Dense Phase Conveying of Fine Coal at High Total Pressures

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1 Powder Technology, 62 (199) Dense Phase Conveying of Fine Coal at High Total Pressures D. GEDART and S. J. NG* University of Bradford, Bradford, BD7 1DP (U.K.) (Received October 19, 1989; in revised form May 8, 199) SUMMARY Two sizes of fine coal have been transported at room temperature at pressures up to 82.7 bar through horizontal lines 6.25, 9.19 and 12.5 mm in diameter and up to 15 m long using nitrogen, hydrogen, and carbon dioxide. Pressure differentials were established between two pressure vessels and solids and gas flow rates were measured. Solids fluxes up to 2 kg/(m2-s) were achieved at superficial gas velocities between.8 and 1 m/s in dense phase conveying. Separate pressure drop measurements were made over the acceleration zone and across bends, and correlations are presented for the contributions made to the total pressure drop by bends, acceleration and solids friction. NTRODUCTON The experiments reported here were carried out at British Gas, Midlands Research Station, Solihull, England and the work formed part of a Ph.D. dissertation [ 11. Two powdered coals were used in three diameters of line using three different gases at high total pressures in the extrusion and dense phase conveying modes. n this paper, the results of the dense phase conveying experiments are presented. The work differs from that of earlier researchers in a number of respects, but primarily because the separate contributions to the total pressure drop of bends, acceleration zones and, in particular, solids friction have been evaluated. Also, the pressure drops in the pipeline were only a small fraction (usually less than *Present address: C Films plc, Drungans Works, Dumfries, U.K /9/$3.5 7%) of the total operating pressure. Thus, the system operated at virtually constant gas velocity during each run, whereas in many so-called dense phase conveying systems the velocity increases significantly at the delivery end. THEORETCA BACKGROUND AND PREVOUS WORK There is increasing interest in transporting powders and granular solids pneumatically in the so-called dense phase conveying mode and a comprehensive review of the field has been published recently [ 21. The two major motivations for this are: (i) much smaller quantities of the carrier gas are required and this may be essential if the solids are being fed into a chemical reactor, coal gasifier or comb (ii) lower superficial gas velocities are used, thus reducing particle degradation. Experimental data in pneumatic conveying systems may be obtained by setting a solids feed rate M, (kg/s) and measuring the pressure drop across the line over a range of gas flow rates, starting at a high value and gradually decreasing. A new solids rate M, is selected and the experiment repeated. The data are usually plotted as total pressure drop per unit length APT/ against gas flow rate Q (m3/s) or superficial velocity U (m/s), with solids flow rate M (kg/s) or solids flux G, (kg/(m2-s)) as a parameter. A typical plot for horizontal conveying is shown in Fig. 1; the regions well to the right of the minima are defined as lean or dilute phase conveying and those near to the minima, and to their left, are said to be operating in the dense phase mode. These data can be presented in the form shown in Fig. 2 by reading off the solids fluxes Elsevier Sequoia/Printed in The Netherlands

2 244 and solids/solids friction) + Ap(bends) Total PWSS W drop unit N/m3 per length Fig. 1. Total pressure drop through a pneumatic conveying line as a function of gas velocity and solids flux. GS kg/(m ~rl Fig. 2. Solids flux as a function of gas velocity and pressure drop. corresponding to any given pressure drop and plotting them against gas velocity. The minima of the curves on Fig. 1 and the maxima on Fig. 2 are called the saltation velocities and will form the subject of a separate paper. When doing experiments at a high total pressure, it is convenient to set the pressure drop between the feed and delivery vessel and then measure the solids rate as a function of the gas flow rate, thus obtaining data which can be plotted directly in the form of Fig. 2. Virtually all powders can be conveyed in the dilute phase mode and design methods are available, though none is particularly accurate and overdesign is the rule. n pneumatic conveying, the total pressure drop is expressed as the sum of a number of separate contributions thus: Ap(tota1) = A&acceleration of gas) + Ap (acceleration of solids) + Ap(gas/wall friction) + Ap(solids/wall + Ap(net static head) (1) The contribution of the solids friction term is obtained experimentally by subtracting all the other pressure drops (some of which are calculated, not measured) from the total pressure drop. Consequently, the scatter is enormous and the accuracy of correlations for Ap (solids friction) is low. Fortunately, the size of this contribution to ApT is small in dilute phase conveying. n horizontal dense phase conveying, however, the solids friction component of the total pressure drop dominates and extrapolation of dilute phase correlations to the dense phase operational mode can lead to wildly inaccurate predictions, especially if the dense phase conveying is to be done at high pressure. Most of the work published on horizontal dense phase conveying relates to feeding devices in which the powder is fluidized or minimally aerated in a vessel and then flows along a relatively short line under a differential pressure without the addition of extra gas - so-called extrusion flow. Those correlations which were developed relate the overall pressure drop to system variables without separating the various contributions which play a role in dilute phase systems. The Wen-Simons [ 31 correlation, for example, which is widely used, assumes that a fixed quantity of gas is associated with any specified solids flux and pipe diameter. Moreover, in their work, since the pipe lengths were short, acceleration effects may have made a significant but unknown contribution. Dense phase conveying at high pressure does not require the use of additional transport gas, but its use reduces the pressure drop required to convey a specified solids mass flow rate; alternatively, a higher solids flow rate is possible with the same pressure drop thus allowing another degree of freedom in the design and operation of the equipment. EQUPMENT AND METHOD The main vessels of the experimental equipment are shown in Fig. 3. The equipment was capable of conveying batches of up to 15 kg of fine coal using hydrogen or

3 245 Fig. 3. The experimental equipment. nitrogen at total pressures of up to 83.7 bar abs. or carbon dioxide up to about 3 bar abs. During an experiment, coal was transferred between PVl and PV2, which were identical feeder or receiver vessels. Experimental results could also be taken during the return transfer of coal between PV2 and PVl. The conveying line, 24 mm in diameter, passed vertically downwards out of the feeder vessel into a ball valve. Fluidizing gas could be added just above this valve, if required. Below the valve was a 12.5-mm diameter line which joined the main horizontal pipe at a tee-piece and the main supply of transport gas was added at that point. n those experiments in which the transport line was smaller than 12.5 mm diameter, a reduction piece smoothed the transition to either 9.19 or 6.2 mm in internal diameter. The smallest pipe blocked easily and consequently was used for only a few experiments. All three sizes of pipe followed the same route and contained a sloping section 5 m long at an angle 2 to the horizontal since the inlet to PV2 was above the outlet from PVl. When flowing from PVl to PV2, the solids had to pass up a vertical section 8.4 m long, whilst when flowing from PV2 to PVl the inlet and outlet were at the same level. About half the experiments were made in lines 8.8 m long from PVl to PV2; the longest conveying line was 15 m in the direction PV2 to PVl and contained 37 bends, all of which were.3 m radius. With the exception of one set of experiments carried out in a stainless steel line, all the pipes consisted of sections of smooth mild steel joined with compression fittings. The principal measurements made were the solids and gas flow rates and the differential pressures across the feeder vessel and across the conveying line. The coal feed rate was obtained from weigh cell chart records and the gas flow rate was calculated by timing integrating gas meters in the vent lines. All pressure drops were measured using differential pressure cells whose calibration was checked for each set of experiments. Batches of up to 15 kg of coal could be transferred between the vessels and this usually allowed solids flow rates to be measured for up to six different transport gas flows per batch at any specified pressure drop. Corrections were made in the calculations to allow for gas displaced in the receiver and the gas required to replace coal leaving the feeder vessel. Because TABE Basic properties of coals used Property Coal F Coal M Median size, pm Surface/volume size, pm 8 26 Particle density, kg/m Tapped bulk density, kg/m Aerated bulk density, kg/m Tapped/aerated ratio,

4 246 high total pressures were used, the effect of gas expansion along the line was negligible; in general, the superficial gas velocities were in the range.8 to 5 m/s for nitrogen and carbon dioxide, and 2 to 1 m/s for hydrogen. At high operating pressures, the solids to gas mass ratio is a poor criterion for judging whether the mode of flow is dense or dilute phase and the voidage E of the flowing suspension is to be preferred. n these experiments, E ranged from.55 to.985 with most of the values being in the range clearly dense phase conveying. Two grades of fine coal, F and M, were used, and both coals had extremely wide size distributions. Coal F contained 2.9% under 5.8 pm and 1.2% larger than 113 pm; coal M had 3.6% under 5.8 E.trn and 4.7% above 262 pm. Other relevant data are given in the Table. Gas was added at the pipe cross on the feeder outlet and also at the tee, where the direction of flow changed from vertically downwards to horizontal. The small proportion of gas added at the pipe cross gave a smoother discharge of coal from the feeder and since both sources of gas appeared to have the same effect on the line pressure drop no distinction has been made between them in the results. Differential pressures between.67 and 6.9 bar were employed, giving mass fluxes ranging from 7 to 22 kg/(m *s). EXPERMENTA RESUTS A capacitance flowmeter was installed between DP cells in two straight horizontal sections of pipe near the beginning and also Nitrogen Fig. 4. Chart recordings from a capacitance solids flowmeter using coal F. Total pressure 52.7 bar abs. near the end of one of the pipelines. Results were similar for both positions. As Fig. 4 shows, flow appears to be smooth and nonslugging using nitrogen at 52.7 bar, whilst the signals imply that slugging occurs at the same pressure when using hydrogen. The pressure signals reflected the capacitance traces in that those monitoring experiments with nitrogen were completely steady whilst those monitoring hydrogen were unsteady and showed a periodicity, from which it was deduced that the slugs were about 1 m long and occurred about every 2 s, travelling at about 2 m/s. These apparent differences in flow behaviour were reflected in the numerical results in so far as the pressure drop due to the solids/wall friction was found to be less dependent on velocity with hydrogen. This is discussed further in a later section. Acceleration pressure drop The acceleration pressure drop in the mm diameter pipe was obtained by measuring the pressure difference between a tapping in the transport gas injection line just before the vertical line from PVl and another 1.8 m away in the horizontal line. Since this distance of more than 1 pipe diameters is much higher than the acceleration length predicted by the Rose and Duckworth [ 41 correlation, it is reasonable to suppose that the flow was fully accelerated at the end of this section. The pressure over a l-m straight section in the same pipe line was also measured simultaneously for coal F transported by nitrogen at 52.7 bar abs. Values were measured for conditions ranging from 94 kg/(m*-s) at 1 m/s to 656 kg/(m**s) at 2.72 m/s. The theoretical pressure loss required to accelerate the solids from rest to their final velocity is G,V,, where V, is given by U/E minus the slip velocity, usually taken to be V,, the particle terminal velocity. n these dense phase conveying experiments using fine coal, V, was very small and E averaged so that there is little error in assuming that V, = U. The additional pressure losses caused by the acceleration (corrected for the small difference in pipe length) are plotted in Fig. 5 against G&Y. t is surprising to find that the experimental values of the acceleration pressure drop are approximately twice the expected value. The least-squares fit gives a slope of 1.91 with an intercept of 152 N/m*

5 *E % ; : 2? 6._ 15 fii i : : E t 1.5 Y Ef * 1. E j, h.5 2 d G, U. kn/m2 Fig. 6. Correlation of bend pressure loss G,U, N/m2 Fig. 5. Correlation of acceleration pressure drop. at zero velocity, whereas the line must pass through the origin. The following simpler equation can be used with little loss of accuracy : Apa,, = 2GJJ (2) Pressure drop across bends Pressure drops were measured over two two-meter sections of 9.19-mm internal diameter pipe. Each section comprised a l-m straight section immediately before a bend of radius.3 m and 1 m long. Both sections were in positions where the flow was fully accelerated before entering; one was near the end of a 38.5-m length and the other near the beginning. Coal F was used in nitrogen at 52.7 bar abs. at superficial gas velocities between.78 and 5.84 m/s and solids fluxes from 457 to 1649 kg/(m**s). By subtracting the pressure drop across the straight section from that across the bend, the additional pressure loss across the bend was calculated, and the values are plotted in Fig. 6 against GJJ. The basis for using G,U, an acceleration term, is that the solids are centrifuged out in the bend and must be re-accelerated immediately afterwards [ 51. The additional pressure loss ranged from 15 to 65% of that across a straight section of the same length and is correlated by Pressure drop due to gas/wall friction t is generally assumed that this pressure drop term has the same value in the presence of flowing solids as it has in their absence and in dilute phase conveying the use of the Fanning equation with the Blasius or Nikuradse friction factor gives satisfactory results. At the high velocities used in horizontal dilute phase conveying, this contribution is of a similar magnitude to the other terms in the total pressure drop equation, but at the low velocities used in our dense phase work its role is of much less significance. n any event, we have followed convention and used Arms = 2fP,U2 D where f =.79Re-*25. Pressure drop due to the static head The long lines used in this study contained a vertical section 8.4 m long up which the (3) (4)

6 248 solids had to flow when passing from PVl to PV2, though not in the opposite direction. The pressure difference required to lift the solids through a vertical height H is that due to the hold-up in the vertical pipe, and may be calculated from &solids head= Pm@ (5) where pm is the in situ mixture density, or hold-up, of the suspension in the pipe. n the case of dense phase conveying using fine particles, as in this work, the slip velocity is small and the as-fed mixture density, which is virtually the same as pm, has been used throughout in our calculations. n the more general case, the vertical head loss may be calculated from [6] (f-5) where V, is the solids velocity. Note: There are several correlations for solids (or particle) velocity, the most commonly used being those of Yang [7] (eqn. (11)) and Hinkle [S], (eqn. (2)). Pressure loss due to solids/solids and solids/wall friction t is impossible to differentiate between the interparticle and particle/wall friction; indeed, it is not possible to measure directly their combined contribution to overall pressure drop. The residual pressure drops obtained after all the other contributions discussed earlier have been subtracted are deemed to be the solids friction pressure losses. n dilute phase conveying at high velocities, its contribution is relatively small compared with gas/wall friction and this has made it difficult for researchers to obtain consistent data for use in correlations. Published correlations for the solids friction term in dilute phase vertical conveying have been summarised by eung and Wiles [9] and use the concept of a friction factor f, for use in an equation of the Fanning type (eqn. (4)) but with the velocity and density of the particles substituted for those of the gas. Correlations for the friction factor f, fall into three categories: those which quote a single constant value for the particles used; those which use the form f,= $ S (7) and those which try to take into account gas and particle properties. Correlations for the friction factor in dilute phase horizontal conveying have been reviewed by Soo [lo]. Most use a combined friction factor fm for the gas/solid mixture, which is then inserted in the Fanning equation written to include the superficial gas velocity, not the solids velocity. For example, the correlation of Dogin and ebedev [ 111 is f, = fe +B (d,) *1Re.4Fr.5PpM* u Pl3 where Re and Fr are based on the superficial gas velocity and the pipe diameter, and B is an adjustable parameter given a value of 2 X lop6 to correlate data on coal dust. The second term on the right-hand side is the solids friction factor f, and when combined with the Fanning equation gives AP, 4 X 1-6d,o* p,g,u2*4 -= g.5pg.6p.4~1.2 (9) The correlation of Yang [7] is of a quite different form: r f, =.292 G (1) where Re, = pg( U/E - VS)d,/p V,= $ -&VT (11) Manipulation of these equations leads to e2 f, =.627 X 1O-3 (l_ e)o.353 (12) Now, for fine particles in dense phase flow, V, = U and.8 < E <.95. f, becomes virtually independent of flow conditions or particle properties and assumes values between 1.74 X 1O-3 and 4 X 1-3. Yang quotes the Fanning equation for solids friction pressure loss as -= AP, 2fsG,V, D Substituting for f, in eqn. (13) gives AP, UGS - = (1.74 x 1-s to 4 x lo-s) 7 (13) (14)

7 aps N/m3 A G kg/(m -r). GS / * GB 13-16, Nitrogen. coal M. pipe dia. = 9.19 mm Opsratlng pressure : 52.7 bar 1 l--e=: GS PP vs 249 (16) (remembering that for dense phase flow V, x U, the gas velocity). This suggests that in homogeneous dense flow the particle concentration at the pipe wall controls the energy loss due to sliding friction in contrast to dilute phase flow, in which the kinetic energy of the particles, which increases as gas velocity increases, controls. Our results show that the solids friction pressure drop was much less dependent on gas velocity when the coal was conveyed with hydrogen and this is consistent with the fact that, as shown by the AP u s (N/sm? U m/s Fig. 7. Solids friction pressure drop VS. superficial gas velocity. 1 1 n summary then, the correlations of earlier researchers for the pressure drop due to solids friction in dilute phase conveying predict a direct dependency on the solids flux, and on the solids or gas velocity, and an inverse dependency on pipe diameter. Some also include gas properties and particle size. By plotting our results as Ap,/ against the superficial gas velocity U, with all other parameters constant, it became quite clear that in contrast to eqns. (9) and (14) the solids friction pressure drop in our dense phase conveying system decreased as gas velocity increased (Fig. 7), and this is true for all the conditions and configurations investigated. Thus, AP, 1 -a- u (15) The following hypothesis is proposed to explain this observation. The capacitance traces (Fig. 4) indicate that the flow regime was probably homogeneous when using nitrogen. Now, for a fixed solids flux, as the solids velocity rises the solids concentration falls, according to the relation 1, 1 1 Gas density kg/m3 Fig. 8. Correlation of Ap,U with gas density. Ap, (N/s-m*) 1 1 1x16 1x1 Fig. 9. Correlation of Ap,lJl kinematic gas viscosity. against p/cc, the inverse

8 25 capacitance traces (Fig. 4), the flow regime was probably in the form of plugs or dunes, within which the solids concentration would change little. Plots of Ap,U/ against gas density pg (Fig. 8) and against p/p (Fig. 9) show that the latter gives the best straight line, though as expected the hydrogen data show the greatest scatter. The correlation is of the form ApJ pe -.4 -cx - i /J 1 (17) t is also clear from Fig. 9 that Ap,U/ increases with increasing G, and the next step would be to plot A~,U/[P~/~]~ ~ against G,. However, data taken under otherwise identical conditions in and 9.19-mm diameter pipes (Fig. 1) show that under any given total pressure differential the delivered solids flux G, (kg/(m2*s)) is significantly higher in the larger pipe. Since most of the pressure loss is caused by the solids friction contribution, it seemed logical therefore to plot Ap,U/ X [pa/p] 4 against G,/D. Figure 11 shows all the data (6 points) relating to the solids friction pressure drop. There is considerable scatter in the data, which are best correlated by two lines which coincide at G,/D equal to 47. The least-squares equations are AP, --- where K = 16 and m =.83 for G,/D < 4.7 X lo4 and K =.838 and m = 1.28 for G,/D > 4.7 x 14. t will be noticed that no attempt has been made to include the particle properties in the correlation. Firstly, only two coals were tested; secondly, the size distribution was so wide that it is difficult to know which size to use (see the Table); and thirdly, no clear difference in solids frictional pressure drop between the two coals was apparent at low solids fluxes, though there was a tendency towards a lower solids friction pressure drop at higher loadings with the coarser coal (coal M). PREDCTON OF TOTA PRESSURE DROP The overall correlation for dense phase flow is given below 7 Ei Coal F, operating prcssvrc z 52.7 ciosed points 9.19mm Dia. bar ApT = 2GJJ + 2fPJJ2 +K(i) (:r; D (1) (2) (3) + G,Hg + ;GJJ (19) VS ;i 6 E z r 5 d 4 w % E t 2 B 1 A lo5 1 lo5 1= Superficial gas velocity u m/s GJD Fig. 1. Solids flux as a function of total pressure Fig. 11. Solids friction pressure loss function paramdrop, gas velocity and pipe diameter. eter Ap,U/~(p/p)~*~ as a function of GJD.

9 251 where f =.79Re-*25 K = 16, m =.83 for G,/D < 47 kg(m3*s) k =.838, m = 1.28 for G,/D > 47 kg/(m3*s) H = net vertical distance between the feed and exit positions n = number of bends V, = U(l-.638dPo 3p,o 5) [8] (2) inlet); Term (5) is the pressure loss caused by n bends of radius.3 m. The pressure drops predicted by eqn. (19) are compared with measured values in Fig. 12. The agreement is moderately good. CONCUSONS Extensive experiments have been done on the high-pressure dense phase conveying of fine coal in long, small-diameter pipelines. The separate contributions to the pressure loss caused by bends, acceleration and solids/ wall friction have been evaluated and a correlation (eqn. (19)) is presented based on more than 6 data points. Term (1) is the pressure loss due to the acceleration of the mixture from rest to a superficial velocity U; Term (2) is the gas/pipe friction; Term (3) the mixture/wall friction loss; Term (4) the pressure lost in lifting the solids through a net vertical height H (note that H will be negative if the powder leaves the system at a lower elevation than the 7 *k / $7 t / / a Experimental ApToTA (bar) Fig. 12. Predicted versus experimental total pressure drop. / ST OF SYMBOS A B d, D :b, fm, fs 3 Fr GS H K ii M* 2\PT AP aw2, APbend. AP gas> AP, Q Re Ret Rep cross-sectional area of pipeline, m2 constant in eqn. (8) mean particle size, m internal diameter of pipeline, m Fanning friction factor, - Fanning friction factor for gas, mixture and solids, respectively, - acceleration due to gravity, 9.81 m/s2 Froude number = U2/g mass flux of solids, kg/(m2as) vertical distance between powder discharge and feed points constant pipe length, m exponent in eqns. (18) and (19) mass flow rate of solids, kg/s solids to gas mass flow rate ratio, - exponent total pressure drop across system, N/m2 pressure drops due to acceleration, bends, gas/wall, and solids/ wall friction respectively, N/m2 volumetric flow rate of gas, m3/s Reynolds number = pgud/p, - Reynolds number at particle terminal velocity = pgvtdp/p, - Reynolds number of particle = P,[(U/e) - Vsldpl~, -

10 252 u VS vt superficial velocity of gas = Q/A, m/s solids velocity, m/s terminal velocity of single particle, m/s Greek symbols E voidage, - P gas viscosity, kg/(m* s) 6% Pm9 Pp density of gas, mixture, and particle respectively, kg/m3 ACKNOWEDGEMENT The experimental work reported in this paper was carried out at the Midlands Research Station of British Gas plc. The authors would like to thank British Gas for the facilities provided and for permission to publish this paper. REFERENCES S. J. ing, Ph.D. Dissertation, Univ. of Bradford (1988). K. Konrad, Powder Technol., 48 (1986) 193. C. Y. Wen and H. P. Simons, AZChE J., 5 (1959) 263. H. E. Rose and R. A. Duckworth, Engineer, 227 (1969) 433. R. D. Marcus, J. D. Hilbert and G. E. Klinzing, Bulk Solids Handling, 5 (1985) 121. T. M. Knowlton, in D. Geldart (ed.), G-as Fluidization Technology, Wiley, Chichester, 1986, p W. C. Yang, J. Powder Bulk Solids Technol., 1 (1977) 89. B.. Hinkle, Ph.D. Thesis, Georgia nst. of Tech. (1953).. S. eung and R. J. Wiles, Znd. Eng. Chem. Process Des. Dev., 15 (1976) 552. S.. Soo, in G. Hetsroni (ed.), Handbook of Multiphase Systems, Hemisphere, Washington, 1982, p M. E. Dogin and V. P. ehedev, Znd. Eng. Chem., (USSR), 2 (1962) 64.

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