Article pubs.acs.org/IECR
Scaling-up a Confined Jet Reactor for the Continuous Hydrothermal Manufacture of Nanomaterials Robert I. Gruar, Christopher J. Tighe, and Jawwad A. Darr* Department of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street, London, WC1H 0AJ, U.K. S Supporting Information *
ABSTRACT: A confined jet reactor (mixer) is presented as a novel solution for the scalable continuous hydrothermal flow synthesis (CHFS) of nanoceramics. In CHFS, nanoceramics are formed upon mixing of two streams consisting of an aqueous metal salt solution at room temperature with a flow of less dense supercritical water (at 240 bar and 450 °C). Upon mixing, hydrolysis and dehydration occurs, resulting in the particles being formed in a continuous manner. The confined jet mixer used herein overcomes previous designs of mixers that can accumulate material internally and block. A method for scaling up the jet mixer (reactor) is described, to determine the size of jet mixer (internal mixer diameter 13.5 mm) prior to its use in a newly commissioned pilot plant designed to process flow rates 40 times greater than the equivalent laboratory-scale process (internal mixer diameter 4.6 mm). It was confirmed that the pilot plant scale mixer allowed safe and continuous operation with no blockages at much higher concentrations (i.e., higher molarity) of metal salt precursor than laboratory scale because of the higher velocities and larger physical dimensions of the mixer. Consequently, the pilot plant was used to manufacture nanoparticles at a rate >400 times that of the laboratory-scale process. The synthesis of zinc oxide nanoparticles was used as a model to compare the properties of particles produced on different production scales. The same model system was also used to assess the limitations of a scale-up strategy based on mass (i.e., increasing the molarity of the metal salt).
1. INTRODUCTION Strong investment in the nanotechnology sector has seeded the development of many new synthesis approaches.1 With the increasing costs of energy and waste disposal, the manufacturing industry is striving to become more efficient.2 However, many nanomaterial synthesis methods are often intensive in terms of labor, chemicals, and energy, and can generate relatively large amounts of waste when scaled up (e.g., use of organic solvents to make Q dots).3 Furthermore, many processes are inherently difficult to scale-up due to changes in process behavior, e.g. time dependent changes in heat/mass transfer, giving different conditions of nucleation and growth of nanoparticles.4 Selected synthesis processes may also require very high temperatures/pressures (∼1000 °C, >500 bar), which may give agglomerated nanoparticles with low surface areas or unwanted secondary phases.5 Similarly, many processes require inconvenient and potentially risky nanoparticle capture methods (e.g., some flame processes).6 The latter may involve collecting nanoparticles from the gas phase and pose a greater risk of nanoparticle exposure for workers.7 Thus, developing scaled-up nanoparticle synthesis processes from the laboratory (g h−1) up to a production plant (e.g., multitonnes pa) is often made difficult by the aforementioned factors. Drawing heavily from the 24 principles of green engineering and green chemistry,2 the authors suggest that sustainable scaled-up nanoceramics synthesis processes should have the following key attributes: (1) the process should offer consistent quality of nanoparticles (1 kg h−1 by dry mass) should yield similar or better particle properties than those obtained for the equivalent smaller laboratory-scale process (this level is defined here as typically ca. 1−10 g h−1 by dry mass). Ideally, any nanoparticles made on any scale should be readily dispersed and functionalized appropriately to minimize agglomeration, and, thus, readily useable in inks/formulations or as dispersions. This may be achieved if the process allows surface functionalization in situ.9 Finally, in terms of safety, any scaled-up process must pose a low risk of exposure (e.g. via inhalation, skin penetration, etc.) to operators; this will be determined largely by the chemistry, production scale, and methods used for the recovery of the product.8 This final point is one of the biggest concerns to scientists and the general public as the fields of nanotechnology and green chemistry move closer toward commercialization.2,3 The authors have extensive experience of developing a laboratory-scale nanoceramics synthesis process known as continuous hydrothermal flow synthesis (CHFS).10−15,15,16 Similar processes were first developed in Japan, in the 1990s, and by others since.3,17 The CHFS process works by bringing Received: Revised: Accepted: Published: 5270
September 20, 2012 January 22, 2013 February 12, 2013 February 12, 2013 dx.doi.org/10.1021/ie302567d | Ind. Eng. Chem. Res. 2013, 52, 5270−5281
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Figure 1. Flow diagram of the continuous hydrothermal pilot plant.
numerous descriptions of mixers are presented in the literature, many of the fundamental processes of heat and mass transfer occurring within them have received little attention, with the notable exception of a counterflow mixer which has been studied by measuring the distribution of temperature at conditions analogous to those used for the synthesis of nanoparticles, modeling and through in situ high energy diffraction techniques.14−16 Herein, we present our findings on the development a new type of reactor (mixer) for CHFS reactors and assess its suitability for the manufacture of nanoceramics at different process scales. The mixer relies on mixing the flow of supercritical water and an ambient temperature precursor flow in a coaxial arrangement, where the inlet of the preheated water flow is confined within the annulus of the outlet. The purpose of this report is to demonstrate that this mixing arrangement is suitable for the production of nanoparticles. The novel use of such a mixer in this application was evaluated on both the laboratory and pilot-scale using the synthesis of zinc oxide nanoparticles (to study the effects of scale-up). Particle properties were compared on the different scales and with increasing molarity of the metal salt precursor. A brief investigation into the mixing processes was carried out using in situ temperature measurements. This yielded information on the likely heat and mass transfer processes responsible for efficient operation of the mixer over all process scales.
together a feed of low-density supercritical (or superheated) water with a higher-density ambient-temperature stream of aqueous metal salts. This is usually carried out using a particular reactor (mixer) arrangement resulting in rapid conversion of the metal salts into metal oxides via instantaneous hydrolysis and dehydration.18 Importantly, the reactor and conditions used should not result in blockages of the pipes due to material accumulation.15 This often occurs due to substantial preheating of the incoming metal salt precursor stream prior to being brought into contact with the much hotter water stream.19 In CHFS, nanoparticles are formed in the reactor (mixer) itself and then cooled via in-line (pipe-in pipe) cooling and then pass through a back-pressure regulator device, allowing products to be continuously produced and harvested as an aqueous nanoparticle slurry at atmospheric pressure and temperature. However, in CHFS-like processes, there appears to be relatively few reactor (mixer) designs that avoid blockage of the mixer within a few hours, limiting the capacity for sustained nanomaterial production.15 In 2009, the authors began to develop a scale-up pilot plant CHFS reactor (20× scale-up on volumetric flow rate alone) for which we now report among the first results. Prior to this report, the authors are aware of at least two organizations in the world that possess a pilot-scale or larger continuous hydrothermal plant capable of manufacturing >1 kg h−1 of nanopowder product.3 In particular, Hanwha Chemical Corporation, Republic of Korea, have not only a pilot plant, but also the world’s first full industrial scale continuous hydrothermal production plant for nanoceramics which is set to have a capacity of >200 tpa by 2012.3 However, to our knowledge, relatively few results from these processes have been published in the general academic literature. In the use of continuous hydrothermal reactors, the method by which superheated water is contacted with the solution of metal salt (precursors) can influence particles properties (e.g., size, size distribution, etc.) and reproducibility of synthesis.20 Consequently, this field of literature provides many descriptions of mixers which are suitable for the synthesis of nanomaterials, including tee pieces,21 cross pieces,22 vortexinducing devices,23 and a counterflow mixer.20 Although
2. EXPERIMENTAL SECTION 2.1. Continuous Hydrothermal Apparatus. Figure 1 is a simplified flow diagram of the continuous hydrothermal flow synthesis pilot plant used in this study. The laboratory-scale CHFS process is similar to the pilot plant but on a smaller process scale, and the former has been described in detail in our previous publications.15,16 The pilot-scale process was controlled and monitored (e.g., temperatures, pressures, pump flow rates and tank inventories) by a standalone real-time PC (CompactRio, National Instruments) into which various input/output (I/O) modules were plugged. Process control (heater pressure and outlet temper5271
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seen later). The mixture subsequently flowed upward (3/4 in. OD, 0.103 in. wall thickness, length 0.5 m), before entering two cooled pipe sections in series (1/4 in. OD, 0049 in. wall thickness, length 1.5 m each). Around these cooled pipes, cooling water flowed cocurrently (flow rate 100 L min−1, inlet T = 15 °C). Due to the ∼100 times higher flow rate of cooling water compared to product being cooled from the reactor, the temperature of the cooling water only rose by ∼5 °C, whereas the outlet temperature of the product stream was ∼40 °C. The cooled product then entered a back-pressure regulator (BPR, Swagelok KHB series), automatically adjusted using a PID algorithm to control the pressure in the process (measured at the outlet of pump P-1). The cooled products exiting the BPR at atmospheric pressure were collected. The pilot plant was designed with several safety features including safety relief valves (SRV) set at 276 bar (Parker HPRV type rated category IV under the EU pressure equipment directive 97/23/C) to overpressuring in the case of a partial or complete blockage and assurance that backflow would not occur in the event of a pump stopping. A shutdown system also operated automatically if the temperature at certain points within the system was exceeded. Further details of the safety considerations associated with the use of this equipment can be found in the Supporting Information. 2.2. Measurements of Temperature Profiles. To measure temperature profiles in the confined jet reactor, fine thermocouples (J-type, 0.5 mm OD, length 1.5 m, stainless steel sheath) were inserted into the CHFS process using a highpressure feed-through fitting (Spectite MF, 1/4 in. OD, Viton seal). The precursors for nanoparticle synthesis were substituted with DI water for these experiments to avoid fouling of the thermocouples due to deposition of material. The thermocouples were inserted into the cold inlets of the “precursor” feed line, terminating at various axial positions (defined as z in Figure 2) within the confined jet reactor, from the annulus between the inner and outer pipes and further up into the combined flow of products leaving the reactor. The tips of the thermocouples were left to float freely in the flow. Four temperatures at different locations (z) were simultaneously recorded every 500 ms and time-averaged over 30 s in the laboratory-scale process, while eight simultaneous measurements were taken in the pilot-scale process. The response time of the thermocouples was given by the manufacturer as 125 ms, defined as the time to reach 90% of the steady state temperature following a step change. 2.3. Continuous Hydrothermal Flow Synthesis of ZnO Nanoparticles. The hydrothermal reaction of aqueous zinc nitrate solution (Zn(NO3)2·6H2O (Sigma Aldrich, Dorset UK)) forming ZnO, was used to compare the nanoparticles produced from the laboratory-scale reactor with the reactor used on the pilot plant. Various concentrations of precursors and flow rates were used, and these are listed in Tables 2 and 3 for the laboratory-scale CHFS process and the pilot plant, respectively. To investigate the effect of increasing the total concentration of precursors on particle properties (on the pilot plant), zinc nitrate and KOH solutions (both at 1.0 M) were continuously mixed in a tee-piece and then diluted in line with a further flow of DI water. Aqueous potassium hydroxide, KOH (Fisher Scientific), with the same concentration as the metal salt was used as a mineralizer and was controlled by the output flow rate of the pump. At the highest concentration, this experiment was not feasible on the laboratory-scale process, because the BPR quickly failed due to becoming clogged.
ature and pump flow rates) and data logging functions were carried out within this real-time PC. The controllers and the front panel were programmed using LabView software (National Instruments). Up to four high-pressure diaphragm pumps (Milton Roy, Primeroyal K) were used simultaneously, each with a maximum flow rate of 666 mL min−1 and maximum outlet pressure of 300 bar. The flow rates from each pump were controlled remotely using a linear actuator, by adjusting the length of each stroke at a constant stroke rate (120 rpm). In practice, the first pump (P-1 in Figure 1) pumped deionised (DI) water, at a rate of up to 400 mL min−1 and a pressure of 240 bar, through four thermally insulated electrical heaters (Watlow Cast X 2000) providing a total thermal input of 24 kW. The remaining three pumps (P-2−4 in Figure 1), identical to P-1, were used to pump the precursors with a combined flow rate of up to 400 mL min−1. In the continuous hydrothermal process, these precursors are typically water-soluble metal salts, acids, and bases. In this study, precursors were stored in large tanks (stainless steel, volume 50 L). The precursors were mixed with the supercritical water in a confined jet reactor as shown in Figure 2. The dimensions indicated in Figure 2 are given in
Figure 2. Geometry of the confined jet reactor.
Table 1 for the different size reactors used on both the pilot and the laboratory-scale processes. In the process, supercritical water entered the inner pipe at the bottom of the reactor, and the precursors were fed in below the outlet of the supercritical water, which exited the inner pipe as a turbulent jet (as will be Table 1. Maximum Flow Rates and Confined Jet Reactor Geometry for Laboratory- and Pilot-Scale Processes Qsw/mL min−1 Qp/mL min−1 di/mm do/mm (do − di)/do L/mm
lab scale
pilot scale
20 20 0.99 4.57 0.78 13
400 400 3.97 13.51 0.71 40 5272
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Table 2. Conditions Used for Laboratory-Scale Synthesis of ZnO Nanoparticles flow rates/mL min−1 ID
Qsw
QZn
QK
0.05M40 0.10M40 0.20M40 0.05M30 0.10M30 0.20M30 0.05M20 0.10M20 0.20M20
20 20 20 15 15 15 10 10 10
10 10 10 7.5 7.5 7.5 5 5 5
10 10 10 7.5 7.5 7.5 5 5 5
concentrations/M Qd
Resw,z=0
temperatures/°C
[Zn(NO3)2]
[KOH]
Thw
Tp
Tm
×10−4
(Frsw,z=0)2
0.05 0.10 0.20 0.05 0.10 0.20 0.05 0.10 0.20
0.05 0.10 0.20 0.05 0.10 0.20 0.05 0.10 0.20
450 450 450 450 450 450 450 450 450
20 20 20 20 20 20 20 20 20
335 335 335 335 335 335 335 335 335
1.51 1.16 0.75 1.51 1.16 0.75 1.51 1.16 0.75
159 97 39 159 97 39 159 97 39
Table 3. Conditions Used for Pilot-Scale Synthesis of ZnO nanoparticlesa flow rates/mL min−1
a
ID
Qsw
QZn
QK
0.05M800 0.05M600 0.05M400 0.10M800 0.10M600 0.10M400 0.20M700 0.30M700 0.40M700 0.50M700 0.60M700 0.70M700 0.80M700 0.90M700
400 300 200 400 300 200 350 350 350 350 350 350 350 350
200 150 100 200 150 100 35.0 52.5 70.0 87.5 105.0 122.5 140.0 157.5
200 150 100 200 150 100 35.0 52.5 70.0 87.5 105.0 122.5 140.0 157.5
Qd
conc/M
Resw,z=0
Conc [Zn(NO3)2] or [KOH]/M
×10−4
(Frsw,z=0)2
0.05
7.44 5.55 3.73 7.44 5.55 3.73 6.57 6.57 6.57 6.57 6.57 6.57 6.57 6.57
62 34 16 62 34 16 48 48 48 48 48 48 48 48
0.10
280 245 210 175 140 105 70 35
1.00
Note: Tsw,in = 450 °C, Tp = 20 °C, Tm = 335 °C. The base and zinc precursor concentration are matched in each case.
Typically, for each set of conditions, 100 mL of nanoparticleladen slurry was collected from the laboratory-scale process, and 2.0 L was collected from the pilot plant (plus an additional ca. 50 mL sample to determine the yield of ZnO). In all cases, these samples were then centrifuged and diluted with DI water to remove residual ions (e.g., unreacted precursors, and potassium and nitrate ions). This process was repeated at least six times and then the concentrated slurries were then frozen to −60 °C and freeze-dried (100 × 10−3 bar vacuum for 12 h). 2.4. Characterization of ZnO Nanoparticles. Powder Xray diffraction patterns were collected using a Bruker D4 diffractometer (Cu Kα1, λ = 1.54 Å). Yttria (Y2O3) was used as standard for the estimation of instrumental peak broadening. Samples were prepared for TEM by dispersing the particles ultrasonically in DI water (>10 mega Ohms) and dropping onto carbon film grids [300 mesh] (Agar Scientific, UK). A JEOL 1200 Transmission Electron Microscope (120 KeV accelerating voltage) was used for generating images of particles produced on the pilot-scale CHFS process. Images and SAED patterns were captured using a Coupled CCD camera (GATAN). Image analysis was performed using freely available software ImageJ. Materials produced on the laboratory-scale process were evaluated using a JEOL 100CX TEM (100 KeV accelerating voltage), and images of the particles were captured on EM film (Kodak). Samples were prepared for HRTEM by dispersing the particles ultrasonically in ethanol 99.99%, (Sigma Aldrich, Dorset UK) and dropping onto Holey carbon film grids [400 mesh] (Agar Scientific, UK). A JEOL 4000×
Transmission Electron Microscope (350 KeV accelerating voltage) was used for generating HRTEM micrographs and SAED patterns of particles and images were captured using a coupled CCD camera (GATAN). Brunauer−Emmett−Teller (BET) surface area measurements were carried out using N2 in a Micrometrics ASAP 2420 instrument comprising of six parallel analysis stations. Samples were degassed at 150 °C for 12 h under vacuum before the adsorption isotherms were measured. The samples were identified with labels according to the initial metal salt concentration, and the total flow rate for the process (e.g., the pilot plant scale run at 0.05 M for Zn precursor at a total process flow of 800 mL min−1 is identified as sample 0.05M800.
3. RESULTS AND DISCUSSION 3.1. Confined Jet Reactor Construction and Optimization. The inlet position for the confined jet reactor was chosen based on the results of preliminary experiments, performed on a laboratory-scale mixer. Briefly, it was shown by both in situ temperature measurements and materials synthesis that significant inlet preheating in the annuli used to feed precursors into the mixer was observed over broad Tsw,in (350−450 °C) and, Qp (5−20 mL min−1), Qsw (10−25 mL min−1) when the inlet of the superheated water was below the precursor inlets. These observation suggested significant dilation of the jet of supercritical water issuing from the inlet leads to zones of recirculation if the inlet was not confined (discussed later) which was likely due to the jet not being able to entrain sufficient cold flow leading to recirculation.24 5273
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Figure 3. Temperature profiles in lab scale (Δ Qsw = Qp = 15 mL min−1, ○ 20 mL min−1) and pilot plant reactors (ΔQsw = Qp = 300 mL min−1, ◊ 400 mL min−1) for a supercritical water temperature, Thw of (a) 400 and (b) 450 °C. Dashed lines in a represent Tm,out ′ [Qsw = Qp, Tsw,in = 400 °C] and in (b) [Qsw = Qp, Tsw,in = 450 °C]. Measurements performed at Qsw = 200 and Qp = 200 have been omitted for clarity in the figure.
downstream of the outlet of the supercritical water. This represents a mixing time of ∼40 ms at the highest flow rates (assuming the theoretical mixture temperature occurs at the terminus of the inlet (Rt(s) = πdoz/Vmix), thus demonstrating that complete mixing occurs on incredibly small time scales. In order to satisfy ourselves that the aforementioned results are due to efficient mixing, we considered the contribution of heat conduction between the flows. It is possible that when the two streams meet, the temperature may reach T′m,out simply by conduction of heat between the supercritical water and aqueous precursor feed, even if no mixing occurs between the two streams. Consequently, the distance required to closely approach T′m,out through conduction alone, i.e. from the heat between the inlet of supercritical water (diameter di) and the annulus in which the precursors flows (width do − di) was estimated. This situation represents a simple cocurrent heat exchanger model (pipe-in-pipe), the heat loss from the preheated water flow must equal the heat gained by the precursor flow (as determined by enthalpy balance). For stratified flows of steam and water, it has been reported that Uo can vary between 3 and 30 kW m−2 K−1 giving a length to ′ in this case of z/di = 400−40, approach within 20 °C of Tm,out respectively, i.e. much greater than the measured z/di = 5.26 In other words, it is expected that thermal equilibrium on the basis of heat transfer alone would occur at a minimum of a ca. 40 (inner) pipe diameters from the annulus of the inner pipe, rather than 5 (inner) pipe diameters as measured. Consequently, the observed temperature profiles must be the result of mixing events occurring near the terminus of the preheated water inlet, where mixing events apparently occur in similar geometric space and on similar time scales between the mixers (Figure 3). The observation of complete mixing between the two streams at and beyond z/di = 5 is consistent with the findings of previous studies where highly turbulent jets are
Similarly, it was also shown by preliminary investigations that a longer than needed inlet length showed a greater degree of heat transfer occurring between Qp and Qsw. These preliminary investigations are not discussed further as they do not meet the idealized reaction conditions of near instantaneous heating of the precursor and preheated water feed highlighted as a requirement for the synthesis of nanoparticles in CHFS.20 3.2. Temperature Profiles within Confined Jet Reaction Points of Different Scales. Figure 3a and b compare the temperature profiles measured in the lab scale confined jet mixer with those from the pilot-scale confined jet mixer, the relevant dimensions of each geometry are given in Table 1. The dashed lines in Figure 3a and b show the maximum theoretical temperature calculated for the outlet of the mixer (i.e., the inlet to the cooler) assuming mixing is complete and any heat losses from the outer pipe to the surroundings are negligible. This was determined from the overall enthalpy balance (eq 1): ′ , 240 bar) hm,out(Tm,out =
Gsw hsw,in(Tsw,in , 240 bar) + Gphp,in(Tp,in , 240 bar) Gsw + Gp
(1)
Where, Gsw and Gp are the mass flow rates of the superheated water and “precursors”, respectively. The specific enthalpies hsw,in and hp,in and the temperature, Tm,out ′ , at which the specific enthalpy is hm,out at 240 bar were determined from IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use and the associated FLUIDCAL software.25 This software was also used to calculate the density and viscosity of water for any given temperature at 240 bar. It can be seen in Figure 3a and b that on both the lab and the pilot-scale plant, the measured temperature reached the maximum theoretical value within five inner pipe diameters 5274
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the outlet of the mixer is driven by momentum as (Frsw,z=0)2 ≫ 1. Interestingly, if the process were to be scaled on the Re number of Qsw, the reaction point geometry would have to be much larger owing to the significantly greater values of Gsw + Gp. For the process to be scaled on Reynolds number, the di of the inlet tube containing would have to be significantly larger ca. 8 times that reported to yield numbers of similar magnitude to the laboratory-scale process. However, scale-up on a Re basis would yield a reduction in the (Frsw,z=0)2 which would approach 1 and behave more as a buoyancy driven plume rather than momentum dominated jet. The reported work of Ricou et al. suggested that significant entrainment of the coflowing fluid under conditions similar to those presented in this work was expected at
