Integrated Microreactor System for Gas-Phase Catalytic Reactions. 3

Ind. Eng. Chem. Res. 2007, 46, 8319-8335

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Integrated Microreactor System for Gas-Phase Catalytic Reactions. 3. Microreactor System Design and System Automation D. J. Quiram and K. F. Jensen Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Martin A. Schmidt† Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

P. L. Mills,* J. F. Ryley, M. D. Wetzel, and D. J. Kraus DuPont Company, Central Research & DeVelopment, Experimental Station, Wilmington, Delaware 19880-0304

A novel microreactor system for gas-phase catalyzed reactions is described for which the process components consist of modular circuitlike boards that fit the slots of a commercial computer chassis. The reactor board contains two parallel reactor channels on a single multilaminate silicon chip with independent controls for the gas flow rate and the reaction temperature. All fluidic interfaces between the various boards are performed using standard metal tubing and fittings. System automation was performed using a process logic controller with a human-machine interface for display of sensors and monitoring of process control loops. It was found that the control loop could successfully operate at 500 Hz with all 24 microreactor heaters being under closed-loop PID control. 1. Introduction Microreaction technology (MRT) and miniaturization of microprocess devices, which has applications ranging from fundamental research to small-scale commercial production, has been a subject of increasing technical and business interest over the past decade. A recent market analysis for microelectromechanical (MEMS) and microsystems technology (MST), which includes a spectrum of 26 different product areas, forecasted that sales will increase to $57B by 2009, corresponding to an average growth rate of 11% since 2005.1 Key technical advances and developments in MRT over the past decade, which is one of the product areas within the MEMS/MST arena, have been largely reported at the annual International Conference on Microreaction Technology (IMRET-1 to IMRET-9)2,3 and the annual International Conferences on Microchannels and Minichannels (ICMM-2003 to ICMM-2005),4-6 which now includes nanochannels (ICNMM-2006 and ICNMM-2007).7,8 Some other sources of recent developments have been captured in special journal issues,9,10 dedicated monographs,11,12 edited books,13 and the references cited therein. A review of this literature indicates that considerable growth has occurred in both the scope and applications of MRT with considerable focus on microprocess component development. However, integration of microprocess system components, such as mixers, heat exchangers, reactors, and separators, into an automated platform with sensors and actuators is one of the major challenges for gaining broader user acceptance and utilization.14-16 The largest number of integrated miniaturized systems has been developed in the area of micro-total analysis systems (MicroTAS), which has gained sufficient research momentum to be the subject of a dedicated journal.17 Progress in the science and technology of MicroTAS * Corresponding author. Current address: Department of Chemical and Natural Gas Engineering, Texas A&M UniversitysKingsville, Kingsville, Texas 78362-8202. E-mail: [email protected]. Tel.: (+1) 361 593-4827. Fax: (+1) 361 593-2106. † Microsystems Technology Laboratories.

since the mid-1990s has also been reported on a nearly annual basis at dedicated conferences18-20 and other related symposia.21 However, one key difference that emerges between MicroTAS and MRT systems is the former initially involved implementation of an analytical method using a packaged approach, which itself could be part of an integrated MRT platform. The scope of MicroTAS has been expanded from primarily chemical analytical applications in microscale channels18 to include life sciences, microfluidics, separations in both microscale and nanoscale channels, and integration with other microprocess components.19,20 A review of MicroTAS within the broader field of process analytical chemistry, which also includes other types of microanalytical and related systems, has recently appeared.22 Although a MicroTAS device does not necessarily contain a microreactor, recent work has produced some sophisticated designs that integrate components using methodologies that could be adapted for use in MRT systems. For example, Sandia has developed µChemLab, which is a handheld analytical instrument for detection of chemical warfare agents, toxic industrial chemicals, and other organic solvents.23-25 The packaged system contains a combined sample collector/ concentrator, a micro gas-chromatographic (GC) column, either a surface acoustic wave (SAW) detector or a microthermal conductivity detector (TCD), and a module for controlling the carrier gas flow. The sample collector consists of a heated microscale membrane that is covered by a chemically absorbent gel. The gel absorbs the desired analytes and then releases them upon heating into the inlet side of the micro-GC column. The SAW detector is actually a dual-function sensor that is chemically selective and generates a signal when exposed to an analyte. The sample collector, GC column, and detector are mounted on a novel printed circuit board (PCB) that provides both electrical and fluidic interconnections. In addition to these previous components, the PCB also contains a miniature threeway valve for bypassing the GC column and a gas inlet/outlet manifold. The components are controlled through a userinterface that communicates to a separate board that contains a

10.1021/ie0702577 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/08/2007

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microprocessor and memory along with the power control. Unfortunately, no documentation exists on design of component integration, such as fluidic interconnections, so that replication of this design would be tedious. Another prototype micromachined reaction system has been described26,27 that employs a mixed circuit board (MCB) whose design features are somewhat similar to those for the Sandia’s µChemLab.23-25 This device has two pumps and flow sensors operating in parallel with a multifluid mixing and reaction chamber that are combined into a single device. The MCB consists of a Pyrex top layer that is anodically bonded to a silicon bottom layer. However, all of the micromachining is performed in the Pyrex layer, and the silicon is used only to seal the channels. The microfluidic components are then mounted on the Pyrex layer by another anodic bonding step. The authors do not describe any special techniques for performing robust electrical interconnections, however. The development of an analytical system that uses a rotary valve to integrate various functions to create a lab-on-a-valve versus a lab-on-a-chip design was recently described for environmental applications.28 This approach has more flexibility for being modified versus lab-on-a-chip devices since the architecture in the latter is created by more sophisticated fabrication processes. Consequently, modifications are not feasible when the nature of the sample is applicationdependent. A more detailed discussion of the above devices and other approaches used for developing microprocess systems based on mixed circuit board concepts and other related MEMS-like technology is available in a separate review.29 The development of novel systems that employ circuit-board technology is actually part of a larger effort on development of microchemical plants, which has been the subject of several dedicated conferences in Japan.30,31 Even though connecting parallel reactors and other microprocess system components together to assemble an integrated system is a straightforward concept, various challenges arise when developing robust designs that also meet design objectives related to fabrication, assembly, operation, maintenance, process automation, and process safety. A multidisciplinary approach must be used that utilizes the various engineering disciplines, computer and materials science, chemistry, physics, and biology to create such a system. In Part 1, the design of a novel gas-phase microreactor was described that consists of a multilayer laminate structure with onboard flow sensors, temperature sensors, and platinum-based heaters.32 Fabrication of the device using a multistep MEMS processing sequence produced a silicon chip-based reactor die with two parallel channels in which a platinum catalyst was deposited as a thin film. Rigorous protocols of electrical and other various tests were devised to evaluate the reactor die performance under cold conditions, which showed that all design specifications were either achieved or exceeded. Development of a robust microreactor packaging scheme and evaluation of its performance was subsequently described in Part 2 of this series.33 The package utilized an existing device based on Known Good Die (KGD) technology as the starting platform, which was originally used for testing of early microcomputer integrated circuits.34 Interfacing of the KGD, which was constructed of an engineering plastic, to heated reactor feed gas and product transport tubing was also demonstrated. The KGD is a key component in a novel integrated system whose platform utilizes a multiple-slot computer chassis with modular boards to perform the required process functions. The latter is the subject of the current paper.

Figure 1. Automated, integrated microreactor system (AIMS) laboratory reactor system.

The main objective of this work is to develop a scale-up microreaction system with integrated process controls and safety-monitoring features using the novel microreactor chip and the chip-packaging scheme that were described in Parts 1 and 2, respectively.32,33 Scale-up is interpreted here as the various processes needed to transform the single microreactor design package into an integrated system where multiple reactors are operated in parallel. This particular definition for scale-up, which is also commonly referred to in microreaction technology as “numbering up”, differs from the conventional one used to describe translation of reactions conducted in laboratory-scale contactors to larger volume pilot plants.9,10,35,36 One motivation for developing this system, which is called the Automated, Integrated Microreactor System or AIMS for short, is to identify challenges that occur when various process functions are implemented using modular circuit boards as the design basis. These key challenges include packaging of microreactors to handle numerous electrical and fluidic connections, methodologies for making electrical and fluidic connections between devices in a scaleable fashion, and the development of a realtime control system for automated operation and safety monitoring. The latter system is a critical component because of the nature of the platform being used, the system complexity, and the safety aspects associated with the chemistries that are used as test reactions. Another objective is to develop solutions to these challenges and to demonstrate how the various circuit boards could be incorporated into a system platform that consists of a commercial computer chassis. The performance of the packaged system is evaluated in Part 4 of this series37 using the oxidation of methane and the oxidation of ammonia as test reactions over a Pt film catalyst in independent sequences of experiments. 2. Microreactor System Design 2.1. Overall Process Configuration. The automated, integrated microreactor system (AIMS) was designed to be functionally equivalent to a heterogeneous gas-phase catalyst testing system with parallel reactors, such as the multiple automated reactor system or MARS.38 The key system components of the AIMS are shown in Figure 1. These components include the following: (i) a feed gas manifold, which consists of pressure regulators and a bank of mass flow controllers (MFCs) for online generation of the reaction feed gas mixture; (ii) a reactor manifold, which consists of dedicated MFCs that meter the feed gas mixture to individual parallel-operating microreactors; and (iii) an online reactor feed and product gas analysis system. A more detailed process flow diagram of the AIMS is shown in Figure 2. The reactor dimensions, catalyst particulars, and ranges for the key operating parameters are summarized in Table 1.

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Figure 2. Process flow diagram of the AIMS scale-up microreactor system. Table 1. Key Reactor System Operating Parameters reactor geometry

duct

width, mm depth, mm length, mm volume, mm3

0.5 0.5 11 2.75 catalyst

type

film

material width, mm depth, µm length, mm weight, ng

Pt 0.5 0.1 11 11.6 max operating limits

parameter

value

T, °C P, bar (abs) Q, sccm GHSVa, hr-1 WHSVb, hr-1

600 1.5 10 1.1 × 1012 6.7 × 107

a

GHSV is defined as (m3 of feed gas)/(m3 of catalyst)‚h. b WHSV is defined as (kg of feed gas)/(kg of catalyst)‚h.

One key difference between the AIMS and the conventional MARS system is the use of MEMS and MST components, such as microvalves and micro-MFCs, to replace conventional components. These components are mounted on standard circuit boards that fit into the backplane of a multislot computer chassis. Another key difference is the greatly reduced footprint of the

Figure 3. Process gas manifold for the microreactor system.

AIMS. Conventional online gas sampling valves and GC analytical equipment were utilized since a microscale analytical system would have required detailed development because of the need to implement a multidimensional method.39 2.2. Primary Feed Gas System. The primary feed gas system allows the user to generate a steady-state reaction composition containing a fuel, an oxidizer, and a process diluent, such as nitrogen, that is continuously supplied to the downstream reactor boards. The feed system consists of gas cylinders, dedicated

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Figure 4. External tubing connections to various AIMS circuit boards.

pressure control elements, MFCs, and various devices for ensuring safe operation, such as preventing overpressurization of critical process elements. Certain key subsystem components, such as gas shutoff and gas MFCs, are located on a dedicated feed gas mixing board. These elements are shown in Figure 2 within the dashed subregion to the right of the gas cylinders. The individual feed gas components shown in Figure 2 are supplied from standard size 1A gas cylinders purchased from commercial sources. Each of the gas supply cylinders are fitted with two-stage pressure regulators where the maximum range for the second stage is set at 200 psig. Pressure relief valves, which are set to open at 80 psig, are connected downstream of each two-stage regulator to prevent overpressurization of any downstream components. In addition, the fuel and oxygen gas cylinders are equipped with flame arrestors. The second-stage pressure regulators used on the gas cylinders are not designed for precise pressure control for any downstream gas flow control devices. In addition, the regulators are connected to the gas cylinders inside a ventilated cylinder closet, which is located some distance upstream from the gas supply connection point for the feed gas mixing board. To provide a more stable gas feed supply pressure to the feed gas mixing board, the effluent gas from each two-stage regulator is connected to a dedicated single-stage forward pressure regulator located adjacent to the AIMS. These regulators and related hardware are shown in Figure 3. The effluent from the singlestage regulators is directed through 0.5 µm in-line filters before entering the board to eliminate or minimize particulate matter from entering the downstream microvalves.

The fuel gases used here were methane (CH4) and ammonia (NH3), since their respective oxidation reactions are used in Part 4 of this series to evaluate the reactor system performance using a Pt film catalyst.37 Other organic and inorganic substrates could be substituted to study other types of homogeneous or heterogeneous catalyzed gas-phase chemistries, such as hydrogenations, halogenations, reforming, and hydration, provided the feed system is configured to meter the given fluid state (gas or liquid) of the chosen reactants. For catalytic reactions, the catalyst could be deposited as a film on the underside of the silicon nitride membrane or loaded in the reaction channel as either small particles or some other form, such as a catalytic mesh or structured packing.40 In the applications discussed in Part 4,37 the fuel is supplied from a single cylinder as a binary mixture diluted with either inert helium (He) or nitrogen (N2), such as 10% hydrocarbon and 90% N2 or 10% hydrocarbon and 90% He. If NH3 is used as one of the reactants, krypton (Kr) is used as the diluent since N2 is one of the oxidation reaction products. The presence of Kr inert gas also increases the dew point temperature of the binary mixture so it cannot condense in the feed manifold. The process diluent is either He or N2 when a light hydrocarbon is used as the reactant or He when NH3 is used as the reactant, so the resulting fuel/oxidizer/inert composition is below the lower flammability limit for safe operation. The oxidant is pure oxygen (O2). Referring to Figure 2, metering of the individual reactor feed gas constituents to generate the desired feed gas composition actually occurs on the feed gas mixing board that is located


Figure 5. MEMS mass flow controller packaging: (a) original Redwood MEMS-Flow mass flow controller and (b) repackaged Redwood flow manifold.

Figure 7. Repackaged electronics for the Redwood flow manifold.

Figure 6. Original electronics package as received from Redwood MicroSystems.

within the AIMS chassis. This board contains three parallel manifolds that control the process gas flow rates using a Redwood Microsystems MEMS-Flow MFC and three Redwood Microsystems normally closed shutoff valves (SOVs).41 The composition of the reactor feed gas mixture is defined by setting the flow rates for each of the process gases using dedicated MFCs. Figure 2 also shows that each flow manifold has two normally closed SOVs in parallel whose exhausts are connected to a common feed of a Redwood MicroSystems MEMS-Flow MFC. The inlet side of the primary SOV is connected to one of the process gases (i.e., the fuel gas, O2, or process diluent) that are supplied from the gas cylinders. The inlet side of the secondary SOV is connected to a common purge gas manifold, while the exit side is attached into the inlet side of the MFC. During startup and any emergency or nonemergency shutdown of the process, gases are purged from the system by closing the SOV that is connected to the process gas supply while simultaneously opening the SOV that is connected to the purge gas valve. Another normally closed SOV that is located after the MFC can also be used to isolate that section of the manifold leg from the rest of the system. The same arrangement is duplicated for each of the three process gas streams. This feature can be readily duplicated for the addition of other reaction gases, such as second fuel gas source or secondary reactant, if desired.

Control of gas flows in the AIMS differs from a conventional MARS parallel reactor system38 since all of the SOVs are configured to be normally closed. This means that, when electrical power is applied, the shutoff valve driver is actuated so that gas can flow through the valve. Ideally, the system should purge itself with inert N2 if there is a power failure, i.e., all valve operators should fail open in the absence of electrical power (so-called normally open) so that reactive gases are not isolated between any two closed valves. The particular SOVs used here (Redwood MicroSystems) were not available in a normally open configuration, which could have resulted in trapped process gases in each leg of the feed gas mixing board in the event of power failure or emergency stop (so-called e-stop) condition. However, the pressure downstream of the feed gas mixing board is vented through a normally open valve. Although some process gases could remain trapped in the system, the small volume of the lines helps to minimize the safety hazard if this situation occurs. Another difference in the configuration of the MFC feed leg when compared to that used in a conventional reactor system is the absence of forward- and back-pressure regulators on either side of the MFCs. In a typical MFC gas supply leg used in laboratory-scale reactors or larger pilot-scale units, two pressure regulators are often used to maintain a constant gas supply pressure and constant pressure drop across the MFC. Maintenance of these constant conditions ensures that the MFCs provide proper control in the presence of either upstream or downstream pressure fluctuations since the control element is based on a small, constant gas flow through a capillary bypass leg. This regulator arrangement cannot be used with the Redwood MicroSystems MFCs because the control principle upon which they operate is based on adjusting the pressure drop

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Figure 8. Temperature controller board.

across a flow restrictor. For this reason, their operation is sensitive to the inlet and exit pressures of the mass flow controller. In the AIMS feed gas mixing board, the Redwood MFCs operate with fixed upstream and downstream pressures, but these are set by their design specifications and are not normally adjusted during operation. 2.3. Reactor Feed Gas System. Inspection of Figure 2 shows that the fuel, oxidant, and process diluent gases exit their respective legs of the feed gas mixing board and converge to a single line so the resulting feed gas mixture can be directed to a common manifold. The manifold serves as a feed gas source for the downstream reactor boards, the feed gas analysis system, and for venting excess feed gas during regular operation or during an emergency situation. To ensure that the individual gases are blended to generate a uniform composition, they are passed through a 0.5 µm in-line porous filter before entering the reactor boards. Online GC analysis of the feed gas also provides independent confirmation of the feed gas composition uniformity. Figure 4 is a photograph that shows the external tubing connections between the feed gas mixing boards and the reactor boards. The first process element connected to the feed gas manifold is an external back-pressure regulator (BPR). The BPR maintains a constant pressure at the inlet side of the mass flow controllers used to meter feed gas to each microreactor channel. This feature also allows the user to vary the residence time of the reaction gas by specifying a series of feed gas flow rates without altering the flow rates of the upstream supply gases. In order for the BPR to maintain a stable pressure, the total flow rate generated by the feed gas mixing board should exceed the sum of the maximum total flow rates through all of the reactors by ca. 50 sccm. The actual minimum and maximum flow rates are defined by the orifice performance of the particular BPR. The downstream pressure is set at 35 psig according to the MFC manufacturer’s specifications. An external normally open valve is also located on the feed gas manifold and is used to exhaust any excess gas that could exist in the manifold tubing in the event of a power failure or an emergency-stop condition. Both the external BPR and the valve are isolated from the system by 1/ psig check valves. One check valve is placed upstream of 3 the vent valve, while the other check valve is located downstream of the back-pressure regulator. The check valves also

prevent any backflow of gases to the feed stream that could otherwise alter its composition. They also reduce the potential for atmospheric oxygen entering the feed gas and possibly forming an explosive mixture. Figure 2 shows that each reactor board has two separate connections where reactor feed gas can be introduced from the feed manifold. These connections serve as independent feed inlet points for two parallel Redwood flow manifolds (RFMs) that are used to control the gas flow rate to each channel of the microreactor die. A total of four microreactors can be operated since each die has two independently controlled reaction channels. This arrangement is an integral part of the scale-up system design. The RFMs are identical to those used for the feed gas mixing board except for their inlet and exit operating pressures. After exiting the RFMs, the feed gas flows to the DieMate manifold and microreactor. These devices are heated to 200 °C to prevent product condensation. The product transfer line that exists between the DieMate manifold and the exit of the system chassis is heat-traced. Once the product gas exits the transfer line, it is introduced to the online gas analysis system. A power failure or so-called e-stop condition results in process gas being trapped in the system. The pressure downstream of the RFMs in the reactor boards is essentially atmospheric, since these lines ultimately exhaust to vent. A small amount of pressurized mixed feed gas is trapped within the RFM, but its volume is only 0.270 cm3. Furthermore, the microreactor heaters have a time constant that is ∼3 ms, so they return almost immediately to the temperature of the microreactor die. A process hazards analysis was developed and provides a detailed safety analysis of the system. 2.4. Component Descriptions. 2.4.1. Redwood Flow Manifolds. The other MEMS components in the system, besides the microreactors, are the microvalves and micro-MFCs (Redwood MicroSystems, Menlo Park, CA).41 In order to reduce their size, the system components were not used as supplied in the standard manufacturer’s packages. Instead, the MFC was combined with three SOVs on a Type 316L stainless steel flow manifold (referred to hereafter as the Redwood valve manifold) that was specially designed for this particular application. The original MEMS-Flow MFC package as supplied by the manufacturer is shown in Figure 5a, while the final Redwood flow manifold


Figure 9. Side view (right) of the feed gas mixing board.

after repackaging with the onboard SOVs and MFC is shown in Figure 5b. The new manifold design allows the MFC and the associated SOVs to be mounted within nearly the same footprint as the original MFC package supplied by the manufacturer. The electronics for the Redwood flow manifold were also repackaged by mounting them on a custom-designed PC board. Figure 6 shows the original electronics packaging from Redwood MicroSystems, while Figure 7 shows the repackaged electronics. The custom-designed PC boards are fabricated using FR-4 laminate, which consists of multiple plies of epoxy-resinimpregnated woven glass cloth. The SOVs are normally closed valves, and they have a specified leak rate of 0.1 mL/min. The MFC proportional valve is normally open. The design inlet and outlet pressures of the feed gas mixing MFCs are 60 and 35 psig, respectively. The

design inlet and outlet pressures of the reactor MFCs are 35 and 0 psig, respectively. Both the MEMS-Flow MFCs and the microvalves have a design pressure of 150 psig and a burst pressure of 250 psig. Other than the microreactors, these devices are the most likely to fail due to an overpressurization condition. The operating temperature range of these components is between 0 and 55 °C. 2.4.2. System Boards. Packaging of the Redwood flow manifolds and microreactors is achieved by mounting the devices on standard 6U Compact PCI boards. The designation 6U refers the height of the board since the Compact PCI standard supports many different board heights. The boards are then inserted in a chassis, and the electrical connections between the boards are made through the backplane, ribbon cables, and rear I/O boards. Further details on the chassis design are given later in this section. All boards in the microreactor chassis are again

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Figure 10. Side view (left) of the feed gas mixing board.

fabricated using FR-4 laminate. The upper operating temperature limit of this material is 130 °C, and the UL flammability classification is 94V-0, which means samples self-extinguish within 10 s after flame application.42 An overview of the design basis for each board is described below. 2.4.2.a. Temperature Controller Board. The temperature controller board is shown in Figure 8 and consists of five CAL 3300 temperature controllers.43 These devices are used to control the temperature of the DieMate manifolds and the product transfer lines by manipulating the power input to their respective heaters. Two controllers are used for the DieMate manifolds themselves, and another two are used for the transfer lines from the DieMate manifold to the front panel. Two controllers per DieMate manifold are required since each reactor board requires a dedicated controller. 2.4.2.b. Feed Gas Mixing Board. The feed gas mixing board controls the composition of the feed gas to the reactors. The gas flow rates for each of the three feed gases (fuel, O2, and

diluent) are controlled by three independent Redwood flow manifolds. All of these gases enter the system at 60 psig. A right-side view of the board is shown in Figure 9, while a leftside view is shown in Figure 10. Figure 10 shows that the front side of the board contains the system feed gas inlet connections for fuel, oxygen, diluent, and purge gas. The tubing assemblies were constructed from 1/16 in. type 316L stainless steel tubing using Valco fittings. The T-junctions were custom-fabricated for this application. Silver solder was used to attach the 1/16 in. tubing to the T-junctions. Serial communication to the Redwood flow manifolds are available for diagnostic purposes through three four-conductor RJ-11 jacks on the front of the card. 2.4.2.c. Reactor Boards. Two reactor boards are used in the current version of the AIMS. This number was selected to demonstrate independent operation of two parallel reactor systems. Each board contains a microreactor die with two independent reaction channels along with two Redwood flow manifolds to control the feed gas flow rate to each channel. As


Figure 11. Isometric view of a reactor board that shows all external connections.

described in Part 2, the microreactor is mounted in a DieMate socket with fluidic connections made through a DieMate manifold.33 The boards are identical and interchangeable in the system chassis. Each reactor channel has separate controls for flow rate and temperature so that the same reaction can be studied using different parameters. The maximum reactor operating pressure is defined by the microreactor membrane material, which is 1.5 bar (abs) in the current design. An isometric view of the reactor board is shown in Figure 11, while side views are shown in Figures 12 and 13, respectively. The front of the board has gas inlets for the reactor feed and the purge gas. In addition, there are two gas outlets, one for each reaction channel. The inlet and outlet fittings are 1/ 16 in. type 316L stainless steel obtained from a commercial source (Valco Instruments, Houston, TX). Figure 13 shows the tubing layout for gas flow in the system. The tubing assembly was constructed from 1/16 in. type 316L stainless steel tubing and fittings, where the latter were also obtained from a commercial source (Valco Instruments, Houston, TX). The tubing junction that is visible in Figure 12 was machined from type 316L stainless steel. The tubing was attached to the junction and the DieMate manifold using silver solder. The most pressure-sensitive component of the tubing assembly is the microreactor membrane, which will fail in the vicinity of 7 psig.44 The reactor board contains the electronics for the Redwood flow manifolds as daughter and granddaughter cards, which are visible in Figure 12. The electronics needed to operate six temperature sensors on each microreactor channel lies underneath these cards. The ribbon connector, which is also visible in Figure 12, is used to transfer electrical signals directly from the reactor board to the heater circuit board. There are two fourconductor RJ-11 jacks on the front for serial communications with the Redwood flow manifolds. The product transfer lines from the microreactor are heated through the heating cartridges in the DieMate manifold using a clamshell heater that encases the tubing to the front of the card.

A photograph of the clamshell heater is shown in Figure 14, which was custom-fabricated from type 316L stainless steel. A flexible Kapton heater that is wrapped around the clamshell provides heating power through a 24 VDC input. The heaters were obtained from a commercial source (Technical Industrial Products, P/N KHLV-101/5).45 The temperature of the clamshell is monitored using two 100 Ω platinum resistive temperature devices (RTDs) from the same commercial source. One of the RTDs in the clamshell required a two-wire configuration (P/N RTD15-100L-SS116-1-PVC24-12-2) since it was connected to the CAL controller. The other RTD required a four-wire configuration (P/N RTD15-100L-SS116-1-PVC28-12-4) since it was connected to the process-control computer. Both types were 1/16 in. o.d. and had an overall length of 1 in. Both the DieMate manifold and the clamshell heater are insulated using ceramic fiber strip obtained from a commercial source (McMaster-Carr, P/N 87575K87).46 This insulation is visible in Figure 13. The fiber strip is 1 in. in width and has a thickness of 1/8 in. The manufacturer-specified thermal conductivity is 0.38 (BTU‚in)/(h‚ft2‚°F), and its maximum temperature rating is 2300 °F. 2.4.2.d. Heater Driver Circuit Boards. Each reactor board in the AIMS has a dedicated heater driver circuit board. One of these circuit boards is shown in Figure 15. It contains the electronic circuitry necessary to power 12 microheaters on a microreactor die. Both of the cards used in the system are interchangeable. Again, the left-most board when installed in the chassis is referred to as heater driver circuit board 1. The heater circuits are voltage-controlled so that the power input is reduced to the microreactor heaters as the temperature increases. This occurs because increasing temperature increases the resistance of the microheaters, so the current supplied to the microheaters is reduced to maintain a constant voltage. This is an important feature since the temperature increase due to ignition, such as the magnitude that can occur in a catalytic oxidation reaction, can be >100 °C and can occur within a few milliseconds.

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Figure 12. Side view (right) of a reactor board.

2.4.2.e. System Chassis. The AIMS is housed in a standard Compact PCI chassis manufactured by Kaparel.47 Figure 16 (front view) and Figure 17 (rear view) show the final AIMS chassis with all of the boards installed. This particular chassis is an integrated subrack (model no. 9U × 84 HP PS6090) that measures 19 in. wide × 15 3/4 in. high × 11 3/4 in. deep. It has a maximum capacity of 14 6U, 64-bit, or 32-bit Compact PCI cards and up to 2 doublewide (8HP) 6U power supplies. The chassis includes dual 12V DC blowers to cool the system components and to reduce the accumulation of process gases inside the chassis. They are located in the top compartment of the chassis as shown in Figure 16. These particular blowers are rated for 440 CFM of air and were obtained from a commercial source (Rittal Corporation, RiCool P/N SK 3344012).48 Electrical interconnections between the boards are made using a backplane and ribbon cables between rear I/O cards. In addition, there are ribbon cables on the front side of the chassis that connect the reactor boards to the heater driver circuit boards. The rear I/O cards with the ribbon cable connections are visible in Figure 18, which shows a rear view of the AIMS without the back cover panels. The rear I/O cards are custom-built PC boards that are used to transfer signals from cables connected to the control computer to the system boards on the front. The connectors for the cables leading to the control computer are

visible in Figure 18. Ribbon cables are used to transfer connections between the rear I/O cards because there were not enough lines available in the backplane to make all of the connections needed. The backplane, which is visible in Figure 19, is a H-110 Compact PCI telephony backplane obtained from a commercial source.47 Two PS4400 backplane boards were used along with one PS1150 board. The backplane PCBs were slightly modified by removing selected chip capacitors that were not compatible with the amount of current being carried on the backplane. Only analog signals were transmitted by the backplane, so the digital communications capability of the backplane bus was not utilized. The details of all of the electrical connections are beyond the scope of this work and omitted for brevity. Detailed I/O net lists were developed of the connections that were needed between the boards and the location of each end of the connection. In addition, these lists cover all of the connections to the system-control computer and any external electrical signals such as those needed for the external normally open shutoff valve. Appendix A in the Supporting Information provides a list of engineering design drawings used for fabrication of all system components.


Figure 13. Side view (left) of a reactor board.

3. Microreactor Component and System Testing Procedures The various AIMS components and circuit boards were subjected to a series of electrical tests both on the absence and presence of electrical power. Some minor issues were identified, but these were readily corrected and incorporated into the final design. The detailed procedures used with more explanations are provided in Appendix B of the Supporting Information. 4. System Control and Process Monitoring 4.1. Overview. The integrated system consisted of various modular electronic boards contained within a computer chassis with essentially no external manually operated control devices. For this reason, a human-machine interface (HMI) was required before the system could be operated and used for any practical application. The control hardware and software presented special challenges that exceeded those typically encountered for a more conventional laboratory reactor system since it involved the use

of prototype modular boards for all key process functions. The primary challenges included managing the rather large number of input/output signals and meeting the high control-loop cycle rate without degradation of performance. In addition, the control computer was responsible for both safety monitoring and activating interlocks when dictated by alarms. A control computer was used for the process control loop while another system provided the operator interface, which is standard industrial practice for any reactor systems involving potentially explosive reactions. This section provides an overview of the system hardware, the design principles associated with the process control software, and the various process-control human-machine interfaces. Additional details are provided in Appendix C of the Supporting Information. 4.2. System Hardware. The process control and monitoring system consisted of two major components: (1) a programmable logic controller (PLC); and (2) the human-machine interface (HMI). A PLC was selected since it was based upon an

8330


Figure 14. Clamshell design used for heating the product transfer lines from the DieMate manifold to the front of the board.

industrially hardened computer that operated under a deterministic real-time operating system. Because the PLC was responsible for process-safety aspects, reliability was the most important factor. This factor eliminates any machine operating under a Windows OS. The PLC was a National Instruments (NI) PXI-1010 chassis with an embedded controller. This computer was responsible for monitoring all of the discrete and analog inputs from the AIMS as well as providing the discrete and analog outputs for process control. It is actually composed of two coupled NI PXI chassis: (1) a PXI-1010 and (2) a PXI-1000B. Two chassis were required because of the number of boards that were required for ata logging and process control. 4.3. Process Control Software. The control program for the PLC was written in G using LabVIEW version 6.03. G is the proprietary graphical programming language of National Instruments. The control program executing on the PLC is responsible for the following actions: (1) monitoring process variables and transmitting them to the HMI; (2) examining the process variables for abnormal conditions; (3) executing interlocks based on the status of the process variables; (4) performing closedloop control of the microreactor heaters (24 heaters in total);

and (5) receiving and implementing operator input from the HMI. All of these actions are considered part of the control loop and must be executed within a specified time period for the controller to be operating in real time. Thus, the speed of the backplane, processor, and communications are all factors in determining the maximum loop execution rate. The HMI provides the connection between the AIMS hardware and the user. It is responsible for display of data to the user, thereby allowing the user to enter new process set points, and historical logging of data. The other task of the HMI is to log all of the process variables at a designated time interval. For this task, the combined LabVIEW Datalogging and Supervisory Control module was installed on the HMI computer. The maximum data-storage rate supported by the LabVIEW data-logging module is 4 Hz, which is much slower than the time constants of the microreactor heaters. However, recording the heater temperature data during experiments that lasted hours at a rate in excess of 1000 Hz would have generated enormous files and provided only a small level of improved process information. In addition, most of the AIMS experiments in this work were not focused on studying the very fast dynamic processes of the microreactors. Even at a data storage rate of 4


Figure 15. Heater driver circuit board.

Figure 16. Front view of the AIMS without external fluidic connections.

Hz and using database-compression techniques, a single days worth of data would have required about 250 Mb of storage space because of the large number of process variables being monitored and the small dead band used for data logging. This points to the issue of data management when using parallel systems of microreactors involving fast process dynamics. 4.4. Process Control Interface. The AIMS HMI is organized around a central VI panel that allows the operator to quickly view many of the process variables and to easily access more detailed panels. Panel selection is performed by selecting various

tabs that are associated with the operator interface. The various tabs include the following: (1) run-time engine tab; (2) process gases tab; (3) thermal blocks tab; (4) microreactors tab; and (5) sampling tab. An overview of the reactor-control panel within the microreactors tab is given here since this is the system engine and is illustrative of the other panels. Appendix C in the Supporting Information provides additional details on the process control interface. The reactor-control panel allows the operator to view detailed information on the operation of an individual microreactor

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Figure 17. Rear view of the AIMS without external electrical connections.

Figure 18. Rear view of the AIMS with the back cover panels removed.

channel. It also allows the operator to introduce changes to various process set points. Figure 20 shows the panel for channel A of the microreactor on reactor board 1. The two key subpanel

components are the flow rate controller for the feed gas or purge gas and the microreactor sensing and control package. These panels allow the operator to choose the gas stream flowing to


Figure 19. Front (left) and rear (right) views of the PS4400 backplane PCB.

Figure 20. Process control panel for one of the reactors in the AIMS operator interface.

the microreactor (either feed gas or purge gas), specify the gas flow rate, define set points for each of the reactor heater zones, view the local temperatures along the reactor, specify resistances

for the onboard platinum temperature sensors, and examine the amount of power (current) applied to each of the reactor heater zones.

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5. Automation Testing Results and Discussion

6. Summary and Conclusions

The majority of the HMI and PLC control program components operated as expected, and minimal troubleshooting was required. The key observations are summarized below. 5.1. Temperature Sensors. The temperatures measured by the RTDs using the SCXI-1121 boards were subject to inaccuracies. The inaccuracies were attributed to some poor electrical connections inside the AIMS chassis. In particular, the connectors between the front system boards to the backplane were identified as a large contributor to the problem. It was noted during testing that removing a board and reinserting it could dramatically improve or degrade one of the RTD values. The backplane connectors themselves are not completely at fault since the frequency of board removal and replacement in the AIMS was much higher than originally anticipated. The RTD temperatures measured by the CAL controllers in the AIMS chassis had a higher accuracy than those measured using the SCXI-1121 board. Occasionally, a temperature measurement would be clearly inaccurate, but these values generally reflected expected behavior during most of the test work. Although difficult to verify by probing directly, other indicators such as the power output by the controllers to maintain a set point temperature could be monitored to infer actual temperature. These controllers may have functioned better since a smaller number of electrical connections were required between the temperature controller board and the reactor boards. Thermocouples were not used for temperature measurements since the various metals used in the backplane and PCB would not have been compatible with their use. In this case, it would have been necessary to place a voltage amplifier on the thermocouple signals so that their signals could have been measured as an analog input. 5.2. Temperature Controllers. The CAL controllers exhibited some other operational problems. In particular, the quality of the OPC (OLE for Process Control) connection to the HMI did not always meet system requirements. It was often difficult to initiate communication with the CAL controllers, and data errors were sometimes transmitted, e.g., set points would be changed to incorrect values or other controller parameters would be unintentionally altered. The difficulty was likely due to the particular converter used, but poor connections in the serial links could have also been to blame. Because of these problems, it was preferred to make changes to the CAL controllers on the controllers directly without using the HMI. 5.3. Process Control Loops. Various values for the control loop rate were used during testing to evaluate the ability of the system to maintain real-time proportional-integral-derivative (PID) temperature control on the microreactor heaters. It was found that the control loop could successfully operate at 500 Hz with all 24 microreactor heaters being under closedloop PID control. At faster rates, the processor did not have sufficient time to complete the transmission control protocol/ internet protocol (TCP/IP) communications with the HMI, thereby resulting in data loss. The PID algorithm did appear to be adequate in maintaining microreactor temperature controller. Somewhat conservative controller parameters were used because of the noise in the temperature-measurement signal, but a stable temperature was maintained at its set point under a wide variety of conditions (including reaction). The dynamics of the closed-loop control algorithm were not explored since the rate of data logging (4 Hz) was not fast enough to perform these types of evaluations. This would be possible with program modifications, but this problem was not the emphasis of this project.

Development of the prototype AIMS chassis and various circuit boards was challenging since component assembly required a significant amount of custom design and fabrication owing to the large departure of the system design basis from conventional laboratory reactor system practice. A multidisciplinary team involving various engineering disciplines, computer and material scientists, and skilled technicians was required for successful execution, which points to a growing trend for new product engineering research. Use of several off-the-shelf MEMS system components, such as the electronic mass flow controllers and shutoff valves, required repackaging before they could be properly integrated as onboard devices into one of the modular process circuit boards. The particular devices used here were manufacturer prototype versions that did not perform according to the design specifications during initial testing, but the defects were later corrected within reasonable limits. Another source of difficulty was concerned with obtaining reliable temperature measurements using the onboard reactor platinum RTDs and the associated data-capture circuit board. An alternate board capable of more precise measurements of the corresponding low-level RTD voltages would produce more accurate temperature data. The operational issues associated with the electronic valves and temperature-measurement devices point to the need for improved MEMS components for these specific applications so that microreactor scale-up can proceed at a faster pace with a greater degree of process reliability. Despite these issues, the remaining system components performed quite well when evaluated using a standarized set of electrical tests. Incorporation of microfluidics and other MRT process functions into a multislot computer chassis using modular cards was proved to be feasible. The process automation required assembly of special-purpose hardware and a human-machine interface. From a hardware perspective, commercially available PLC systems and various signal acquisition and control cards were used without any modifications. Installation of various process sensors, actuators, and other related devices at strategic locations within the microreactor system allowed the development of a humanmachine interface so the system could be completed operated from the computer system. Acknowledgment The authors wish to thank the anonymous reviewers for their useful comments that helped improve the final manuscript. Supporting Information Available: The Supporting Information includes the component design drawings, the results of the various microreactor electrical tests, and information on the process control and automation. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Salomon, P. Product-Technology Roadmap for MicrosystemssA Nexus Task Force Report; WTC Wicht Technologie Consulting: Berlin, Germany 2005. (2) Ehrfeld, W., Ed. Microreaction Technology: Proceedings of the 1st International Conference on Microreaction Technology; Springer-Verlag Publishers: Berlin, Germany, 1998. (3) Schu¨tte, R., Matlosz, M., Renken, A., Liauw, M., Langer, O.-U., organizers. Presented at The Ninth International Conference on Microreaction Technology (IMRET 9), Potsdam/Berlin, Germany, Sept 6-9, 2006; http://events.dechema.de/imret9. (4) Kandlikar, S. K., Ed. Proceedings of the 1st International Conference on Microchannels and Minichannels (ICMM 2003), American Society of

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ReceiVed for reView February 18, 2007 ReVised manuscript receiVed August 22, 2007 Accepted August 29, 2007 IE0702577

Integrated Microreactor System for Gas-Phase Catalytic Reactions. 3

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