Chemical Process Optimization needs Out of the Box Thinking

Actual Unit -- Figure 2. This continuous vacuum belt filter with 9.0-m2 filter area was installed in 2016.
A continuous vacuum belt filter with 9.0-m2 filter area.

Loyal readers of this blog know how much I value innovation and creativity. So, you can’t be surprised that I want to share with you a success story in which we partnered with a client to develop an optimized filtration process for a zinc oxide product.

As discussed in a coauthored article for Chemical Processing, Madison Industries and BHS-Sonthofen Inc. worked together on laboratory and field pilot testing. Engineers from both firms showed creativity and “outside-the-box” thinking in looking at the process from new vantage points in their quest to find a better option than the installed batch filter press.

Our efforts led to the selection of continuous vacuum filtration. The continuous filter, which was installed in 2016, provides maximum filtration efficiency and improves product quality while increasing yield and reducing operating and maintenance costs.

Schematic Operation Of Filter -- Figure 1. Technology, based on fixed vacuum trays, features step-wise movement of filter media.
Figure 1. Technology, based on fixed vacuum trays, features step-wise movement of filter media.

Case Background

Madison Industries, based in Old Bridge, N.J., supplies copper and zinc compounds such as copper sulfate, copper carbonate, zinc sulfate, zinc chloride, zinc orthophosphate and phosphoric acid as well as other chemical products containing copper and zinc. Applications include animal feed, water treatment, dairy farming, food and pharmaceutical processing, and pool and wood preservative chemicals, among others.

The Madison facility was using a plate-and-frame filter press to filter a zinc oxide slurry made from a mix of various zinc feedstocks. The solids were mixed with water to form a slurry of 20% solids and then filtered. The cake was bagged in 2,000-lb totes, moved to another area of the plant and reslurried in sulfuric acid for further processing.

Madison wanted to expand production and replace the present labor-intensive process with a continuous operation.

Crucial Tests

BHS process engineers began laboratory evaluation of the process. Madison was open to all ideas and formed a team to brainstorm different approaches.

BHS conducted several weeks of testing and evaluated both pressure and vacuum filtration based upon the specific characteristics of the solids and slurries. The testing led to the following observations:

• Filtrate clarity: The most-appropriate filter cloth is a double-weave 12-micron polypropylene.
• Filtration rate: Vacuum filtration produced the maximum filtration flux rate at a cake thickness of 6 mm.
• Cake washing: Maximum displacement washing was achieved with wash ratios of 2.6:1.
• Cake moisture: Although not a critical parameter because the cake is reslurried, cake moisture is approximately 35%.

Based on its creative testing, BHS’s process engineers recommended continuous-indexing vacuum filtration as the optimum option.

Why Continuous Indexing

The BHS continuous-indexing vacuum belt filter 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 (see Figure 1). In practical terms, the belt filter operates similarly to a series of Buchner funnels.

At each indexed belt position, washing and drying efficiencies are maximized with the stopped belt and the mechanism of plug flow for gases and liquids. Cake pressing and squeezing further enhance drying. Finally, the fixed trays allow for the mother liquor and the wash filtrates to be recovered individually and recirculated/recovered/reused for a more efficient operation. The design also can integrate steaming as well as counter-current washing.

Successful Switch

Madison and BHS installed the vacuum belt filter in 2016. The unit has met all product quality specifications. Madison has realized a 50% savings in wash liquids per batch as well as a reduction in labor and operating costs because the vacuum belt filter operation is fully automatic. Since the installation, Madison has optimized the operation, improving yields and minimizing costs.

The Madison and BHS collaboration illustrates a successful relationship between client and technology supplier. The BHS approach of lab and pilot testing, coupled with idea-generation, fosters identifying the optimal option for critical and difficult solid/liquid separations.

Selecting the Right Type of Filtration for Solid-Liquid Separation

solid-liquid separation
Photo by Picturepest on / CC BY

Filtration selection, if we think back to Sherlock Holmes, means “not jumping to conclusions.”  There is no “one size fits all” process solution.  Selecting a filtration technology requires a systems approach incorporated with other solids processing such as reactors, dryers, solids handling, etc.  You could gain an objective overview by filling out an application data sheet (like the ones I use for new or existing applications) that can help identify what’s involved in the specific solid-liquid separation.

Ultimately, the process has three components:

  • Material properties, which I’ll describe in more detail below
  • Separation performance objectives including, for example, filtrate quality (conductivity or residual solids) cake dryness, flowability of the cake, crystal breakage /fines generation and conditioning of the cake for further processing
  • Mechanical properties — The specification must be clear in terms of material of construction, temperatures/pressures, FDA validation, cleaning procedures, manufacturing codes, etc.  Each equipment type will have its own mechanical specifications that must be satisfied.

These three considerations are combined and ranked choices are then evaluated for operational, economic, and plant (internal and external) objectives.

Finding the Best Filtration Procedure

Your examination of material properties considers the solids and the liquids.  For solids, the engineer needs to know the total suspended solids (TSS) and solids concentration, particle size distribution (PSD), and particle shape.  The PSD should be based upon particle counts at different sizes rather than by weight or volume.

The particle shapes can vary:  spheres, rounded, angular, flaky, or thinly-flaked are among the examples.  Shape will influence the filtration rates for the process and also impact the PSD due to the nature of particle size measuring equipment.

Knowing this, the solid-liquid filtration system further requires a systems approach to incorporate other solids processing such as reactors, dryers, and solids handling, etc.  The full scope should include the actual upstream and downstream.

Consider this typical example of a chemical process including all of the associated processing steps:

  • Chemical synthesis and Crystallization:
    • Types of catalysts
    • Solvents
    • Continuous or batch
    • Temperature
    • Flashing
    • Inerting
  • Filtration
  • Drying
  • Dissolution
  • Hydrogenation
  • Secondary crystallization
  • Filtration
  • Final drying
  • Solids and slurry handling in all steps

General Guidelines to Selection

So, the question is where to begin to make the preliminary filtration technology choices for solid-liquid separation?  Here are some general guidelines to selection:

Filter Press Continuous Vacuum and Pressure Nutsche  Filter & Filter-Dryer Clarification
Solid content of the suspension (%) 5 to 30 10 to 40 10 – 40 < 5
Maximum Pressure Difference 100 bar -1 to 6 bar 6 bar 10 bar
Cake Thickness (mm) 5 to 50 5 to 150 5 to 300 20
Average Particle Size 1 to 100 micron 1 to 100 micron 5 to 200 micron 1 to 50 micron
Type of Operation Batch Continuous Batch Batch
Comments Good for slow filtration and can produce dry filter cakes; Excellent cake washing and pre-drying Good when reactor batch times equal to total cycle times Disposable for low flows; candle and plate filters for large flows

Let me know if this is helpful to you.  My idea is to do a series of types of filtration systems for solid-liquid separation for various applications.  What is troubling you?