Uniflow cyclone efficiency study

Powder Technology,

217

62 (1990) 217 - 225

Uniflow Cyclone Efficiency Study T. A. GAUTHIER Ecole Nationale Superieure du P&role et des Moteurs, Znstitut Francais du Petrole, B.P. 311, 92506 Rue&Malmaison Cedex (France) C. L. BRIENS, M. A. BERGOUGNOU Department of Chemical and Biochemical Ont. N6A 5B9 (Canada)

Engineering,

The University of Western Ontario, London,

and P. GALTIER Znstitut francak du P&role, C.E.D.Z., B.P. 3, 69390 (Received August 25,1989;

Vernaison (France)

in revised form February 12, 1990)

SUMMARY

A 0.05-m diameter uniflow cyclone, developed for the ultra-rapid fluidized reactor (URF) process, has been tested under cold modeling conditions at inlet velocities ranging from 9 to 31 m/s and solid loadings ranging from 1.0 to 6.0 wt.solids/wt.gas. The separation length was varied from 1.0 to 10.5 cyclone diameters. Glass beads with a Sauter mean diameter of 29 um were used. Interferences in the inlet region greatly impaired the solids separation at short separation lengths. They were successfully eliminated by inserting a downward helical roof in the inlet region. Excellent collection efficiencies (over 99.99%) were thus achieved. When the separation length was increased, the vortex intensity in the gas exit region was reduced and the collection efficiency dramatically decreased.

BACKGROUND

The high temperature ultra-rapid fluidized (URF) process reactor is being developed at the University of Western Ontario. In its pyrolytic embodiment (ultra-rapid pyrolysis), a cold carbonaceous feedstock is subjected to thermal and mechanical shocks by rapid and efficient mixing with hot thermofor particulate solids. The reaction occurs in a tubular downflow transported fluid&d reactor at residence times which can be set between 70 and 500 ms. The gaseous products are then 0032-5910/90/$3.50

separated from the thermofor solids in a fast separator and quenched to prevent thermal degradation of the products. ,This technique has been applied in a mini-pilot plant to the ultrapyrolysis of biomass [l], heavy oil and Cold Lake bitumen [Z]. Most standard gas solid separators (including gravity settling chambers, electrostatic precipitators and high-temperature filters) are not practical for the URF process, which requires high solid loading, high temperature and short separator gas residence time. Cyclones allow high solid loadings and high temperatures and could thus be applied to the URF process, if the gas residence time could be kept small with a narrow residence time distribution. In a conventional reverse-flow cyclone, a centrifugal force induced by a tangential inlet forces the solids to migrate to the wall. The cleaned gas reverses direction and flows out upward through a central gas outlet in the upper part of the cyclone, while the collected particles exit at the bottom of the cyclone. In a uniflow cyclone, gas and solids exit in the same direction. The gas exit can be a downward central pipe inside the cylindrical separation chamber. The gas is thus not forced to reverse its flow before exiting. This results in a shorter separator gas residence time and avoids the progressive leakage of gas into the central core which spreads the gas residence time distribution in the reverse-flow cyclone. A uniflow cyclone was, thus, selected as separator of the URF process. 0 Elsevier Sequoia/Printed in The Netherlands

218

A novel uniflow cyclone was developed by Sumner et al. [3, 41 to meet the specific demands of the URF process. Their preliminary cold model study on a 0.05 m I.D. cyclone demonstrated that excellent collection efficiencies (over 99.9%) could be achieved with relatively low pressure drops with silica sands of 55 and 105 pm Sauter mean diameters. The separation time was short but not clearly defined. It was nominally of the order of 30 ms. With a constant air inlet velocity (Vi = 25 m/s) and a constant solid loading (5/l wt.solids/ wt.gas), they found that, by decreasing the gas outlet diameter, the collection efficiency was improved, and that the optimal separation length was around 1 cyclone diameter. Gas underflow (Le., gas removed with the solids stream) also improved the separation efficiency. However, interferences in the inlet area and erosion at the wall were encountered and operating parameters such as solid loading and inlet gas velocity were not investigated. A new experimental study was thus undertaken to investigate, in a cold model, the effect of variations in solid loading and inlet gas velocity on cyclone collection efficiency. Changes in cyclone geometry were also made to improve its collection efficiency.

EXPERIMENTAL

EQUIPMENT

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CYCLONE

AND METHOD

A modular uniflow cyclone, allowing easy geometry changes, was built with an inside diameter of 0.0508 m (Fig. 1). The cyclone inlet was machined from stainless steel. In the inlet region, a removable hardened insert was installed, flush with the cyclone wall, to minimize cyclone wall erosion. Two cyclone roofs were built, one in Plexiglas to observe A'

the inlet region, and the other in stainless steel to mitigate electrostatic problems. Central cylinders and cones could be easily screwed to the cyclone roof. A volute (downward helical roof) was thus inserted (Fig. 2). The cyclone cylindrical body was 0.5 m long. Such a length made it possible to study patterns around the gas exit pipe. The position of the gas exit could be varied by adding stainless steel tubes (0.025 m O.D., 0.022 m I.D.) to the gas outlet. The separation length L,, which is defined as the distance between the cyclone roof and the tip of the gas exit tube, could thus be varied from 1 to 10.5 cyclone diameters. Figure 3 illustrates the overall flow diagram. Filters removed particulates and oil droplets from the compressed air. According to measurements with a wet bulb-dry bulb hygrometer, the relative humidity of the air remained between 8 and 14%. The air was introduced through a network of three sonic nozzles which kept the air flow rate constant and independent of any fluctuations in downstream pressure. Upstream pressure was regulated to 300 - 400 kPa, depending on the required gas flow rate. The calibrated sonic nozzles were designed according to Arnberg’s criteria [ 51, with throat diameters of 0.0036 m, 0.0031 m and 0.0025 m. Gas flow rates

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219

from 3 to 15 g/s could be obtained by adjusting the upstream pressure with various combinations of sonic nozzles. The inlet gas velocities could thus be varied from 9 to 31 m/s at the cyclone inlet conditions. A screwfeeder delivered a glass beads flow rate which could be varied from 5 to 130 g/s. Trial runs showed that particles, pushed by the screw, were flowing as intermittent dumps (due to the cohesivity of the powder) into the injection cone. The problem was solved by inserting a screen (0.014 m square mesh) in front of the screw before the solids fell into the injection cone. By dividing the stream of solids, dumps were eliminated and stable solids flow rates were achieved. Trial runs also established that steady state was reached in 20 s. The gas-solid mixture was thus directed to a hopper during the first 30 s of each run. It was then shifted to the uniflow cyclone with the diversion valve. A special acceleration line was installed upstream of the cyclone inlet to provide a well-established gas-solid flow to the cyclone. It had the same cross-sectional area as the cyclone inlet and its length was equal to 80 hydraulic diameters. Separated solids were collected and weighed in a 20-l tared bottle fixed to the cyclone by a quick connection. Particles entrained with the exiting cleaned gas were removed with a high-efficiency reverse-flow cyclone, collected in another tared bottle, and weighed. A sock filter cleaned the gas exiting to the atmosphere. For experiments with gas underflow, the flow rate of gas was monitored with two calibrated rotameters protected by a filter. Particles were spherical glass particles with a Sauter mean diameter of 29 pm. Figure 4 shows the size distribution of the particles, which did not change over the duration of the study, as shown by size analyses performed on a Malvern laser particle sizer 2600. Electrostatic effects were important in the original Plexiglas equipment, and had to be eliminated to ensure significant and reproducible results. Air humidification (up to 80% relative humidity) failed, since surface condensation made the particles cohesive before complete elimination of electrostatic effects could be achieved [6]. Electrostatic effects were successfully reduced by using a grounded copper cyclone and injecting 30 -

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60 wt.ppm of dry ammonia in the compressed air [ 61, as done by Nieh and Nguyen [ 71. The performance of the cyclone was evaluated by measuring the collection efficiency E, which is the ratio of the mass of the solids collected by the uniflow cyclone M, to the total mass of solids introduced in the cyclone M, + M, (the mass of particles collected in the sock filter was less than 0.3% of the mass of the particles collected in the secondary cyclone, and could thus be neglected) : E=lOO

MC

(1)

M, +% When the measured collection efficiency was over 99.99%, the mass of particles collected by the secondary cyclone was less than 1 g, which was near the experimental error. All efficiencies higher than 99.99% were therefore set to 99.99%. As a cyclone becomes more efficient, a given incremental increase in collection efficiency becomes harder to obtain. Collection efficiencies were thus converted into numbers of transfer units (N.T.U.) for a better representation:

Since the duration of each run had no significant effect on the measured collection efficiency, the start-up and shutdown procedures were assumed to have no effect on

220

cyclone performance. The duration of each run varied from 80 to 800 s, but was mainly about 150 to 200 s. The reproducibility of the collection efficiency was good, the maximum experimental error on the number of transfer units being + 3% (except at solid loadings of less than 1 wt./wt.). EXPERIMENTAL

RESULTS

AND DISCUSSION

Initially, the effect of the solid loading on collection efficiency was investigated for separation lengths ranging from 1 to 10.5 cyclone diameters. Five inlet velocities, measured at cyclone inlet conditions, were investigated: 9, 13, 18, 25 and 31 m/s, corresponding to gas flow rates of 3.4, 5.2, 7.4, 10.8 and 14.1 g/s, respectively. The

solid loading was varied from 1 to 6 wt./wt. To stabilize the vortex, a central cone (base diameter = 0.5 D,, height = 0.5 D,) was fixed to the roof (where the gas exit would normally be in a reverse-flow cyclone). Experimental results are reported in Fig. 5, where the effect of the solid loading at various separation lengths (1,2, 3,5 and 10.5 D,) on the number of transfer units is shown for the five inlet velocities. No clear trends appear from these figures. This was not due to experimental errors, as shown by the duplicate experimental results reported on Figs. 5(c) and 5(d), at least for loadings over 1 wt./wt. (at lower loadings, the feeding system was operating in its minimum range and its fluctuations could not be well controlled). 9

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For short separation lengths (below 30,), collection efficiency was excellent (over 99.8%). The effect of the solid loading was not strong compared with the effect. of the separation length L, , except at low loadings. The optimum separation length ranged from 2 to 3 cyclone diameters (see Fig. 5). When the separation length became longer, particles losses dramatically increased, and the collection efficiency decreased to 95 - 98%. Visual observations of the top of the cyclone with a Plexiglas roof showed complex flow patterns due to interferences during the first,rotation of the gas-solid stream entering the cyclone (Fig. 6). Incoming solids were forced to the wall as soon as they entered the cyclone. The gas-solid spiral flowed along the wall during its first rotation, and then split into three streams when it came back to the inlet region and encountered the incoming stream : - In stream 1, the particles kept spiralling down along the wall below the tangential inlet. - In stream 2, the particles began a new rotation at the top of the cyclone in the vena contracta region, and finally flowed down for more than one and a half rotations inside the cyclone. While they were flowing downward, those particles were not at the wall but midway between the wall and the conical insert. - In stream 3, the particles were entrained by an eddy which resulted from the interaction of the vena contracta with the conical insert. These phenomena were also,observed when the cone was replaced by an insert composed of a cylinder (0.50, in diameter, 0.50, long) prolonged by the cone. With this new insert, interferences were stronger than with the

cone. Sumner [ 31, with similar equipment, but with no insert in the inlet region, reported the existence of the first,two streams. He also found that the optimum separation length was around 1 cyclone diameter. Experimental results from Fig. 5 were interpolated to investigate the direct effect of the separation length on the collection efficiency. Figure 7 shows the influence of the separation length at various solid loadings for the five inlet,velocities studied. When the separation length was decreased from 2 cyclone diameters to 1 cyclone diameter, there was always a sharp decrease in collection efficiency. This can be attributed to the detrimental effect of the interferences in the inlet region. As the separation length increased, these interferences were dissipated and normal flow patterns were established. Figure 8 shows that the optimum separation length increased from 2 to 3 cyclone diameters to 3 to 4 cyclone diameters when the inlet velocity was increased from 13 m/s to 31 m/s. This indicates that the interferences were stronger for high inlet velocities. Once the separation length had reached its optimum value, efficiency decreased, and became a function of inlet velocity and solid loading, as shown on Figs. 7(b), 7(c), 8(a) and 8(b). However, when the loading became high, (e.g., L, = 6 wt./wt., on Fig. 8(c)), the collection efficiency became independent of the inlet velocity. When the separation length was equal to 10.5 cyclone diameters, the collection efficiency was not as strongly affected by inlet velocity and solid loading as at smaller separation lengths (Figs. 7 and 8). Most cyclone models assume that increasing the separation length increases the residence time of the particles within the vortex, and thus, allows more particles to migrate to the wall. This should result in higher collection efficiencies at longer separation lengths. Figures 7 and 8 show that this was verified only when the separation length was very short. However, visual observations suggested that the interference phenomena affected the operation of the cyclone: the solids migrated quickly to the wall, and were then partially reentrained by the interferences. It is thus likely that the relatively low collection efficiencies observed at low separation lengths result from the interfer-

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Fig. 7. Effect of separation length on collection efficiency at constant inlet velocity for various loadings. (a), Vi = 9 m/s; (b), Vi = 13 m/s; (c), L$ = 18 m/s; (d), Vi = 25 m/s; (e), 4 = 31 m/s; F,, = 0%. Interpolation from Fig. 5.

reaccelerated by the vortex and forced to the wall. For reverse-flow cyclones, the gas vortex is forced to reverse before exiting, the vortex length, thus, greatly affects cyclone performance and the length of the cyclone body is an important design criterion [9,10 1. In the uniflow cyclone, the gas flow patterns in the gas exit region are not presently clearly understood, particularly when the gas exit diameter is small, as in our study. Part of the gas may continue to flow around the gas exit with the solids, before reversing and exiting, instead of going out directly to the gas exit. Increased losses of particles with the cleaned gas, as the separation length is increased, may, thus, be attributed to (see Fig. 9): (i) particles being reentrained by the gas above the gas exit;

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(ii) particles being reentrained by the gas below the gas exit; (iii) the weakening and eventual disappearance of the gas flow reversal below the gas exit. This flow reversal probably has a beneficial effect by promoting the disengagement of solids from the gas through inertial effects. The present experimental results do not allow to identify which of these three mechanisms predominates. New measurements are needed to complement the cyclone efficiency measurements. However, it seems that the vortex strength depends on solid loading and

inlet gas velocity. The efficiency, and presumably the vortex strength, were greatly affected by solid loading and inlet gas velocity at intermediate separation lengths (3 to 10.5 D,), as reported on Figs. 7(d), 7(e) and 8(a). Such effects on the vortex may have been present at shorter separation lengths, but their impact on collection efficiency was then masked by the effect of the interferences at the cyclone inlet. The importance of the vortex length was confirmed by a few experiments conducted with 9% gas underflow. Gas underflow did not have any influence when the separation length was short (Fig. 10(a)). However, when the separation length was long (Fig. 10(b)), the effect of underflow became very significant, and increased with loading. Gas underflow probably modified gas flow patterns around the gas exit. When the separation length was short, losses were attributed to interferences, which pushed particles into the central core. This phenomenon should not be affected by gas underflow, because it occurs in the central core. This was confirmed by the lack of gas underflow effect on the collection efficiency. When the separation length was long, increased particle losses were earlier attributed to particle reentrainment or the weakening of the flow reversal around the gas exit. In this case, the underflow improved the collec-

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tion efficiency, probably by increasing and stabilizing the gas flow below the gas exit. To eliminate the detrimental interferences in the inlet region, a downward helical roof (volute) was inserted in the inlet area, forcing the spiralling stream below the inlet and thus eliminating streams 2 and 3 of Fig. 6. As shown in Fig. 2, the height of the volute after one rotation was 0.5 D, (the same dimension as the inlet height), and the conical insert was now attached to the volute. To avoid penetration of the tip of the cone into the gas exit, separation lengths of 1.5 D, and 2.5 D, were studied instead of 1.0 and 2.0 D,. Experimental results obtained with a separation length of 1.5 cyclone diameters (Fig. 11) show that, with the volute, the collection efficiency was very high (except at low loadings) and well over the detection limits of the system (99.99% collection efficiency). A comparison of these results (Fig. 11) with the results obtained without the volute (Fig. 5) shows that the volute eliminated the interferences which were thus the main cause of the low collection efficiencies at short separation lengths.

Inlet problems have already been studied in reverse-flow cyclones. Many of them have a scroll tangential inlet [ll] to gradually reduce the vena contracta. Alden [12] reported some interference problems in the inlet region and used a downward helical roof which increased efficiency as in the present study. However, Stern et al. [ 131 considered that in a reverse-flow cyclone, the beneficial effect of a volute on the inlet interferences was balanced by its detrimental effect on the vortex. On the other hand, all those studies were conducted at very low solid loadings, which prevented interferences from being strong enough to clearly show the advantage of the downward helical roof. Indeed, it is likely that the interferences observed in our study could also be observed in a reverse-flow cyclone operating at high solid loadings.

CONCLUSION

The cold model study clearly showed that cyclone collection efficiency was greatly influenced by the separation length. Interferences in the cyclone inlet region reduced the collection efficiency when the separation length was short. These interferences were totally eliminated by inserting a downward helical roof. Thus, collection efficiencies of 99.99% or more were achieved at solid loadings from 1 to 6 wt./wt. with a very short separation length of 1.5 cyclone diameters, i.e., with very short estimated separation durations in the range of 15 to 60 ms (depending on the inlet velocity). According to results obtained at longer separation lengths, the collection efficiency is likely to be affected by the reduced vortex

225

intensity in the gas exit pipe region where part of the separation probably occurs. Future work will include a study of cyclone pressure drop. The gas flow patterns inside the cyclone will be studied and the residence time of the gas inside the separator will be measured.

ACKNOWLEDGEMENTS

The generous help of the Institut Francais du P&role (E.N.S.P.M. and C.E.D.I.) is hereby gratefully acknowledged. Without it, this work could not have been done.

LIST OF SYMBOLS

cyclone diameter, m Sauter mean diameter of the particles, m uniflow cyclone collection efficiency, 70 gas underflow flow rate (wt.% of main gas flow rate solid loading, wt.solids/wt.gas separation length, m mass of collected particles in uniflow cyclone, kg mass of collected particles in reverse-flow cyclone, kg

N.T.U. Vi

PS

number of transfer unit cyclone gas inlet velocity, particle density, kg/m3

m/s

REFERENCES 1 R. G. Graham, B. A. Freel, R. P. Overend, L. K. Mok and M. A. Bergougnou, Proc. 5th Engineering Foundation Conference on Fluidisation, Elsinore, Denmark, May 18 - 23, 1986, p. 473. 2 A. L. Vogiatzis, S. Afara, C. L. Briens and M. A. Bergougnou, Proc. 2nd International Conference on Circulating Fluidised Beds, Compiegne, France, March 14 - 18, 1988, p. 483. R. J. Sumner, Master’s Thesis, Univ. of Western Ontario (1986). R. J. Sumner, C. L. Briens and M. A. Bergougnou, Can. J. Chem. Eng., 65 (1987) 471. B. T. Arnberg, J. Basic Eng. (Trans ASME), (1962) 447. T. A. Gauthier, These de doctorat, E.N.S.P.M., France (1990). S. Nieh and T. Nguyen, Part. Sci. Technol., 5 (1987) 115. F. A. Zenz, in J. McKetta (ed.), Encyclopedia of Chemical Processing and Design, Marcel Dekker, New York, 1976, Vol. 14, p. 82. 9 R. McK. Alexander, Proc. Aus. Z.M.M., 152 - 153 (1949) 203. 10 S. Bryant, R. W. Silverman and F. A. Zenz, Hydrocarbon Process., (1983) 87. 11 R. Jackson, Mechanical Equipment for Removing Grit and Dust from Gases, The British Coal Utilisation Research Association, Leatherhead, 1963. 12 J. Alden, Design of Industrial Exhaust Systems, Industrial Press, New York, 1939. 13 A. C. Stern, K. J. Caplan and P. D. Bush, Cyclone Dust Collectors (Report on Removal of Particulate Matters from Gaseous Wastes), A.P.I., 1955.