Fixed bed centrifuges

Fixed-Bed Centrifuges. The simplest of the fixed-bed centrifuges is the perforated basket centrifuge (Fig. 31) which has a vertical axis, a closed bottom, and a Hp or overflow dam at the top end. Some domestic machines of this type are ia use for straining vegetable or fmit juices. Ia the iadustrial versioas, the basket housiag is oftea supported by a three-point suspeasioa called the three-column centrifuge. 

Air is compressed to modest pressures, typically 100 to 200 kPa ( 15-30 psig) with either a centrifugal or radial compressor, and mixed with superheated vaporized butane. Static mixers are normally employed to ensure good mixing. Butane concentrations are often limited to less than 1.7 mol 1 to stay below the lower flammable limit of butane (144). Operation of the reactor at butane concentrations below the flammable limit does not eliminate the requirement for combustion venting, and consequendy most processes use mpture disks on both the inlet and exit reactor heads. A dow diagram of the Huntsman fixed-bed maleic anhydride process is shown in Figure 1. 

The most reliable recycle reactors are those with a centrifugal pump, a fixed bed of catalyst, and a well-defined and forced flow path through the catalyst bed. Some of those shown on the two bottom rows in Jankowski s papers are of this type. From these, large diameter and/or high speed blowers are needed to generate high pressure increase and only small gaps can be tolerated between catalyst basket and blower, to minimize internal back flow. 

It is convenient to classify centrifugal filters into two broad classes, depending on how the solids are removed fixed bed or moving bed. 

Centrifuges, 22 288. See also Hydrocyclones basket, 11 389-390, 391 filtration equipment, 5 542-548 fixed-bed, 11 389-391 materials of construction and operational stress, 5 522-524 moving-bed, 11 391-392 noise, 5 524-525... 

Enzymes, when immobilized in spherical particles or in films made from various polymers and porous materials, are referred to as immobUized enzymes. Enzymes can be immobilized by covalent bonding, electrostatic interaction, crosslinking of the enzymes, and entrapment in a polymer network, among other techniques. In the case of batch reactors, the particles or films of immobilized enzymes can be reused after having been separated from the solution after reaction by physical means, such as sedimentation, centrifugation, and filtration. Immobilized enzymes can also be used in continuous fixed-bed reactors, fluidized reactors, and membrane reactors. 

An alternative approach is taken in the production of monosodium glutamate (MSG) which, unlike interferon, is secreted into the fermentation broth. The stages of downstream processing for MSG are shown in Figure 14.3. Again, a variety of unit operations, including centrifugation, crystallization, vaporization, and fixed-bed adsorption, are used in this process. 

The ester is then fed directly to the low-pressure, vapor-phase hydrogenation system where it vaporized into an excess of hydrogen in the vaporizer (3) and fed to a fixed-bed reactor (4), containing a copper catalyst. The reaction product is cooled (5) and condensed (6) with the hydrogen being recycled by the centrifugal circulator (7). 

Reaction conditions (1) 70 ml of 0.32 mol/1 EAHQ solution produced in a fixed-bed reactor with a Pd/Al203 catalyst was oxidized with O2, and 20 mmol H2O2 was formed. (2) EAHQ fresh catalyst = 300 1. At the end of each cycle, the catalyst was separated by centrifugation, washed with toluene, and used in the next reaction wilii addition of fresh catalyst to make up for losses. 

Electrokinetic mobilities can be measured by direct observation of the particle movement by use of a microscope or of the boundary between suspension and clear electrolyte separated from the suspension by centrifugation (moving boundary method). When electrolyte is forced through a fixed bed, e.g., of carbon fibers, a potential builds up between the ends of the bed. This streaming potential can also be used for the measurement of -potentials. Details of these methods are described in textbooks of colloid chemistry. 

The ammoxidation reaction can, on the other hand, be performed continuously in fixed-bed and fluid-bed reactors, and by-products (particularly CO2) can be easily removed. The fluidized bed has some advantages in terms of heat transfer but demands are made on the mechanical durability of the catalyst and so catalyst choice is limited. The long-term stability of the catalysts is also important and so multicomponent systems are recommended [e.g. 1,12]. The separation of the nitrile formed can be achieved by condensation, centrifugation, filtration, or rectification. Sometimes the formation of hazardous by-products (HCN, CO) must be considered. 

The major disadvantage of homogeneous transition metal catalysts is the difficulty of separating the catalyst from the product. Heterogeneous catalysts are either automatically removed in the process (e.g., gas-phase reactions in fixed-bed reactors), or they can be separated by simple methods such as filtration or centrifugation. In the case of homogeneous catalysts, more complicated processes such as distillation, liquid-liquid extraction, and ion exchange must often be used [3]. 

Separation and reusability - the solid catalyst can either be used in a fixed bed configuration or simply filtered or centrifuged from a stirred tank reaction and then, in many cases reactivated for reuse (e.g., zeolites used in petroleum refining can be reactivated and reused for years before disposal). 

In this process, fixed bed reactors could be used advantageously. This would prevent any loss of catalyst and avoid the expensive equipment needed for stirring, centrifuging and filterir. Two reactors in series would be necessary to obtain a complete conversion. In the first one heat can be removed by cooling and recirculating the liquid. In the second one which could approach piston flow behaviour, the required conversion could be achieved. 

Centrifugal filters can be classified into two fundamentally different classes discontinuously fed, fixed bed machines, and continuously fed, moving bed machines. 

Starting with the fixed bed, batch or cychc centrifuges, these have a cyhndrical screen onto which the feed suspension is fed and the cake is stationary. This feature makes it possible to separate relatively finer particles than in the continuous moving bed machines. The perforate basket (or three point ) centrifuge and the peeler type centrifuge are examples in this category. 

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