The driving force for separation in a filtering centrifuge is centrifugal force, unlike filtration where pressure or vacuum is used. The basic principals of Poiseuilles' cake filtration equation can be applied by substituting P (pressure) with the stress or pressure induced by the bowl contents, which is a function of centrifugal acceleration. (See Eq. 8.)
10.1 Cake Washing
A wash can be introduced to fulfill different requirements:
1. Remove original slurry liquid (mother liquid)
2. Dissolve impurities
3. Alter pH
4. Displace mother liquor with another liquid (often to facilitate drying of a solvent with a lower vapor pressure or eliminate toxic solvent)
Distinctly different types of wash are:
1. Displacement—Removal of one liquid in favor of another
2. Diffusion—Dissolved materials retained in the capillary liquid and in the surface liquor are transported by the wash medium
3. Dissolution—Components of the solid which is composed of different materials of varying solubility are dissolved in the wash medium
One or all of the steps can be used on a product or occur simultaneously. Several steps can be used often, firstadisplacementwashto remove mother liquors and any associated impurities, followed by a dissolution or diffusion wash.
Two different methods of introducing the wash are:
1. Flood Washing—With this method, the wash medium is fed at a faster rate than the case is dewatering, thus a liquid level forms on the top of the cake. This ensures distribution of the wash fluid over the entire cake. Positive displacement is the most effective form of washing. Carried out in a plug flow manner, clean wash fluid contacts the solids without backmixing. Except where retention time is required to allow for mass diffusion of the impurities through the solids, positive displacement washes are more efficient then reslurrying. Redilution of the impurities occurs as reslurrying backmixes impurities into the fresh medium. The cake should be even to achieve this displacement wash, as the wash fluid will seek the path of least resistance on nonuniform cakes. Vertical basket centrifuges in particular have uneven cakes due to the feeding method and can require copious quantities of wash to compensate for the uneven cake.
2. Spray Washing—Liquid is supplied via spray nozzles. It is the only effective way of working cakes which are uneven from the top to the bottom for basket centrifuge. Peeler centrifuges, being unaffected by the force of gravity in the distribution of the cake, tend to have more even cakes, although nonuniformity can still occur due to the feeding mechanism.
11.0 VERTICAL BASKET CENTRIFUGES
Vertical basket centrifuges have been the "work horse" for the pharmaceutical industry for many years for intermediate and final filtration steps. Chemical and specialty chemical productions use this type of equipment in a wide range of applications
In fermentation, typically, post-crystallization steps are processed on this unit, crystalline products that are free-draining. Different designs are available, the simplest being a manual design or under-driven top-discharge. A perforated basket with cloth or screen is the filter media. Filtrate passes through to a filtrate chamber as shown in Fig. 3. Solids must be dug out or the entire filter bag hoisted out with the solids. Labor intensive with operation exposure a problem, this design is more often employed in pilot than production plants. G-forces up to 800 g's are attainable. Introduction of a traversing plough mechanism to this design enables automatic solids discharge through the basket bottom. Speeds are variable in this design through 800 g's.
The cycle is similar to that of a household washing machine, filling, spinning or dewatering, washing, a final spin and unloading. The feed is fed off to one side. This, coupled with the force of gravity in the vertical basket design, can cause an uneven slurry distribution, i.e., a thicker cake at the bottom of the basket or in the middle where the feed pipe is located. Hence, reduced filling speeds may be required depending upon the product. Cones for 360° feed distribution are available for more even loading.
Operating batch-wise, basket centrifuges in general are optimized best when operated almost continuously by feeding the machine at the same rate as the slurry is dewatering, thus maximizing solids concentration. Loading the basket infinitely fast will only produce a basket with the same solids concentration as the feed tank.
The wash can be quantified by mass or volumetric flow to ensure product quality. The intermediate and final spins are usually timed if the basket is automated, however, if out of balance conditions exist during the feed cycle, an operator is often required throughout the operation. Operator judgement is often required to determine when the liquid level on the cake disappears. This may be required, for example, to remove all mother liquor, introduce the wash, particularly for difficult filtrations, or to be sure the liquid level remains on the cake to prevent cracking and preferential channeling of the wash liquid. Variable solids concentrations or particle sizes distributions will make it more difficult to fully automatic a standard basket centrifuge as operator judgment may be required at several points in the cycle. Consistent, uniform batches with every filtration can, however, be automated on a time basis.
For solids unloading, a plough cuts out the solids at reduced speeds of 40-70 rpm, and traversing action must be slowed down to prevent the basket from stalling. For certain products, the cake can be sufficiently difficult to remove that the plough cannot remove all of the solids due to tolerances and possibility of damaging the filtercloth, thus a residual heel of solids is left for some products. For some products, this is not a problem and the next cycle can begin. For others, the heel can glaze over and reduce filtration rates on subsequent batches. It must be scraped out manually, dissolved, or an air knife can be used, depending upon the hardness of the heel. Depending upon how problematic the residual heel, even the automated vertical basket can be labor intensive.
An average cycle would be filling at 600 rpm, washing at 800 rpm and dewateringat 1000 rpm. With a 48" basket, these are g forces of 240,426 and 667, respectively. Discharge by a plough occurs at less than 100 rpm, or, if manually unloaded, at zero speed.
If there are significant out-of-balance operating conditions, mechanical parts such as the plough or cake detection can vibrate loose. The bearings and shaft seal components will also have limited lifetimes, depending on the operation.
12.0 HORIZONTAL PEELER CENTRIFUGE
The horizontal peeler centrifuge (Fig. 4) is a variation of the vertical basket. Up to 80 inches in diameter and producing as much as 100 tons per day of product applications, this machine has been prevalent in the isolation of beet sugar and starch. The design, characterized as TerMeer, after the inventor, is sometimes used in bulk pharmaceutical productions. Dedicated productions of relatively easy filtrations being processed are applications for this type of equipment.
Solids Discharge. This is carried out by an automatic plough or knife. Since the knife cannot contact the filter medium, a heel of product remains in the basket after each discharge. This can prevent fines from passing, but, like in a vertical basket, may become glazed and impervious to filtration. Backwashing the heel or redissolving may be possible. Even changing the depth at which the blade cuts the cake may help. In the Ferrum design, high pressure air forces the cake off the screen during discharge. This will work in some applications. Solids exit a chute, but can also discharge by screw conveyor.
Figure 4.- Peeler centrifuge. 12.2 Operation
Feed Mechanism. A cake detection device, pneumatic in some designs, activates the feed valve closure when the desired cake depth is reached. This cake depth is monitored by a proximity switch. This device can also act as a cake distributor to level the load during feeding. Alternative feed designs are available depending upon the vendor.
Wash. The entire cycle is operated automatically on a pre-programmed basis, all by time. The wash can, of course, be by time or volumetric basis, monitored by air in-line flow meter and totalized.
Operational Speeds. During the entire sequence of loading, deliquoring, and unloading, a constant bowl speed is maintained. "Dead time," associated with acceleration and deceleration, is minimized. Maximum operating speeds depend upon bowl diameter, the larger the diameter the lower the speeds. Sizes range from pilot scale 450 mm to 1430 mm diameter with g-forces from 3200 to 1200. Basket speeds range from 3000 rpm to 1200 rpm. Specially designed vibration-damping systems will minimize plant structural supports required. Each manufacturer's design must be evaluated as to what is required. Special cement foundations are often a necessity.
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