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Ind. Eng. Chem. Res. 1999, 38, 3606-3613
A Membrane Process To Recover Chlorine from Chloralkali Plant Tail Gas K. A. Lokhandwala, S. Segelke, P. Nguyen, R. W. Baker,* T. T. Su, and I. Pinnau Membrane Technology and Research, Inc., 1360 Willow Road, Menlo Park, California 94025-1516
Chlorine is manufactured by the electrolysis of brine. The chlorine product is a gas, which is collected, dried, compressed, and cooled to produce a liquid. This paper describes the development and field demonstration of a membrane process to recover chlorine from the liquefaction tail gas of chloralkali plants. The tail gas consists of about 20% chlorine and 50-70% air, with the balance being hydrogen and carbon dioxide. A number of membrane materials can achieve a selectivity of 20 or more for chlorine from nitrogen, but degradation of the membrane materials in the presence of high concentrations of chlorine gas often occurs. However, modified silicone rubber membranes are stable to chlorine gas streams. Silicone rubber composite membranes were prepared and formed into modules of 1-2 m2 membrane area. The modules were tested in the laboratory and in a field test on a slip stream from a chlorine liquefaction unit. In the laboratory, chlorine/nitrogen membrane selectivities of more than 40 were obtained, but selectivities of 6-10 were measured in the field test. This decrease in selectivity was caused by low gas flow rates through the modules, which resulted in concentration polarization effects. However, the membrane maintained essentially constant fluxes and selectivities in field tests lasting more than 1 month. Calculated plant designs based on a selectivity of 8 are able to recover more than 95% of the chlorine in the tail gas. Typical project payback times based on the value of the recovered chlorine and avoided caustic scrubber chemical use are expected to be 1-2 years. Introduction Chlorine ranks among the 10 most important commodity chemicals.1 Total production of chlorine in the United States in 1991 was reported to be about 12 million metric tons. Most of this chlorine is produced by the electrolysis of brine, which produces sodium hydroxide and chlorine gas. Almost half of the chlorine gas produced is used on-site; the rest is liquefied for shipment to the point of use. The chlorine liquefaction step produces a vent gas or “tail gas”, which is the principal source of chlorinecontaining waste gas from chloralkali plants. In the past, tail gas was treated by absorption in carbon tetrachloride, followed by stripping of the chlorine. However, approximately 30 lb of carbon tetrachloride/ ton of recovered chlorine was lost to the atmosphere in this process.2 It is estimated that about 8.8 × 106 lb/ year of carbon tetrachloride was emitted to the atmosphere by chlorine liquefaction tail-gas treatment. Because of the high ozone depletion potential of carbon tetrachloride, the EPA has mandated that these emissions be eliminated; carbon tetrachloride production has now ceased. The chloralkali industry is, therefore, actively looking for alternative tail-gas treatment technology. Substitution of carbon tetrachloride with alternate absorbents is one solution, low-temperature condensation followed by caustic scrubbing is another, and membrane separation is a third. A flow diagram showing the essential features of the conventional chlorine liquefaction and absorption tailgas treatment step in a chloralkali plant is shown in Figure 1. Chlorine and caustic soda are produced in the * Corresponding author. Phone: (650) 328-2228. Fax: (650) 328-6580. E-mail:
[email protected].
electrolytic cells from brine. The chlorine produced in the electrolytic reaction is collected from the cells and dried by contacting the gas countercurrently with concentrated sulfuric acid in a tower. The dried chlorine gas contains other contaminants, including oxygen, nitrogen, hydrogen, and carbon dioxide. After additional treatment to remove traces of explosive components such as nitrogen trichloride, the gas is compressed. The discharge pressure of the compressor ranges from 30 to 200 psig. The hot compressed gases are cooled in a staged process, first with cooling tower water, followed by chilled water, and then by refrigerant coolant. In each cooling step, liquid chlorine is produced and separated from the gas. The off-gas from the final cooling step is called the tail gas. Depending on the operating conditions, tail gas may contain between 10 and 50 vol % chlorine. This gas is treated with a carbon tetrachloride absorption unit to reduce the chlorine concentration to less than 1%. Finally, the residual gas is scrubbed with caustic soda to produce a vent stream essentially free of chlorine. When the gas is scrubbed with caustic solution, both the chlorine and the caustic solution are lost. Therefore, it is desirable to reduce the amount of chlorine in the tail gas. The membrane process we3 and others4 are developing is designed to replace the current tail-gas treatment system, shown in the enlarged portion of Figure 1. A flow diagram of the membrane process is shown in Figure 2. The exit stream from the current liquefaction train forms the feed stream to the membrane process. The membrane preferentially permeates chlorine, while rejecting the inert gases. The chlorine-enriched permeate stream is recycled to the front of the second-stage liquefaction condenser for further recovery. The chlorine recovered in the membrane process is liquefied and
10.1021/ie9900364 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/27/1999
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3607
Figure 1. Flow diagram of the chlorine production process showing the condensation-absorption-stripping tail-gas treatment unit in detail.
Background
Figure 2. Flow diagram showing the application of a membrane system to tail-gas treatment and chlorine recovery.
collected in the existing chillers. The residue (treated) gas stream from the membrane system is sent to a caustic scrub tank to chemically fix the remaining small amount of chlorine, prior to venting the inerts. The membrane system illustrated in Figure 2 is similar to systems already developed for the separation and recovery of organic vapors from effluent air or petrochemical vent streams. More than 100 systems of this type have been installed during the past few years.5,6 The principal technical objective of this project was to develop a membrane and membrane module with appropriate selectivity for chlorine from the inert gases hydrogen, nitrogen, and oxygen, which would be stable in the presence of high concentrations of reactive chlorine gas.
Synthetic polymer membranes separate gas mixtures because gases permeate membranes at different rates. The most widely used model of membrane transport is the solution-diffusion model. In this model, it is assumed that gas at the high-pressure side of the membrane dissolves in the membrane material and diffuses down a concentration gradient to the opposite interface which is maintained at a lower pressure, where the gas is desorbed. Transport of a single component through these membranes is generally described by a simple Fick’s law equation
J ) DS∆p/l
(1)
which can be further simplified to
J ) P∆p/l
(2)
where J is the membrane flux [cm3(STP)/cm2‚s], D is the diffusion coefficient of the gas in the membrane (cm2/ s) and is a measure of the gas mobility, S is the solubility coefficient linking the concentration of the gas in the membrane material to the pressure in the adjacent gas (cm3(STP)/cm3‚cmHg), ∆p is the partial pressure difference across the membrane (cmHg), l is the membrane thickness (cm), and P is the permeability, which is a measure of the rate at which a particular
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Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999
Figure 3. Schematic diagram of the gas-mixture permeation test system. Samples can be withdrawn from the feed, residue, and permeate for GC analysis.
gas moves through a membrane of standard thickness (1 cm) under a standard partial pressure difference (1 cmHg). The permeability unit, 1 × 10-10 cm3(STP)‚cm/ cm2‚s‚cmHg, is often called a Barrer, after R. M. Barrer, a pioneer in membrane permeation studies. The transport of any gaseous component through a membrane is characterized by a normalized permeation flux, Q [cm3(STP)/cm2‚s‚cmHg], defined as
Q)
P J ) l ∆p
(3)
or
J ) Q∆p
(4)
A measure of the ability of a membrane to separate two gases or vapors (1 and 2) is the selectivity, R, defined as the ratio of their permeabilities:
R)
[ ][ ]
P1 D1 S1 ) P2 D2 S2
(5)
or, in terms of the individual normalized permeation fluxes,
R ) Q1/Q2
(6)
The intrinsic selectivity of a polymer material is established by measuring the permeabilities with pure gas or vapor samples and then calculating the ratio. This easily measured coefficient is useful in the initial screening of different membrane materials, but the actual selectivity obtained in permeation measurements with gas mixtures is required for process design calculations. Selectivities obtained in gas mixture experiments are often significantly lower than intrinsic selectivities calculated from pure gas measurements. For example, sorption of the more permeable component leads to plasticization of the membrane, increasing the permeability of the less permeable component. The selectivity defined in eq 5 is made up of two terms. The ratio D1/D2 is the ratio of the diffusion coefficients of the two gases and can be viewed as the mobility selectivity, reflecting the different sizes of the two molecules. The ratio S1/S2 is the ratio of the Henry’s law sorption coefficients of the two gases and can be viewed as the sorption, or solubility, selectivity, reflect-
ing the relative condensabilities of the two gases. The mobility selectivity, D1/D2, for chlorine vapor over nitrogen or hydrogen will always be less than 1 (for glassy polymers, much less than 1), reflecting the large size of the chlorine vapor molecule compared to hydrogen and nitrogen. However, the sorption selectivity will normally be greater than 1, reflecting the high condensability of chlorine vapors compared to nitrogen and hydrogen. The balance between the sorption selectivity and the mobility selectivity determines whether a membrane material is selective for chlorine vapor or nitrogen or hydrogen. Material Selection/Membrane Stamp Measurements The membrane process shown in Figure 2 requires membranes that are more permeable to chlorine vapors than to the other gases present. In rubbery membrane materials the sorption selectivity term usually dominates the mobility selectivity term, so this type of membrane material was selected for most of our initial material screening experiments. Chlorine is an extremely reactive gas and rapidly degrades many polymers, such as urethane, polyesters, and polyamides. However, we found a number of polymers that showed no obvious effects of chlorine attack in short-term permeation experiments. Pure gas and gas mixture permeability tests were performed in the permeation test system shown in Figure 3. Chlorine/nitrogen mixtures containing 3%, 10%, 20%, and 100% chlorine were used as the feed. The permeate was maintained at atmospheric pressure. The residue and permeate streams were passed first to flow meters and then to a 5 wt % sodium hydroxide scrubber. During each test, the permeate and residue flow rates were measured and the feed, permeate, and residue concentrations were analyzed by gas chromatography. In these membrane stamp tests, the residue flow rate was always more than 100 times larger than the permeate flow rate so minimal concentration polarization occurred in the test cell. The permeabilities of chlorine and nitrogen through some of the materials evaluated are listed in Table 1. On the basis of these results, three polymers were selected for a more detailed series of screening tests with chlorine/nitrogen gas mixtures: silicone rubber (SR), ethylene-propylene-diene polymer (EPD), and ethyl-
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3609 Table 1. Chlorine and Nitrogen Pure Gas Permeabilities Measured with a Number of Potential Membrane Materials permeability (10-10cm3(STP)‚cm/cm2‚s‚cmHg) polymer
chlorine
nitrogen
chlorine/nitrogen selectivity
PTFE-co-ethylene (Hostaflon ET) ethylcellulose modified silicone rubber (SR) poly(1-trimethylsilyl-1-propyne) ethylene-propylene-diene polymer (EPD)a ethylene-propylene copolymer (EPM)a
1.2 36 2700 29400 100 200
0.2 2.9 120 1280 5 5
6 12 22 23 20 40
a
Measured with chlorine/nitrogen mixtures.
Table 2. Permeation Properties of Ethylene-Propylene-Diene (EPD), Ethylene-Propylene Copolymer (EPM), and Silicone Rubber (SR) Membrane Stamps Determined with a Chlorine/Nitrogen Gas Mixture (Temperature, 23 °C; Membrane Area, 12.6 cm2; Stage-Cut,
