Apparatus for Preventing Particle Deposition From Process Streams on Optical Access Windows

This invention relates to apparatus and methods for preventing contamination of optical windows used for in-situ measurements of process streams.

In-situ measurements of species in process streams are preferred over extractive methods of analysis because of the uncertainty that extracted samples remain representative of the process streams once they are removed and because of the extended time required for laboratory analyses. In-situ optical methods of analysis require the installation of windows on a process stream pipe, duct, or other housing, through which a light can enter and/or exit the process environment. This enables spectroscopic determinations of atomic or molecular species concentrations or measurements of physical characteristics of process streams such as temperatures, velocities, or particulates.

An inherent problem with the use of optical windows in making such measurements is the tendency of solid particles to become deposited on the interior surface of the windows. Deposits on the windows may arise from sources such as dust, process particles, or condensible vapors carried in the process stream. A layer of particles builds up on optical windows, attenuating light beams and interfering with optical measurements. A solution to this problem is especially needed for prevention of window contamination in apparatus used for study of coal conversion processes, where high temperatures, pressures, and contamination levels are frequently encountered, and where a need exists for unobstructed access for extended time intervals. For such applications, the design used to minimize window contamination must avoid producing chemical and physical changes to the process stream in the region being monitored.

Various approaches have been taken to remove or compensate for particle deposition on optical windows of process stream equipment. The attenuation of light beams by particulate deposits has been compensated for to some degree by using as a reference a parallel light source which is not absorbed by the process stream constituents. Both the signal and reference beam will be attenuated to the same degree by the layer of particles on the window. Thus, the degree of attenuation due to particles on the window is known, and the signal beam intensity measurements can be adjusted accordingly. This technique is useful for thin layers of deposits; however, as deposition on the windows continues, optical measurements become impossible, and the windows have to be cleaned. Cleaning the windows usually requires shutting down the process. Thus, frequent cleaning of optical windows is inconvenient and, in many cases, impossible.

Clean gas jets impinging on the interior surface of windows have been used to reduce the rate of particle deposition. However, this technique is often ineffective. This technique has the further disadvantage that the gas jet used to purge the windows mixes with the process stream, changing its composition and temperature profiles. Relatively large volume flows of purge gas are required to substantially reduce the rate of particle deposition.

The present invention is directed to an electrostatic precipitator positioned inside the window of a gaseous process stream viewing port. Deposition of solid particles carried the process stream onto the window of the port is prevented by collection of the particles on a precipitator electrode. The precipitator includes a discharge electrode and a collecting electrode operably connected to a source of unidirectional voltage, the electrodes being disposed around the periphery of the viewing port so as to avoid interference with optical access. Viewing ports for which the invention may be used typically are located at the ends of tubular housing perpendicular to a pipe or duct in which the process stream is carried, with transparent windows disposed across ends of the housing.

In operation, a strong electrical field is developed by initiating a voltage differential between the electrodes. Air molecules are ionized at some critical voltage, causing a flow of negative and positive gas ions to the collecting and discharge electrodes, respectively. The air ions become attached to the particles, giving them a charge and also causing them to migrate toward the electrodes. In this way, particles are removed from the gas stream and are prevented from depositing on the window. The gas adjacent to the windows is stationary, and the particles move through the gas by electrophoresis. Thus, the particles have low velocity, facilitating their removal using this device and method.

Devices embodying the invention prevent particle deposition on optical windows used on process streams or reactor apparatus to a much greater degree and more efficiently than any other method available. A further advantage is that the device does not require the use of gas jets which would mix with the process stream and change its composition or temperature profiles. This facilitates making optical measurements and keeps sight windows clean so as to allow undiminished viewing of the process being observed.

It is, therefore, an object of this invention to provide apparatus for preventing deposition of solid particles on optical windows used for viewing process streams.

Another object is to provide such apparatus that keeps optical windows clean without requiring use of purge gases.

Other objects and advantages of the invention may be seen by reference to the following detailed description and the appended claims.

FIG. 1 is a view taken in section showing a precipitator embodying the invention installed in a pair of viewing ports of a process stream pipe.

FIG. 2A is a pictorial view, partly broken away, of the area shown by line 2A--2A of FIG. 1.

FIG. 2B is a side view showing one embodiment of an inner electrode ring.

FIG. 2C is a side view showing a toothed ribbon electrode embodiment.

FIG. 3 is a pictorial view, partly broken away, showing an alternate embodiment of the invention wherein the discharge electrode has a plurality of radially extending fingers.

Referring to FIG. 1 of the drawings, there is shown a tubular reactor containment chamber 10 adapted to have a particle-containing or particle-producing process stream moved longitudinally therethrough. The chamber is defined by a metal pipe 12 and refractory liner 16 on the inside of the pipe. Radially extending viewing ports 14, 14a are provided opposite from one another on the pipe wall, the ports being contained within refractory-lined tubular housing 18, 18a joined to the pipe. The ports are in axial alignment with one another to enable light to be passed through the chamber from one side, with observations or measurements being made at the opposite side.

Transparent windows 20,20a are disposed across the housing ends in a sealed assembly that prevents escape of process stream gas. The windows are supported in circular plates 21, 21a, which in turn are secured to flanges 22, 22a. The windows are made of a material selected to provide transparency at the spectral range of the investigations, and they are preferably sized to accommodate the F number of the optical apparatus involved.

Precipitators 24 and 24a of the present invention have a pair of electrodes 26, 28 and 26a, 28a which may be made of any conductive metal compatible with the process temperatures and atmospheres involved. Outer electrodes 26, 26a are provided in the form of conductive metal bands embedded in the inner surface of the refractory liner and are connected to ground. Inner electrodes 28, 28a may comprise conductive metal wire rings as shown in FIGS. 2A and 2B supported by stiff conductive wires 30 that extend outwardly through apertures in the refractory liner, the electrodes being spaced apart from one another by the refractory material. Insulated high-pressure feedthroughs are used for passage of the electrode wires through the end plates to avoid leakage of the process stream gas. The inner electrodes are electrically connected to a current-limiting high volta source 31 selected to provide a potential sufficiently high maintain a steady corona discharge, but below the voltage which would produce an unstable condition bordering on flareover. The maximizes ionization of gas molecules in the process gas, initiating particle migration. The electrodes may be made of electrically conductive material suitable for use at the temperatures and atmospheres presented in a specific process stream. Either of the electrodes may be given a positive or negative charge, with the other electrode being given the opposite charge.

FIG. 3 shows an embodiment in which electrode 32 is a met band embedded in the refractory liner in axial alignment with electrode 26. Conducting fingers 30 connected to electrode 32 have an L-shaped structure, extending radially inward for a short distance and then axially away from the window, the fingers being disposed near the periphery of the window so as not to interfere with viewing. This structure provides for enhanced ionization of gas molecules in the viewing port.

The invention is described above with reference to its application to apparatus having a pair of viewing ports disposed on opposite sides of a reaction chamber, which enables light to be passed through both ports for making and recording observations. It is to be understood that the precipitator of this invention may also be used for single viewing ports. Other equipment such as lenses for focusing light into the desired viewing area may also be used in combination with the apparatus shown.