TECHNOLOGY
High-capacity units widen centrifuge use Machine size alone now makes some applications feasible, lowers installation and maintenance costs for others A widening world of applications, half of them coming into existence in just the past five years, is bringing new prominence to centrifuges. Not the least of the factors responsible— though it's only one of several—is the development of high-capacity, high-G force machines. Like most process equipment, centrifuges have been feeling demands for higher operating temperatures, higher operating pressures, and higher capacities. Applications that require these conditions, either singly or in combination, have opened up to centrifuges as machines that meet them have come along. Ability to handle higher temperatures-up to 250° F. for standard designs, 350° F. for special ones—has meant, for example, that centrifuges can now be considered for materials that would have been too viscous at previous operating limits. And with designs to handle pressures up to 150 p.s.i.g., centrifuges can now be used to handle materials that would foam or vaporize under atmospheric pressure conditions. While temperature- and pressurehandling developments have thus grappled with process conditions, high-capacity developments have made a direct assault on economics. In areas where centrifuges are already being used, high-capacity machines are more economical for new plants or added capacity. In others, the alternative of large batteries of smaller, older machines had been too expensive to make centrifuges feasible. But by catching up to the size of the streams in some industries—pulp and paper plant waste streams, for example—centrifuges are now finding a new niche. Recent market. High-capacity, high-G centrifuges have come on the market only within about the past seven years. The three major U.S. makers are Sharpies-Equipment Division of Pennsalt Chemicals (Philadelphia), De Laval Separator (Poughkeepsie, N.Y.), and Dorr-Oliver (Stamford, Conn.). In addition, Westfalia machines from Germany are sold in the U.S. by Centrico, Inc. (Englewood, N.J.). Among the high-capacity models available are Sharpies' DH-5, De 46 C&EN JUNE 20, 1966
Laval's QX-412, Dorr-Oliver's Merco H-30, and the Westfalia SIG-15007. The DH-5 develops up to 6500 G with a bowl inside diameter of about 25 inches. The QX-412 has a bowl inside diameter of about 24 inches and develops 7300 G. Bowl diameter and force for the Merco H-30 are about 30 inches and 4600 G, and for the SIG15007 about 25 inches and 7200 G. All have a maximum capacity of about 500 gallons a minute.
The three U.S. machines are of the nozzle bowl type and can be used as liquid/liquid separators with or without continuous discharge of solids through peripheral nozzles. They can also be used as clarifiers for single liquid streams or as sludge concentrators. The SIG-15007 is also a nozzle bowl machine but is designed for clarification and solids concentration. It's the combination of size and G force that actually distinguishes the
Light phase Heavy phase
Solids (centrifuge may have discharge nozzles or ports for solids)
Feed
High-capacity centrifuges:
t h e principle
Disk centrifuges rely on a stack of conical disks, spaced about V20 inch apart, as an aid in gaining separation efficiency. The disks separate feed into strata, which greatly reduces the distance a particle must travel before it reaches a solid surface. Feed enters the bottom or top of the centrifuge, depending on how the unit is supported and driven, and is distributed through channels formed by matching holes in the assembled stack. Light phase moves toward the center of the stack and flows up and out a discharge. Heavy phase moves to the wall of the bowl where it is displaced upward and out another discharge. Solids, if they are present in the particular operation, impinge on the underside of the disks and move outward to the wall. The disks generally form an angle of 35° to 45°—enough so that the solids will slide along the surface as a result of the difference between their density and that of the liquid. In a continuous machine, the solids discharge through nozzles located around the periphery of the bowl. In this case, the bowl is designed with a wall that slopes toward the zone containing the nozzles
high-capacity machines. Other industrial types develop much higher forces. Tubular centrifuges, for example, can reach 13,000 G, but their bowl diameters are limited to 4 to 5 inches. Because of the small diameters, flow rates in tubular units are relatively low—generally 2 to 16 gallons per minute. As a result, all of the high-capacity centrifuges are of the disk design, since this gives the best combination of capacity and separation. In this design a stack of conical disks breaks the liquid up into strata. This reduces the sedimentation distance that a particle must travel before it reaches a solid surface. Centrifuge manufacturers didn't jump in one step from the small tubular units to the high-capacity machines, but went through an intermediate stage—what are now termed medium-sized units. These units,
High-capacity centrifuges:
with bowl diameters of 15 to 20 inches, have capacities three to four times greater than those of tubulars. The current large machines, however, have four to five times the capacity of a medium-sized unit. Development of the high-capacity centrifuges had to await not only industry need and demand but metallurgical and mechanical developments as well. Stress, stability, and power consumption are all critical factors in centrifuge design. Probably the most critical factor, and the limiting one, is the stress that develops in the bowl wall. For the large-diameter, high-force machines, it becomes quite high. In tubular centrifuges, for example, hoop stresses can run 15,000 to 20,000 p.s.i. In the disk machines, however, stresses can reach as high as 45,000 p.s.i. As a result, bowls in the high-capacity machines require high-strength al-
t h e operation
A typical example of high-capacity centrifuge application is this crude soybeanoil refining operation. The high-capacity machine (right) is a De Laval Model SRG-214 with a 24-inch bowl. It takes little more floor space than the smaller disk unit (left) installed about five years earlier, but it semirefines 7.5 tons of oil an hour, compared to 2.3 tons for the smaller unit. Both centrifuges are hermetically sealed to improve recovery and because pressurized operation permits a closed, continuous flow process. At atmospheric pressure, De Laval points out, the centrifuging operation produces a residue of gums and waxes that is 60% insoluble in acetone (Al), but at 80 p.s.i. gives a residue 75% Al. Specific gravities of the light and heavy phases of the unrefined product are 0.92 and 1.07. The position of the interface in the bowl is held constant in each centrifuge by varying the back pressure on the light phase outlet. The adjustment, De Laval says, can be made with the units running at full capacity. The machines are cleaned thoroughly by hand each week but require a shutdown of only two hours for each
loys that won't crack or stretch under the high stress. Stainless steels such as high-strength 316, 329, or 431 are commonly used. The general profile of high-capacity machines arises from the design approach to stability, which is especially important at high speeds. As the ratio of bowl length to diameter approaches 1, a centrifuge becomes more unstable. Tubular centrifuges are designed to be well above 1, with length-to-diameter ratios generally ranging from 4 to 8. The approach with disk machines, however, is to have the diameter larger than the height to keep the ratio below 1. Bearings are also a critical factor. Those used for high-capacity machines are generally a very select group from a bearing manufacturer's production. Increasing power. Power consumption is still another factor, and one which is becoming more important, particularly with the number of pressure and vacuum operations increasing. Power increases in proportion to the cube of surface speed and the square of diameter. Thus, there is a design and application limit imposed by the need to keep power input at reasonable levels. Overheating can become significant in a pressure operation where the atmosphere is dense. High-capacity machines have had a significant effect on the economics of centrifugal separation. Equipment, installation, maintenance, and operating costs are all generally lower than for equivalent overall capacity with batteries of small machines. And less floor space is required. De Laval estimates that for equal capacity, overall cost of a high-capacity centrifuge installation is a third to a half that of a battery of smaller machines. In a phosphoric acid application, for example, two high-capacity machines would cost less than $100,000 installed. With 60 tubular centrifuges to handle the same capacity, cost of the machines alone would be about $250,000. Installation would push this up to well over $300,000. Considerable savings also come about from auxiliary equipment and maintenance. There is less plumbing, fewer motors to take care of, and few centrifuges to service. Sharpies, De Laval, and Dorr-Oliver all point out that because of this, replacement would be economically sound in many cases. One company, for example, reduced its maintenance costs by more than $20,000 per year by replacing a battery of small units with several high-capacity machines. High-capacity centrifuges are now being used in a continually growing number of applications: separating wash water from fats and vegetable or fish oils and removing fatty acid in JUNE 20, 1966 C&EN
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caustic refining of various vegetable oils; acid desludging of petroleum stocks; producing antibiotics; refining uranium; clarifying wet-process phosphoric acid; concentrating starch and gluten; processing beverages; classifying clay; processing polymers; and clarifying fuel oils. Dorr-Oliver says that it alone has sold more than 130 Merco H-30's. Among the users of high-capacity machines are the U.S. Navy, Du Pont, Celanese, Mallinckrodt, Corn Products, American Cyanamid, and Sonneborn Chemical. For the Navy, for example, De Laval developed its Model B-214A jet fuel purifier. The Navy had found that water was passing through coalescing filters previously used aboard aircraft carriers to purify the jet fuel. The water turned to ice in a plane's fuel tanks at high altitude, blocking fuel lines, and causing flame-outs. The B-214A has a 24-inch-diameter bowl and a capacity of 12,000 gallons per hour of JP-5 jet fuel. A civilian version of the machine, the Model B214, is now used in one chemical plant to reduce contamination of steam condensate with steam cylinder lubricant. It reduces the contamination from 60 to 80 p.p.m. to less than 1 p.p.m. at a rate of 10,000 gallons per hour. Acid clarification. In fertilizer production, high-capacity centrifuges came into use about seven years ago to clarify phosphoric acid. Among the companies involved in this application are International Minerals & Chemical, Central Phosphate, and Occidental, which use Dorr-Oliver centrifuges, and American Cyanamid, which has some De Laval machines. American Cyanamid's phosphoric acid plant at Brewster, Fla., is typical of the present use of high-capacity centrifuges. The plant has a two-stage clarification system. Phosphate rock dust is digested with sulfuric acid in large tanks for several hours. The resultant slurry contains about 32% P 2 0 5 and small, irregularly shaped crystals of calcium sulfate (gypsum). Solids are removed in the two stages. First is a filtration of the weak acids/solids slurry from the digester. In the second stage, three high-capacity centrifuges are used following an evaporation step in which the concentration of weak initially filtered phosphoric acid is raised to 54% P2O-,. All production passes through the centrifuges before final storage. Another application for high-capacity centrifuges is in water pollution control, where a few machines can clean up very large volumes of water. More than 2 million gallons a day can be handled with three 500 I gallon-a-minute machines.
Phillips has three new ways to process plastics Phillips Petroleum has three new plastics-processing techniques—a process for making foam-molded parts from polyethylene; a nonscrew extruder which can handle ultrahigh-molecularweight, high-density polyethylene; and a method for converting film into fiber. All were revealed during the 11th National Plastics Exposition, in New York City. The first of these should provide plastic replacements for many articles now made of wood, cork, or leather, according to R. Vernon Jones, director of Phillips' plastics services division. The second will broaden the range of engineering plastics available for industrial parts. The third, by eliminating spinning, could cut capital investment and labor costs in half for some fibers. Foam-molded polyethylene articles, such as corks for wine bottles, innersoles for shoes, and handles for tools, are being made by Phillips using a process invented in West Germany by Thomas Engel. Phillips has acquired worldwide rights to the process, and will license, sell materials and machinery, or supply custom-fabricated plastic parts. One of the problems which the new process solves is that of foaming agents decomposing at temperatures below the plasticization temperature of the material to be foamed. Although the company gives few details of the process, it says that the process involves mixing a well-known foaming agent with plasticized material under pressure. The pressure prevents expansion of the melt until it leaves the die. Since the pressure needed to fill the mold doesn't exceed 120 p.s.i.g., molds can be made of relatively inexpensive materials, according to Phillips. Aluminum, brass, or epoxy compounds can be used, for example. Other advantages Phillips cites are high production rates and applicability to other polyolefins and thermoplastics. The company has just received its first German-made machine and is having others manufactured in the U.S. Shapes extruded. Phillips' second advance, the nonscrew extruder, was put into commercial production in 1963 but kept secret until the plastics exposition this month. It allows precision shapes to be made economically from ultrahigh-molecular-weight polyethylene, one of the toughest of the thermoplastics. This hasn't been possible before because of difficulty in extruding the resins. They remain so viscous and rubbery, even at 600° F., that conventional extrusion equipment
