Batch filter cycles

Batch esterification, 10 478—480 Batch experimental reactor, 21 352 Batch extraction, 10 756 Batch extractor, holdup in, 10 764 Batch fermentation, 10 267 Batch filter cycles, 11 344, 345-346 Batch furnaces, 12 288—289 Batch gasoline blending, 12 413 Batch hydrogenation, 10 811 Batching, ceramics processing, 5 648 Batch injection analysis (BLA) technique, 9 586-587... 

In this chapter the equations and models previously described in detail (Wakeman and Tarleton, 2005 a) are summarised and brought together so that the various phases in a batch filter cycle can be related to one another and meaningful process engineering models obtained. These models facilitate sequential calculations to provide the basis for computer simulations. Whilst there is scope to predict the performance of most of the filter types described in Chapter 1, those shown in Table 6.1 are presented in sufficient detail to model their filter cycles. 

Table 6.2 Potential operations in a relatively complex batch filter cycle with single washing and deliquoring phases (adapted from Purchas and Wakeman, 1986).

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. 

By operating cycle. Filtration may be intermittent (batch) or continuous. Batch filters may be operated with constant-pressure driving force, at constant rate, or in cycles that are variable with respect to both pressure and rate. Batch cycle can vary greatly, depending on filter area and sohds loading. 

Scaling Up Test Results The results of small-scale tests are determined as dry weight of sohds or volume of filtrate per unit of area per cycle. This quantity multiplied by the number of cycles per day permits the calculation of either the filter area reqiiired for a stipulated daily capacity or the daily capacity of a specified plant filter. The scaled-up filtration area should be increased by 25 percent as a factor of uncertainty. In the calculation of cycle length, proper account must be made of the downtime of a batch filter. 

A suspension of aluminum hydroxide in water is to be filtered imder constant pressure in a batch Nutsch filter having a filtering area of 1 m. Each filter cycle is estimated to separate out 0.5 m of suspension. The operating temperature is 25° C. The following expression for the cake resistance was empirically determined from pilot tests ... 

Filter cycles, 11 344-346 batch, 11 344, 345-346 continuous, 11 344-345 Filter drag model, 26 712 Filter housing, 11 322 Filter media, 11 322, 325-326. See also Media... 

The material to be filtered is fed into the vessel under pressure, and separation takes place with the solids being deposited on the leaf surface, and the liquid passing through the drainage system and out of the filter. Cycle times are determined by pressure, cake capacity or batch quantity. Where particularly fine solids must be removed, a layer of precoat material may be deposited on the leaves prior to filtration, using diatomaceous earth, Perlite, or other suitable precoat materials. 

Bag filter batch load cycle and clean intermittent shaking, reverse pulse, reverse blow ring or sonic cleaifing. Load filter until the gas pressure drop across the filter is >1.5 kPa, then clean. Choice of fabric is critical static charge on fabric, operating temperature, potential for fumes to absorb with moisture to deteriorate bag, and need to select dust removal option to keep the Ap across the bag of 0.5 to 1.5 kPa. Felted material gives higher gas flow rate per unit area than woven, costs 3 to 4 times more, and cannot be cleaned by... 

Cake thickness is an important factor in determining the capacity and design of a batch filter and it determines the cycle of operation. In constant-pressure filtration, the integration of Equation 58.1, with the medium resistance neglected, yields... 

Chapter 11 deals with the application of these concq>ts to process calculations in large-scale vacuum and pressure filters used in continuous and batch processes, hi all the examples presented, the importance of the consideration of the fiill filter cycle is stressed. Along with the development of modem fitters capable of operating in thin-cake conditions, units are also included for the dewatering of cakes by gas displacement and squeezing. 

These units are sometimes described as pressure vessel filters the vessels used to contain the filter dements are cylindrical designs, positioned vertically or horizontally. These vessels operate in batch mode, with filter cycles similar to plate and fimne filters. The variations available in the type of elements (leaf or candle) and its position in the pressure vessel, lead to a large number of available designs [Purchas, 1981], General operating features of pressure vessel filters have been described [Bosley, 1986] these units are considered to be cleaner and more reliable than other pressure filters, provided adequate attention is given to filter cake properties. 

For batch filters the time dependency of parameters such as mass of solids, liquids and solutes (both extracted and retained), cake thickness and moisture content and fractional solute recovery can be obtained for the whole cycle, or individual phases where appropriate. The production rates are also given for both products and potential waste streams. In the case of continuous filters throughputs rather than masses are calculated. 

Section 6.1 describes the principal features of filter cycles in relation to common batch filters while Sections 6.2 and 6.3 present the equations required to... 

Table 6.1 Batch filters and presses and potential phases in their operational cycles.

Depending on the type and mode of operation of a batch filter, a cycle may comprise up to two filtration phases followed by a number of compression, gas deliquoring and washing phases, essentially in any order, as well as a cake discharge operation (see also Tarleton and Hancock, 1997 , Tarleton and Wakeman, 1994c, 2005b Wakeman and Tarleton, 1994a,b, 1999). 

As described in Section 6.1, the cycle for a batch filter can comprise one or more cake formation phases followed by any sequential combination of consolidation, displacement washing and gas deliquoring phases. While a complex batch cycle may involve the list of operations shown in Table 6.2, a more typical cycle can be represented by... 

When constant pressure/vacuum filtrations are performed in batch filters the rate of filtration is generally a maximum at the start and then progressively decreases. A decision must be taken as to when the filter cycle should be ended. In Section 6.2.1.1, for example, the end of filtration was specified by the formation of a known mass of cake solids and the duration of subsequent phases was specified via a time or specific cake condition. An alternative way of making these decisions is to define an average filtration rate on the basis of the total cycle time. By incorporating equation (6.9) the overall production rate (F) can be written as... 

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