Lab Experiment 12 Filter Press

CHET 409

DEWATERING OF SLUDGE BY PLATE AND FRAME FILTER PRESS

Name & No.:

Date:

Section:

Score:

LAB EXER. 12 / 1

DEWATERING OF SLUDGE BY PLATE AND FRAME FILTER PRESS 1.1.1 PERFORMANCE OBJECTIVE Upon completion of this experiment, the student will be able to : 1. Explain the effect of using two reactors working in series on the anaerobic process. 1.2 1. 2. 3. 1.3

EQUIPMENT Disposable plastic gloves Lab-coat or rubber apron Dissolved oxygen meter MATERIALS

1. Wastewater sludge 1.4

DISCUSSION

The plate and frame filter is one of the most commonly used filter design. It has two end filter plates to which the filter medium. Between the two end plates are frame plates and wash plates. Slurry is pumped into the frame plates from the inlet and filtered solids collect there. Wash plates have a filter medium on both faces and have a particular port arrangement to enable slurry washing procedures. The number of frame plates and wash plates can be changed depending on the separation area and solids capacity required. All of the plates are heldpressed together to form a seal which enables the filter unit to be operated at pressure, providing drive force to push the slurry through the filter cloths. To obtain the solids after each filtration, the filter and frame filter has to be dismantled. In industry a wide variety of filter media are used, from wire mesh screens and plastic sheets such as nylon to loose powder media such as diatomaceous earth or perlite. 1. DEMONSTRATION OF PRECOAT FILTRATION

Yanbu Industrial College

18 June 2010

CHET 409


LAB EXER. 12 / 2

There are many types of filter media. Coarse nylon filter medium with 45 m nominal pore size. However, the sizes of most particles recovered by filtration are much smaller than this. It is possible to use filter media with much smaller pore sizes; for example polyester material with a pore size of 3 m is commercially available. Although such a small pore size may not be sufficient for the recovery of many solids. Moreover, the area of the filter that is actually pores, termed the open area, reduces reasonably as the pore size reduces which in turn leads to much reduced filtration rates. Both problems can be overcome by using precoat of loose solids on a coarse medium. There are a few types of precoat can be used in different filtration process. These precoats can maintain the open area even for particles as small as 1m. Another important characteristic of precoat is their high porosity which means that there is little added resistance to the filtration process. This is important for the efficiency of a filtration process. Diatomaceous earth and expanded perlite are two most common precoat to be used. Diatomaceous earth is the fossilised remains of microscopic plants which are several million years old. While, expanded perlite is derived from a type of volcanic rock. Preparation of expanded perlite involves crushing, grinding, screening and calcining of the mineral. In the process of filter aid, these precoating materials are added to feed solution to aid the filtration process by increasing the porosity of the filter cake. 2. DEMONSTRATION OF DARCY’S LAW

Filtration can be described by Darcy’s equation: Q

P L

where Q is the volumetric flow rate of filtrate, ∆P is the driving pressure and L is the bed or cake thickness. The equation is often written in the following form: Q

KAP L

or

Q

AP  RT

where K is a constant referred to as the bed permeability, A is the filtration area,  is the filtrate viscosity and RT, equal to L/K, is termed the resistance. The filtration operation can be run either at constant feed pressure or at constant feed flow rate. As the filtration proceeds solids are deposited onto the filter surfaces to form a cake, the thickness of which increases with time.

3. DETERMINATION OF MEDIUM AND CAKE RESISTANCES


18 June 2010

CHET 409


LAB EXER. 12 / 3

The resistances to filtration, the medium resistance and the cake resistance are determined as an inclusive term, RT. It is more useful to determine these resistances individually. Thus: Q

PA  ( R  RC )

1.4.1 where R and Rc are the medium and cake resistances respectively. The cake resistance increases with time as more solids are deposited on the filtration surfaces. The medium resistance is assumed to be constant although this may not be true due to blocking of pores with time. The medium and the cake resistances are important in helping to design and scale up the filtration procedures. For incompressible cakes, the cake resistance is directly proportional to the amount of cake deposited, therefore, RC   w

where w is the mass of cake deposited per unit area and  is the specific cake resistance. Therefore, by substitution: PA  w   R For batch filtration, w is a function of time and is given by: Q

wA  cV where c is the mass concentration of solids (expressed in terms of the unit volume of filtrate), and V is the cumulative filtrate volume. Thus the general filtration equation can be given by: PA Q  c(V / A)   R 1.4.1.1 3.1 Constant rate filtration Now for constant rate filtration it becomes: Q

P(t ) A  c(V (t ) / A)   R

V = Qt


18 June 2010

CHET 409


LAB EXER. 12 / 4

Hence, P 

 cQ 2t A



2

 RQ A

If,

a  c

b  R

and

then, P  av 2t  bv

where v is the approach velocity of the filtrate: v

Q A

1.4.1.1.1 Hence, a plot of ∆P against t should give a straight line with a gradient of av 2 intercept of bv. From this values of  and R can be calculated.

and an

3.2 Constant pressure filtration For constant pressure filtration it can be shown that: t

 cV 2 2 A P 2



 RV AP

If, a1 

Then,

 c 2 A P 2

and b1 

R AP

t  a1V  b1 V

High initial flow rates through a clean medium are usually avoided to prevent penetration of solids through the clean medium which lead to contamination of the filtrate. Therefore a period of constant flow rate is normally adopted prior to constant pressure filtration. Thus, the equation must be modified:


18 June 2010

CHET 409


LAB EXER. 12 / 5

t  ts R  a1 (V  Vs )  V  Vs AP

where ts and Vs are the time and the volume of filtrate prior to the constant pressure filtration period. t  ts Thus a plot of vs V+Vs gives a straight line from which the medium and cake V  Vs resistances can be calculated.

4. DEMONSTRATION OF FILTER CAKE WASHING AND DEWATERING It is often to wash the filter cake before solids are removed from the filter either to recover valuable mother liquor or to remove impurities. Important parameters to determine are the volume of wash liquid required for efficient washing and the time the procedure takes. There are two basic methods used in industry for filter washing: (a) Immediately apply the wash liquid to the cake just after the filtration ends, with all the pores within the cake are still completely full with liquid from the original feed slurry, ie. when the cake is saturated. (b) Air is blown through the filter cake prior to wash liquid application, ie. the cake is unsaturated prior to washing.

PROCEDURE:

1. SLURRY PREPARATION 1.1 PREPARATION OF PRECOAT SUSPENSIONS 1. 0.5% (w/v) solution is used. Mixing 5g to a litre of water can make this. 2. With the same ratio, add water to 20g of precoat and make up to 4L. 1.2 PREPARATION OF CALCIUM CARBONATE SUSPENSIONS 1. 4.0% (w/v) solution is used. Mixing 40g of CaCO3 to a litre of water can make this. 2. With the same ratio, add water to 320g of CaCO3 and make up to 8L. 3. In many of the exercises instructions are given to make a 8L solution of CaCO3 and add it to the feed tank which already contains 4L of water from recirculation of the precoat


18 June 2010

CHET 409


LAB EXER. 12 / 6

suspension. The final concentration of the 12L solution must be 2.67% (w/v) CaCO3. In order to reach this concentration, 320g of CaCO3 are added to 8L of water which is then added to the feed tank. 1.3 GENERAL START-UP 1. Check that the filter media and gasket are properly arranged between the filter plates. Tightened the bolts and nuts. 2. Make sure all the filter plates are packed together in closed position with the moving platen pressed against it and the recessed indent of the plate handles are resting neatly on the sidebars. 3. Check that all instruments are installed properly and in good condition. 4. Power-up the equipment by connecting the electrical plug to the nearest electrical power supply. Then switch on the main switch.

2. EXPERIMENTAL PROCEDURES

2.1 DEMONSTRATION OF PRECOAT FILTRATION Note: Lead water to drain the pump for the first time to avoid any oxidised deposits to enter the pump. 1. Prepare 8L solution of 4% (w/v) calcium carbonate and add to the feed tank. Turn on the feed tank stirrer to maintain a homogeneous feed solution. 2. Arrange the valves so that filtrate is returned to the feed tank. With the controller on manual, set the feed flow rate to 40L/hr and begin pumping. Start the logging of data when the slurry is entering the filter. Switch valve V5 to allow filtrate to collect in the filtrate vessel. Bleed any air from the filter frame plates. Observe the filter for any build up of filter cake. After five minutes sample the filtrate and stop the pump. Stop data logging. 3. Discharge the calcium carbonate from the feed tank, remix with the filtrate and retain for later. 4. Prepare 4L of 0.5% (w/v) precoat and add to the feed tank. Turn on the feed tank stirrer to maintain a homogeneous feed solution. Arrange the valves so that filtrate is returned to the feed tank. With the controller on manual set the feed flow rate to 40L/hr and begin pumping. As the precoat suspension enters the filter frame plates, begin logging of data. Bleed any air from the filter frame plates. 5. Continue pumping for 10-15 minutes by which time most of the precoat will be retained on the filter surfaces within the filter. Stop data logging.


18 June 2010

CHET 409


LAB EXER. 12 / 7

6. Add the calcium carbonate from the previous experimental run to the feed tank. As the calcium carbonate suspension enters the filter frame plates, begin logging of data. Switch valve V5 to allow filtrate to collect in the filtrate vessel. 7. Continue the experiment until the pressure reaches 1.5 bar and then stop the pump and stop data logging. Determine the extent of precoat deposition onto the filter surfaces. 8. Measure the volume of filtrate. 9. Plot ΔP against V and comment on the relationship. Identify the major resistances to filtration. 2.2 DEMONSTRATION OF DARCY’S LAW 1. Prepare 4L of 0.5% (w/v) precoat and add to the feed tank. Turn on the feed tank stirrer to maintain a homogeneous feed solution. Arrange the valves so that filtrate is returned to the feed tank. Set the controller to 1.5 bar but set to manual pump control. 2. Set the pump to give a feed flow rate of 40L/hr and begin pumping. Bleed any air from the filter frame plates. Continue pumping for 10-15 minutes by which time most of the filter aid will be retained on the filter surfaces within the filter. 3. Prepare 8L of calcium carbonate solution, add to the feed tank and make up to 12L. The final concentration of calcium carbonate in the feed tank should be 2.67% (w/v) and 0.17% (w/v) filter aid. As the calcium carbonate/filter aid begins to enter the filter plates, begin data logging. Switch valve V5 to allow filtrate to collect in the filtrate vessel. As the pressure reaches 0.20 bar switch the controller to automatic. 4. As the volume of material in the feed tank becomes low stop the pump by adjusting the controller set point to 0.0 bar. Stop data logging. 5. Disassemble, clean and re-assemble the filter before performing the next experimental run. 6. Plot a graph for Q against t. Determine the gradients for the constant pressure periods of operation and comment on the results. Calculate K and RT. Comment on the results for the different feed compositions. 2.3 DETERMINATION OF MEDIUM AND CAKE RESISTENCES Constant rate filtration Note: When using the pump for the first time of the day it is best to lead water to drain so that any oxidised deposits in the pump body do not enter the filter. 1. Prepare 4L of 0.5% (w/v) precoat and add to the feed tank. Turn on the feed tank stirrer to maintain a homogeneous feed solution. Arrange the valves so that filtrate is returned to the feed tank. 2. Set the pump to give a feed flow rate of 40L/hr and begin pumping. Bleed any air from the filter frame plates. Continue pumping for 10-15 minutes by which time most of the filter aid will be retained on the filter surfaces within the filter.


18 June 2010

CHET 409


LAB EXER. 12 / 8

3. Prepare 8L of calcium carbonate solution, add to the feed tank and make up to 12L. The final concentration of calcium carbonate in the feed tank should be 2.67% (w/v). 4. As the calcium carbonate/filter aid begins to enter the filter plates, begin data logging. Switch valve V5 to allow filtrate to collect in the filtrate vessel. Continue the filtration until the liquid level in the tank becomes low (~ 3L). Stop data logging. 5. Clean the filter and tanks before repeating the next experiment. 6. Repeat the experiment at 48 L/hr. 7. Plot all the data on a single graph of pressure vs time. From this plot determine the gradient and the intercept. This enables calculation of the medium and cake resistances. Constant pressure filtration 1. Prepare 4L of 0.5% (w/v) precoat and add to the feed tank. Turn on the feed tank stirrer to maintain a homogeneous feed solution. Arrange the valves so that filtrate is returned to the feed tank. Set the controller to 1.0 bar but set to manual pump control. 2. Set the pump to give a feed flow rate of 40L/hr and begin pumping. Bleed any air from the filter frame plates. Continue pumping for 10-15 minutes by which time most of the filter aid will be retained on the filter surfaces within the filter. 3. Prepare 8L of calcium carbonate solution, add to the feed tank and make up to 12L. The final concentration of calcium carbonate in the feed tank should be 2.67% (w/v). 4. As the calcium carbonate begins to enter the filter plates, begin data logging. Switch valve V9 to allow filtrate to collect in the filtrate vessel. As the pressure reaches 1.0 bar, switch to automatic control. Continue the filtration until the liquid level in the tank becomes low (~ 3L). Stop data logging. 5. Plot all the data on a single graph, plotting (t-ts) / (V-Vs) vs V+Vs. The data for the constant pressure filtration period should give a straight line although there may be a slight curve to the data. This enables the medium and cake resistances to be calculated. 2.4 DEMONSTRATION OF FILTER CAKE WASHING AND DEWATERING 1. Prepare 4L of 0.5% (w/v) precoat and add to the feed tank. Turn on the feed tank stirrer to maintain a homogeneous feed solution. Arrange the valves so that filtrate is returned to the feed tank. 2. Set the pump to give a feed flow rate of 40L/hr and begin pumping. Bleed any air from the filter frame plates. Continue pumping for 10-15 minutes by which time most of the filter aid will be retained on the filter surfaces within the filter. 3. Prepare 8L of calcium carbonate solution, add to the feed tank and make up to 12L. The final concentration of calcium carbonate in the feed tank should be 2.67% (w/v). As the calcium carbonate/filter aid begins to enter the filter plates, begin data logging. 4. Switch valve V5 to allow filtrate to collect in the filtrate vessel. Continue the filtration until the filter reaches its solids capacity. Stop data logging. 5. Discharge the feed and fill again with 3L of water and allow the water to recirculate through the filter and tank for 5 minutes. After this period drain the water from the tank. Results


18 June 2010

CHET 409


LAB EXER. 12 / 9

Demonstration of Precoat Filtration Results: Temperature of filtrate: Time, t

°C

Pressure drop,  P

Volumetric flow rate of

Volume of filtrate, V

filtrate, Q (L/min)

(L)

(bar)

(min)

Demonstration of Darcy’s Law Results: Temperature of filtrate:

°C Pressure drop,  P (bar)

Time, t (min)

Volumetric flow rate, Q (L/min)

Determination of Medium and Cake Resistances Results: A) Constant rate filtration Time, t (min)

Feed flow rate = 40L/hr

Feed flow rate = 48 L/hr

Temperature =

Temperature =

°C

Pressure drop,  P (bar)


°C

Pressure drop,  P (bar)

18 June 2010

CHET 409


LAB EXER. 12 / 10

B) Constant pressure filtration Results: Temperature of filtrate : °C Time

Pressure drop,

Volumetric flow

Volume of

(min)

 P (bar)

rate, Q(L/min)

filtrate, Vf (L)

Cumulative volume, V (L)

t  ts V  Vs

1.4.2 FINAL CHECKLIST 1. Clean your equipment and work benches before you leave. 2. Return all equipment to their proper storage area. 3. Submit your answers to the review questions along with your technical report to your instructor before the next laboratory session.


18 June 2010

Lab Experiment 12 Filter Press

Recommend Documents