Chemistry Filtration: Processes, Techniques And Experiment

Synopses

Describe about the Chemistry Filtration?

It is common in various process operations to obtain slurry which contain solid articles immersed in a liquid and thus need to be separated. The most common method to ensure the same is to filter the slurry using a filter cloth having a file mesh which results in the solid particles being deposited on the cloth while the liquid would be filtered. In the above process either of the products or both may be useful (Wakeman & Tarleton, 2005). Once the solid cake starts depositing on the cloth mesh it results in greater resistance which thus requires higher pressure for forcing the liquid through the mesh. In case a vacuum filter is deployed, the above mentioned problem could be resolved by continuously removing the cake from the cloth but it considerably slows the overall process and results in a loose cake. To counter these loopholes, a higher pressure filtration may be more suitable but it needs a PRESS in which the whole system needs to be constrained and repeatedly opened for removing the solid dense cake (Dickenson, 1997).

Filtration may be defined as the process in which solid matter is separated from the liquid constituents by passing the mixture through a filter which causes the liquid (called filtrate) to pass which retaining the solid on the pores. The filter material has many choices namely unglazed earthenware, sand, asbestos, slag- or glass-wool, cloth or cotton-wool, asbestos or any material which is porous. In case of using filter press, a slurry pump is employed which deploys the pressure drive principle for guiding the overall filtration process and measuring the overall resistance provided both by the solid cake and the filter employed (Tien, 2006). Whenever a filter press is employed, after regular volume the operation needs to be stopped for discharging the solid cake deposited on the filter and then only the next batch can be processed. Skeleton and filter pack are the two prominent components of filter press.  The role of the skeleton is to ensure that under the pressure developed in the filter chamber, the filter pack is held together (Matteson & Orr, 1987). However only a specific volume of the cake can be held by the same.

1^st part

The first step is to weigh the filter papers. After this the screws are unscrewed for opening the filter. Further to the filter, the filter paper is added after which it is sealed to ensure the filter paper remains there. The filter is then screwed back. Now the pressure is set for conducting the experiment. Further the valves are opened so as to allow the chalk solution to be passed through the filter. At particular heights and points, the volume of the solution needs to be measured along with the time taken to reach these volumes. The valves need to be closed after reaching a particular value. Once such a volume is reached, the filter is unscrewed and the filter paper is dried in an oven.  The filter paper need to be weighed after nearly a week to determine the mass of filtrate. For a particular pressure level, the above process needs to be repeated thrice to enhance accuracy. Further the same procedure needs to be carried out for different pressure levels.

Experimental Technique

The filter material needs to be made wet and then weighed. The bolt needs to be unwind so as to loosen the filter machine. The filter material is then inserted between four plates after which the bolt is tightened. Then the red valve needs to be closed while the valve 1 is opened. The filter procedure is then conducted for a pre-determined amount of time (for instance 300 seconds). After the time is finished, the chalk solution is stopped by opening the red valve and closing the valve 1. Further on each run, at a particular time the flow rate needs to be measured. For a particular pressure level, the above process needs to be repeated thrice to enhance accuracy. Further the same procedure needs to be carried out for different pressure levels.

Filtration may be defined as the process in which solid matter is separated from the liquid constituents by passing the mixture through a filter which causes the liquid (called filtrate) to pass which retaining the solid on the pores. The filter becomes effective only when an initial deposit of the solid cake happens on the filter. For experimental purposes, in a laboratory a Buchner funnel is deployed and additionally a vacuum source is deployed so as to suck liquid out of the thin particle layer. However for simpler experiments, the apparatus is much simpler and only a conical funnel containing a filter paper is used for filtering the solution (Sutherland & Chase, 2008).

In case of filtration at constant pressure, there is a decrease in the rate of filtration as the resistance of the filter continues to increase as the solid particles keep on depositing on the filter. This trend is also validated from the results obtained from the experiment. Additionally it is also notices that the resistance provided by the solid cake is directly proportional to the pressure level. Further the cake thickness is directly proportional to the total amount of filtered feed and consequently the filtrate. However, there is an inverse relationship between the cake thickness and the filter area. Further, there is a direct relationship between filtration pressure and filtration time. The slope and resulting intercept can be known from the graph between [v/(t)] and [v(t)].

The constant pressure filtration has certain advantages and disadvantages which are mentioned below (Dickenson, 1997).

• Sizable amount of experimental studies have been conducted
• Procedure is simple and data can be easily recorded
• Increase in the flow rate when the medium is approached by liquid which results in the filter cake being compressed

• Consistency is lacking especially when tests are performed by different researchers and hence results need to be revalidated.
• During the period of medium blocking, there may be issues with maintaining a constant level of pressure on the filter cake.
• There is a paucity of time for the study of initial filtration period.

The apparatus in this case primarily requires a set of square plates which in order to create compartments are segregated using hollows frames. The plates after being covered with filter medium are stacked in a vertical manner by the use of a screw. There is a projection of the plates along the circumference. After this, the plate and frame portion slurry is introduced. It is apparent from the graph of ‘ Filtration volume vs Time’ (attached in the Appendix) that the filtrate volume encounters a gradual reduction as filtration proceeds. This is primarily because as the layer of the cake formed thickens, the resistance offered to filtration also increases. Further it is apparent from the ‘ Filtration pressure vs Time’ graph that there is a direct relationship between the filtration time and filtration pressure. Further, it is also noted in he experiments that while the outermost frames has the majority of the filter cakes while the cake formation in the middle frames is primarily incomplete. Hence the filtration process throughout the plate is non uniform and thus accounts for experimental error. The experiment determines the relationship between cake thickness, filtration pressure and filtrate volume to filtration time in a plate and frame filter operation. It is apparent that there is an inverse relationship between filtrate volume and filtration time due to which there is a decline in the collection of volume as the process of filtration proceeds (Tarleston,1998).

The experimental results may be considered theoretically correct till the correlations obtained are the same as determined by theoretical calculations. However during this experiment especially in the first stage certain errors were committed primarily because of lack of enough volume readings and thus the correlations obtained experimentally for certain variables do not match with the theoretically expected relations. In order to ensure experimental accuracy certain anomalous readings were ignored (Tien, 2006).

Conclusion

It may be concluded from the experiment that the results should be considered acceptable till the time that these are in line with the theoretical framework and obtain all required variables. It was found that there is a direct relationship between the equivalent resistance of the cake filtration and the differential pressure. Due to certain experimental errors, the experimental compressibility factor is on the higher side as compared to the expected value. This cannot be correct since the experimental errors cannot be accurately measured and their source primarily unidentified as to whether it is the commitment of the student or issues of quality of the equipments used in the apparatus.

References

Dickenson, T.C. 1997. Filters and Filtration Handbook. 4^th edn, Elsevier Advanced Technology, Oxford

Matteson, M. & Orr, C. 1987. Filtration: Principles and Practices, 2^nd edn, CRC Press, New York

Sutherland, K.S. & Chase, G. 2008. Filters and Filtration Handbook. 5^th edn, Elsevier Advanced Technology, Oxford

Tien, C. 2006. Introduction to Cake Filtration, 1^st edn, Elsevier BV, Amsterdam

Tarleston, E.S. 1998. ‘Predicting the performance of pressure filters’, Filtration and Separation, Vol.35, No.2, pp. 293-298

Wakeman, R. & Tarleton, S. 2005. Principles of Industrial Filtration, 1^st edn, Elsevier Advanced Technology, Oxford