There are several filtration technologies that are used in industrial processes for liquid-solid separation. In most cases, the solid that is obtained is the desired product, however there are instances (though less common) when the filtered liquid is the valued product. Regardless of what material you are looking to harvest or whether your process runs as a continuous or batch operation, it can be overwhelming when you are trying to determine what equipment will best fit your process requirements. Since the type of material being separated can vary based on particle size and product density as well as other application-specific details, there is not a “one size fits all” design that will carry out every type of filtration need.
Two of the most prevalent separation technology options available are centrifuges and filter/dryers. In this post we will define each of these units and review some of their pros and cons through a product comparison. While there are certain situations where these units can compete with one another, there are also many applications that favor one technology over the other based on their capabilities and limitations.
Before we dive into equipment basics, let’s talk about who exactly is performing filtration at an industrial level. The six most prominent industries that utilize large-scale separation equipment are:
- Wastewater Processing – mechanical separation of suspended solids, toxic heavy metals, and fatty organic materials
- Chemical Processing – filtration of raw materials such as acids, salts, and polymers
- Pharmaceutical and Biotechnology – production of various drugs, active pharmaceutical ingredients (APIs), vaccines and medicines
- Fuel and Biofuel – processing of ethanol, synthetic fuels, and biodiesel
- Food Processing – refining of vegetable oils, dairy products, and alcohol
- Mining and Mineral Processing –purifying of coal, tar sands, and precious metals
There are, of course, other industries and use cases for these filter technologies, but this list is meant to give you a general idea of the widespread utilization of centrifuges and filter/dryers across the gamut of processes that are out there.
A centrifuge’s operation is based on the core principle of centrifugal force – an outward force that is created by the fast, circular rotation of the device via an electric motor. The objective of this equipment is to separate, extract and/or wash a product in either a continuous or discontinuous (batch) operation using high-speed gravitational pull that is generated from the rapid RPMs. The filtered product then falls to the bottom of the centrifuge where it is discharged from the vessel.
There are two categories for industrial centrifuges – sedimentation and filtering centrifuges. Our focus, filtering centrifuges, uses the centrifugal force mentioned above to pass a liquid through filtration media (such as wire mesh or cloth) while the solids are captured by the media. This category is subdivided into three main types: perforate basket centrifuge (think washing machine design), pusher centrifuge (featuring a basket that rotates on a horizontal axis to enable “moving bed” continuous filtration and discharge), and peeler centrifuges (for batch or continuous processes and featuring a peeler knife mechanism that facilitates complete product discharge).
The filter/dryer is another logical choice in the quest for equipment that provides efficient filtration suited for critical, high-purity separations. As with centrifuges, there are various types of filters available, a prime example being Nutsche filter/dryers, which employ a batch filtration technique through the use of vacuum and/or pressure in a closed vessel. The pressure/vacuum is used in combination with a filter plate fitted with the appropriate filter media to force liquid through the media, forming a solid bed often referred to as “cake”. The process steps for Nutsche filtration include:
3. Displacement Washing
4. Reslurry Washing
For more detailed information on the seven basic steps of Nutsche filtration and drying, check out our post that specifies the operational sequence.
So now that we’ve outlined the basics for these two formidable opponents, how do they measure up to each other? In the series of credentials listed below, we’ve determined who would come out as the top qualifier for each criterion.
A filter/dryer is totally enclosed and pressure tight (rated to 50 psig/FV at a minimum), a design that eliminates operator exposure and environmental contact with the product (including protection against solvent vaporization). The closed system that is created is especially ideal for high purity, toxic, or flammable materials.
Maintenance and Operational Costs
With no high speed components and a low motor HP when compared to the centrifuge, the filter/dryer scores more points when it comes to maintenance and operational costs. Additionally, the agitator is only used during drying, smoothing, reslurry washing, and discharge, whereas a centrifuge motor is always in operation.
Reduced Moisture Content of Wet Cake
A centrifuge’s typical LOD (limit of detection) difference is 5% to 10% (depending on time and product). This data assumes there is no extended blow through in the filter.
A filter/dryer allows for extended residence time during displacement washing with the added capability of a reslurry washing if the process requires it (with the added benefit of using a reduced amount of wash fluid to effectively complete this step).
Although centrifuges and filter/dryers are available in roughly the same size range, filter/dryers enable possible increased capacity as the units are typically sized to handle the entire batch solids load.
Ability to Handle Extremely Difficult Products
Especially for compressible cakes, centrifuges promise better results than separation in conventional filter/dryers. Centrifuges also prove to be a good solution for cakes not suitable for deep bed filtration.
When it comes to discharging capabilities, both centrifuges and filter/dryers offer their own unique advantages. For complete product discharge, a filter/dryer has some heel associated with agitator to vessel tolerances whereas a centrifuge allows for complete discharge (though this can depend on the centrifuge design). Filter/dryers specialize in flexible discharge which enables the expulsion dry solids, wet solids, slurries, or liquid. In short, depending on your discharge objectives one type of equipment may be more advantageous to you than the other.
With “dryer” as part of its name, it’s no surprise that the filter/dryer takes the lead in this category. Utilizing vacuum or blow through drying, the filter/dryer makes it possible to obtain completely dry material out of the filter, which reduces overall process time.
Live Load Imposed on Filter
There is no live load associated with the filter/dryer thanks to the low RPM agitator (8-20 RPM on average), making the installation cost lower than that of the centrifuge.
It may appear that the filter/dryer is the champion in our mock competition, but as with most industrial processes, it really depends on your specifications in order to assess what separation technology will best fit the needs of your specialized application. De Dietrich Process Systems has an online Filtration Questionnaire that will give you a complimentary valuation of your filtration process to help determine a solution for your separation requirements.