Pressure Filtration Practical

DURBAN UNIVERSITY OF TECHNOLOGY DEPARTMENT OF CHEMICAL ENGINEERING CHEM PLANT 302 PRACTICAL: PRESSURE FILTRATION

GROUP: SERVESHAN NAIDOO KEOLIN PILLAY NICHELLE NAIDOO MANIKA NAIDOO DATE OF SUBMISSION: 18 OCTOBER 2010

SUMMARY The objective of conducting this practical was to determine the filter medium resistance (Rm) and the specific cake resistance (ave) for the filter medium provided. In addition it is also required to recommend the total cycle time for the run. The following equipment provide vital in order to conduct this practical effectively: an electrically operated agitator, slurry tank, diaphragm for slurry, air hydraulic pump, hydraulic cylinder, Larox pressure filter, water tank, air hydraulic pump for pressing water, cake washing liquid tank. Slurry containing water and lime was mixed in the slurry tank by means of an electrically operated agitator. A vacuum was created using a hydraulic pump. The slurry was allowed to filter through the filter cloth, with the solids accumulating on the filter cloth and the liquid passing through. At timed intervals samples were collected and analysed. Once the filtration was complete, the filter cake was compressed and then air dried.

TABLE OF CONTENTS INTRODUCTION THEORY EXPERIMENTAL METHOD APPARATUS RESULTS DISCUSSION CONCLUSION AND RECOMMENDATIONS NOMENCLATURE REFERENCES APPENDICES

INTRODUCTION An experiment was conducted investigating continuous pressure filtration in order to determine the filter medium resistance (Rm) and the specific cake resistance (ave) for a pilot scale Larox pressure filter. In addition, it is required as a second objective to recommend the total cycle time for the run. Filtration can be defined as the process of separating dispersed particles from a dispensing fluid by means of porous media.the dispersing fluid in this respect is a liquid(water). The term solid-liquid filtration covers all processes in which a liquid containing a suspended solid is freed of some or all of the solid when the suspension n is drawn through a porous medium, in the case of this practical the porous medium is the filter cloth. The choice of a filter design for a given process depends on many factors, amoung which are the properties of the solid particles to be removed, i.e. particle size and shape distribution and state of aggregation; the fluid viscosity, density, and interaction with structural materials; the quantity of material to be handled; the dryness of the cake produced; the concentration of solids in the suspension; the value of materials being processed; whether the material to be retained is the solid, liquid, or both; and whether the product need be washed. In terms of this practical the valuable material was the cake obtained after filtration. In cake filtration the solid material accumulates on the surface of the medium so that after a short initial period, filtration is through the bed of the deposited cake. This process will continue until the pressure drop across the cake exceeds the maximum permitted by economic or technical considerations or until the space is filled. This method of filtration is most employed in process industries and s very well suited to the filtration of concentrated suspensions and the recovery of large quantities of solid. The most important factor in cake filtration is the permeability or resistance of the filter cake, and this may be controlled by altering the particle size distribution of the material or by altering the state of aggregation of the solid.

THEORY Constant pressure filtration is a cake filtration that is carried out with a constant total pressure difference across the filter; this does not imply that the pressure difference across the filter cake is constant during the period of filtration. As the cake forms so the filtrate flow rate tends to reduce, hence also known as declining rate filtration. The information listed below was used in conjunction with data obtained from conducting the practical in order to meet the objectives of conducting the practical.

EQUATIONS USED CONSTANT PRESSURE FILTRATION P = (K1V + K2)q

(1)

Where, P = pressure drop across filter q = filtrate flowrate V = filtrate volume K1 and K2 = filtration constants

q = dV / d

(2)

K1 = (s   ave) / (1-ms)A2

K 2 = Rm  / A

Where s = mass fraction of solids in slurry Rm = filter medium resistance

(4)

 = filtrate density  = filtrate viscosi1ty m = mass of wet cake/mass of dry cake A = filter area ave = average specific cake resistance (m/kg)

FILTRATION TIME θt

Vt

0

0

∫ dθ = ∫

( K 1V + K 2) dV P

(5)

For constant pressure filtration, the time required to obtain a given filtrate volume, Vf is thus obtained as,

t =

K 1Vf 2P

2

+

K 2Vf P

(6)

To find the filtration constants, K1 and K2,

θt

v

f

=

K1 Vf + KP2 2P (7)

Plot

θt

v

f

vs

V

f

Intercept = K2/P Slope = K1/2P Total cycle time = pumping time + pressing time + drying time + ‘dead tim’(cake discharge, cloth washing, etc) Filter area = 0.1m2in order to determine the mass of wet cake/ mass of dry cake, take a sample of wet cake, weigh it,then place it in the oven to dry. Thereafter weigh the dry sample.

APPARATUS APPARATUS Slurry tank agitator Filter cloth Foot treadle Ball valves Diaphragm pump Stop watch Three way valve Air hydraulic pump Water tank Cake wash water tank

DESCRIPTION Contains water + lime mixture Mixes the mixture in slurry tank Filtration medium Used to control hydraulic pressure Open/close streams Record time Open/close streams -

TABLE 1: APPARATUS USED TO CONDUCT PRACTICAL

EXPERIMENTAL METHOD AND APPARATUS EXPERIMENTAL METHOD:  Fill the slurry tank with 70 litres of water and 10 kg lime  Switch on the agitator  Place the filter cloth in place and close the filter using the foot treadle on the filter side  The hydraulic pressure must be atleast 600 bar before the filtration can start  Open the drying valve(C)  Close the slurry discharge valve (E)  Air should now be flowing into the filter, penetrate the filter cloth and come out of the filtrate hose  Close the drying valve (C)  Place the 5litre container under the filtrate hose FILTRATION

        

Adjust the diaphragm pump speed to maximum Take a 250ml sample from the slurry in order to measure the dry solids content Close the circulation valve (A) Open slurry valve (B) immediately and start the stop watch Record the filtrate volume collected at various time intervals (1 minute intervals used) Pump the slurry for 20 – 25 minutes (the cake thickness should be near maximum) Close slurry valve(B) Slop the stop watch Open circulation valve (A)

 Adjust the pump speed for reaching a suitable circulation COMPRESSION OF FILTER CAKE

     

Start the diaphragm pressing by turning the 3-way valve (P) to position on The water pump (8) starts and pumps the water above the diaphragm When the pressure rises above 5 bar, start the stop watch Note how the filtrate flows out and how it gradually weakens When there is no more filtrate, stop the stop watch Open slurry discharge valve (E)

AIR DRYING

          

Carefully open the drying valve (C) Air now flows out of the slurry discharge valve (E) and cleans the pipes Adjust the air flow speed Close the discharge valve (E) Turn the diaphragm pressing –way valve (P) to position off The water flows back into the water tank (E) Start the stop watch when air flows through the filtrate hose Watch the filtrate flow as the drying proceeds When the filtrate flow stops, close the drying valve (C) Stop the stop watch Open the slurry discharge valve (E) to remove the pressure fro the chamber. Only after this can the chamber be opened safely.

DISCHARGE

   

Remove the pressure from the hydraulics The filter chamber opens Rinse the cake with the cloth Rinse the cloth and the filter chamber

FIGURE 2: SCHEMATIC DIAGRAM OF APPARATUS

RESULTS, DISCUSSION AND CONCLUSION

NOMENCLATURE P S K1 , K2 T   V  ave A Rm Q

Pressure Mass fraction of solids in slurry Filtration constants Temperature Filtrate density Filtrate viscosity Filtrate volume time Average specific cake resistance area Filter medium resistance Filtrate volume flowrate

REFERENCES

1. pressure filtration." Encyclopedia Britannica. 2010. Encyclopedia Britannica Online. 15 Oct. 2010 http://www.britannica.com/EBchecked/topic/343175/filtration_seperation 2. Kopecký F., Kaclík P., Fazekaš T.: Laboratory manual for physical chemistry, Farmaceutical faculty of Comenius University, Bratislava, 1996 3. Matteson MJ, Orr C. (1987). Filtration principles and practices. Second edition. New York. New York and Basel publishing. 4. filtration. Home of Separation. 2001.online.14 October 2010 www.et.byu.edu/~rowley/pfilt/final/simulator.htm

APPENDICES