University of San Carlos School of Engineering Talamban, Cebu City, Philippines
CHE 512L
Chemical Engineering Laboratory 2
Filtration
The Plate-and-Frame Filter Press
A laboratory report submitted to
Luis K. Cabatingan CHE 512L Instructor
by
Serge Lorenz Villasica
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1. Introduction
Filtration is a unit operation that separates liquid from a suspension with the use of a porous medium (Coulson & Richardson, 2002). Filtration has these important elements: the slurry, the filter medium, the filtrate and the filter cake. The slurry is the suspension from which the liquid is to be separated through a filter medium. A filter medium is usually porous and allows for selective passage of the particles that make up the slurry. The particles in the suspension are large enough to pass through the pores on the medium so they are retained. Water (or other liquids), on the other hand, passes through the filter medium with ease. Retained solids starts to accumulate with time. They become large enough that the flow of liquid is obstructed. Filtration ends when there is no longer significant effluent being collected due to the accumulation of the filter cake.
Filtration process can be attributed to the motion of a fluid through a packed bed (in this case, the bed is the filter medium and the filter cake) (Geankoplis, 2003). Flow is caused by the difference of pressure between two points inside the filtration equipment. The flow is resistant mainly by the filter medium and the growing filter cake
m f R p Adt dV (1) A V c p Adt dV s c (2)
The sum of the two resistances results to the equation
) ( m s c R A V c p Adt dV (3)
In cases where the pressure drop is maintained, the rate of effluent varies with time because filter cake slowly builds up as more solid particles are retained in the filter medium, increasing the cake resistance. Manipulating the equation, it would result to a linearized form with 𝑉𝑡 as a function of 𝑉, which is;
B V K V t p 2 (4) Where 𝐾𝑝 = 𝜇𝛼𝑐𝑠 𝐴2(−Δ𝑝) and 𝐵 = 𝜇𝑅𝑚 𝐴(−Δ𝑝)
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The linearized form of the filtration equation will allow us to approximate quantities like the specific cake resistance and the filter medium resistance through experimentation (Geankoplis, 2003).
2. Objectives of the Experiment
Determine the specific cake resistance and filter medium resistance in the filtration of aqueous CaCO3 slurries with different concentrations performed at constant-pressure condition.
3. Methodology
3.1. Methodological Framework
Calcium carbonate slurry was the feed for the filtration experiment. The filtrate was collected and its time of collection was measured at different weight recordings. In the processing of data, the volume as a function of time was converted to 𝑉𝑡 as a function of 𝑉 so that linear regression could be carried out. From there, and using equation (4), specific cake resistance and filter medium resistance were obtained.
Figure 1. Framework of the filtration experiment
𝐶𝑎𝐶𝑂3 slurry Filtrate Measurement of mass of filtrate collected as a function of time Conversion of 𝑉(𝑡) to 𝑡 𝑉(𝑉), linear regression of data
Specific cake and Filter medium resistances
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3.2. Materials
Two calcium carbonate slurries were prepared, 10% and 14% in weight,. Large pail was placed on top of the bathroom scale. Stopwatch was prepared in order to measure the time as filtrate is being collected.
3.3. Equipment
The equipment that was used in the experiment was the plate-and-frame filter press. Slurry tank is where the slurry is fed and the solid particles are kept from settling by the agitator. The slurry then passes through the outlet valve and is pumped to the filtering media. Some feed slurry is recycled to the slurry tank and this can be adjusted with the use of the recycle valve. Adjusting the recycle valve changes the gauge pressure reading so this must be monitored in order to maintain a constant pressure filtration. The slurry then goes to the plates and frames and cakes are formed on the filter cloth. Filtrate then exits the plates and is collected.
3.4. Procedure
18 kilograms of water was poured into the slurry tank and the agitator was switched on. 2 kilograms of powdered Calcium carbonate was added to the water inside the slurry tank. While the suspension is being mixed well, the plates were covered with clean filter cloths and the pates were attached onto the frames and they are locked together tightly with the screw.
The outlet valve was opened and as soon as the first drop of filtrate had been collected with the large bucket on a bathroom scale, filtration time was started. Time was recorded for every 400 grams of filtrate that was collected, meanwhile the recycle valve was regularly adjusted so as to maintain a constant pressure reading in the pressure gauge. When slurry tank had run out of the slurry, a gallon of water was poured into the tank. Filtration was continued until there was no filtrate that would be collected, in which time, the agitator and the pump were switched off and the remaining slurry was drained. The screw in the press was loosen and the frames were withdrawn. The filter cloth was removed and the two wet filter cakes were collected carefully. They were put on a tray and their dimensions and initial mass were measured. The trays were put into a convective oven and were dried overnight after which they were taken out from the oven and weighed. The data that was obtained was mass as a function of time. The filtrate mass was converted into volume by assuming that the density of the slurry is equivalent to the density of water. Then table 𝑉𝑡 as a function of 𝑉 was constructed and from there the data was linearly regressed according to the working
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equation (4). Quantities 𝐾𝑝 and 𝐵 were determined from the equation of the line formed from linear regression and 𝐾𝑝 and 𝐵 yielded 𝛼 and 𝑅𝑚 which are the specific cake resistance and filter medium resistance. Another run of the experiment was done with 14% slurry. This was obtained by adding 2.9 kilograms of Calcium carbonate into the tank with 18.1 kilogram of water. Determined 𝛼 and 𝑅𝑚 were compared with the previously calculated values and the quantities were further compared by manipulating one quantity to arrive at the same units, as referred from equation (3).
4. Results and Discussions
Specific cake resistance and filter medium resistance
In the filtration operation, two resistances are commonly associated with the process. These are the specific filter cake resistance which is a property of the cake formed during the filtration operation and the filter medium resistance which is a property of the filter medium. Shown in Table 1 are the resistances in the filtration of aqueous CaCO3 using the plate and frame filtration equipment performed at constant pressure condition.
Table 1. The specific cake resistance (α) and filter medium resistance (Rm) at constant pressure condition
Resistance For 10 wt % CaCO3 slurry For 14 wt % CaCO3 slurry
%
difference Average
α (m/kg) 1.47 x 109 1.69 x 109 13.0 1.58 x 109 α (m/kg) -> (m-1) 9.83 x 1010 1.24 x 1011 20.7 1.11 x 1011 Rm (m-1) 5.39 x 109 6.06 x 109 11.1 5.73 x 109
It was found out that the specific cake resistance, when transformed to the same units of the filter medium resistance using equation 1, is greater compared to the resistance contributed by the filter medium.
𝛼 (𝑚−1) = 𝛼 (𝑚 𝑘𝑔) (
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑖𝑒𝑑 𝑐𝑎𝑘𝑒
𝑎𝑟𝑒𝑎 𝑜𝑓 𝑑𝑟𝑖𝑒𝑑 𝑐𝑎𝑘𝑒) (4)
It is because the cake formed during the filtration process creates a less permeable filtration medium which resists the flow slurry. The specific cake resistance is comprised by the resistance due to the deposition of
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residues from slurry. As filtration goes, it creates a more hindered flow of the slurry to the filtering medium resulting in the increase of the interval of time of the filtration between measurements.
The difference between the filter medium resistances calculated for the 10 and 14 wt % CaCO3 feed slurry can be attributed to the execution of the washing and cleaning of the filter cloth of the plate and frame filtration equipment. If there are residues left in the cloth, it will affect the initial porosity of the filter medium.
5. Conclusions
The specific cake resistance and filter medium resistance in the filtration of aqueous CaCO3 slurries performed at constant 5 psi pressure are 1.58 x 109 m/kg (1.11 x 1011 m-1) and 5.73 x 109 m-1.
References
Coulson, J., & Richardson, J. (2002). Coulson and Richardson's Chemical Engineering (Vol. 2). Woburn, MA: Elsevier Science.
Geankoplis, C. J. (2003). Principles of Transport Processes and Separation Processes (4th ed.). New Jersey, USA: Pearson Education, Inc.
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