demand for efficient, continuous methods of Increasing drymg . . filter cakes, sludges, and slurries with high solids
Heavy Paste Dispersion Drying System -.
S. J. BARAN
A new& designed nozzle f o r dying heavypastes, sludges, and slurries substantial& increases the capacities of conventional dying systems and offers better control o f the product quality
contents has resulted in the widely accepted spray and dispersion drying techniques. Virtual elimination of product losses and contamination, inherent in the older batch and semicontinuousmethods, has been particularly valuable in the pigments and chemicals industries. The successful operation of a dispersion or spray drying system depends principally on the degree and uniformity of atomization. General methods of atomization presently in use include pressure nozzles, pneumatic nozzles, and centrifugal, disk atomizers. Pressure nozzles, which produce atomization by forcing the feed through a n orifice under pressures up to 8000 p.s.i., are limited to feeds of low viscosity which exhibit neither thixotropic nor dilatant characteristics. Capacity of a single nozzle is seldom more than 2 gal./ min., with greater capacities being obtained through operation of several nozzles in parallel Multiple nozzle systems usually require duplicate pumping units and are not suitable for pastes where excessive pressures are required or there is a marked tendency to clog the orifice. Pneumatic, or two-fluid, nozzles achieve atomization by internal mixing of the feed with a second fluid, usually steam or air. Ultimate mixing and subsequent break-up of the feed occurs immediately after the feed leaves the nozzle. Although pneumatic nozzles are adaptable to a much broader range of feed characteristics than pressure nozzles, particle size is difficult to control and high steam or air rates are necessary for atomization in the 10- to 20-micron range. In addition, pneumatic nozzles suffer the disadvantages of limited capacity and the necessity for a second fluid.
Fagwe STEAM OR AIR LINE
-PNEUMATIC FLOW INDlCAllNG TRANSMIIIER HEAVY PASTE NOZZLE
-RATIO RECORDING CONROLLER
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I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
r
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Centrifugal atomization involves coating the surface of a high speed, rotating disk with the feed which is accelerated to the peripheral velocity. The feed leaves the periphery of the disk in a thin sheet which breaks up into fine droplets upon striking the air. 'Bisk atomizers are adaptable to feeds with viscosities of 100,000 centipoises but often have high power requirements at useful angular velocities. In large capacity systems, disk design becomes very complex and costly if the usual difficulties of overloading and the resultant erratic atomization patterns are to be avoided. Disk atomization is generally used for a wide range of feed viscosities and feed types including solutions, emulsions, dispersions, and slurries. T o overcome the natural limitations of existing methods of atomization, a heavy paste dispersion nozzle has been designed and successfully incorporated into a drying system which functions efficiently with feeds having viscosities above 100,000 centipoises and without the necessity of reducing viscosity either through dilution or deflocculation. Materials successfully treated thus far include titanium dioxide, calcium carbonate, clays, stearates, corn starch, and organic and inorganic pigments. The new nozzle contains a central core thus requiring the paste to exit through a n annular region. We shall refer to more advanced, coreless nozzles subsequently. Advanced coreless nozzles are still under development.
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'
/I
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Figure 2 Basic Nozzle Design and Operating Characteristics
The heavy paste nozzle is of the pneumatic type using either air or steam as the second fluid. The unique feature of the nozzle is a series of internal expansion
t
Figure 1. Automated drying systems incorporating the heauy parte nozzle. The primary control ekment is the or;f.. plate i n the stream or air lim Figwe 2. Asmnbkd, exterior oiew of thi two-fluid, pneumatic heavy parte nozzle. O w - a l l diminsionr for the 8000 lb./hr. prototype are dimnetcr of 9 inches, and height 20 inches Figure 3. Cut-away view of the hemy parte nozzle showing the prernin'ng chamber and the annular conuerging-diwging section which completes the mixing of the gasre and secondary fluid
,J, Baran i s Product M a n a p for Proctor and Schwortz, Inc., Philadelphia, P a . AUTHOR S.
on STEAM
igura 3
VOL. 5 6
NO. 1 0 O C T O B E R 1 9 6 4
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stages where complete mixing of the feed and secondary fluid occurs prior to atomization. I n tests conducted thus far, uniform particle sizes down to 4 microns have been obtained. The essential features of the new nozzle are shown in Figure 3. The nozzle may be quickly disassembled for maintenance and the surfaces subject to wear may be easily replaced. The rated capacity of the prototype is 8000 lb./hr. with a maximum capacity approaching 10,000 lb./hr. , Unlike conventional pneumatic nozzles requiring a feed-to-air or -steam ratio of 0.5 to 1, the heavy paste nozzle operates efficiently at ratios of 8 to 1 for air and 3 to 1 for steam over a broad range of capacities. For optimum operation, secondary fluid pressures of about 100 p s i . are preferred with the feed pressure being maintained at +20 p.s.i. of the secondary fluid pressure, measured at the inlet to the nozzle. A unique advantage of the heavy paste nozzle is its adaptability to scale-up in ratios up to 10 to 1. The basic standard nozzle with a normal capacity of 750 lb.,/hr. has been scaled up to a capacity exceeding 8000 lb./hr. and placed in commercial use. Current designs call for capacities exceeding 10,000 Ib.,/hr. A comparison of the heavy paste nozzle with corresponding conventional nozzles is shown in Table I. A recently developed modification of the heavy paste nozzle incorporates a coreless design which allows even greater capacity to size ratios with basically the same economy of operation enjoyed with the core-type nozzles. The coreless nozzles are intended for capacities under 2000 lb. feed/hr. The use of coreless nozzles also opens other fields of application for heavy paste drying systems. Heavy Paste Dispersion Drying System
The potential of the heavy paste nozzle can be best exploited by combination with an automated drying system. Such a system is shown in Figure 1, indicating the instrumentation necessary for automated control. Allowing for the normal fluctuations of plant steam or air pressure, a constant ratio of feed to atomizing medium is maintained by regulating solids flow rate with the flow rate of the air or steam. The primary control element is an orifice meter in the steam or air line which actuates the variable-speed transmission driving the solids feed pump through a ratio controller. A self-cleaning strainer in the solids line ensures uninterrupted nozzle TABLE I.
COMPARISON OF NOZZLE CHARACTER ISTICS
performance and there is also provision for premixing of the feed to ensure homogeneity of the solids feed. The dryer is of conventional design discharging into a total bag collector which delivers the product to either intermediate storage or to packaging equipment. The dryer is equipped with safety purges and flame-failure controls as well as with temperature controls. Nozzle instrumentation is independent of the dryer operation and the inlet dryer temperature varies in response to the drying load. Economics
Since the cost of spray or dispersion drying per pound of product is an exponential function of the water content of the feed, incorporation of the heavy paste nozzle in the conventional drying system permits substantial reduction in the operating costs. Further cost reduction results from the possibility of mechanically dewatering the feed before drying. The process is continuous with low maintenance costs and automation requires only a single operator regardless of the size of the installation, thereby minimizing labor costs. Since minimum particle size and maximum temperature differential are realized, efficiencies of 1800 to 2200 B.t.u. per pound of water evaporated are typical. Use of the heavy paste nozzle eliminates the need for subsequent grinding of the product and the narrow particle size distribution allows close control of the product moisture content and outlet humidities. Nozzle wear is nominal and, in cases of extreme abrasion, special materials may be used on those components bearing the burden of solids flow Nozzle maintenance costs range from 0.002 to 0.01 cent per pound of product. Operating costs of a typical system are shown in Table 11. T h e data assume the feed to contain 50% solids and exclude capital investment and depreciation. Conclusions
Although the heavy paste nozzles are primarily intended for high-solids slurries, feeds with solids contents as low as 157, have been profitably handled. Installations employing the heavy paste nozzles have ranged in capacity from 300 lb./hr. to over 15,000 lb./hr. Additional installations are currently under consideration for the titanium dioxide and food industries. TABLE II. OPERATING COSTS OF A TYPICAL DRYER5
Item
36
I
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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$/ib.firoduct
1
2.50
0.00312
Fuel ($0.80/mm./B.t.u.l
!
1 .28
0.001 60
0.10
0.000125 0,000237
Nozzle replacementb
1
0.19 0.08
Tota! costs
1
$4.15
Power
I
.$/hr.
tabor i@s$2.50/hr.l Atomizing air
min
~
I
0.00010 -
$0,005182
a Based on an evafiorati~tecapacity of 800 Ibs.)hr. operatzng hr. and 140 coating costs.
Based on 500
