SEPARATION OF INSOLUBLE PRODUCTS

BY FILTRATION

Shruti GuptaNCAAH

Introduction

Filtration is commonly the mechanical or physical operation which is used for the separation of products like solids from fluids by interposing a medium through which only the fluid can pass.

The fluid that passes through is called a filtrate.

Filtration is the best established and most versatile method for removing insoluble from dilute streams like fermentation broths.

Darcy's law is a phenomenological derived constitutive equation that describes the flow of a fluid through a porous medium.

Darcy's law is a simple proportional relationship between the instantaneous discharge rate through a porous medium, the viscosity of the fluid and the pressure drop over a given distance.

The total discharge, Q (units of volume per time, e.g., m3/s) is equal to the product of the intrinsic permeability of the medium, k (m2), the cross-sectional area to flow, A (units of area, e.g., m2), and the pressure drop (Pb - Pa), (Pascals), all divided by the viscosity, μ (Pa·s) and the length over which the pressure drop is taking place (m).

General theory of filtration

Types of filtration equipment

The industrial filtration equipment differs from laboratory filtration equipment only in the amount of material handled and in the necessity for low-cost operation.

Fig. Simple laboratory filtration apparatus

Filters can also be classified by operating cycle. Filters can be operated as batch, where the cake is removed after a run, or continuous, where the cake is removed continuously.

In another classification, filters can be of the gravity type, where the liquid simply flows by means of a hydrostatic head, or pressure or vacuum can be used to increase the flow rates.

Some of the most important types of filters

1. Bed filter

2. Plate-and-frame-filter

3. Leaf Filters

4. Continuous rotary filters

a. Continuous rotary vacuum-drum filter

b. Continuous rotary disk filter

c. Continuous rotary horizontal filter

Such filters are useful mainly in cases where relatively small amounts of solids are to be removed from large amounts of water in clarifying the liquid.

Fig. Bed Filter

1. BED FILTER

2. PLATE-AND-FRAME-FILTER

The feed slurry is pumped which flows through the duct. The filtrate flows through the filter cloth and the solids build up as a cake on the frame side of the cloth. The filtrate flows between the filter cloth and the face of the plate through the channels to the outlet.

Fig. Plate-and-frame-filter press

3. Leaf Filters

This filter is useful for many purposes but is not economical for handling large quantities of sludge or for washing with a small amount of fresh water. The wash water often channels in the cake and large volumes of wash water may be needed.

Fig. Leaf Filter

4. Continuous rotary filters

A number of such filters are as follows:

a. Continuous rotary vacuum-drum filter

b. Continuous rotary disk filter

c. Continuous rotary horizontal filter

A. CONTINUOUS ROTARY VACUUM-DRUM FILTER

Fig. Continuous rotary Vacuum drum filter

B. CONTINUOUS ROTARY DISK FILTER

The filter consist of concentric vertical disks mounted on a horizontal rotating shaft. The filter operates on the same principle as the vacuum rotary-drum filter. Each disk in hollow and covered with a filter cloth and is partly submerged in the slurry.

C. CONTINUOUS ROTARY HORIZONTAL FILTER

This type is a vacuum filter with the rotating annular filtering surface divided into sectors. As the horizontal filter rotates, it successively receives slurry, is washed, is dried, and the cake is scraped off.

Removal of Insoluble Products

Rotary Vacuum Filtration

slurry is pumped into the trough

The vacuum draws liquid and air through the filter media and out the shaft hence forming a layer of cake

An agitator is used to regulate the slurry if the texture is coarse and it is settling rapidly

Solids that are trapped on the surface of the drum

washed and dried, removing all the free moisture

Microfiltration/ Ultra filtration

Microfiltration usually serves as a pre-treatment for other separation processes such as ultrafiltration, and a post-treatment for granular media filtration.

The typical particle size used for microfiltration ranges from about 0.1 to 10 µm.

In terms of approximate molecular weight these membranes can separate macromolecules generally less than 100,000 g/mol.

The filters used in the microfiltration process are specially designed to prevent particles such as, sediment, algae, protozoa or large bacteria from passing through a specially designed filter.

More microscopic, atomic or ionic materials such as water (H2O), monovalent species such as Sodium (Na+) or Chloride (Cl-) ions, dissolved or natural organic matter, and small colloids and viruses will still be able to pass through the filter.

References:

Sparks, T. (2012) Solid-Liquid Filtration - A Users’ Guide to Minimizing Costs and Environmental Impact; Maximizing Quality and Productivity, Elsevier.

 Haug, G. (1999) Aspects of Rotary Vacuum Filter Design & Performance, Eagle-Picher Minerals Inc.

Crittenden, J, Trussell, R, Hand, D, Howe, K & Tchobanoglous, G. 2012, Principles of Water Treatment, 2nd edn, John Wiley and Sons, New Jersey. 8.1.

Perry, RH & Green, DW, 2007. Perry's Chemical Engineers' Handbook, 8th Edn. McGraw-Hill Professional, New York. p. 2072.

Baker, R 2000, Microfiltration, in Membrane Technology and Applications, John Wiley & Sons Ltd, California. p. 279

Kenna,E & Zander, A 2000, Current Management of Membrane Plant Concentrate, American Waterworks Association, Denver. p.14

Perry, RH & Green, DW, 2007. Perry's Chemical Engineers' Handbook, 8th Edn. McGraw-Hill Professional, New York. p 2072-2100

Seadler ,J & Henley, E 2006, Separation Process Principles, 2nd Edn, John Wiley & Sons Inc. New Jersey. p.501

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