Containment of Slurries in Continuous and Batch Operations

Slurries
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In the 1970s, the chemical operations used acetone and benzene for the main slurry solid-liquid separation process. Next, there was a push to minimize solvent use. We looked to use water as the process liquid, but still had open filter presses and rotary drum filters; the entire plant was white from titanium dioxide or pharma stearates.

Today, we all know that processes remain open with filter presses, vacuum filters, and centrifuges. Our job is finding solid-liquid separation process solutions that can be contained for high solids slurries (greater than 10% solids) during filtration, cake washing, and dewatering/drying. This discussion considers your options for both batch and continuous operations.

Batch operations

When it comes to batch operations there are many possible ways to go.

Nutsche filter-dryers. Sized to take the complete batch from the reactor and process it to completion (final dryness). The nutsche filter contains an agitator, normally three blades, sealed to the vessel and moving up and down, clockwise and counter-clockwise. The agitated nutsche filter can conduct pressure filtration, cake smoothing, cake washing (displacement and reslurry washing), vacuum and pressure drying, and then automatic cake discharge.

The agitated nutsche filter-dryer is based upon thick cakes from 5–7 cm up to 30 cm and higher. For this type of filter to be successful, the cake permeability must be able to accept a deep cake without compression. Circular or rectangular filter media with a drainage layer is installed on a perforated filter plate.

Contained filter-presses. A contained unit does not require a process change and can operate at a cake thickness down to 2.5 cm, which is not possible in a nutsche filter-dryer. In a typical contained filter-press design a housing seals the plates. Improved designs include pressure filtration up to 1m Pa, cake washing in the forward and reverse direction, cake drying in the forward and reverse direction using pressure blowing and vacuum, as well as automatic cake discharge.

Contained centrifuges. These vary in design depending upon the operation and the type of centrifuge (such as horizontal peeler, inverting basket, and disk centrifuges). Centrifuges can be blanked or inerted for operation as well as sealed designs.

Continuous Operations

In continuous operations with slurries new options surface.

Rotary pressure filters. A continuous pressure filter designed for thin cake to deep cake filtration with cake depths from 6–150 mm. A slowly rotating drum (6–60 rph) 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. 

Pressurized vacuum drum filters. A rotating drum inside a pressure vessel. The unit consists of a filter drum, slurry trough, agitator, wash bars, and a pressure let-down rotary valve. The process begins by closing the pressure vessel, pressurizing the vessel with compressed gas. The rotary valve is also pressurized for sealing, and the filter trough is filled via the suspension feed pipe. The agitator is started to keep the solids in suspension. Filtration, cake washing, and drying are by vacuum operation.

Indexing vacuum belt filters. Provides for vacuum filtration, cake washing, pressing, and drying of high solids slurries. The technology is based upon fixed vacuum trays, a continuously-feeding slurry system and indexing or step-wise movement of the filter media. In practical terms, the operational features of the belt filter can be viewed as a series of Buchner funnels.

For the process operation, due to the stepwise operation of the belt, washing and drying efficiencies are maximized with the stopped belt and a plug-flow mechanism for gases and liquids. Cake pressing and squeezing further enhances drying. Finally, the fixed trays allow for the mother liquor and the wash filtrates to be recovered individually and recirculated, recovered, or reused for a more efficient operation. 

Final Thoughts

Process engineers have many choices to contain an operation. The decision is not easy:

  • Is the process batch or continuous?
  • Is it a thin-cake or thick-cake operation?
  • What is the filter media (synthetic or metal)?
  • What are the critical process steps?
  • What about maintenance and other parameters?

The design questions go on and on. In the end, whatever you choose, involve process, production, operations, and maintenance in your decisions.

This blog is an adapted version of my article for The Chemical Engineer. Read the full article here!