On the Study of the Peat Filtration Process
Abstract Generated abstract
This paper examines the preliminary dewatering of highly moist peat hydromass by filtration through an artificial porous partition. Using sedge-hypnum peat from the Tagan deposit with varying degrees of decomposition, the authors analyze filtration rate, sediment resistance, and pore clogging behavior, treating peat sediment as a compressible layer whose resistance increases during filtration. Graphical comparison of standard filtration coordinates suggests that peat filtration can be described approximately as either an intermediate filtration process or as partial clogging of the filter pores. The study concludes that lower peat dispersity reduces the rate at which total filtration resistance increases with filtrate volume.
Full Text
On the Study of the Peat Filtration Process
V. N. Ponomarev, S. I. Smolyaninov, M. I. Fomchenkov
(Recommended by the scientific-methodological seminar of the Chemical-Technological Faculty)
Peat is a typical porous medium that offers a certain resistance to the flow of liquid moving through it. Moreover, the filtration of water through peat often differs significantly from the process of filtration through other systems, which has been studied in considerable detail [1–3]. This is due to the complexity of the multicomponent peat system, which, along with true solutions of low- and high-molecular substances, contains hydrophobic sols and hydrophilic semicolloids that are in dynamic dispersion equilibrium.
In developing technological processes in which the initial peat is present in the form of a hydromass with a moisture content of 95% and higher, the process of rapid preliminary dewatering of the hydromass is of primary importance.
To determine the regularities of dewatering of peat hydromass, we carried out experiments on hydromass filtration. A metal mesh with an aperture size of 0.25 mm was used as the filtering partition. The hydromass was prepared from peats of the Tagan deposit with various degrees of decomposition. The peat was sedge-hypnum.
At one time, experiments by L. S. Evstaf’eva on hydromass filtration showed that the fine particles deposited above the underlying filtering layer and directly within this layer create a zone with a sharply reduced filtration coefficient and thereby hinder its further process. The filtration coefficients of this main layer for various types of peat are approximately equal to \(K = 1.23 \cdot 10^{-5}\) cm/sec [5].
A similar process also occurs in the case of filtration of peat hydromass through artificial porous partitions. The decrease in filtration rate at a constant pressure difference is also caused by an increase in the resistance of the sediment as a result of the increase in its thickness. Simultaneously with the increase in the thickness of the sediment layer, the sediment is compacted as a result of a decrease in its porosity and deformation of the particles (especially when the pressure difference is increased), which also leads to a decrease in the rate of the filtration process.
Naturally, determining the resistance of the sediment while taking into account all factors affecting the course of the filtration process—namely: the surface of the filtering partition, the magnitude of the sediment layer, its porosity, the size and shape of the particles, the viscosity of the liquid phase, etc.—is very difficult and at present does not appear possible.
A continuous increase in the resistance of the peat forming the sediment and of the filtering partition leads to a decrease in the rate of the filtration process (Fig. 1).
The rate of peat filtration is expressed in the general case as follows:
\[ W=\frac{dV}{S d\tau}, \]
where \(V\) is the volume of filtrate, \(\mathrm{cm}^3\);
\(S\) is the filtration surface, \(\mathrm{cm}^3\);
\(\tau\) is the duration of filtration, sec.
Neglecting the resistance of the filtering partition, since it is incomparably small in comparison with the resistance offered by the peat sediment formed during filtration, the dependence of the filtration rate in differential form may be expressed as follows:
Fig. 1. Dependence of the filtration rate of peat hydromass on time
\[ \frac{dV}{S d\tau}=\frac{\Delta P}{\mu R_{\mathrm{oc}}}, \]
where \(\mu\) is the viscosity of water;
\(R_{\mathrm{oc}}\) is the resistance of the sediment layer;
\(\Delta P\) is the pressure difference.
The resistance of the layer of peat sediment formed in the filtration process can be expressed by the known relation
\[ R_{\mathrm{oc}}=r_o\cdot h_{\mathrm{oc}}, \]
where \(h_{\mathrm{oc}}\) is the height of the sediment layer;
\(r_o\) is the average specific resistance of the sediment.
In the case of the peat filtration process it must be taken into account that the value of the specific resistance of the sediment, which is compressible, is a variable quantity and increases from the boundary of the filtered system with air to the boundary with the filtering partition.
As is known, four types of filtration through porous partitions are distinguished: with complete clogging of pores, with partial clogging of pores, filtration of an intermediate type, and filtration with formation of sediment. The functional dependence of the filtration process is expressed by a straight line in the following coordinates (4):
| Type of filtration | Coordinates |
|---|---|
| With complete clogging of pores | \(W — q\) |
| With partial clogging of pores | \(W — \dfrac{\tau}{q}\) |
| With formation of sediment | \(\tau — \dfrac{\tau}{q}\) |
| Intermediate | \(\tau — \dfrac{1}{W}\), |
where \(q\) is the amount of filtrate obtained from \(1\ \text{m}^2\) of surface;
\(W\) is the filtration rate.
Figures 2 and 3 present graphical relationships for the filtration process of processed raw peat with a degree of decomposition of 35% in various coordinates.
From the graphical data presented it follows that the peat-filtration process may, with sufficient approximation, be assigned to the intermediate type and to a process with partial clogging of the pores of the filtering partition.
Fig. 2. Functional relationship of the peat-filtration process
1 — in coordinates \(W\)—\(q\),
2 — in coordinates \(W\)—\(\dfrac{\tau}{q}\).
The intermediate type is characteristic of the case of filtration of peats of a low degree of decomposition; in this case the intensity of increase of the total resistance as the amount of filtrate increases is proportional to this resistance [4], i.e.
\[ \frac{dR}{dq}=K\cdot R, \]
where \(R\) is the total resistance of the system.
The filtration process of processed peats and peats of a high degree of decomposition is more consistent with the type with partial clogging of the pores, for which there is characteristic
Fig. 3. Functional relationship of the peat-filtration process
1 — in coordinates \(\tau\)—\(\dfrac{1}{W}\),
2 — in coordinates \(\tau\)—\(\dfrac{\tau}{q}\)
the gradual clogging of one pore of the filtering partition by many solid peat particles, and the intensity of increase of the tot-
the total resistance as the amount of filtrate increases is proportional to this resistance to the power \(3/2\), i.e.
\[ \frac{dR}{dq}=K' \cdot R^{3/2}. \]
Conclusions
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The filtration processes of peats of varying degrees of dispersity may be assigned to an intermediate type and to a process with partial clogging of the pores of the filtering partition.
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As the dispersity of peat decreases, the intensity of the increase in total resistance as the amount of filtrate increases is reduced.
Literature
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Yu. M. Shekhtman. Filtration of Concentrated Suspensions, Academy of Sciences of the USSR, 1961.
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N. E. Zhukovsky. Collected Works, vol. 2, Hydrodynamics, Gostekhizdat, 1949.
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I. I. Lyashko. Solution of Filtration Problems by the Method of Integral Transforms. Kiev University Press, 1963.
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V. A. Zhuzhikov. Filtration, Gostekhizdat, 1961.
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V. G. Goryachkin. Fundamentals of the Technology of Peat Production, GEI, 1953.
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B. V. Deryagin, N. V. Churaev. The influence of surface forces on the movement of moisture in porous bodies. In: Studies in the Field of Surface Forces. Academy of Sciences of the USSR, 1964.