Introduction to Filtration

These days, I feel like learning topics related to solid processing. After I learned about agitation process and its scale-up, I want to learn about filtration process.

So, in this post I want to share you general preview of filtration process. Honestly in my career until now, I already handled one project related to filtration equipment, which is my very first project: pre-feasibility study of carboxymethyl cellulose plant. The filtration equipment used in the project was rotary vacuum filter.

I hope by learning and writing what I read here in this blog will refresh my knowledge and hopefully useful in the future projects.

Definition and Principles

Filtration is the process of separating solids from liquid by passing or filtering the slurry through some form of porous filter medium. It is widely used separating process in chemical and other process industries.

The most commonly used filter medium is woven cloth, but there are many varieties of other media is also used. In some cases, filtration of slurries is difficult. Therefore, filter aids are often used to increase the rate of filtration. They are either applied as a precoat to the filter coat or added to the slurry.

Filtration is essentially batch process. Even with those filters designed for continuous process, for example rotating drum filters, periodic stoppages are necessary to change filter cloths. Batch filters can be coupled to continuous plant by using several units in parallel, or by providing buffer storage capacity for the feed and product.

The driving force of filtration may be gravity, vacuum, pressure, or centrifugal.

1. Gravity – this can be very useful to reduce large quantities to more manageable proportions.
2. Vacuum – this is the simplest driving force available. Vacuum filters can be batch operated but are normally continuous. Some vacuum filters offer the best washing possibilities.
3. Pressure – cake washing can be excellent, and the final cakes are usually as dry as can be expected without heat input.
4. Centrifugal – this can sometimes offer a compromise between vacuum and pressure filtration. The nature and behavior of the solids will play a great part in the success or failure of centrifugal filters.

Several Types of Filtration Systems

In general, filters are divided into three main groups:

1. Cake filters
2. Clarifying filters
3. Crossflow filters

Cake filters separate relatively large amounts of solids as a cake of crystals or sludge. They often include provisions for washing the cake and for removing some of liquid from the solids before discharge. Figure below illustrates cake filters.

Clarifying filters remove small amounts of solids to produce a clean gas or sparkling clear liquids, such as beverages. The solid particles are trapped inside the filter mediums or on its external surfaces. Clarifying filters differ from screens in that the diameter of filter medium are much larger than the particles to be removed. Figure below illustrates the clarifying filters.

In a crossflow filter, the feed suspension flows under pressure at a fairly high velocity across the filter medium. A thin layer of solids may form on the surface of the medium, but the high liquid velocity keeps the layer from building up. The filter medium is a ceramic, metal, or polymer membrane with pores small enough to exclude most of the suspended particles. Figure below illustrates crossflow filters.

Basic Selection of Filtration Systems

When selecting a suitable filtration system, we must remember that there is no “one size fits all” process solution. Selecting a filtration technology requires a system approach that must be incorporated with other solids processing such as reactors, dryers, solids handling, and others. The engineer’s job is to select and specify the equipment needed for a particular process while consulting with the vendors to ensure that equipment is suitable.

Filtration process has three components that must be considered: material properties, mechanical properties, and separation performance. These are combined and ranked choices must be evaluated weighing operational, economic, and plant objectives.

The material properties examine the solids and the liquids. For solids, the engineer will need to know the total suspended solids (TSS) and solids concentration, particle size distribution (PSD), and particle shape. For the liquids, the typical parameters include viscosity, temperature, vapor pressures, pH, ionic strength, and other conditions

In terms of mechanical components, the engineer will need to provide information about material of construction, temperature, and pressure, seal information, and compatibilities of the solids, liquids, and cleaning solutions.

As for separation performance, includes capacities or throughputs of solids, liquids or dry solids, filtrate quality, dryness, washing liquids, conductivity, wash ratios, and other.

References:

1. McCabe, Warren L, and Julian C. Smith, “Unit Operations of Chemical Engineering Seventh Edition”, McGraw-Hill, 2005.
2. Perlmutter, Barry A, “Solid-Liquid Filtration, Practical Guides in Chemical Engineering”, Elsevier, 2015.
3. Sinnot, R K, “Coulson & Richardson’s Chemical Engineering”, Elsevier, 2005.