SU1754191A1 - Method of modifying membranes for separation of gas mixtures - Google Patents

Description

When processing membranes, you can use both pure (undiluted) fluorine and mixtures of fluorine and helium, argon, nitrogen or another inert gas, since the processing time required to form a fluorinated layer of a certain thickness depends only on the partial pressure of fluorine and does not depend on from the partial pressure of helium, argon, or nitrogen, even when the partial pressures of these gases are five times higher than the partial pressure of fluorine.

The difference of the invented method lies in the fact that membrane-based (PMP) membranes are taken for modification and treated in a fluorine medium to a certain depth determined by the fluorine partial pressure and fluorination time. The use of PMP as a membrane for separating N2-02 mixtures is known. However, this membrane does not allow separation of mixtures of CO2 and H2S, since the selectivity (by selectivity a is understood as the ratio of the gas permeability of carbon dioxide Pc and hydrogen sulfide PH: Pc / Ph) is close to unity: a-1.2 ± 0.2 (measuring held in FI-NEPHF Academy of Sciences of the USSR by the authors of the invention). The proposed dependence of the thickness of the modified layer on the fluorine partial pressure and processing time is a necessary and sufficient condition for ensuring the quality of the finished membrane.

Our attempts to use the well-known polystyrene membranes for the separation of H2 and C02 gas mixtures did not lead to success: the selectivity of polystyrene membranes with a profirator of 5p 0.82; 1.49 and 3.64 µm, is 1.86.2, O3 and U1, respectively, i.e. the increase in selectivity compared to the original polystyrene membrane, 79) did not exceed 10% and was significantly less than the selections obtained by us for fluorinated PMP (see below)

In the known solution, the process of modifying the membranes is carried out with periodic monitoring of the degree of hydrogen substitution by fluorine and the transport properties of the membranes. However, as shown by studies conducted at the Institute of Physical and Industrial Physics of the Academy of Sciences of the USSR, the dependence of selectivity on the thickness of the modified layer is non-monotonous and reaches its maximum in a narrow range of thicknesses of the modified layer. The drawing shows the dependence of selectivity a (curve 1) and the relative decrease in carbon dioxide permeability / 3 ( curve 2) on the thickness of the fluorinated layer CTMP (5p).

As is illustrated in the drawing, the implementation of membrane modification according to a known method before replacing at least 10% of hydrogen with fluorine does not ensure the achievement of the maximum selectivity value.

According to the proposed method, flat films or bundles of hollow fibers from PMP are taken as a membrane, introduced into a closed volume, evacuated to a pressure of at least Torr, after which gaseous fluorine is introduced into the closed volume at a pressure of Pp not higher than atmospheric (atm). After a period of time t, the fluorine is removed.

from the closed volume, and the modified membrane is ready for operation. The PMP fluorination process is diffusion-controlled, i.e. the process speed is limited by the penetration of fluorine through

a layer of fluorinated polymer to the layer of the original polymer. In the layer of fluorinated polymer, almost all hydrogen atoms are replaced by fluorine atoms, and the initial and fluorinated polymer layers are separated.

a narrow transition zone, the thickness of which does not exceed 0.1 μm with a thickness of the fluorinated layer up to 10 μm. The time t required to form a layer of fluorinated polymer with a thickness of d (is determined from relation (1).

The permeability of the membranes was measured according to a standard procedure: a pressure differential (150 Torr) was created on the membrane and the rate of gas passing through the membrane was recorded from the pressure increase in a calibrated volume. The selectivity of a was determined as the ratio of the fluxes (or permeabilities) of CO2 and H2S at the same pressure drop.

on the membrane. The dependence of the value “on the thickness of the fluorinated layer is shown in the drawing (curve 1). Curve 2 shows the relative decrease in PC permeability with an increase in 5p. From the data in the drawing it can be seen that when exposed to fluoride on the PMP, the selectivity of a varies from 1 to 4, and the maximum max 4 is reached at 7 ± 0.2 μm, and the performance of the membrane with carbon dioxide 4 (t .e. the value of Pc) decreases (as compared with the unmodified membrane 80-85 µm thick used by us) by about 1.6 times. Consequently,

membrane productivity in CO2 is insignificantly reduced, while selectivity increases in magnitude close to unity (, 2) to a 4. Thus, to obtain a membrane with

the maximum value of selectively max 4 must be exposed to fluorine at a partial pressure of its PF for a period of time t according to the formula t For example,

with a fluorine pressure of Pp 1 atm, the membrane processing time is 24.6 minutes, and at 25 atm, 98.4 minutes. The modification time decreases with increasing fluorine partial pressure. In the known method, the partial pressure of fluorine does not exceed 0.5 atm, according to the proposed method, the pressure of fluorine reached atm, i.e. the time required to form the fluorinated layer of the required thickness is halved. It is not recommended to raise fluorine pressure above atmospheric pressure in accordance with safety requirements.

If it is necessary to obtain a membrane with a maximum gas permeability value, then it is necessary to sacrifice the selectivity value and choose the fluorination time in accordance with formula (1) and the drawing.

One more important feature of the membrane modified by the proposed method should be noted: the permeability of C02 is higher than the permeability of HaS, i.e. the mixture passing through the membrane is enriched with carbon dioxide. The aforementioned feature of fluorinated PMP membranes allows using it for cleaning gas discharges of enterprises burning natural gas: in the gas passing through the membrane, the concentration of hydrogen sulfide (which is much more toxic than carbon dioxide) will be reduced by about 4 times.

The advantage of the proposed method is that fluorinated membranes have a significantly higher chemical resistance compared to the original hydrogen-containing membranes. This fact is explained by the fact that in the fluorinated membrane layer all hydrogen atoms are replaced by fluorine, i.e. in terms of resistance to chemically aggressive substances, this layer is close to polytetrafluoroethylene.

Note that both flat films and polished products can be used as membrane elements, since the fluorination process occurs in the gas phase for several tens or several minutes and fluorine decreases due to a chemical reaction with the membrane material.

due to the diffusion of kz from one point of the reaction volume to another. v v%

EXAMPLE 1 A flat film of PMP is introduced into a closed volume, evacuated to a Torr pressure, after which gaseous fluorine is introduced into a closed volume at a partial pressure of atm. After a period of time, min fluorine is removed from the closed volume. The thickness of the fluorinated layer, 83 MKiOi, wiliness a 1.98, the relative decrease / permeability of COa, through the modified membrane (Pc) o), where Pc and (Pc) c is the permeability of C02 through the modified and initial membranes respectively,, 33.

PRI mme R 2. Analogously to example 1, only, 25 atm,, 5 min, 50 microns, GH2.42, /0.99; ,

PRI me R 3, analogously to example 1, only ATM,, (, 3 μm, No. 1.93, p 0.34. /,;:

PRI me R 4. Analogously to example 1, only, 5 atm, min, (, 2 microns, oK}, 09, / 3 0.57. X-.7

PRI me R 5. Analogously to example 1, only, 5 atm, min, 0 microns, 74, 87. j

Example Analogously to example 1, only ATM, .5 min,, 59 microns, "КЗ.88, / 3 0,69.v;

PRI me R 7. Analogously to example 1, only, 25 atm, min,, 07 microns,, 94, / 0,56.

Example Similar to the irimer 1, only, 25 atm,, 36 min,, 20 μm, OM, 87, / 3 1.0.

PRI me R 9. Analogously to example 1, only ATM,, 5 min,, 80 microns,, 06,, 53.

Example 10. Analogously to example 1, only ATM, 4 min,, 50 microns, (, 42, / 0.99.

Thus, the technological features of the proposed method in comparison with the known provide:

- obtaining the maximum value of selectivity a 4 with a slight drop in membrane productivity (not more than 1.6 times);

-membranes are modified by dry technology, i.e. the membranes do not contain liquid components and, therefore, the performance of the membranes is determined only by their mechanical strength and exceeds the performance of the composite membranes with liquid carriers;

-y modified by the proposed method, the membrane permeability of COA you 0

five

0

five

0

five

0

five

0

five

above, we have HaS permeability, i.e. after passing through the membrane, the mixture is enriched with carbon dioxide;

- membranes modified by the proposed method have a significantly higher chemical resistance.

Claims (1)

Claims of Invention A method of modifying membranes for separating a mixture of gases, comprising treating a polymer membrane in fluorine, characterized in that. with the aim of I ST, - 0 increase the selectivity of membranes in the separation of a mixture of hydrogen sulphide and carbon dioxide; a poly (4-methylpenthen-1) membrane is used as a polymer membrane, and its processing is carried out for a time determined from the ratio of "-eight, where t is the processing time, min; , 5-3.0 - the required thickness of the fluorinated layer, micron; PF - fluorine partial pressure, atm. about J