Ind. Eng. Chem. Res. 2003, 42, 6389-6395
6389
Permeation Equipment for High-Pressure Gas Separation Membranes Shilpa Damle and William J. Koros*,† School of Chemical Engineering, Georgia Institute of Technology, 778 Atlantic Drive, Atlanta, Georgia 30332-0100
This paper describes the development of a constant volume, variable pressure permeation system for use in testing membrane materials up to pressures of 276 bar with both a vacuum and pressurized downstream. In addition, a membrane masking technique that is well-suited for use with this system in high-pressure applications is also described. To illustrate the utility of the equipment, permeation and selectivity results for a commercially available polyimide, Matrimid, under pure and mixed gas conditions are presented and discussed in relation to studies reported at lower feed pressures with more conventional systems. Introduction
Background
Membranes offer an attractive potential technology to offset traditional bulky and often less environmentally friendly means of separations.1 Membrane-based separations of oxygen from nitrogen, methane from carbon dioxide, and propane from propylene are under active development. Characterization of materials developed for these separation processes include testing of productivity (permeability) and selective efficiency (selectivity). Typically, these materials have been characterized for feed pressures below 69 bar. Recently, high-pressure separation applications that require development of novel robust membrane materials have attracted interest2 as alternatives to traditional processes. Separations involving supercritical carbon dioxide are of great interest and development of CO2 technology for use in industrial processing has become a research focal point. Worldwide, billions of pounds of organic solvents are used as processing tools, cleaning agents, and dispersants.2 Concern for workers’ safety and a drive to protect the environment has prompted industrial and academic research efforts to develop acceptable solvent alternatives. Carbon dioxide has the potential to be the alternative of choice because it is nontoxic, inexpensive, nonflammable, widely available, and has a broad range of dissolving power, especially in the supercritical region (critical point: T ) 31 °C, P ) 74 bar).3 For the CO2 to be recycled and reused in processing applications, separation of the CO2 from small organic solutes will be necessary. Membrane technology offers an inexpensive and efficient potential method for this separation so that CO2 can be recycled for use in further processing. As well, interest in high-pressure “down-hole” natural gas processing and enhanced oil recovery requires the development of membranes for high-pressure applications.4 This paper describes modifications to conventional low-pressure permeation equipment, sample mounting, and testing protocols developed in our group to enable testing materials at high feed pressures and with large transmembrane pressure drops often needed.
Measuring the permeability of dense films with lowpressure feeds has been done for decades, and several papers have been written describing the equipment designed for such applications.5-10 On the other hand, permeation through materials under high-pressure environments is an active topic that is of increasing importance.11-20 Both inorganic and organic materials have been considered for these applications. Several researchers have begun to investigate both inorganic and organic materials for these applications. Nakamura et al. describes the use of an R-alumina membrane and a composite membrane consisting of a polyimide layer coated with a silicone selective layer to separate supercritical carbon dioxide from poly(ethylene glycol)s (PEG) model solutes.21 Sarrade et al. have investigated systems combining supercritical carbon dioxide extraction coupled with nanofiltration separations.15-17 The materials used in these separations were either inorganic/ organic composites or purely inorganic. Higashijima et al.13 studied hydrocarbon/supercritical CO2 separations utilizing asymmetric Kapton polyimide materials. Both Higashijima and Sarrade briefly describe a high-pressure system that was used to obtain experimental data. A more detailed description of a high-pressure permeation system was given by Afrane and Chimowitz,22 who described a system that is well-suited for investigation of flux through inorganic particles where the solutes precipitate in u-tube ice baths for collection. However, a different setup is required for dense film testing of gas transport. Chiu looks at regeneration of supercritical carbon dioxide from a mixture of caffeine and CO2 and gives a brief description of equipment suitable for material testing.11 The most detailed description of a high-pressure apparatus for measuring transport through both organic and inorganic materials, however, is detailed by Kulcke,14 who studied the permeation of pure gases and mixed gases through a variety of inorganic and organic membranes. Pieces of equipment described in the work of Kulcke as in the other above works have similarities to the equipment described in this paper; however, several important differences exist and will be discussed. None of the published works describe the modifications necessary to the actual membrane testing cell to enable testing of thick dense films at high transmembrane pressures (up to 276 bar) under
* To whom correspondence should be addressed. Tel: (404)385-2684. Fax: (404)894-2866. E-mail:
[email protected]. † Present address: Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712.
10.1021/ie030039n CCC: $25.00 © 2003 American Chemical Society Published on Web 08/29/2003
6390 Ind. Eng. Chem. Res., Vol. 42, No. 25, 2003
Figure 1. Typical high-pressure permeation apparatus: (1) CO2 cylinder, (2) ISCO or Haskell Booster pump, (3) high-pressure valves, (4) ballast volumes, (5) membrane permeation cell, (6) pressure gauge, (7) fan, (8) heating element, (9) and temperature-controlled chamber.
mixed gas conditions. As well, mounting and masking techniques for materials to be tested in high-pressure environments have not been addressed. Most importantly, the combined works do not distinctly describe the considerations that must be taken into account to modify a system for high-pressure permeation experiments with large transmembrane pressure differences. This paper (1) details the basic design considerations for high-pressure permeation equipment having the ability to measure productivity with and without a nonzero pressure downstream and (2) addresses challenges and benefits to different masking and mounting procedures for materials to be tested in high-pressure environments. The utility of the equipment is illustrated by including pure CO2 permeation data to pressures of 103 bar and mixed gas CO2/CH4 permeation and selectivity results of a commercial polyimide, Matrimid, subjected to pressures up to 221 bar. Equipment Design Permeation Cell. The basis for the constant volume pressure rise permeation cell has been discussed previously,5-10 so only a quick review is provided here. In this method, a constant pressure is maintained on the upstream face of the membrane after the entire system has been sufficiently evacuated and degassed, while the rising pressure in the permeate reservoir is recorded during the testing period. A detailed sketch of the highpressure permeation equipment is included in Figure 1. The basic infrastructure is similar to the low-pressure apparatus described elsewhere10 with some significant and noteworthy differences. The major differences are in (1) the valves used, (2) the type of membrane cell used, and (3) the way the feed is introduced to the system. A fourth difference, introduced when the ability to measure permeability with a non-zero downstream pressure is required, will also be discussed. Low-leak, low-pressure permeation equipment often have been reported based on Swagelok Nupro bellows
seal valves, which are rated to 69 bar. For higher pressure applications, where contaminants are not an issue, Swagelok Whitey or Swagelok Whitey Severe Service valves, which are rated to 345 and 689 bar, respectively, are useful. However, the stems on these valves often have either a silicone- or a hydrocarbon-based lubricant on their stem tips that can contaminate the process stream. A cleaner approach that again makes use of a bellows seal valve is the Swagelok Nupro UW series valves. These valves are only rated to 172 bar, but provide a good seal and will not introduce new contaminants into the system. For tests with feed pressures that are
