Microfabricated Packed-Bed Reactor forPhosgene Synthesis

Sameer K. Ajmera, Matthew W. Losey, and Klavs F. JensenDept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

Martin A. SchmidtMicrosystems Technologies Laboratories, Dept. of Electrical Engineering and Computer Science,

Massachusetts Institute of Technology, Cambridge, MA 02139

A silicon micropacked-bed reactor for phosgene synthesis is demonstrated as an ex-ample of the potential for safe on-siteron-demand production of a hazardous com-pound. Complete con®ersion of chlorine is obser®ed for both a 2:1 COrCl feed at 4.52std. cm3rmin and a 1:1 feed at 8 std. cm3rmin. The latter gi®es a projected producti®ityof �100 kgryr from a 10-channel microreactor, with the opportunity to produce signif-icant quantities through operating many reactors in parallel. The ®ersatility of siliconmicrofabrication technology for producing reactors for corrosi®e gases is demonstratedby a protecti®e oxide coating formed during reactor fabrication. The increased heat andmass transfer inherent at the submillimeter reactor length scale pro®ides a larger degreeof safety, control, and suppression of gradients than is a®ailable in macroscale systems.These ad®antages are also explored in the extraction of chemical kinetics from microre-actor experiments. The preexponential factor and apparent acti®ation energy for phos-gene formation are determined demonstrating the utility of micropacked-bed reactors aslaboratory research tools.

Introduction

Microfabricated chemical systems are receiving an in-creased interest in a variety of chemical and biological appli-cations. The microchemical research effort has grown as anatural extension of the expanding field of MicroElectroMe-

Ž .chanical Systems MEMS , which initially developed from in-tegrated circuit fabrication technology. The miniaturizationof chemical devices such as ‘‘micro-total-analysis-systems’’Ž . Ž�TAS is an example of this development Van den Berg et

.al., 2000 . Microchemical technology has also seen a broadrange of development in a variety of unit operations such asheat exchangers, mixers, extractors, as well as chemical reac-

Ž .tors Ehrfeld et al., 1998; Jensen, 1999; Lowe et al., 2000 .

Correspondence concerning this article should be addressed to K. F. Jensen.

The ability to manufacture many reactors in parallel, analo-gous to computer chips in the microelectronics industry, also

Žopens up the door for on-demand chemical synthesis Lerou.et al., 1995 .

Silicon-based microchemical reaction systems with sub-mil-limeter length scales have the potential to realize systems withcapabilities exceeding conventional macroscopic systems. Thesurface-area-to-volume ratio of the reaction channel in-creases as the characteristic length of the reactor is reduced.The larger area for heat transfer reduces thermal gradients,and, for exothermic processes, suppresses the formation ofhot spots that otherwise could lead to reactor runaway. Theability to reduce runaway is not only an inherent safety ad-vantage, but allows the reactor to be run under aggressiveconditions or in regimes that would ordinarily be difficult orunsafe in larger systems. An example of this is ammonia par-

July 2001 Vol. 47, No. 7AIChE Journal 1639

tial oxidation with pure oxygen in a membrane-based mi-Ž .croreactor Franz et al., 1999; Srinivasan et al., 1997 . Sub-

millimeter length scales also enhance mixing, since the char-acteristic time of diffusion scales with the square of length.With characteristic diffusion times of milliseconds or less,mass-transfer gradients are also reduced.

Various microreactor designs have been fabricated utiliz-Žing thin film catalysts, coatings, or other structures Janicke

et al., 2000; Srinivasan et al., 1997; Weissmeier and Honicke,¨.1998 . These designs have small catalyst surface areas, and

do not incorporate the broad range of catalyst supports andpreparation techniques used in practice. A significant body ofresearch and experience exists on catalyst development andsynthesis. Leveraging this knowledge is desirable in imple-menting a catalytic microchemical platform. A micropacked-bed reactor has been demonstrated which utilizes high sur-

Ž .face area catalyst particles 36�75 �m diameter that areŽ .prepared with standard techniques Losey et al., 2000 . At

these dimensions, the reactor operates in laminar flow atReynolds numbers around unity for gas flows. In macroscale

Ž .reactors mm to m , low Reynolds numbers and larger cata-lyst diameters often lead to poor heat and mass transfer inpacked beds due to the lack of turbulent mixing and slowdiffusion over the larger length scales. The micropacked-beddimensions, however, offset these deleterious transport ef-fects and lead to more than a 100-fold increase in the gas-liquid mass-transfer coefficient relative to conventional sys-

Žtems for the multiphase hydrogenation of cyclohexene Losey.et al., 2001 .

Economies of scale usually lead to large facilities fromwhere chemicals are shipped. However, safety and environ-

mental concerns could shift the current model towards smallerplants located near the intended point of application. This isparticularly the case for hazardous and toxic chemical inter-mediates that have serious storage and shipping constraints.One such intermediate used throughout the chemical and

Žpharmaceutical industry is phosgene COCl , carbonyl2.dichloride , manufactured from gaseous chlorine and carbon

monoxide over activated carbon

Cl qCO™COCl y� Hs26 kcalrmol 1Ž .2 2

ŽThe reaction is moderately fast and exothermic Lord and.Pritchard, 1970; Saunders et al., 1953 .

Phosgene is widely used as a chemical intermediate for theproduction of isocyanates used in polyurethane foams and inthe synthesis of pharmaceuticals and pesticides. Processes us-ing phosgene require specialized cylinder storage, environ-mental enclosures, pipelines, fixtures under negative pres-sure, and significant preventative maintenance. Moreover,phosgene is under a variety of transportation restrictions. Asa consequence, most phosgene is consumed at the point of

Ž .production EPA, 1985 . Off-site production often necessi-tates out-sourcing not only the phosgene synthesis, but also aset of sequential processing steps in order to get to a safe,transportable compound. Microchemical systems stand toprovide an opportunity for flexible point-of-use manufactur-ing of chemicals such as phosgene. Banks of reactors can beturned on or off as needed to maintain as close to zero stor-

Figure 1. Microfabricated silicon packed-bed reactor.Ž .a Top-view of reactor partially loaded with 60 �m activated carbon particles. The reactor channel is 20 mm long. The image is spliced to

Ž . Ž .fit the 20 mm reaction channel by omitting the long channel midsection photograph by Felice Frankel, MIT ; b SEM of the 25 �m wideŽ .interleaved inlets; c SEM of the catalyst filter structure.

July 2001 Vol. 47, No. 7 AIChE Journal1640

age as possible. Single reactor failures would lead to ex-tremely small chemical releases. To demonstrate the abilityto produce hazardous compounds from microfabricated de-vices, the DuPont Company has synthesized a number of haz-ardous chemicals such as isocyanates in a microreactor

Ž .formed by bonded silicon layers Lerou et al., 1995 . Further,with the development of multistep microchemical systems,subsequent processing steps can be performed in a single de-vice eliminating the need for storage or transportation.

In this work, we present phosgene synthesis in a silicon-based micropacked-bed reactor as an example of the poten-tial for safe on-siteron-demand production of a hazardouscompound. Preliminary productivity values are presented.The versatility of microfabricated reactors is demonstratedfor hazardous and corrosive gases such as chlorine throughthe use of a glass-like protective coating that is formed dur-ing the reactor fabrication process. The increased heat- andmass-transfer characteristics, which provide inherent safetyand increased productivity in chemical synthesis, are also ex-plored as advantages in the extraction of chemical kinetics.The preexponential factor and apparent activation energy forphosgene formation are determined to demonstrate the util-ity of micropacked-bed reactors as laboratory research tools.

Micropacked-Bed Reactor Design and FabricationA detailed description of the motivation, design issues, and

characterization of the micropacked-bed reactor is describedŽ . Ž .by Losey et al. 2001 . The microreactor Figure 1 is fabri-

cated out of single crystal silicon with standard microfabrica-tion processes developed for integrated circuits and MEMS.The geometry is defined using photolithography and created

with silicon etching. The reactor consists of a 20 mm long,Ž625 �m wide, 300 �m deep reaction channel 3.75 �L vol-

.ume capped by Pyrex. Figure 1b shows a scanning electronŽ .micrograph SEM of the inlet where flow is split among sev-

Ž .eral interleaved channels 25 �m wide that meet at the en-trance of the reaction channel. Perpendicular to the inletchannels are 400 �m wide loading channels used to delivercatalyst particles to the reactor. Catalyst is loaded by placinga vacuum at the exit of the reactor and drawing in particlesthrough the loading channels. An inert gas can be used toload catalyst if contamination or deactivation is an issue. Atthe outlet of the reaction chamber, a series of posts with 25

Ž .�m gaps acts as a filter to retain the catalyst bed Figure 1c .There are also four 325 �m wide channels perpendicular tothe reaction channel along its length for holding thermocou-

Ž .ples or optical fibers . Access ports for flow come from un-Ž .derneath at the inlet not shown in Figure 1 , the reactor exit,

and at the ends of the catalyst loading channels.The thermocouple wells, inlets, the reactor and catalyst

loading channels, and the catalyst filter are etched in a sili-Ž .con substrate 100 mm diameter wafer, 500 �m thick using a

Žtime-multiplexed inductively coupled plasma etch Ayon et.al., 1999 . The wafer is then turned over, patterned, and

etched on its back-side to create the access ports. Since chlo-rine etches silicon, a conformal silicon dioxide film about

˚5,000 A thick is grown around the entire wafer in a wet oxi-dation furnace to protect the reactor. Finally, the oxide coated

Ž .channels are capped by a Pyrex wafer Corning 7740 , whichhas a similar thermal coefficient of expansion as silicon, using

Ž .an anodic bond Schmidt, 1998 . The bonded wafer stack iscut with a die saw to obtain individual devices. A 100 mm

Ždiameter wafer yields twelve single channel reactors 10 mm.�40 mm�1.0 mm after bonding and dicing.

Figure 2. Experimental setup.Flow controllers, purge lines, and other equipment have been eliminated for simplicity.

July 2001 Vol. 47, No. 7AIChE Journal 1641

Experimental SetupThe reactor is compressed with a metal cover plate against

Ž .a thin elastomer gasket 0.8 mm thick Kalrez with punchedthrough-holes to form fluidic connections to a stainless-steelbase. External fluidic connections are made directly to the

Ž .metal base Figure 2 . The metal compression plate containsŽ .cartridge heaters Omega Engineering . Thermocouples

Ž .Omega Engineering are threaded into the side wells of thereactor. Once the reactor is loaded with catalyst, the catalystinlets are closed off by substituting a sealing gasket withoutcatalyst loading through-holes.

The reactorrbase assembly is connected to the rest of theŽ .system via standard fittings, valves, and 1.6 mm 1r16 in. O.D.

stainless-steel tubing. All materials in contact with chlorine,with the exception of the reactor, are stainless-steel,

ŽTEFLON, or Kalrez. The chlorine BOC Gases�99.96% pu-. Ž .rity and carbon monoxide BOC Gases�99.3% purity gas

Žfeeds are controlled by mass-flow controllers UNIT Instru-.ments . The carbon monoxide flow controller is specially fit

with stainless-steel internal components. The chlorine flowcontroller is fitted with Kalrez fluidic seals to minimizedegradation. The gas feeds are mixed in a tee junction, andenter either the reactor or a bypass line. The exit stream isinterfaced via a glass capillary to a mass spectrometer pumpedthrough a two-stage vacuum system for continuous real-timespectrometry. Any flow not drawn into the sampling capillaryis directed to the exhaust, which is bubbled through a NaOHsolution to remove any phosgene. The vacuum typically drawsa larger flow through the capillary than the total flow rateused in these experiments. In order to prevent the differencefrom being drawn from the exhaust, argon purified in an oxy-gen and moisture scrubber is mixed with the exit stream justbefore the sampling capillary. The dry argon flow rate is ad-justed with a mass-flow controller to make the total flow rateat the capillary inlet larger than the capillary draw rate. Aswitching valve is placed before the sampling capillary whicheither directs flow to the mass spectrometer as previously de-scribed, or to other laboratory equipment for further process-ing. High-pressure dry argon also serves as a purge gas at thebeginning and end of the experiments. Before beginning anexperiment, the entire system is heated to approximately150�C for 2 h under a constant flow of dry argon. This re-moves adsorbed moisture which reacts with chlorine to formHCl and corrodes the setup. The entire system is inside aventilated enclosure similar to a standard chemical fumehood. This highlights an inherent benefit of working with mi-croreactors as additional safety structures and cooling mech-anisms, which add expense and complexity to even typicallaboratory work, are not needed.

The experiments were carried out with approximately 1.3mg of activated carbon particles sieved between 53�73 �m

2 Žwith a surface area of 850 m rg DARCO G-60 American Norit.Company . The catalyst was loaded in air. A mixture of 2r3

CO and 1r3 Cl was mixed at a total flow rate of 4.5 std.2cm3rmin and fed into the reactor at room temperature. Someexperiments were also done with a 50r50 stoichiometric feedat a total flow rate of 8.0 std. cm3rmin. The reactor was in-crementally heated, without external cooling, from room tem-perature to approximately 220�C. The absolute pressure at

Žthe inlet of the reactor was approximately 1.35�1.40 atm 132.kPa and was nominally atmospheric pressure at the exit.

Data AnalysisData from the mass spectrometer are recorded on a PC

Ž .with LABVIEW software National Instruments . Since theexit stream is diluted with argon, post-run analysis is neces-sary to back out relevant partial pressures using calibrationsperformed at the beginning of each experiment. The calibra-tion procedure involves flowing pure argon, chlorine, andcarbon monoxide separately and in combination to the massspectrometer and calibrating the intensities of the relevantmass fractions to the partial pressures of the respectivespecies. Using the calibration, the partial pressure of argon iscalculated at each data point during the experiment and is

Žthen used to scale the other mass fraction intensities Mil-.lard, 1978 . We decided not to calibrate the mass spectrome-

ter using pure phosgene from an external gas cylinder be-cause of the potential dangers and increased experimentalcomplexity of having a pressured phosgene source. Instead,the calibration was done using phosgene produced in the mi-croreactor at a complete conversion of chlorine. The overlap-

Ž .ping mass fractions with chlorine MrEs70, MrEs35 andŽ .carbon monoxide MrEs28 were deconvoluted by noting

Žwhere the MrEs70 chlorine mass fraction minimal contri-.bution from phosgene stopped decreasing with increasing

temperature. At this point, complete conversion of chlorinewas assumed and the self-consistency of the calculations basedon that assumption was examined.

At complete conversion, all mass fractions overlapping withchlorine can be calibrated. Stoichiometry and the mass frac-tion intensities of the excess CO can be used to calibrate theremaining phosgene mass fractions. Using the complete setof calibrations, the partial pressures of all three species werecalculated for the entire data set. A correctly deconvolutedsystem gave the same value for conversion regardless of whichof the three species the calculations were based. The conver-

Ž .sion of chlorine � to phosgene was computed from theCl2

carbon monoxide, chlorine, and phosgene mol fractions usingstoichiometric balances

x 1qY y1Ž . x Y q x yYf Ž .Cl f f2 CO CO� s sCl2 x y1x y1 Ž .Ž . ff COCl2

x 1qYŽ .fCO Cl2s 2Ž .x q1Ž .fCO Cl2

where x is the mol fraction of species i at the exit of thef ireactor, and Y is the ratio of COrCl in the feed. Figure 32shows the conversions computed independently from thethree species over the entire temperature ramp from an ex-perimental run. The consistency between the independentlycalculated values validates the original assumption of com-plete conversion made at the beginning of the data analysisand gives confidence to the data analysis procedure.

The equilibrium constant K for Reaction 1 confirms thataŽfull conversion is a reasonable assumption Tester and Mo-

.dell, 1997 . Assuming an ideal vapor mixture at 450 K andŽ .1.35 atmospheres pressure at the reactor inlet , and using

Ž .the heats of formation reported by Chase et al. 1985 , theequilibrium constant indicates that a 2:1 mixture of carbon

July 2001 Vol. 47, No. 7 AIChE Journal1642

Figure 3. Conversion of chlorine to phosgene indepen-dently calculated from chlorine, phosgene,and carbon monoxide mass fractions.The temperature was increased until complete conversionand then ramped down.

monoxide and chlorine would equilibrate at 99.99% conver-sion of chlorine. Complete conversion to the limits of detec-

Ž .tion was also reported by Shapatina et al. 1976 .

Results and DiscussionChemical compatibility

Chlorine etches silicon, particularly at elevated tempera-tures. Consequently, microreactors made from silicon wouldbe corroded, releasing chlorine and possibly phosgene as wellas losing the utility of defined features such as the inlets andcatalyst retainers. Figure 4a shows the deleterious effects ofexposing a silicon reactor to chlorine at 250�C. The reactorinlet and channel are severely etched. In contrast, the reactor

Ž .with a silicon oxide coating Figure 4b shows no visiblechange after 6 h of continuous experiments. This exampleillustrates first, the importance of microfabricating chemi-cally compatible systems, and, second, that even for systemswhere silicon is not suitable, thin chemically resistant coat-ings can be used to render a stable device.

Phosgene productionNo temperature increase could be measured upon switch-

ing flow from the bypass line to the reactor. This is expectedbecause single crystal silicon has a large thermal conductivityŽ .150 Wrm �K and readily dissipates heat from the packedbed. The thermal mass of the stainless-steel packaging is manyorders of magnitude larger than the reactor and provides asignificant heat-sink. Likewise, the energy provided by thecartridge heaters to maintain the temperature of thereactorrpackaging is orders of magnitude larger than the en-ergy generated from the reaction. This gives fine temperaturecontrol over the exothermic reaction. No deactivation as re-

Ž .ported by Shapatina et al. 1976 was observed during theŽ .experimental time-scales 6�10 h possibly due to the high

level of purity in the gas feeds. No side products were ob-

Figure 4. Protective coating of silicon dioxide preventsetching of the silicon reactor by chlorine.Ž .a A microreactor etched catastrophically at the inlets un-der a constant 50r50 Cl rCO flow at elevated temperatures;2

˚Ž .b reactor protected with a conformal 5,000 A oxide layergrown in a wet oxidation furnace. After high-temperature-flow for over 6 h, the reactor shows no noticeable degrada-tion.

served in a full mass spectrum scan, presumably as a result ofsuppressing hot spots common in larger reactors and usinghigh purity feeds. Products such as silicon tetrachloride, whichwould form if chlorine were reacting with silicon, were alsonot detected.

Figure 5 shows the average conversion of chlorine to phos-gene as a function of temperature. The conversion of chlo-rine peaks at about 200�C where it stops increasing with in-creasing temperature. Using the rationale described in thedata analysis, it was confirmed that the tailing off was not aresult of mass-transfer limitations, but from complete conver-sion. At 4.5 std. cm3rmin total feed rate with 33.3% chlorine,

Ž .a phosgene productivity of 3.5 kgryr 0.40 grh is projectedfor a continuously operating single channel device. Experi-ments were also done with an 8 std. cm3rmin stoichiometricfeed. Complete conversion to phosgene was observed with a

Ž .corresponding phosgene productivity of 9.3 kgryr 1.1 grh .The flow rates were not increased due to the limitations ofthe mass-flow controllers, but the ability to control thermalrunaway in these microsystems would safely allow an aggres-

July 2001 Vol. 47, No. 7AIChE Journal 1643

Figure 5. Conversion of chlorine with a 1:2 chlorine/////carbon monoxide feed at 4.5 std. cm3/////min to-tal flow rate.Complete conversion is seen at approximately 200�C giving

Ž .a projected productivity of 3.5 kgryr 0.40 grh of phosgenefrom the single channel reactor.

sive increase in temperature and flow rate into ordinarilydangerous regimes, further increasing productivity. A multi-channel reactor integrating 10 reaction channels with a com-mon inlet and exit onto a single chip has been fabricated and

Ž .is only 1.5�larger in total chip area Losey et al., 2001 . Ex-periments with other systems show that the productivity ofthe multichannel microreactor scales with the number of in-tegrated channels. A single 10-channel device would produce

Ž . 393 kgryr 11 grh of phosgene with an 80 std. cm rmin stoi-chiometric feed at the given temperatures. Again, the oppor-tunity to increase temperature and flow rate exists. Scalingout with multiple devices operating in parallel would provideadditional opportunity to produce substantial amounts ofphosgene in an on-demand fashion.

Isocyanates are derived from reactions of phosgene withamine precursors. The reactions are highly exothermic, andselectivity towards the desired products is strongly governed

Žby reaction conditions, in particular, temperature Othmer,.1982 . Following a procedure for the synthesis of cyclo-

Ž .hexylisocyanate Sheehan et al., 1961 , phosgene generatedby the microreactor at complete conversion was used to drivea small bench-scale experiment. Phosgene was continuouslybubbled through a solution of cyclohexylamine and toluenein a 50 mL reaction flask. Generation of cyclohexylisocyanatewas demonstrated, with complete conversion of the amine re-actant. Even with volumes as small as 50 mL, the flask tem-perature rose 50�C without external cooling, severely impact-ing selectivity. Integrating a microfabricated gas-liquid reac-tor performing the amine phosgenation with the microreactorproducing phosgene into a single microchemical system wouldgive better control over selectivity. Moreover, the phosgenewould be consumed in the same device, as it is produced,further reducing the hazards of working with the toxic com-pound.

Extraction of kinetics from the microreactor systemThe small dimensions reduce thermal gradients that other-

wise would complicate the determination of reaction kineticsfrom packed-bed reactor data. In order to demonstrate themicroreactor as a tool for measuring reaction kinetics, rateconstants for phosgene formation were extracted assuming

Ž .plug-flow PFR . The PFR assumption is reasonable basedon the reactor Peclet number. Calculated values range be-tween 180 and 360. The number of catalyst particles acrossthe reactor diameter is �6.4, whereas a value of 10 or moreis typically desired for plug-flow analysis. In order to examinefurther the potential for gradients in the packed bed, charac-teristic dimensionless numbers were considered on the basisof experimental data.

Ž .Anderson 1963 proposed a criterion for assuring that theobserved reaction rate does not differ from the actual reac-tion rate within a catalyst particle by more than 5% due tointraparticle temperature gradients

N� H NR r 2Er xn p act�0.75 3Ž .

k T RTŽ .p s

The carbon catalyst in the phosgene experiments may be con-sidered isothermal as the lefthand side of the inequalityranges between �0.0006�0.006 using experimental data anda thermal conductivity of �0.27 Wrm �K for porous acti-

Ž .vated carbon Satterfield, 1996 . The Weisz modulus Mwwhich gives the ratio of the reaction rate to the diffusion ratein the porous catalyst is

R r 2 �r xn p pM s 4Ž .w C Ds E

BET nitrogen adsorption and specifications from the catalystmanufacturer indicate that Knudsen diffusion effects are im-portant. With a calculated effective diffusivity �10y7m2rs,the Weisz modulus was determined to be between 0.1�0.5indicating minimal intraparticle mass-transfer limitations,even with the low diffusivity.

In traditional packed beds, low particle Reynolds numberstypically result in poor mass transfer to the catalyst particlesurface. Relationships have been developed for the Sher-wood number in packed beds governing mass transfer from

Žthe fluid film around a particle to the particle surface Kuniiand Suzuki, 1967; Satterfield, 1970; Wakao and Tanisho,

.1974 . Their applicability to microfabricated reactors wherediffusive mixing is fast is unclear. However, an order of mag-nitude analysis can be made by examining the concentrationgradient needed to maintain a flux corresponding to reactantconsumption at the largest reaction rate observed in the ex-periments. In the limiting case of mass transfer, whereReynolds number approaches zero, diffusion is the only formof transport to the catalyst surface. For a 63 micron particleat the fastest rates of phosgene formation observed in theexperiments, the concentration difference is calculated to beapproximately 0.01 molrm3 over a characteristic length of oneparticle diameter. This is negligible compared to the bulkconcentration of �15 molrm3. Therefore, mass transfer to

July 2001 Vol. 47, No. 7 AIChE Journal1644

the catalyst surface is not limiting and the reactant concen-trations at the catalyst surface may be taken as the same asthe bulk concentrations. Furthermore, the interstitial diffu-

Žsion time from one particle to another taken as 1 particle.diameter is on the same order as the single particle resi-

dence time based on the superficial velocity of the gases, thatis, Pe �1. The preceding analyses demonstrate how theparticlesmall microreactor length scale reduces heat- and mass-trans-fer gradients in packed beds. Since characteristic thermal andmass transport times decrease as a square of the characteris-tic length, the micropacked-bed reactor operates in a regimewhere diffusive mixing and heat transfer across the small cat-alyst pellets are fast enough to suppress gradients.

The PFR equation material balance

F d� sy r dW 5Ž .i i io

is integrated, where F is the flow rate of species i inio

molrtime at the reactor inlet, � is the conversion of speciesii, r is the rate of formationrconsumption of species i, and Wi

Ž .is the mass of catalyst. The rate of reaction molrsrg catalystis taken from a rate expression reported by Shapatina et al.Ž .1976

1r4PCl2R skP 6Ž .r xn CO ž /AP q PCO COCl2

Žwhere k is the Arrhenius rate constant molrsratmrg cata-. Ž .lyst , P is the partial pressure atm of species i, and thei

constant A is a temperature-dependent equilibrium con-stant. This expression was obtained from experiments per-formed between 70�130�C. The temperatures in this analysisextend to 180�C, and Eq. 6 is extrapolated outside the rangereported by Shapatina et al. for both data analysis and com-parison. Equation 6 only applies to regions of intermediateCO surface coverage occurring at chlorine mol fractions above

Ž .3% Shapatina and Kuchaev, 1980 . As chlorine partial pres-sure decreases, the reaction passes from a region of interme-diate surface coverage to a region of high coverage where thecatalyst is almost completely covered by adsorbed CO. Theexpression in Eq. 6 does not capture this change in surfacecharacteristic. Low chlorine concentrations occur in the mi-cropacked-bed reactor towards the exit of the reactor at highconversions. Therefore, Eq. 6 was only used to obtain kinet-ics below 80% chlorine conversion to ensure that the chlo-rine mol fraction was always above 3%.

Ž .The Ergun equation Eq. 7 is often used to describe theŽ .pressure drop in traditional packed beds Bird et al., 1960

dP G 1y� 150 1y� �Ž .sy q1.75G 7Ž .3ž /dL �D D�p p

Experiments measuring the pressure drop through the mi-Žcropacked bed were performed using nitrogen 0�20 std.

3 .cm rmin . Equation 7 was integrated assuming a compress-Ž .ible gas, and the void fraction � was determined to be ap-

proximately 0.4 by fitting the data using � as the only vari-able parameter. Using the determined �, the Ergun equationgives a good prediction of the pressure drop for the range of

gas flow studied in the phosgene experiments, justifying itsuse in further analysis.

The reduction in the total number of moles in Reaction 1causes the density of the gas stream to increase with conver-sion. This causes a nonlinear decrease in total pressure alongthe reactor channel. Substituting Eq. 2 and an expression fordensity as a function of conversion into Eq. 7, the pressuredrop along the reactor channel with respect to catalyst weightcan be written as

dP G 1y� 150 1y� �Ž .sy q1.75G3ž /dW � � D D�c 0 p p

�P0

1q� 8Ž .Ž .Cl2P

where P is the total pressure at any point along the reactor,W is the weight of catalyst along the reactor, � is the cata-Clyst packing density in terms of the weight per unit length

Ž .along the reactor channel kg catalystrm channel , is the %change in the number of moles at complete conversion, as

Ž .defined by Fogler 1992 , and � is the conversion of limit-Cl2Ž .ing reagent chlorine along the reactor. sy 1r3 for the

case of a 2:1 COrCl feed. The viscosity of the gas mixture is2assumed to be constant for this analysis. Plugging Eqs. 2 and6 into Eq. 5, the conversion of chlorine along the reactor withrespect to catalyst weight is

1r4d� Yy� 1y� PCl Cl Cl2 2 2sk 9Ž .ž / ž /dW Yq1y� A Yy� q� FŽ .Cl Cl Cl Cl2 2 2 2 O

Equations 8 and 9 together give a system of coupled differen-tial equations in both P and � for which all the parametersare known except the rate constant k. Values for k wereextracted by iteratively solving the coupled system at differ-ent temperatures using a fourth-order Runge-Kutta algo-

Ž .rithm Hoffman, 1992 with the constraint that conversion atthe exit of the reactor matches the data shown in Figure 5.Using BET measurements on the catalytic surface area, therate constants with respect to surface area are calculated andcompared to values from Shapatina et al. in Figure 6. Theexperimental error bars are calculated using statistical analy-sis on the repeatability of the data at 95% confidence. Shap-

Ž .atina et al. 1976 reported catalyst surface area before andafter each experiment. The post reaction surface area is usedin Figure 6. A reasonable agreement is seen between the ex-perimental values and the values reported by Shapatina et al.given that they did not report error analysis and that thisanalysis extrapolated their expressions outside of their re-ported temperature range. The apparent activation energyfrom the experiments of 7.6 kcalrmol compares with the 8.6kcalrmol reported by Shapatina et al. The rate constants ex-trapolated from the microreactor also favorably compare tothe previously published values. The data falls on a straightline, even at the higher temperatures, giving further evidencefor the lack of mass-transfer limitations.

July 2001 Vol. 47, No. 7AIChE Journal 1645

( )Figure 6. ln k vs. 1/////T for the phosgene experiments( )and Shapatina et al. 1976 .

The experimental activation energy of 7.6 kcalrmol com-pares favorably to 8.6 kcalrmol from Shapatina et al.

ConclusionsA silicon-based micropacked-bed reactor has been pre-

sented for the heterogeneous gas-phase production of phos-gene. The microreactor utilizes high surface area catalystparticles synthesized by standard procedures. By using cata-lyst particles instead of thin-films or coatings, current indus-trial catalysts can be utilized for a wide range of applications.An example of the robustness of microfabrication technologyfor chemical processing is demonstrated through the use of asilicon-dioxide layer to protect the reactor from chemical at-tack.

Phosgene production was used as an example of the poten-tial for safe on-siteron-demand production of a hazardouscompound. The production of phosgene, normally requiringsignificant investment in infrastructure, was easily performedin a standard ventilated enclosure. Even with test-tube scalereactors, catalyst temperature control and thermal gradientsare difficult to control. With toxic compounds such as phos-gene, a bank of these ‘‘macroscale’’ reactors for catalyst test-ing would require special enclosures and expensive safetyprecautions. In contrast, a bank of microreactors would offerminimal structural investment beyond a small ventilated en-closure or fume hood while providing fine temperature con-trol and reduction of both thermal and concentration gradi-ents.

Preliminary productivity values for a single reaction chan-Ž .nel device yielded 9.3 kgryr 1.1 grh of phosgene for an 8

std. cm3rmin stoichiometric feed of carbon monoxide andchlorine. Although the flow rates were not increased due tolimitations in the mass-flow controllers, opportunity exists forincreasing productivity as temperature and flow rates can beaggressively increased without compromising safety. A multi-channel device integrating 10 reaction channels onto a singlechip has been fabricated that would increase productivity bya factor of ten. A scaled-out system with multiple parallelten-channel reactors could yield substantial amounts of phos-

gene. Bench-scale synthesis of an isocyanate from an aminewith phosgene produced in the microreactor was demon-strated. Although only performed in a 50 mL flask, tempera-ture control was an issue as significant temperature excur-sions were noted when external cooling was not applied.These excursions negatively impact selectivity and are an ex-ample of the difficulty of thermal control with these types ofreactions on the industrial scale. Future work involves inte-grating the microreactor producing phosgene with a secondmicroreactor designed for gas-liquid contacting. The com-plete microchemical system would perform both the phos-gene production and the subsequent phosgenation, eliminat-ing safety issues with handling phosgene and enhancing con-trol over selectivity. The suppression of hot spots and in-creased mass-transfer capabilities were also explored as ad-vantages for the extraction of chemical kinetics. Quantitativeanalysis indicates that the catalyst particles are small enoughto be isothermal and that mass-transfer resistances are notsevere for the phosgene experiments. Rate constants forphosgene formation were extracted from the packed-bed dataand compared favorably with literature, demonstrating thepotential for micropacked-bed reactors as practical labora-tory research tools.

AcknowledgmentsThe authors would like to thank Felice Frankel, Justin T. McCue,

Dr. Shinji Isogai, Dr. Aleks J. Franz, and Dr. Cyril Delattre for theirhelp during the course of this work. The assistance and expertise of

Ž .the personnel at the Microsystems Technology Laboratory MIT ,where the microreactors were fabricated, are also gratefully acknowl-edged. S. K. Ajmera would like to thank the National Science Foun-dation Graduate Fellowship Program. The authors would also like to

Ž .thank the DARPA Micro Flumes Program F30602-97-2-0100 forpartial financial support.

NotationC sconcentration of limiting reactant at the catalyst surface,s

molrm3

D sdiameter of catalyst particle, mpŽE rRTsArrhenius group activation energy, universal gas con-act.stant, temperature

Ž 2 .Gssuperficial mass velocity, kgrm � s� Hschange in enthalpy of forward reaction, Jrg mol

k scatalyst thermal conductivity, Wrm �KpP smeasured pressure at the inlet of the reactor, atmor scatalyst particle radius, mpT scatalyst surface temperature taken as the bulk fluid tem-s

perature, K� sfluid or gas density, kgrm3

f� scatalyst density, kgrm3

p� sviscosity of species i, Pa � si

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Manuscript recei®ed Feb. 1, 2001.

July 2001 Vol. 47, No. 7AIChE Journal 1647