I n d . E n g . C h e m . R e s . 1988,27, 1553-1555
Acknowledgment The authors acknowledge financial support from the Direcci6n de InvestigaciGn, Universidad Cat6lica (DIUC), through Grant 36/85. Registry No. COz, 124-38-9.
Literature Cited Chrastil, J. "Solubility of Solids and Liquids in Supercritical Gases". J. Phys. Chem. 1982,86,3016-3021. de Filippi, R. P. "COPas a Solvent Application to Fats, Oils and Other Materials". Chem. Ind. (London) 1982, 385-394. Friedrich, J. P. "Supercritical COz extraction of lipids from lipidcontaining materials". U.S. Patent 4, 446, 923, 1984. Hubert, P.; Vitzthum, 0. G. "Fluid Extraction of Hops, Spices and Tobacco with Supercritical Gases". Angew. Chem., Int. Ed. Engl. 1978,17, 710-715. Pickering, S. F. "p-v-TRelations in the Gaseous State for Substances which are Gases at 0 "C and 1 atm". In international Critical Tables of Numerical Data, Physics, Chemistry and Technology, 1st ed.; Washburn, E. W., Ed.; McGraw-Hill: New York, 1928; VOl. 3, p 3. Rizvi, S. S. H.; Benado, A. L.; Zollweg, ,J. A,; Daniels, J. A. "Supercritical Fluid Extraction: Fundamental Principles and Modeling Methods". Food Technol. 19868,40(6), 55-65.
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Rizvi, S. S. H.; Daniels, J. A.; Benado, A. L.; Sollweg, J. A. "Supercritical Fluid Extraction: Operating Principles and Food Applications". Food Technol. 198613, 40(7), 57-64. Stahl, E.; Shilz, W.; Schutz, E.; Willing, E. "A Quick Method for the Microanalytical Evaluation of the Dissolving Power of Supercritical Gases". Angew. Chem., Int. Ed. Engl. 1978, 17, 731-738. Stahl, E.; Schutz, E.; Mangold, H. K. "Extraction of Seed Oils with Liquid and Supercritical Carbon Dioxide". J.Agric. Food Chem. 1980,28, 1153-1157.
Jose M. del Valle Department of Food Science University of Illinois a t Urbana-Champaign 3820 Agricultural Engineering Sciences Building 1304 W e s t Pennsylvania Avenue Urbana, Illinois 61801
Jose M. Aguilera* Department of Chemical Engineering Pontificia Universidad Catolica de Chile P.O. Box 61 77 Santiago, Chile Received f o r review November 19, 1987 Revised manuscript received April 18, 1988 Accepted April 30, 1988
A Simple Meter with Zero Pressure Drop for Gas Flows A simple and inexpensive meter with zero pressure drop for gas flows has been developed. A number of these meters have heen in continuous operation for several years. The working principle, as well as operating experience and design rules are presented in this paper. Measuring the rate of flow of a gas produced by microorganisms is a delicate task. Maintaining a gas-producing system a t a constant pressure is difficult because gas meters in general cause a pressure drop. The problem is further aggravated if the system hai3 to be kept a t atmospheric pressure; the need for atmospheric pressure may be called upon in order to prevent gas, from leaking out to the surroundings. The present paper describes the experience with a simple meter developed primarily for laboratory and pilotplant experiments; see Figure 1. The meter determines the on-line rate of flow with virtually zero pressure drop. The operation of the apparatus can be adjusted in such a way that the system pressure can be maintained on an arbitrary level. The pressure can be selected as negative, relative to the atmosphere, which enables the use of the gas meter as an inherent gas pump.
Description of the Flow Meter
A schematic diagram of the meter is shown in Figure 2. The meter consists of a gas cell and two valves at the inlet and outlet control, respectively, of the gas to be measured. The box is both an electronic control and recording unit. Figure 3 displays how the gas cell works. The gas enters the cell from the top and passes through the inner tube. As the gas enters the jacket, the liquid level sinks a t the same rate as gas is supplied. When the level of water in the jacket reaches the lower level electrode, the inlet valve closes and the outlet valve opens. Hence, the gas is passed out by means of an aspirator. At the same time, the water level increases in the jacket until it rieaches the upper electrode. Here, the control unit switches the valves again, and the cycle is repeated. 0S8S-5SS5/S8/2627-1553$01.50/0
In the present illustration, Le., Figure 3, the level of the water in the external beaker has been adjusted to the same level as the exit hole of the inner tube. Thus, the gas is maintained a t atmospheric pressure throughout the system. On the other hand, if the water level of the external communicating vessel is below the exit hole of the inner tube, the gas meter works as an inherent pump. Hereby a negative pressure is maintained in the flow meter. Similarly, the meter can be kept at an excess pressure by adjusting the level of the communicating vessel upward. The determination of the rate of flow is based on the number of cycles per time unit. Furthermore, the volume between the two level electrodes inside the jacket has to be known very accurately. The quickest and easiest way to determine this volume is to connect a syringe to the gas inlet and inject air until the valve closes. The accuracy of the meter is in this case determined by the accuracy of the syringe. The gas meter shown in Figure 2 has been equipped with a counter, which records a pulse for each cycle. The rate of flow can thus be obtained by manually reading the number of counts as a function of time. The control and recording unit can also be connected to a device such as a strip chart recorder or a desk top computer, which enables the time for each pulse to be registered.
Operating Experience Several meters have been tested on various types of equipment for biogas production from wastewaters and energy crops. One meter has been in continuous operation for over 2 years to measure gas production in an anaerobic fixed-film reactor. The rate of flow of gas varied up to 50 L/day (6 X m3/s) for this system. Figure 4 shows the record for continuous operation of the meter over a period 0 1988 American Chemical Society
1554 Ind. Eng. Chem. Res., Val. 27, No. 8, 1988 .1
...~
,,.
~
.. ~
* .' i .
...
..
Gas inlet valve
Gar "bf e erence elenmde
TO aspirator
Edernal mmmunicating
Figure 3. Working principle of the meter. 50 7
Figure 1. Photograph of the gas flow meter 0
100
200
i 0
Elapsed lime (days)
Figure 4. Example of operating record of the meter.
most critical parameter is the cross-sectional area of the space between the inner tube and the jacket ( A in m2J. In practice, the following condition must be met
Figure 2. Design of the gss flow rate meter. Weight, 1800 g. Dimensions, 120 X 120 X 250 mm. Equipment cost less than USD 300.
of about 8 months. The inserted curve represents the number of metering cycles as a function of time. The curve is hence a direct measure of the accumulated volume of gas passed through the meter. The rate of flow shown by the lower curve was simply estimated as the fixed volume, defined hy the meter, divided by the time for a metering cycle. Three meters connected to a desk computer have been in continuous operation on three CSTR pilot plants for biogas production from energy crops. AC Biotechnics AB has been testing and using three of these meters for several years. The meter has been tested for flows of up to 160 L/day. The design and geometry for a given capacity is based on the fact that the time for keeping the gas inlet valve clased must be less than the time for a metering cycle. This is done in order to preventpressure from building up. The
where G (mS/s) is the maximum rate of flow of gas and V (m/s) is the velocity of the liquid rising inside the jacket from the lower electrode to the upper electrode. For maximum reproduction, it has been observed that the suction pressure of the aspirator should be adjusted so that V 5 0.02 m/s. In the example displayed hy Figure 4, the parameters had the following values: A = 4 X IO4 m2and V = 0.02 m/s. Hence, the largest G/(AV)value was about 0.05 during the time period data were recorded. The requirement given above can thus be considered to be fulfilled. Due to its inherent nature, the meter has unlimited turn down. This property was, for instance, a necessity for recording the rate of flows obtained in connection with the microbially produced gas in the previous example. The level-control electrodes are made of ordinary welding electrodes. The most reliable design is to insert them from the top, via a standard laboratory screw cap. Melt-sealing the electrodes has also been tested, which has led to crack formation in the glass. The reference electrode and, depending on the liquid level, one of the other two electrodes work as contactors and the liquid as a contact medium. Tap water is sufficient for obtaining electrical signals from the electrodes. In order to enswe lack of signal, small amounts of potassium chloride may he added to the water.
Ind. Eng. Chem. Res. 1988,27, 1555-1556
The level-control unit has worked without any problems. Both stainless steel solenoid valves and a simpler type of pinch valves have been tested. The latter type works fairly well but needs to be adjusted a little too frequently to close and open properly. Although the solenoid valves are exposed to wet and corrosive gases, the valves work extremely well. They only have to be cleaned from corrosion products 2-3 times a year. The problem with gases that are readily soluble in water can be handled in various ways. Tests with biogas, containing almost 50% C02,have shown that the fraction of dissolved COz can be neglected even when fresh water is used. Eventually, the water will be saturated with respect to soluble gases. In order to prevent leakage of gases out via the external vessel, the following simple method was used. By connecting the lower tube of the jacket, through which water is pumped out and in, to the inflatable rubber
1555
insert of a football, a completely closed system was obtained. In this case, the external vessel and the jacket communicate via the rubber membrane.
Acknowledgment This work was supported by the Swedish Board for Technical Development. Mikael Carlsson is gratefully acknowledged for technical support. Britt K. Nilsson,* Ingemar Bjerle, Hans T. Karlsson Department of Chemical Engineering ZI Lund Institute of Technology P.O. Box 124 5’-221 00 Lund, Sweden Received for review June 29, 1987 Revised manuscript received March 11, 1988 Accepted March 29, 1988
CORRESPONDENCE Comments on “Effects of Hydrogen Treat Rate and Hydrogen Mass Transfer in SRC-I1 Liquefaction of Coal” and “Kinetics of Liquefaction of Coal Catalyzed by Coal Minerals” Sir: Recent articles by Singh (1987) and Singh and Carr (1987) have reported findings that have been interpreted by these researchers as evidence for inhibition of the rate of coal liquefaction by hydrogen sulfide. These findings are principally evidenced by an increase in coal conversion as a function of hydrogen treat rate in the liquefaction reactor. These authors hold that a decrease in H2S partial pressure alone is responsible for the increase in liquid yield found with an increase in hydrogen treat rate. While the correlations that they have developed seem to substantiate this finding, several alternate interpretations for this effect are possible. The literature is not in universal agreement on the inhibition of hydrogen sulfide in hydrogenation/ hydrogenolysis reactions as suggested by the authors. Hydrogen sulfide has been found to inhibit catalytic hydrosulfurization as might be expected (Morooka and Hamrin, 1979; Satterfield and Roberts, 1968), and inhibition of hydrogenation of aromatics such as biphenyl over sulfided catalysts has also been reported (Sapre and Gates, 1982). However, the presence of H2S has been also found to markedly increase hydrogenolysis reactions in the HDN of pyridine (Goudriaan et al., 1973) and quinoline (Shih et al., 1977; Yang and Satterfield, 1984) without significantly affecting the rate of hydrogenation of these species. A very recent study on the HDN of pyridine has shown the rate of hydrogenolysis of the saturated intermediate (piperidine) to increase with an increase in the ratio of the partial pressure of H2S to hydrogen (Hanlon, 1987). In the same study, the rate of hydrogenation of pyridine to piperidine was found to be independent of hydrogen sulfide concentration. These citations merely serve to illustrate the point that it is extremely difficult to extrapolate from the findings of experiments on model systems with typical industrial hydroprocessing catalysts to coal liquefaction conditions with a catalyst such as iron 0888-5885 18812627- 155X$O1.5010
pyrite. Unpublished work from Auburn University (Guin et al., 1988) on the catalytic hydrogenation of naphthalene to tetralin and decalin has recently shown that the yield of the partially saturated hydrogen donor intermediate (tetralin) is strongly increased by an increase in the partial pressure of H2S. These data have further shown that the rate of formation of decalin from tetralin can be reduced by maintaining the proper partial pressure of hydrogen sulfide (not zero). Hence, the formation of undesirable saturates such as decalin a t the expense of desirable hydroaromatics such as tetralin can be suppressed by control of hydrogen sulfide partial pressure at some intermediate level. Finally, it is difficult to reconcile the negative effect of hydrogen sulfide reported by Singh with the positive influences of H2S on the reactions of model coal mimics such as diphenylmethane hydrocracking (Ogawa et al., 1984; Hattori et al., 1987)and on coal liquefaction (Baldwin and Vinciguerra, 1983; Hirschon and Laine, 1985; Willson et al., 1985; Sofianos, 1987). An alternate interpretation on the positive effect of hydrogen treat rate can be found in the effect of this variable on the phase equilibria and resulting residence time distribution in the liquefaction reactor. A similar effect of hydrogen treat rate on the yield of liquids from brown coal liquefaction catalyzed by red mud was found by researchers at Bergbau-Forschung (Strobe1 and Friedrich, 1987). The effect of increased hydrogen treat rate was in this case, however, ascribed to a marked increase in the average residence time of the coal in the reactor caused by preferential removal of the more volatile species from the reaction vessel. By employing a tracer technique, these researchers were able to directly measure the effect of gas treat rate on the mean residence time in the liquefaction reactor, which was completely backmixed. These results showed that doubling the gas treat rate more than doubled the average residence time in the liquefaction 0 1988 American Chemical Society
