AU704686B2 - Method for the purification of chlorine gas - Google Patents

Description

The invention concerns a method for the purification of chlorine gas.

By electrolytic production of magnesium metal from magnesium chloride, chlorine gas is formed at the anode. This gas contains between 1 and 10 weight air, most usual The process for the production of magnesium demands, at a certain stage in the process, dry, pure chlorine gas. In the actual process there will in a subsequent step be a combustion of chlorine and hydrogen to form hydrodiloric acid gas. The air, and 0. especially oxygen, therefore has to be removed to prev/ent the formation of water. There S0: are several ways of doing this using well known standard methods, but they all demand a fairly large and complicated process equipment modules.

One method is condensation of the chlorine gas and another drying of the HCI gas by help of a drying agent (for example magnesium chloride particles). Both these methods demand large investments and large working expenses due to a large energy consumption.

The main object of the invention is to increase the chlorine concentration in a chlorine gas contaminated with air by reducing the oxygen content to below 0.2 weight 0 Another object is to obtain a simple and rapid method ,or the separation of the gases. A third object is to develop a gas separation method especially suitable for use in a process for production of magnesium.

One or more of these and other objects of the invention may be achieved by the method described below and the invention is further described. The invention will be further described with reference to the Figures 1 2 Figure 1 shows the experimental set-up for tte permeability measurements of pure gases Figure 2 shows the experimental set-up for the permeability measurements of the mixed gases Figures 3 A,B show IR-spectra of membrane surface of PDMS before and after being exposed to 150°C and tested with Ci, SFigures 4 A,-D show the permeability curves for the pure gases N,,CI, and H, Figure 5 show the possible membrane process for the purification of CI gas.

*0 S S"S The Invention concerns a method for the purification of chlorine gas with a content of L. 1 t 10 weight air wherein the contaminated chlorina gas is supplied to the high pressure side of a membrane separator. The chlorine gas is permeated through a rubbery non-porous membrane material and the impurities withheld. The pressure ratio between the high pressure side and low pressure side of the membrane is kept at 3-7.

Preferably there is used vacuum on the low pressure side. It was found that 0polydimethylsiloxane is a suitable selective membrane material. It is preferably used a composite membrane comprising a selective membrane, a microporous substrate and a support material. Polyvinylidene fluoride (PVDF) or PVDC are suitable as microporous substrates.

T: he invention also concems a method for purification of chlorine gas from the electrolytic production of magnesium from magnesium chloride. Chlorine gas with a content of LP to weight air is supplied to a set of membrane modules, whereby the chlorine gas is permeated tllrough the membrane material and the contaminations withheld. The purified gas is thereafter supplied to a unit for chlorine combustion. The oxygen content of the chlorine gas is reduced to below 0.2 weight It is preferred to keep the pressure ratio over the membrane at 3-7. Polydimethylslloxane Is a preferred selective membrane material.

Membranes are used In separation of gases in general. However, use of membranes in connection with chlorine gas has not earlier been repoited. For the whole magnesium production process it would be a great advantage if it was possible to use membrane technology for the gas purification. The energy consumption for such processes is small and the investments will be smaller than for the alternative methods. Chlorine however, is a very aggressive gas and great demands must be msde to the selection of materials.

Using a membrane process, this can theoretically be done in two ways: 1. by using a membrane with high selectivity for the! permeation of 02 compared to CI thus producing a dry, clean Cl, gas stream as retenate, or i 2. by using a membrane with high selectivity for the permeation of CI 2 compared to 00 0 02; thus producing a dry, clean Cl, gas stream a!i permeate These two alternative process solutions demand fundazmentally different types of membranes ("glassy or rubbery" polymers) and different process solutions. The membranes must be able to separate the gases in question at fairly high temperatures (preferably 80-120oC) and moderate to low pressures i 1-2) bara.

0 e The chosen membrane materials for the process have to be tested as follows: e S1 The selective membrane layer. Permeability and selectivity for the following gases:

C

2 02,, N 2

H

2 at fairly high temperatures (35-120 and moderate pressures (up to 1-2 bara).

2, The selective membrane: Any possible degradat on or interaction with C12.

3. The substrate and the support material of the composite membrane: Any possible degradation or reaction with C12 at given process conditions.

4. The durability of the composite membrane over time must be documented.

Three fundamentally different materials have been tes:ed for the actual process. The experimental set-up used for the permeability measurements the for the pure gases through the actual membranes is shown in Figure 1 and for the mixed gas in Figure 2.

The membrane 1 was mounted in a membrane cell (separator) 2. The reference numbers i, 1I and III describe a chlorine tank, a high pressure tank and a vacuum tank respectively and 3 is a vacuum pump. A reduction vafve 4 is mounted on the pipes from the tank I and a two-way valve 5 is mounted to be able to purge the apparatus with nitrogen. The letters A-K refers to valves. The pressure on the high-pressure side was varied in the range 1-2 bar absolute. On the low pressure side there was a pressure of about 0.15 0.65 bara. The pressure ratio over the membrane was kept at 3-7, preferably 5. During the experiments the pressure variation related to time was registered for the amount of gas which permeated through the membrane. From this information the permeability could be calculated.

0 Permeation of gas through a non-porous membrane is commonly described as a mechanism including solubility and diffusion and the permeability, P, is given by the e following equation: C 9 P= DS where D [m2/ diffusitivity of the gas S [m3 gas m3 solid] solubility of the gas The membrane material's capacity in separation of the pure gases A and B is given as selectivity, a, which is the ratio between their permeabilities: a

PA/P,

Calculation of the selectivity in the mixed gas measurements (gas A and B) are based on the molfractions for the components in feed and premeate aAB (VA/YB) (XA/XB) For calculation of permeabilities, reference is made to M. Mulder, "Basis Principles of membrane Technology", Kluwer Academic Publ., 1991. For calculation of selectivities of mixed gas reference is made to Paul, D.R. and Morel, Membrane Technology", Kirk Othmer Encyclopedia of Chemical Technology, ed, Vol 15, p. 92, Wiley Son, New York 1981.

Example 1 A Teflon membrane was tested for what was expected to be the alternative 1 solution (permeation of 02). The Teflon AF T M (supplied by GKSS) material was chosen due to its excellent thermal stability (Glass transition temperature, Tg 130 and high chemical resistance, which would be excellent for the given process condition. Results for permeabilities and selectivities are given in Table 1. In addition durability was checked by exposure to 120°C and C12 over 4 days with good results.

Table 1: SELECTIVITIES: a PA/P Material: Teflon AFM Temperature: 350C PERMEABILITY; given as P/I m(STP) m 2 h bar] Pressure Selectivity Selectivity Permeability Permeability Permeability (bara) Pog/P, PgPc,, 02 N2_ Cl2 go 2 2.0 1.5 0.262 0.128 0.177 .3 2.1 1.5 0.269 0.135 0.198 2.1 1.2 0.298 0.140 0.255 6 2.1 0.29C 0.138 p 000' As can be seen from the results, the permeabilities and selectivities are too low to be of interest as a membrane for use in purifying chlorine. The material is too "porous".

.0 0 Example 2 A PDMS (poly-dimethyl-siloxane) membrane was tested for the alternative 2 solution (permeation of Cl 2 The material was chosen due to the well documented properties of the standard rubbery polymer (Tg -123°C) such as high permeabilities for certain chlorinated hydrocarbons and comparable low permeabilities for 0, N 2 (Ref. M.Mulder, Basic Principles of Membrane Technology; KluwerAc. Publ. 1991). However, data for the permeability of pure Cl 2 in PDMS and the effect of temperature on a gas mixture like in the process considered, has never been reported. It was used a composite membrane supplied by GKSS, comprising 2 Im PDMS (polydimethylsiloxane) as selective membrane, 30 gm PVDF (polyvinyliden fluoride) as a microporous substrate and 100 mLm PP (polypropylene) as porous support material. The support material is now 6* 9 *4 e *4 6 6 being replaced with other materials which have prover resistant to CI, and high temperature.

Some of the permeability-selectivity results are reported in Table 2 and Figures 4 A-D.

Table 2: SELECTIVITIES; a P/P, Material: PDMS, Standard Temperature range 25 120°C Temperature. Pressure PoP' P/ Po pcPH2 Pc P02 C) (bara) 1 2.01 1.24 16.92 20.91 2 2.10 1.24 17.85 22.19 3 2.14 1.22 20.30 24.76 35 1 2.09 1.28 13.64 17.45 2 1.90 1.42 14.55 20.79 3 1.90 1.44 16.63 23.91 1 1.97 1.38 10.40 14.45 2 1.94 1.44 10.48 15.08 1.95 1.41 10.97 15.52 1 2.05 1.40 7.57 10,56 2 1.66 1.71 7.61 13.05 3 1.72 1.70 7.73 13.13 1 1.87 1.68 4.91 8.23 2 1.85 1.72 4.70 8.08 3 1.82 1.67 4.84 8.08 100 1 1.78 1.70 3.64 6.19 2 1.72 1.72 3.50 3 1.78 1.76 3.30 5.80 120 1 1.73 1.75 2.73 4.76 2 1.79 1.75 2.54 4.44 3 2.43 The tendency is that PDMS is a very good membrane material forthe process at low temperatures 351C) and that the selectivity will go down at higher temperatures unless the polymer is crosslinked, Mixed gas experiments over time weeks) seem to show a decrease in flux of 0-20 The selectivity of the mixed gas is kept at 8-9 when calculated according to Paul et al.

The molfractions of the mixed gas streams were measured by absorption titration of CI, and measured by GC for 02 and N 2 Example 3 For testing the durability of the membrane and the support material a special constructed cell was used (not shown) in which a sample material is exposed for chlorine gas over a longer period of time at various temperatures Any changes in the material is then investigated by electron spectroscopy and FT-IR analysis. In figure 3 A and B the IR-spectra of the membrane surface of PDMS before and after being exposed to 1500C and tested with CIl are shown.

Og As have been demonstrated by the examples, it was surprisingly found that oxygen can g 0" be removed from chlorine gas by use of a certain type of membrane and at a defined pressure ratio, A chlorine gas from a magnesium electrolyzer will also contain some chlorinated hydrocarbons. Some of these molecules will be removed in the same way as the oxygen (will not permeate the membrane), which iU a great advantage for the process.

The method can also be used for other applications where chlorine gas has to be purified.

OO f4* Example 4 4 Simulations have been performed for a full-scale membrane process. The proposed membrane process is shown in Figure 5. Table 3 shows the input data for the process.

These are common data for all the simulations, Table 4 shows the results (necessary permeation area and composition of gas streams) from the simulations for 3 different temperatures. The program used for simulation is developed for hollow fibre membrane modules.Two modules are shown in serie with resircu ation of permeat 2 to the feed stream. This system is then run with a parallel, Table3 0* a4 *00 as.

0 04* -0 Table 4 The claims of the present invention are as follows: 1. Method for the purification of chlorine gas with a content of up to 10 weight air, wherein the contaminated chlorine gas is supplied to the high pressure side of a membrane separator whereby the chlorine gas is permeated through a rubbery non-porous membrane material and the impurities withheld, and where the pressure ratio between the high pressure side and low pressure side of the membrane is kept at 3-7 and the temperature between 35 and 120 C.

2. Method according to claim 1, wherein there is used vacuum at the low pressure side of the membrane.

3. Method according to claim 1, wherein polydimethylsiloxane is used as selective membrane material.

4. Method according to claim 3, wherein composite membrane is used comprising a selective membrane, a microporous substrate and a porous support material.

Method according to claim 4, wherein PVDF or PVDC are used as microporous substrate.

6. Method according to claim 1, wherein the contaminated chlorine gas is supplied to a set of membrane modules, and where the purified gas is further supplied to a unit for chlorine combustion.

7. Method according to claim 1, wherein the clorine gas to be purified originate from the electrolytic producion of magnesium from magnesium chloride.

8. Method according to claim 1, wherein the oxygen content of the chlorine gas is reduced to below 0.2 weight 9. A method according to claim 1 substantially as hereinbefore described with reference to any of the examples.

DATED: 9 February 1999 PHILLIPS ORMONDE FITZPATRICK Attorneys for: NORSK HYDRO

ASA

Claims (1)

1. 9. Method according to claim 6, c h a r a c t e r i s e d i n t h a t 'polydimethylsiloxane is used as selective membrane material. 0000 0 0000 S 000600 0 *0 5 00 09 05 4 0 00 00 00 0 0~0 4 DATED: l0th May, 1996 PHILLIPS ORMONDE FITZPATRICK Attorneys for: NORSK HYDRO a-r-s M S. S. I #0 So S 0600 0000 @0 00 j 0 ce 00 0 000 y to 00 00 0 0 4 @0000 0 Abstract Method for the purification of chlorine gas with a content of until 10 weight air, wherein the contaminated gas is supplied to the high pressure side of a membrane separator. The chlorine gas is permeated through a rubbery non-porous membrane material and the impurities withheld. The pressure ratio between the high pressure side and low pressure side of the membrane is kept at 3-7. It is preferred to use vacuum on the low pressure side. Polydimethylsiloxane is suitable as selective membrane. It is preferred to use a composite membrane. a, S b 9a at S