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Evans, J. F. M.S. Thesis. University of Kentucky, Lexington, KY, 1978. Gavrllova, A. A. Tr. Inst. ooryuch,Iskop. Moscow 1972, 28, 167. Hall, K. R.; Eagleton, L. C.; Acrlvos, A,; Vermeulen, T. Ind. Eng. Chem. Fundem. 1908, 5 , 212. Hasatmi. M.; Wen, C. Y. "Reacthmity of Iron Oxide for H2S Removal from Hot Low-Btu Gas"; Report to U.S. DOE, Washington, DC, Sept 1977. Jones, C. H.; Donohue,J. M. "Comparatlve Evakratbn of Hlgh and Low Tem Cycle Power perature Gas Cleaning for Coal Gasiflcatlon+Mned Systems"; Rept. EPRI-AF-416, Electric Power Research InstLute, Paio Alto, CA, Apr 1977. Joshi, D. K.; Leuenberget, E. L. "Hot Low BTU Producer Gas Desulfurizatbn In Fixed-Bed of Iron Oxide-Fly Ash"; US. DOE Report, Washlngton, DC, FE-2033-19, 1978. Kerr, R. K.; Paskall. H. G. Energy Procew. (Canada) 1978, 69(2) 38. Klemantaski, S. J. J. Iron Steel I n s t . 1952, 171, 176. Olsson, R. G.; Turkdogan. E. T. Metall. Trans. 1974, 5 , 21. Oldaker, E. C.; Poston, A. M., Jr.; Farrlor. W. L., Jr. "Removal of Hydrogen Sulfide from Hot Low-BTU Gas with Iron OxMe-Fly Ash Sorbents";
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Received for reuiew November 17, 1980 Accepted December 28, 1981
Pressure Drop Losses Due to Etectrostatk Generation in Pneumatic Transport Eugene E.
Smettzer,' Mark L. Weaver,? and George E. Kllnzlng'
Chemical and Petroleum Enginwring Department, Unlversity of Pittsburgh, Pittsburgh, Pennsylvania 1526 1
The pressure losses in a dilute phase transport of solids by a gas has been measured experimentally with the presence of electrostatics. Low relative humidities and differences In surface properties of the solis and the conveying tube wall caused a sizeable electrostatic effect up to 70% Increase in the overall pressure drop. A modified dilute phase transfer theory of Yang was developed to interpret the electrostatic effect. The increased pressure losses are seen with Increased solids loadlng, decreasing relative humidity of the gas stream, and decreasing particle diameters.
Introduction Over several decades electrostatics in pneumatics conveying systems has been recognized mainly as a problem. Potential hazards as well as increased energy losses have been noted. Usefulness of such a phenomenon has been sparse except for Vollrath's (Vollrath, 1932) power generation proposal and King's (King, 1973) electrostatic flow meter. All in all the presence of electrostatics in pneumatic system as well as other situations has had the aura of some kind of black magic and great measures have been taken to eliminate the effect. The most common technique for reducing electrostatics in a flow system has been to humidify the transport gas to relative humidities of 75% or greater. Theory In the transport of solids by a gas stream the effect of electrostatic forces is seen by the macroscopic increase in the energy requirement to move the solids. The overall pressure drop in such a flow system can be substantially increased over situations where electrostatic forces do not exist. Analysis of these effects can be made by modification of the overall momentum balance on the system. Considering the unified analysis of Yang (1977) for a dilute system of solids in a gas, one has the balance of forces as Westinghouse Rssearch and Development Corp., 1340 Beulah Road, Pittsburgh, PA 15235. 2Gulf Research and Development Corp., P.O. Drawer 2038, Pittsburgh, PA 15230. 0198-4305/82/1121-0390$01.25/0
In this expression d F d is the drag force, dF, is the gravitational expression, dFf is the frictional term, and the new term dF, accounts for the electrostatic force. The term Am, is the differential element of mass of particles on which the forces act. These preceding terms can be expanded to give
dF, = gAm,
(3) (4)
Me= E,qAm,
(5)
For a steady state, the first term in eq 1 is zero and the balance reduces to yield the velocity of the particle as
Up = Uf - Ut[
(( + -) 1
fPUpa
+
112
?)e4.7]
(6)
for vertical flow. The term &q represents the product of the electric field in the flow direction times the charge on the particle. In considering the overall pressure drop for the gas and solid phases, one finds 0 1982 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 3, 1982 301
With a knowledge of the term E,q, one can predict a priori the overall effect of electrostatics on the flow. Note that fp
= 4fw
Past and recent data taken in our laboratory permits us to assess the contribution of electrostatic forces to the energy requirements for pneumatic transfer under certain fixed conditions which are well identified. In order to utilize this equation, a few of the terms must be evaluated. For the dilute phase transfer considered here calculations of the particle velocity was determined by the Yang implicit expression and a modified Hinkle form given by IGT. The results of the particle velocity by these two techniques varied less than 2 % , Thus for simplicity, the correlation given by IGT in a modified Hinkle (Institute of Gas Technology, 1978) format was used to evaluate the particle velocity
I air f l l t e l
a n d dryer
Up = Uo(l - 0 . 6 8 D , 0 . 9 2 p , 0 . 5 p f ~ . 2 D ~ ' ~(8) ) It should be noted that in this equation the constant (0.68) has units in agreement with the SI system. One can also note that for fine particles, less than 40 pm in diameter, the particle velocity can be expressed as the difference between the gas velocity and the terminal velocity of the particle. Little difference was seen for the particle velocity of the 25-pm diameter particles determined in this manner and those found from eq 8. The voidage, e , can then be evaluated with 6
= 1 - [4W,/(PP - Pf)TD2U,1
(9)
The fluid velocity is given by u, = U,/t The solid friction factor can be calculated from Yang (1977) as
where the terminal velocity, Ut is given by
u, =
0.1530,'~'4$.71(p, - ~ 3 " ~ ' (12) /.4°.43PP29
for (Re), = DpUppf/pand 2.0 < (Re), < 1000. By experimentally measuring the pressure drop of the gas alone at several velocities, an analysis can be performed to obtain the functional relationship between the pressure drop and velocity. It is important that this term be evaluated as accurately as possible, since it is the major contribution to the total pressure drop under dilute phase transport conditions.
Experimental Arrangement The experimental flow system used to study the overall energy losses as measured by the pressure drops is shown in Figure 1. The transport air is supplied by an air blower capable of 9.2 X 10-2m3/sof air flow. The air is brought to the desired humidity value in the humidity control chamber. This chamber consists of two parallel sections, one for humidifying with packed damp sponges and the other for dehumidifying with packed Drierite. A hygrometer measured the final humidity. An absolute filter by MSA removes any particulate matter from the flow stream before entering the flow rotameter and test section. Air
Figure 1. Experimental apparatus.
flow is measured through a rotameter whose maximum reading is 1.98 X 10-2m3/s. The solids are fed to the system by a calibrated belt feeder enclosed in a chamber which is slightly pressurized to 34 to 69 kN/m2 in order to obtain a uniform solid flow from the belt feeder to the test section. Solid to gas feed rates from 0.01 to 1.0 have been investigated in this study. Grounded copper tubes, the same diameters as the test section (0.0254 m), are provided both before and after the test section to eliminate the electrical effects before and after the test section. A Whitby ion gun immediately before entrance to the test section provided charge equilibrium for the particles. This ion gun is fed with a low flow rate of nitrogen gas and loo00 V across the charging gap. The entrance and exit effects are also reduced by these copper sections. A 1.5-m development section before the test section provided adequate length to eliminate the entrance effect. This length agrees will with recent experimental work of Shimizu et al. (1978). In addition, studies in our laboratory have shown that the pressure drop per unit length over the test section (3.0 m) is constant, indicating that no acceleration is present. Pressure taps flush with the Plexiglas test section tube wall are packed loosely with glass wool to prevent solids contamination with the pressure transducer. After passing through the test section the particles are removed from the air stream by a cyclone and MSA absolute filter. After each run any accumulative charge on the particles is removed from the solids by contacting them with a grounded copper rod. The pressure drop measurements were taken by use of a micromanometer in the case of the Silastic tube test section. The micromanometer is able to detect pressure change to k2.5 N/m2. A Viatran differential pressure transducer was employed for the Plexiglas tube test section. The transducer has a linear response with a sensitivity of k 2 . 5 N/m2. In running the experimental tests, flow is begun at high humidities to establish the base point for nonelectrostatic
392 Ind. Eng. Chem. Process Des. Dev., Vd. 21, No. 3, 1982
READ
0
0 05
%
QATE %G/SLC
x 1000
Figure 3. ElectrostaticAP vs. bead flow rate for R.H.= 65%: (0) beads 75 pm; (0) beads 150 pm.
Figure 2. Excess pressure drop vs. bead flow rate: 25-pm particles; low humidity -25%.
effects and then the humidity is lowered to obtain the electrwtaticinfluence. A maximum pressure drop increase of 70% i s attributable to the electrostatic contribution. The overall test arrangement has the flexibility of interchanging test sections and bead types. The present analysis is for the case of glass beads with a diameter of 25,75, and 150 pm.
Analysis To test the theory developed, the data of Peters (1971) were first analyzed. He had investigated the combined effects of solids addition and a compliant wall to obtain drag reduction in a vertical flowing air stream. The test section used for this study is similar to that shown in Figure 1. During his tests of transferring 25pm nominal diameter glass beads through a 0.025-m i.d. Silastic tube, -2.1 m long, the presence of electrostaticswas noted when low humidity air was used as the transporting gas. Using Peters' data for the pressure drops of air only through the Silastic tube at varying velocities, a relationship for the pressure drop with an average error of about 2% was found. A *TIL was calculatedfor all the teats conducted for low and high humidity runs for 25-pm beads with the electrostatic effect being seen for the low humidity cases (
