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Multiple Automated Reactor Systems (MARS). 1. A Novel Reactor System for Detailed Testing of Gas-Phase Heterogeneous Oxidation Catalysts Patrick L. Mills* DuPont Company, Experimental Station, E304/A204, Wilmington, Delaware 19880-0304
Jacques F. Nicole Catalytica Energy Systems, 430 Ferguson Drive, Mountain View, California 94043-5272
An automated reactor system for a detailed performance evaluation of gas-phase heterogeneous oxidation catalysts that utilizes a parallel array of six fixed microreactors called the Multiple Automated Reactor System, or MARS, is described. The key MARS components include a gas manifold that safely generates a light hydrocarbon oxidation feed composition, an array of six fixed-bed microreactors with dedicated components for control of individual reactor feed gas flow rates and temperatures, an integrated gas sampling and gas chromatography system for online analysis of feed and product gas compositions, and a process automation control package based on process logic controller technology. The addition of one or more liquid feed components, such as steam or organometallic catalyst surface modification agents, is also possible through a dedicated liquid feed vaporizer subsystem. The automation package contains all of the elements needed for logging of process sensors, monitoring of all process alarms, control of all process variables, interlock sequencing, and communication between the operator and automation hardware through a human-machine interface. These features allow a user-defined catalyst testing protocol to be downloaded from the automation so that the system can safely operate 24/7 in an unattended mode. Two versions of the MARS are described that mainly differ in the fixed-bed microreactor configuration and the length of the heated zones used for transport of product gases from the catalytic zone to the online gas sampling system. One version employs a classical U-tube fixed-bed microreactor, whereas the second version uses a straight-through fixed bed. An overview of key operating characteristics is provided. It is shown in part 2 of this series of papers that the contribution of metal-wall-catalyzed and homogeneous gas-phase reactions on the observed hydrocarbon and oxygen conversions can achieve significant yet different levels in each reactor configuration for 1,3-butadiene oxidation. The need to first assess the role of undesired reactions and then to either eliminate or minimize their contribution to the desired solid-catalyzed reaction in parallel microreactors is emphasized. 1. Introduction A central theme of the Chemical Reaction Engineering Laboratory’s (CREL’s) research plan over the past 30 years under the direction of Professor M. P. Dudukovic´ has revolved around the quantification of the interaction between transport effects, hydrodynamics, and reaction kinetics for multiphase reaction systems.1-3 Notable early work from the CREL group examined the extraction of the apparent kinetics for gas-solid noncatalytic reactions by the analysis of radial-flow fixedbed reactor performance data4 and by the modeling of transient microbalance data for single pellets.5 Later efforts shifted to the study of intrinsic and apparent kinetics for three-phase solid-catalyzed reactions at moderate temperatures and pressures under gaseous reactant-limited conditions using novel slurry- and basket-type laboratory reactors.6 This prototypical investigation6 and others that followed7-9 demonstrated that three-phase trickle-bed reactor performance data for either nonvolatile or volatile liquid reactant systems could be accurately predicted using fundamental reac* To whom correspondence should be addressed. Tel.: (302) 695-8100. Fax: (302) 695-3501. E-mail: Patrick.L.Mills@ usa.dupont.com.
tion engineering models without resorting to the use of adjustable transport or pseudokinetic parameters. Kinetic model discrimination and parameter estimation were also demonstrated for a complex gas-liquid-solidcatalyzed reaction network by analysis of data collected from a laboratory-scale trickle-bed reactor operating under high pressures and elevated temperatures.10,11 Imbedded within these works was the development of novel experimental methods for laboratory-scale threephase reactors that minimized nonideal effects so that the kinetic parameter estimation was simplified. Within the past decade, efforts by the CREL group have been primarily focused upon the development of novel experimental tools for noninvasive measurement of fluid mechanical parameters in multiphase reactors and on the modeling of multiphase-flow hydrodynamics using computational fluid dynamics.12,13 Coupling of these advanced methods for flow modeling of multiphase systems with local transport/kinetic models provides the fundamental tools required for either scale-up of new reactors or troubleshooting of existing ones.14,15 The work described here follows in the spirit of early CREL group efforts from the perspective of reaction kinetics because it is concerned with the development and application of a novel parallel reactor system for
10.1021/ie048908b CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005
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detailed performance evaluation of gas-phase solidcatalyzed selective oxidation chemistries. These reactions have notable commercial importance16,17 because selective oxidation products are primarily utilized as the basic building blocks for various addition, condensation, and other classes of polymers used to manufacture highperformance plastics, textile fibers, and other synthetic materials.18 In addition, oxygenated products account for about 60% of the total annual production rate of chemical intermediates that are manufactured using either soluble organometallic19 or heterogeneous20 catalytic technologies. A recent review of catalytic technologies that have been commercialized in the past decade21 suggests that invention of either new or improved selective oxidation process technologies is being driven by the following key factors: (i) many existing processes generate byproducts and waste streams whose end-ofpipe treatment would require significant capital investment to satisfy new or anticipated environmental regulations; (ii) existing raw materials, which are typically based on olefinic feedstocks, account for 60-70% of the total production costs and must eventually be replaced by less expensive alkanes to maintain economic viability; and (iii) many older processes are approaching obsolescence, and they cannot be readily modified to take advantage of newer reaction engineering concepts, such as unsteady-state operation,22-25 that utilize novel reactors and operational approaches to obtain improved product yields. These and other related drivers for the invention of new processes, especially from an environmental perspective, have also been described.26 2. Historical Perspective and Literature Review The parallel reactor system described in later sections was developed within the DuPont Central Research Department through evolutionary efforts that spanned nearly 2 decades and involved new catalyst invention and process development for a variety of gas-phase oxidation systems. A prototype system for heterogeneous oxidation catalyst testing called the Automated Reactor Facility (ARF) was first described in the mid1980s by Blackstone and Lerou.27 This design contained six parallel fixed-bed microreactors and incorporated independent controls for the reactor feed composition, reactor flow rates, and catalyst bed temperatures with process automation based upon a PDP11/44 computer. However, the online gas chromatography (GC) method used by the ARF was based on then-existing 1980s-era packed column technology. This GC method was not capable of resolving and quantifying certain organic catalytic reaction products produced from the partial oxidation of n-butane, such as maleic anhydride, acrylic acid, and acetic acid,28 to the higher degree of accuracy and repeatability that is now possible with more sophisticated mega- or microbore capillary column technology.29 As a result, time-consuming offline wet chemistry analytical work was needed to quantify the organic acids for the development of reliable product distribution and overall mass balance data. These offline efforts reduced some of the catalyst testing productivity gains that were otherwise obtained from parallel operation of the six fixed-bed microreactors and suggested the need for analytical debottlenecking. The second-generation automated reactor system that evolved from catalyst research experience with the ARF was called the multiple automated reactor system, or MARS-1 for short. It was initially developed in the early
1990s and was based upon dual parallel fixed-bed microreactors.30 Initial data generated from the MARS-1 provided the basis for the invention of novel heteropolyacid (HPA) catalysts for n-butane to maleic anhydride as alternatives to well-known vanadium phosphorus oxide (VPO) catalysts.31 The HPA catalyst synthesis produced a crystalline oxide in which some of the molybdenum and/or tungsten at the HPA octahedral sites was stoichiometrically substituted with vanadium and a transition-metal or main group cation. Under anaerobic conditions, the metal-substituted HPA compositions exhibited improved lattice oxygen capacity when compared to standard VPO catalysts. This feature is attractive for circulating fluidized-bed reactor operation where n-butane is converted to maleic anhydride in the absence of gas-phase oxygen via lattice oxygen on the riser side of the process. The resulting oxygen vacancies are then replenished by contacting the reduced HPA catalyst with air in a bubbling fluidized bed located on the regenerator side of the circulating fluidized-bed reactor system.32 To accelerate the catalyst screening methodology for C4 hydrocarbon partial oxidation reactions using the newer MARS-1 design, a multidimensional GC method was developed during the early to middle 1990s that was based on both packed and newer capillary columns.33 The prototype online technique, which used four integrated GC columns connected to multiport gas switching valves in a single GC oven, was fully automated and provided a full quantitative account of all hydrocarbons, organic oxygenates, and inorganic combustion products generated from the MARS-1 catalyst evaluation experiments, including H2O. An enhanced version of this method was later demonstrated that employed dual-column ovens with independent temperature programs for improved peak resolution and faster overall analysis time.34 The method also included the capability of positive identification of all reaction species by online injection to a Hewlett-Packard 6890 GC system with a Hewlett-Packard 5973 mass selective detector. The latter instrument could be operated in parallel with the multiple-oven process GC system on an as-needed basis. The need to increase the catalyst research efficiency led to the development of an in-house automated roboticlike system for parallel synthesis of heterogeneous catalysts along with a novel reactor system for highthroughput screening. A recent publication35 describes the basic design features of this novel reactor system, illustrates its application to screening of promoted bismuth-molybdenum oxide catalysts for the selective oxidation of 1,3-butadiene to furan, and compares catalyst rankings with those obtained from the MARS-2 system. The reactor design was configured to provide a first level, or so-called stage 1, catalyst library screening36 with a maximum throughput rate of 64-80 catalysts/day, which was within the range of the robotic catalyst library synthesis rate of 72 catalysts/day. Experience in using this new stage 1 screening methodology showed that the generation rate of new catalyst leads exceeded the rate at which stage 2 detailed screening36 could be performed using the dual-reactor MARS-1 system. To meet the new throughput rate demands for the stage 1 rapid screening protocol, nextgeneration reactor systems were developed for detailed evaluation of oxidation catalysts that were called MARS-2 and MARS-3. These internal company developments
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involving robotic catalyst library generation, rapid catalyst screening, and MARS detailed catalyst evaluation occurred over the past decade in parallel with external developments in the field of combinatorial heterogeneous catalysis.37-41 The findings are consistent with various reports from a recent symposium that was focused on reaction engineering aspects of catalytic process development42 in which the serial approach is being replaced, with a few exceptions, by parallel testing of multiple catalyst samples using combinatorial techniques. The combinatorial methodology can be viewed as an integral part of the total catalyst invention process where various activities focused on discovery, development, and commercialization occur simultaneously through a series of three interacting cycles.43 Each successive cycle is designed to evaluate the catalyst performance with increasing scrutiny so that the resulting catalyst package has the robustness required to operate within a commercial environment to meet the required process economic objectives.36,44 Detailed performance evaluation of gas-phase partial oxidation catalysts using parallel reactor systems creates special challenges because of the unique nature of this class of reactions. Design and operational challenges that occur include the following: (1) the hydrocarbon reactants and products form flammable mixtures in the presence of air or O245 so safe operational features and safety interlocks must be integrated into the design; (2) online addition of various catalyst modification or reaction-altering agents to the feed gas may be required, e.g., water in the form of steam46 or other additives, such as organophosphorus compounds;47 (3) the activity versus selectivity behavior of catalysts can vary significantly as a function of time on stream owing to slow structural and morphological changes that are induced by contact with the gas-phase reaction mixture or through the catalyst thermal history;48 (4) the selective oxidation product may experience thermal degradation in the postcatalytic zone, which can lead to a decrease in the actual catalytic selectivity; (5) the product gas composition can vary significantly as a result of changes in the process conditions or catalyst behavior, which can place stringent demands on the online product analysis system operation and data analysis; and (6) reactor system surfaces, such as those associated with reactor tube walls, sample valves, and product transfer lines, can provide sites for undesired wall-catalyzed reactions. Collectively, these various challenges require the incorporation of special features into the reactor system design and proper specification of process conditions used for the catalyst performance evaluation. Parallel reactor systems for detailed performance evaluation of gas-phase heterogeneous catalysts have received increasing attention as part of the growth in the use of combinatorial methods. Moulijn and coworkers49 described a system containing a parallel array of six fixed-bed microreactors and summarized a number of applications involving environmental-type catalyst screening and kinetic studies. A summary of the key working equations used for the estimation of intraand interparticle gradients to verify that the observed kinetics are not corrupted by nonideal transport effects was also provided. A recent publication from this same group50 focuses upon key fundamental and practical aspects of the detailed evaluation stage in heterogeneous catalyst development. Various examples of parallel reactor systems are also given along with key aspects
Table 1. Differences between the MARS-2 and MARS-3 Systems MARS-2
MARS-3
gas-flow direction heating system steam generation
upflow sand bath gas saturator
reactor design
VCR type tube fitting type 316 SS
downflow split-tube furnace custom liquid pump with mixer vaporizer 6.35-mm (1/4-in.) o.d. tube Hastelloy C
reactor material
MARS-2 reactor no. total length of product gas transfer lines
1 2 3 4 5 6
length [m] 0.81 0.58 0.36 0.38 0.64 1.04
MARS-3 reactor no. 1 2 3 4 5 6
length [m] 1.3 1.3 1.3 1.3 1.3 1.3
of technology development and its utilization. Systems that allow the parallel evaluation of 2N catalysts where N ) 1, 2, ..., 6 or so have been patented51 and are also commercially available.52 However, these are general purpose systems for gas-phase solid-catalyzed systems that do not specifically address various issues associated with testing of gas-phase oxidation chemistries for safe, unattended operation using various operating modes. The primary objective of Part 1 of this series of papers is to describe two related versions of a six-flow-reactor system design called MARS-2 and MARS-3 that are intended for detailed evaluation of gas-phase heterogeneous catalyst oxidation systems. A secondary objective is to briefly illustrate the performance of key system components. Part 2 of this series of papers provides an assessment of the role of gas-phase homogeneous reactions and the effect of reactor system materials of construction on the observed product distributions for selected C4 partial oxidation reactions. Part 3 of this series of papers considers various applications, such as the partial oxidation of n-butane to MAN and the partial oxidation of 1,3-butadiene to furan, where the MARS have led to new discoveries and insights. Both reactions have commercial significance and have features that can lead to various complications in laboratory-scale operations. 3. Experimental Section This section describes various aspects of the MARS. Included here are details on the various system components, an overview of the analytical methods used for identification and quantification of the reactor feed and product gases, definitions for the various experimental protocols, and descriptions of the methods used for reduction and processing of the raw experimental data. 3.1. MARS. Two versions of the MARS that are referred to as MARS-2 and MARS-3 are described in this section. The MARS-3 was largely based upon the earlier MARS-2 prototype design. The main differences between the two systems were in various reactor design aspects, which are compared in Table 1. The MARS-2 was mainly dedicated to detailed performance evaluations for new n-butane oxidation catalyst compositions that emerged from stage 1 rapid screening tests. Catalyst candidates that successfully emerged from short-term detailed testing protocols (e.g.,
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Figure 1. Schematic of the MARS-2 and MARS-3 systems.
up to a few days) were sometimes evaluated for several hundred to several thousand hours of continuous operation using either the original dual-reactor MARS-1 system or the six-reactor MARS-2 system. The MARS-2 system was also used to provide a performance snapshot of catalyst samples that were withdrawn from commercial plant operations on a daily basis over several years of plant operation. The MARS-3 system was developed later after operating experience was gained with the MARS-2. The MARS-3 was mainly used for other exploratory heterogeneous oxidation catalyst chemistries. More details on these aspects are provided in parts 2 and 3 of this series of papers. 3.1.1. General Description. A simplified schematic of both the MARS-2 and MARS-3 systems is shown in Figure 1. Photographs of these two systems are shown in Figures 2 and 3, respectively. Descriptions of the key system components are provided in the following sections. The working concept for the MARS allows six catalyst samples to be simultaneously evaluated using a continuous steady flow rate of reaction gas under isothermal conditions. Each catalyst sample is contained in a small fixed-bed microreactor whose inlet flow rate of reaction gas, inlet gas composition, and reactor tem-
Figure 2. MARS-2 system.
perature can be independently controlled. By variation of the flow rate, composition, and temperature over a predetermined range of conditions, both the gas-solid contact time and reaction rate can be adjusted so that a large span of reactant conversions and product selectivities can be obtained. Proper selection of the catalyst particle size, feed gas composition, and inlet gas flow rate allows intrinsic kinetic data to be collected where both inter- and intraparticle mass and heat transport effects are negligible.49,50,53 3.1.2. Feed Manifold. Referring to Figure 1, the MARS feed manifold consists of seven mass flow controllers (MFCs). The first four MFCs allow nitrogen (N2), n-butane (n-C4) or 1,3-butadiene (1,3-C4d), oxygen (O2), and helium (He) to be blended online to generate the desired composition of mixed reaction feed gases. The remaining three MFCs in the manifold are used to introduce a constant flow rate of dilution N2, purge N2, and calibration gas to various system elements. The exit ports of the first four MFCs are connected to a four-port valve so the resulting gas mixture can be directed to either a common manifold that supplies the reactor MFCs (on position, as shown in Figure 1) or to the vent (off position). When the four-port valve is rotated to the on position, the supply pressure of the feed gas mixture is maintained constant by a primary back-pressure regulator (BPR). The feed gas that is not consumed by the reactor MFCs represents an excess gas flow that is directed to the BPR vent. The vent feature allows the flow rate through each reactor to be varied without changing the set points of the various MFCs in the reactor feed manifold. The maximum flow rates used in all six reactors are set so that their sum is at least 40 mL/min less than the sum of the flow rates of the first four MFCs that are used to generate the desired reactor feed gas composition. A small slip stream taken from this vent gas source is directed to one port of the multiport sampling valve for subsequent GC analysis. When the four-port valve
Ind. Eng. Chem. Res., Vol. 44, No. 16, 2005 6439 Table 2. Feed Gas Types and Composition (in vol %). lean hydrocarbon rich hydrocarbon extra-rich hydrocarbon
Figure 3. MARS-3 system.
is rotated to the off position, the feed gas mixture is sent directly to the vent while a constant flow rate of purge N2 is introduced to the feed gas manifold. A secondary BPR is located on the vent with a set point so the resulting pressure matches the one generated by the primary BPR. This ensures that the flow rate of the feed gas does not undergo any significant surges due to pressure fluctuations when the four-port valve is rotated back and forth between the on and off positions. The purge N2 supply is used as one of the reactor system safety features. If an unsafe condition is detected, the automation system will rotate the four-port valve so that the purge N2 is directed to the manifold that supplies feed gas to the reactor MFCs. Examples of unsafe conditions are high or low reactor temperatures, excessive reactor feed gas supply pressures, or a hood crash. The purge operation causes the reaction gas that is present in each leg of the reactor feed system to be displaced by inert N2. Consequently, any hydrocarbon and O2 components are safely purged from the system to eliminate the possibility of a deflagration. Independent experiments are performed prior to the initiation of any detailed catalyst testing campaign to assess the potential contribution of homogeneous gasphase reactions. If homogeneous gas-phase reactions are present at appreciable levels in the product gas transfer lines, a dilution N2 flow can be introduced into the exit line of each reactor as a means of reducing the bulk gas concentrations of O2 and various organic reaction products. The dilution N2 stream can also be diverted to vent or halted before the product gas is sampled so that the actual gas composition can be determined. Experience with the dilution N2 feature shows that its usage is application-dependent.
n-C4 or 1,3-C4d
O2
1 9 25
10 10 25
The calibration gas MFC is maintained at a constant flow rate (30 mL/min) of a gas mixture containing a known composition of n-C4 or 1,3-C4d, CO, CO2, N2, and He to the multiport sampling valve. The calibration gas is used to evaluate the GC analytical system performance during the course of a catalyst test. It also provides data needed for online evaluation of the thermal conductivity detector (TCD) relative response factors using N2 as the reference component. To conserve consumption of the calibration gas, a flow is only established 2 min before the start of the GC analysis cycle to fill the sample loop. The flow is then halted once the calibration gas sample is injected. 3.1.3. Feed Gas Composition. Figure 1 shows that the C4 reactant (e.g., n-C4, i-C4d, i-C4, or 1,3-C4d) is supplied as one component in a ternary gas mixture, where N2 and He are the remaining two components. The composition of the ternary mixture was typically specified as x% C4/x% N2/(100 - 2x)% He (balance) where x ) 1, 10, or 25. The presence of N2 at the indicated concentration allows it to be used as a GC internal standard without saturating the TCD output signal of the GC analytical system. Because helium is also used as the GC column carrier gas, its elution to the TCD does not result in any overlap with other possible coeluting components. Previous work with the MARS-130 used a binary mixture of either 10 vol % n-C4/ 90 vol % N2 or 10 vol % 1,3-C4d/90 vol % N2. However, the resulting GC peak for N2 that eluted from the molecular sieve column saturated the TCD output signal at the given detector sensitivity setting because of the high concentration of N2 relative to the other components. This prevented N2 from being correctly quantified in the GC peak integration method. Hence, N2 could not be used as an internal standard. For this reason, the external standard method was used, but this approach introduced a larger degree of uncertainty in the GC peak quantification because a reference component is not used. Three different feed gas types are often used in MARS applications, although this is not a limitation. Typical compositions when n-C4 or 1,3-C4d was used as the C4 hydrocarbon with a cofeed of O2 are shown in Table 2. Other feed gas compositions are generated by blending helium and steam with the n-C4 or 1,3-C4d and O2 mixture. 3.1.4. Sampling System. The final key mechanical component in the MARS is a heated valve oven enclosure (model HVE3, Valco Instruments Co., Inc., Houston, TX) that houses a 17-port stream-selection valve, a 4-port flow-directing valve, and a 10-port gas sampling valve. Figure 4 is a photograph of the oven enclosure with the cover removed that shows the multiport valve arrangement. This ensemble of multiport valves provides a means for online sampling of the product gas from each of the six reactors, the feed gas, and the calibration gas. Detailed explanations of the stream flow paths and other key aspects are provided in a related publication devoted to the online GC method,34 so only a brief overview of the sampling system is given here. The 17-port valve contains a total of eight positions. This allows one of eight possible input streams to be
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Figure 4. Heated valve enclosure that shows the installation details of the various multiport valves used for directing gas flows and collecting gas samples for GC analysis.
selected and then directed to a single output port. The remaining seven streams are then directed to their respective vent ports. The input streams include the reactor product gas streams (six positions), the reaction feed gas stream (one position), and the calibration gas stream (one position). Because each of these streams requires a single inlet port and a single outlet port on the 17-port valve, the total number of ports utilized is 8 samples × 2 ports/sample ) 16. The last port (port 17) is assigned to the common outlet port. The common outlet port of the 17-port valve is connected to a 4-port valve. The 4-port valve further directs the selected process gas stream to either the inlet port of a two-position 10-port GC sampling valve (on position) or to a vent (off position). When the 4-port valve is rotated to the on position, the sample stream simultaneously fills two 500-µL loops on the 10-port valve. Rotation of the 10-port valve connects the GC helium carrier gas to the sample loops, which simultaneously flushes the two samples to the GC system injection ports. This time also marks the start of the GC temperature program. The valve enclosure oven is typically maintained at 200 °C, with temperature control provided by an independent proportionalintegral-derivative (PID) system. Oven heating is provided by a 150-W cartridge element that is situated inside an aluminum block. In the original MARS reactor operating protocol, the 4-port multiport valve shown in Figure 1 was rotated at the onset of an experiment so that purge N2 was introduced to all six reactors. This valve position also directed the feed gas to the 17-port valve, which was then directed to the GC for subsequent analysis as explained above. In one of the commonly used catalyst testing protocols, two feed samples were collected before the 4-port multiport valve was rotated back in the opposite direction. The latter valve rotation allowed feed gas to be reintroduced to all six reactors. Product gas samples were then sequentially taken from each reactor and analyzed by the GC. The 17-port valve was rotated for three cycles so that a total of 18 product samples were analyzed (6 samples per cycle × 3 cycles). The 4-port multiport valve was then rotated to introduce N2 to all six reactors, and two feed gas samples were collected again. This cycle of 2 feed samples followed by 18 product gas samples was then repeated until a certain condition or total elapsed run time was achieved.
Figure 5. Prototype MARS-2 fixed-bed microreactor: (a) assembled; (b) disassembled.
Experience in using this approach showed that intermittent exposure of the catalyst samples to N2 followed by the reaction gas resulted in a modification in the catalyst performance as determined by comparing conversion versus selectivity data obtained before and after the introduction of N2. For this reason, the 4-port valve was set as shown in Figure 1 so that the catalyst was always exposed to the reaction gas for the duration of the experiment. To obtain a feed gas sample without exposing the catalyst to N2, the excess feed gas vented from the 4-port multiport valve was connected to the 17-port multiport valve and maintained at a constant flow rate. In addition, a constant flow rate of calibration gas was also maintained to the 17-port multiport valve. The sampling sequence that was used in the modified method followed the order calibration gas, feed gas, reactor 1 product gas, reactor 2 product gas, ..., reactor 6 product gas. 3.1.5. Reactor Designs. Three different reactor designs were used during the course of the study. Two of these designs were associated with MARS-2, while the third design was used with MARS-3. In all three of these designs, the catalyst sample was maintained as a fixed bed versus a fluidized bed. a. Original MARS-2 Reactor Design. The original reactor design for MARS-2 was a U-tube configuration and is shown in Figure 5 in both assembled and disassembled forms. The catalyst zone consisted of a standard 6.35-mm (1/4-in.) o.d. type 316 stainless steel (SS) tube with a wall thickness of 0.889 mm (0.035 in.), which translates to an internal diameter of 4.572 mm. The tube length in the catalyst zone was 51 mm (2 in.), which corresponds to an empty tube volume of 0.84 mL. The reactor feed gas was introduced through a 3.175mm (1/8-in.) o.d. type 316 SS tube, while the product gas exited through a larger 6.35-mm (1/4-in.) o.d. type 316 SS tube. The various tubes were connected using commercially available Swagelok® fittings. Heating of the reactor tube was achieved by placing it in an isothermal fluidized sand bath in which silicon carbide (Agsco Corp., No. 240) was used as the fluidized heattransfer medium. The inlet and exit gas tubing could be easily connected for installation and removal because the reactor had the general shape of a U-tube. The external wall temperature of the microreactor at the midpoint of the catalyst bed was used as the input signal for the reactor temperature controller. The temperature was measured using a 1.5875-mm (1/16-in.) o.d. type J
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Figure 6. Improved MARS-2 fixed-bed microreactor: (a) assembled; (b) disassembled.
thermocouple that is held in place by copper wire. The inner reactor temperature is monitored using another 0.508-mm (0.02-in.) o.d. type J thermocouple whose tip is located in the upper one-third part of the catalyst bed. In a typical experiment, the reactor was packed with about 0.50-2 g of 250-425-µm particles (-40 to +60 mesh). The particles were held in place by sandwiching them between thin layers of glass wool. Experience in using the reactor described above showed that the various tube fittings would leak if the reactor was reused after running a typical reaction protocol having a duration of 18-20 h for several cycles. These leaks were attributed to the thermal expansion and contraction of the tube fittings, which were significant between room temperature and 500 °C. Another secondary leak source was attributed to the exposure of these fittings to the abrasive silicon carbide fluidized sand bath particles. The presence of leaks was verified by a postreaction pressure test on a used reactor tube using house air at 621 kPa (90 psig). Attempts to tighten the fittings beyond the manufacturer’s (Swagelok®) number of turns failed to eliminate the leaks. Hence, an alternate reactor design was developed, which is described below. b. Improved MARS-2 Reactor Design. The improved MARS-2 fixed-bed microreactor design is shown in both assembled and disassembled form in Figure 6. The reactor body that holds the catalyst consists of a standard Swagelok® 6.35-mm (1/4-in.) VCR type fitting with an inside diameter of 4.83 mm (0.19 in.) and an overall length of 56.6 mm (2.23 in.) (Swagelok®, Cat. No. SS-4-VCR-61). The empty tube volume is ca. 1 mL. The body is connected to the inlet and outlet tubes using VCR type nuts. A 0.508-mm (0.02-in.) o.d. thermocouple is inserted through the side of the VCR body at the midway point and sealed using a 1.5875-mm (1/16-in.) graphite ferrule (Chrompack, Cat. No. 4763). Sealing between the reactor body and these fittings is performed by using a type 316 SS flat gasket (Swagelok®, Cat. No. SS-4-VCR-2-VS). Experience in using this alternate reactor design showed that the fittings could be easily removed, even after conducting 10-15 catalyst testing cycles having a duration of at least 18-20 h for each cycle. The metal seals could typically be reused three to five times before they were replaced. The graphite ferrule was checked for leaks prior to each run and replaced as needed. Minor tightening of the ferrule nut
Figure 7. MARS-3 fixed-bed microreactor: (a) assembled; (b) disassembled.
Figure 8. Method for packing the MARS-3 reactor using glass wool as a catalyst bed support without any inert filler. The sketch on the left shows the placement of the MARS-3 reactor in the furnace, while the one on the right shows details on the method used for packing the reactor.
was usually sufficient for eliminating a leak. When this procedure did not eliminate the leak, the ferrule was replaced. c. MARS-3 Reactor Design. A photograph of the fixed-bed microreactor design used in the MARS-3 is shown in both assembled and disassembled forms in Figure 7. The two methods used to pack the MARS-3 reactor with catalyst are illustrated in Figures 8 and 9. The main differences are the magnitudes of the void volumes of the reactor sections that exist upstream and downstream of the catalyst zone. The reactor consists of a straight 25.7-cm (10.125-in.) section of standard 6.35mm (1/4-in.) o.d. × 4.572-mm (0.18-in.) i.d. type 316 SS tubing. The body is connected to the feed gas tube (top) and the product gas tube (bottom) using Swagelok® tees with 6.35-mm (1/4-in.) fittings. A 1.5875-mm (1/16-in.) o.d. type J thermocouple is used as the temperature control input. It is inserted through one leg of the top tee and sealed using a 3.175-mm (1/8-in.) port connector. Figure 8 shows that the distance from the thermocouple tip to
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Figure 9. Method for packing the MARS-3 reactor using glass beads as both a catalyst bed support and an inert filler to reduce the reactor voidage. The sketch on the left shows the placement of the MARS-3 reactor in the furnace, while the one on the right illustrates details on the reactor packing method.
the top of the reactor tube is ca. 15.24 cm (6 in.). The tip is positioned so that the thermocouple penetrates the catalyst bed to a depth between 1.5875 and 2.38 mm (1/16 and 3/32 in.). The thermocouple tip is maintained in the center of the reactor tube by using a spoked metal disk whose hub contains a 1.5875-mm (1/16-in.) drilledthrough opening. The outer diameter of the disk was selected to have a sliding fit inside the reactor tube. The open area between the spokes and the center hub accounts for more than 80% of the total reactor crosssectional area so that any undesired effects, such as additional pressure drop or gas flow maldistribution, are negligible. A second 1.5875-mm (1/16-in.) o.d. type J thermocouple is inserted through the bottom tee and used for sensing of the catalyst bed temperature at about one-third the distance down the bed. The top section of the reactor was either maintained empty (Figure 8) or filled with spherical glass beads with a dp of 425-500 µm (Figure 9). In the latter case, the lower-sensing thermocouple was removed and the bottom of the reactor was also filled with 1.76 g (1.12 mL) of 425-500-µm spherical glass beads that were supported by a 20-µm type 316 SS sintered-metal frit. Another 20-µm type 316 SS sintered-metal frit was located on the top of the glass bead section to prevent the catalyst particles from fluidizing at high gas velocities during gas upflow operation. The use of glass bead packing as an inert filler (Figure 9) was adopted during catalyst testing for the partial oxidation of 1,3-butadiene to furan. The effects of both reactor voidage and reactor materials of construction were investigated as part of an effort to minimize homogeneous gas-phase and heterogeneous wall-catalyzed reactions that were identified as being operative (see part 2 of this series of papers). In some cases, experiments were conducted using the reactor packing method illustrated in Figure 8. Here, the catalyst bed was supported using a glass wool plug that was inserted from the bottom of the reactor tube. Because the temperatures of the empty reactor volume above the catalyst bed and below the glass wool plug were similar to those maintained in the catalyst bed, homogeneous gas-phase reactions involving both the feed and product gases were shown to occur in these zones. Part 2 of this series of papers compares the differences in performance
Figure 10. Fluidized sand bath design used for heating an individual MARS-2 reactor.
that are obtained when the two different schemes for packing of the reactor as shown in Figures 8 and 9 are utilized. 3.1.6. Reactor Heating. The MARS-2 U-tube and the MARS-3 straight-through reactor designs, which are shown in Figures 6 and 7, respectively, required the use of special-purpose heating hardware so the reactors could be easily installed and removed. The temperature of each MARS-2 U-tube reactor could be individually controlled using the fluidized sand bath design shown in Figure 10. Custom-designed furnaces were used for the MARS-3 reactors, which are shown in Figure 11. Detailed mechanical design drawings for both the fluidized sand bath and furnace are included in Appendix A of the Supporting Information. 3.1.7. Liquid Feed Vaporization. Safe and reliable methods for vaporization of liquid reactants, or for the addition of one or more nonvolatile components to the feed gas, such as water, are often required for detailed catalyst performance evaluations. Steam was initially generated in the MARS-2 by contacting the dry feed gas mixture for a particular reactor with water using a gas saturator. A typical saturator design fabricated from glass is illustrated in Figure 6.28 by Anderson and Pratt.54 The temperature of the gas-liquid dispersion was selected to produce a known vapor pressure of water. Assuming that thermodynamic equilibrium was achieved in the gas phase, Raoult’s law was then used to estimate the vapor-phase mole fraction of water. In practice, the mole fraction of water was selected and the temperature required to achieve the target vapor pressure was subsequently determined using Antoine’s vapor pressure equation with the appropriate constants. Online GC analysis of the resulting water peak was used to quantify the resulting vapor composition. If necessary, the saturator temperature was adjusted to approach the desired target composition. The gas saturator approach described above, in which a carrier gas stream is bubbled through the liquid of interest, is a commonly used approach for introducing nonvolatile or volatile liquids in laboratory-scale reactor systems. However, saturator operation is not readily
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Figure 13. Vaporizer designs used for generating steam or other vapors from nonvolatile liquids: (a) vaporizer with upflow of gas and liquid with an internal thermocouple for temperature control; (b) vaporizer with downflow of gas and liquid with an external thermocouple for temperature control.
Figure 11. Split-tube furnace design used for heating an individual MARS-3 reactor.
Figure 12. Typical breakthrough response of 1,3-butadiene from a gas saturator used in the MARS-2 system. Conditions: water volume in the saturator ) 250 mL; flow rate of the dry gas at the saturator inlet ) 25 cm3/min (at NTP); dry gas composition at the saturator inlet ) 2 vol % 1,3-butadiene, 10 vol % N2, 88 vol % (balance) He.
automated, especially when various feed compositions are required as part of the catalyst testing protocol. In addition, experience using the above approach showed that up to a few hours of operation were sometimes required for the exit vapor composition to approach a steady state after a new set point was introduced to either the carrier gas flow rate or the saturator temperature (Figure 12). This transient behavior lengthened the testing protocol and also introduced complications into modeling of the intermittent catalyst performance data. Finally, because O2 is one of the components present in the carrier gas and it contacts a hydrocarbon liquid in the saturator, measures must be taken to ensure that a flammable vapor mixture cannot be formed during any of the reactor system operational stages. This aspect is particularly important during startup and either normal or emergency shutdown
where the concentrations of O2 and hydrocarbon are variable. For these reasons, a direct method for vaporization was developed where liquid was metered to a vaporizer from a micropump and then mixed with other reaction gases to create a homogeneous feed gas containing the vaporized component of interest. The quality of liquid vaporization in laboratory-scale heat exchangers is primarily affected by the following factors: (1) delivery of low liquid flow rates (e.g.,
