Ind. Eng. Chem. Res. 1994,33, 993-1000
993
Hydrodynamics of Sawdust and Mixtures of Wood Residues in Conical Spouted Beds Martin Olazar,’ Maria J. San Jose, Ricardo LLamosas, and Javier Bilbao Departamento de Zngenierh Qulmica, Universidad del Pais Vasco, Apartado 644, 48080 Bilbao, Spain
It has been proven that conical spouted beds allow for stable operation with sawdust and with wood residues, even with mixtures of these materials of wide particle size range and without being diluted with an inert solid. Peculiar hydrodynamic characteristics for sawdust have been observed; a great hysteresis in the pressure drop vs velocity curve, a pronounced peak of maximum pressure drop, and a difference between the velocity for which spout and fountain are formed and the velocity of the fully spouted bed. From the hydrodynamic study of sawdust, the ranges of the contactor geometric factors (cone angle, inlet diameter/base diameter ratio, inlet diameter/particle diameter ratio) for which operation is stable have been determined. Original correlations for calculation of minimum spouting velocity, of stable operation pressure drop, of maximum pressure drop, and of minimum voidage of complete spouting have been proposed. 1. Introduction Due to the decreasing supply and increasing costs of fossil fuels, interest has grown in the use of wood and wood residues among those industries dealing in wood extraction and treatment (Deglise and Lede, 1982). Different methods can be followed for the use of these biomass feedstocks. One of them is combustion (La Nauze, 1987)and subsequent use in the factory itself of the energy produced. Other methods of use are gasification and pyrolysis (Prasad and Kuester, 1988; Bilbao et al., 1988; HBmatie et al., 1989; Stiles and Kandiyoti, 1989; Arauzo et al., 1992). Habitually, these processes are carried out in fluidized bed contactors, in which the use of an inert material (generally sand) is required in order to facilitate the fluidization of biomass. Different authors have studied the hydrodynamics of mixtures of sawdust and sand and have calculated hydrodynamic correlations for treatment of these mixtures in fluidized beds (Mascarenhas et al., 1990; Aznar et al., 1992). In the literature, the benefits of fluidized beds have been compared with those of cylindrical spouted beds (Bhattacharya and Shah, 1987; Zak and Nutcher, 1987). In the spouted bed the combustion efficiency is higher, the capital cost is lower, and the pressure drop, and consequently the operation cost, is much lower (on the order of 20%). Moreover, the spouted bed allows for the treatment of more diverse material (of wide particle size distribution and of irregular shape) and with a high moisture content. On the other hand, the three “T’s” needed for good biomass treatment, that is to say, short time, temperature control, and high turbulence, can be achieved with the spouted bed (Zak and Nutcher, 1987). The three requirements are not achieved with any other contact method, as very few designs provide proper turbulence. In this paper, the spouted bed in exclusively conical contactors has been applied to the treatment of pine sawdust and of mixtures of this material with other wood residues. The conical contactors, which have the advantage that they do not require the handling of sand or another inert solid, have already been successfully used in processes in which the solid size is changing with the residence time in the contactor, such as coal gasification (Tsuji et al., 1989; Uemaki and Tsuji, 1986, 1991) and catalytic polymerizations where a solid that is sticky and that has a wide particle size distribution is handled (Bilbao et al., 1987,1989). In previous papers, the hydrodynamics OSSS-5885/94/2633-0993$04.50/0
and the design factors for spouted beds in conical contactors have been studied (Wan-Fyong et al., 1969; Kmiec, 1983; Olazar et al., 1992, 1993a,b;San Jose et al., 1993; Olazar et al., 1993~).Nevertheless, these studies correspond only to granular materials (glassspheres, seeds and grains, ceramic materials, different plastics, wood cubes, and gravel), so the hydrodynamic correlations are not applicable to sawdust or other biomass material, as their hydrodynamic characteristics are peculiar. The treatment of sawdust fractions of different particle size range (consequently the shape factor also changes) and the treatment of mixtures of different size sawdust fractions with other woodwastes have been approached in this paper. The objective has been to take into account the nonuniform nature of these materials, which are byproducts of wood industries. The ranges of the geometric factors of the contactors and of the operating conditions required to carry out the operation in stable regime have been delimited. Under these conditions, original hydrodynamic correlations for calculation of the minimum spouting velocity, of the maximum pressure drop, of the pressure drop in stable operation, and of the voidage corresponding to the minimum spouting velocity have been obtained. 2. Experimental Section
The materials used are sawdust, shavings, pine wood chips, and several binary and tertiary mixtures. These materials, whose characteristics are set out in Table 1, belong to groups A, B, and D of the Geldart classification (1973, 1986). The equipment used at the pilot plant, the probes for pressure and velocity measurement, their usage, and the general conditions for the experimental work have been detailed in a previous paper (Olazar et al., 1992). The evolution of pressure drop with air velocity has been measured by a computerized data acquisition system, which uses a PC-Lab-718 card provided with PCLS-711 software. The plant allows for working under different regimes (fluidized or spouted bed), according to the contactor (of poly(methy1 methacrylate)) and to the air velocity range used. In the fluidized bed contactor, the column diameter is 0.153 m and different gas distributors have been used: three perforated plates and a 0.5-mm mesh. The perforated plates are made of stainless steel plates with 1.5,5.5, 0 1994 American Chemical Society
994 Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 Table 1. Properties of the Solids Used material sawdust shavings woodchips
size range, mm 0.8-1.0 1.0-2.0 3.0-4.7 5 X 15 X 0.0515 X 35 X 0.05 (orientative) 25
mean diameter, d,,mm 0.95
density,
shape, 4 0.95
voidage,
242 242 242 242
moisture content, % 9 9 9 9
0.90 0.16
0.60 0.70 0.87
242
9
0.90
0.47
ps, kg/m3
1.5
4.2 25
I
Figure 1. Geometric factors of the contactor.
and 10 mm diameter holes punched on square pitches of 10mm, 15mm, and 20 mm, respectively. The multiorifice plates obtained have a free area, with respect to the whole section, of 1.25, 5.6, and 8.9%, respectively. In this way, the ratio between plate pressure drop and bed pressure drop is high (always over 80%) in order to ensure stable operation. The behavior of the fluidized bed has been compared with that of the cylindrical and conical spouted beds. The cylindrical contactors used have been of conical bottom (angle of 45O) and of flat bottom. The column diameter of these contactors is 0.153 m. Five conical contactors, whose geometric factors are defined in Figure 1,have been used. The dimensions of the conical contactors are as follows: upper diameter of the contactor (column diameter, Dc),0.36 m; base diameter of the contactor (Di), 0.06 m; heights of the conical section (HJ, 0.36, 0.40, 0.45, 0.50, and 0.60 m; cone angle (y) corresponding to the previous heights, 45, 39,36,33 and 28". With each contactor the study has been extended to four values of the inlet diameter, Do:0.03,0.04,0.05, and 0.06 m. 3. Hydrodynamics of Sawdust 3.1. Comparison of t h e Different Contactors. From the results of the tests in fluidized beds when different fractions of sawdust and shavings have been used, it is observed that an acceptable fluidization is not achieved if no additional inert material is used, which is due to the formation of preferential paths and to the lack of homogeneity of the bed. Consequently, the pressure drop of the bed once fluidized decreases sharply with air velocity and stabilizes at a value that is much lower than that corresponding to the bed weight by unit section. The behavior of the cylindrical contactors, with either flat or conical bottom, is very similar when different fractions of sawdust and shavings are used. In both contactors, the air opens a central crater that provokes a permanent stagnation of the annular zone, without solid circulation, which is proof that cylindrical contactors are not suitable for treatment of this kind of biomass. In
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Archimedes Geldart no., Ar classification 6.7 X 103 A 2.6 X 10" B 5.8 X los D 1.2 X lo8
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order to attain an adequate functioning, the sawdust must be mixed with a relatively high percentage (over 60% in weight) of inert material (sand). Nevertheless, in the spouting regime in conical contactors, there is no need to use any kind of inert material to operate without stagnation when different sawdust fractions, shavings, mixtures of these materials, and mixtures of these materials with wood chips are used. The only limitations are those inherent to the geometric factors and to the operating conditions for bed stability and those caused by the hydrodynamic peculiarities of small particles (d, < 0.8 mm). The study on the hydrodynamic peculiarity of small particles in conical spouted beds is beyond the scope of this paper and has been previously approached for cylindrical spouted beds (Rooney and Harrison, 1974; Chandnani and Epstein, 1986; Epstein and Chandnani, 1987). 3.2. Pressure Drop Evolution with Air Velocity. The evolution of pressure drop with air velocity is different from that observed for other materials (Olazar et al., 1992). It is peculiar that once the bed has been spouted, when the air flow is cut, the random order of the particles is not restored, but the particles stay reordered in the same situation as in the spouting regime, which is due to the fact that a central crater created in the spouting operation had remained. As a consequence of this bed behavior, the maximum pressure drop that is obtained in a second spouting operation is much lower than that measured in the first operation. This phenomenon does not affect the measurement of stable pressure drop, which is the same as in the first operation. This hydrodynamic peculiarity has its origin in the deficient fluidity of sawdust and in cross-linkage between particles due to the fact that they are mainly long and irregular. In order to obtain a reproducible measurement of pressure drop evolution,avoidingthe previously mentioned problem, the measurements have been carried out once the solid was fed from a sufficiently high altitude (1m). These measurements are totally reproducible. In Figure 2, the trace of pressure drop is shown as an example, for a sawdust fraction of particle size between 1.0 and 2.0 mm, in the contactors of y = 45O (Figure 2a) and y = 28O (Figure 2b). Both plots correspond to the same values of inlet diameter, Do = 0.05 m, and of stagnant bed height, Ho = 0.20 m. Although the stable operation pressure is small, a sharp peak of the maximum pressure drop is observed in Figure 2. Ascertainingthis peak value is important for the starting operation (the blower must be designed to overcome it). The pronounced hysteresis increases as the angle is greater. It has been observed that the contactor angle has a great influence in the starting of the spouting regime. While for the contactor angle of 2 8 O (Figure 2b) incipient spouting and fountain onset coincide with the cyclic movement of the whole bed, for the contactor angle of 45O (Figure 2a) only the cyclic movement of the bed at the interface zone between the angular and spout zones is observed and
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 995
AP (pa)
l < d p 5 / 6 , slugging phenomenon is produced. Contactor angles of 36', 39", and 45'; 2/3 < Do/Di < 1. The lower limit corresponds to stagnation of the bed in the annular zone, which is not eliminated by increase of air velocity. It has been proven that the influence of stagnant bed height on stability is not noticeable, so the previous stabilityranges of DJDi are valid for the H,range between 0.05 and 0.45 m, in which experimentation was carried out. The ratio between inlet diameter and particle diameter, Ddd,, must be lower than 60. Above this value no fountain is formed and the regime of spouting does not take place but a random movement of the solids, which is more characteristic of fluidized beds, is formed. 3.4. Minimum Velocity for Complete Spouting. The experimental values of minimum velocity for complete spouting obtained for sawdust have been fitted by the Complex method for nonlinear regression to the equation proposed in the literature of Olazar et al. (1992) for calculation of the minimum spouting velocity for granular materials and glass spheres:
(Re,)ms = 0.126Ar0.50(DdD,)
tan(y12) 1
(1)
The fitting of eq 1 is suitable, with the regression coefficient r2 = 0.93 and a relative error lower than 7.5%. The adequacy of the fitting is shown in Figure 4,where the experimental values of minimum velocity for complete spouting have been plotted for the contactor angle of y = 36' vs the values calculated using eq 1. Each point in Figure 4 corresponds to one value of stagnant bed height (H,) in the range between 0.03 and 0.20 m. In Figure 5, the values of [(Reo)m$Ar0.51 modulus
Ho (4 Figure 5. Values of [(Re,)mJAfl.6]modulus calculated using eq 1 vs stagnant bed height, H,. calculated using eq 1have been plotted vs stagnant bed height (H,). When the parameters corresponding to gas flow and solid particles are grouped in the modulus studied, the effect on the hydrodynamics can be studied by this plot. Each curve corresponds to a different combination of contactor geometric factors, of cone angle, and of inlet diameter. The stretchesdrawn as a dashed line correspond to unstable operation conditions for the particle diameter studied in the example, between 1 and 2 mm. In Figure 5, it can be appreciated for all geometries that the modulus studied increases more than proportionally with the stagnated bed height and more sharply as the angle is greater and the inlet diameter is smaller. It is clearly seen that the effect of the inlet diameter is more important than that of the cone angle for small stagnant bed heights. For high values of heights, the effect of the angle on the hydrodynamics is increased and is qualitatively as important as that of the inlet diameter. 3.5. Pressure Drop i n Stable Operation. The pressure drop in a stable spouting regime for beds of sawdust has very low values, considerablylower than those predicted by the equation proposed for granular materials and glass spheres (Olazar et al., 1993a). Using the same dimensionless moduli (Olazar et al., 1993a),the values of stable pressure drop for sawdust have been fitted by the Complex method and the following is obtained: A D
--- - 0.04[tan(y/2)l-0~1'(Re,),~20(H~Do)1~10 (2) - 0
Hops
The regression coefficient is r2 = 0.97, with a relative error lower than 6 % The adequacy of the fitting to eq 2 is shown in Figure 6, in which the experimental values of stable pressure drop for the contactor of y = 36' have been plotted vs the values calculated using eq 2. Each point in Figure 6 corresponds to one value of stagnant bed height (H,)in the range between 0.03 and 0.20 m. Comparing eq 2 with the one proposed for the other materials (Olazar et al., 1993a), it is observed that the angle has the same influence (same exponent). The Reynolds number has an opposite and more pronounced influence. The influence of the H,lD, ratio in eq 2 has the same tendency but in a more pronounced way. In order to analyze the effect of the geometric factors of the contactors, the values of the pressure drop parameter (-(AP$H,pbg) have been plotted in Figure 7 against stagnant bed height (H,) for a sawdust fraction. Studying this parameter, the effect of particle and fluid characteristics is isolated. The stretches drawn as a dashed
.
Ind. Eng. Chem. Res., Vol. 33, No. 4, 1994 997 Q
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