RU2179064C2 - Method of modifying porous structure of inorganic membranes with pyrocarbon - Google Patents

Abstract

FIELD: inorganic membranes. SUBSTANCE: hydrocarbon stock (C1-C18-hydrocarbons, benzene and its homologues) is fed into reaction zone and pyrolyzed at 700-1100 C. Resulting carbon is fixed on membrane thereby modifying its porous structure. Modification of porous structure of anisotropic membranes on metal support is carried out by predeformation and slow heating limiting temperature rise rate by 6,2 C/min. EFFECT: enabled formation of separating membranes with specified pore size and therefore required selectivity and permeability. 3 cl, 2 tbl, 3 ex

Description

 The invention relates to a technology for the manufacture of inorganic membranes with a given pore size and the required porosity, which allows to obtain separation elements with the selectivity and performance necessary for a particular process, namely for ultrafiltration separation of liquid and gaseous mixtures when they are purified or the target component is isolated, and can be used in the oil refining industry, in the processes of purification and separation of gases, in the food industry.

при этом авторы полагают, что, используя при пиролизе углеводороды, начиная с C₃, они предотвращают нежелательное снижение пористости, которое имеет место при использовании метана. Данное положение весьма спорно, поскольку при указанных размерах пор подложки разница в размерах молекул C₁ и C_3-8, составляющая не более 10

незначительна и не является определяющим фактором.A method [1] for producing anisotropic membranes is described in which a selective layer is formed by depositing pyrocarbon on a porous ceramic or carbon substrate. As a porous base used products with a pore size of 2000-50000

Moreover, the authors believe that using hydrocarbons in the pyrolysis, starting with C ₃ , they prevent the undesirable decrease in porosity that occurs when methane is used. This position is very controversial, since at the indicated pore sizes of the substrate, the difference in the sizes of C ₁ and C _3-8 molecules, which is no more than 10

insignificant and not a determining factor.

 In addition, the authors argue that by passing a pyrolyzable hydrocarbon over a substrate, they form a selective layer on its surface, which, taking into account the above ratios of pore sizes and pyrolyzable chemical compounds, is quite controversial and requires correct justification (measurement of the thickness of the pyrocarbon layer formed, change control pore volume, etc.).

In addition, using a propane-butane mixture as a pyrolyzable substance at 1000 ^o C, the authors actually work with its degradation products, i.e., with C _1-2 hydrocarbons, especially since, despite the absence of a description of the installation in the patent, assume that the working gas mixture heats up gradually.

A known method [2] of increasing the selectivity of hollow fiber carbon membranes used in gas separation processes by depositing carbon on their surface obtained by the pyrolysis of carbon-containing chemical compounds (trichloroethane, tetramethylmethane) in the vapor phase. Moreover, first to increase the permeability of the membrane is activated by treatment with an oxygen-containing gas at 200-320 ^o C.

 One of the drawbacks of this work is that the authors do not control the changes in porosity and pore size that occur during the deposition of carbon on the membrane surface, but only record differences in their permeability before and after treatment.

 In addition, the periodic carbon deposition procedure is unsuccessful: a reactor with a membrane filled with a pyrolyzable substance is heated to the reaction temperature, flushed with an inert gas, cooled, etc. Thus, only carbon obtained from the decomposition of those molecules of the pyrolyzed compound is deposited on the membrane surface that are either adsorbed on its surface or in contact with it.

поскольку, если использовать его для мембран на основе пористого углерода, размер пор которых составляет 0,1-0,01 мкм, то для заметного изменения их пористой структуры эту процедуру необходимо повторять десятки раз, что связано с многократным нагревом-охлаждением, развитием процессов спекания и т.п.In this regard, this method of improving the selectivity of carbon membranes should be applied only to their very narrow class, characterized by a pore size of up to 8-10

since, if you use it for membranes based on porous carbon, the pore size of which is 0.1-0.01 μm, for a noticeable change in their porous structure, this procedure must be repeated dozens of times, which is associated with multiple heating-cooling, the development of sintering processes etc.

 This method is the closest analogue.

 The objective of the invention is the creation of separation elements with the required selectivity and permeability, which is achieved by modifying the porous structure of inorganic anisotropic membranes with pyrocarbon.

The problem is solved by modifying the porous structure of an inorganic anisotropic membrane by depositing carbon obtained by the pyrolysis of hydrocarbons on its selective layer deposited on a metal substrate. The modification is carried out by covering the membrane with a mechanical load of 400-900 g in a carbon cell, the design of which allows pyrocarbon to be deposited only on the selective layer of the membrane, without affecting its substrate, at a controlled temperature rise rate of not more than 6.2 ^o C / min.

 Oxides of Al, Ti, Zr, Hf, Th, La, Ca, etc. are used as the separation layer.

The modification is carried out at a temperature of 700-1100 ^o C, a pressure of 5-760 mm RT. Art., using gaseous and liquid hydrocarbons C _{1-6 of} normal isostructure as a pyrolyzable component, as well as benzene and its homologues supplied to the reaction zone with a bulk velocity of 2.5-10 l / min.

 It should be noted that the sign "regressing" is used in a rather narrow field of scientific and technological developments, in particular, in the calculation and manufacture of springs and indicates the application of mechanical load to the object. In the book of S. D. Ponomarev, V. L. Biderman, K. K. Likharev, V. M. Makushin and others. “Strength calculations in mechanical engineering”. M .: Mashgiz, 1956, v. 2, p. 593-594 we read: "Springing of springs is considered as a method of testing springs with a long load and the period of grating is determined by their role in a particular design." In the same place: "For cylindrical compression springs, the gouging operation consists in compressing them under the action of a mechanical load until the coils touch and hold in a deformed state for 6 to 48 hours." The above definitions of the sign of "overwhelming" show that it is applicable in our case too, since when modifying the membranes we also exert mechanical stress on them and withstand it for the duration of the process.

We use anisotropic TiO ₂ / ZrO ₂ membranes (ZrO ₂ <10%) on porous steel, the average pore diameter of which is 0.07 μm, with an average pore diameter of the substrate of 1.9-2.0 μm and a porosity of ~ 30- 33%

) для органических и неорганических ультрафильтрационных разделительных элементов.The analysis of the porous structure of membranes is carried out by the quasi-equilibrium method of dynamic desorption porosimetry [3], which allows one to determine the porosity and pore size (from 10 to 10 ³

) for organic and inorganic ultrafiltration separation elements.

 The pyrolyzable component is supplied to the reaction zone both using a carrier gas (nitrogen, helium) and in pure form.

 The method is illustrated by the following examples.

Состав газа на выходе из реактора, об.%: CH₄ - 87,10; C₂H₆ - 0,70; C₂H₄ - 1,01; C₂H₂ - 0,62; H₂ - 10,57.Example 1. Modification of the porous structure of TiO ₂ / ZrO ₂ membranes is carried out using methane in the following compositions, vol.%: CH ₄ - 98.93; C ₂ H ₆ 0.28; C ₂ H ₈ - 0.07; N ₂ - 0.72. The deposition of pyrocarbon spend 1050 ^o C, a pressure of 25 mm RT. Art., volumetric feed rate of methane 4.0 l / min and exposure time 15 minutes The load of gouging is 850 g, and the rate of temperature rise is 5.2 ^o C / min. As a result of the treatment, the average radius of the membrane transport pores decreases from ~ 325 to ~ 70

The composition of the gas at the outlet of the reactor, vol.%: CH ₄ - 87,10; C ₂ H ₆ 0.70; C ₂ H ₄ - 1.01; C ₂ H ₂ 0.62; H ₂ - 10.57.

где q - объемный поток жидкости или газа в единицу времени через единицу поверхности пористого слоя толщиной L;

градиент давления потока в направлении его оси; η - динамическая вязкость жидкости или газа; K - коэффициент проницаемости пористой среды, характеризующий ее свойства (размер пор, проницаемость и т.п.). Естественно, что при модификации пористой структуры мембран коэффициент их проницаемости должен изменяться. Это было прослежено на примере фильтрации через мембраны тетралина и декана. В табл. 1 приведены результаты, полученные при обработке мембраны при 900^oC, давлении 28 мм рт. ст., объемной скорости подачи метана 4,2 л/мин. Скорость подъема температуры и нагрузка заневоливания, как в примере 1.Example 2. In most cases, the filtration of Newtonian fluids through a layer of porous material obeys Darcy's law:

where q is the volumetric flow of liquid or gas per unit time through the surface unit of the porous layer of thickness L;

pressure gradient of the flow in the direction of its axis; η is the dynamic viscosity of a liquid or gas; K is the coefficient of permeability of a porous medium, characterizing its properties (pore size, permeability, etc.). Naturally, when modifying the porous structure of membranes, their permeability coefficient must change. This was traced by the example of filtration through tetralin and decane membranes. In the table. 1 shows the results obtained by processing the membrane at 900 ^o C, a pressure of 28 mm RT. Art., volumetric feed rate of methane 4.2 l / min. The rate of temperature rise and the load of regressing, as in example 1.

The average gas composition at the outlet of the reactor, vol.%: CH ₄ - 97.32; C ₂ H ₄ 0.25; C ₂ H ₆ 0.32; H ₂ - 2.11.

K - коэффициент проницаемости, м²;
τ - время обработки, ч.Based on the experimental data, dependencies are obtained that describe the change in pore size and permeability coefficient on processing time at 900 ^o C:
In R = 5.36-0.06 τ; (2)
In K 10 ^-16 = 3.10-0.20τ, (3)
where R is the radius of the transport pores,

K is the permeability coefficient, m ² ;
τ is the processing time, h

K_э с 1,44 до 4,14, а K_р с 1,49 до 4,31.Example 3. To assess the sintering process that occurs when the microporous bodies are heated due to the energy intensity of their surface, the membrane is treated in an inert medium (helium). As a result of 1-hour exposure under the conditions of example 2, R _k increased from 206 to 265

K _e from 1.44 to 4.14, and K _p from 1.49 to 4.31.

Example 4. In the table. 2 shows the results of the modification of the porous structure of the membrane with pyrocarbon, which is carried out at a temperature of 900 ^o C, a pressure of 32 mm RT. Art. at a methane feed rate of 4.4 l / min. The load of mowing is 700 g at a rate of temperature increase of 5.0 ^o C / min. The first column shows the data for a membrane that has not undergone pyrocarbon modification, in the 2nd after 4-hour treatment, and in the 3rd calculated according to equations 2,3. The quite satisfactory convergence of the experimental and calculated results shows that, using the obtained dependences, it is possible to control the change in pore size and porosity of the membranes when they are modified with pyrocarbon. A similar procedure can be carried out for other temperature conditions.

Sources of information
1. Pat. RF N 2096073.

 2. European Pat. N 0617997.

 3. Pat. RF 2141642.

Claims (3)

1. A method of modifying the porous structure of an inorganic anisotropic membrane, including the deposition of carbon obtained by the pyrolysis of hydrocarbons on its selective layer, characterized in that the anisotropic membrane is modified, the selective layer of which is deposited on a metal substrate, the modification is carried out by covering the membrane with a mechanical load of 400-900 g in a carbon cell, the design of which allows pyrocarbon to be deposited only on the selective layer of the membrane, without affecting its substrate, under controlled the rate of temperature rise is not more than 6.2 ^o C / min. 2. The method according to p. 1, characterized in that the modification is carried out at a temperature of 700-1000 ^o C, a pressure of 5-760 mm RT. Art.  3. The method according to p. 1, characterized in that as the separation layer using oxides of aluminum, titanium, zirconium, hafnium, thorium, lanthanum, calcium.

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