Continuous filter cycles

In this chapter the equations and models previously described in detail (Wakeman and Tarleton, 2005a) are used to provide process engineering models for continuous filter cycles. The models facilitate detailed calculations and provide a platform for the development of computer simulations. While there is scope to predict the performance of many of the continuous filters described in Chapter 1, those shown in Table 7.1 are discussed in sufficient detail to model and simulate their filter cycles. 

In Section 7.1 the principal features of common continuous filter cycles are described, while Section 7.2 presents the equations required to model these cycles. Section 7.3 provides detailed example calculations for the horizontal belt filter and the rotary drum filter as these are representative of typical continuous cycles. Section 7.4 shows how computer simulations can be used to examine in detail the effects of process variables on... 

To describe the washing phase on a continuous filter cycle the dispersion model is preferred (Wakeman, 1986a Wakeman and Attwood, 1988,1990). A dispersion number (D ) that characterises the cake washing process is calculated and for a known number of wash ratios (IT) the fractional solute recovery (F) can be found. The applied vacuum (ApJ is fixed throughout washing and several properties of the cake are assumed to remain constant and equal to the values at the end of the previous phase in the cycle, i.e. K=Lpr==( <,vV Qv=(CJpr = 1 (ejpr nd M/t)=(M,(t))p ... 

It is both convenient and reasonable in continuous filtration, except for precoat filters, to assume that the resistance of the filter cloth plus filtrate drainage is neghgible compared to the resistance of the filter cake and to assume that both pressure drop and specific cake resistance remain constant throughout the filter cycle. Equation (18-51), integrated under these conditions, may then be manipulated to give the following relationships ... 

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... 

Each filter has demonstrated the capacity to filter the full brine flow of 195 m3 h 1. The pressure drop through the filter medium is measured and monitored continuously. Typically, it is nearly constant over a 2-h filtration at 195 m3 h 1. Back-pulse cleaning restores the initial pressure drop from cycle to cycle, with only a slow increase over time. After 12 months running time, the initial pressure drop at the beginning of the filter cycle had increased by 0.6 bar. The filter membranes were chemically cleaned with 5 % hydrochloric acid. After a cleaning time of 2 h the filter was started again and the pressure drop was less than 0.1 bar greater than that of new filter socks. 

Filter and dry. Check PEG-bound product by NMR and repeat glycosylation if necessary. Remove temporary protecting group and continue second cycle. 

Light Exposures. Silk fabric samples, 0.25 m x 0.17 m, were mounted in Atlas Electric Devices aluminum sample holders according to AATCC Test Method 16E-1982 (7). An Atlas Ci-35 Weather-Ometer xenon-arc was used on continuous light cycle. Exposures were conducted at an irradiance of 0.35 W/m2 measured at 340 nm and the irradiance was monitored and controlled automatically. Borosilicate inner and outer filters were used to simulate the solar spectrum. The relative humidity was maintained at 65% and the black panel temperature was 50°C. The actual fabric temperature during the irradiation was measured, using small thermocouples threaded into the fabric, and was found to be 35 C. Control samples for these tests were kept in the dark at 35°C and 65% RH for the same time period as the illuminated samples. 

A continued cost is associated with purchase and disposal of the filter medium, which is usually discarded at the end of each filter cycle. 

Time, h or s tp, residence time in centrifuge , cycle time in continuous filter... 

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. 

Filter presses are not at all suitable for the filtration of the large quantity of solids involved in the processing of low-grade ores. The large labour requirement for these filters can be reduced by designing them to operate on a continuous automatic cycle, but their throughput is still relatively low. 

EXAMPLE 14.2-4. Filtration in a Continuous Rotary Drum Filter A rotary vacuum drum filter having a 33% submergence of the drum in the slurry is to be used to filter a CaCO slurry as given in Example 14.2-1 using a pressure drop of 67.0 kPa. The solids concentration in the slurry is = 0.191 kg solid/kg slurry and the filter cake is such that the kg wet cake/kg dry cake = m — 2.0. The density and viscosity of the filtrate can be assumed as that of water at 298.2 K. Calculate the filter area needed to filter 0.778 kg slurry/s. The filter cycle time is 250 s. The specific cake resistance can be represented by a = (4.37 x 10 ) (—Ap) , where —Ap is in Pa and a in m/kg. 

Throughput in Continuous Rotary Drum Filter. A rotary drum filter having an area of 2.20 m is to be used to filter the CaCO slurry given in Example 14.2-4. The drum has a 28% submergence and the filter cycle time is 300 s. A pressure drop of 62.0 kN/m is to be used. Calculate the slurry feed rate in kgslurry/s for the following cases. 

Table 1.3 Typical filter cycle data for continuous vacuum filters (adapted from Purchas and Wakeman, 1986).

The use of computer control allows sequential filter cycle data to be acquired in a repeatable and reliable manner with a minimum of operator interference. By defining the desired cycle phases through a software algorithm, a cake formation phase can be directly followed by the chosen combination of washing and deliquoring. The real time measurement of experimental parameters also allows continuous display of results and the use of on-line analysis techniques as an experiment proceeds. 

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. 

Table 7.1 Continuous filters and potential phases in their operational cycles.

As previously described, the cycle for a continuous filter typically comprises a cake formation phase followed by a combination of sequential displacement washing and gas deliquoring phases, potentially in any order. If a cycle is assumed to comprise the sequence filtration-washing-deliquoring and the subscripts /, d and w, respectively denote values for these phases, then the total time (tj) devoted to a cycle is given by... 

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