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RESEARCH NOTES Gravimetric Method for the Determination of Liquid Holdup in Pressurized Trickle-Bed Reactors Damjan Nemec and Gorazd Bercˇ icˇ Laboratory for Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, P.O. Box 3430, SI-1001 Ljubljana, Slovenia
Janez Levec* Department of Chemical Engineering, University of Ljubljana, P.O. Box 537, SI-1001 Ljubljana, Slovenia
A gravimetric method for the determination of liquid holdup was successfully employed in a high-pressure-operated trickle-bed reactor. The method has proven to be a reliable, fast, and relatively simple way of measuring liquid holdups under high pressures, despite opposite indications of previous works done in this field. A detailed description of the apparatus and measuring procedure is given. Introduction The study of the hydrodynamics of trickle-bed reactors has received much attention in the past decade. However, the still unclear view on hydrodynamics of tricklebed reactors is mostly due to the lack of reliable experimental data especially under high-pressure operation. This comes as a consequence of complex experimental work with trickle-bed reactors. If the measurement of the pressure drop of two-phase flow in a packed bed is rather straightforward, the same cannot be said for the determination of liquid holdup. A review of the existing methods for the determination of liquid holdup can be found in the literature;1,2 however, most of them have only been used for the atmospheric pressure operation. Until now only two methods for high-pressure operation, namely, the tracer technique and the so-called “stop flow” or drainage method, have been used. Both have proven to be pretty reliable1 but are time-consuming, and in the case of the “stop flow” method, it cannot be performed without disturbing the operation of the system. Other types of methods for possible application for high-pressure operation include the tomographic methods such as gammametry. Although these types of methods have the advantage of continuous (on-line) and accurate determination of liquid holdup,2 the problems of such methods are mostly associated with the cost of measuring equipment and safety aspects. One method, which has proven to be both reliable and fast under atmospheric pressure operation, is the socalled gravimetric method where the column is weighed during continuous operation. From the obtained weight, the weight of the dry column is subtracted so that the value of the liquid holdup can be obtained. The method * Corresponding author. Tel: +386-1-4760-280. Fax: +3861-4259-244. E-mail:
[email protected].
permits the determination of both dynamic and static holdup. However, its application encounters two difficulties: one is linked with the effect of auxiliary equipment (connections with the fluid inlet and outlet lines), and the other is linked with the inaccuracy of measuring differences between heavy weights. Nevertheless, based on experimental work performed at atmospheric pressure, this method has been recommended over the methods mentioned previously.3-5 Until recently, the method had not been tested for high pressures. The work of Al-Dahhan and Highfill1 in a 2.2 cm diameter column with pressures of up to 50 bar led the authors to the conclusion that the use of this method is questionable under high-pressure operation. What they observed was that when gas was introduced at high pressure, the weight recorded by the load cell was significantly more than the added mass of the dense gas induced. Furthermore, the recorded weight increased with an increase of both the superficial gas velocity and the total pressure. However, the recorded weights at two-phase flow operation were lower than those recorded at gas-only flow operation. The phenomena had been explained as being due to the impingement of the gas inlet jets on the bed in which the gas phase imparts a downward force in addition to the body force when it collides with the packing. The force increases with gas density, causing the load cell to read a larger value than the mass of the gas induced (gravity force). The lower recorded weights during the two-phase flow operation would then be due to the presence of the liquid flow, which dissipates the gas jets and, hence, reduces the force imparted by the flowing fluids on the system. In the end it would be virtually impossible to extract the body force that represents only the liquidphase weight from such measurements to obtain a reliable value for holdup at high-pressure operation, and therefore the use of such a method for the determination
10.1021/ie010061l CCC: $20.00 © 2001 American Chemical Society Published on Web 06/23/2001
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Figure 1. Schematic representation of the trickle-bed experimental system.
of liquid holdup under high-pressure operation would be ill-advised. However, Al-Dahhan and Highfill1 admit that it is not clear whether, by redesign of the reactor/separator setup or the distributor, such phenomena would be minimized or eliminated. We have used a reactor with an inside diameter of about twice the size of the one used in the above-mentioned work, that is, 4.1 cm, and have succeeded in measuring the liquid holdups under highpressure operation as will be clearly shown later in the paper. Experimental Section Apparatus. The high-pressure trickle-bed experimental system used in this investigation is shown in Figure 1: the system consists of a high-pressure packed column, liquid and gas delivery systems, and a data acquisition system. The stainless steel trickle-bed reactor with an inside diameter of 41 mm and a length of 70 cm is made to operate at pressures of up to 7 MPa. This allows low- and high-pressure experiments and measurements of liquid holdups and pressure drops simultaneously over a broad range of conditions. An upstream electronic mass flow controller (F212ACFA22V by Bronkhorst High-Tech) and a downstream electronic backpressure controller (P512CFA100A by Bronkhorst High-Tech) were used to obtain and maintain the
Table 1. Operating Conditions Employed condition
range
reactor pressure (MPa) gas superficial velocity (cm/s) liquid superficial velocity (cm/s) temperature (K) particle size (mm) bed porosity
0.5-5.0 0.2-6.7 0.05-0.67 298 2.10 and 2.05 0.38 and 0.44
desired flow of gas and operating pressure. The range of operating conditions is given in Table 1. The gasliquid distributor has been designed similarly to the one described by Levec et al.3 It consists of 42 capillary tubes at 5 mm pitch with 0.9 mm i.d. and 2.5 cm length, through which water is introduced into the column. The capillaries are held between two plates 1 cm apart. The bottom plate has circular holes around the capillaries: the holes are slightly larger in diameter than the outer diameter of the capillary tubes. The gas phase was introduced into the chamber formed between the plates, and it exited the distributor through these holes. The top of the packing was 0.5 cm below the distributor, while the packing itself was kept in place by a stainless steel mesh placed at the bottom of the column. The bottom of the reactor was connected to the gasliquid separator. The separator was connected at the top and the bottom to a differential pressure transducer (3051CD4 by Fisher Rosemount), which monitored and
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controlled the liquid level to a resolution of 1 mm by using a high-pressure solenoid valve. The liquid phase was delivered to the reactor by a high-pressure membrane metering pump (Mf-S30/22 by Orlita), while the gas phase was delivered to the reactor from highpressure gas cylinders. Before entering the reactor, the gas was saturated by passing through a high-pressure saturator (bubbler), to prevent evaporation in the reactor catalyst bed. The pressure drop across the packed bed was measured with a high-pressure differential transducer (3051CD2 by Fisher Rosemount) with an adjustable range (a maximum of 20 bar), which was connected to the top and the bottom of the reactor bed. Liquid holdup was measured by the so-called gravimetric technique, by weighing the whole column during continuous operation. The column was suspended on a load cell, and a straight vertical position was assured. Flexible tubes, capable of withstanding high pressures, were used to connect the column to the gas and liquid inlet and outlet lines. A load cell with a sensitivity of 3.185 mV/V at a maximum weight of 75 lb (ZA1-75 by Sentran LLC) was used. The load cell is excited with 10 V so the resulting signal at 34.02 kg is 31.85 mV, or 31 850 µV. Using the high-performance digital indicator (KA2, Sentran LLC), capable of resolving 0.5 µV, this means that one is able to measure the weight of the column to a 0.5 g accuracy. Two types of packings had been used. Nonporous 2.1 mm glass beads with a bed porosity of 0.38 and porous 2.05 mm alumina extrudates with a bed porosity of 0.44. For the gas and liquid phases, nitrogen and distilled water were used, respectively. Procedure. The liquid holdup was measured by weighing the column in two different ways. For reasons of clarity we will name them methods 1 and 2 (the reason for using two methods will become clear later in the paper). First the bed was packed with particles during continuous shaking of the column. In this way reproducibility of the measured pressure drop and liquid holdup data was obtained. Once the column was packed, in the case of method 1 the separator was connected to the column bottom and the gas-liquid distributor was attached to the top. The whole assembly was suspended onto the load cell, situated above the reactor. After all necessary connections were made, the liquid in the separator was brought to a certain level from the bottom up, which would be maintained throughout the process of measuring the liquid holdup, after that the weight of the whole (dry bed) assembly was recorded and the load cell was calibrated. Then the gas was introduced into the reactor to bring it up to a certain pressure in order to see what the recorded weight of the gas at a specific pressure was. After that, the column was slowly flooded from the bottom and the bed was left to soak overnight. After the bed was extensively prewetted, the reactor was operated first in a high interaction regime (pulse and/or bubble flow regime), i.e., at high liquid and gas mass velocities, which were then reduced to the desired level at which the pressure drop and liquid holdup were measured. This procedure, besides ensuring a uniform gas-liquid distribution at the bed entrance, minimizes the liquid maldistribution and prevents the hysteresis effect in the measured pressure drop and liquid holdups.3 The operation was assumed to be stable for taking the data when the reactor pressure, the pressure drop, and the liquid
Figure 2. Recorded weight of the dry packed column as a function of pressure in the column.
holdup did not change for at least 5 min. Once the weight of the operating column was recorded, the value that had been recorded before soaking the bed (the weight of the dry bed) was subtracted, and the weight of the liquid corresponding to the total liquid holdup was obtained. The end effects (liquid trapped on the mesh and in the distributor) were also accounted for. During the operation the level of liquid in the gas-liquid separator situated at the bottom of the column had to be maintained at the level at which the weight of the dry column had been recorded. This was done by using the before-mentioned level indicator, a solenoid valve, and a software especially designed for such level control. However, we had noticed that the control of the liquid level in the separator had become quite inaccurate at pressures above 20 bar, affecting the recorded weights of the liquid holdups, and we were therefore forced to turn to another solution. In the case of method 2, the separator was connected to the bottom via a highpressure flexible tube (Swagelok). By doing so, we separated the problem of level control in the separator from the weighing of the column to determine the liquid holdup. The whole procedure regarding the recorded weight of the dry column, the flooding, and operation in pulsing flow was identical with the one described for method 1. However, the weight of the gas recorded at gas-only flow at a certain pressure was not the same as the one recorded in method 1 because of the deforming of the flexible tube under high-pressure conditions (a Bourdon tube type effect). Nevertheless, this changed only the initial recorded weight value, which was subtracted from the obtained weights during continuous operation. Other characteristics of the system stayed the same, as described in method 1. Again, the end effects had been accounted for; however, this time they were more severe because of the liquid trickling down the flexible tube into the separator. Results and Discussion Figure 2 represents the recorded weight of the gas phase in the reactor as a function of the total pressure measured by method 1 where the separator was connected directly to the bottom of the column and the whole assembly was weighed. One can see that there is a linear dependence between the recorded weight and the pressure in the reactor when only gas is present in the reactor. This is in disagreement with the findings of Al-Dahhan and Highfill.1 Moreover, from the slope
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Figure 3. Continuous tracking of the recorded weight and pressure drop as a function of time and operating conditions for a typical experimental run (P ) 10 bar, 2.05 mm alumina extrudates).
of the linear dependency (which equals VM/RT), one is able to calculate the apparent volume of the reactor/ separator assembly and gets a value of about 0.5 L, which is equal to the actual volume (obtained by flooding the reactor/separator system) to within (5%. Therefore, one can conclude that no impingement of the gas inlet jets on the bed or similar types of phenomena as reported by Al-Dahhan and Highfill (1999) was observed, but rather the recorded increase of the total weight is simply due to the presence of the gas in the reactor at a certain pressure. This was further supported by the fact that the recorded weight was independent of (not affected by) the velocity with which the gas had been introduced into the reactor at a constant pressure. Once the liquid was introduced into the reactor at a certain pressure, an increase in the weight was detected. The increase of weight was simply due to the body force of the flowing liquid present in the bed. To further support such a claim, continuous tracking of the recorded weight was performed as a function of time and different operating conditions; this is represented in Figure 3. The prewetted bed was exposed to a flow of the gas phase with three different superficial velocities (vγ), each lasting 200 s. As one can see, there was no change in the recorded weight, although the gas flow had been substantially increased each time. The “humps” shown are due to the 13.0 g calibrated weight, which was added every time the gas flow rate was changed in order to check whether the sensitivity of the weighting device changed during the process of increasing the gas flow rate. Because of the fact that we were always able to detect the 13 g change within the (0.5 g detection limit, we concluded that the sensitivity of the weighing was independent of the gas flow rate through the column. After 600 s, the liquid was introduced into the column with a mass flow velocity of 3.08 kg/m2‚s, while the gas was still flowing with the same rate (vγ ) 6.8 cm/s). The recorded weight increased substantially because of the liquid holdup in the packed bed. When the recorded weight reached a stable value, the gas flow was decreased; therefore, the increase in holdup was detected each time. Again, during each change of the gas flow rate, the sensitivity of the weighting had been checked using the 13.0 g calibrated weight. After 1200 s, both the flows of gas and liquid were reduced to zero and the draining of the column began. A curve representing the continuous tracking of the pressure drop during the experiment is also presented in Figure 3. As
Figure 4. Measured total liquid holdup as a function of the liquidphase Reynolds number values at different pressures and gas flow rates for 2.05 mm alumina extrudates.
one can see, the pressure drop followed the expectation for one- or two-phase flow in packed beds. From the experimental evidences, we are able to conclude that it is indeed possible to measure gravimetrically the liquid holdup in a trickle-bed reactor operating under high pressures. The recorded weight can be attributed to both gas and liquid present in the column, and in order to obtain the holdup, the presence of gas as well as the end effects has to be properly accounted for. The strength of the gravimetric method lies in its speed and accuracy of measurement. For illustration Figure 4 represents the measured total liquid holdup values for a typical experiment. The presented set of holdup values were obtained in a single-day measurement, which is considered as fast or even faster than most conventional methods mentioned earlier. In light of the relative simplicity of the nonintrusive way of measuring liquid holdup and guaranteed accuracy, the gravimetric method may be preferred over other methods for measuring liquid holdup in pressurized tricklebed reactors. Conclusions Despite the indications of others, we have demonstrated that the gravimetric method is a reliable, fast, and relatively simple way of determining the liquid holdup in pressurized trickle-bed reactors. Hopefully, through the use of the method presented here, the experimental studies of the hydrodynamics of pressurized trickle-bed reactors will become more attractive for researchers investigating the hydrodynamic parameters of two-phase flow through packed beds. We are confident that if a procedure similar to the one presented in this work is followed, then the determination of liquid holdup under high pressures using the gravimetric method can be done in an accurate and relatively fast manner. Nomenclature M ) molar mass (g/mol) P ) total pressure (bar) Re* ) modified Reynolds number V ) volume (m3) v ) superficial velocity (cm/s)
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Greek Letters β ) liquid phase (subscript only) γ ) gas phase (subscript only) ∆m ) difference in recorded weight (g) β ) total liquid holdup
Literature Cited (1) Al-Dahhan, M. H.; Highfill, W. Liquid Holdup Measurement Techniques in Laboratory High-Pressure Trickle Bed Reactors. Can. J. Chem. Eng. 1999, 77, 759-765. (2) Yang, X. L.; Wild, G.; Euzen, J. P. Study of Liquid Retention in Fixed-bed Reactors with Upward Flow of Gas and Liquid. Int. Chem. Eng. 1993, 33, 72-84.
(3) Levec, J.; Sa´ez, A. E.; Carbonell, R. G. Hydrodynamics of Trickling Flow in Packed Beds. Part II: Experimental Observations. AIChE J. 1986, 32, 369-380. (4) Lakota, A. Hydrodynamics and Mass Transfer Characteristics of Trickle-bed Reactors. Ph.D. Thesis, University of Ljubljana, Slovenia, 1991. (5) Holub, R. A.; Dudukovic´, M. P.; Ramachandran, P. A. Pressure Drop, Liquid Holdup, and Flow Regime Transition in Trickle Flow. AIChE J. 1993, 39, 302-321.
Received for review January 22, 2001 Revised manuscript received April 25, 2001 Accepted May 9, 2001 IE010061L
