MXPA99007217A - Improved mass transfer device - Google Patents

Abstract

A mass transfer device is provided in the form of a tray having perforations spanned by bridge members shaped to divert liquid flowing in the design flow direction around the perforation such that gas rising through the perforation encounters the liquid flow essentially at rightangles.

Description

DEVICE FOR IMPROVED MASS TRANSFER BACKGROUND OF THE INVENTION This invention relates to equipment for chemical processing in which a liquid is brought into contact with a gas backflow. This equipment can be for a variety of purposes such as removing a component from a liquid stream or absorbing a component in a liquid stream. More generally, this invention relates to equipment designed to facilitate the transfer of mass and / or heat between phases. The type of equipment to which this invention refers specifically employs trays for cross-flow fractionation connected by descent tubes. In said equipment, a tower is provided with a plurality of fractionating trays generally arranged horizontally within the tower. Each tower has a perforated platform and at least one channel, called a descent tube, in which a liquid flowing over the platform can be collected and channeled to the lower tray. During use, a gas or vapor is introduced into the base of the tower and passes upwards through the perforations in the platforms of the trays for fractionation. Meanwhile, a liquid is introduced into the upper part of the tower and filtered down passing over the trays for fractionation and down through the lower tubes to the lower tray.

Upon reaching the tray, the liquid flows through it in what is described here as the "design flow direction", which indicates the direction in which the liquid is intended to flow when the tray is operating under optimum conditions. of design. The tray is provided with a plurality of perforations through which the gas bubbles continuously at a pressure which, under normal operating conditions, prevents the liquid from passing through the perforation. These perforations, together with the associated covers, are referred to as "valves" and are designed to allow contact for efficient mass transfer between the gas and the liquid. These valves are divided into two main groups: fixed and mobile. The fixed valves have non-moving parts and the moveable ones are adapted to respond to upstream gas pressure by opening or closing the valve. The present invention relates to fixed valves. In the ideal design of the procedure, the liquid should be prevented from passing through the valves in the trays by the pressure of the gas passing through the perforations in the upward direction. This is a finely balanced procedure since, if the pressure is too high, the gas will have a shorter transit time inside the tower and a less efficient contact with the liquid as it flows through the trays and down the tower . The high velocity of the gas can also cause droplets to be carried to the upper tray, thus reducing the separation efficiency as a result of remixing. On the other hand, if the flow velocity of the gas is too low, the liquid will penetrate through the valves in the trays, (known as "drip"), and cause a short circuit in the flow patterns that are designed to increase to the maximum the liquid / gas contacts. Some designs of mobile valves actually allow the valve to close if the pressure drops too much. However, these valves can cause problems if they remain closed or only partially open. They are also too expensive. Other (fixed) valve designs simply place a cover over the perforation in the platform of the tray to prevent the liquid from falling into the perforation with sufficient velocity to penetrate it, even when the pressure is at design levels. In a typical moving valve, a perforation (eg, a round hole) is drilled out of the tray material. This is then typically covered by the complementary shape (a metallic disc in the example), supported on extremities or perhaps in a box where it is tried that the valve opens or closes with the pressure of the gas. When the cover is supported on limbs that move within the bore so that the cover is adapted to rise or fall with the pressure, it is often found that, when a pressure discharge or excessive vibration occurs, the entire valve is boot of the tray and from there on is permanently open. Problems can also occur easily if the valve remains open or closed.

In a typical fixed valve structure, the cover is made of the material of the tray. Usually, this is done by cutting a couple of slots and deforming the surface of the tray upwards to create a perforation and a cover for the perforation at the same time. This fixed valve structure is described in USPP 5,468,425 and 3,463,464. As will be seen, in said fixed valve structures, the dimensions or opening dictate the dimensions of the cover which, due to deformations or cutting procedures, will barely cover, or will not cover in its entirety, the opening. Furthermore, it is not possible to create openings with advantageous shapes that are different from those of the cover. For example, it is impossible to provide a round opening by means of the standard deformation technique or, using the material removed when the opening is cut, a quadrilateral covering that will completely cover the opening. It is also not possible to configure the hole to create a Venturi effect when drilling through the tray material from the top to create a relatively narrow opening in the bottom opening in a wider channel in the upper surface of the hole. the tray. It has been found that Venturi effects are often desirable characteristics of fixed valves. The disadvantage of said methods of the prior art is that the shape of the perforation governs the shape of the cover. Therefore, there are limitations that are inherent in the production process. The present invention provides a way to carry out mass transfer by contacting devices in which the shapes of the perforation and the cover can both be manipulated to produce the advantage of optimal procedure and efficiency of operation of the device. The present invention provides a simple mass transfer contact device which can be easily installed and which provides highly effective means for contacting the liquid flowing on and around the valve with the gas flowing up through the liquid. valve. Equally important, the design is such that the size and shape of the perforations and covers can be optimized independently for the specific application. In addition, there are no moving parts in the valve that can adhere or stop functioning properly and can be configured to fit any pierce.

GENERAL DESCRIPTION OF THE INVENTION The present invention provides a fractionating tray having at least one perforation therein and a design flow direction in the vicinity of said perforation and, including the perforation, a bridge member comprising first and second connected support ends. by a cover member oriented in the direction of design flow in the vicinity of the perforation and wide enough to completely cover the perforation at all points, the limbs being adapted to be releasably fixed to the tray to encompass the perforation, the first limb being a solid member located upstream of the perforation in the direction of design flow and with a width that is at least 5% wider, and preferably at least 10% or 20% wider, than the width Larger drilling cross-sectional to the direction of design flow. The other support limb may also be a solid member, although this is not an essential feature. It may comprise, for example, a plurality of vertical support elements which together constitute the second end of the bridge member. The total width of the second limb, as constituted, is preferably at least 10%, and preferably at least 20% narrower than that of the first limb. The end of the bridge member that is located downstream in the direction of flow is preferably provided with one or more openings passing through the wall. These can be provided by holes cut into a solid member or can be created in effect by having the extremity comprise two or more vertical support members separated by a space or spaces that provide the opening (s). During use, a portion of the gas that flows upward through the perforation passes through the openings to minimize any opportunity for the liquid to remain near the surface of the limb. It is also possible to provide, instead of, or in addition to said openings, that the width of the limb be reduced until it is narrower than the wider dimension of the hole at right angles to the direction of flow. This has the same effect of reducing any fluid that remains in the region of the downstream limb. The limbs of the bridge member preferably terminate in tabs that are adapted to cooperate with slots cut in the platform to secure the bridge member in place. Subsequently the tabs can be bent to prevent easy removal of the bridge member. Alternatively, other means can be provided to achieve the same objective. For example, the tabs may be provided with holes aligned to receive a bar that passes through both, or each tab may be twisted slightly on the underside of the tray so that the tabs are no longer aligned with the holes through the openings. of which pass. The cover member connecting the limbs can be a simple and horizontal flat plate, or it can be provided, at least in the lower part, with a rib member aligned along the design flow direction. The purpose of the rib would be to direct the flow of gas that emerges through the perforation off the sides. In other embodiments, the cover member may comprise a large number of components that together serve to protect the perforation against the access of liquid approaching the perforation from above. In this way, the cover member can for example comprise two plates that together form a cover with a transverse V-shaped (or inverted V) shape in the direction of design flow. It is preferred that the cover member, at all points, be at least 10% wider than the width of the perforation at the point directly below it. The cover member may also be provided with an opening configured such that a gas flow through the opening is directed essentially horizontally and in the design flow direction. The opening can be created for example by making a groove in the cover member parallel to the plane of the first end and deforming the side of the groove that is located near the first end upwards. In use such an arrangement is particularly useful if the valve is located below the drop tube so that a flow of liquid falls directly on the cover member. Said slot opening helps to initiate the flow of the liquid in the desired design flow direction. The cover member may be horizontal but it is often preferred that it be tilted so that the end next to the first limb member, i.e., the upstream end in terms of the design flow direction, is greater than the end. end next to the second extremity or to the extremity downstream. This has the effect of creating high pressure at the downstream end of the valve and lower pressure at the downstream end, thereby encouraging and increasing the flow in the design flow direction. The degree of inclination is a matter for optimization depending on the desired volume and velocity of the flow of the liquid in the direction of design flow. However, it is often preferred that the first support limb be at least 5% longer, about 5 to 25% longer, than the second support extremity. The perforation in the platform of the tray can have any convenient shape and this includes round, elliptical, and polygonal. Venturi perforations in which drilling is done by drilling a suitable figure, generally round, and generally in the downward direction, are particularly advantageous for use with the bridge members of the present invention. In use, a liquid flowing in the direction of design flow through the platform meets the first limb member. Since this limb is solid, the liquid flow is divided and directed to either side of the perforation. As it passes on each side of the borehole it encounters gas flowing at right angles to the direction of liquid flow. This causes a very efficient gas / liquid contact. The different directions of flow can make it unlikely any dripping of liquid through the perforations. Since the cover member is preferably at least 10% longer than the perforation at all its points, the effect of gas flow direction is increased and the possibility of dripping is reduced. Generally the valves are arranged in staggered arrangements so that each row of valves through the direction of flow is staggered relative to the valves in the rows in front and back, so that they remain (in relation to the direction of flow), between pairs of adjacent valves in these rows.

DESCRIPTION OF THE DRAWINGS Figure 1 is a cross section of a valve according to the invention. Figure 2 is a plan view of the valve shown in Figure 1. Figure 3 is a flat plate that can be flexed to provide the valve cover shown in Figures 1 and 2. Figure 4 is a cross section of the valve according to the invention, showing ends of different length and a flow direction opening in the cover member. In the drawings, the direction of the design flow is from right to left.

DETAILED DESCRIPTION OF THE INVENTION The invention is now described in more detail with respect to the drawings, the purpose of which is to illustrate the invention, but they should not be taken as implying any essential limitations on the scope of the invention. The device illustrated in Figures 1 and 2 comprises a bridge member that includes a bore 6 in the tray 5, and comprises the first and second ends, 1 and 2 respectively, connected by a cover member 3. The ends end in tabs 4, which pass through slots 7, which cut into the tray 5. The tabs below the level of the tray are flexed to prevent removal of the bridge member. Figure 3 shows a flat metal plate molded to provide a bridge member according to the invention when flexing along the dotted lines to form the first and second limbs. Opening 8, in the narrowest part of the two extremities (the downstream end), is intended to allow a gas to flow through the opening to minimize the accumulation of fluid on the face downstream of the extremity . In FIG. 4, the end 1 is greater than the end 2, so that the cover member 3 tilts down towards the second end. The cover member also has a direction groove 8, formed by cutting a groove in the cover member and deforming the side of the groove next to the first end upwards. During use, the first end of the bridge member is located at right angles to the design flow direction. In this way, the liquid flowing through the tray encounters the face of the first limb and deviates laterally and around the perforation.

This decreases the flow velocity and ensures that the gas contact with the liquid, unlike the gas flowing through the openings in the second end, is essentially at right angles. The extremities shown in the drawings are of similar height but this is not an essential feature and in fact it is often preferred that the cover member be biased downwards towards the second end. For example, it may be advantageous to make the first limb larger than the second limb to intercept more flow amount and to minimize the risk of significant amounts of liquid passing over the bridge member and deflecting the gas contact zones in each side of the bridge member. During operation, a tray has a large number of perforations that are usually circular, although other figures such as ellipticals and even polygonal ones are useful. The preferred locations of the perforations on the tray are in lines through the design flow direction with adjacent lines staggered so that the perforations in a line are between pairs of perforations in the lines on each side along the line. design flow direction. This ensures that the flows are repeatedly divided and combined to ensure that no liquid flow develops, ie that it does not come into contact with rising gas.

The operation of the connection device for mass transfer illustrated in Figure 1 of the drawings was compared with a standard single-weight mobile valve with the same dimensions of perforation., in identical areas of the tray and under identical liquid flow conditions. The prior art mobile valve comprised a flat cover greater than the bore and provided with three peripheral and evenly spaced extremities extended perpendicular to the cover and below the lower surface of the tray with stops in the lower extremities to prevent the cover rise beyond the predetermined point. It was found that it is useful to compare the "flood point" for each device on a scale of gas flow velocities. The flood point is reached when the integrity of the liquid and gas flows are lost and the tower is filled with liquid. In this way the Flood point defines an extremity of the permitted scale of operation for a tray. The flood point for the device according to the invention was compared to the standard prior art valve on a scale of the gas flow velocities. The flood point figures measured for the two devices are shown in the table below. It is also useful to compare the operations at the opposite end of the permitted operating scale, ie the pressure drop required across the tray at low gas flow rates before the prior art valves close and do not allow more mixed or where excessive dripping renders the device in accordance with the invention ineffective. In the following table, the relation of the mentioned parameter for the device of the invention to the same parameter for the mobile valve of the prior art at the same speed of the flow of the liquid is given. In general, for? P, the lowest is the best and for Fp, the highest is the best. In general it will be seen that the operation of the valve of the invention is as good as, or better than, that of the best valve of the prior art.

RELATION OF LFR INVENTION INVENTION RELATIONSHIP OF? P RELATIONSHIP OF Fp 1 Minimum 0.90 1.00 Maximum 1.21 1.17 2.5 Minimum 0.87 0.91 Maximum 1.02 1.10 3.0 Minimum 0.80 0.80 Maximum 0.96 1.08 4.0 Minimum 0.88 0.84 Maximum 0.90 1.03 5.0 Minimum 0.94 0.88 Maximum 0.92 1.04"LFR" means the liquid flow rate normalized in liters per minute, per centimeter of the pourer. The "LFR RELATION" is the ratio of the actual flow velocity to the lower flow rate that was used. "? p" means the pressure drop across the tray in cm of water.

"Fp" means the gas flow rate normalized in cm / seconds based on the area of active bubbles. "Minimum" and "Maximum" refer to the operating parameters indicated at the opposite ends of the permitted scale of the gas flow rate and the pressure drop for a given liquid flow rate in the standardized test tower used in the evaluations. From the above information it can be seen that at minimum gas velocities and liquid flow rates, the device according to the invention worked rather efficiently than the conventional valve which can be closed since the lower pressure drop and the Gas flow velocities were possible for efficient operation. Perhaps most important, the compliance device of the invention operates at high speeds and with lower pressure drop before reaching the Flood point. In a second series of tests the same fixed valves of the invention that were used in the first series of comparisons were compared with fixed valves made by deformation of the material of the tray to form a trapezoidal bridge member of essentially the same dimensions as the member of bridge in the present invention as illustrated in the drawings. This valve is described in USP 3,463,464. The difference is that the shape of the perforation in the valve of the prior art is stipulated by the shape of the bridge member. As in the previous comparisons, the orientation and separation of the valves in the tray were the same and the materials used, and the gases and liquids put in contact were the same. The results obtained were the following: LFR RELATION INVENTION INVENTION RELATIONSHIP OF? P RELATION OF Fp 1.0 Minimum 1.05 0.96 Maximum 1.22 1.08 2.5 Minimum 0.99 0.82 Maximum 1.33 1.22 3.0 Minimum 0.95 0.79 Maximum 1.24 1.20 4.0 Minimum 0.98 0.84 Maximum 1.07 1.16 5.0 Minimum 0.96 0.86 Maximum 1.09 1.20 The advantages of Previous operations were obtained in tests in which the devices operated in problem-free mode. However in the real world the prior art / moving valves often cause problems by remaining either open or closed. The absence of moving parts together with the retention of an equivalent normal operating scale is a great advantage for the devices of the invention in which said problems with the stuck valves are totally eliminated.

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A tray for mass transfer having a large number of similar perforations therein and a design flow direction through the tray beyond the perforations and, spanning each perforation, a bridge member consisting of first and second support legs connected by a solid cover member oriented in the direction of design flow in the vicinity of the bore and large enough to completely cover the bore, the ends of said bridge member adapted to join the tray to encompass the perforation, the first limb is a solid member located upstream of the perforation in the design flow direction and has a width that is at least 5% wider than the largest width of the perforation found in shape transverse to the direction of design flow, and the second extremity is located downstream of the perf sentence.

2 - A tray for mass transfer according to claim 1, further characterized in that the first end is at least 10% wider than the greater width of the perforation in the direction of design flow.

3. - A tray for mass transfer according to claim 1, further characterized in that the first support limb is longer than the second support extremity, whereby the cover member is tilted down in the design flow direction .

4. A tray for mass transfer according to claim 1, further characterized in that the supporting ends of the bridge member are provided with tongue extensions that cooperate with openings in the tray to provide attachment means for maintaining the member of the tray. bridge in place over the perforation.

5. A tray for mass transfer according to claim 1, further characterized in that the second support extremity is provided with openings that pass through the end.

6. A tray for mass transfer according to claim 1, further characterized in that the first and second support legs have a similar length.

7. A tray for mass transfer according to claim 1, further characterized in that the perforations are Venturi.

8. A tray for transfer of dough according to claim 1, further characterized in that the cover member is provided with an opening designed to direct the flow of gas through it essentially horizontally in the direction of flow of design.

9. - A tray for mass transfer according to claim 1, further characterized in that each perforation has a shape different from that of the cover member covering it.