As we announced in March, BHS has acquired AVA-GmbH. The AVA technologies provide for turbulent mixing, reacting and drying of wet cakes as well as powders and process slurries. The vertical and horizontal technologies are vacuum or atmospheric, batch and continuous, for final drying to “bone-dry” powders. They are an essential part of our “Continuum Approach.”
What do I mean by that? This blog briefly reviews our first ground-breaking study showing the benefits of a “Continuum Approach” to final drying and upstream solid-liquid filtration, cake washing and dewatering.
Most often when analyzing a new process development approach, engineers take a “silo” approach and look at each step independently.Our article illustrates that by taking a holistic approach and looking at each step not individually but as a continuum, the process solution becomes much more efficient.
In the manufacturing of the specialty chemical involved, the crystals coming from the reactor in a methanol slurry had to be filtered, washed and dewatered and then dried to a final moisture of less than 1.0 (<1.0 %).The standard approach would be to first look at the solid-liquid filtration step and optimize this step for the maximum washing and drying efficiency. Then, with this information, we’d optimize the downstream drying.The operating company, however, took a different approach and looked at the process as a continuum from solid-liquid filtration through cake washing and dewatering to final drying.The Continuum Approach” resulted in operational energy and nitrogen savings as well as lower capital and installation costs for a more efficient and reliable process.
You can read the full technical article, but the overall result was a 50% decrease in the filtration area, elimination of a nitrogen recovery system with a 30 minute increase in batch drying time at a lower temperature for better product quality.
Contact me to optimize your current drying and filtration process.Let’s get more efficient together!
Our approaches to process engineering must always be evolving. Otherwise, we’ll never grow and innovate. Recently, I contributed a feature to The Chemical Engineer on making the change from batch to continuous processing. Here’s an edited version of that article for my loyal blog readers.
As there is a push to become more efficient, many process industries have begun thinking about continuous processing. Many specialty and fine chemical operations are batch operated. It is easy and typically uses filter presses, vacuum nutsche filters, filter-dryers, plate and leaf filters, and batch centrifuges.
Yet batch processing significantly lacks flexibility in scaling capacity, and typically requires larger manufacturing footprints and less efficient use of space. So, I’ve been seeing more of a shift from batch to continuous processing.
In my career, I’ve helped engineers move to continuous operations for such applications in pharma and biochemical, specialty polymers, starch and cellulose, aromatic acids and fly ash wetting.
Why? In continuous processes:
a filter is typically one-third the size of a batch filter
the process can increase yield and optimize quality
there are fewer reslurry/holding/buffer tanks
transfer pumps can be eliminated
complications from solids handling can be minimized
it can be easier to maintain constant flows, pressures and temperatures
Applications of Continuous Processing
In the article, I shared several examples of continuous processing applications in my career. I’ll recap a couple of them here too.
In a specialty chemical polymer application, a client wanted to transition to continuous processing to eliminate solids handling and reslurry tanks. Eliminating the liquid ring vacuum pump required for vacuum filtration would also cut energy costs. At BHS Filtration, we did lab and pilot testing to determine the rotary pressure filter was the best option.The continuous pressure filter saw a 16% increase in filtration rate; maintaining the temperature at -5oC resulted in a higher capacity. Secondly, we saw a more efficient washing due to less cake cracking in the thin cake (5 mm) as compared with 150 mm (6-inch) cake.
For a pharmaceutical client, BHS was involved with a transition to fully-automated continuous processing in extracting phospholipids from egg yolk for preparation as a pet food additive. After consulting with the client and testing, the choice was a continuous-indexing vacuum belt filter for vacuum filtration, cake washing, and dewatering of the cake. The technology is based upon fixed vacuum trays, a continuously feeding slurry system, and indexing or stepwise movement of the filter media. In practical terms, the operational features of the belt filter can be viewed as a series of Buchner funnels. Making that change to the filter validated, as a GMP installation, for pharmaceutical production has increased the yield of the phospholipids by 3–5%.
In doing this kind of work, we’ve run into different challenges. We’ve been reminded that process scale matters and what works in the lab may not work in the plant. We’ve seen the need to silo both batch and continuous processes in the same line as a continuum. We’ve been reminded of the need to understand how one upstream decision will impact downstream processes.
We must also remember making the transition from batch to continuous processing requires more than just new equipment. The entire manufacturing operation and the mindset of staff need transformed.
Process engineers have many choices to transition to a continuous operation. Continuous can be more challenging, but the benefits are there. Just be ready for some unexpected consequences along the way, and always test, test, test!
Of course, if you want to read the entire article, and I hope you will, it’s available! I’d be happy to discuss any of the ideas or possible applications of these insights with you. Reach out to me today!
As the chemical industry changes and becomes more integrated worldwide, there is a need for information exchange. This must include not only principles of operation but practical knowledge transfer. That’s why I have agreed to edit a new book for Elsevier, “Integration and Optimization of Unit Operations.”
As my readers know, in 2015, I published the “Handbook of Solid-Liquid Filtration” with Elsevier, UK. This new project offers up-to-date and practical information on chemical unit operations from the R & D stage to scale-up and demonstration to commercialization and optimization.
For this exciting and unique book to work, I need your help. I’m currently seeking contributing authors who have skills at each stage of the process from lab-scale/R&D, through pilot plants to full-scale production and finally optimization or as I call it, Putting-It-All-Together, for actual case histories / war stories. We will also cover decommissioning of plants. Check out the preliminary Table of Contents.
Currently, most books look unit operations, each in a silo. In this book, at each stage, the information presented differs as the technology and issues faced at the lab scale differ through commercialization and optimization. So, we will move from a silo approach to an integrated – holistic approach.
Why this Book is Needed
This book addresses a need for engineers with a broader training background. In the early 70’s, companies wanted staff with an I-shaped skill level. Someone with I-Shaped Skills is a person with a deep (vertical) expertise in one area and practically no experience or knowledge in other areas. This person is typically known as a specialist.
Then, in the 1980s, the industry wanted T-shaped professionals. The vertical bar on the T represents strong knowledge in a specific discipline. The horizontal bar represents a wide (horizontal) yet shallow knowledge in other areas. This allows the person to be able to collaborate across other disciplines and acquire new skills or knowledge.
Yet what we need today, with the rapid proliferation of technological advances and the cross- disciplinary nature of our work, is key-shaped engineers who can address several areas of expertise with varying degrees of depth.
This book aims to address the needs of engineers who want to increase their skill levels in various disciplines so that they are able to develop, commercialize and optimize processes. The engineers must be able to ask questions of experts to develop creative solutions.
What Can You Contribute?
Contributing authors should be able to discuss unit operations at each stage and then relate how these technology/process decisions impacts the next stage. I am targeting the first draft by the end of the year. I will provide technical guidance and assistance as well as from my associate who is skilled in technical writing along with the Elsevier requirements.
The book will be listed on ScienceDirect, Elsevier and others and chapters will receive individual indexing so they can be searched. Review the preliminary Table of Contents and let me know what interests you to write about!
Solids handling is not a unit operation. Therefore, it’s not covered in engineering courses. This leaves process engineers struggling to understand the “flowability” of bulk solids. This blind spot is huge. So, let’s talk about dryer selection and bulk solids handling.
Recently in The Chemical Engineer, Grant Wellwood described bulk solids handling as the biggest industrial activity on the planet. The article estimated “that >70% of everything we use or consume involves bulk solids handling somewhere in its lifecycle.”
Mishandled, this process can quickly and efficiently destroy product value, careers, projects and even organizations. Yet, bulk solids flow is often an afterthought once the separation and drying equipment is selected. This article aims to bring bulk solids handling to the forefront.
Bulk Solids Handling Parameters
Bulk solids are defined as materials (solids) handled in various volumes and counts. Their flowability is impacted or controlled by friction (particle-particle or particle-surface). During the drying process, solids go through different phases such as free moisture, bound moisture, thixotropic and finally (and hopefully) free flowing.
The selected dryer must be able to handle each phase without creating fines, balls that can trap liquids, and without adding additional heat due to friction.
Here are some of the process and design parameters engineers need to consider for dryer selection:
Dryer Process: Batch, Continuous, Atmospheric/ Vacuum, Turbulent, Gentle, Ring-Layer, Feeding(Volumetric or Gravimetric), Upstream and Downstream Equipment
Recipes: Number of ingredients, Frequency of campaigns, Cleaning operations, Product integrity (fines generation) after drying andResidence time
Product Characteristics: Quality, Bulk density, Tendency of segregation & agglomeration, Thixotropic phase, Shape, Size, Homogeneity, Risk of separation, Flow properties, Abrasiveness, and Moisture & Temperature
Mixer design: Material of construction,Surface quality, Heating/cooling, Liquid feeding, Type of mixing tools, Speed of mixing tools and degree of back mixing
Dryer Integration: Material flow, Physical space, Process sampling, safety requirements, etc.
It’s a lot to think about. Westwood observed in his thorough article, “When handling bulk solids, it’s always important to take a holistic or systems view because of the complex dependencies.”
BHS & Bulk Solids Handling
As my readers know, BHS provides for thin-cake filtration, cake washing and dewatering based upon pressure or vacuum, for batch or continuous operations from high solids slurries to clarification applications with solids to 1% and trace amounts.
In 2018, BHS acquired AVA mixers and dryers based in Herrsching (Munich) Germany.VA is in the unique position to provide both vertical and horizontal technologies providing for turbulent as well as gentle mixing, reacting and drying of wet cakes, powders and process slurries. The technologies are vacuum or atmospheric, batch and continuous, for final drying to “bone-dry” powders. The BHS technical article, Dryer Selection, explains the designs as well as selection parameters.
We know that solids change when processed from a wet-cake to bone-dry powder. Process engineers need to do the tests and trial and error to better understand these changes. As I often say, we can’t jump to conclusions.
Our process engineers would be happy to help at the BHS test center. With an understanding of how the flow properties change, depending on “complex interactions between particle size and distribution, moisture content and distribution, process history (time and manner), mineral composition, surface texture and condition as well as ambient conditions, just to name a few…” the dryer selection can begin in an educated manner.
Good luck and feel free to contact me for help with your bulk solids handling questions.
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.
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.
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.
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!
Recently, I discussed the five management lessons that we can learn from the Apollo lunar landing in 1969. Continuing on this theme, an article in The Chemical Engineer, “Houston-We have a checklist” a UK magazine that I write for, had an interesting take on the lunar landing and engineer checklists. I was intrigued, of course, as I periodically invoke Sherlock Holmes and the benefits of checklists for testing, analysis, etc.
The magazine article, written by Mark Yates, looks at the checklists used both at Mission Control and in space. He takes us through the Apollo missions where there could be two spacecraft both operating remotely 240,000 miles from Earth and out of communications contact with Earth for significant periods of time.
Checklists and cue cards covered everything from mission rules, abort criteria, emergency procedures and activation of backup systems in the event of a total failure of a primary control system for example. These checklists and procedures went everywhere. In fact, each Moon-walking astronaut would have a book of procedures strapped to his left wrist that he could follow out on the lunar surface.
In fact, all of the Apollo crews would each log over 100 hours familiarizing themselves with the numerous procedures and checklists. Apollo 11’s Command Module Pilot Michael Collins called them the “fourth crew member.” These checklists were also one of the first examples of digital computers and man being able to operate together seamlessly. One of the actual checklists used by the Apollo 11 crew is shown below:
Chemical Engineering Checklists
How do we use checklists in chemical engineering? We have many uses for them. For example, if you visit an earlier blog, you’ll find checklists and application details for filtration testing.
Being a process engineer is all about making choices. When it comes API filtration technologies, many different types of equipment can be used for removing catalyst residues. While conventional filtration equipment is operated manually, I recently worked with PharmTech on an article outlining how both candle filters and pressure plate filters are operated as automated systems. This article reviews what we discussed.
Pharmaceutical manufacturers are increasingly looking for automated equipment with in-line process control. Well, automated candle and pressure-plate filtration equipment for removing catalyst residues from API slurries are operated in a closed system. This automated filtration also meets the demand for improved safety and reliability by removing the manual operation.
Conventional or traditional filters can be defined as bag filters, cartridge filters, manual plate filters, and plate and frame filter presses. These are all manually operated filters. They are not really sealed—especially not when solids get discharged.
Candle filters and pressure plate filters are improvements over these types in terms of reproducible quality, multiple process steps, cleanable and reusable filter media, and full containment for solids recovery.
A major difference is that the operation of plate filters and candle filters is 100% automated. Solids discharge is provided in a sealed and safe way.
When to Use Candle or Pressure Plate Filters
Deciding between candle and pressure plate filters depends largely upon the cake structure developed by the process solids.
Cake structures that can maintain their integrity in a vertical form are suited for candle filters. If the cakes themselves are too dense or too light or tend to crack, a horizontal plate filter is the better choice of technology. Thickness of the cake structure is another decision parameter. Candle filters typically have maximum cake thickness of 20 mm, while plate filters can handle up to 75 mm.
Generally, the candle filters and pressure plate filters can be used interchangeably based upon the cake structure itself. Some cakes can be handled in either vertical or horizontal form. In that case, the process dictates the choice.
When it comes to deciding the best filtration type for continuous or semi-continuous processing, consider the upstream and downstream equipment. Both candle filters and pressure plate filters are batch operations. For continuous or semi-continuous operations, either multiple units are required or buffer/holding tanks can be installed.
Pharma Disposal or Recycling
We also discussed best practices for disposal or recycling. For non-hazardous disposal, the cakes can be first washed to remove all of the toxic or hazardous compounds and then dried to a standard of no free liquids. The cakes can be fully discharged in a contained and dust-free manner to totes or drums.
For recycling, the process solids can be reslurried within the candle filter or pressure plate filter to be pumped back as a slurry to the process. The process liquids or filtrates can also be pumped back to the upstream reactors for reuse.
Questions about alternative API filtration technologies? Other decision parameters I didn’t think about? Let me know, I’m always ready to chat.
This is an edited version of an article I wrote for The Chemical Engineer. I hope you enjoy this take on automated clarification technologies.
Throughout my career in the solid-liquid separation market space, I have seen some interesting solid-liquid separation solutions. At one melamine resin facility, the slurry was in a formaldehyde process. The operators were wearing masks, opening up a manual plate filter in a room with residential floor fans, to dig out the cake from the paper filter media.
In another case for zeolites, the client had multiple bag filters to clarify the filtrates following a vacuum belt filter. When the filtrates, the final product, remained cloudy, to my surprise, the client decided to add another set of bag filters!
A clarification system is employed after coarse-particle filtration or as a stand-alone system to remove fine particles at low concentrations. These particles are typically less than 5 µm and are in concentrations less than 5% solids down to ppm levels.
Process engineers struggle to clarify process liquids. But there are ways to automate the clarification processes to improve filtration and minimize operator exposure. The cake solids structure and the nature of the process will determine which types of pressure-filtration, automated clarification technologies are best for you.
A candle filter is a pressure vessel filled with tubular filters (called filter candles). A typical filter candle comprises a dip pipe to flow the filtrate and pressurized gas, a perforated core with supporting tie rods, and a filter sock.
As the cake builds during operation, the candle filter’s removal efficiency increases, enabling removal of particles as small as approximately 0.5 μm.
During operation, a feed pump or pressure from the reactor or feed tank forces the slurry into the bottom of the pressure vessel. The solids build up on the outside of the filter sock, while the liquid filtrate flows into the candle, through the registers, and out of the vessel. This process continues until the maximum pressure drop, design cake thickness, minimum flow, or maximum filtration time is reached. The cake is washed to remove impurities and residual mother liquor, and then dried by blowing gas through the cake. Next, low-pressure gas enters the individual candles and expands the filter socks.
This process breaks apart the dry cake, which detaches from the filter sock and falls into the vessel cone. Candle filters are used for thin-cake (5–20 mm) pressure filtration applications. They are best suited for filter cakes that are vertically stable.
Pressure Plate Filters
Like the candle filter, pressure plate filters comprise filter elements contained within a pressure vessel. However, instead of vertical filter candles, the vessel containshorizontal filter plates. These elements are slightly sloped, conical-shaped metal plates that support a coarse-mesh backing screen covered with filter cloth.
An opening in the centre of the plate allows the filtrate to travel between plates and throughout the vessel. The filter cloth can be synthetic, as in the candle filter, or metallic as the cake discharge is by vibration or spinning.
Operation is similar to that of a candle filter. For cake discharge, there are two main designs. In one, two unbalanced motors vibrate the filter plates to dislodge the cake from the filter cloth. In a second design, the plates spin so that the cake can be ‘thrown’ off the plates. Pressure plate filters are used for filtration of cakes up to 75 mm thick.
Sintered Porous Metal Cartridges
Another type of automated clarification technology is based upon sintered porous metal cartridges. These can be used in a variety of process flows such as inside-out filtration. After each cycle, solids are backwashed off the inside of the elements and discharged as a concentrated slurry or wet cake. They can also operate in a conventional outside-in filtration. Porous metal cartridges are used for high temperature applications greater than 200oC where the solids are well-defined hard crystalline shaped.
Filter aids are generally the last resort. Often in clarification applications, the solids are very fine or amorphous, so can be difficult to filter. When filtered, the solids will create a thin, impermeable coating over the filter media and immediately reduce the filtration rate to an unacceptable level. In these difficult cases, filter aid pretreatment can be used to improve filtration properties and efficiently remove the fine solids from the process liquids. The types of filter aid include diatomite, perlite, and cellulose.
In the precoat method, filter aid is used to generate a thin layer of solids on top of the filter media. Once formed, the filter aid cake functions as the primary filter media. Therefore, the filter cloth is no longer the real filter when precoat is used. For that reason, the filter cloth should be as open as possible while still retaining the filter aid material.
The precoat process is achieved by mixing the filter aid into clear liquid or mother filtrate in a precoat tank. This slurry is then recirculated through the filter where the solids are captured by the filter media. The clean filtrate is recirculated back into the precoat tank. The precoat should be thick enough to ensure that the entire media surface is coated but thin enough so that it does not provide significant resistance to filtration.
In body feed filtration applications, the filter aid is blended with the slurry feed either by dosing the concentrated filter aid suspension into the slurry feed with in-line mixing or by mixing the filter aid into the entire slurry batch and maintaining agitation. By adding filter aid into the slurry feed, the resulting filter cake is more porous, allowing higher and longer sustained flow rates. Body feed also helps to restrict solids movement which improves filtrate clarity.
Needless to say, the use of filter aid improves filtration but requires more equipment, more process control, and results in more solids for disposal.
Final Thoughts on Automated Clarification Technologies
How can the process engineer be successful? When confronted with a clarification process, don’t simple throw more bag filters at the problem. Conduct lab testing to analyze the cake structure, filter media, filtration pressure and cake thickness. With the data in hand, you can evaluate the different technologies and design a more reliable and cost-effective clarification process. Find a different approach!
P&IDs are par for the course in process engineering. Recently, I was poring over P&IDs and process planning for several projects. Each project was multinational, multicultural, and extremely complex. For one specialty chemical filtration application, part of a plant expansion in the southern United States, the engineering company is in the Southeast while the existing processes were from the Netherlands and Austria. In another project, with a similar scope, the plant expansion and the engineering company were both in the Northeast U.S., yet the current processes operate all throughout the UK.
As you can imagine, the piping and instrumentation diagrams (P&IDs) had many changes, each shown in a different color —the Christmas Trees of P&IDs.There were extensive e-mail threads of comments and questions and, of course, questions/comments about the comments/questions. Plus, the projects required equally fun conference / video calls accounting for time zone differences, various languages and accents, and varied engineering cultures and operating philosophies. You’ve been in this situation too, I’ll bet.
The discussion, though, is invigorating. The idea exchange goes well beyond solid-liquid separation to encompass types of valves, types of pumps, where to put the pumps, how to handle the solids, operator safety, disposal, and on and on and on.I even had a question about desalination and how to operate the DAF (Dissolved Air Flotation) units (that’s a topic for another blog).
Developing A New Process Path with P & IDs
After one of these calls, I had an “A-Ha” moment about the true value of our plentiful rounds with P&IDs and process. This is where the innovation happens. The P&IDs are idea development in action. This is where we, as I wrote in one of my earlier blogs, clear our path of unknowns.
Anyone who’s read my blog consistently will recognize this is what is excites me about process engineering and all we do in this role. I’ve decided to take my own early 2019 advice and stretch myself in new directions with the birth of “P&ID-Perlmutter Idea Development” which you can find at perlmutter-ideadevelopment.com.
To me, these two sites work together like a candle filter functions better with the right filter sock. I’m excited to see how this idea develops, and eager to see what my readers, colleagues, and fellow bloggers will want to add and change and discuss (after all, it’s a P&IDs and process we’re talking about here).
Filter aid pretreatment can improve filtration properties and efficient removal of fine solids. Whether the filter aids are used in Plate-and-frame filter presses, horizontal and vertical pressure leaf filters, candle or tubular filters, Nutsche filters, or rotary vacuum drum filters, these practical tips can help this part of the process run smoothly.
We typically see diatomite, perlite and cellulose filter aids today. They meet the requirements of a filter aid in that they:
Consist of rigid, complex shaped, discrete particles;
Remain chemically inert and insoluble in the process liquid.
You’ll want to test different approaches to determine the best aid for your process and which of the methods — precoat or body feed — offers the greatest benefits. Once you’ve done so, though, it’s important to keep these troubleshooting tips in mind.
Practical Pointers for Using Filter Aids
Whether the process is precoating or body feeding, the filter aid slurry tank and pump are critical to the operation.
In precoating, the mix tank should be a round, vertical tank with a height twice its diameter. Set the usable volume of the precoat tank at ≈1.25–1.5 times the volume of the filter plus the connecting lines. Use a mixer or agitator with large slow-speed impellers to avoid filter aid degradation and the creation of fines — otherwise you’ll dramatically change the filter aid process filtration.
The precoating pumps almost always are centrifugal pumps because they produce no pulsations to disturb precoat formation and their internal parts usually have hardened surfaces and open impellers to reduce wear. For body feeding, you’ll use positive displacement pumps.
Yet even when the feed tank and pump are correct, several typical issues with filtration/filter-aid systems can arise.
Bleed-through is common where the filter aid is bypassing the filter media. It may stem from mechanical, operational or process causes. Check a couple of mechanical points:
Is the filter medium secured to the filter correctly?
Does the filter medium have a tear or pinholes?
Is the type of filter aid correct for the filter medium mesh size and the particle size distribution of the process solids?
Another issue may be reduced filtration cycles — i.e., the time to reach the maximum pressure drop becomes shorter and shorter. This may occur:
if the cake isn’t being discharged completely, then each new batch has residual solids in the filter, resulting in lower capacities. Increasing precoat height or lengthening cake drying time may help improve cake discharge.
if the precoat doesn’t completely cover the filter medium, then the process solids may begin to blind the medium.
if you’re using body feed, inadequate mixing with the process solids may result in filter medium blinding. This also can happen if the velocity in the filter vessel is too low, which will allow the filter aid to settle out before reaching the filter elements. A bypass at the top of the filter vessel can help keep the solids suspended within the vessel.
On filters with vertical elements, precoat pump flowrate or pressure may cause loss of the precoat from the filter medium, Improper valve sequencing creating a sudden change in the pressure or flowrate may also be to blame. Finally, a mechanical issue with the filter may prompt a pulsation or pressure change that impacts the cake structure.
Apply Filter Aids Wisely
Employing filter aids to help filtration is tricky; most process operations try to eliminate or minimize their use. However, sometimes they are unavoidable.
To succeed with filter aids, a process engineer should take three essential steps:
Conduct lab testing to examine the filtration operation (vacuum or pressure), cake thickness, filter aid quantities, filter medium and other parameters that are crucial to the process design;
Ensure correct mechanical design to provide optimum precoat or body feed handling and distribution; and
Arrange for operator training on the filtration technology as well as on filter aid operation.
This blog is an edited version of an article I co-authored with Garrett Bergquist, BHS-Sonthofen Inc. for Chemical Processing.