Pressure vs. Vacuum Filtration: A Guide to Proper Filtration Equipment Selection to Optimize Your Operation

First, let’s examine a range of critical questions for you to address as you look at filtration equipment selection. Critical considerations are operational as well as process-related:

• Is the process a Batch or a Continuous operation? What is the operating time per cycle to separate the solids phase from the liquid phase? How many Batches are produced per 24 hours?
• What level of Automation is required for the Filtration Equipment? 
• Are there special safety concerns, such as with solvents or oils, or mineral acids at high temperature? 
• Must the process temperature be maintained during the separation step?
• What is the viscosity of the slurry at the process temperature?  (Note:  the filtration rate is inversely proportional to viscosity.)
• Is a filter aid, such as diatomite, perlite, or cellulose, permitted to assist filterability?
• What are the solids loading rate (dry kg/hr/m²) and the hydraulic loading rate (l/hr/m²)?
• What are the properties of the optimized, formed filter cake, e.g., the cake resistance value? 
• What is the optimum cake thickness, to achieve maximum cake porosity at the required production rate?  What is the cake formation time?  What is the cake wash time?  What is the final cake dewatering time?
• What is the particle size distribution (PSD) and specific gravity (SG) of the slurry?  What is the density of the solids phase?
• If cake washing is required, should it be a co-current or counter-current cake washing scheme?
• What is the final cake moisture target?  What is the cake discharge quality at the target moisture?
• What is the degree to which the process conditions may vary hour to hour, shift to shift, day to day?
• Will the filter fabric see blinding conditions, such as with colloids or oils?  If so, what is the required frequency of fabric cleaning to mitigate blinding?

While a detailed analysis is called for, based on the prior set of questions, here are some generalized Selection Guidelines: 

Batch vs. Continuous Process?

One of the most significant differences between Pressure and Vacuum Filtration is whether the process needs to run continuously, or in batch mode. Vacuum Filters typically operate as a continuous process, whereas Pressure Filters typically operate as Batch processes. However, two or three Pressure Filters can be efficiently used, in series, to process a continuous stream.

Filter Cake Characteristics

Vacuum Filtration is generally best when there is a low Cake Resistance Value. Pressure Filtration tends to be more favorable in instances where there is a high Cake Resistance Value.

Particle Size Distribution

Particle size distribution (PSD) is another important factor.  Wide particle size distributions, with a significant fraction in the large particle range, at a high density value, tend to favor the use of Vacuum Filters.  Tight particle size distributions, with a significant fraction in the small particle range, are better suited to Pressure Filters.

Cake Moisture

In most instances, Pressure Filters will achieve a lower moisture filter cake than with vacuum filters, due to the higher driving forces.  Low cake moisture is desirable in many cases because of downstream material handling concerns, or for efficient thermal drying in the next process stage.

Process Variability

Pressure Filters can also be more forgiving when it comes to upstream process changes or variability. While it is important to control the process within key PSD and SG parameters, if there is likely to be some variability in particle size distribution, the amount of total suspended solids, the slurry specific gravity, and the particle morphology, it is likely that a Pressure Filter will perform more reliably in the wide operating ranges than a Vacuum Filter.

Operator Intervention

Vacuum Filters generally require less direct operator intervention than most conventional Filter Presses. Of course, this can be mitigated with Filter Press features such as Hydraulic Plate Shifters, Automatic Cloth Washing, etc. There are some filter presses such as Micronics’ Rollbar Filter Press which is explicitly designed for fully automatic and unattended operation to minimize manpower requirements.

Filtrate Quality

Pressure Filters typically offer better solids capture and hence, better filtrate quality.  Filter Presses typically capture greater than 98% of the solids processed. 

Next Steps to Optimize Your Filtration Process

These Guidelines are generalized.  A liquid-solid separation expert from Micronics can examine your process, and help find the best technology solution to precisely meet your separation needs.  We will custom-design and engineer a pressure or vacuum filtration solution to meet your process objectives. Micronics will help you select the filtration equipment with the lowest CAPEX/OPEX, sized and designed specifically for your process, and with filter media that is optimized for your process.  

The Micronics Engineered Filtration Group also offers in-house laboratory testing for both Pressure Filtration and Vacuum Filtration.  Micronics also provides pilot-scale rental units! Learn more about our innovative MicroPress™ here.

Contact Us to speak with one of our liquid-solid separation experts and benefit from our deep technical expertise.