How to calculate permeability
is a parameter that characterizes the ability of soil or rock to conct liquid. Its size is related to the porosity, the geometry of the pores in the direction of liquid permeability, the particle size and the arrangement direction, but not to the properties of the liquid moving in the medium. Permeability (k) is used to indicate the permeability
when the pressure gradient is 1, the penetration velocity of the liquid with dynamic viscosity coefficient of 1 in the medium. There are two ways to measure and calculate the rock permeability with dimension L2:
1< 2. Indirect method (logging method or reservoir engineering method)
the permeability of rock is expressed by the numerical value of permeability, which can be expressed in three ways: absolute permeability, effective permeability and relative permeability
absolute permeability
when the cross-sectional area of single-phase fluid is a, the length is l, and the pressure difference is 0 Δ When a section of porous medium of P flows in layers, the viscosity of the fluid is 0 μ， Then the amount of fluid passing through the rock pore in unit time is q = K Δ PA/ μ L [4] When the single-phase fluid flows through the porous media in layers, the liquid flow rate per unit time through the rock cross-sectional area is directly proportional to the pressure difference and cross-sectional area, but inversely proportional to the length of the liquid through the rock and the viscosity of the liquid
where: Q -- flow rate of fluid through rock in unit time, cm3 / S; A -- cross sectional area of liquid passing through rock, cm2< br /> μ—— Viscosity of liquid, PA · s; L -- length of rock, cm; Δ P -- pressure difference before and after liquid passing through rock, MPa
the absolute permeability of rock is the measured permeability when there is only one fluid (single phase) in the pores of rock, the fluid does not react with the rock in any physical or chemical way, and the fluid flow conforms to Darcy's linear percolation law
since the gas is obviously affected by pressure, when the gas flows from (high pressure) to (low pressure) along the rock, the volume of the gas will expand, and the volume flow is variable when it passes through the cross-sectional area, so the volume flow in Darcy's formula should be the average flow through the rock.
When water flows in soil, it will cause head loss. This kind of head loss is e to the energy consumption of dragging soil particles when water flows in soil pores
naturally, when the flow drags the soil particles, it will give the soil particles a certain drag force, and the drag force exerted by the seepage flow on the soil particles in the unit soil is called seepage force. The seepage force is directly proportional to the hydraulic gradient, and its direction is consistent with the seepage direction
extended data
the main factors affecting the permeability of sandy soil are the size, shape, gradation and density of permeable fluid and soil particles. The main influence of seepage fluid is viscosity, which is affected by temperature. The higher the temperature is, the lower the viscosity is and the higher the seepage velocity is. The influence of soil particles is that the finer the particles are, the lower the permeability is; For the well graded soil, because the fine particles fill the pores of the large particles, the pore size is reced and the permeability is reced
When the density ofsoil increases, the porosity decreases and the permeability decreases. The main factors affecting the permeability of cohesive soil are the mineral composition, shape and structure of particles (pore size and distribution), and the interaction of soil water electrolyte system. The shape of clay particles is flat and has directional arrangement, so the permeability has significant anisotropy
The capillary model ofpermeability shows that the seepage velocity is proportional to the square of pore diameter, and the unit flow rate is proportional to the fourth power of pore diameter. The permeability of clay with the same porosity is much higher than that of clay with uniform pore size. The microstructure and macrostructure of clay have great influence on the permeability
without accurate calculation:
osmotic concentration C = C (Na +) + C (PO43 -)
= 3 × 0.1 + 0.1 = 0.4 mol · L-1
osmotic pressure Π = CRT
by substituting C, universal gas constant R, and absolute temperature T, we can get the penetration coefficient Π.
The calculation formula of osmotic pressure is as follows π= cRT
Where, π Is the osmotic pressure of dilute solution, C is the concentration of solution, R is the gas constant, n is the amount of solute, t is the absolute temperature
According to the law of van der Waals, the osmotic pressure is directly proportional to the concentration and temperature of the solution, and its proportional constant is the constant R in the equation of state of the gas
extended data:
put the solution and water in the U-shaped tube, and place a semi permeable membrane in the middle of the U-shaped tube to separate water and solution. Water can be seen running to one end of the solution through the semi permeable membrane. If pressure is applied at the solution end, and this pressure can just prevent water from penetrating, it is called osmotic pressure, The osmotic pressure is related to the molarity of solution, solution temperature and solute dissociation degree
Therefore, sometimes the molecular weight of macromolecules can be deced by the osmotic pressure and other conditions. Van terhoff won the first Nobel Prize in Chemistry for his research on osmotic pressure and chemical kineticsThe osmotic pressure of solution refers to the attraction of solute particles to water. The osmotic pressure of solution depends on the number of solute particles in unit volume of solution: the more solute particles, the higher the concentration of solution, the greater the attraction to water, and the higher the osmotic pressure of solution
On the contrary, the less solute particles, that is, the lower the concentration of solution, the weaker the attraction to water and the lower the osmotic pressure of solution. It is related to the content of inorganic salt and protein The results showed that Na & # 8314 was the most abundant inorganic salt in extracellular fluid; And Cl & # 8315;, More than 90% of the osmotic pressure of extracellular fluid came from Na + - 8314; And Cl & # 8315;. At 37 ℃, the osmotic pressure of human plasma is about 770 kPa, which is equivalent to the osmotic pressure of intracellular fluid According to the number property:because the equilibrium osmotic pressure follows the law of ideal gas (the interaction of solute molecules is ignored in dilute solution), the mathematical derivation process is omitted here, and finally the van terhoff relation can be obtained π= CRT (or π= kTN/V N / V is the molecular number density)
It can be seen from the formula that the osmotic pressure of solution is only determined by the molecular number of solute, so the osmotic pressure is also a number dependent property of solution. This relation does not give the real pressure, but the pressure that may be needed to stop the osmotic flow, that is, the pressure difference needed for the system to reach equilibriumπ V = NRT or π= cRT
from a chemical point of view, π V = NRT or π= CRT, where π Is the osmotic pressure, V is the volume of solution, n is the amount of substance, C is the concentration of substance, R is the ideal gas constant, t is the thermodynamic temperature. The unit is Kelvin, K, t / k = t / ℃ + 273.15, that is, the thermodynamic temperature is always 273.15 higher than the temperature value of the centigrade temperature scale, and t cannot reach 0k, which is often said that absolute zero cannot be reached
The size of osmotic pressure depends on the concentration of total solute particles (molecules and ions), which is called osmotic concentration. For non electrolyte solution, the osmotic concentration is consistent with the molar concentration, and the osmotic pressure is approximately proportional to the molar concentration, but the electrolyte solution is different
extended data:
precautions:
infiltration is the movement of water according to water potential gradient. In other words, the direction and velocity of water flow through the membrane are not only determined by the concentration gradient or pressure gradient of water, but also by the sum of these two driving forces. Permeation refers to the movement of solvent molecules through a semi permeable membrane
in an osmotic system, the direction of water movement depends on the water potential of the solution on both sides of the semi permeable membrane. The water in the solution with high water potential flows to the solution with low water potential
you can use the calculation software of reverse osmosis system to design
which contains this part of data