Gas separation: processes – Filtering
Reexamination Certificate
2000-06-06
2003-12-23
Smith, Duane (Department: 1724)
Gas separation: processes
Filtering
C095S055000, C095S056000, C055S423000, C055S484000, C165S060000, C165S110000, C210S321750, C210S321840, C261S104000, C261S153000, C261S154000
Reexamination Certificate
active
06666909
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes and devices that utilize capillary forces to separate fluids. Several of the inventive embodiments are limited to microcomponent or microchannel devices that utilize capillary forces.
BACKGROUND OF THE INVENTION
Compact systems for capturing and/or separating fluids are desirable in a variety of applications. For example, hydrogen-powered vehicles could utilize fuel cells that recycle water. As another example, efficient and lightweight systems for recovery and reuse of water in spacecraft has long been recognized as a requirement for human space exploration. The present invention provides methods and apparatus for efficient fluid capture and separation.
SUMMARY OF THE INVENTION
The invention provides methods and apparatus for separating fluids and/or heat exchange. One process separates fluids by passing a mixture of at least two fluids, comprising a first fluid and a second fluid, into a device having at least one channel. The channel has an open area and a wicking region. The first fluid is either a liquid (such as a droplet or liquid particle) that is sorbed by the wicking region, or a gas that, under separation conditions, forms a liquid in the wicking region. The first liquid travels through the wicking region to a liquid flow channel and then exits the device through a liquid exit channel. The second fluid is a gas that passes through the gas flow channel to a gas exit, and exits the device through the gas exit.
The invention also provides a process of contacting fluids in which at least two fluids are passed into a device having at least one channel. The channel has an open area and a wicking region and an interface between the wicking region and the open area. During operation, at least one fluid flows through the wicking region, and at least one other fluid flows through the open area. At the interface between the wicking region and the open area, one fluid contacts at least one other immiscible fluid, and there is mass transfer occuring through the interface between the at least one fluid flowing through the wicking region, and the at least one other fluid flowing through the open area.
The invention further provides a method of condensing a liquid in which a gas passes into a device having at least one channel. The channel has an open area and a wicking region and is in thermal contact with at least one microchannel heat exchanger; and a heat exchange fluid is passed through the microchannel heat exchanger(s). During operation heat is removed from the gas stream causing some part to condense to form a liquid. Said formed liquid is sorbed into the wicking region, travels through the wicking region to a liquid flow channel and then exits the device through a liquid exit channel.
The invention further provides an apparatus having at least one channel comprising an open area and a wick. The wick in the channel is connected to an exit wick, and the open area is connected to a gas exit. This apparatus is useful for many of the processes described herein.
The invention also provides a liquid condenser comprising at least one channel; wherein the channel comprises a gas flow channel and a wick. The channel is in thermal contact with at least one microchannel heat exchanger. Both the apparatus and condenser are particularly well suited for use in a chemical reactor.
The presence of wicks and optional pore throats and capture structures are common to multiple embodiments of the invention. A wick is a material that will preferentially retain a wetting fluid by capillary forces and through which there are multiple continuous channels through which liquids may travel by capillary flow. The channels can be regularly or irregularly shaped. Liquid will migrate through a dry wick, while liquid in a liquid-containing wick can be transported by applying a pressure differential, such as suction, to a part or parts of the wick. The capillary pore size in the wick can be selected based on the contact angle of the liquid and the intended pressure gradient in the device, and the surface tension of the liquid. Preferably, the pressure at which gas will intrude into the wick should be greater than the pressure differential across the wick during operation-this will exclude gas from the wick.
The liquid preferentially resides in the wick due to surface forces, i.e. wettability, and is held there by interfacial tension. The liquid prefers the wick to the gas channel and as long as there is capacity in the wick, liquid is removed from the gas stream and does not leave in the gas stream.
The wick can be made of different materials depending on the liquid that is intended to be transported through the wick. The wick could be a uniform material, a mixture of materials, a composite material, or a gradient material. For example, the wick could be graded by pore size or wettability to help drain liquid in a desired direction. Examples of wick materials suitable for use in the invention include: sintered metals, metal screens, metal foams, polymer fibers including cellulosic fibers, or other wetting, porous materials. The capillary pore sizes in the wick materials are preferably in the range of 10 nm to 1 mm, more preferably 100 nm to 0.1 mm, where these sizes are the largest pore diameters in the cross-section of a wick observed by scanning electron microscopy (SEM). In a preferred embodiment, the wick is, or includes, a microchannel structure. Liquid in the microchannels migrates by capillary flow. The microchannels can be of any length, preferably the microchannels have a depth of 1 to 1000 micrometers (&mgr;m), more preferably 10 to 500 &mgr;m. Preferably the microchannels have a width of 1 to 1000 &mgr;m, more preferably 10 to 100 &mgr;m. In a preferred embodiment, the microchannels are microgrooves, that is, having a constant or decreasing width from the top to the bottom of the groove. In another embodiment, the microchannels form the mouth to a larger diameter pore for liquid transport.
The wick is preferably not permitted to dry out during operation since this could result in gas escaping through the wick. One approach for avoiding dryout is to add a flow restrictor in capillary contact with the wick structure, such as a porous structure with a smaller pore size than the wick structure and limiting the magnitude of the suction pressure such that the non-wetting phase(s) cannot displace the wetting phase from the flow restrictor. This type of restrictor is also known as a pore throat. In preferred embodiments, a pore throat is provided between the wick and the liquid flow channel and/or at the liquid outlet. In some embodiments, the wick can have a small pore diameter such that is serves to transport fluids from the gas channel and also prevents gas intrusion, thus serving the dual purpose of a wick and a pore throat.
A pore throat has a bubble point that is greater than the maximum pressure difference across the pore throat during operation. This precludes intrusion of gas into the pore throat due to capillary forces (surface tension, wettability, and contact angle dependent). The pore throat should seal the liquid exit, so there should be a seal around the pore throat or the pore throat should cover the exit in order to prevent gas from bypassing the pore throat. The pore throat is preferably very thin to maximize liquid flow through the pore throat at a give pressure drop across the pore throat. Preferably, the pore throat has a pore size that is less than half that of the wick and a thickness of 50% or less than the wick's thickness; more preferably the pore throat has a pore size that is 20% or less that of the wick. Preferably, the pore throat is in capillary contact with the wicking material to prevent gas from being trapped between the wick and the pore throat and blocking the exit.
Flooding can result from exceeding the flow capacity of the device for wetting phase through the wick; the flow capacity is determined by the pore structure of the wick, the cross-sectional area for flow, or the pressure drop in the wi
Gauglitz Phillip A.
Stenkamp Victoria S.
TeGrotenhuis Ward E.
Wegeng Robert S.
Whyatt Greg A.
Battelle (Memorial Institute)
Harrington Todd J.
Pham Minh-Chan T.
Rosenberg Frank
Smith Duane
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