Microreactors

Quak Foo LeeDepartment of Chemical and Biological Engineering

The University of British Columbia

Introduction

In the last two decades powerful processes have been developed for the fabrication of three-dimensional microdevices from a wide variety of materials.

Processes

Bulk micromachining of monocrystalline materials, e.g., silicon, by anisotropic wet chemical etching.

Dry etching with low-pressure plasma of ion beams (reactive ion etching, reactive ion beam etching)

A commbination of deep lithography, electroforming, and molding

Micromachining with laser radiation Anisotropic etching of photosensitive glass Advanced mechanical milling, turning, sawing, and

drilling processes based on precision engineering Isotropic wet chemical etching, e.g., of metal foils with a

resist pattern

Advantages of Microreactors

Faster transfer of research results into production Earlier start of production at lower costs Easier scale-up of production capacity Smaller plants for production at distributed sites Lower costs for transport, materials, and energy More flexible reaction to market demands

Microreactor Components

Micromixers

Micromixers

Injection of Multiple Microjets

Schematic of a micromixer with injection of multiple microjets into a mixing chamber

1. The central element of the mixeris a sievelike structure with a largenumber of regular holes.

2. During operations, the mixing are is filled with one liquid, and the other liquid is injected into themixing volume through a multitudeof microholes.

3. Numerous microjets are generatedand increase the contact surface between the two liquids.

4. The holes are positioned in rows10-100 μm apart, which resultsin short diffusional paths betweenthe jets.

5. Typical flowrates are in the μL/s, the hole diameter is 10 μm, and theheight of the mixing chamber some 100 μm

Micromixers

Injection of Multiple Microjets Experiment studies

Microscopic observation of the jets showed that a homogeneous mixture was established with a few seconds.

Complete mixing at a flow rate of about 1μL/s was found within a mixing time of about 1s.

Micromixers

Multiple Flow Splitting and Recombination

Mixing units of a static micromixer with multiple slit-shaped injection openings

1. Application: industrial chemical sensor2. Flow range: 0.01 – 0.1 μL/s3. Highly viscous flow with a Re < 14. The whole system consists of a

silicon/glass sandwich connected by anodic bonding.

5. One channel structure is etched into glass and the other into silicon.

6. In the region where the channels overlap, they are separated by a structured plate defined by an etchstop layer.

7. Max. width = 300 μmmax. depth = 30 μm

8. The thickness of the structured plate for separating the channels in the glass and the silicon wafer is 5 μm and the slit width 15 μm.

Micromixers

Multilamination of Fluid Layers

Multilamination of streams in channels with corrugated walls, leading to fast mixing by diffusion

Scanning electron micrographs of a mixing element based on multilamination of thin fluid layers. The device consists of 2 × 15 interdigitated microchannels with corrugated walls, fabricated by LIGA technology

1. The fluid to be mixed are introducedinto the mixing elements in counter-flowand stream into an interdigitated channelswith corrugated walls.

2. Typical channel widths = 25 or 40 μm3. The channel configuration leads to a

periodical arrangement of flow lamellaeof the two fluids.

4. The lamellar flow leaves the deviceperpendicular to the direction of the feedflows and, because of the thinness of thelamellae, fast mixing takes place by diffusion.

5. The corrugated channel walls increase thecontact surface of the lamellar streams andimprove the mechanical stability of theseparating walls.

Microreactor Components

Micro Heat Exchangers

Micro Heat Exchangers To effectively transfer heat from one flowing fluid across a solid boundary

to another flowing fluid, sufficiently large contact areas and temperature gradients are required.

Another important criterion is the ratio of heat transfer to pressure loss.

Fluid stream is split into many partial streams of small dimensions (e.g. plate heat exchangers).

These partial streams are characterized by low Reynolds numbers, i.e. they show viscous rather than turbulent behavior.

Decreasing the dimensions of the fluid flow increases the temperature gradients and the exchange surface to volume ration, that is miniaturization leads to a better heat exchange.

The decreased flow dimensions are inevitably associated with increased viscous losses, but the overall heat transfer to pressure loss ratio is improved.

Micro Heat Exchangers

Cross-Flow Heat Exchange in Stacked Plate Devices

Central component of a cross-flow micro heat exchanger consisting of a stack of crosswise oriented metal platelets (source: Forschungszentrum Karlsruhe)

Micro Heat Exchangers

Cross-Flow Heat Exchange in Stacked Plate Devices

1. About 100 platelets, several square centimeters in size and containing rectangularmicrochannels, are stacked crosswise and bonded hermetically.

2. Two separate passages for the heat transfer fluid and the process fluid with about4000 microchannels are formed.

3. The cross section of a single microchannels is 100 × 80 μm, the material thicknessbetween the two fluids in the crossing channels is 20-25 μm.

4. The stack is fitted with top and cover plates and connected to fittings for the inletand outlet ducts of the heat exchanger fluids.

5. The active volume of such a micro heat exchanger is typically 1 cm3 with an innersurface area of 300 cm2 and a heat transfer surface of 150 cm2.

6. The passages are helium-tight both with respect to each other and to the outside.7. Because of the small dimensions of the channels and the strong bonding, a relatively

high operating pressure can be applied (25 bar).8. Example: At a mean temperature difference of 59.3 K, a power of 19,2 kW was

transferred in an active volume of 1 cm3. The overall heat transfer coefficients wereto 25 kW m-2 K-1, corresponding to a volumetric heat transfer coefficient of 0.3 kW cm-3 K-1.

Micro Heat Exchangers

Cross-Flow Heat Exchange in Stacked Plate Devices

Micro heat exchanger with connections for fluid supply (source: Forschungszentrum Karlsruhe)

Micro Heat Exchangers

Counterflow Heat Exchange in Stacked Plate Devices

Schematic of the design and flow configuration of a plate-type counterflow micro heat exchanger

Assembled counter-flow micro heat exchanger with PEEK housing and single platelet

Micro Heat Exchangers

Counterflow Heat Exchange in Stacked Plate Devices

1. Compared to other heat exchange configurations, counterflow is the most efficientfrom a thermodynamic point of view.

2. It consists of platelets with an outer frame and an inner thin membrane, stacked to form a plate-type heat exchanger.

3. The membranes have parallel fines that form channels that guide the fluids and increase the mechanical stability of the device, which is of major importance foroperation at high differential pressure.

4. The platelets comprises openings in their corners, where two diagonal openings alternately from the inlet and outlet ducts for one fluid.

5. The other fluid passes through the remaining openings to the adjacent platelet.6. On both sides of the membranes, which are very thin to ensure efficient heat

transfer, uniform countercurrent flow is established by the parallel fins.7. The total amount of heat transferred in such a stacked device is determined by the

number of platelets, which is limited by the pressure losses in the inlet and outletchannels.

Microreactor Components

Microseparators

Microseparators

Exchange between Immiscible Fluids

Schematic of solute exchange between immiscible fluids in partially overlapping microchannels (left) and scanning electron micrograph of the cross section of the partially overlapping microchannels (right)

Microseparators

Exchange between Immiscible Fluids

Scanning electron micrograph (left) and schematic (right) of an extraction unit with adjacent channels for two fluids with slits, oblique to the flow direction, for exchange between the two phases

Microseparators

Exchange between Immiscible Fluids Extraction processes considered are based on the contact of two

immiscible fluids and solute transfer between the two phases. Miniaturization leads to an increase of surface area to volume

ratio, it results in a corresponding enlargement of the exchange interface.

Stable flow can be achieved within a certain range of flow rates and viscosities.

The stability of the flow is influenced in particular by surface forces, whereas other parameters sich as buoyancy, momentum, and viscous are of minor importance.

Splitting of the contacting fluids by a wedge-shaped flow divider can be performed with a high precision and only minor mixing of the two phases.

Microseparators

Filtration, Diffusion, and Aerodynamic Separation

Scanning electron micrograph of a cross-flow filter consisting of lamellae arranged at an angle of attack to the flow direction

Microseparators

Filtration, Diffusion, and Aerodynamic Separation In the macroscopic range, filtration and sieve structures are often carefully

designed with regard to the shape and position of the openings. In the microscopic range usually porous materials with irregular pattern are

applied. Microfabrication methods allow the production of completely isoporous

microfilters from a wide variety of materials, whereby the size, shape, and position of each pore can be designed.

Typical microfilters have pore dimensions in the micrometer range. Special configurations allow the realization of cross-flow filters for

concentrating suspended particles or cells. In the case of membrane units, microfabricated devices are useful as

carrier structures with integrated inlets and outlets for fluids. Integrated membrane units are being developed for gas-phase microreactors, and microfabricated carrier structures could become important in fuel cells.