Download - Design and operation of micro-chemical plants—bridging the gap between nano, micro and macro technologies

Computers and Chemical Engineering 29 (2004) 57–64

Design and operation of micro-chemical plants—bridging the gapbetween nano, micro and macro technologies

Shinji Hasebe∗

Department of Chemical Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, Kyoto 606-8501, Japan

Received 11 June 2004; received in revised form 27 July 2004; accepted 28 July 2004Available online 11 September 2004

Abstract

In the last decade, much effort has been devoted to developing micro devices for reaction, mixing and separation. Many researchers havereported the advantages of using such micro devices for production. However, few micro chemical plants are used for real production. In thisresearch, the design and operation problems of micro chemical plants are discussed from the viewpoint of process systems engineering. First,the design problems of micro chemical plants are classified into two sub-problems: the design of the micro unit operations and the design oft which mustb of the microc l plants hast©

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he entire micro plant. For each of the sub-problems the features of the micro systems are explained and the dominant problemse solved by process systems engineers are pointed out. Then, the characteristics of the instrumentation and control problemshemical plants are summarized. Finally, it is pointed out that the research on the design and control problem of micro chemicahe possibility to act as a bridge between chemical engineering science and process systems engineering.

2004 Elsevier Ltd. All rights reserved.

eywords:Micro chemical plant; CFD simulation; Numbering up; Optimal design

. Introduction

Advanced chemical industries always move in the direc-ion of producing high value-added products and reducinghe process development period. The problem is what kindf strategy can be taken to achieve these targets. The produc-

ion of high value-added products requires the precise controlf temperature, flow pattern and residence time, etc., but it isifficult for conventional plants to satisfy these requirements.ven though computer-aided design systems have been used

or the process design, many steps are still needed for scalingp from laboratory size to industrial size. To overcome theseroblems, many researchers have discussed the availabilityf the micro-systems for production.

The research on micro systems is classified into fourelds. The first is related to the efficient operation ofhemical laboratories. The research called “lab-on a chip” orcombinatorial chemistry” belongs to this field. The second

∗ Tel.: +81 75 383 2667; fax: +81 75 383 2657.E-mail address:[email protected].

field is the development of the analytical devices usingmicro technologies—usually called “micro-TAS.” The thinvolves the micro fabrication techniques. The techniqdeveloped in the field of micro-electro-mechanical syst(MEMS) are widely used to develop micro devices. Theis the research on the development of the production sysconsisting of micro devices. This manuscript is limitedthe problems related to the last field. Micro-chemical pl(MCPs) consist of micro devices such as micro reacmicro mixers, and micro heat exchangers. The developof these devices is indispensable to constructing MCPs.much as there are exhaustive reviews on the presentof the development of these micro unit operations (Hesse& Loewe, 2003a, 2003b, 2003c) and introductory bookof micro process engineering (Ehrfeld, Hessel, & Loewe2002b; Hessel, Hardt, & Loewe, 2003), in this researcthe problems related to individual unit operation arediscussed.

Few micro-chemical plants (MCPs) have been usedreal production; therefore, engineers have neither the erience in designing MCPs, nor any systematic tools for

098-1354/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.compchemeng.2004.07.020

58 S. Hasebe / Computers and Chemical Engineering 29 (2004) 57–64

design and control. One of the dominant characteristics of theMCPs is that the research results can be transferred into pro-duction much faster. Thus, it is very important to elucidate theproblems which occur in the design and control of MCPs andpropose solutions for those problems. From this viewpoint,the emphasis is placed on explaining the characteristics ofthe MCPs and pointing out the future research subjects to besolved; the concrete techniques to solve the problems are notexplained.

2. Possibility of MCPs

When an MCP is developed for real production, the rea-son for using micro devices must be clear. It is meaninglessto say that a product which can be produced in a conven-tional plant can also be produced in an MCP. We must aimat the production of materials that cannot be produced inconventional chemical plants, or the production efficiency ofwhich is drastically improved by using an MCP. Over the lastdecade, many kinds of materials have been produced throughmicro devices.Hessel and Loewe (2003a, 2003b, 2003c)of-fer a comprehensive list of the materials produced by microsystems. However, the number of products which really sat-isfy the above conditions is not clear. Scientists and engineerse theirr

s isl tubei ratiob vice.T anyb ll be-c evicec s cana t oft oldsn viceb ingt hersa t taket t,2 1R isd 0,000t te iso liedt cusst

medt ofd teriala daysa cessip uced

Fig. 1. Conventional batch plant.

Fig. 2. Micro chemical plant.

equally. What is the same amount of products is produced inan MCP? If one train of MCP is assigned to one product, theflow rate of each production train which is enough for thesame amount of production with the batch plant is around2000 cm3/h. This amount of production can be attained by16 square channels each of which has a cross-sectional areaof 600�m× 600�m, if the average flow speed is 0.1 m/s (seeFig. 2). However, it should be mentioned that the residencetime is 1 s if the length of the device is 10 cm. From thisexample, it becomes clear that the residence time, not the sizeof the device, is the problem. In this case, much effort shouldbe devoted to increasing the reaction rate. One of the featuresof micro devices is that the temperature can be controlledeasily. If the device can be operated at a higher temperatureby using this characteristic, the reaction rate can be increased.This example shows that if a sufficient reaction rate can beachieved in micro-devices, the MCPs can be used not onlyfor the production of small volume specialty chemicals butalso for the production of commodity chemicals of mediumvolume.

3. Design problem of MCPs

The design problem of MCPs is classified into two sub-p n oft hesep

ngaged in the research of MCPs must always evaluateesults from the viewpoint of real production.

One of the dominant characteristics of micro devicearge surface to volume ratio. When the diameter of as decreased to one hundredth, the surface to volumeecomes one hundred times larger than the original dehe increase in the surface to volume ratio exhibits meneficial characteristics. Heat transfer through the waomes efficient, and as a result the temperature in the dan be controlled precisely. The surface of the channellso be used efficiently. This results in the improvemen

he efficiency of catalytic reaction. Because the Reynumber becomes very small, the flow inside the deecomes laminar flow, which is very helpful for controll

he residence time distribution precisely. Many researcre interested in the development of micro systems tha

hese characteristics into account (Ehrfeld, 1999; Basel002; DECHEMA, 2003; Matlosz, Ehrfeld, & Baselt, 200;inard, 2000). However, when the diameter of the tubeecreased to one hundredth, the flow rate becomes 1

imes smaller than the original device. The production rane of the dominant problems of an MCP when it is app

o the production process. Thus, it is meaningful to dishe amount of production of MCPs with an example.

Fig. 1 shows a conventional batch plant. It is assuhat the batch size is 1.0 m3 of product, and that ten kindsifferent products are produced by changing the raw mand/or production conditions. It is also assumed that 2re required for one batch of production, and the pro

s operated 320 days in a year. In this case, 16 m3 of eachroduct is produced in a year when every product is prod

roblems: the design of micro devices, and the desighe entire plant. Here, the dominant characteristics of troblems are explained.

S. Hasebe / Computers and Chemical Engineering 29 (2004) 57–64 59

Fig. 3. Effect of shape on flow distribution.

3.1. Design of micro unit operations

Many types of micro devices have been proposed for eachof the micro unit operations (MUO), but there is no standardshape for each type of MUO. Thus, the general characteristicsof the design problems of micro unit operations (MUOs) areexplained in this section.

3.1.1. Formulation of design problemIn a conventional design problem, the unit operations

are modeled by using terms such as perfect mixing, pistonflow, steady state, and total heat transfer coefficient. In otherwords, each unit operation is modeled as a lumped parametersystem. Recently, computational fluid dynamics (CFD)simulation has been gradually introduced in the designalgorithm. However, much effort is required to adjust thesimulation parameters, because the flow inside the vessels isso complicated. The desirable characteristics of MUOs arisefrom the precise controls of the temperature, the residencetime distribution and/or the degree of mixing. Thus, the

ithm of

design problem of MUOs usually includes the constraints onthe temperature profile, residence time distribution and/orsize of segments to be mixed. To satisfy these requests, theshape of the device must be included in the design variables inaddition to the size of the device. As the flow inside the devicebecomes laminar flow, it is possible to execute precise CFDsimulation, and it can be embedded in the design algorithm.

In order to show how the shape of the device affects theflow pattern in the device, the design problem of a plate fintype micro device is used as an example (Commenge, Falk,Corriou, & Matlosz, 2002; Ehrfeld et al., 2000a). The platefin type micro device is a typical micro reactor for catalyticreaction. When the catalytic reaction occurs in the channels,it is desirable to make the residence times at all channelsuniform. Fig. 3 shows the mass flow rate at each channelof the plate fin type micro devices (Tonomura et al., 2003).The black circle in the figure represents the flow rate at eachchannel of a Type B-1 device. The white circle shows theflow rate of a Type B-2 device. From the viewpoint of flowuniformity, Type B-2 is better than Type B-1. The differencebetween these two devices is the size of the outlet manifoldarea. That is, the magnification of outlet manifold area makesthe flow distribution uniform.

The shape of the device has a large degree of freedom;thus, it is almost impossible to derive the best shape if noc ctiono enceo mustc thed rou sat-i izet

3nt of

d nsate

Fig. 4. Design algor
micro unit operations.
onstraints are added to the shape. However, the introduf the constraints to the shape interferes with the emergf the devices designed by a completely new idea. Wearefully select the constraints which are embedded inesign problem.Fig. 4 shows a design algorithm of micnit operations. In this algorithm, the shape of the device

sfying all constraints is gradually improved so as to optimhe given objective function.

.1.2. Design marginWhen chemical equipment is designed, some amou

esign margin is added to each variable so as to compe


Fig. 5. Effects of design margin.

for unforeseen variations. This method can be used whenthe design margin always shows a beneficial effect on theproduction efficiency of the device. In MUOs, the size ofa device strongly affects its function, that is, the functionaldesign and physical design cannot be executed separately.In this case, the design margin may not work as well ex-pected. In the simple micro device shown inFig. 5, thecross-sectional area and the residence time are assumed tobe the dominant factors which affect the function of the de-vice. It is obvious that the function expected from the deviceis not satisfied when the cross-sectional area is increased.If the device is lengthened or the number of channels isincreased, the residence time is also increased. It is clearfrom this example that in the MCPs the uncertainties of themodel and parameters may not be compensated by the de-sign margins. Thus, the disturbance should be compensatedfor by the adjustment of the operation as well as the designmargin.

Laminar flow characteristics are exhibited in a channel of amicro-device. Recent advances of computer technology andsimulation algorithms enable us to simulate the flows withreactions in the micro devices by using a CFD simulator.The advance of the CFD simulation algorithm creates newpossibilities for embedding the CFD simulator into the designsystem and designing devices which do not require any designm

3on-

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Fig. 6. Schematic view of counter-flow plate-fin micro heat exchanger.

flows. Under the conditions shown inFig. 6, CFD simulationsusing Fluent® code were performed to analyze the influenceof physical properties of materials on the heat transfer per-formance. Three types of materials—copper, stainless steel,and glass—were examined. Their thermal conductivities are388, 16.3, and 0.78 W m−1 K−1, respectively. Temperaturechanges of heat transfer fluids were used to evaluate the per-formance of micro heat exchangers. The simulation resultssummarized inTable 1show that the heat transfer efficiencyof micro heat exchangers made of stainless steel or glass ishigher than that of copper.

When the copper micro heat exchanger is used, the tem-perature profile inside the wall (device itself) becomes flatin the longitudinal direction due to high heat conduction asshown inFig. 7(left). The stainless steel or glass micro heatexchanger generates an appropriate temperature gradient in-side the wall in this case study, as shown inFig. 7 (right).Therefore, higher heat transfer efficiency is not necessarilyachieved by using materials with higher thermal conductivity,which leads to the conclusion that the heat transfer behavior

Table 1Heat transfer performance achieved by using three kinds of materials

Materials Thermal conductivity(W m−1 K−1)

Temperature change (K)

CSG

argin.

.1.3. Reevaluation of the neglected termsThough a micro device is fairly small compared with c

entional chemical equipment, it is still very large compaith atoms or molecules. In principle, the physical laws

ablished in the conventional world can be used to deshe behavior in the device. However, it is probable that serms which have been neglected in the conventional dannot be neglected in the design of MUO. As an examhe results of an efficiency analysis of heat exchangerxplained.

Plate-fin-type micro heat exchangers are represenf micro heat exchangers.Fig. 6 shows the counter-flolate-fin-type micro heat exchanger investigated in this wTonomura, Kano, Hasebe, & Hashimoto, 2002). A numberf plates are stacked, and on each plate a hot or cold s

opper 388 59.4tainless steel 16.3 72.7lass 0.78 64.8

Fig. 7. Temperature profiles of heat transfer fluids.


depends largely on the longitudinal heat conduction insidewalls (Stief, Langer, & Schubert, 2000). This result does notmean that materials with lower thermal conductivity are suit-able for achieving higher heat transfer performance. In de-signing micro heat exchangers, it is necessary to carefullydesign the shape and size of the devices and select appro-priate operating conditions that maximize their performance.Longitudinal heat conduction inside walls is ignored for thedesign of conventional macro heat exchangers. However, theeffect of longitudinal heat conduction cannot be neglected indesigning micro heat exchangers, because the ratio of wallvolume to channel volume is large. The radiation from thehousing is another term which must be embedded in the heattransfer model.

3.2. Design of micro plants

It is said that the production capacity is easily increasedby operating the micro plants in parallel. However, from theviewpoint of operating the process economically and safelyfor a long period, the decision of plant structure is not aneasy task. Over the last decade, many plant structures havebeen proposed (Felcht, 2002; Hessel & Loewe, 2003c; Lohf,Loewe, Hessel, & Ehrfeld, 2000; Quiram et al., 2000). Thosecan be classified in four types shown inFig. 8. Fig. 8a showsa Thep mberos tiona UOd tiono forts esa

The optimal structure must be determined by taking thefollowing terms into account:

(a) Types of MUOs suitable for the process: The MUOs orthe train of MUOs which is suitable for the given processmust be made clear. If a conventional unit operation isused in a part of the process, the structure shown inFig. 8bmust be adopted.

(b) The time allowable for the material to transfer to the nextdevice: In an MCP, the residence time in the connectiondevice cannot be neglected. If a shorter residence timein the connection device is desirable, the structures inFig. 8a and c are better than that inFig. 8b.

(c) Operating temperature: As is shown inSection 3.1.3,the heat transfer in the longitudinal direction cannot beneglected in the micro device. Thus, it is not desirableto aggregate two MUOs which are operated at differenttemperatures in a device. If two MUOs are operated atdifferent temperature, the structures shown inFig. 8band c are better than that inFig. 8a.

(d) Operating pressure: There are some proposals for themicro pumps. However, from the viewpoint of flowcapacity and reliability, conventional pumps with lessmicroseisms may be used to transfer the materialsamong devices. When the operating pressures of MUOs

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typical case of the numbering-up of unit micro plants.roduction rate can be increased by increasing the nuf unit micro plants operated in parallel.Fig. 8b shows thetructure in which the micro devices having the same funcre connected to one aggregated device. If an efficient Moes not exist for a type of unit operation, a combinaf micro devices and a conventional device is possible

he structure inFig. 8b. The structure shown inFig. 8c isimilar to that inFig. 8b, but the flow pattern is different. Thtructure shown inFig. 8d is the hybrid structure ofFig. 8and b.

Fig. 8. Four types of plant structures.

are different, a pump is requested at every interseof unit operations. For such a case, the structures sin Fig. 8a and c are not recommended.

(e) Flow uniformity: When precise flow control of each uoperation is needed, the structures shown inFig. 8a andc are better than that inFig. 8b.

(f) Flexibility of the production rate: The residence timethe device is an important design factor which affectsperformance of the device. Thus, a large change oproduction rate is difficult for the structures inFig. 8band c. When the structure shown inFig. 8a is adoptedthe production rate can be easily changed by chanthe number of unit micro plants to be operated.

g) Flexibility of the replacement of unit operations: Fthe structures inFig. 8b or c, the structure of the placan be changed by changing the connections amthe devices. This feature can effectively be used fmulti-purpose plant in which the structure of the plchanges product to product. For catalytic reactionsreactor must be replaced periodically by the deactivaof the catalyst. It is easy to replace the device whenstructure inFig. 8b or c is adopted.

h) Ease of instrumentation and control: This termexplained more precisely in the next section.

The structure of the micro plant should be decidedaking into account the many factors mentioned aboveresent, there is no quantitative unified index that cased to evaluate each factor. Thus, to derive the qualitxpressions of those factors is the first step of the formulf the design problem of MCPs.


4. Instrumentation and control of MCPs

4.1. Instrumentation of MCPs

A catalyst may progressively degrade with time, and theenvironmental temperature and pressure also change withtime. In order to produce desirable products, information onthe precise operating condition of the plant is indispensable.In many researches in micro reaction technology, measure-ment systems have been embedded in the device in order toanalyze the behavior of the micro systems. However, most ofthem do not entail continuous operation of the micro systemsfor a long time.

For conventional chemical plants, it is possible to add newmeasurement devices after the construction of the plant. Butit is almost impossible to add new measurement devices to theMCP after construction. Thus, the plant and the instrumen-tation and control systems must be designed simultaneously.At present, it is difficult to discuss the allocation problem ofthe instrumentation devices, because there are no standardsensors and actuators suitable for MCPs. Thus, the possibil-ity of inference system of unmeasured variables is mentionedin this section.

Fig. 9 shows a micro plant consisting of parallelproduction lines. In this plant, it is assumed that reactant A isc thec ethero alysto uctc ffi-c thep ver,i stedh mayb tingc

•• ional

• od-

ari-a stemse .

Fig. 10. Feedback control of product temperature.

4.2. Control of MCPs

As mentioned in the previous section, the design of aplant and the design of the control system should be dis-cussed simultaneously. In this section, two examples areshown in order to explain how the design and the control arerelated.

4.2.1. Reduction of controlled variablesIn conventional chemical plants, feedback control is pre-

dominantly used to keep the plant at the desirable condition.For example, the temperature of the reactor is controlled asshown inFig. 10a. The temperature of the liquid in the reac-tor is measured, and the flow rate of the heating medium isadjusted so that the temperature of the liquid in the reactorbecomes the predefined value. Since MCPs have more de-vices than conventional chemical plants, it means that theyhave many variables to be controlled. When the number ofcontrol variables increases, it becomes economically diffi-cult to install a feedback control system in every place to becontrolled (seeFig. 10b). Thus, the control variables must beselected carefully, taking the variation of the conditions andthe effect of each variation into account.

One possible approach is to adopt an indirect control sys-tem. One of the dominant characteristics of micro devices ist har-a quidi per-a ctures ber

.

onverted to product C by catalytic reaction. Activity ofatalyst gradually deteriorates. Thus, the problem is whr not we can estimate abnormal deterioration of the catf particular production line without measuring the prodomposition of every production line or not. It is usually diult to estimate the product composition of each line fromroduct composition of the aggregated outlet flow. Howe

f the plant is designed so as to satisfy the conditions liere, the product composition of each production linee estimated by collecting the data for different operaonditions:

Flow rate at each line can be estimated.Changes of the flow rate at each line are not proportto the changes of total flow rate.Effect of flow rate change to the reaction rate can be meled.

It is a typical estimation problem of unobserved state vbles. The methods developed in the field of process syngineering can be applied effectively to such a problem

Fig. 9. Estimation of product compositions.

he good thermal conductivity in the device. Using this ccteristic of the micro devices, the temperature of the li

n the micro device can be controlled by keeping the temture of the heating medium constant. By using the struhown inFig. 11, the number of controlled variables caneduced drastically.

Fig. 11. Feedback control of the temperature of heating medium


4.2.2. Robust design and controlIn a conventional chemical plant, a feedback control sys-

tem is used to keep the plant at the desirable state. In somecases, it is difficult to measure and control the flow conditionin the micro devices, although the flow condition is affectedby the change of a physical property of a fluid such as theviscosity. Therefore, the micro device should be designed sothat the effect of the change of the physical property to theflow condition is as small as possible. As the flow in the mi-cro channel is laminar, CFD simulation is effectively used toestimate the sensitivity of disturbances to the flow condition.The optimal shape of the device should be derived by takingthe various operating conditions into account.

How the design affects the sensitivity of the disturbancecan be explained by using an example. In the system shownin Fig. 11, the actual controlled variables are not controlleddirectly. Thus, if some disturbances are added to the pro-cess, it is not assured that the controlled variables keep theirset points. For example, if the flow rate decreases, the out-let temperature increases as shown inFig. 12a. Therefore,a robust design for dominant disturbances is indispensablewhen the control structure inFig. 11is adopted. In this case,by increasing the heat conductivity of the wall and settingthe outlet temperature close to the inlet temperature of theheating medium, the effect of flow rate change on the outlett

5e

de-s ned.A andc s int in thed erings n. In

Fig. 13. Bridge between the micro and macro engineering.

most cases, it is possible to assume laminar flow in microdevices. The development of a precise model is thus mucheasier than that of conventional plants. That is, the designof micro chemical plants can be regarded as a most appro-priate subject for applying a new design method based onchemical engineering science, systems science and computertechnology. There is no standard shape for each of the microunit operations. Therefore, by using such a design methoda completely new shape of the device or new structure ofthe plant may be generated systematically from the informa-tion of functions to be embedded in the device or plant. Atpresent, even in the field of micro reaction systems most of theinnovative results are obtained by trial-and error approaches.For process systems engineers to play a key role in the de-velopment of advanced new materials, the development of asystematic design method based on the precise models is es-sential. The author believes that micro chemical plants act asa bridge between chemical engineering science (micro area)and process systems engineering (macro area) as shown inFig. 13.

Acknowledgments

A part of this research has been performed in the Projecto ndM thes lantT trialT

R

B ngC imal

D 0

E ects.

emperature can be reduced as shown inFig. 12b.

. Conclusion—bridge between the micro and macrongineering

The dominant problems to be solved in the field of theign and control of the micro chemical plants were explailthough there is rapid progress in the field of chemistryhemical engineering science, the irregularity of the flowhe devices discourages us from using precise modelsesign problems. The latest results on chemical enginecience have not been reflected sufficiently in the desig

Fig. 12. Sensitivity of the system.

f Micro-Chemical Technology for Production, Analysis aeasurement Systems. The author would like to thank

upport of the Research Association of Micro Chemical Pechnology (MCPT) and the New Energy and Indusechnology Development Organization (NEDO).

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