Weighing Alternative API Filtration Technologies

API filtration technologies
Image source: Google images

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.

First, though, you need to understand the difference between candle filters and pressure-plate filters and how they differ from tradition filters. 

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. 

Clear thinking about automated clarification technologies

automated clarification technologies
Photo by SplitShire on Pexels.com

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.

Candle Filters

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 

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!

 

 

Removing Catalyst Fines From Raney Nickel Catalyst Reactions

 

Raney nickel catalysts
Image source

Whether you call it Raney nickel or Raney mud, this alloy of aluminum and nickel is a reagent common to many organic processes. Currently, most Raney nickel catalyst slurries are clarified with the use of manual plate or nutsche filters, bag filters, or cartridge filters. 

Yet any of these approaches require manual operations for cake discharge and cleaning between batches or campaigns. At the same time, they accrue high labor, maintenance and disposal costs  and  expose operators and the environment to toxic and hazardous solvents, solids and contaminated filter tools.  

BHS developed a more contained, cost-effective approach using batch-operated, pressure-filtration systems candle filters. 

A Candle Filter Primer

A candle filter is a pressure vessel filled with tubular filters called candles. The candle is comprised of a filtrate pipe, a perforated core with supporting tie rods, and a filter sock.

The filtrate pipe runs the length of the candle and ensures high liquid flow, as well as maximum distribution of the gas during cake discharge. The tie rods create an annular space between the filter sock and the perforated core, which helps maintain a low pressure drop during operation and promotes efficient expansion of the filter sock during cake discharge. The filter sock, made of various synthetic materials, is installed over the candle and can remove particles smaller than 1 micron (μm).

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, pressure from the reactor 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 filtration time is reached. 

For concentrated cake discharge, low-pressure gas enters in the reverse direction through the registers and into the individual candles and expands the filter socks. This process breaks apart the cake, which detaches from the filter sock and falls into the vessel cone. The cake is then discharged as a concentrated slurry. 

Raney Nickel Catalyst with Candle Filters for Slurry Discharge

In this application, the current process after the reactor is gravity separation, hydrocyclones and then followed with cartridges and bag filters.  The specification for the process liquid (diamine and water) is less than 3 ppm catalyst.  This recovery process was inefficient and exposes the operators to the diamine and catalysts creating a safety hazard.  The average particle size is 2 um and amorphous crystals.   

Lab testing and pilot testing was conducted to determine a processing scheme that eliminates solvent exposure, reduces the maintenance and operation requirements of the current scheme and recovers the catalyst to less than 3 ppm.  The final design was a BHS slurry-discharge candle filter with 19 m2 of filtration area. 

Candle Filters for Raney nickel Slurry Discharge

BHS developed this approach working with a client whose process after the reactor included gravity separation, hydrocyclones, then followed with cartridges and bag filters. The specification for the process liquid (diamine and water) was less than 3 ppm catalyst. The average particle size was 2 um and amorphous crystals. Yet, this recovery process was inefficient and exposed operators to the diamine and catalysts, which created a safety hazard.  

BHS conducted lab  and pilot testing to determine a processing scheme that eliminated solvent exposure, reduced maintenance and operation requirements, and recovered the catalyst to less than 3 ppm. The final design was a BHS slurry-discharge candle filter with 19 m2 of filtration area. Learn more about this application in this article.

Novel Filtration Technologies for Pharmaceutical Hydrogenation

pharmaceutical hydrogenation
Image source

When it comes to removing catalyst fines from pharmaceutical hydrogenation reactions, BHS Filtration has come up with a novel approach.

Currently, most hydrogenation slurries are clarified with the use of manual plate or nutsche filters, bag filters, or cartridge filters. All of these require manual operations for cake discharge and cleaning between batches or campaigns. At the same time, these units suffer from high labor, maintenance and disposal costs as well as the exposure of the operators and the environment to toxic and hazardous solvents and solids, used and contaminated filter cloth, bag filters, and filter cartridges.  

A new approach uses candle filters which are batch-operated, pressure-filtration systems.

Understanding Candle Filters

A candle filter is a pressure vessel filled with tubular filters called candles. The candle is comprised of:

  • filtrate pipe —  runs the length of the candle and ensures high liquid flow, as well as maximum distribution of the gas during cake discharge.
  • perforated core with supporting tie rods —  the tie rods create an annular space between the filter sock and the perforated core, which helps to maintain a low pressure drop during operation and promotes efficient expansion of the filter sock during cake discharge
  • filter sock — installed over the candle, and made of various synthetic materials, the filter sock is capable of removing particles smaller than 1 micron (μm). As the cake builds during operation, the candle filter’s removal efficiency increases, enabling removal of particles as small as approximately 0.5 μm.

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Candle Filter in Action

During operation, pressure from the reactor 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 filtration time is reached. The cake is then washed to remove impurities and residual mother liquor. Finally, the cake is dried. 

For cake discharge, low-pressure gas enters in the reverse direction through the registers and into 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. The cake can also be discharged as a concentrated slurry. 

Pharmaceutical Hydrogenation Application 

In the pharmaceutical catalytic hydrogenation application, the current process after the reactor is metal bag filters for slurry discharge into manual nutsche “clamshell” filters for vacuum filtration and drying.  The process was time-consuming and required handling of liquids and solids including final “manual dig out” of the filters.  The process solvent was tetrahydrofuran (THF) and ethanol.

Lab testing was conducted to develop a new, one-step process for filtration and drying.  The BHS candle filters with pharma designed candles and cGMP compliance allowed for a revamp of the operation with two filters, one-on/one-off for continuous operation.  Read the full article and let me know what you think!

Filtration more art than science?

batch filtration
Photo credit: Timothy Neesam (GumshoePhotos) via Foter.com / CC BY-NC-ND (Yep, that’s Toronto in the background)

At BHS-Filtration, our Art and Science of Filtration (AS0F) newsletter is now in its eighth year. Instead of experiencing a seven-year-itch, we’re perhaps facing an eight year one as I started off the latest installment wondering if filtration isn’t sometimes more of an art than a science.

As we continue into 2017, the newsletter will focus on the art of filtration through case studies and creative problem solving. To start us out, issue 8.1, focuses on the challenges of continuous processing as well as batch filtration and removal/clarification examples.

Continuous Filtration and Scaling Up

In the chemical process industry (CPI) as well as new technology arena (NTA), there are inherent risks and benefits to scaling up from the laboratory / bench top through pilot, demonstration and then finally commercial scale. ASoF provides an article following the process filtration approach developed for each technology stage gate starting from batch filtration in the lab to continuous filtration for the full-scale commercial operation.

The article is the basis for a BHS-led discussion at The 2017 Process Development Symposium organized by AICHE. Join us June 6 – 8 in beautiful Toronto to exchange wisdom, knowledge, tips, and personal experiences in the development and scale-up of chemical and related processes.

In our seminar you’ll learn it’s important to consider all of the steps in process scale-up:

  • First, it is critical to obtain the correct data from all prospectives including reaction, filtration, solids handling, drying as well as all of the other upstream and downstream equipment and systems. The team must know the process, observe the testing, and deduce the solution only from what is observed (and nothing more). Partnering with suppliers with a proven track record in similar applications will shorten the technology scale-up cycle.
  • Second, always allow time for fine-tuning even after the scale-up seems complete.
  • Next, the start-up and commissioning at each step will also have unknowns associated with these activities.
  • Finally, all that matters is the premises (process definition, requirements and testing objectives) and how the testing unwinds the crucial from the incidental (what is the critical process parameter), and ending up in the logical conclusion (optimum process filtration solution).

Removing Catalyst Fines From Slurries

BHS also shares in its latest newsletter a new technical paper reviewing coarse particle filtration and existing equipment for catalyst fines removal / recovery. The article also covers new approaches of candle and pressure plate filters including testing and selection.  Three case studies are illustrated examining (1) Raney Nickel Catalyst, (2) Pharmaceutical Hydrogenation, and (3) Palladium Catalyst Filtration, Washing and Drying.

After all, many chemical, petrochemical and pharmaceutical manufacturing processes involve reactions of solid- and liquid-phase reactants to produce a slurry. The slurry typically needs to be separated into its component parts — the mother liquid and the solid. The article considers the many choices of technologies and all candle filters and pressure plate filters provide for higher quality filtration, improved yields with fully automated and contained operations.

Finally, BHS is undertaking a collaborative project to look at the relationship of filter aids, filter media, and filtration technologies. There is an abundance of information on filter aids, filter media, batch filtration, and filtration technologies in the marketplace. Yet there is no one overall comparison from which to develop a process solution.  BHS seeks to fill the void by undertaking research and testing to develop a comprehensive approach to filter aid usage, filter media and wrapping all of this into filtration technology selection.  Our work is targeted for a completion date of August 2017.  We’d love to hear your ideas — let me know what you think!