ieee transactions on magnetics, vol. mag-14, no. 5, september 1978 ...

407

IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-14, NO. 5, SEPTEMBER 1978 APPLICATION AND MODELING OF HIGH

GRADIENT

MAGNETIC

FILTRATION IN A PARTICULATE/GAS

SYSTEM

C. H. Gooding,* T. W. Sigmon," L. K. Monteith,** D. C. Drehmel*** ABSTRACT

A theoretical and experimental investigation was fine particles efficiently at a high gas velocity, a conducted to assess the potential use of high gradientlow pressure drop, and a low magnetic field if it is to be competitive with other control methods. magnetic filtration as a method of collecting particulate matter from industrial stack gases. Preliminary Early in this study bench-scale experiments were calculations and bench-scale experiments were utilized to demonstrate the ability of HGMS to collect fine gas- conducted with dispersed dust from a basic oxygen borne particles andto provide guidelines for the de- steelmaking furnace (BOF). The dusty air stream was passed through a loosely packed steel wool matrix in sign of a pilot-scale unit. An experimental program was then conducted in the pilot plant using dispersedthe bore of a solenoid. Figure 1 shows the typical dramatic reductionin particle penetration through the dusts obtained from two steel industry processes. A matrix that was achieved with the required low magnetic theoretical particle trajectory model was used to analyze the results and to study the effects of parti- field, high gas velocity, and low matrix pressure drop. cle characteristics and system operating parameters. The preliminary experiments indicated that the HGMS The results indicate that high-efficiency collection process could be used to collect gas-borne particles and provided data from which to design a larger unit of fine particles can be achieved with operating conditions that project to reasonable full-scale capital and a more systematic experiment. costs and power requirements. INTRODUCTION Interest in the application of high gradient magnetic separation (HGMS) has grown rapidly since it achieved commercial acceptance in the clay industry. Numerous experimental investigations have been conducted to improve the fundamental understanding of the particle capture process and to evaluate potential applications in liquid process systems. Mathematical models have been developed by several research groups using a theoretical particle-trajectory calculation with physical assumptions directed toward particular applications. Well over 100 reports, patents, and technical papers dealing with various aspects of the HGMS process have been published in the last decade. The unique objective of this work towas evaluate the potential utilization of HGMS as a method of removing fine particles from selected industrial stack gases.l Conventional methods of industrial particulate emission control include electrostatic precipitation, wet scrubbing, and fabric filtration. HGMS -0 B 0.2 0.4 .0.6 0.8 P . 5 offers the opportunity of combining magnetic forces PARTICLE DIAMETER, prn with conventional filtration phenomena to enhance the collection process and to reduce the cost of particuFig. 1. Bench-scale collection of B9F dust with a late emission control. velocity of 8 . 4 m/s and a pressure drop 1.7 of kPa. PRELIMINARY WORK PILOT-SCALE EXPERIMENTS The competitive methodsof particulate emission The layout of the pilot-scale HGMS system is control delineate practical constraints on the design of a HGMS device for stack gas applications. Technol- depicted schematically in Fig.2. Dusts from a BOF as well asan electric arc steelmaking furnace (EAF) were ogy is currently available to control particulate dispersed into a wind tunnel, and 0 . 8 am3/s slipstream emissions from most industrial sources with a capital was drawn off and processed through the HGMS pilot investment of less than $8500/m3/s (uninstalled plant. A low-efficiency cyclone, upstream of the HGMS, primary equipment cost/gas flow capacity). Power removed uncharacteristically large agglomerates that consumption is normally less than 3.2 kW/m3/sin were not adequately broken up by the dispersion system. precipitators and fabric filters, but can be many times The magnetic separator consisted of 0.30-m a diameter greater in difficult applications where high-energy wet of loosely scrubbers must be used. From these general constraints iron-bound solenoid surrounding a canister and the cost and power requirements of large magnets, packed 430 stainless steel wool. Ranges of the experiit is evident that HGMS must be capable of collecting mental operating parameters are givenin Table I. The experiments were systematically designed s o that effects of individual parameters could be studied. Manuscript received March 1 4 , 1978. After operating conditions were established for a * Research Triangle Institute, Research Triangle Park, particular run, the fractional penetration of dust N.C. 27709 particles through theHGMS wasdetermined as a function ** North Carolina State University, Raleigh, N.C. of particle size by using cascade inertial impactors to 27607. *** Industrial Environmental Research Laboratory,U. S. measure the concentration and size distribution of the dust upstr-am and downstream of the matrix. Since the Environmental Protection Agency, Research Triangle impactors required approximately90 minutes to collect Park, N.C. 27711. an adequate sample, an optical (light-scattering) 0018-9464/78/0900-0407$00.75; 01978 IEEE

408 particle sizing device was used to ensure that no significant upsetsor transients occurred during the sampling period. TABLE I.

APPLIEDFIELD.

2500

-

-

0.# I

T

0.4

0.3

0.2

RANGES OF OPERATING PARAMETERS IN HGMS EXPERIMENTS

Applied Field Matrix Packing Density Matrix Length Superficial Gas Velocity Gas Temperature

0-0.4

T 0.005-0.010 0.15-0.30 m 5.2-11.1 m/s 25-1100 C

A run of several hours duration was made with each dust using the optical particle counter to study the transient matrix-loading characteristics. Theoretical arguments and experimental observations of paramagnetic particle capture in liquid systems have shown that collection efficiency decreases as the IO collected articles fill the regionof high field APPLIED FIELD, A/m x gradients.$33 In the first loading experiment the Fig. 3 . Magnetization curves of dust samples. penetration of BOF particles did not change significantly from a clean-matrix condition even though the To analyze the experimental data, Watson's pubmatrix collected twice its own mass in particulates. lished results were used to locate several Rc contours T h i s behavior was apparently due to the relatively at low values of K with appropriate values of A Gand high permeabilityof the dust particles (Fig.3 ) , which = 0. These contours were then extrapolated to larger would tend to extend the region of significant field K using the curvature of the lines in Fig. 4 as a gradients outward as the wire is coated. 'The penetration of EAF particles was less stable and showed anguide. From the extrapolated contours, approximate 4 increase of 50 to 100 percent over the clean-matrix penetration under equivalent loading conditions. c

To BAGHOUSE

F40M

WINDTUNNEL

-BAGHOUSE

HGMS

SAMPLE

CYCLONE

b--

1

AIR

I

+

WASTE

Fig. 2.

Schematic represention of BGMS system. ANALYSIS OF RESULTS

-2

-I

0

I

2

3

log K

Fig. 4 . Dependence 0 1 dimensionless sing1.e--fiber The experimental data were analyzed with the aid of theoretical results previously obtained by Watson475 capture radius on system parameters.6 and Lawson, e t . 6 , Watson mathematically determind the trajectoriesof spherical paramagnetic parti- values of Rc were determined for all the of expericles under the influence of magnetic and viscous mental data. By conventional arguments the singleforces near a cylindrical ferromagnetic in wire a homofiber capture radii were then extended to a differengeneous applied field. Lawson conducted a more comtial layer of hatrix and integrated to yield preplete analysis including inertial and gravitational dictions of total-matrix penetration in the form forces. His thesis results include a generalized graphical presentation of the dimensionless singleP = exp[-EFLR,/a(l-F)]. (1) fiber capture radius, Rc, in terms of four dimensionless groups (Fig. 4 ) . Lawson's results are identical An empirical "effectiveness factor," E, was retained to to those of Watson for low values of K where inertial allow for deviations from the idealized assumptions cnncerning matl-ix and particle characteristicsand effects are insignificant. The CurvatUTe of t h e l i n e s in Fig. 4 is a result of the inertial effects. With interactions. Reasonable assumptions and geometric the experimental conditions used in this work, G arguments predict the value of E to be 4 / r 2 (or 0 . 4 1 ) , approaches zero,A is estimatedto range from 1.5to but the BOF and EAF data were better fit by lower E 2 . 4 , log W ranges from approximately -2.2 to - 0 . 6 , values of 0 . 0 9 and 0.07, respectively, implying some and log K ranges from approximately-1.0 to 3.0. It is loss of effectiveness dueto non-idealities. The lover obvious from Fig.4 that inertial effects are important E for EAF dust implies that matrix loading effects may for nearly all of the experimental data. Inertial also have been significant for that dust. Figure 5 effects are beneficial to particle collection for Rc shows typical experimental data and predicted curves values less than1.0 but detrimental for Rc values for the two dusts at identical operating conditions. greater than 1.0.

409

magnets) of 3.2 kW/m3/s. FXF dust collection would be more expensive and would consume more power. The theoretical model provides a valuable tool to screen potential applications, evaluate alternative system designs, plan experiments, analyze data, and conduct economic analyses. Additional pilot plant data are being obtained, and theoretical work is underway to verify the Rc contour extrapolations and to investigate other theoretical effects. A mobile pilot plant is being designed so that experiments can be conducted at industrial sites. NOMENCLATURE [The implied magnetic relationships assume usage of (SI).] the International System of Units

a A b Bo 0

OK8

a1

I

0.2

3

I

0.5

1.0

2

5

PARTICLE DIAMETER, !dm

Fig. 5. Experimental data and theoretical prediction -of dust collectionby HGMS.

E F g G HO K

L

M The effects of individual parameters on particle P collection were found to be in reasonable quantitative RC agreement with the mathematical model. The particle V size and magnetic permeability dependence are illus- W trated in Fig. 5. In other runs at lower fields and higher velocities,the penetration of larger particles rl tended to be greater than the theoretical predictions,UO P indicating that the extrapolated Rc contours may underestimate detrimental inertial effects. Velocity had a P f relatively small effect on the collection of fine parti-X cles; higher velocities may actually be beneficial to particle collectionin the highK region where Rc contours less than1.0 have large negative slopes. 1

matrix wire radius dimensionless group,M/(2Ho) particle radius flux density of applied field effectiveness factor matrix packing density acceleration of gravity dimensionless group, ag(l-pf/p)/VZ strength of applied field dimensionless group, 2b2pV/(9ar1) matrix length magnetization of wire fractional penetration of particles through matrix dimensionless capture radius of wire superficial fluid velocity dimensionless group,~.loxHZ/[ (1+x/3)pV2] dynamic viscosityof fluid permeability of free space particle density fluid density magnetic susceptibilityof particle REFERENCES

Gooding, C. H., T. W. Sigmon, and L. K. Monteith, Application of High Gradient Magnetic Separation to Fine Particle Control, EPA-600/2-77-230 (NTIS No. PB 276633/AS), November 1977. Luborsky, F. E. and B. J. Drummond, "High Gradient Magnetic Separation: Theory Versus Experiment," IEEE Trans. Magn., Vol. MAG-11(6), pp. 1696-1700, November 1975. Cowen, C., and F. J. Friedlaender, "Single Wire Model of High Gradient Magnetic Separation Processes 111," IEEE Trans. Magn., Vol. MAG-13(5), pp. 1483-1485, September1977. Watson, J. H. P., "Magnetic Filtration," J. Appl. Vol. 44(9), pp. 4209-4213, September 1973. Watson, J. H. P . , "Theory of Capture of Particles 5. in Magnetic High-Intensity Filters," IEEE Trans. Increasing the operating temperature could adVol. MAG-11(5), pp. 1597-1599, September versely affect particle collection since the gas vis1975. cosity would be increased and magnetization of the Lawson, W. F., The Dynamics of Paramagnetic 6. particles and matrix may be diminished, no butsignifiParticles Near a Magnetized Wire, M.S. Thesis, cant effectof temperature could be discerned from four Dept. of Physics, West Virginia University, 1976. experimental runs that were made at the higher tempernLawson, W. F., W. H. Simons, and R. P. Treat, "The 7. ture. Dynamics of a Particle Attracted by a Magnetized Wire," J. Appl. Phys.,Vol. 48(8),- pp. 3213-3224, CONCLUSIONS AND CONTINUING WORK August 1977. Higher magnetic fields enhance the collection of particles, but the effect is diminished as both the particles and matrix approach saturation. With dusts 2. exhibiting magnetic properties similar to those shown in Fig. 3, collection efficiency can probably be improved more economically by increasing the density or length of the collection matrix rather than increasing 3. the applied field beyond0.4 to 0.5 T. Experimental data confirmed the effects of matrix density and length expressed in Equation(1) and showed that collection efficiences greater than 99 percent at 1the pm parti4. cle size can be achieved with operating parameters within the ranges given in Table I.

Phys.

w.,

The results of this investigation indicate that HGMS may be an efficient and economical method of particulate emission control in selected applications where relatively high permeability dust must be .collected. Based on currently available HGMS equipment, projections for a full-scale, high-efficiency BOF dust collection device predict an uninstalled capital cost of $8200/m3/s and power requirements (for fans and