Powder Technology_ 31i (1983) i9 - 57 i9

Vertical Gas-Solid Transition Flow with Electrostatics

S. JOSEPH and G. E. KLINZING

Cl~emical/Pflroleum Engineering Department, L’nir?ersity of Pitisburgh, I’ittsbuqh. P-4 15261 jUSA_)

(Received December 9, 1982; in revised form March 2s. 1993)

Choking in vertical pneumatic transport in the presence of electrostatics was examined using a 0.0254 m dia. pIeA-ig2as tube and 150 pm dia. glass particles. Electrostatics was found to make the choking transition more violent and occur at a higher air velocity_

INTRODUCTION

The choking region in vertical pneumatic transport has been defined as the sharp transition between dilute and slugging dense phase flow. This region represents an unstable flow condition, and studies carried out to date deal with choking in the absence of electrostatics_

Electrostatic forces present in gas-solid transport cause energy losses to increase substantially [l] _ The additional pressure drop in the transport line due to electrostatics could modify the choking transition in a way which has not been investigated as yet.

The purpose of this study is to determine whether electrostatics has the potential to make this transition less violent.

LITERATURE

Flow patterns in pneumatic conveying The three main types of flow patterns in

vertical pneumatic conveying have been defined by Leung and Wiles [ 2 J as follows: (-4) Dilute phase flow in which the solids are carried upward in an apparently evenly dispersed suspension with a low volumetric solid concentration (generally less than about 50/o)_ (B) Dense phase flow: two types of dense phase flow are distinguished depending on

whether or not slugging occurs in a particular gas-solid system_ Slugging dense phase flow occurs when solids are carried up by air slugs- For systems where fine powders are con- cerned, slugging does not occur and solids are carried upward in a dense phase with con- siderable internal solid recirculation_ This fiow is defined as dense phase flow without slugging_ (C) XloGng bed flow where solids are carried upward as a packed bed at a voidage corre- sponding to that of a packed bed 1vit.h hardly any relative motion between particles_

The fiow patterns that can arise in vertical pneumatic transport have been classified by Leung [3]_ For systems in which slugging dense phase floxv occurs, the transition from dilute to slugging dense phase flow is referred to as the choking point-_

This phenomenon of choking was first described by Zenz and Othmer [a]_ A typical esperiment to determine the choking condi- tion is to set the solid rate at a given value and t-hen decrease the velocity of the gas_ The pressure drop decreases as the velocity of the gas is reduced. Tine concentrat.ion of the particles per unit volume begins to increase, since the velocity of the particles also de- creases- Soon a point is reached when the rate of increase in pressure drop due to the in- crease of solids head esceeds the rate of decrease in pressure due to wall friction losses and this increases the overall pressure drop_ Decreasing the fluid velocity at this point causes the solids velocity to decrease to such an extent that t-he entire suspension collapses and is then transported up the tube in slug flow-. The collapse of the disperse flowing suspension into a condition of slug flow is referred to as choking.

The determination of the choking point is not precise due to the pressure fluctuations in the line operating near the choking condition_

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Yousfi and Gau [5] visually observed this phenomenon and defined choking as the state in which solid slugs estend over the entire pipe cross-section. Capes and Nakamura [S] have shown that choking is not a clear-cut phenomenon but involves a whole range of instabilities. It was thought that choking occurs in all systems, but the observation by Kehoe and Davidson 173 on the gradual breakdown of the slug regime in a fluidized bed shows that choking does not occur in all gas-solid systems_

Choking and non-choking systems The properties of the solid and gas and the

size of the tube determine whether a system is a ‘choking type’ system or a ‘non-choking type’ system- Leung [S] has suggested t.hat, as a general guide, fine particles in large tubes eshibit the non-choking phenomenon while coarse particles in small tubes exhibit the choking-type system_ This has been confirmed by Canning and Thompson [S], who observed that strong, nat.urally occurring plug flow was always obtained with coarse (3 mm) poly- ethylene pellets over a wide range of velocities in a range of pipe sizes containing both horizontal and vertical sections. However, with fine powders, plug flow was not ob- tained naturally, i.e. without pulsing or by-passing the air flow. Yang [9] has also shown that the slugging mode of transport could be eliminated simply by increasing the diameter of the pipe or decreasing the particle size and density.

Solids fluidized by gases have been classi- fied by Geldart [lo] into four groups which are characterized by the density difference (p, - pf) and mean particle size. Canning and Thompson [S] have used Geldart’s fluidiza- tion chart to piot a slugging diagram. They have shown how the slugging diagrams could be used to choose the most suitable type of powder for dense phase conveying.

iUechanis.m of choking The collapse of the disperse flowing suspen-

sion into a condition of slug flow has been defined as choking by Zenz and Othmer [4]_ The tendency of particles to aggregate has been postulated to be the reason for the collapse of the suspension. The experiments of Capes [ll] conclusively prove that particle aggregation is the main cause of this phenom-

enon. The experiments were performed both in an empty line and in a line cont.aining screen packing. Choking was observed in the conventional empty line as soon as the gas velocity was decreased. But st.able operation was possible in the screen-packed line from the dilute phase down through the dense phase to the fluidizing region. The packings prevented particle aggregation.

The process by which clusters mere initi- ated has been esamined by Grace and Tuot 1121, who have characterized the factors which promote cluster fomlation, by using Jackson’s [13] linear stability analysis to calculate the growth distance of clusters for a number of systems- It was shown that decreasing t.he particle density, increasing the gas density, increasing the pressure and increasing the superficial gas velocity had a st-abilizing influence, Le. to increase the distance required for the disturbance growth io occur. Growth distances were predicted to increase as the voidage went to unity_ This explained why clustering was not a serious factor in very dilute systems.

Matsen 1141 has presented a novel approach on t.he mechanism of choking which is based on the slip velocity-voidage relation_ The assumption of a uniform particle size and negligible wall friction leads to the slip velocity being a function only of the voidage of the suspension_

Matsen [14] defined a relationship, in which the slip velocity steadily increases above the terminal velocity as the voidage decreases, to take into account the experi- mentally observed voidage discontinuity and t.he coexistence of dilute and dense phases for choking. This results in a flow map which exhibits an envelope limiting the solids rate at any given gas rate. In a pneumatic transport system, below the envelope either gas or solids rate can be changed slightly with only a corresponding slight change in voidage. However, as the envelope is reached, the limiting or choking voidage is reached by either decreasing the gas rate or increasing the solid rate. The system must change to a totally different voidage-slip line in order to accommodate any further decrease in gas rate or increase in solid rate_ Choking occurs due to the sudden step increase in the volume fraction of solids. Though this approach explains the mechanism of choking quite

Sl

well, the assumption of negligible wall friction may be too restrictive in pneumatic transport where solids are transported at a high velocity in relatively small pipes.

Chokincr criteria Quantitative analyses are available in the

literature to classify a particular system as a choking or non-choking type of system. One of the first criteria for choking was proposed by Zenz [15]. The choking velocity could be determined from a curve derived experimen- tally by plotting the dimensional term U,‘/

gdpps’ against the solid loading R, a dimen- sionless ratio of solid flow to gas flow. By assuming that the gas-solid slip velocity at choking is equal to the terminal velocity of a single particle, the solids concentration at choking was calculated.

Doig and Roper [16] suggested the following empirical equations for predicting choking:

log u,

(gdJ’-’ = 0_03OU, -I 0.25 log R

tit < 3.05 m/s ill

1% u,-2

(gd:O-j = 28 - -+0_25logR

U, > 8.05 m/s (2)

Leung et al. 1171 have pointed out the inadequacies of both the above correlations_ Based on the observations for various systems that the voidage at the onset of choking lies within a narrow range and that the slip velocity at the onset of choking is equal to the terminal settling velocity of a single particle, Leung et al_ [17] proposed a method for calculating flow rates at the onset of choking. However, their assumption of a constant voidage at choking, e = 0.97, could involve larger error [ IS]-

The assumption of the slip velocity at choking being equal to the terminal velocity was retained by Yang [18] but instead of assuming a constant voidage at choking, he assumed that the solid friction factor fp was constant at choking.

(fp)c = 5wt(E-4-7 - 1) = o_o1

[(U,), - u,j* (3)

Punwani et al. [ 191 modified this equation to correlate the high pressure choking data of Knowlton and Bachovchin [ 2Oj _

The stabilitv of unflow of a uniform unbounded suspension was considered by Yousfi and Gau [5], who assumed that choking would occur when the uniform suspension was unstable_ Their analysis led to the criterion

r/,2 > 140

@P (4)

for choking to occur. By analogy with the stability of a single

bubble in a fluidized bed, Yang [9 J con- sidered the stability of a rising slug and arrived at the criterion

= > O-173 gD, -

for choking to occur_ Smith 1211 considered bubble stability in

terms of velocity of the continuity wave and suggested that the bubbles (slugs) could not rise at a velocity higher than t.he wave propa- gation velocity_ He assumed that the bubbles should be stable for choking to occur and showed that

LT,e”_%2(1 - E) > o ;21

WVO-’ - (6)

for choking to occur_ Leung [3] analyzed the criteria of Yousfi

and Gau [5], Smith [21] and Yang [9] and recommended that Yang’s criterion be used for predicting choking and non-choking systems since it incorporat.ed the important parameter of pipe diameter and was not an implicit equation like Smith’s crit-erion.

Yang [ 223 refined his mathematical defini- tion of choking by assuming that the friction factor at choking was 0.04 rather than O-01. This seemed to correlat-e the majority of choking data in the literature better_

ESPERIMENT-4L APPAR-TUS AND PROCEMJRE

The experimental apparatus used in this study was essentially the same as the one on which Ally [l] determined t-he electrostatic effects in dilute and dense phase pneumatic transport_ A schematic diagram of the appara- tus can be found in a recent publication [ 233 _

The presence of electrostatics in pneumatic transport is dependent on both particle and

T Iii I-J-l I!

Fig. l_ Particle size nnnlysis of POOS glass beads.

tube characteristics. It was decided to use the system plcsiglas tube-PO08 glass p‘articles. me plesiglas tube was chosen to afford visual observation of the choking phenomenon. By using Yang’s criterion (eqn. (5)), it was confirmed that the plesiglas tube-PO08 glass particles constituted a choking type of system. The average particle size of the glass particles was determined using an Omnicon Alpha Image Analyzer in conjunction with a Bausch and Lomb Balplan microscope. The particle size analysis is shown in Fig. 1.

The main test section consisted of a plesi- g1a.s tube 3.05 m long with an internal diam- eter of 0.0254 m. Flanges made of plesiglns were used to attach the tube to the rcmninder of the equipment. Pressure taps were made in the flanges.

A 1.83 n section of copper tubing was provided at the entrance section to ensure that the solid particles were fully accelerated when they entered the test section. The particles were separated from the air by means of a cyclone separator from which they dropped into a storage unit and were fed back to the bin feeder.

The air supply unit consisted of a pres- surized tank 3.05 m high and 0.61 m in diameter. The air supply was obtained from the house compressor and the tank could be pressurized to 620 kPa to serve as a source of non-fluctuating air supply.

The humidity of the air could be controlled by splitting the air flow between a drying and a humidifying section. The drying section consisted of a cylindrical Plexiglas tube filled with Drierite. The humidifying section was

packed with layered sponges interposed with layers of sponge fragments and this whole arrangement was saturated with water. The air from the humidifying and drying sections was combined and the humidity was monitored by means of a hygrometer (Hydrodynamics Model 15-3050).

The hydrometer measured the moisture content of the humidified air by means of minute changes which occurred in t.he con- ductivity of salt fihns on the surface of sensing elements. The two sensors used spanned a humidity range of 12 to 65% relative humidity (R.H.) with an accuracy of 1.5% R.H. The air flow rate was measured with a rotameter.

The solids were contained in a bin and were fed Grect.ly into the air stream by means of a screw feeder. The amount of solids fed could be changed by adjusting the speed of the screw feeder.

The surface of the particles had to be as clean as possible since electrostatic forces arise primarily due to surface-surface intcr- actions. *rhc glass particles were thoroughly rinsed with distilled wntcr twice. ‘I’iwy wwc’ subsecpcntly soaked for a period of 34 h in nitric acid. At the end of the 24 h period, the csccss acid solution was drnhicd off and the particles rinsed thoroughly with distilled water. Following this procedure, the glass particles wcrc soaked once ngnin in methyl nlcohol so that any residual acid would renct. The cscess alcohol was drained and the particles rinsed again with distilled water. Finally, they were dried at 100 “C in an oven for a period of 3 to 4 days before use.

The pressure drop for the two-phase flow system was measured using a differential pressure transducer (Viatran Model 119). The range of pressures that could be measured was 0 - 30 in WCD (Water Calibrated Differential). An advantage of using a Vintran pressure transducer is that the system need not be calibrated time and again by applying a known pressure input. A 22.72 in WCD output signal was produced by shunting an external resistance of 49.9 ka across the transducer and this wns used as the reference pressure drop.

The output signal from the transducer was coupled to a Hewlett-Packard low-level preamplifier Model 8803 A. This coupling enabled the fluctuations in pressure drop to

be directly displayed on a Hewlett-Packard strip chart recorder Model 7702 B. Ally [ 1 ] has shown how to check the calibration of 22.72 in WCD specified on the transducer with a manometer arrangement.

The humidity of the air was chosen as being the most important parameter to investigate the effect of electrostatics on the choking transition. The experiments were devised in such a manner as to vary only the humidity of the air while the solids flow rate was kept con&ant. Three different solids flow rates were examined: (i) 0.006 kg/s, (ii) 0.021 kg/s. (iii) 0.027 kg/s. The relative humidity of the air was kept at 16% for the low humidity runs (electrostatic forces dominant) and at 55% for the medium humidity runs (electrostatic forces extremely small). Ally 111 has sl~own that if the humid- ity of the air was such that. 0 ratio of more bhan 0.1 kg water per kg of solids is obtained, then little or no elcctrostutic charge was generated.

‘The air vclocily was slowly decreased while keeping the solids flow rute constiu~t.

At sulI?cicntly high uir velocities, the soli& were conveyed in dilute plrnsc! with low solids conccnt.rntion. Pnrticlc-wall iirter~Mhms in this phnse were sninll nnd the :unount of c!luirc(c pcncrntc,d wns also sinnll. Since nralting nntl hret~king of surface conMet was the prinx~ rquiremcnt for static elcctrificution. it was cspcctccl thnt the cluctrostntic forces would hc smnll in the dilute plinsc. As the nir velocity wns dccrcnscd. the pressure drop dccrcnscd hut the sali& canccntrntim in- creased. This cs~ltinucd until the prcssurc drop renclicd n minimum iu the trnnsition region where the frictiannl nnd stntic licnd contributions to tlic pressure drop were equnl. Wl~cii tlic gas velocity was dccrcascd further. the nmount of solids in the line increased rapidly and, since the static head pEXioilli-

nated over the frictionnl rcsistnncc, tllc prcssurc drop rose. In this dense phase mode of transport, particle-wall interactions were cstrcmcly rnpid nnd the electrostatic pressure drop was n major contributor to the total pressure drop.

The pressure drop was measured nt all such air velocities. The humidity of the air was then increased to nullify the electrostatic charging and the same procedure as described above was followed.

a3

RESULTS AND DISCUSSION

Eicctrostatic effect OIZ ckokirzg trarlsitiort As stated in the section on Experimental

apparatus and procedure, three different solids flow rates were esamined- The results are shown in the state diagrams, Figs. 2, 3 and 4. A state diagram shows the pressure drop against the average air velocity with the mass fiow rate as parameter. The effect of electrostatic forces can be clearly seen in the case of transport with low humidity air. In the dilute phase, where part.iclc-wall colli- sions were rclat.ivcly sparse, the electrostatic pressure drop was minor. However, as t.ho dense phase was approached, particle-wall illtcractiolls ~~ccninc n1Orc? inlport.nilt. and the clectrostutic: pressure drop rose. This rise in the electrostatic pressure clrol, caused the choking tr:msition to occur at a higher air velocity than it. did with air at high humidity. Table 1 shows the rise* in the velocit.its at min- inrum pressure drop wht*n Lhe air humidity was lowered.

.- ;

i

Fig. 2. State diagram of pressure drop/unit length LWSIIS nir velocity for solids flow rate of 0.006 kg/s.

I -

1 -

,- a

1 J

I

AS%- vssncsty. nrs.c

Fig. 3. State diagram of pressure drop/unit length LETSUS air velocity fur solids flow rate of 0.021 kg/s.

2 4 6 3 10 12

Fig. 4_ State diagram of pressure drop/unit length L*TS~LS air velocity for solids flow rate of 0.027 kgls.

pressure drop increases not only due to the increased solids inventory in the line but also due to the increased electrostatic forces. Transport with high humidity air results in a significant reduction in the electrostatic

TABLE 1

Increase in velocity at minimum pressure drop due to electrostatics

Solids flow rate Velocity at minimum pressure drop

(kg/s) (m/s)

Air at Air al R.H. 55% R-H. 16%

0.006 4.2 7.0 0.021 6.0 8.0 0.027 6-5 9.0

pressure drop and hence a lower air velocity at the choking transition.

The rise in the velocity at minimum pres- sure drop can be quite det.rimental to the optimal operating conditions in vertical transport. Vertical pneumatic conveying is generally carried out in the dilute phase regime. Operation in this regime requires low air flow rates from considerations of energy requirements, pipe erosion and particle attrition_ The minimum air flow rate at which solids can be conveyed in the dilute phase regime is set by the air velocity at minimum pressure drop in the state diagram. Air velocities lower than this would cause the solids to be conveyed in a possibly unstable slugging dense phase regime. Though the dense phase mode of transport has recently attracted attention due to its reduced gas flow rate, many workers recommend that dense phase flow be avoided due to the erratic nature of the flow, the pressure fluctuations, high pressure drops, and pipe line vibration. Sandy et al_ 1241 have shown that for the vertical transport of alumina particles the power requirement for dilute phase conveying was less than that for dense phase conveying by a factor of 10. However, the gas require- ment for the dilute phase transport was roughly 20 times greater than that for dense phase transport. Thus, the economic advan- tage lies with dilute phase conveying as long as air can be used as the conveying gas.

Rizk [25] has shown that it is possible to relate and transfer experimental data from the laboratory to large-scale plants by the use of characteristic numbers of similarity, the loading and the F’roude number at minimum pressure drop. The characteristic of a state diagram is the minimum which arises in the pressure drop. The connecting curve to all

Fig. 5. Pressure minimum diagram showing solids loading LXVSUS Froude number.

minima points yields the pressure minimum curve_ Figure 5 show-s the pressure minimum curve on a log-log scale in a dimensionless form, the loading LW-sus the Froude number. This curve is interpreted with the following approximation:

R = -$ Fr” (‘7)

Rizk has stated that the esponents 6 and s are dependent on the equivalent particle size and the pipe material_ But this study shows that the humidity of the air is also an im- portant parameter which influences the exponents 6 and x_

j+L 101.97

Fr2.51

for transport with air of R-H_ 55%

RCA- 104.96

&4.3”

(8)

for transport with air of R-H_ 16% In both cases, the pipe material and particle size were the same.

The importance of humidity in determining the parameters 6 and x cannot be overempha- sized_ The humidity of the air controls the electrostatic charging of particles in pneu-

matic transport_ Hence, it would seem that the problem can be resolved merely by ensuring that the humidit.y of the air be kept at a sufficiently high value to nullify the electrostatic forces_ But the answer is not as simple as that_ In most solid processing systems like direct combustion and polymer processing, dry transport gas is used which makes the presence of electrostatic forces inevitable. Thus, a more fundamental ap- proach is required. Electrostatic charging depends on both particle and wall charac- teristics One of the conditions for electro- statics to be present is that the chemical nature of the substances involved should be such that electron transfer is possible_ Physical conditions such as low humidities and un- grounded apparatus are also necessary. Thus, in designing a pneumatic conveying system, a knowledge of whether electrostatic forces would be present in the system and the magnitude of such electrostatic forces would be helpful in reducing energy requirements_

Fluctuations in pressure drop The fluctuations in pressure drop about the

mean pressure drop can be used to determine t.he onset of the slugging transition. As seen in Fig_ 6, the fluctuations increase with decreasing ve1ocit.y until a masimum is

a, , i

. . . 0 2 4 6 a 10 12 14 16

LIZ- r*3oc:ty. mfsec

Fig. 6. Pressure fluctuations in the choking region_

86

reached which corresponds to the slugging transition_ The fluctuations decrease after the maximum has been reached. This region corresponds to the dense phase mode of transport which is inherently unstable_ A slight decrease in air velocity at this point could lead to plugging of the solids in the line.

The fluctuations in pressure drop observed are quite similar to the pressure fluctuations observed by Yerushalmi and Cankurt [26] across a fluid bed. In their work, the transi- tion from bubbling to turbulent fluidization was characterized by an air velocity at which the pressure fluctuations peaked.

CONCLUSIONS

This work reveals the significance electrostatic forces in determining the

of chok-

ing transition_ As stated in the introduction, most investigators have studied choking in the absence of electrostatics. This was done by increasing the humidity of the air so as to nullify electrostatics [lo]- This study points to a number of conclusions.

Compared with gas-solid transport in the absence of electrostatics, the pressure drop at choking increases to a much higher extent with electrostatic forces present.

The velocity at minimum pressure drop increases in the presence of electrostatics_ This has the effect of increasing the optimal operating conditions and thus the power requirements for gas-solid transport_

The hmn%ityoftheZiiraiTdthep~ and wall characteristics have to be taken into consideration, in addition to the particle size, when transferring experimental data from the laboratory to large-scale plants on the basis of the pressure minimum curve.

pressure fluctuations in the choking region are much more violent in the presence of electrostatics_ The pressure fluctuations can indicate the onset of the slugging transition both in the presence and the absence of electrostatics_

In designing pneumatic conveying systems, care should be taken to minimize electrostatic effects, as these forces have an adverse effect on the optimal operating conditions_

LIST OF SYMBOLS

particle diameter, pm tube diameter, m particle friction factor at choking Froude number, U,/(gD,)“-5 gravitational acceleration, cm/s2 esponent in eqn. (6) solids loading, (kg solids/s)/(kg air/s) superficial gas velocity, m/s superficial gas velocity at choking, m/s terminal falling velocity of a particle, m/s exponent in eqn. (7) exponent in eqn (7) voidage density of solids, g/cm3

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M. R. Ally, Ph.D. Dissertation. University of Pitts- burgh, 1981. L. S. Leung and R. J_ Wiles. Znd. Eng_ Chem_ Process Design Dewtop.. 15 (1976) 652_ L. S. Leung, Proceedings of Pneumotransport 5. BHRA Fluid Engineering. London. Apn‘116 - I8. 1980.

4 F_ A. Zenz and D. F. Othmer. Ftuidization and Fluid-Particle Systems, Reinhold, New- York, 1960, pp- 318 - 319. Y. Yousf; and G. Gau, Chem. Eng. Sci. 29 (1974) 1939.

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