SE448682B - PROCEDURE FOR REMOVING PARTICULATE MATERIAL FROM A FLUID OF COMPRESSED GAS - Google Patents

Abstract

A liquid-tight enclosure (10) is partially filled with water (14). A container (16) supports a porous bed of sand and gravel that is partially submerged in the liquid. The lower end (22) of the porous bed is spaced upward from the bottom (11) of the enclosure. A gas delivery duct (23) feeds gas and entrained aerosols to a location vertically beneath the porous bed to enable a stream of gas to pass upwardly through the porous bed, drawing water with it to continuously clean the gravel and to remove aerosols from the stream of gas, which is discharged at 26. <IMAGE>

Description

448 682 in the stream.

Sand or gravel filters are built up using layers of size-grained grain material with the largest grain sizes usually stored on the bottom of the filter and the following layers of successively smaller grains arranged upwards from the bottom layer. Aerosol-laden gas passes from the bottom of the filter bed to its top. The aerosol particles are removed by inertial forces, diffusion, capture, gravitational forces, grain growth and deposition. Due to the layers of finely sorted grains at the upper parts of the bed, sand or gravel filters show a high cleaning effect for aerosols.

However, since such a bed has a limited void volume, the filter can only handle a small amount of collected material per unit volume of filter material which must then be replaced, flushed or otherwise purified.

The hybrid washer described consists of a porous bed which is at least partially immersed in a basin of the washing liquid. The porous bed may contain granular material such as sand or gravel. The liquid may be water. A stream of aerosol-laden compressed air or other gas is introduced into the bottom of the bed. It distributes itself over the bed and flows up through the irregular channels formed in the spaces of the bed. The bed is continuously cleaned of the gas entrained in the gas. The pure gas leaves the porous bed at its top.

In a passive self-cleaning aerosol washer for removing particles from a compressed gas, a liquid-tight containment vessel is used which is partly filled with washing liquid. A container with gas-tight side walls has been placed in the containment vessel, which extends from a lower end immersed in the liquid to an upper end. The upper end of the container and the porous bed may be below, level with or above the liquid surface. The porous bed extends vertically upwards from a submerged bottom level which is higher than the lower end of the side walls of the container. A gas supply line extends down into the containment vessel and empties below the bed to direct aerosol-laden compressed gas to the bottom of the bed. The bed is open at the top so that liquid, which came with the gas, can be returned to the liquid in the containment vessel.

The washed gas passes through the top of the bed and is discharged through an outlet.

The process according to the invention will be described in more detail below with reference to the accompanying drawings.

Fig. 1 shows a schematic cross-sectional view through the aerosol washer used in the process, and Fig. 2 shows a modified washer.

The washer shown in Fig. 1 basically consists of a porous bed which is at least partially immersed in a liquid pool. Upward gas flow is initiated by extracting gas from the outlet line at the top of the appliance. Aerosol amount of gas flows down through the inlet line and up through the irregular porous channels in the bed. Liquid jets down through the bed due to the difference in density of the liquid outside the bed and the apparent density of the gas-liquid mixture inside the bed. By this spraying of liquid, the porous bed is washed free of collected aerosol. As shown in the drawing, a practical embodiment of the device consists of a liquid-tight containment vessel 10 with bottom 11 and connecting vertical side walls 12. The container is preferably completely closed by a roof 13, but can also be open as will be described below.

The containment vessel 10 is partially filled with a liquid 13 to a level 14. The liquid may be water or another desired liquid. 448 682 In the containment vessel 10 has been placed a container 16 open at the ends with gas-tight side walls extending from a lower end 18 to an upper end 20. The lower end 18 of the container 16 is immersed in the liquid 14, while its upper end 20 is either located next to, above or below the liquid level 15 in the containment vessel 10.

A porous bed 21 of gravel or other filter material arranged in the container 16 extends up into the container 16 from a bottom position which lies above the lower end 18 of the vertical side walls 17 of the container 16. This position is delimited by means of a transverse space between the side walls 17 arranged porous or perforated plate 22. In the preferred embodiment shown, the porous bed 21 is located at about half its height below the liquid level 15 and is thus immersed in the liquid 14. An inlet line 23 is arranged to direct a stream of compressed gas with particles suspended therein to a location below the porous bed 21. The conduit is shown as a vertical tube of gas-tight material extending through the center of the porous bed Z1. The inlet conduit 23 terminates at an open bottom end 24 lying at a level between the bottom of the porous bed 21 and the lower end of the container walls 17.

The porous bed 21 is shown covered by a porous or perforated roof plate 25. Although such a roof plate is desirable, it is not always necessary to keep the bed 21 enclosed. The upper surface of the porous bed 21 is open for the passage of liquid so that the liquid droplets which followed the gas flow over the sides of the container 16 can flow back down into the liquid 14 in the containment vessel 10.

Different materials can be used in the porous bed 21. The porous material must be insoluble in the liquid. It can be sand or gravel, fibrous materials or other packing material that is usually used in dry or wet filters.

An outlet line 26 is connected to the containment vessel 10 at a level above the liquid level 15. After the passage of the gas through the porous bed Z1, it is discharged through the line 26. 448 682 By the presence of gas in the porous bed 21 enclosed by the container 16, it is reduced. apparent density of the liquid in the bed nooks and crannies. During the flow of the gas up through the bed 21, therefore, according to the principle of the mammoth pump, the liquid from the containment vessel 10 flows or is "pumped" into the bottom of the bed, up through the bed and drains over the top. Collected aerosol in the porous bed 21 is thereby continuously washed away therefrom. This passive, self-cleaning function of the porous bed 21 is one of the new features of this device.

With the device shown, aerosols are effectively removed from a gas stream. The efficiency of aerosol purification can be adjusted by changing the depth of the bed 21, the grain sizes of the packing materials of which the bed is composed, and the flow rate of the gas as it passes through the bed. The only limitation on the amount of collected material that can be accommodated in the apparatus is the volume of the liquid 14 in the containment vessel 10 and the solubility of the removed aerosol materials in the liquid. Another limitation is the volume of soluble particles that can be accommodated in the containment vessel 10.

The washer is a three-phase washer. The solid phase, which consists of the porous material in the bed 21, is stationary, while the gas and liquid phases are conducted in countercurrent through the bed 21.

Due to the complex nature of the system, studies were conducted to develop the concept and measure the performance of the washer.

A prototype of the washer was constructed substantially in the manner shown in Fig. 1. The bed was 300 mm in diameter and 610 mm deep and was packed with basalt gravel sieved between + 9, 5 mm and -12.7 mm. The bed's free passage area for gas passage was 6.9 dmz. The cavity portion of the bed was 0.45 + 0.050.

The basalt gravel used in the experiments was characterized by the grains not having smooth sides. It was held in the container 16 between horizontal plates 22 and 25 made of solid flat plates with holes evenly distributed over the surface and a central opening for the inlet pipe 23. Another suitable type of support plate could be made of a suitable 448-682 screening material.

The lower end 18 of the side walls 17 of the container 16 is provided with openings 27 arranged at a level above the bottom of the containment vessel 10 to prevent the return of insoluble particles to the liquid and gas flowing into the bed from below. The openings 27 enable the inflow of liquid from below into the bath 21. The area between the openings 27 and the bottom of the bed Z1 forms a space in which incoming gas is briefly collected before it flows up through the bed 21.

As can be seen from Fig. 1, the horizontal transverse area of the container 16 is considerably smaller than the transverse area of the container 16. The transverse area and depth of the liquid 14 in the containment vessel is a function of the storage capacity required to care for the aerosol removed from the gas stream. The vertical side walls 12 of the containment vessel 10 are spaced outside the side walls 17 of the container 16. As a result, liquid can flow into the container 16 through the openings 27 from all sides, and the liquid leaving the container drains around the entire periphery.

In the particular example, concentric cylindrical walls arranged coamially with the vertical gas inlet line 23 are used for the containment vessel 10 and the container 16.

A series of experiments were performed using different aerosols of sodium compounds to measure the efficiency of aerosol purification using water as washing liquid.

Table I lists the test parameters showing the ridge velocity based on a free flow area of the bed of 6.9 dmz.

During each run, the volume flow of the gas was varied and aerosol samples were periodically taken from both the inlet line 23 and the outlet line 26. A conventional dry filter was placed in the outlet line 26 downstream of the scrubber to be leached and analyzed for sodium after each run.

The experimental results are summarized in Table II. The total efficiency was calculated from the total amount of sodium collected in the washing solution 448 682 and on the fibrous filter.

The average efficiency was calculated by the arithmetic mean of individual efficiencies determined from measurements of the instantaneous gas concentration. These values show that a change in the grain sizes in the bed Z1 has no effect on the collection. When the bed height was reduced by half, the aerosol penetration increased by a factor of 7.

TABLE I Test conditions Flow test Aerosol source Aerosol type Test time, h speed m / min l Spray fire Na2CO3 9.7 2 - 30 2 Pool fire Na202 4.3 1.2 _ 22 3 Pool fire Naz fl z 2.7 2 - 27 4 Pool fire Na202 4.0 0.8 - 20 S Spray fire NaOH 24 17 - 27 TABLE II Exam result Total efficiency- Average Collected Tested efficiency efficiency amount Na, g parameter 99.8 99.18 748 Type of aerosol 99.8 99.97 590 Type of aerosol 98 , 6 98.78 401 Reduced bath height (305 mm) 4 99.8 99.97 S66 Reduced ator- play of packing (634 '° 915 mm) 5 99.996 99.998 4517 Fibrous fi filter uillßa fifi In the final test 5 Qom reported in Tables I and II, the aerosol scrubber was supplemented with a polypropylene fiber filter (Fig. 2) to remove the accompanying mist 448 682 and improve the capture of small particles. The filter had 610 mm outer diameter, 460 mm inner diameter and 610 mm length.

It was placed directly above the porous bed 21, from which the effluent gas flowed up into the central region of the filter 23 and out horizontally through the fibrous filling. The droplets of water which accompanied the gas leaving the bed 21 continuously washed away the collected aerosol from the filter. The special test 5 reported in Tables I and II was performed using a NaOH aerosol. The pressure drop across the filter 28 remained constant during the test, indicating that the fibers therein had been sufficiently washed by the entrained water. This test shows the increased cleaning effect that can be achieved by combining the hybrid washer with an available fibrous filter.

Hydraulic tests were performed without aerosols for different bed configurations to measure the effects of pressure drop, water circulation and water level. The pressure drop in the porous bed Z1 was found to be independent of volume flows corresponding to flow velocities between 0.002 and 0.5 m / s. The pressure drop in the apparatus was found to be primarily dependent on the hydrostatic pressure at the submerged bottom end of the inlet line 23. The volume flow of the internal water circulation was found to be a function of the gas volume flow, bed depth, grain size and inlet depth. When checking the effect of the gas flow on the water flow, it was found that water was pumped with decreasing volume flow to the same extent as the water level dropped approximately to half the depth of the bed.

The test parameter with the greatest influence on the water flow was the immersion depth of the inlet pipe 23.

The apparatus according to the invention is capable of handling a gas stream at a pressure of 70-350 kPa, which is typical of vessels used in nuclear reactor installations. Gas streams discharged from such containment vessels can, if necessary, be throttled to meet the limitations of the volume flow in a special washer. No other pumping is required for the gas stream, so that any energy requirement for the activation of the washer 448 682 is eliminated.

The tests performed on the experimental model show that a passive self-cleaning washer can be based on a gas flow rate of 0.5 m / s and a bed depth of 600 mm. The efficiency of aerosol purification can be predicted to exceed 99% for such aerosols that can be expected to occur in a nuclear power plant. The efficiency of aerosol purification would exceed 99.9% for all probable grain distributions if a passive fibrous filter were included as shown in Fig. 2.

In this device a mammoth pump is used to circulate washing liquid through the porous bed 21. This is kept clean during the use of the washing machine without the need to use an external liquid pump which in turn would require an energy source. This is of the utmost importance when electric power is not available.

A scrubber can be designed so that its efficiency for aerosol purification obtains the value required for a particular use. A high efficiency for the removal of small particles can be realized. This is a definite advantage compared to submerged pipes, where large bubbles lead to low efficiencies for the removal of finer particles.

Since the porous bed 21 is constantly kept wet, entrained dust will either dissolve or be washed away from the bed material. Stoic amounts of airborne particles can therefore accompany the gas without the porous bed 21 becoming clogged.

Compared to a simple submerged pipe, this apparatus has a much lower pressure drop in a device designed to provide the same efficiency for purification. This is because the gas stream is dispersed into small bubbles when it enters the porous bed 21.

All the flow channels in the porous bed, which are more or less narrow, are washed by the liquid and are therefore not clogged. The inlet line 23, which is not washed, can be of the size desired to prevent clogging. This means a great improvement compared to other bubble developing devices, e.g. narrow bubble tubes immersed in the washing liquid. 448 682 "10 'The device according to the invention can be built of simple and easily accessible parts which are easy to manufacture, and should therefore be able to be designed at relatively low costs.

The appliance is reliable and can be put into operation immediately after being idle for several years. Only the fluid level needs to be checked periodically. No complicated parts or controls are required for the function, nor is any auxiliary power required.

Claims (1)

1. n; 117 'DO G0 P0 CLAIMS A method for removing particulate matter from a stream of compressed gas, characterized by the measures: a) obtaining a container (16) open at the end with gas-impermeable, upright side walls (17); b) filling the end open container (16) with a porous bed (21); c) enclosing the end-open container (16) with a liquid-tight housing (10) to form an annular space between the end-open container (16) and the liquid-tight housing (10); d) partially filling the liquid-tight casing (10) so that the porous bed is immersed in the liquid to at least half its height; e) passing a stream of gas through the porous bed (21) to remove the particulate material and reduce the density of the liquid (14) inside the porous bed (21) and cause the liquid (14) in the porous bed (21) to pumped up over the side walls (17) of the container (16) and into the annular space due to the density difference between the liquid (14) in the porous bed (21) and in the annular space and to further cause liquid (14) to the annular space flows into the porous bed (21) through the bottom of the end-open container (16) to replace the pumped liquid (14) and thereby effect the circulation of liquid (14) between the porous the bed (Zl) and the annular space.