Mass transfer contact apparatus.

This invention relates to a mass transfer contact apparatus, for example a gas or liquid scrubber for removal of solid or liquid particles entrained in a gas stream and/or for removal of soluble gaseous constituents of a gas stream. It is also generally applicable to any like gas treatment operations which require basically an intimate contact between a gas and liquid. Such operations may include effecting heat exchange between gases and liquids in general; mass transfer; the drying of a liquid saturated or partially saturated gas stream by contact with a chilled liquid; stripping; distillation; and others.

In the chemical engineering art, in addition to the gas scrubbing process there are many instances where mass transfer reactions, energy transfer reactions, and chemical reactions or combinations of these are carried out by bringing a gas or vapour and a liquid into intimate contact with each other, usually within a vessel. In continuous processes, liquid and gas feed stock streams are continuously introduced to the gas-liquid contact vessel, and gas and liquid product streams are continuously withdrawn. The flow paths of the two streams through the vessel are in most instances of the counter- current type, with the liquid being introduced at or near the top of the vessel, and withdrawn at the bottom, and with the gas being introduced at or near the bottom of the vessel and withdrawn at the top. In some instances, co-current flow, with both streams moving through the vessel in the same direction, is employed.

It is the practice to mount within the gas-liquid contact vessel passive apparatus or structure intended to ensure that the liquid and gas achieve the desired degree of contact with each other so that the planned reaction occurs at the designed rate. The internal structure is passive in the sense that it is not power driven and has few or no moving parts. (Those parts that do move do so under the influence of the gas or liquid moving through the vessel.) Various kinds of structures have been employed, including bubble trays, packed columns, and grids. Ideally, the design of the vessels must include process, operational and constructional considerations. The contact reaction must be effective and efficient from a quality (purity) and quantity (yield) point of view. Further, it must use energy supplied efficiently as well as accomplish the reaction with a minimum pressure drop. Also, the apparatus should be simple and economical to build and easily cleaned and maintained. Further, vessel size is important. Desirably, the vessel should be as small and short as possible.

The foregoing considerations may be summarized as follows: it is desirable that gas-liquid contact apparatus produce good product quality and yield, at a good energy efficiency and low pressure drop; that it be practical and simple to construct and maintain; and that its size and the tower size be minimized while throughput capacity is maximized.

It is therefore an object of this invention to provide a gas-liquid contact apparatus which substantially satisfies these considerations providing high efficiencies at lower, installed and operating costs.

According to the invention there is provided a gas-liquid contact device having a specially contoured grid member(s) interposed in the gas flow of the device. Ideally the grid member(s) has a cross-sectional profile which would equal the algebraic sum of the theoretical velocity pressure profile of the gas flowing in an open vessel and the static and dynamic heads of the contacting liquid. This is practically approximate by the free hanging catenary shaped grid described. An additional depth is preferably added to the grid, approximately 5 to 40% more, to create an energy imbalance in favour of the introduced liquid, thereby allowing it to drain. The grid member (s) can be made of rigid or flexible materials, the latter facilitating cleaning. The grids can be arranged singularly, or in pairs separated by a predetermined distance. The latter arrangement provides further improved particulate removal. The device can be used on countercurrent and co-current systems, i.e. the contacting liquid injected opposite the gas flow, or with the gas flow. Grid members are effective with grid open areas upwards of 75% and even higher.

. Although the catenary shaped grid provides the least structural complexity in approximating the parabolic shape of the velocity pressure profile of the gas stream, the desired grid shape can be approximated, e.g., through the use of a shallow cone or series of cones, or by a formed shape, with varying degrees of efficiency. Further, in existing vessels where it is desirable to replace prior art devices with an apparatus embodying the present invention, and where existing structure make it impractical to utilize a one piece grid a plurality of grid modules may be employed. Each of these modules is designed including a shaped grid or grid pair approximating the velocity pressure profile of the gas stream in each such that the net effect substantially approaches what would be achieved if the single grid or grid pair were suspended in the vessel.

IN THE ACCOMPANYING DRAWINGS:

<ul id="ul0001" list-style="none"><li>FIG. 1 is an elevation view partially in section depicting a gas liquid contact device, a gas scrubbing device, employing the principle of the present invention.</li><li>FIG. 2 is a schematic representation depicting the relationship between the velocity profile of the flowing gas and the grid member design.</li><li>FIG. 3 is a graphical presentation based on empirical data, reflecting the liquid to gas ratio required to create a fluidized condition in the device for various gas velocities.</li><li>FIG. 4 is a graphical presentation based on empirical data, depicting the pressure drop in inches of water versus the liquid to gas ratio, for varying gas velocities.</li><li>FIG. 5 is a schematic representation depicting the gas-liquid activity about a grid pair designed and disposed according to the principles of the present invention.</li><li>FIG. 6 is a plan view of a quarter section of the grid module version of the invention.</li><li>FIG. 7 is a perspective view of the section shown in FIG. 6.</li><li>FIG. 8 is aielevation view taken along lines 8-8 of FIG. 6.</li><li>FIG. 9 is a front elevation view, partly in section, of the grid module/version depicting a second embodiment including a different way for dispensing the liquid onto the grid structure.</li></ul>

It is to be understood that the principles of the present invention have broad application to many types of gas-liquid contact devices. For purposes of illustration, however, the following description will elaborate on a gas scrubbing device.

Figure 1 shows a device 10 which includes a housing member 12 having an inlet and outlet, 14 and 16, for passing the contaminant gas. Between these is the scrubbing vessel 18 or ductwork.

Items found on the vessel essentially unrelated to the present invention include a drain outlet 20; liquid connection 22 for recirculation purposes; and a chevron or mist eliminator 24.

Suspended within the vessel are a plurality of grid members 26 to 36. These are hung from the sides of the vessel by suitable means, not detailed, such as a circular rim piece. The grid members are orientated such that they are substantially perpendicular to the direction of gas flow.

In this figure, the grid members are shown in pairs. However, as will be seen from the subsequent discussion, the principles of the invention apply for a single grid member as well.

It is apparent from Figure 1 that the grid members 26 to 36 are contoured, having an apparent, shallow, parabolic shape. The particulars of the design and how it is arrived at, are explained hereinafter.

In the embodiment shown, positioned above each grid pair are means 38 for dispensing the scrubbing liquid onto the concave side of the upper grid member in each pair. The dispensing means include headers 40, 42 and 44 which are connected to a scrubbing liquid supply. Internal piping 46, 48 and 50 distribute the liquid to a plurality of, typically, low pressure (3.45 x 10⁴to 6.9 x 10⁴_Pa, or 5 - 10 psig) nozzles, of open mixing design, including 52, 54 and 56. These dispense the liquid in droplet form across the breadth of the vessel, onto the upper surface of the top grid member 26, 30 and 34. A variety of liquid injection means (spray, freeflow, tangential, weirbox, etc.) can be employed since the method of liquid injection has little to do with the performance of the invention disclosed.

THEORY

When gases pass through a duct or scrubber vessel, a velocity pressure profile results which appears to approximate a shallow parabola. The velocity pressure is maximum in the centre of the vessel, decreasing to nearly zero at the wall.

The unit mass flow rate at any given point can be described (assuming a homogeneous mixture) as: <ul id="ul0002" list-style="none"><li>M_x = V_Gx ℓ where M_x = Mass flow rate (lb/ft²/sec) V = Velocity of the gas (feet/sec) Gx at point x</li></ul>

^e= Density of the gas (lb/ft³) Thus, the unit mass flow rate is greatest at the centre of the vessel.

As the tower gas velcoity increases (above 10-15 feet/second), this effect becomes more pronounced. The mass flow rate in the centre can be hundreds of times that at the wall.

If scrubbing liquid is sprayed or distributed uniformly across the diameter of the vessel, the ratio of the scrubbing liquid to mass flow at any pont x is:<maths id="math0001"><img file="EP0099656A2_D0001.tif" /></maths>Where L = Liquid rate (gps) <ul id="ul0003" list-style="none"><li>M = Mass flow rate (lb/ft²/sec) at point x.</li></ul>

This provides an indication that the absorption rate of gas scrubbers operating at a fixed temperature is related to the specific quantity of scrubbing liquid exposed to the contaminated gas.

Thus, the rate of absorption, impaction or interception varies across the diameter of the scrubber. It is actually lower at the centre than at a point midway between the wall and the centre, since the gas velocity is greater at the centre (M is greater, therefore _L/M_x is lower).

If one were to proportion the spray across the vessel diameter to compensate for the foregoing, the performance would decrease. This results from the contact time of gas and liquid being lower at the centre given the higher gas velocity. The fact that the spray falls due to the combined initial velocity plus the acceleration of gravity, further aggravates the situation.

Or, if one were to spray more liquid downward against the gas (in the case of counter current flow) or less liquid with the gas (in the case of co-current flow) in the centre zone, distribution and gas mixing problems would occur.

If, however, an appropriately shaped grid has its perigee located at a sufficient depth below the contact point of the grid with the vessel side wall, the cross-sectional profile of the grid approximating the velocity pressure profile (in the mirror image) of the contaminant gas rising in the vessel, the effects of the velocity pressure on the rate of absorption etc. can be largely cancelled out. As a result, more scrubbing liquid is provided in this high velocity zone and also, by creating more central zone resistance to gas flow, the flow of gas nearer the wall can be increased. Thus, the entire vessel is better utilized.

The grid can be made of metallic or non-metallic, flexible materials. It can be preformed grid pressed into a shape approximating the velocity profile. Or, the shape required can be approximated through use of a shallow cone mesh or segments of cone. meshes connected together to approximate the shape. A non-rigid grid can be mechanically flexed to facilitate cleaning, if necessary. No internal beams or other structural members need be used.

The net open area of the grid is selected to accommodate various conditions. For example, the open area will tend to be lower where it is necessary to conserve on the amount of scrubbing liquid to be used; to reduce abrasion of the grid member; or increase its structural strength (for large vessel spans). Conversely, it is desirable to have a greater open area where energy demands need to be restricted; or where large amounts of scrubbing liquid are employed (where gaseous contaminant concentrations are high); or to facilitate the drainage of scrubbing liquids containing higher percentages of solids, such as limestone slurry, or where dense particulate contamination is found. Generally, it has been found that the net open area normally runs between 50% and 75%, although an open area of the order of up to 85% to 90% has been used. However, at this high end, the performance of the grid scrubber approaches that of the conventional spray tower, with its attending problems.

In setting the shape of the grid, its depth at any point across the vessel, d_X, is given by the following equation:<maths id="math0002"><img file="EP0099656A2_D0002.tif" /></maths>where P_vx = velocity pressure of the gas at point x, <ul id="ul0004" list-style="none"><li>^H_SL = the static head of the scrubbing liquid,</li><li>H_{DL =} the dynamic head of the scrubbing liquid.</li></ul>

Note, this is an algebraic sum and the relative signs of the components must be taken into consideration, so that the equation has application both to co-current as well as counter current application. If pressures acting upward are taken as negative, then downward pressures would be taken as positive.

H_SL and H_DL are relatively constant across the vessel, Since a flexible grid, if supported at its perimeter will hand in a substantially parabolic curve (a catenary shape and the approximate shape of the ideal grid profile, the calculation is simplified if only the depth of the grid, d_c, at the centre of the vessel is calculated.

<maths id="math0003"><img file="EP0099656A2_D0003.tif" /></maths>

Using the following conventional pitot tube equation, the velocity pressure of the gas in inches of water can be determined.<maths id="math0004"><img file="EP0099656A2_D0004.tif" /></maths>where V is the average of the gas velocity G AVG <ul id="ul0005" list-style="none"><li>Q is the flow rate in ft³/min AVG</li><li>A is the vessel cross sectional area in ft² is the carrier gas density in lbs. per ft³ for the temperature and pressure conditions in effect.</li></ul>

Solving for P _vAVG , knowing the flow rate and vessel area:<maths id="math0005"><img file="EP0099656A2_D0005.tif" /></maths>

The value arrived at for P_AVG is then converted in standard procedure to the gas pressure at the centre, P_vc by multiplying by 1.1, since formula 3 represents the average gas velocity pressure, not the pressure in the centre.

Therefore<maths id="math0006"><img file="EP0099656A2_D0006.tif" /></maths>H_SL, the static head of the scrubbing liquid can be determined as follows:<maths id="math0007"><img file="EP0099656A2_D0007.tif" /></maths>where L is the liquid flow rate in gal/min. Ce is the density of the liquid in gal/ft³<ul id="ul0006" list-style="none"><li>A is the grid open area in ft² OG</li><li>R is an empirically determined retention time in min. for the grid, which typically is a minimum of 0.1 sec. or .0017 min. 60 min.</li></ul>

H_SL, in feet of water, is then converted to inches of water by multiplying by 12.

^H_DL, the dynamic head of water, is a small factor and not of serious practical concern. It is the velocity pressure of the scrubbing liquid if applied by spray nozzles at close proximity to the grid.

If the perigee of the catenary grid is set exactly at d , an energy equilibrium condition exists between the liquid and the gas. Liquid introduced to the top of the grid is suspended, fluidized, but does not drain.

Experimentation has shown, however, that practically speaking, it is desirable to create an equilibrium imbalance in favor of the scrubbing liquid. This is achieved by further deepening the shaped grid. This depth increase, d₊, can vary depending on many things including: <ul id="ul0007" list-style="none"><li>whether the contaminant gas includes particulate matter or not;</li><li>the contaminant gas density;</li><li>viscosity of the scrubbing liquid;</li><li>tower gas velocity;</li><li>acceptable degree of abrasion or erosion of the grid member;</li></ul> and others.

For example, tests conducted have shown that the increased depth can vary from 5% (for particulate-free gas) to 40% (for entrained particles), over the figure, d , calculated above.

A schematic diagram depicting the liquid and gas interface around a single grid is shown in Figure 2. The scrubbing liquid 58 is shown being dispersed towards the interface from above. As noted earlier, it can be introduced onto the top surface 60 of the grid 62, in a variety of ways. This is not critical to the operation of the fluidized zone described hereinafter.

The flowing gas is depicted as having a velocity pressure profile 64. When the gas reaches the catenary shaped grid 62 a zone of high turbulence 66 is created on and below the grid. Further, a gas-liquid contacting surface 68 results which is uniformly distributed above and across the top surface of the grid.

Above this contact surface 68, there is also crafted a fluidized bed or zone in which droplets of the scrubbing liquid and randomly dispersed, contaminant gases efficiently interact. This results in the improved absorbative characteristic of the present invention. The height of the fluidized zone will be a function of the droplet size and the velocity of the gas. The higher the fluidized zone, the greater the gas absorption and particulate removal. For a properly dished grid the top surface of the zone, 72, is substantially flat as shown. With a shallower dish configuration, the centre area of the top surface rises up higher than the area near the vessel sides reflecting less dispersement of the gases towards the sidewall and less effective scrubbing of the gas. In fact, flow through of the gases can occur for dished grids which are too shallow.

Because of the energy imbalance in favour of the scrubbing liquid, the liquid droplets in the fluidized zone will eventually collect and drain after a certain "operating" time in the zone. Drainage occurs primarily near the centre of the grid. The drained liquid collects in a sump below for extraction, reprocessing and reuse. Operating time of a droplet of liquid in the fluidized zone vary depending on many factors, and have been experimentally determined to be as high as 3 to 4 seconds.

The above effects have been verified by photographs and laboratory tests, field pilot testing and analysis.

Because of the shaped grid, the velocity pressure profile of the flowing gases becomes more uniform, with gases from the centre dispensing towards the vessel walls. Liquid flow above the grid counters this movement, on a path 74 in from the walls towards the centre.

In situations where the primary contaminant is gaseous in nature, the catenary grid scrubber with a single grid has been found effective.

Pilot tests using S0₂ as the contaminant and limestone slurry as the scrubbing liquid have shown significant improvement in S0₂ removal efficiency over the venturi scrubber for comparable liquid to gas ratios. S0₂ removal with a shaped grid was 85% to 92% using L/G values of 15 to 25 (G is defined as the gas flow rate in cfm) A venturi scrubber obtains 30% - 55% S0₂ removal at L/G values of 10 to 30. Similar efficiencies are obtained in tower scrubbers but only with significantly higher L/G ratios of 50 - 100.

Figure 3 reflects empirically derived data where L/G is plotted against gas velocity for the catenary shaped grid. The area 76 above the curve reflects the various L/G ratios for available gas velocities wherein the fluidization bed 70 results. Below the curve, in area 78, fluidization does not occur. For the contaminant removal efficiencies available with the present invention, for a given gas velocity, an L/G ratio is selected which will place the system's operation in zone 76.

Figure 4 depicts various pressure drops for a single grid catenary scrubber at various gas velocities and liquid to gas ratios.

The pilot tests reveal that gas velocities in the catenary scrubber can be higher than those in a typical spray tower scrubber. Face velocities of up to 20 ft/sec produced no irregularities. Although no upper figure has been established yet, it is believed that face velocities of upwards of 25 fps are obtainable with a single grid. The higher gas velocities permit smaller, more economical towers thus saving initial investment expense and operating space.

Above 25 ft/sec, the pattern of the liquid flow will become discontinuous. Here the turbulent, fluidized scrubbing zone breaks down. Therefore, the pressure drop curves of Figure 4 become discontinuous as shown by the dashed lines. Optimum gas velocity (based on best fluidization at the lowest L/G ratio) in the catenary appears to be about 16 ft/sec compared with 8-12 ft/sec in tower scrubbers. Good gas-liquid contacting is obtained under these conditions.

The lower pressure drops in wet operations as well as in dry operation permits temporary by-pass directly through the vessel in emergency conditions. Dry losses are, typically, only 0.2" - 0.4" water per grid.

In the area of particulate removal, (+1 micron), the single grid has been found to be approximately 65% efficient. An extension of the principles of the basic invention includes the placement of a second grid in parallel with the first, separated by no more than a maximum prescribed distance. Figure 5 depicts a grid pair which exhibits enhanced particulate removal.

Here the upper grid 80 of similar shape to the lower grid 82 is positioned up to an empirically determined distance 84 above the lower grid. In theory there is no set minimum spacing between the two, since with the one grid physically on top of the other, the performance of the system would be as a single grid.

The maximum distance 84 between the two will be a function of many variables including gas velocity, gas density and grid open area. For example, pilot test runs with a gas face-velocity of 30 fps (higher velocities are possible with the grid pair); grid open area of 60%; and, a gas density of 0.075 lbs/ft³, resulted in a maximum spacing of approximately six inches.

Spacings generally between three to six inches result in improved performance. Generally, if gas density or velocity decreases, the spacing between the grids decreases. If grid open area decreases, spacing increases.

The primary consequence which controls the spacing is the creation of a turbulent zone in the area 86 between the grids. If the grids are too far apart, the flowing gas is able to re-establish the velocity pressure profile that it presented as it approached the lower grid.

The lower grid can be perceived as an energy dissipator. The random movement and reduced energy level of the gases in the zone 86 minimize the possibility of a turbulent zone immediately above the upper grid as is the case in a single grid arrangement. As a result the scrubbing grid forms a film on the top surface of the upper grid, with movement, again, towards the centre of the vessel along paths 88.

With the scrubbing liquid film right on the top surface of the grid, enhanced particulate removal is obtained. Pilot runs indicate that with the double grid arrangement, 99.7% of + 1 micron particulate matter is removed. This is for a total pressure drop of approximately 1.9 inches w.c. across the pair. This compares with a venturi scrubber which requires a drop of 8 to 10 inches w.c. for a cyclonic scrubber.

Again, a fluidized bed or zone 90 is created in the volume immediately above the upper grid. For all the same parameters, conditions and grid design, the upper surface 92 of the fluidized zone 90 is substantially the same distance above the upper grid as in the case of the single grid approach. Again, the fluidized zone is a controlled high density of scrubbing liquid per unit volume with enhanced liquid-particulate interaction, and gas absorption through increased surface area of scrubbing liquid.

Again the gas tends to redistribute itself towards the walls, as the liquid moves into the centre of the vessel. The liquid penetrates and drains through the upper grid into the turbulent zone 86 where further reaction between the gas and scrubbing agent takes place. Finally, the liquid penetrates the lower grid 82 and falls into the bottom of the vessel, from where it is processed or recirculated.

The present design affords a very high turndown, i.e. the ratio of maximum tolerable gas flow to minimum gas flow, of three to one.

The present invention may be used with a single or plurality of single grids; or, a single pair of plurality of pairs; or, a mixture of single grids and pairs, to provide the required degree of gas and/or particulate absorption.

Although, the design of the grid reflects the mirror image of the gas-velocity profile summed with the static and dynamic heads of scrubbing liquids, it is to be understood that grid shapes approximating this design (such as the catenary) and employing the teachings of this invention can be utilized with expectedly, varying degrees of success in improved absorption and particulate removal.

Although the embodiment discloses a counter-current application of the principles of the invention, the design also has application to co-current scrubbers. In the latter, the gas flow and scrubbing liquid are applied to the concave side of the grid. In this application, typically, the grid will have less open area. Further, since the liquid and gas are co-current the grid can be shallower.

The present design affords a degree of self-scouring because of liquid turbulence, a benefit where solids buildup can occur.

Further, with respect to retrofitting of existing systems, in many cases this can be done easily and relatively inexpensively, because no special tools, fixtures or rigging are required to install the grids. In fact, the design requires less space than conventional absorption devices.

However, in other existing systems it becomes impractical to use the grid configurations described above for retrofitting. Thus a modular concept has been developed employing the theoretical principles described which results in improved performance in the retrofitted towers, while remaining economically feasible.

Figures 6 to 10 depict the grid module approach employing the principles of the invention. Fig. 6 and Fig. 7 depict a quadrant or segment of a typical installation. A plurality of cylindrical modules such as module 100 are mounted on a plate 102 which includes a mounting flange 104 use to secure the plate to the vessel side walls or suspended plate support structure.

Each module includes a housing 105 which is fabricated from suitable material based on the application. The housings typically run up to two feet in length and can vary in diameter, e.g. 6", 8", 12", 18", 20", 24", depending principally on the gas flow rate (cfm), G, in the vessel and the vessel diameter. Generally, the housings in a given vessel are uniform in length and diameter.

The velocity profile of the gas flow for each module is empirically determined knowing the gas velocity in the unmodified vessel (i.e., without the module plates). Generally the gas flow rate in each module will be the gas flow rate in the unmodified vessel divided by the number of modules to be used.

The design of the grid structure for the individual housings is based on the liquid and gas characteristics within each module; in accordance with the design criteria set forth above for a vessel grid structure.

Fig. 8 (showing two grid structures per module although the module adaption employs the single grid structure as well), depicts one way of introducing the liquid medium onto the concave side of the grid. The liquid 108 is dispensed onto the plate 110 to which the modules are secured. The liquid reaches the top 112 of the housing and spills over into the module towards the concave side of the upper grid structure 114. The entering gas stream 116 interacts with the liquid medium above the grid structure(s) and the fluidized bed or zone 117 created, resulting in the desired mass transfer reaction between the two. The height of the housing walls above the grid(s) is sufficient to accommodate the height of the fluidized bed. The flow rate of the liquid cascading down onto the grid(s) will be a function of the rate at which the liquid is dispensed onto the plate 110, and must be at least sufficient to result in the controlled fluidization bed above the concave side of the grid for the gas velocity within the module. This again is in accordance with the design criteria discussed above. After an interaction period with the gas medium, the liquid droplets in the fluidized bed eventually coalesce and drain down through the grids and into the bottom of the vessel where the liquid can be recovered, processed and then reused.

Fig. 9 shows yet another embodiment depicting an alternative way of introducing the liquid medium above the grid structures. Here the liquid deposited on the plate 118 enters the module (e.g. 120) through openings such as 122 and 124. The height of the liquid on the plate is above the openings to provide a hydraulic seal. This presents gas by-pass through the liquid entry points. Again the openings are designed so-that a sufficient liquid flow rate onto the grid(s) occurs, resulting in a controlled fluidization bed above the grid pair.

This grid module variation described consists of a grid or grid pair (or multiple grid or grid pairs) in a cylindrical housing. A square housing or similar configuration can likewise be employed.

Other ways of dispensing the liquid medium onto the grid structure should also be obvious in view of the teachings herein.