Liquid Holdup by Gravimetric Recirculation Continuous Measurement

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Liquid holdup by gravimetric recirculation continuous measurement method. Application to Trickle Bed Reactors under pressure at lab scale Juan García-Serna, Gianluca Gallina, Pierdomenico Biasi, and Tapio Olavi Salmi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01810 • Publication Date (Web): 22 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017

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Industrial & Engineering Chemistry Research

Liquid holdup by gravimetric recirculation continuous measurement method. Application to Trickle Bed Reactors under pressure at lab scale Juan García-Sernaa*, Gianluca Gallinaa,b, Pierdomenico Biasib* and Tapio Salmib a

High Pressure Processes Group (hpp.uva.es), Department of Chemical Engineering and Environmental Technology, University of Valladolid, 47011 Valladolid, Spain

b

Johan Gadolin Process Chemistry Centre, Laboratory of Industrial Chemistry and Reaction Engineering, Åbo Akademi University, Biskopsgatan 8, Turku/Åbo, FI-20500, Finland Corresponding authors: E-mail: [email protected] (J. García-Serna) E-mail: [email protected] (P.D. Biasi)

ABSTRACT Liquid holdup is a crucial parameter when operating catalytic trickle bed reactors related to wetting efficiency and residence time. Most of the classical methods are expensive, time consuming or not valid for high pressures. Liquid Holdup by Gravimetric Recirculation (LHGR) measurement method determines the average total liquid holdup using a close loop

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with total recirculation of liquid. The recirculating liquid is contained in a vessel that is weight instantaneously with a known amount of liquid. The difference between the initial amount of liquid and the instantaneous weight gives the liquid holdup. Main benefits are: first, the measurement takes only the time of stabilization that at lab scale is 2 to 5 minutes, and second, the system is ready for the next measurement without any modification (the bed is not disturbed nor modified), third, it works for high pressures, and finally, it gives values comparable to classical techniques like RTD or conductivity methods.

Keywords Trickle bed reactor; fixed bed; heterogeneous catalysis; residence time distribution; recirculation; holdup.

1. Introduction Multiphase Chemical Reactors (MCR) are the most extended in the industry nowadays, where high performance and waste reduction is a priority. Heterogeneous catalysts have improved considerably the atom efficiency of chemical reactions compared to homogeneous reactions, as the use of raw materials is minimized and the catalyst recovery is easier in general. Trickle Bed reactors (TBR) and Slurry Bubble Column reactors (SBR) are the most widely used among the different continuous reactors designs available. Focusing on TBRs, they are mainly composed of a fixed bed reactor where: the solid catalytic phase is inside and static, the liquid phase down flows and the gas phase can either up or down flow. A dual mass transfer occurs, from gas to liquid phase and from liquid phase to the active centers in the solid phase.


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Hydrodynamics of TBRs are complex and highly dependent on the particle size, reactor diameter, catalyst surface characteristics and liquid physical properties. Alsolami et al. pointed out very clearly that scale matters, thus, laboratory and pilot TBRs behave completely different than industrial scale TBRs as they use catalyst particles smaller than 2 mm in general. This motivates that, at small scales, capillary actions prevail over inertial, gravitational and viscous forces1. The analysis for each particular case can be done using Eötvös number, that compares the ratio between gravitational and capillary (surface tension) forces. The criterion indicates the Eö=1 is the limit to consider the importance of capillary forces. This occurs around particle sizes between 2 and 4 mm depending on the physical properties. Liquid holdup represents the total amount of liquid in the reactor volume at any time. Liquid saturation refers to the amount of liquid per void volume. The liquid holdup can be divided in external and internal (for porous materials) to the particles. The external liquid holdup can be static (near the particle surface creating a thin static layer) and dynamic (between the static layer and the gas phase). The different percentage combinations of the three phases, i.e. solid, liquid and gas provides the unique flow types of multiphase fixed bed reactors: pulse flow, spray flow, trickle flow and bubbly flow2. TBRs operating under high pressures are hardly made of a transparent material, so liquid holdup measurement techniques cannot rely on visual image, nuclear magnetic resonance or similar methods. Al-Dahhan and Highfill reviewed all the main measurement methods classifying them depending on the extension of the bed measured: (a) Integral, for the whole bed, (b) Semi-integral, for parts of the bed and (c) Local measurements at specific spots3. Within the integral measurement methods, Al-Dahhan and Highfill classified: (1) draining methods, (2) tracer, (3) closed-loop and (4) electrical conductivity methods. Boyer et al. comprehensively


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reviewed all the measuring techniques applicable to gas-liquid and gas-liquid-solid reactors, dividing them into invasive and non-invasive. Particularly, they explained a so-called technique “device for liquid collection” that is basically a tray with 60 small compartments that sample liquid and gas, so it can help in the mapping of the TBR at the outlet4. The draining methods imply the sudden shut-off of the flow using on-off valves before and after the reactor, the physical disconnection of the system and the weight of it. This is time consuming and causes degradation on the closures after a certain number of trials. The close loop method was mentioned by Charpentier: the liquid holdup in the packed-bed is determined as the difference of loop volume and the liquid outside the packed-bed when operating in closed loop5,6. As far as we know, this method has not been reported later on since 1968. Larachi et al. studied the influence of pressure up to 8.1 MPa for non-porous particles using a tracer technique for the water/nitrogen system liquid saturation7. They found values of liquid saturation, βL, from 0.36 (at liquid flow rate of L=1.88 kg·m-2·s-1) to 0.72 (L=24.5 kg·m-2·s-1) at low gas flowrates. The value of βL was almost independent of pressure. Liquid saturation decreased along with increasing the gas flowrate and increased by decreasing the particle size (capillary effect). Similarly, βL decreased by more than half by adding 1% ethanol (foaming effect). Urrutia et al. studied thoroughly the drainage of TBRs, determining the liquid holdup at different liquid velocities8. They used a closed loop method where the TBR was flowing downflow for gas and liquid. The liquid collected at the bottom was returned to the entrance vessel (closed loop). The experiment started when a constant mass in the scale of the entrance vessel was achieved, then the flow was stopped and the “dynamic” holdup was measured by measuring the water flowing down from the TBR.


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Lange et al. used a drainage technique on a 3.4 cm inner diameter reactor for crushed pellets of γ-Al2O3 and SiO2 beads9. The total liquid holdup for γ-Al2O3 pellets was almost double (εL = 0.38-0.48) than the SiO2 beads. (εL = 0.18-0.23) at similar conditions. Similarly, dynamic liquid holdup, εLd, increased along with decreasing particle size, due to the capillarity effect. Transparent reactors allow the continuous local measurement of maldistribution and holdup, as the work done by Liu et al10. Electric Capacitance Tomography (ECT) was used to demonstrate that non-steady state operation of periodically operated TBRs affect local liquid holdup. However, these types of measurement are very difficult at lab scale operation under high pressures. The aim of our work is to develop a simpler and faster technique to determine the liquid holdup of TBRs operating under high pressures. We present the Liquid Holdup by Gravimetric Recirculation continuous measurement method (LHGR), a simple integral method using the advantage of operating in closed loop together with total recirculation that as far as we know, has not been used before for such purposes. We believe that this LHGR method simplifies enormously the lab procedure, increases the number of experiments per unit of time and their reliability. Our experimental system has similarities to the one used by Urrutia et al. but our purpose was to determine the instant holdup and to measure the holdup variation with liquid and gas flow and pressure8. In addition, it has similarities with the collecting device described by Boyer et al.4 It differs in the recirculation step and that, in our case, only one collecting device is required at lab scale. The method can be used at reaction conditions if needed. For this purpose, the conditions selected were in the range of H2O2 direct synthesis11–13, although the reaction experiments are out of the scope of this paper and will be presented in a future work.


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2. Materials and methods 2.1. Materials The reactor was charged with a bed of 150-200 µm SiO2 particles (Sigma-Aldrich). Deionized water was used as the model reaction liquid phase. Premium grade nitrogen (99.999%) was purchase from AGA (Linde Group, Finland) and used as the model reaction gas phase.

2.2. Experimental set-up The TBR presented has been used in previous works for the study of the hydrogen peroxide direct synthesis using different catalytic beds, e.g. SiO2(inert)+Pd/C(catalyst)12,14. The set-up of the system was simplified and connected in closed loop and total recirculation to determine the holdup dynamically as exemplified in Figure 1. LIQUID RE CIRCULA TION

FCV-01

VOID VOLUME

RS

P-01

INE RT BED

PCV-01

D-02

R-01

N2

OFF GAS

RR

D-01

GLA SS WOOL

000.00 g

Figure 1. Trickle Bed Reactor in close loop total recirculation mode A stainless steel AISI 316 Trickle Bed reactor (R-01) was used with a nominal length of L=65 cm and an internal diameter ID=1.5 cm. The reactor was charged with SiO2 particles, filled up to


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L=60 cm (real bed length). The TBR had a heating jacket with tempered water from a bath that acted as a refrigerant or heating fluid depending on the bath set temperature. Similarly, to other studies carried out with this rig, the N2 gas was held in a high-pressure cylinder, controlled by a Brooks gas controller and introduced in the top of the reactor (gas phase downflow). The liquid phase was pumped inside the reactor from the top (liquid downflow). The TBR pressure was controlled using a back-pressure valve, PCV-01, at the outlet of the reactor; this means that the valve operated depressurizing a mixture of liquid and gas that was separated afterwards in the liquid collecting vessel (D-01) that acted as a recirculation vessel. This unit was located over a scale that measured the weight of the liquid collected in D-01 at all times. The pipes and fittings were selected and designed to minimize the water retention that might disturb the measurement; water retention inside the pipes and fittings was measured and discounted to assure precision.

2.3. Experimental procedure The holdup experiments were carried out using a novel idea, as indicated in the introduction section. The system operated in closed loop for the liquid and in open mode for the gas. Moreover, the liquid operated under full recirculation, which meant that the liquid coming out the reactor was collected in a vessel (D-01) and pumped back (P-01) to the top of the TBR (R-01). The start-up and operation exhibited a behavior exemplified in Figure 2. It can be seen that, first the system is charged with a known amount of liquid (around 197 g in the example of Figure 2). When the recirculation pump was turned on a constant amount of liquid was extracted from the reservoir vessel (D-01) and pumped into the reactor system (R-01), that is seen in the linear decrease from time=0 min to around time=18 min. That meant that the liquid was wetting the TBR (R-01). Around time=18 min the liquid started


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coming out the reactor and the vessel received liquid in recirculation continuously, so the weight stabilized around time=20 min. The system was left to stabilize and after three stable measurements the pump flowrate was modified at around time=35 min, and so on with the rest of the measurements planned. The detailed experimental steps for the start-up and operation procedure are listed next: 1. Prepare the TBR reactor with the bed length desired (as explained in section 2.2). 2. Tight the reactor and fittings. 3. Start the liquid refrigerant and wait until temperature stabilizes. 4. Tare the scale with D-01 weight. 5. Load D-01 with a known amount of deionized water (around 200 g). 6. Flush the system using N2 flowrate set by the gas controller (FCV-01). 7. Set the desired pressure using the back-pressure control valve (BPV-01). 8. Start the pump (P-01) at the desired liquid flowrate (i.e. 1 mL/min). 9. Register and plot the mass values from the scale. 10. At some point the mass stabilizes indicating that the system is operating at steady state in liquid phase. Take three mass measurements (every 5 min) and calculate an average value. Check that temperature and pressure are stable. 11. Change the independent variable considered (liquid or gas flowrate, pressure, temperature) and wait until stabilization. 12. Repeat steps 9-11 for each constant reactor length until all the planned experiments are carried out. 13. Stop the system, depressurize and detach the reactor.


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Figure 2. Unsteady state of mass stabilization during the experiments The determination of the holdup is done by determining the volume of the liquid from difference between the initial mass of the liquid (mL0) and the instant mass of the liquid in the scale (mL) as shown in Eq. 1 (using water density, ρL at temperature and pressure of the run).

(1)

3. Results and discussion The system was tested with 31 tests (runs #01 to #31)were liquid flow, gas flow and pressure were varied, as listed in Tables S1 and S2 (Supporting Info). All of these 31 experiments were conducted at 15 ºC for a reactor length of 60 cm. Each run was repeated three times and the average error in mass was lower than 1%. This indicates that the measurement was very stable and precise. The stabilization time between different runs were lower than 10 minutes. Indeed, after the first data point that took a bit more time, the stabilization was under 3 minutes (see


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Figure 2). This demonstrates that the method proposed can be very useful for measuring the liquid holdup during a reaction. All the experiments were conducted under conditions of trickle flow, although in general all the criteria has been generally developed for bigger systems with respect to the lab plant reactors. Indeed, capillary forces for small particle diameters are of great importance. Alsolami et al. defined a criterion to identify the flow regime using the Eötvös number1. They considered that trickling flow regime started when Eö>1. However, the combination of the different criteria from several authors do not always provide exact results, as the regions have a margin of uncertainty. In a system with nitrogen and water, the trickling will start from particle diameters between 2.5 and 3 mm. Particle diameters smaller than 2.5 mm will show effect of capillary forces against the gravitational force, as in our case. This was also indicated by Wammes et al. establishing a safer limit of 2 mm15. Considering the Reynolds number criteria (listed in Table S2) one can see that the trickle region occurs at ReL < 102 and ReL

Liquid Holdup by Gravimetric Recirculation Continuous Measurement

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