water was monitored as a function of time. The wet and dry 10.5 to 16% in the high-pressure dewatering, and the final DS weights of the peat cake were also determined at completion. Experimental Design. Using experimental design and (18)Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for multivariate techniques,'8 an optimization of a dual Polymer Experimenters, an Introduction to Design, Data Analysis and Model system using first a anionic polymer and then a cationicpolymer Building; Wiley-Interscience: New York, 1978.
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956 Energy & Fuels, Vol. 8, No. 4, 1994
Table 1. Floc Size and Settling Characteristics settling settling treatment floc sizea velocityb volumec (mL) no polymer anionic
very small
small large
slow slow
125 130 240 200 180
fast cationic dual anionic/cationic quite large fast dual cationidanionic quite large intermediate a The relative size of the flocs, Figure la-e. * Relative velocity. c Final volume after 2 days. content of the peat cake after completion of the high-pressure dewatering, were analyzed using response surface modeling (RSM)by means of multiple linear regression (MLR). The data analysis used to model the system relates the independent variables (factors x ) and dependent variables (responses Y) to a mathematical model.
where e = residual. The model includes the individual factors (XI, IC"),the interaction factors (~1x2,x(,+l)xn), and to be able to model nonlinearities,the squared factors (x12,~ " 2 ) . MLR estimates the coefficients (60, 61, ....) in the model. The Coefficients give information on the influence of respective factors. As the factors are scaled orthogonally, so that the high level is transformed to +1 and the low level is transformed to -1, the MLR estimated coefficients can be compared. A higher value means a higher influence of the factor. All responses were transformed to their logarithms. This was done in order to limit problems with structural inadequacy of the models, and errors not having a constant variance and not being normally distributed. The model used consists of 19 experiments, containing 5 replicates in two different points. The condition number of the design is 4.74, indicatingthat the layout of the design is adequately orthogonal. The use of replicates enables analysis of the variance (AN0VA)la to be conducted. The ANOVA divides the total variance into components due to regression, lack of fit, and pure error. Thus enabling a validity check on the calculated model.
Results and Discussion Flocculation and Low-Pressure Predewatering Filtration. The effect of the addition of anionicor cationic polymers and the combination of these chemicals on floc size and shape were first studied (Figure 1 a-e). These experiments were followed by an investigation of the settling velocity and the settled volume of the flocculated peat as can be seen in Table 1 and Figure 2. The next step in the investigation was the low-pressure predewatering filtration experiments. Experiments using different degrees of agitation and the deterioration in filterability thereof were used as a measurement of the floc strength (Table 2 and Figure 3). Finally, a high-pressure dewatering was applied on the peat samples after predewatering. Without Polymers. At pH 3, using no polymer a t all, the peat slurry consists of very small flocs or unflocculated peat particles. The settling velocity as well as final settled volume was low. A predewatering low-pressure filtration gave long filtration time. The structure was easily ruptured by agitation that caused increasing filtration times. The high-pressureapplication caused very slow dewatering with a low final DS content (20.8%) in the peat cake. Anionic Polymer. Addition of the slightly anionic polymer to peat formed small flocs at optimum amount according to filtration time. This image was confirmed by a slow settling velocity and a small final settled volume.
The predewatering low-pressure filtration time was quite high. The flocs formed were easily ruptured during agitation. Pressing on peat treated with an anionic polymer gave a rather slow water removal and a rather low final DS content (21.4%). CationicPolymer. The highly charged cationic polymer formed large "fluffy" flocs, causing a high rate of settling and a high final settled volume. Water removal during low-pressure filtration was high and the flocs were, to a minor extent, ruptured by agitation. Stronger flocs were achieved with the cationic polymer than with the anionic polymer. This result agrees with measurements performed by Forsberg and Alden.* High pressure application on these samples gave high final DS content (26.6 5% ). Anionic/Cationic Dual Polymer System. The dual polymer system, where the addition of the anionic polymer was followed by the cationic, formed quite large flocs. The rate of settling was high and a quite large settled volume was reached. The filtration time was short and the flocs were to a minor extent ruptured by agitation. Also, highpressure application on these samples was successful; a high final DS content (27.3%)was achieved. CationiclAnionicDual Polymer System. In paper pulp the addition of cationic polymer followed by addition of anionic polymer gave the best retention and drainage and a very high resistance to shear forces compared to the single polymer system. Addition of the cationic polymer produced, via a patch model mechanism, flocculation of the particles. When the anionic polymer was added, the negatively charged chain created bridges between the primary flocs. If the floc sizes before the anionic dosage were too large, the anionic bridges could not improve the strength of the internal bondings inside the In the peat slurry,the cationic /anionic polymer system gave quite large flocs, an intermediate settling rate, and a slightly smaller settling volume than the opposite sequence of addition. The filtration time was high and the flocs were to some extent ruptured during agitation. The floc size after addition of the cationic polymer was very large in the peat slurry, which can be the reason for the weak aggregates in the cationic/anionic polymer system. However, when a high pressure was applied on these samples, a high final DS content (27.0%)was achieved. The reason for the rather long filtration time, but high final DS content achieved after pressing, cannot be explained from measurements conducted in this investigation. During filtration the anionic/cationic polymer system gave the shortest filtration time, addition of cationic polymer the second best, (Table 2). The anionidcationic polymer system gave the same floc strength (measured as filtration time after agitation) as the cationic polymer alone. The aggregates in the cationidanionic polymer system were more sensitive to agitation than the opposite sequence of addition. When a high pressure was applied, the cationic and the two dual polymer systems yielded a high final DS content. A t this pressure the sequence of addition in the dual polymer system was of no importance, contrary to the results in the filtration experiments. Surface load is an important factor when polymers are used as dewatering agents in peat dewatering. The surface load in the described high-pressure experiments were 2.8 kg DS/m2. Here the differences in DS content after a high-pressure dewatering between cationic and anionic
Energy & Fuels, Vol. 8, No. 4, 1994 957
Dual Polymer System in Peat Dewatering
Table 2. Floc Strength Measured as Increase of Filtration Time after 5 min Agitation. Low- and High-Pressure Dewatering of Differently Treated Peat
filtration time, dewatering capacity: final DS treatment floc strength' floc strengthb low pressure (s) high pressure (kg DS/m2h) content (7%) 22.3 20.8 1.1 195 no polymer weak 26.3 21.4 1.0 65 weak anionic 63.7 26.6 strong 0.3 23 cationic 71.2 21.3 strong 0.4 17 dual anionidcationic 58.9 27.0 intermediate 0.6 52 dual cationic/anionic a Estimated from filtration curves. * Increase of filtration time due to agitation (A(fi1trationtime)/A(agitation time)). Capacity to reach 20.8% DS in the peatcake (the highest DS content reached using no polymer). Table 3. Experimental Design Layout and Response Data Used in Regression (Polymer Amounts in wt % DS)
Ik,
+ Cationic ~
400
\
-
~
AnioniciCationic I t CationiciAnionic I
\\
I
I 100~
40
160
120
80
Time (mm)
Figure 2. Graph of the interface volume vs time during the
settling of the differently flocculated peat particles.
exp
anionic
cationic
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
0.06 0.06 0.06 0.06 0.00 0.06 0.00 0.06 0.00 0.04 0.04 0.08 0.08 0.06 0.04 0.08 0.02 0.02 0.02
0.00 0.02 0.05 0.06 0.00 0.00 0.05 0.05 0.03 0.03 0.05 0.03 0.05 0.05 0.00 0.00 0.03 0.05 0.00
lo+
highb
DSc
70.9 129.9 259.8 259.8 41.0 88.2 246.7 274.5 119.9 129.9 173.2 129.9 194.9 194.9 73.1 83.5 123.0 146.0 47.2
69.3 113.2 135.8 154.3 53.9 13.8 149.8 169.8 125.8 161.7 159.2 130.6 154.3 156.7 76.0 81.5 114.5 128.9 62.9
22.0 24.5 26.3 25.8 20.8 21.4 26.6 27.3 26.0 26.8 27.3 25.8 27.2 27.2 22.0 22.3 24.9 25.6 21.6
Capacity (kgDS/m2h) in predewatering low-pressure filtration. Capacity to increase the dry solids content from 10.5 to 16% in a high-pressure dewatering. Final dry solids content ( % ) in peatcake after the high-pressure dewatering. 0
b
+ Cationic
-
AnionicICationic
1
2
3
4
5
Stirring (min)
Figure 3. Graph of the filtration time vs the stirring time.
Prolonged filtration times show floc breakdown due to shear forces. polymers are clearly seen. At lower surface loads the differences are less pronounced.lg
Optimization of the AnionicKationic Dual Polymer System. The previously described experiments, with polymer amounts a t the optimum for the individual polymers,showed that specificallythe dual polymer system using first the anionic followed by the cationic polymer gave improvements on the dewatering characteristics.The improvements are vast compared to the anionic polymer alone, but less pronounced when compared with the cationic polymer alone. The anionic polymer is, because of the lower cost, more interesting to use than the cationic. To investigate how the dual polymer system behaves a t (19) Liljekvist,J. 0. Farawattning med Kemikalietillsatser (Predewatering with Addition of Chemicals). Swedish State Electeric Board, Report no. VU-V-91:6 (in Swedish).
varying polymer concentrations an optimization of a dual polymer peat dewatering system using first anionic followed by cationic polymer was conducted, using experimental design and response surface modeling. The experimental domain were set to below the determined optimum for the individual polymers. Three responses were measured, capacity in the low-pressure filtration, capacity to increase the DS content from the initial 10.5 to 16% in the high-pressure dewatering (the early stage in the high-pressure dewatering) and the final DS content of the peat cake after the high-pressure dewatering. The capacity (kg DS/m2 h) express the amount peat that can be produced per square meter gauze and hour. Experimental design layout together with the responses measured are listed in Table 3. The capacity of the low-pressure filtration was dependent on the amount of both anionic and cationic polymer added. The amount of cationic polymer being of greater importance. The interaction coefficient (Table 4) between the two polymers are opposite in sign compared to the main terms, which means that the sums of the positive effect of the polymers used alone are reduced when they are used together. However, as can be seen in Figure 4, below 0.05 wt % cationic polymer, a positive effect was reached when the dual polymer system was used. The correlation coefficients that are significant a t the 95% confidence interval are cationic, anionic, and the interaction term. The capacity to increase the DS content from the initial 10.5 to 16% in the high-pressure dewatering stage depends on the amount of both the anionic and cationic polymer
Ringqvist and Igsell
958 Energy & Fuels, Vol. 8, No. 4, 1994
Table 4. Multiple Linear Regression on Low-Pressure Capacity, High-pressure Capacity, and Final DS Content. low high final dry pressure pressure solids content fconf fconf konf term coeff intb coeff int coeff int 0.049 1.414 0.012 2.129 0.090 constant 2.125 0.036 0.033 0.006 0.008 0.068 0.061 anionic 0.042 0.009 0.168 0.035 0.307 0.064 cationic anionic (2) -0.015 0.103 -0.024 0.056 4.001 0.014 cationic (2) 4.011 0.118 -0.118 0.064 -0,037 0.016 anionic * -0.092 0.086 -0.035 0.047 -0,003 0.012 cationic R2 values 0.925 0.948 0.952 a Table of the multiple linear regression determined coefficients for the different factors of the different response models (capacity in a predewatering low pressure filtration, capacity to increase the dry solids content from 10.5 to 16% in a high-pressure dewatering and the final dry solids content after a high-pressure dewatering). The coefficientsare given with their 95% confidence interval. In the last row are the correspondingR2valuesfor themodels. Allresponses have been transformed to their logarithm. Confidence interval. 0 06
0 00
0 02 0 04 0.06 Anionic polymer concentration (wt% of DS)
0 08
Figure 5. Response surface plot of the by MLR determined coefficients, using capacity to increase the DS content in the peatcake during pressing from 10.5 to 16% as response. Units are in kg DS/m2 h.
I
250
; ; n 005
' .!
0 06
220 --
-
004 .-
Ep
003
8
sg b,
Ai
-a 0 0 2 a
.-uC
s
002
.-0C
'f 0 0 1 0
001
0 00
,
0 00 0 00
0 02
0 04
0 00
0 06
0 08
Anionic polymer concentration (wt% of DS)
Figure 4. Response surface plot of the by MLR determined coefficients, using low-pressure predewatering filtration as response, units are in kg DS/m2 h. added. The interaction coefficient term is not significant; the individual polymer terms are, however, significant. As can be seen in Figure 5, using two polymers have a positive effect on the capacity; the highest capacity is reached using two polymers. The squared term of the cationic polymer is significant and of opposite sign as compared to the main term. This shows that the model is not linear but has an optimum as cationic polymer concentration is concerned. This is also clearly seen in the contour plot. The final DS content of the peat cake depends almost compleatly on the amount of cationic polymer added (Figure 6). The squared term is also here significant and of opposite sign as compared to the main term. A very slight additive effect between the two polymers can be seen, but the coefficient of the anionic polymer is not significant. The ANOVA analysis of each of the three responses (Y) produce more or less identical results. The three models are significantly different from the mean square error; i.e., the probability that the models could be explained by pure error alone is virtually zero. The residual SS (sum of squares) broken down into pure error and model lack of fit shows that the models only have a moderate lack of fit. The sum of squares of pure error and model lack of fit is of the same magnitude for all responses. Given these
0 02
0 04
0 06
0 08
Anionic polymer concentration (wt% of US)
Figure 6. Response surface plot of the by MLR determined coefficients, using DS content in the peatcake after high-pressure dewatering as response, units are in % . facts, and the high R2 values for the models (Table 41, it can be concluded that the models accurately describe the experimental domain. The most important factor in the dual polymer peat dewatering system tested was the amount of cationic polymer, regardless of what responses are used. In a predewatering stage, and early in a high-pressure dewatering stage, a positive effect of a dual polymer system is seen. In order to reach the highest capacity in the predewatering and in the early stages of the high-pressure dewatering, the use of a dual polymer system could prove useful. The cost of treating a peat slurry to a given capacity is approximately the same whether a single polymer of either anionic or cationic character or a dual polymer system is used. The rather low DS content achieved in this study after high-pressure dewatering can be further increased if the dewatering time is prolonged. Previous studies using 60 min pressing time a t 60 bar pressure have resulted in 35-5076 DS content.2 In conjunction with the low-pressure filtrations the turbidity of the filtered off water was measured. The turbidity was greatly reduced when a dual polymer system was applied on the peat slurry. The reduction was from around 40 NTU (nephelometric turbidity unit) if no polymer or only an anionic polymer was used to under 5 NTU if a small amount of cationic polymer was added.
Dual Polymer System in Peat Dewatering
Conclusions Addition of anionic polymer alone gave rather small aggregatesand cationic polymer alone gave large aggregates in the peat slurry. When these polymers were used together the aggregates formed were of intermediate size. The sequence of addition influenced the size to a minor extent. The aggregates in the cationiclanionic dual system were more easily ruptured during agitation than the opposite sequence of addition. From this study it can also be concluded that the cationic polymer has stronger influence on the dewatering properties than the anionic polymer. This can be seen both when the polymers are used alone and in the anioniclcationic dual system. However, the effect of the anionic polymer is dependent on the surface load used; a t low surface load the difference in dewatering properties between the two polymers was small. The optimization of the dual polymer system showed that the effects on the dewatering properties are large when compared to the use of a single anionic polymer and small when compared to the use of a single cationic polymer. Some synergistic effects are reached by the use of a dual polymer system. In the low-pressure filtration a positive effect of the dual polymer system was seen a t low concentrations of cationic polymer. At high concentrations the effect of the anionic polymer was neglectable. In the initial phase of the high-pressure dewatering stage
Energy & Fuels, Vol. 8, No. 4, 1994 959 the effect of the dual polymer system on the dewatering properties was significant. The use of a dual polymer system was required in order to reach the highest capacity. The final DS content after completion of the high-pressure dewatering depended almost compleatly on the amount of cationic polymer added. The total chemical cost of treating a peat slurry to a given filterability is approximately the same whether a single polymer is used of either anionic or cationic character or a dual polymer system is used. The results show that using a dual polymer system instead of a single polymer system has a positive influence on the dewatering properties of a peat slurry. The improvements are small compared to addition of cationic polymer alone. The price of cationic polymers is about twice that of anionic polymers. The use of a dual polymer system can in fact prove to be competitive compared to a single anionic polymer system, especially when a high surface load is used during the dewatering operation. Mechanical handling (pumping, loading a press, etc.) of a slurry treated with the dual polymer system only affects the dewatering properties to a minor extent, compared to a slurry treated with anionic polymer alone.
Acknowledgment. Financial support for this project was provided by the National Energy Administration Sweden through grant 216 089-2.
