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Ind. Eng. Chem. Res. 1990,29, 1568-1570
Table I. Area Response for GC Analysis Using G-Cal Permeation Devices 0.87 ppm 1.30 ppm dimethyl sulfide methyl mercaptan 169600 72 060 170 300 67 760 67 000 170 600 70 200 170300 69 480 169 200 68 660 167 300 171 800 73 780 av 169 900 69 850 2 400 SD 1400
response functions are based on approximately the square of the analyte concentration. Results and Discussion The chromatogram obtained for running the three permeation tubes in series is shown in Figure 3. The scan shows excellent resolution of component peaks and the absence of noise or extraneous peaks. Similar results were obtained for calibration gases at all concentration and for analyzed samples. By contrast, Figure 4 shows a chromatogram obtained from the analysis of a compressed gas standard containing 20.4 ppm of methyl mercaptan. This analysis shows a significant amount of dimethyl disulfide which forms as the dimerization product of methyl mercaptan in the presence of oxygen. This dimerization may occur in situ catalyzed by active metal surfaces of the regulator or cylinder or it may result from the presence of the gas standard. The presence of this amount of dimethyl disulfide tends to cast a doubt on the validity of the standard. Conclusions The use of permeation devices has been shown to be a suitable alternative to the use of compressed gas standards for sulfur gas analyses. The advantages of these devices are ease of use and reduced hazards compared to working
with compressed toxic gases. Since the devices can be used to generate a range of calibration concentrations and in tandem to generate multicomponent mixtures, they are less expensive to use than a collection of compressed gas standards. Make-up gas composition can also be altered to prepare calibration standards that match the matrix of samples. G-Cal permeation devices have the added advantage of not requiring precision thermostating for routine analyses. Registry No. H2S, 7783-06-4; methyl mercaptan, 74-93-1; dimethyl sulfide, 75-18-3.
Literature Cited Chand, R. Gas Emitting Device Stores Gas as Liquid Which Permeates into Second Chamber for Controlled Permeation through Dimethylpolysiloxane. US. Patent 4,399,942, 1983. Greer, D. G.; Bydalek, T. J. Response Characterization of the Melpar Flame Photometric Detector for Hydrogen Sulfide and Sulfur Dioxode. Environ. Sci. Technol. 1973, 7, 153-155. MacTaggart, D. L.; Kagel, R. A.; Fanvell, S. 0. Validation of ppb/ppt Sulfur Gas Standards by Independent Analytical Methods. J.Air Pollut. Control Assoc. 1987, 37, 143-148. Scaringelli, F. P.; O’Keeffe, A. E.; Rosenberg, E.; Bell, J. P. Preparation of Known Concentrations of Gases and Vapors with Permeation Devices Calibrated Gravimetrically. Anal. Chem. 1970, 42, 871-876. Stevens, R. K.; Mulik, J. D.; O’Keeffe, A. E.; Krost, K. J. Gas Chromatography of Reactive Sulfur Gases in Air a t the Partsper-Billion Level. Anal. Chem. 1971, 43, 827-831. Supelco, Inc. Analysis of Sulfur Gases: Trace Quantities. GC Bulletin 7225, Supelco, Inc.: Bellefonte, PA, 1983. Yeh, J. T. Y. Online Composition Analyzers. Chem. Eng. 1986, Jan 20, 55-68.
Daniel G . Flowers Norton Company Chemical Process Products P.O. Box 350 Akron, Ohio 44309-0350 Received for review November 27, 1989 Revised manuscript received March 27, 1990 Accepted March 30, 1990
An Equilibrium View Cell for Measuring Phase Equilibria at Elevated Temperatures and Pressures An equilibrium view cell has been designed and constructed for measuring fluid-phase equilibria
at temperatures t o 673 K and pressures t o 350 bar. Depending on the size of the interchangeable inner chamber, the cell has an internal volume of 15,33, or 50 cm3. Sealing a t elevated temperatures is achieved by using the appropriate number and configuration of Belleville spring washers such that a net pressure of a t least 76 N/mm2 (11000 psi) is maintained on the graphite gaskets for the cell windows a t all times. Because of its fixed volume, the cell is most appropriate for use in conjunction with an equilibrium flow apparatus. Introduction Fluid-phase equilibrium measurements a t elevated temperatures and pressures are of interest for a number of applications including coal liquefaction processes, reactions in supercritical water, and fractionation of coalderived and petroleum pitches with supercritical fluids. An equilibrium flow apparatus is usually required to investigate such systems, since thermally induced degradation or polymerization reactions can occur. Another advantage of the flow apparatus is that large quantities of samples can be produced for further analysis and processing. Several researchers have developed view cells for use in a flow apparatus that are capable of operating at elevated 0888-588519012629- 1568$02.50/0
temperatures and pressures. Wilson and Owens (1977) describe a view cell that has been operated to 755 K and 200 bar. The cell consists of a horizontal steel tube with windows mounted on the ends and has a volume of approximately 1 cm3. Thies and Paulaitis (1984) operated a view cell of the liquid level gauge type (30 cm3) to 580 K and 150 bar. A view cell of the horizontal cylinder type with an internal volume of 10 cm3was designed by Lin et al. (1985) and is rated for 710 K and 250 bar. Niesen et al. (1986) describe a cell that is conceptually similar to that of Lin et al. but that has a volume of approximately 250 cm3;operating limits are stated to be 623 K and 100 bar. To meet our research needs, a new view cell has been developed subject to the following constraints: (1)the cell 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1569 Top Phase Port
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can be used to study phase equilibria for systems containing near-critical and supercritical water, (2) the design should be of the liquid level gauge type for maximum visibility and to minimize the possibility of phase entrainment, and (3) the cell volume should be large enough to allow the operator to easily control the fluid interface at throughput flow rates of 300-700 cm3/h. Such flow rates will be required when relatively large samples (e.g., 50-100 g) are needed for analysis or further processing. Herein we describe the design of a new view cell that, when incorporated into an equilibrium flow apparatus, can be used to measure phase behavior to 673 K and 350 bar.
Discussion A frontal view and cross section of the view cell are shown in Figures 1and 2. The cell is conceptually based on the design of a liquid level gauge, such as the Jerguson Model T-40 (Jerguson Gage and Valve Co.), which is rated for 350 bar at ambient temperatures. The volume of the cell shown in Figure 1is approximately 15 cm3. Two additional inner chamber pieces of greater depth (3.8 and 6.1 cm) are also available and can be used to increase the volume of the cell to either 33 cm3or 50 cm3, respectively. The cover plates and inner chamber were machined from age-hardened Carpenter Custom 450 stainless steel (Carpenter Technology), which has approximately 5 times the yield strength of 304 stainless steel. The gasket surfaces of the inner chamber are polished to improve both sealing and resistance to pitting. To further improve the pitting resistance of the gasket surfaces, the cover plates and inner chambers were passivated in an aqueous 10 vol % nitric acid solution for 15 min. The cell windows are tempered aluminosilicate liquid level gauge glasses (Almax-11, T1 size, Hoya Optics). Due to the internal stress that is induced by tempering, gauge glass is capable of withstanding both high clamping and operating pressures. The maximum operating temperature of the view cell is defined by the temperature at which internal stress relaxation of the gauge glass begins to occur. For normal service, a tempered aluminosilicateglass should not be operated a t temperatures exceeding 673 K (McLellan and Shand, 1984). The gauge glasses are mounted on 0.38-mm-thick (0.015-in.-thick) monolithic graphite gaskets (Grafoil, GTJ grade, UCAR). The use of a monolithic gasket reduces extrusion into the inner
chamber during service and therefore minimizes contamination of the cell contents with graphite fines. The GTJ-grade graphite contains a passivating inorganic inhibitor and only 50 ppm leachable chloride, both of which reduce pitting compared to conventional gaskets. The graphite cushions between the cover plates and windows are identical with the gaskets described above. The maximum operating temperature of the graphite gaskets is 723 K in an oxidizing atmosphere; however, if an inert environment is maintained (e.g., our cell is in a nitrogen bath), there is no practical upper temperature limit. The maximum operating pressure of the gaskets is 690 bar. The cell is assembled with 12.7- X 1905" (0.5- X 7.5-in.) high-temperature service B7 studs and 2H heavy hex nuts. Belleville spring washers (Associated Spring) of 17-7 P H stainless steel are used on the studs to limit loss of the required load on the window gaskets due to differential thermal expansion.
Design Calculations . The following design calculations are for our operational requirements of 350 bar and 673 K. However, this procedure also applies to view cells of this type or a similar type at different operating conditions. The number and arrangement of Belleville spring washers are crucial for sealing a view cell of the liquid level gauge type at elevated temperatures. The gasket manufacturer, UCAR, requires a continuous net pressure of at least 76 N/mm2 (11OOO psi) on the gaskets to seal at operating pressures of 350 bar (UCAR, 1986). The initial load applied by the tightened studs on the gasket surface is reduced by two factors: (1) the force exerted by the fluid contained in the view cell (known as end load) and (2) the loss of preload on the tightened studs, which occurs due to differential thermal expansion. Although the first of these factors is relatively small in magnitude even at cell pressures of 350 bar, the second can result in virtually a complete loss of gasket loading at elevated temperatures. The appropriate number and arrangement of Belleville spring washers for maintaining the required net gasket pressure are calculated iteratively as follows: 1. The required gasket net pressure is multiplied times the gasket area to determine the initial preload of the studs. This initial preload is then added to the end load
1570 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
due to the cell operating pressure, yielding the total initial load; dividing this value by the number of studs yields an estimate for the total initial load per stud, from which the stud diameter can be determined. From this first estimate for the total initial load per stud, a preliminary number of nested (stacked in parallel configuration) Belleville spring washers is determined. 2. Next the effect of differential thermal expansion on gasket loading is calculated. The expansion that the cell components under compression (cover plates, inner chamber, gaskets, gauge glasses, and Belleville spring washers) will undergo for a temperature increase from ambient to 673 K is determined. Similarly, the expansion of the studs (under tension) due to the same temperature increase is calculated. The difference between the two expansions (i.e., the differential thermal expansion) results in a loss of stud preload and, thus, gasket loading. This loss is governed by the spring constant (k = F / x ) of the Belleville spring washer stack, since it has by far the lowest spring constant of the parts under compression. 3. If these calculations indicate that the loss of gasket loading would decrease the net pressure on the gaskets to less than 76 N/mm2 (11000 psi), an additional stack of nested Belleville spring washers identical with the first is added in serial configuration to each stud (see Figure 2). Now the spring constant of the two Belleville spring washer stacks in series is one-half that of the original, and the loss of gasket loading due to differential thermal expansion is also halved. The final required gasket preload is then obtained by summing the initial preload required to achieve the desired gasket net pressure of 76 N/mm2 (11000 psi), the end load due to cell contents at 350 bar, and the loss of gasket loading due to differential thermal expansion. The final required preload of the studs should be between 60% and 90% of their proof load (Shigley and Mitchell, 1983). If the final preload exceeds the elastic load limit for the nested Belleville spring washers, additional washers are nested (parallel configuration) to each stack as required. In addition, the final gasket preload should not exceed the maximum allowable pressure limit of 165 N/mm2 (24000 psi). If all loads are within allowable limits, the torque to achieve the desired final preload is calculated from existing correlations (Shigley and Mischke, 1989). For our cell, a double stack of seven nested Belleville spring washers (Part B1000-073-S, Associated Spring), 0.5-in. studs, and a torque of 102 Nm (75 f t lb.) are required for sealing at 673 K and 350 bar.
Applications The cell is currently being used to measure phase equilibria for mixtures of supercritical toluene and petroleum pitch (Hutchenson et al., 1990a) and was recently used to measure vapor-liquid equilibrium for the toluene-phenanthrene system a t temperatures to 675 K (Hutchenson et al., 1990b). Future work will include phase equilibrium measurements for high molecular weight nparaffins (e.g., dodecane) with water at temperatures to 673 K and pressures to 300 bar. For this work, mica shields will be used to protect the cell windows from the etching effects of water a t elevated temperatures. Safety Considerations Extreme care has to be taken when the cell is assembled. All gasket surfaces must be cleaned thoroughly, especially the sealing surfaces of the inner chamber. All cell pieces
should be perfectly aligned before the nuts are tightened. Good alignment of the gauge glasses in the recesses of the inner chamber and the cover plates is provided by wrapping the gauge glass circumference with 0.25- X 12.7-mm (0.01- X 0.5-in.) adhesive graphite tape (Grafoil, GTF grade, UCAR). The tape also prevents any possible contact between the gauge glass and the inner chamber or cover plates. The 2H heavy hex nuts should be tightened in 7 Nm (5 ft lb.) increments, starting from the inner nuts and then proceeding in a diagonal pattern to the outer nuts. After the cell has been fully assembled, it should always be hydrostatically tested a t ambient temperatures. Our cell was hydrostatically tested at 450 bar and ambient temperatures for several days and then pressure tested with silicone heat-transfer liquid (Part Q2-1132, Dow Corning) for several days at 673 K and 350 bar; no leakage was observed. Since most of our work involves flammable materials, our cell is kept in a nitrogen bath to prevent a possible fire should cell leakage occur. In addition, the cell bath should always include a blowout port or wall for pressure release in the event of gauge glass failure. Finally, the contents of the cell should never be directly viewed; we use a polycarbonate shield and a mirror for our work.
Acknowledgment We thank Dennis M. Robinson of UCAR for his helpful suggestions and for supplying the gasket material.
Literature Cited Hutchenson, K. W.; Roebers, J. R.; Thies, M. C. Vapor-Liquid Equilibrium for Phenanthrene/Toluene Mixtures at Elevated Temperatures and Pressures. Fluid Phase Equilib. 1990a, in press. Hutchenson, K. W.; Roebers, J. R.; Thies, M. C. Fractionation of Petroleum Pitch by- SuDercritical Fluid Extraction. Carbon 1990b, in press. Lin, H. M.; Kim, H.; Leet, W. A,; Chao, K. C. New Vapor-Liquid Eauilibrium AoDaratus for Elevated Temwratures and Pressures. mh. Eng. Chem. Fundam. 1985,24,26&262. McLellan, G. W.; Shand, E. B. Glass Engineering Handbook, 3rd ed.; McGraw-Hill: New York, 1984; Chapter 15. Niesen, V.; Palavra, A.; Kidnay, A. J.; Yesavage, V. F. An Apparatus for Vapor-Liquid Equilibrium at Elevated Temperatures and Pressures and Selected Results for the Water-Ethanol and Methanol-Ethanol Systems. Fluid Phase Equilib. 1986, 31, 283-298.
Shigley, J. E.; Mischke, C. R. Mechanical Engineering Design, 5th ed.; McGraw-Hill: New York, 1989; Chapter 8. Shigley, J. E.; Mitchell, C. R. Mechanical Engineering Design, 4th ed.; McGraw-Hill: New York, 1983; Chapter 8. Thies, M. C.; Paulaitis, M. E. Vapor-Liquid Equilibrium for 1Methylnaphthalene/Methanol Mixtures at Elevated Temperatures and Pressures. J. Chem. Eng. Data 1984,29, 438-440. UCAR Grafoil Flexible Graphite, Catalog Section G 8816, UCAR, Cleveland, OH, 1986. Wilson, G. M.; Owens, R. S. High Temperature Vapor-Liquid Equilibrium Studies on Synthetic Fuel Systems. Presented at the Symposium on High Pressure Phase Equilibria-Experimental Data and Methods of Prediction, 70th Annual AIChE Meeting, New York, Nov 1977.
Johannes R. Roebers, Mark C. Thies* Department of Chemical Engineering Clemson University 125 Earle Hall Clemson, South Carolina 29634-0909 Received f o r review December 11, 1989 Accepted April 16, 1990
