Gas Analyzer With Diffusion Filter

This invention relates to gas analyzers and more particularly to analyzers having filters to separate liquids and particulate solids from the gas to be analyzed.

Most gas sensors and analyzers, e.g. electrochemical, catalytic combustion, semiconductor, infrared, ultraviolet, flame ionization, gas chromatographs and mass spectrophotometers, are adversely affected by water and particulate solids. It is conventional practice to flow such dirty gas streams through water traps and/or filters, usually paper or fritted glass or metal filters, to provide a clean gas stream to the analyzer. Many gas streams have condensable liquids that together with entrained particulate matter form a solid sludge that plugs filters. Examples of particularly difficult sample streams are stack analysis for combustion control and incinerator exhaust analysis. In these cases, both condensables and particulates exist, and frequently dissolved corrosive gases, such as SO.sub.3 and HCl. Conventional filtering techniques suffer from entrainment of contaminants or plugging of filter media. Even filters using permeation driers, inertial filtering, water wash and other methods, have proven to require very frequent maintenance and are marginal at best.

FIG. 1 is a schematic diagram of the gas analyzers with the diffusion filter of this invention.

FIG. 2 is an elevation of the diffusion filter of FIG. 1.

FIG. 3 is a section on line III--III of FIG. 2.

In accordance with this invention a gas permeable diffusion membrane separates a flowing stream of dirty sample gas from a closed circuit clean gas stream circulated through an analyzer. The dirty and clean streams are preferably flowed in opposite directions along the diffusion membrane. The membrane does not pass liquids or particulate solids but does permit rapid diffusion of gases so the gas composition in the closed circuit rapidly reaches equilibrium with the sample gas.

The membrane is preferably a synthetic, gas porous material that does not readily wet, to minimize clogging. Preferred membranes are preferably Zitex.RTM. or Goretex.RTM. porous fluorocarbon membranes. Other membrane materials may be used provided they have characteristics similar to Zitex and Goretex, including not becoming wet by water or other liquid in the sample gas, having a large number of pores (e.g. 50% porous) which in turn are of small pore size and being thin enough to permit a sufficiently fast gas diffusion rate that gives a substantial equilibrium between the sample and closed circuit gas streams.

With reference to FIG. 1, a diffusion filter 2 comprises a housing 4, having a chamber divided by membrane into a first elongate compartment 8 and a second elongate compartment 10. The first compartment has a sample gas inlet 12 and a sample gas outlet 14, so that sample gas can be flowed lengthwise through the compartment. Compartment 10 likewise has an inlet 16 and an outlet 18 connected in a closed circuit loop with the inlet 20 and outlet 22 of an analyzer 24. Pump 25, or a pump incorporated in the analyzer, circulates gas in the closed circuit.

In operation, fluid or particulate solids in the sample stream are prevented by the membrane from passing from compartment 8 into compartment 10, and are discharged with the gas stream from outlet 14. Gas from compartments 8 and 10, however, are free to diffuse through the membrane, so the composition of gas in the closed circuit is in equilibrium with and substantially the same composition as the gas phase of the sample stream. The area and thickness of the membrane are selected to provide a diffusion rate sufficient to give a desired response time to reach equilibrium.

Exemplary of the invention, using a wet sample stream of CO.sub.2 in air and a Goretex No. S10755 (expanded PTFE; thickness, 0.010 inches; density, 0.7 g/cc; porosity, 68%) membrane having 4.5 square inches of surface exposed to the sample and analytical gas streams, the interposition of the diffusion filter between a CO.sub.2 electrochemical analyzer and the sample stream increased response time less than 5%.

FIGS. 2 and 3 show the now preferred embodiment of the diffusion filter in more detail.

Housing blocks 26 and 28 have matching serpentine semicircular grooves 30 and 32 respectively. A diffusion membrane 34 is secured between the housing blocks by bolts 36 and nuts 38. The grooves 30 and 32 form two compartments separated by the membrane 34. Inlet and outlet connections to each compartment are provided by bores 40, 42, 44 and 46. This construction provides a long flow path with good mechanical support of the membrane.

It is understood that this invention may be practiced otherwise than as described above within the scope of the appended claim.