Ind. Eng. Chem. Res. 1999, 38, 3817-3821
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Operating Pressure and Gas-Phase Properties Effects on the Liquid Flow Nonidealities in Small-Scale Upflow Reactors Antony Thanos,† Philip Menagias,† Pierre Galtier,‡ and Nikos Papayannakos*,† Department of Chemical Engineering, National Technical University of Athens, 9, Heroon Polytechniou Str., Gr-157 80 Zografos, Athens, Greece, and Institut Francais du Petrole, C.E.D.I., B.P. 3, 69390 Vernaison, France
Pilot-scale hydrotreatment reactors often operate with gas-liquid upflow. For this type of operation there is a lack of information concerning the nonideal characteristics of the liquid flow, especially at elevated pressures. In this study, the Peclet number and liquid holdup of a pilot-scale upflow reactor were derived by residence time distribution experiments. The experiments were carried out at various operating pressures with different feed gases. The liquidphase mixing was found to be identical for constant feed volumetric superficial velocities calculated at the system pressure. The liquid holdup diminished when elevated pressures were used. The use of very high gas flow rates during the shift of the operating pressure can intensify the liquid-phase nonidealities. Introduction Upflow three-phase reactors are often used in laboratory- and pilot-scale studies for assessment of the performance of large-scale reactors, catalyst testing and kinetic studies. The continuous liquid phase and the high holdup values1-6 ensure complete catalyst wetting and better heat transfer compared to trickle flow conditions. However, it is well-known that data collected from upflow small scale reactors are affected by liquid-phase dispersion.5,7-10 The higher the conversion level, the stronger the influence of the nonideal flow on the kinetic data. Decoupling of kinetics from flow nonideality effects is therefore indispensable for successful scale-up and determination of reaction kinetics. The literature about the nonideal flow characteristics in upflow reactors is limited and rarely concerns the dispersion of the liquid phase.5,9-11 The few works on upflow hydrodynamics present data collected from gasliquid systems which hardly resemble the systems used in specific applications such as hydrotreating, and the extrapolation of the results to the conditions prevailing at reaction conditions is questionable. It is also noted that the data and the correlating equations presented in the literature should be used very carefully. The correlating equations of the experimental data are often presented in the form (Pe or Hl) ) function(Rel, Reg, Ga).1,9,11 This expression relates the axial dispersion parameters with the feed properties (d, µ, σ), but the respective parameters have been derived from data obtained with one feed system in each work and larger particle size than the size of the commonly used catalysts. The great majority of the papers refer to experiments at ambient pressure and temperature. The information about the hydrodynamic characteristics of the liquid flow at elevated pressures is restricted. In the paper of Papayannakos et al.,10 experiments were presented * To whom correspondence should be addressed. † National Technical University of Athens. ‡ C.E.D.I.
under a constant high pressure. The work of van Gelder and Westerterp11 includes experiments with methanol and hydrogen as the feed at several pressures (P ) 2.110 bar), using cylindrical extrudates with a diameter of dp ) 3.8 mm and length l ) 4.8 mm as bed packing. The high variance of Peclet number and liquid holdup at constant operating conditions (feed velocities and pressure) does not allow the extraction of conclusions about the effect of pressure on the axial dispersion parameters. The work of Larachi et al.2 deals with the study of liquid holdup with several different sets of gas and liquid feeds at ambient temperature and variable pressure (P ) 3-51 bar). The results of their work indicate that the liquid holdup is invariable for constant feed volumetric superficial velocities calculated at the operating conditions for a specific feed gas. In the same work it is mentioned that for different gas feeds at proper pressures (for which the different feeds have the same density) the liquid holdup is the same for constant feed volumetric superficial velocities calculated at the operating conditions. However, the effect of gas-phase properties and working pressure on liquid flow nonidealities has not yet been clarified to the extent that it can be safely used in small-scale reactor simulation. The scope of this paper is to provide information about the way that the operating pressure and the physical properties of the gas phase affect the nonideal behavior of the liquid-phase flow in small scale upflow reactors. The liquid phase used was toluene and cylindrical nonporous extrudates with dimensions close to the ones of typical hydrotreating catalysts were used as packing. The experimental data for the determination of the hydrodynamic characteristics of the flow were collected by tracing the liquid phase. The treatment of the dynamic response of the tracing experiments was carried out with the axial dispersion model. Experimental Section The experiments were carried out in a reactor with inner diameter Dr ) 25 mm and length L ) 50 cm. A coaxial thermowell with outer diameter Dth ) 6 mm was used in the reactor. The packing used was cylindrical
10.1021/ie990242b CCC: $18.00 © 1999 American Chemical Society Published on Web 09/11/1999
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Table 1. Extrudate Dimensions dp, mm l, mm
ex. 12
ex. 15
1.2 2.7
1.5 5.3
Table 2. RTD Beds experimental set
extrudates
gas
P, bar
FA FB RA RB RC RD RE RF RG
ex. 12 ex. 12 ex. 15 ex. 15 ex. 15 ex. 15 ex. 15 ex. 15 ex. 15
N2 H2 N2 N2 H2 H2 H2 H2 H2
1 53 1 2.7 1 53 28 13 83
Table 3. Gas-Feed Properties hydrotreatment
RTD experiments
gas
H2
H2
H2
N2
N2
T, °C P, bar d, kg/m3 µ, Pa‚s(×10-6)
350 70 2.7 14
25 1 0.08 8.8
25 53 4.3 8.8
25 1 1.24 17.7
25 2.7 3.4 17.7
extrudates which were prepared by extrusion and calcination of a clay paste. The extrudates were nonporous, as was confirmed by mercury penetration and nitrogen adsorption porosimetry. The extrudate dimensions are presented in Table 1. The liquid feed used in the residence time distribution (RTD) experiments was toluene. The physical properties (density and viscosity) of toluene at ambient temperature are similar to the properties of typical hydrotreatment liquid feeds at reaction conditions (high pressure and temperature).5 The gas feed, the system pressure, and the bed particles for the experiments presented in this work are given in Table 2. The experimental set FB was carried out after the experimental set FA without unloading the reactor, but only by varying the gas and the operating system pressure. The same bed was used for the sets RA-RG while the experiments were performed sequentially from the set RA to the set RG. The reactor was kept under pressure during experimentation with the beds tested. The calculated physical properties of the gas feeds at working conditions for every bed are shown in Table 3 along with the physical properties of the hydrotreatment gas feed (H2) at reaction conditions. The gas-feed compressibility factor was in the region 1 ( 0.05 for the conditions presented in Table 3, according to Tables 3-163 and 3-165 in Perry and Green12 and therefore it was considered as equal to 1. The gas viscosity was calculated by a nomograph (Figure 3-42) in Perry and Green.12 The pressure effect on viscosity is negligible according to the Reichenberg correlation (Figure 3-47) in Perry and Green.12 The residence time distribution experiments were carried out at ambient temperature with p-nitrophenol as a tracer in the liquid feed. The feed superficial velocities (calculated at operating conditions) were in the range uls ) 0.02-0.1 mm/s and ugs ) 0.5-3.5 mm/ s. The experimental setup and procedure for the atmospheric pressure experiments was the same as the one described in a work of Thanos et al.5 In Figure 1, the experimental setup used for the highpressure experiments is presented. The gas was supplied by high-pressure cylinders (1). The gas feed rate
Figure 1. Flow sheet for the high-pressure experimental system: (1) High-pressure cylinder, (2) back-pressure regulator, (3) flow transmitter, (4) back-pressure regulator, (5) nonreturn valve, (6) pressure indicator, (7) safety valve, (8) liquid-feed tanks, (9) threeway valve, (10) auxiliary valve, (11) metering pump, (12) auxiliary valve, (13) nonreturn valve, (14) auxiliary valve, (15) reactor, (16) auxiliary valve, (17) high-pressure separator, (18) temperature indicator, (19) draining valve, (20) back-pressure regulator, (21) needle valve, (22) and (23) scrubbers, and (24) wet gas meter.
was monitored by a mass flowmeter (3) (Brooks 5860E). The reactor pressure was set to the desired value by a back-pressure regulator (4). The tanks (8a) and (8b) contained pure toluene and a solution of 1% w/w p-nitrophenol in toluene correspondingly. A high-pressure metering pump (11) was used to feed the liquid to the reactor (15). The gas-liquid effluent from the reactor was separated at the high-pressure separator which operated at the reactor pressure. The gas-feed flow was controlled by the combination of a backpressure controller (20) and a metering valve (21). The scrubbers (22) and (23) contained water. A wet gas meter was used to check the mass flowmeter readings. The three-way valve (9) was used to perform step increase and decrease disturbances of the tracer concentration in the liquid feed. At regular time intervals, the liquid accumulated in the high-pressure separator was removed by means of the valve (19). The tracer concentration in the former samples was determined by vis-spectrophotometry at 400-nm wavelength. Treatment of the Data The evaluation of the extent of the liquid-phase dispersion in the systems tested was attempted by the axial dispersion model. The pertinent partial differential equation which describes the transient phenomenon in the bed is the following:
L ∂C 1 ∂2C ∂C ) u ∂t Pe ∂z2 ∂z
(1)
where
Pe )
uL Dax
(2)
The boundary equations used in solving eq 1 were the Danckwerts (closed vessel) ones, and they are expressed
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3819
by eqs 3 and 4:
uC0 ) uC - Dax
∂C , x ) 0+ ∂x
(3)
and
∂C ) 0, x ) L∂x
(4)
The initial condition at the reactor inlet for a step increase disturbance at the three-way valve is the following,
[
C0 ) 0, t < tin C0 ) finl(t), t g tin
]
(5)
Figure 2. Peclet number using N2 as the gas feed for different reactor pressures (RA, 1 bar; RB, 2.7 bar).
where finl(x) represents the transient response of the transfer line from the three-way valve to the reactor inlet. The liquid holdup is defined as the ratio of the liquid contained in the reactor to the bed void volume (6):
Hl )
Vl LQl ) Vvoid uV
(6)
The solution of eq 1 was performed in the time domain by the control volume method.13 The tracer dispersion in the transfer lines from the three-way valve to the reactor inlet and from the reactor exit to the highpressure separator was simulated according to a method described in a previous paper.5 The solution algorithm was coupled with the Marquardt algorithm to determine Pe and Hl.
Figure 3. Peclet number at atmospheric pressure for different gas feeds (RA, N2; RC, H2).
Results The presentation of the experimental data is attempted by using the volumetric superficial velocity of the fluid phases, calculated at operating conditions and defined as
uls,ugs )
Ql,Qg Ar
(7)
The experimental sets RA and RB were carried out using N2 as the feed gas. The system pressure was set to 1 and 2.7 bar, respectively. As seen in Table 3, the gas feed viscosity remained constant while its density increased from d ) 1.24 to d ) 3.4 g/L when the system pressure increased. The Peclet numbers determined for both cases are presented in Figure 2, and they appear practically independent of the system pressure for experiments with constant gas and liquid volumetric superficial velocities calculated at operating conditions. The experiments of the sets RA and RC were carried out under 1 bar operating pressure using different gases (N2 and H2) as the gas feed. The feed gas properties were different, as is shown in Table 3. In Figure 3, it is observed that the corresponding Peclet numbers were practically the same for constant feed superficial velocities, although the feed gas density and dynamic viscosity of the set RC were 1450 and 100% correspondingly higher than those of the RA set. In the case of the experimental sets FA and FB, the system pressure was different (P ) 1 and 53 bar, respectively) while N2 and H2 were used as the feed gas. Although the feed gas properties were different, the
Figure 4. Peclet number for the sets FA and FB (FA, N2 P ) 1 bar; FB, H2 P ) 53 bar) (open symbols, FA; filled symbols, FB).
Peclet numbers for these beds were in good agreement (Figure 4) at constant feed superficial velocities. The above findings indicate that neither the pressure nor the gas-phase physical properties affect the liquidphase nonideal behavior for constant feed volumetric superficial velocities calculated at the system conditions. This implies that the gas phase affects the liquid-phase mixing only by its velocity. It is to be noticed that the physical properties of the gas feed at typical hydrotreatment conditions are within the range of the properties covered in this work as can be seen in Table 3. As a result, the liquid-phase nonideal behavior at low pressure and temperature is expected to be the same as that at reaction conditions when the liquid as well as the gas superficial velocities are the same. In a preceding paper of Thanos et al.,5 it was found that the liquid holdup was independent of the liquid feed
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Figure 5. Liquid holdup for the sets RA-RC.
Figure 7. Liquid holdup for the water-Ar-feed system at different system pressure, uls ≈ 12.8 kg/m2s (data from Figure 2a, Larachi et al.2).
Figure 6. Liquid holdup for the sets FA and FB (FA, N2 P ) 1 bar; FB, H2 P ) 53 bar).
velocity (uls) under ambient conditions and it diminished with increasing gas feed velocity. The same is valid for all the beds tested in this paper. The comparison of the liquid holdup of the sets RA and RC (Figure 5) shows that, at constant operating pressure, the liquid holdup was invariable for constant gas feed volumetric superficial velocity, no matter what the gas feed dynamic viscosity or density was (Table 3). When the operating pressure increased, using either the same (N2 at P ) 1 and 2.7 bar) or different gas feed (N2 at P ) 1 bar and H2 at P ) 53 bar), the liquid holdup shows a tendency to decrease, as is presented in Figures 5 and 6, but it still has high values (Hl g 0.7). Despite the data scatter, the tendency has been indicated statistically significant by applying the t test for RA-RB and FA-FB data (probability 0.4 and 0.7%, respectively, for the hypothesis of equal means). In a paper of Larachi et al.,2 the liquid holdup was found to be the same using water as the liquid feed and different feed gases with constant density at different pressures (N2 and He at P ) 3 and 21 bar correspondingly) under the same gas mass superficial velocity. The liquid holdup was found by the former authors to be constant when water was used as the liquid feed and N2 as the gas feed at operating pressures P ) 3 and 51 bar and ugs < 30 mm/s. For the water-Ar feed system, at different operating pressures, the liquid holdup was correlated with the gas-phase mass superficial velocity (Figure 2a in the paper of Larachi et al.2). Although not stated in the paper of Larachi et al.,2 when the data of the former diagram are treated using the corresponding gas-feed volumetric superficial velocities calculated at the operating conditions, the liquid holdup is found to be independent of the system pressure (Figure 7) for ugs < 225 mm/s. All the above findings are consistent with the results presented in this work, although they
Figure 8. Peclet number for uls ) 0.1 mm/s for the sets RA and RD-RG.
were collected at relatively higher gas and liquid velocities. The experimental sets FA and FB, as well as RA, RB, and RC were carried out in series. In this way, the system operating pressure was maintained constant for all the experiments of every bed tested. In addition, the corresponding gas-phase flow rate in the reactor during the change of the operating pressure was kept small (
