JPH0221288B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a mixed liquid separation membrane. The mixed liquid is placed on one side of the separation membrane, and the other side is evacuated to reduce the pressure, or an inert gas is flowed to maintain a low vapor partial pressure, and the pressure difference causes the liquid to permeate. So-called pervaporation, in which a mixed liquid is separated by evaporation on the low-pressure side, has been studied since the mid-1950s. This separation method was devised for the purpose of separating and purifying chemical liquids (mainly organic solvents, hydrocarbons, etc.) that cannot be separated using normal distillation methods. Examples include fractional separation of azeotropes, solvents with similar boiling points, and isomers (ortho and para, cis and trans). Other applications include the concentration and purification of pyrolyzable liquid mixtures and fruit juices, the removal of traces and impurities, and the removal of water produced during ester reactions.

U.S. Pat. No. 2,953,502 uses a vinyl alcohol polymer membrane to separate an azeotropic liquid mixture by pervaporation, and U.S. Pat. No. 3,726,934 uses an acrylonitrile polymer membrane as a separation membrane. The use of a combined membrane to separate styrene from a styrene-benzene mixture is also described in U.S. Pat. It has been reported that organic mixtures can be separated using a composite membrane consisting of

However, when pervaporation is scaled up to an industrial scale, the temperature of the liquid separation system is significantly lowered due to the latent heat of vaporization of the liquid, the liquid viscosity increases, the mobility of membrane polymer constituent molecules decreases, and in some cases Eventually, the liquid freezes, and the permeability of the membrane decreases significantly (actually, it becomes impermeable), making it impossible to use it industrially if it is a laboratory method. The inventors of the present invention have made extensive studies to resolve this fundamental difficulty in pervaporation, and as a result have come up with the method of the present invention that allows pervaporation to be used industrially. That is, in a method for separating mixed liquids by pervaporation, a plurality of membrane module units are
Introducing the residual liquid (liquid that did not pass through the membrane) from the first module unit into the second module unit,
They are connected in series so that the residual liquid of the n-1th module unit is introduced into the nth module, and the mixed liquid is heated before being introduced into each module unit. This is a mixed liquid separation method. The conditions for heating the mixed liquid in the present invention are such that the following relational expression is satisfied:
That is, the ratio Qf/Qd of the flow rate Qf of the liquid supplied to each module unit and the flow rate Qd of the membrane permeate liquid is determined by the temperature T of the supplied liquid, the specific heat C, the limit temperature P, and the heat of evaporation K of the membrane permeate liquid, Qf/Qd>K/{C・(T-P)}
It is preferable that the relationship is as follows. The critical temperature here is a temperature characteristic value whose upper limit is the boiling point of the feed liquid and whose lower limit is the freezing point, and when the amount of liquid permeated through the membrane becomes substantially zero or the flow rate is less than one hundredth of the maximum permeation rate. It is defined as the temperature that decreases.
The temperature T of the feed liquid is lower than the boiling point and the limit temperature P
is set higher than. Desirably, (T-P) is 2 degrees or more, and more preferably 5 degrees or more.

In the present invention, by heating the feed liquid introduced into each module unit, the mixed liquid can be separated with extremely excellent separation performance and permeability, as is clear from the examples described below.

The method of the present invention and the control method are comparatively illustrated in FIG. 1 in terms of membrane structure. In the control method (FIG. 1a), the membrane 1 with the required membrane area S is incorporated substantially into one membrane module unit 2, and the mixed liquid 3 is at one end of the membrane module unit 2. It is separated into vapor 4 of the component introduced through the membrane and passed through the membrane, and residual liquid 5 that does not pass through the membrane. In order to keep the membrane separation temperature constant, a jacket is provided on the outer wall of the membrane module unit 2 to circulate the heat medium, or the mixed liquid is placed in the heating device 6 immediately before it is introduced into the membrane module unit. The solution is heated beforehand and then introduced into the membrane module unit. Compared to such a control method, the method of the present invention (Fig. 1b)
divides the required membrane area S (for example, 4
The mixed liquid 11 is first introduced into the first heating device 12 and heated in advance, and then assembled into the first, second, third, and fourth module units 7, 8, 9, and 10, respectively. The residual liquid 14 introduced into one end of the first module unit 7 and not passing through the membrane 13 is introduced into the second heating device 15. Similarly, the heated residual liquid of the first module unit is transferred to the second module unit.
The residual liquid 17 that does not pass through the membrane 16 is introduced into the third module unit 9 after passing through the third heating device 18, and the residual liquid 20 that does not pass through the membrane 19 passes through the fourth module unit 8. heating device 2
1 to the fourth module 10. The residual liquid 23 that does not pass through the membrane 22 in the fourth module 10 corresponds to the residual liquid 5 of the control method. On the other hand, the vapor of the liquid component that has permeated through the membranes 13, 16, 19, 22 in the first to fourth module units is collected and becomes the membrane-permeable component 24, or is separated into separate membrane-permeable components 25, 26, 27. ,28. Considering the entirety of the first to fourth module units as one membrane module, the difference between the method of the present invention and the control method can be easily expressed as follows. Control method is membrane module 1
membrane systems consisting of substantially one modular unit per membrane module or a plurality of modular units connected in parallel; This membrane system consists of two modular units and a heating device installed immediately in front of each modular unit.

As described above, in the method of the present invention, even if the system uses membranes having the same area as in the control method, it is essential to divide the system into a plurality of modular units and connect them "in series." Let me explain this clearly.

In pervaporation, as mentioned above, it is essential to vaporize the components that have passed through the membrane. This latent heat of vaporization is usually much larger than sensible heat, and heat is taken away due to vaporization, resulting in a drop in liquid temperature and an increase in liquid viscosity (in some cases, freezing). In order to prevent this, it is possible to heat the liquid supplied to the module unit in advance to a level higher than the latent heat of vaporization, but the temperature of the liquid cannot be raised higher than its boiling point. Therefore, there is a limit to the amount of heat that the liquid supplied to the module unit can possess. Typically, the amount of heat that an organic liquid can possess is less than one third of the amount of heat taken away by latent heat of vaporization. Therefore, in the system of the comparative method shown in FIG. 1a, it is impossible in principle to impart the amount of heat removed by the latent heat of vaporization to the liquid supplied to the module unit in advance. For this reason, the performance of the separation membrane cannot be fully demonstrated in the system of the control method. However, in the method of the present invention, it is divided into a plurality of modular units and a heating device is provided in front of each unit.
Since heat can be applied multiple times, so to speak, in a superimposed manner to the feed liquid, if four units are used, for example, four times as much heat can be applied to the liquid as in the control method. Therefore, the amount of heat given to the liquid can exceed the amount of heat required for the latent heat of vaporization, making it possible for the first time to eliminate problems caused by thickening or freezing of the liquid.

In the present invention, the number n of module units is appropriately determined depending on the type of mixed liquid, the amount of treatment, the type of membrane, the capacity of a single module, etc., but in general, n=
2-100, preferably n=4-60.

The membranes used in the present invention include polyethylene, polyvinylidene fluoride, polyvinyl alcohol (including vinyl alcohol copolymers such as ethylene-vinyl alcohol copolymers), polyvinyl acetate, polymethylsiloxane, polyethyleneimine, polybutadiene, Materials include polyvinyl chloride, cellulose acetate, polystyrene, silicone rubber, and regenerated cellulose.
Although the active layer of these films is so-called non-porous, the entire film structure may be a homogeneous structure or a non-uniform structure. The thickness of the film can be arbitrarily selected, but it is generally from 1 to 500 μm, preferably from 5 to 200 μm.

Furthermore, in the present invention, the mixed liquid refers to a liquid formed by mixing at least two components, such as a liquid or a gas component and a liquid component, such as an organic-organic mixed liquid or an organic-water mixed liquid. It includes not only a state in which each component is uniformly mixed in a completely molecular or ionic state, but also a mixture of molecular association, ionic association, emulsion-like molecular mass, etc. Specific examples of mixed liquids include methyl acetate/methyl alcohol, ethyl acetate/
Ethyl alcohol, benzene/cyclohexane,
methanol/acetone, benzene/methanol,
Examples include benzene/ethanol, acetone/chloroform, methanol/acetone, etc. Further, examples of liquid mixtures with close boiling points include ethylbenzene/styrene, parachloroethylbenzene/parachlorostyrene, toluene/methylcyclohexane, butadiene/butenes, butadiene/butanes, and the like.

In addition to the above-mentioned azeotropic mixture, the "mixed liquid" may include mixed liquids that are difficult to separate, such as water-acetic acid,
Furthermore, it also includes mixed liquids (for example, water-methanol, water-acetone) that can be separated by ordinary distillation.

In the pervaporation method of the present invention,
The pressure on the opposite side of the membrane where the "mixed liquid" comes into contact, that is, the exhaust chamber, must be lower than the mixed liquid chamber.The larger the pressure difference, the more effective it is. The atmospheric pressure is preferably 0.5 to 1 atm, more preferably 0.5 to 1 atm. Further, the pressure on the side in contact with the "mixed liquid" is preferably 1 (atmospheric pressure) to 100 atm, preferably at or around atmospheric pressure. On the other hand, in order to maintain the pressure on the opposite side at a pressure lower than the vapor pressure of the substance permeating through the membrane, the pressure on the opposite side is preferably maintained at a vacuum of at most atmospheric pressure, preferably at most 400 mmHg, and more preferably at most 100 mmHg. There are two ways to keep the pressure low: by pulling a vacuum to reduce the pressure, or by flowing an inert gas to maintain a low vapor partial pressure.

Next, the present invention will be further explained with reference to Examples.
The present invention is not limited in any way by these Examples.

Example 1 An example of a membrane separation system using one membrane module according to the method of the present invention as shown in FIG. 2 will be specifically described.

The membrane module consists of 20 module units 31
~50 and 20 heating devices 51 to 70,
Each modular unit was equipped with a PVA hollow membrane (inner diameter 210 μm, outer diameter 270 μm) with a total membrane area of 8 m ² and an effective hollow membrane length of 800 mm. PVA hollow membrane
It was made of PVA117 manufactured by Kuraray Co., Ltd. and glycerin (20% by weight) and had a thickness of 30 μm.

2Kg/hr of feed liquid to membrane module is benzene 50
% by weight and methanol by 50% by weight, the liquid is supplied from the stock solution tank 29 by a metering pump 30 and heated by hot water sent out from a hot water circulation device 71 before being introduced into each module unit in a heating device. Warmed to 55℃. In addition, the pressure on the membrane permeation side is controlled by a combination of a vacuum pump 75 and a manostat 74.
It was kept at a constant value of 30 mmHg. In this way, the cold trap 73 contains a concentration of membrane-permeable components.
99.9wt% methanol was obtained, while the residual liquid tank 7
In No. 2, benzene with a concentration of 99.9wt% was obtained. That is, by the method of the present invention, the liquid mixture could be almost completely separated into its constituent components.

Comparative Example 1 For comparison, 20 modular units with a membrane area of 8 m ² used in Example 1 were connected in series without passing through a heating device, and benzene/methanol (50 wt. %/50% by weight) was supplied to the membrane module and operated under the same conditions as in Example 1. As a result, no membrane-permeable component was collected even after about 1 hour, and the residual liquid was returned to the original supply. The concentration of the components was the same as that of the liquid, and the mixed liquid was not separated at all.

Comparative Example 2 In the membrane separation system of Comparative Example 1, only the mixed liquid supplied to the first module unit was heated to 55°C.
When the mixture of benzene/methanol (50% by weight/50% by weight) was heated to
hr, in the case of Example 1 of the method of the present invention (Q = 0.997
Kg/hr). On the other hand, the residual liquid contains benzene 60.26wt%, methanol 39.74wt%, liquid volume W
= 1.659Kg/hr. Therefore, it has been impossible to successfully separate the mixed liquid into its constituent components using this method.

[Brief explanation of drawings]

FIG. 1 shows the steps of the method of the present invention and the control method, where a shows the steps of the control method and b shows the steps of the method of the invention. FIG. 2 shows the steps of Example 1 of the present invention. 1, 13, 16, 19 and 22... membrane, 2,
7, 8, 9 and 10...module unit,
6, 12, 15, 18 and 21...warming device,
31, 32 and 50...module unit,
51, 52, 53 and 70...warming device.

Claims (1)

[Claims] 1. In a method for separating a mixed liquid by the pervaporation method, a plurality of membrane module units are used to introduce the residual liquid (liquid that did not pass through the membrane) from the first module unit into the second module unit. and connect them in series so that the residual liquid of the n-1th module unit is introduced into the nth module unit, and the mixed liquid is heated before being introduced into each module unit. A mixed liquid separation method characterized by: