Dewatered cakes

Optimization of Cycle Times. In batch filters, one of the important decisions is how much time is allocated to the different operations such as filtration, displacement dewatering, cake washing, and cake discharge, which may involve opening of the pressure vessel. Ah. of this has to happen within a cycle time /. which itself is not fixed, though some of the times involved may be defined, such as the cake discharge time. 

This allows each section, as it rotates, to go through a series of operations such as filtration, dewatering, cake washing, and discharge. Two basic designs exist, depending on the method of soHds discharge. 

Cake dewatering is related to cake formation in a filter cake, the final moisture content is dependent on many variables that also control cake formation. Pretreatment processes, for example, affect both cake formation and cake dewatering. Cake dewatering is achieved by compacting the soHds ... 

Cake Washing Wash efficiency data are most conveniently represented by a semilog plot of percent remaining B as a function of wash ratio N as shown in Fig. 18-103. Percent remaining refers to that portion of the solute in the dewatered but unwashed cake which is left in the washed and dewatered cake. Since a cake-washing operation... 

When very short cycle times are required for fast dewatering cakes such as phosphate slurry. 

Biosolids are squeezed between two porous cloth belts. The dewatered cake is scraped from the belts by blades.60 This operation results in a typical biosolids concentration of 50% for the primary and 20% for secondary biosolids. 

Sepcific gravities of sludge feed, dewatered cake, and filtrate are 1.02,1.07, and 1.01, respectively. 

The underflow of the cyclone 7 is withdrawn by an eccentric worm pump 8 and delivered to a belt filter press 9 yielding a highly dewatered cake and a filtrate consisting essentially of water but loaded with small concentrations of sulfuric acid, furfural, and by-products. This filtrate is recycled to tank 1 for preparing the feed stock slurry. Due to this scheme, most of the sulfuric acid is recovered and reutilized, the only loss being the quantity contained in the cake. This loss is replaced in tank 1. Analogously, the water leaving the system with the cyclone vapor and the cake is also replenished in tank 1 so that the overall mass balance is satisfied. 

The overall loading capacity of this plant is about to 5 tons per day as dewatered cake with water content 75 % (Fig-l)... 

Five different kinds of dewatered cakes were selected for the pilot test, water content and higher heating value of sample cakes were varied from to 76 from 1,500 to 2,960 kcal/kg D. S. respectively. (Table-I)... 

The dewatered cake was dried to the water content of 30 to -0 % in the indirect steam dryer, and fed into the pyrolytic furnace through the screw conveyor. Combustion air less than the theoretical amounts for sludge is supplied into the furnace for partial combustion of combustible pyrolitic gas. That is, pyrolysis was carried out by Direct Heating method. 

Off gas from furnace is introduced into the combustion chamber to burn its remaining combustibles, and to decompose its pollutant and other odor components. Excess air is automatically controlled by the oxgen concentration at the exit of the combustion chamber. Heat of off gas from the combustion chamber is recovered by the waste heat boiler. Steam generated in the boiler is supplied to the indirect steam dryer. When the amounts of steam generated is not sufficient for drying dewatered cake, auxiliary fuel is used to generate the additional steam. 

In this study, to make a comparison with drying-pyrolysis process, the experiments on direct feed processes, i.e., direct pyrolysis process and incineration process, were conducted. In these processes, the dewatered cake was directly fed into the furnace. The plant was operated at steady state for at least 5 or 6 hours in all runs. 

Performance of Drying-Pyrolysis Process Indirect steam drying operation was available to all kinds of dewatered cakes tested and its efficiency (overall heat transfer coefficient U) varied with the kinds of cakes. 

A pilot plant study was carried out on "drying-pyrolysis" process in order to put the sludge pyrolysis process with multiple hearth furnace to a practical use. Additional studies were carried out on direct pyrolysis process and incineration process in order to compare these three thermal processings. The samples in this study were 5 dewatered cakes obtained from various sewage treatment plants and from various dewatering methods. The pilot plant for the experiments of this study consisted of a four-shaft indirect steam dryer with a total heating area of 10.1 m, a four hearth furnace with a total hearth area of 1.63 and its incidental facilities. 

Dewatering of the pretreated feed. Dewatered cakes and the balance of the filtrate after recycling are fed to the digester. 

Figure 8.38 laical washing curve. Curve (a), flooded cake curve (bX dewatered cake... 

Knife discharge following a previous reverse air blast. Here a reasonably fiiick cakei is required, and sometimes it is necessary to protect the filter medium, hich covers the drum, by using a mbber- or plastic-edged knife. The blow-back technique can create a rewetting of the partially dewatered cake with filtrate v hich remains undrained in the conduits behind the fiber cloth. 

The shape of the washing curve depends on many variables (as shown later) and one important one is the amount of dewatering done on the cake before washing. Figure 10.6 shows a case when no dewatering is done, i.e. the cake is full (saturated) of the mother liquor to start with. If the cake is well dewatered, the curve is much lower because much less mother liquor needs to be removed. Partially dewatered cakes would result in curves in between these two. 

Figure 10.6 Comparison of typicai washing curves with saturated and dewatered cakes...

The compartments are separated by seals under adjustable pressure. As the drum rotates, each cell on the drum passes successively through the series of compartments, thereby undergoing different processes such as cake formation, dewatering, cake washing or cake drying, and these can be carried out in several stages under different pressures or even under vacuum. 

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