FILTRATION CARRIED OUT UNDER A CONSTANT PRESSURE DIFFERENCE
AIM OF EXPERIMENT:
- Getting familiar with the apparatus for the cake filtration process carried out at a constant differential pressure
- Description of the filtration process of whey protein, peat or wheat bran slurries and determining the values describing the filtration process.
INTRODUCTION:
Filtration is a process of separating a solid-liquid system and using a porous partition. We can distinguish 2 main types of filtration: volumetric filtration, where separation takes place on a filter layer (made of gravel, slag or sand) and surface - by filtering cloths (made mainly of wool, cotton, synthetic or mineral fibers).
Another classification of filtration is based on the solid content in the suspension, i.e.:
- Separating - used to separate suspensions with a solid phase concentration greater than 1% in order to obtain a filtrate or valuable sediment (mainly surface filtration)
- Purifying - used for clarifying suspensions with a solid phase content <0.1% in order to obtain a filtrate (usually volume filtration).
A special (the most popular) type of surface filtration is the cake filtration. In this case, the role of the filtration partition is taken over by the sediment retained on the surface (so-called filter cake). The driving force causing the flow of liquid through the filtering partition is the pressure difference. We can distinguish:
1) Gravitational filtration (hydrostatic pressure)
2) Pressure filtration
3) Vacuum filtration
As a result of the filtration process, the retained medium creates a layer (surface filtration) or fills the volume (volume filtration). Compressible sludge- due to low permeability, dramatically reduce the filtration rate. To reduce this effect, a filtration aid is added to the slurry that forms an incompressible scaffold in the filtration cake. As filtering aids, diatomaceous earth, glass wool, activated carbon or sawdust are commonly used. Another way is to apply filtering aid to partition of a few mm thickness, before the slurry is introduced.
The equation of cake filtration with the formation of a non-compressible sludge:
(1)
where:
V – filtrate volume [m3]
A – surface of the filtration partition [m2]
t – duration of filtration [s]
p – differential pressure in front of and behind the partition [Pa]
0 – resistance of the sediment separated on the partition [m·kg-1]
ηc – viscosity [Pa·s]
Cs – solid concentration in the filtered suspension [kg·m-3]
r- amount of separated sludge, ensuring resistance of filtrate flow through this sludge, equivalent to filtrate flow resistance through the filtration membrane itself [kg·m-3]
In real systems, the filter cake is compressible and changes its porosity (the assumption that the cake is incompressible can only be accepted when filtering hard materials, e.g. crystals). In the case of suspensions filtration, where the particle size distribution is wide and the proportion of fine particles is considerable, even in incompressible systems, the porosity of the cake changes - due to the deposition of small particles deep in the pores.
0=·ps (2)
s – compressibility coefficient of the cake
– resistance of compressible sludge [(m·kg-1)·(m·s2·kg-1)s]
For non-compressible sludge s = 0, while for compressible s[0,1].
By linearizing equation 1, an equation describing the filtration with precipitation of compressible sludge is obtained assuming, that the pressure difference is constant and from which it is easy to calculate the filtration constant K - necessary to determine the compressibility coefficient.
(3)
Where constants K and C contain all the parameters difficult to determine from equation 1
(4)
(5)
Graphical determination of the isobaric filtration constant:
Determination of the compressibility coefficient of the sludge:
(6)
K- filtration constant
In order to determine the compressibility coefficient of the filtration cake, min. two filtrations under identical conditions (concentration of the filtered suspension, the same filtration cloth, amount of filtration aid, etc.), while at different pressures. For both pressure values, determine the K and C constants (graphically), that are necessary to determine "s".
Determining the throughput of the filter
The volume filtrate flow rate is expressed by:
(7)
A – filtration surface [m2]
p – pressure during filtration [Pa]
R- resistance generated by the filtration partition, which consists of the filtration partition, filtration aid and filter cake (variable in time).
Filter throughput it is the reverse of resistance:
(8)
Determining the dependence of the resistance of the proper filter cake on the applied pressure:
The resistance of the resulting filter cake, causes that the hydraulic resistance increases during the process. In order to determine the dependence of resistance on filtration time, it is necessary to know the resistance generated by the filtration partition together with the filtration aid.
To do this, measure the filtrate stream (in this case using water) for several pressures and calculate the resistance by converting equation 7. By plotting the filtrate flow versus the applied pressure, you can read the flow value for each working pressure and thus the dependence determine the resistance of the filtration partition with the filtration aid.
For filtration aid (non-compressible, because well-selected filtration aid should be non-compressible), the resistance value should be constant – it does not depend on the applied pressure. Thus, the change in the resistance of the filter layer (which consists of the filtering partition, filtration aid and filtration cake) results only from the fact that particles of the filtered suspension accumulates on it.
EXPERIMENTAL PART:
During the laboratory classes filtration will be carried out on two types of filters:
- Pressure filter
- Vacuum filter
Nutsche filters are the simplest type of filter. It is a tank, usually cylindrical with a perforated bottom, on which there is a filtration partition in the form of a granular layer or fabric. The filtration can occur under the pressure of a liquid column, vacuum or high pressure. Due to the small volume of the apparatus, it is used to filter small amounts of suspensions.
Laboratory installation
Pressure filters are connected to a tank (equipped with a paddle or straight stirrer) (Z1) (Figure 1), where the suspension is located and which, when filtering very large particle sizes, should be continuously stirred. The Z1 reservoir also acts as a positive displacement reservoir, hence it is connected to a compressor (S1), which generates pressure above the slurry in the tank, causing it to move over the filtering partition (and exerting the necessary pressure on it). The filtrate is collected into the Z2 tank. The systems are equipped with measuring points (manometers) P1 and P2, which indicate the working pressure.
Fig. 1: Installation for pressure filtration
In the initial state, all valves (from 1 to 6) should be open - the installation should remain depressurized. We start the filtration by closing the valve 1 (!!!) and 3 and pouring the slurry into the tank Z1. The filtration is started after the Z1 tank and the tank containing the filter (F1) have been carefully sealed, by tightening the lids and all outlet valves (1 and 5) and opening the flow valves - 3, 4 and 6. After the compressor starts, the slurry is directed to F1, and under the influence of overpressure liquid begins to pass through the filter and appear in the filtrate tank Z2.
The installation used for vacuum filtration is equipped with a water pump (VP) (Fig. 2), which generates the necessary negative pressure behind the filtration partition, which is connected directly to the filtrate tank (Z1). The filtrate tank is connected to the vacuum nutsche, which during the process may remain open (tightness is required behind the filtration partition, which is provided by the filtered suspension).
Fig. 2: Installation for vacuum filtration
Similarly to the previous work, we start by opening all valves (1, 2 and 3). We start the filtration by closing the valves 1 and 3. Then pour the suspension into the tank F1. After starting the water pump (VP), open the valve 3. In the case of vacuum filtration, the filtrate is collected in a sealed tank Z1, which is invisible.
EXPERIMENT:
During laboratory classes, it is necessary to filter the malt suspension obtained during the production of beer using pressure and vacuum filtration methods.
During each filtration, we measure the filtrate stream every 10 minutes. Using the information contained in the introduction, calculate the compressibility coefficient of the cake, its hydraulic resistance and throughput. Additionally, knowing the resistance generated by the filtration partition with the filtration aid (see section "Determining the resistance of the filter cake to the pressure applied"), calculate (and plot) the change in hydraulic resistance resulting from the formation of a filter cake. The amounts of solid required for the suspension and filter aid (diatomaceous earth) will be given by the lecturer.