BHS-Sonthofen Inc.

BHS Website A Review of the Rotary Pressure Filter for the Filtration of a Liquefied Gas Slurry


Source: BHS-Sonthofen Inc.

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In the manufacturing of this specialty chemical, the pilot plant examined the process to determine the most effective method for filtration to develop a production process. A study was undertaken, by plant engineering and development, to examine the use of a rotary pressure filter to develop the overall process operations for product quality and production rates.

This paper reviews the basics of two alternatives: rotary pressure filter and pressurized drum filters. The technologies are reviewed for the filtration of a liquefied gas slurry.

The article continues with the discussion of the process testing in the laboratory and in the field to evaluate the Rotary Pressure Filter. The lab testing used a BHS pressurized pocket leaf filter with 20 cm2 of filter area while the fieldwork was conducted on a Rotary Pressure Filter with a filter area of 0.18 m2.

The paper discusses test procedures, data collection, filter media selection and other process parameters. Scale-up data includes process conditions and production rates.

The paper conclusion is two-fold: a review of the unique specifications of the rotary pressure filter required for the process to meet production rates, product quality and site environmental and safety concerns. Finally, the paper demonstrates the creative process solution that is developed with close collaboration between the operating company and the vendor.

A liquefied gas is a clean, colorless gas that is easy to liquefy and transport. It is gaseous at normal temperature and pressure, but changes to a liquid when subjected to modest pressure or cooling. This easy liquefaction makes it easy to transport and store. This and other properties of certain liquefied gases, including a high oxygen content, lack of sulfur or other noxious compounds, and ultra clean make these versatile and promising process solvents. The most important point is to keep these gases, under pressure, so they behave as a liquid during filtration, cake washing and drying. The plant engineers reviewed the Rotary Pressure Filter and the Pressurized Drum Filter as two possible alternative solutions.

The rotary pressure filter, shown in Figure 1, is a continuous pressure filter designed for thin cake to deep cake filtration with cake depths from 6 mm to 150 mm. The slowly rotating drum (6 - 60 rph), in Figure 2, is divided into segments (called cells) each with their own filter media (synthetic cloth or single or multilayer metal) and outlet for filtrate or gas. The outlets are manifolded internally to a service/control head where each stream can be directed to a specific plant piping scheme or collection tank. In this way, the mother liquor can be kept separate from the subsequent washing filtrates and drying gases. This allows for better process control as well as reuse and recovery of solvents and the gases. The service/control head, for this application, is pressure rated so the liquefied gas can be kept under pressure, acting as a liquid.

Figure 3 shows a typical operation as follows. The feed suspension enters each cell, under constant pressure, to form a filter cake. Internal divisions of the housing allow the cake to be processed in completely separate zones. Each zone can operate under different pressures depending upon the compressibility of the cake at each stage of filtration, washing and drying.

For example, pressure filtration is conducted up to a maximum differential pressure of 6 bar. Pressure filtration has the added benefit of eliminating post-precipitation of the solids if the process solvent is prone to flashing under vacuum filtration. In this case, the 6 bar pressure differential allows for a back-pressure of 2-bar or more, such that filtration can be at 4 bar and the gas can be a liquid.

A second benefit of the slow-rotating pressure filtration is the reproducible cake depth and the ability to control the residence time (by the speed of the drum). The slow rotation also results in much lower maintenance and energy costs.

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