Apr-2012

Combination mechanical slurry conditioning and filtration

A novel definition of combination filtration provides process engineers with a framework for analysing operating bottlenecks

Barry Perlmutter
BHS-Filtration

Article Summary

Filtration experts, over the years, have discussed combination filtration and debated its definition. In the realm of cartridge filtration, simply defined, a combination filter is one that does at least one other processing job at the same time as filtering a suspension. For example, this could be a carbon canister that removes both suspended and dissolved components. In water applications, a combination filter removes bacteria, sediment, chlorine taste and odour, and scale. In lubrication oil filtration, combination filtration refers to full-flow and by-pass flow filtration. Finally, for small-scale process filtration, combination filtration is installing bag and cartridge filtration systems in series.

There is, however, a new definition of combination filtration that transcends the standard approach and will assist process engineers with troubleshooting and idea generation. The approach relies upon slurry analysis and testing to uncover the process symptom and then develop a process solution called combination mechanical slurry conditioning and filtration. This article discusses slurry testing and process analysis, and then considers seven example installations showing combination mechanical slurry conditioning and filtration.

Overview of bench top testing in the laboratory
BHS’s bench top testing is conducted using the BHS Pocket Leaf Filter (see Figure 1). The testing analyses cake depths, operating pressures, filter media, washing and drying efficiencies, and qualitative cake discharge. The data collection sheets are shown in Table 1.

Combination mechanical slurry conditioning and filtration from bench top testing
The typical testing program is shown in Figure 2. The results and summary of the testing show that the most effective approach for the process filtration is a two-stage process.

The primary filtration is by candle filters on a continuous basis. These filters are able to retain particles down to 0.5 microns and discharge these solids as a concentrated sludge/slurry. In order to reduce utilities, no drying or washing is necessary in the larger candle filters.

The concentrated slurry is then filtered on a secondary pressure plate filter to recover the remaining solvent and discharge the dry solids. The secondary filtration is a batch process that incorporates filtration, counter-current washing, drying and dry cake discharge. The overall result is a very dependable process with high-quality filtrate and minimal utility air and water usage. Each of these processes for the primary and secondary filtration has been tested to confirm that the overall design requirements can be met.

Combination filtration technologies
There are, without doubt, many technologies that can be applied in combination, including the use of chemicals such as flocculants and coagulants, which would be the subject of a filtration-chemical handbook/textbook. These already exist in the marketplace. However, from a practical viewpoint, for the basis of this article, a brief description of five types of filtration technologies is presented based upon general operating conditions at chemical plants.

High-solids slurries: continuous pressure operation
Rotary pressure filter technology provides for thin-cake, continuous production in a single unit. Filtration is conducted via pressures of up to 90 psig. Positive displacement washing or counter-current washing follows filtration. Of course, multiple washing steps as well as solvent exchanges, steaming and extraction can also be accomplished. Finally, the cake is dried by blowing hot or ambient-temperature gas through the cake. The filter has a uniquely designed discharge system, which provides for atmospheric discharge from pressure filtration. After automatic cake discharge, the filter cloth is washed; the clean filter cloth then re-enters the feeding/filtration zone, thereby continuing the process. All solvent and gas streams can be recovered separately and reused in the process to minimise their consumption.

As for the operation, each process zone (typically, there are five to seven zones) is isolated by a separating element. The pressure in the specific zone can be adjusted to meet the process parameters. The separating elements are sealed to the rotating drum via a gas membrane pressure seal. This drum is sealed to the outer housing by drum packing. The drive system with self-sealed roller bearings provides for high stability and low stress on the drive.

High-solids slurries: continuous vacuum operation
High-solids slurries can be defined as up to 50-55% solids in the slurry feed. In some cases, for high solids applications, the slurries can be better handled using vacuum filtration rather than pressure filtration. An example of a continuous, thin-cake technology is a continuous  indexing vacuum belt filter. This technology consists of fixed vacuum trays, continuously feeding the slurry system and indexing or step-wise movement of the filter media. The filter media is indexed by pneumatic cylinders located on the exterior of the unit. The pneumatic operation and fixed trays eliminate the need for a motor and variable-speed drive, and there are no rails/rollers and rubber carrier belt.

Low-solids slurries for clarification and recovery: batch pressure operation
Candle filters and pressure plate filters are installed for clarification and recovery applications from liquids with a low solids content. These units offer full containment, fines removal in the 0.5 micron and finer range, and can be fully automated. The candle filters are vertical candles, while the pressure plate filters are horizontal plates. The major difference between the two units depends on the cake structure that is formed. Some cakes are better handled in the horizontal and some in the vertical. Cake discharge is automatic either by gentle gas expansion of the filter sock in a candle filter or via plate vibration with gas assist for the pressure plate filter. 

Low-solids slurries for clarification and recovery: conventional vs contained filter presses
Conventional filter presses are open design with a series of vertically mounted plates. The filter cloth is hung over each plate, extending from top to bottom, and is held in place by eyelets that fit over the cloth pins on top of the plates. The plates are compressed by hydraulic closing pressure. Cake discharge can be manual or automatic.

Contained filter presses consist of circular filter plates that are sealed in a movable housing. The housing is sealed to 150 psig and full vacuum. The housing is then contained in an outer enclosure. The filter plate design allows for forward and reverse flow operations and can be automatically cleaned to less than 1 ppm residual cross contamination. Process steps can be pressure or vacuum, and cake discharge is automatic using scraper knives.