Dry Solids Operation

It has just been shown that the perft)rmance of a dry solids decanter is related to conveyor torque achievable, bowl speed, pond depth, and Uoccuiant usage. Once the cake dryness has been fixed, it is useful to be able to assess the maximum capacity possible on a given decanter. 

It is shown in Section 4.3.1 that the clarification capacity of a decanter is the product of the Stokes settling velocity (equation (4.28)) and the Sigma value of the centrifuge (equation (4.32)). The Stokes velocity is a function of the process material s physical parameters, while Sigma i.s a function of mechanical features of the centrifuge. 

An alternative approach to this version of compaction will now be made, which is based on conventional compaction theory [18] where processing capacity is found to be a function of volume rather than area. 

After time t the interface which develops between the settling sludge and the clear supernatant will be at a height H above the base of the cylinder. This interface will have a velocity v. 

After an infinite time, settlement will cease when v will be zero, and the height of the interface will be at height Hoc- At this point the weight of the cake is no longer sufficient to express any more liquid from between the pores of the cake. 

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Two-phase systems of fine dispersed solids in a continuous gas stream are quite common in many processing industries dealing with dry solids operations. In diverse processes such as the dry milling of grains, some combustion reactions, or the grinding of raw materials in the cement industry, fine solids may be released to the surrounding air as part of their normal operation. Tiny solid particles (dust) produced in many processes may become airborne and carried out by air currents. There are many problems associated with dust emissions, such as health hazards, abrasion damage to equipment, impaired... 

The decanter centrifuge lends itself to a wide range of theoretical treatments, both process and mechanical. Reif and Stahl [1] observed that the decanter incorporates three extensive fundamental problems , dewatering of the solids, clarification of the liquor, and conveyance of the cake produced. These three major subjects will be covered here, together with separate sections to deal with specific decanter processes, such as thickening, classification, three-phase separation and the latest technology of dry solids operation. 

In t he dewatering of compressible cakes, as much pressure as possible is put on the cake, before adversely affecting capacity or centrate clarity, in dry solids operation, which will he discussed in more detail in other sections, it has been reported [1] that increasing cake height, and thus pressure in the centrifuge bowl, improves dryness capability. Thus, for such applications, it would be advantageous to maximise pond depth. To estimate the pressure within the pond, refer to equation (4,61),... 

It has been found [15] that, in dry solids operation, the cake dryness produced is proportional to the torque developed by the conveyor or vice versa. 

The laboratory tests will advise the operator as to the quantities of polymeric flocculant that will be required. The equipment will be set up capable of supplying in excess of this quantity by at least 50% and perhaps 300% when dry solids operation is planned. It will be seen in subsequent chapters that three times the normal amount of polymer can be used in dry solids operation when the driest cakes are required. 

Each test run will be conducted with, as near as can be judged, the same flocculant dosage level. The setting of differential speed and/or torque for each run will depend upon the type of test being conducted. For a simple dewatering lest, a fixed differential speed may be cho.sen. However, if it is thought that the decanter may be solids capacity limited, then the same feed rale/differential speed ratio could be chosen for each test. For dry solids operation, a fixed torque would be more likely to be chosen. Alternatively, for each test the minimum differential is found, where the best dryness is achieved without deterioration of centrate quality. 

Thirteen models, based on eight bowl diameters, available with variable spccilications, including three-phase operation, and designs for optimal sludge thickening and high dry solids operation... 

Typical operating data for cocurrent rotary dryers are given in Table 12-18. (Note that the driving force AT must be based on wet-bulb depression and not on material temperatures. Use of material temperatures, particrrlariy when the dry solids are superheated after drying, will yield conservative results.)... 

Dry Solids or Filtrate Rate Filtration rate, expressed either in terms of diy solids or filtrate volume, may be plotted as a function of time on log-log paper. However, it is more convenient to delavthe rate calculation until the complete cycle of operations has been defined. 

The multiple-hearth incinerator (Fig. 16-6) can accept sludges containing between 60 and 75% water. The operating costs run between 0.50 and 5.00 per ton of dry solids, with total costs between 8 and 14 per ton. Design information is given in reference 62. When the sludges contain more water, fluidized-bed incinerators are sometimes used. Their operating costs run between 11 and 21 per ton of dry solids and capital costs are 15/ton.6 See reference 63 for more details. All incinerators must have the proper air-pollution abatement devices attached. 

Sometimes, instead of incinerators, so-called oxidation processes can be used. In this context oxidation differs from combustion in that no flame is present and the temperatures are much lower. A high-pressure process (1,750 psi or 123 kg/cm2) that operates at a temperature of 525°F (275°C) has total costs around 33/ton of dry solids.62 Others operate at a pressure of around 600 psi (42 kg/cm2). They can treat sludges containing up to 99% water. 

Dextrose yield, however, can be increased by conducting saccharification at a lower solids level where the reverse reaction is minimized. For instance, dextrose yields of 98.8, 98.2, 97.5, and 96.9% dry basis can be achieved at solids levels of 10, 15, 20, and 25%, respectively (10). Low solids operation, however, is not used commercially owing to problems associated with microbial contamination and cost of water removal. Dextrose level can be increased by 0.5—1.5% at normal reaction solids by using an enzyme such as pullulanase (11) or a B. megaterium amylase (12) in conjunction with... 

The drying of ethereal solutions is an operation frequently met with. In most cases it is advisable to dry an ethereal extract before evaporating off the ether. Again, to dry a moist solid, it is often convenient to dissolve it in ether and to dry the ethereal solution with a dehydrating agent. The dry solid is then obtained by evaporation. For the drying of organic liquids by dialysis, see U. S. P., 1,885,393. 

Optical techniques were developed - both in situ and on line - over a broad range of applications and various types of light sources. Main examples are solids concentration measurements in particle fluidisation [6,7], as well as gas-solid flow [8], or sample composition in dry blending operations, this latter beginning from the pioneering work of Harwood et al. [9] up to more recent advances in this scientific field [10-12], The main interest of these techniques lays in the fact that optical probes are easily available on the market, so that qualitative monitoring of mixers is possible at an industrial scale. However, these probes only provide local information of the mixture (typically 1/40 of a tablet), so that in essence, they still sample the powder flow in a way that may be intrusive and not always representative of the overall stream. 

Since many chemicals are produced wet but sold in dry, solid forms, one of the more common manufacturing steps is a drying operation, which involves removal of a liquid from a solid by vapourisation of the liquid. Although the only basic requirement in drying is that the vapour pressure of the liquid to be evaporated is higher than its partial pressure in the gas stream, the design and operation of dryers... 

Frequently, filtration, washing, and drying operations are integrated especially where noxious substances are being handled or when a crystal slurry of an API is being processed to a dry solid in a controlled environment room (CER). In the latter case, all types of combinations of filters and dryers are used (Figure 13). 

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