RU2198012C1 - Contact device - Google Patents

Abstract

FIELD: various industries, for instance, oil refining, gas processing, petroleum chemistry, by-product coke industry, power engineering, food and other industries. SUBSTANCE: invention deals with contact devices for heat-mass-transfer apparatuses designed for organization of direct contact of vapor or gas with liquid phase in processes of rectification, distillation, absorption, desorption direct heat transfer, and also gas scrubbing. Contact device has vortex plate whose periphery has cuttings-through over circumference, and draining appliance for liquid overflow from upper plate to lower plate. Vortex plate is made of thin sheet metal foil 0.1-0.5 mm thick with radius of 10-50 mm. Draining appliance has a row of draining channels located over periphery, and cuttings-through are made so that their edges are bent upward and downward to form inclined unidirectional channels for passage of gas or vapor and imparting of rotary motion only in one direction to two-phase mixture on plate. EFFECT: increased efficiency of interaction of vapor (gas) and liquid phase, extended stable operation of heat-mass-transfer apparatuses, reduction of their metal usage; simplified design. 5 cl, 16 dwg

Description

 The invention relates to contact devices for heat and mass transfer apparatus, designed to organize direct contact of steam or gas and liquid phases in the processes of rectification, distillation, absorption, desorption, direct heat transfer, as well as gas flushing used in various industries, for example, in oil refining, gas processing , in petrochemistry, chemistry, coke chemistry, energy, food and other industries.

 A contact device for heat and mass transfer apparatuses is known, containing a vortex plate, along the periphery of which there are grooves arranged around the circumference, and a drain device for draining the liquid from the upper plate to the lower one (SU 845309 A, June 15, 84).

 In the known device, along the radius of the plate, many cuts are made arranged in a number of circles, which leads to an increase in hydraulic resistance by steam or gas, while the blades in the form of an Archimedes spiral placed above the plate prevent the creation of high speeds of the two-phase mixture on the plate. The implementation of the drain device in the form of a locally placed drain pipe with a water seal seriously complicates the design.

 The technical result of the invention is to increase the efficiency of heat and mass transfer, expand the range of stable operation of the device in heat and mass transfer apparatuses, reduce the hydraulic resistance of the device, reduce metal consumption and simplify the design of heat and mass transfer column apparatuses.

The specified technical result is achieved due to the fact that in the contact device containing a vortex plate, along the periphery of which there are perforations located around the circumference, and a drain device for draining the liquid from the upper plate to the lower one, the vortex plate is made of thin-sheet metal foil with a thickness of 0.1- 0.5 mm and has a radius of 10-50 m, the drainage device contains a number of peripherally located drainage channels, and the notches are made so that their edges are bent up and down with the formation of inclined unidirectional channels for the passage of gas or steam and giving the two-phase mixture on the plate rotational motion in only one direction, while the dimensions of the cuts, the area of the passage sections of the channels for the passage of gas or steam and channels for draining the liquid are connected with the size of the plate by the following ratios:
L / R = 0.3-0.6;
S _g / S _t = 0.1-0.4;
S _w / S _t = 0.01-0.07;
where R is the radius of the vortex plate, mm;
L is the length of the cuts (the length of the channels for the passage of gas), mm;
S _t - the area of the vortex plate (S _t = πR ² , π = 3,14), mm ² ;
S _g - the total area of the passage sections of the channels for the passage of gas or steam, mm ² ;
S _W - the total area of the passage sections of the channels for draining the liquid, mm ² .

 For ease of attachment to the inner surface of the heat and mass transfer apparatus, the device may have bent up or down plates bent up or down.

 In a preferred embodiment, the drain channels of the contact device are made in the base of the plate in the form of holes with downward curved edges, which are placed so that they abut against the inner surface of the peripheral sides of the plate. These channels can be formed by concave sections of the peripheral sides of the plate and the inner surface of the heat and mass transfer apparatus adjacent to them. The vortex plate can be mounted in relation to the inner surface of the heat and mass transfer apparatus body with a gap, in this case the drain channels are formed by the internal surface of the heat and mass transfer apparatus and the outer surface of the peripheral sides of the plate.

The invention is illustrated by drawings, where:
in FIG. 1 shows a contact device, a top view, with drain channels in the form of holes with downward curved edges, with the peripheral sides of the plate pointing upward;
figure 2 is the same side view;
figure 3 is a contact device, top view, with drain channels formed by recesses in the peripheral sides of the plate, which are directed downward, and the notches are not strictly radially, but at a certain angle to the radius;
figure 4 - the same, in the case of placing a vortex plate with a gap with respect to the inner surface of the heat and mass transfer apparatus (hereinafter simply a column), and the peripheral sides of the plate are directed downward, and the edges of the cuts are not straight but arbitrary shape;
in FIG. 5 is a section A-A of FIGS. 1, 3 and 4, which shows a transverse profile of inclined unidirectional channels for the passage of steam or gas (hereinafter simply referred to as steam);
in FIG. 6 shows a view B of the passage section of unidirectional channels in pairs, having the shape of a rectangle with rounded corners;
Fig.7 is a view of B in the passage section of unidirectional channels in pairs, having an arbitrary shape;
in FIG. 8-10 - section bb In figure 1; GG of FIG. 3 and DD of FIG. 4, respectively, which depict variants of drain channels;
figure 11 shows the operation of the device in the column;
on Fig - diagram of the passage of steam through the inclined unidirectional channels of the device;
on Fig - the process of formation of vapor bubbles in a stationary liquid;
on Fig - the process of formation of vapor bubbles in a moving fluid;
in FIG. 15 is a velocity field in cross section EE of FIG. 11 of the lower fluid layers above the contact device;
in FIG. 16 is a velocity field in section FJ of FIG. 11 of the upper liquid layers above the contact device.

 The contact device for heat and mass transfer apparatuses (columns) is a vortex plate having a base 1 and bent (upward - figures 1 and 2 or down - figures 3 and 4) peripheral sides 2 perpendicular to it. Using these boards, the plate is attached to the inner surface 3 columns. On the periphery of the base there are circumferential radial notches 4, made so that their edges 5 and 6 are bent up and down, respectively, with the formation of inclined unidirectional channels 7 for the passage of steam. Figure 5 shows a cross section of these channels.

To increase the efficiency of this contact device, improve its operational characteristics (for example, erosion of deposits) or for other reasons (for example, the thermal properties of the contacting liquid and vapor phases), the cutouts 4 may have:
- not strictly radial position, as in figure 1, but to have a certain angle β with the radius of the plate itself, as in figure 3;
- not a strictly rectilinear shape, as in FIGS. 1 and 3, but an arbitrary, for example arcuate, shape, as in FIG. 4.

 For the same reasons, and especially to improve the fragmentation of the steam loops 8 into individual small bubbles, the gaps of the inclined unidirectional channels 7 can have a different shape - rectangular with rounded corners, as in Fig.6, or with wavy upper 5 and lower 6 edges of the notch as in Fig.7.

 The drain device of this contact device is made in the form of peripherally located drain channels. In figures 1 and 8, these channels are made in the base 1 of the plate in the form of holes 9 with the edges bent down and arranged so that they adjoin the inner surface of the sides 2. In figures 3 and 9, the drain channels are formed by sections that are concave to the center of the vortex plate peripheral sides 2 and adjacent to the inner surface 3 of the column and are similar to the segment openings 10. In figures 4 and 10, the vortex plate is mounted relative to the inner surface 3 of the column with a gap 11, which is provided by the attachment points 12, and drain to Nala formed by the inner surface of the column and an outer peripheral surface of the bead plates 2 itself.

Due to the small diameter of the plate D = 2R (R = 10-50 mm), it is possible without loss of stiffness and strength to produce it from thin-sheet metal foil with a thickness of 0.1-0.5 mm. Then the metal consumption of such a plate will be only 1-4 kg / m ² (against 10-40 kg / m ^{2 of the} best samples of existing plates).

 To describe the operation of the contact device and to clearly substantiate its absolute and relative sizes, it is first necessary to explain the requirements for any contact devices in heat and mass transfer apparatuses, as well as to explain the processes themselves occurring on the plates in these devices. We will carry out these explanations and a description of the work using an example of a distillation column.

 A distillation column is a countercurrent column apparatus of Fig. 11, in which heat and mass transfer between the flowing down reflux (liquid) 13 and steam rising upward 14 is realized throughout the height. The process of heat and mass transfer consists in the continuous exchange of temperature and individual components between the liquid and vapor phases. The liquid phase is enriched with a higher boiling component, and the vapor phase is enriched with a lower boiling component. In order to intensify the process of heat and mass transfer, contact elements (nozzles, plates) are used. In the case of the use of plates, the vapor in the form of bubbles 15 passes through a layer of liquid 16 located on the plate 17.

The movement of the liquid promotes the formation of smaller vapor bubbles. This is explained by the fact (see FIGS. 13 and 14) that the vapor bubble is separated from the channel (hole) 7 when the surface tension holding force F _mon is equal to the tearing force. The surface tension force F _PN depends only on the size of the channel 7 for the passage of steam and the properties of the liquid itself, and therefore the magnitude of this force is the same for a mobile and stationary fluid. In a stationary fluid (Fig. 13), the breaking force is only the Archimedes force F _A , and in a moving fluid (Fig. 14), a sufficiently large hydraulic effect of the fluid on the bubble F _Ж is added to it. That is why in the latter case, the bubble 15 comes off a smaller size. Direct contact and interaction of phases is carried out at the interface, i.e. on the border of the vapor bubble and the surrounding fluid. In this regard, the smaller the bubble size, the higher the area of heat and mass transfer between the phases.

 The driving "force" of this exchange at the boundary of two phases is the tendency of the liquid and vapor phases to their equilibrium state. The equilibrium state of phases is called their coexistence, in which there are no visible qualitative or quantitative changes in these phases. The phase equilibrium is considered achieved only when two conditions are simultaneously satisfied: the phase temperatures are equal and the partial pressures of each component in the vapor and liquid phases are equal. The second condition means that the process of transition through the phase boundary of each component from the liquid phase to the vapor phase and vice versa is completed, i.e. the compositions of the liquid and vapor phases stabilized, and the concentrations of the components in a single phase are the same at each point in its volume.

 The equilibrium state of the phases rapidly sets in at the very interface, and the propagation of the altered temperature and concentration of the components deep into the phase volume depends on the local velocities of the vapor and liquid near this interface. The efficiency of heat and mass transfer increases sharply with increasing relative phase velocity, i.e. while increasing their turbulent mixing. In conventional plates, the relative velocity of the two phases is small, almost equal to the bubble ascent rate in the standing liquid, and depends only on the size of the bubble and the properties of the liquid. The larger the bubble, the higher the speed of its movement in the same liquid. Thus, for ordinary plates there is some internal theoretical limit to increasing the efficiency of heat and mass transfer: a smaller bubble - a larger area, but less mixing speed and vice versa.

 A measure of the perfection of any plate from the point of view of the heat and mass transfer process just described and those factors that affect its efficiency is the degree to which the states of vapor and liquid after their interaction on the plate differ from their possible, theoretically achievable equilibrium state at this temperature. This value is called the efficiency (coefficient of performance) of the plate and is measured in percent. The average level of efficiency of the plates currently used is 50-70%.

 Bubbles of vapor 15, passing through the liquid layer 16, go out onto its surface 18 and form a foam layer 19 on it. In conventional plate contact devices, the thickness of this foam layer is quite large and depends on the degree of foaming of the liquid. It is taking into account the size of the foam layer that the distance between the plates is selected. Usually this distance is 250-600 mm. The distance between the plates is a very important indicator, as the height of the entire distillation column as a whole depends on it.

 The foam layer is independently destroyed with the formation of small splashes of liquid 20, which are carried away by the steam stream 14 and transferred to the next highest plate. Such intertellar fluid transfer significantly reduces the efficiency of the plate. For conventional plates, the inter-tray fluid transfer is about 10%.

 Another indicator characterizing the efficiency of the plate is its resistance (pressure drop across the pair). The value of this resistance consists of three components: the resistance of the dry plate (the shape and relative area of the channels 7 for the passage of steam), the additional resistance of the wet plate (overcoming the surface tension forces in the channels 7) and the resistance of the liquid layer on the plate (static pressure of the liquid column) . The total resistance of the plates is usually measured in the height of the water column and for the plates currently used is 50-100 mm. The maximum component of this resistance is the static pressure of the liquid layer on the plate. Reducing this layer for conventional plates leads to a significant decrease in their efficiency.

 The last indicator of a plate can be its cost, which depends on the intensity and complexity of the design of the plate.

 The proposed contact device as part of a heat and mass transfer apparatus (distillation column) works as follows.

From above, liquid 13 enters the vortex plate 17, and from the bottom, steam 14. The notches 4 made in the base 1 of the plate 4 with the formation of inclined unidirectional channels 7 turn the plate into a kind of stationary "turbine" (see Fig. 12). Steam passing through this "turbine" exits tangentially to the base 1 and spins (ω _g ) the liquid 16 located on the plate.

 The steam loop 8 exits the inclined unidirectional channels 7 and is crushed into many small bubbles 15 by the oncoming liquid flow. These bubbles mix with the liquid 16 and form a two-phase fine rotating mixture with it. The two-phase mixture does not acquire complete uniformity immediately, but only at a certain distance from the vortex plate — in the upper layers, and in its lower layers there are two regions of “clean” (without vapor bubbles) liquid 16. This is the central cone-shaped region 21 and the peripheral region 22 truncated torus form. To describe the operation of the device, these areas are not really important, but the processes that led to their formation.

 The rotation of the liquid on the plate is a rather complicated process, which is determined by two main and opposing factors: the presence of a source of rotation near the lower layers of the liquid, on the one hand, and the inhibition of this liquid on the inner surface of the column, on the other.

The lower layers of the liquid, located in close proximity to the source of rotation (section EE of FIG. 15), practically do not experience braking on the inner surface 3 of the column. And therefore, they have a classical velocity profile of 23 rotating bodies (just such a velocity profile is realized in a liquid located in a glass rotating around its own axis). Classical centrifugal forces act inside these fluid layers, resulting in the formation of a characteristic meniscus of the surface 18, while the fluid pressure near the axis is less than at the periphery. Steam bubbles formed near each slot 4 in the form of loops 8 move in the direction of the joint action of the gravitational acceleration component (g = 9.81 m / s ² ) upward and the centrifugal acceleration component (ω $\genfrac{}{}{0ex}{}{\text{2}}{\text{well}}$ • R) - to the axis of rotation. In this case, some erosion of the bubble plumes 8 along the radius occurs.

The upper layers of the liquid (section FJ), to a greater extent "feel" braking on the inner surface 3 of the column and additionally work on the unwinding of reflux 13, the film flowing down the inner surface of the column from the upper plate. In this case, a turbulent velocity profile is realized 24. Because of this, in addition to the main rotation 25, “brake” (turbulent) vortices 26 also develop in the fluid. In addition, there is some “slippage” between the fluid layers caused by a decrease in the angular velocity ω _w with distance layers of fluid from a source of rotation. This leads to the formation of “high-altitude” vortices (not shown) with ascending and descending flows of the two-phase mixture. All this complex and schematically described movement of the upper layers finally completes the process of uniformly mixing the vapor bubbles with the liquid. Only two small areas 21 and 22 remain free of bubbles.

 The vapor bubbles, moving together with the liquid, are carried away by turbulent vortices from the high-pressure region (at the periphery) to the low-pressure region (at the center), their volume and shape pulsate, which causes the vapor to move inside the bubble. As a result of the processes described above, an intensive and vortex (turbulent) mixing of the two phases occurs, from where the name of the contact devices themselves comes from.

 Bubbles of vapor 15, passing through a layer of liquid 16, go to its surface 18 and form a foam layer 19 on it. However, the size of this layer is small due to its rapid destruction under the influence of centrifugal forces. The main destruction of the foam layer occurs on the surface of the peripheral part of the meniscus 18 and near the inner surface 3 of the column. Splashes of liquid 20 from bursting bubbles immediately settle onto the inner surface of the column 3 and flow down in the form of a film 13, replenishing a two-phase suspension. The peripheral destruction of the foam layer causes its directed and accelerated movement from the axis of rotation to the periphery. And in accordance with Bernoulli's law, its thickness decreases from the center to the periphery. As a result, the foam layer as it were fills only the meniscus recess on the surface 18.

The foam layer is destroyed and spontaneously with the formation of small splashes 20 of liquid. These liquid droplets are carried away by the steam stream, which retained the residual rotation energy ((ω _g, see Fig. 11). Due to the presence of centrifugal acceleration in the pair (ω $\genfrac{}{}{0ex}{}{\text{2}}{\text{g}}$ • R) there is a separation of droplets that settle on the inner surface of 3 columns and flow back onto the plate. Thus, the inter-tray entrainment of fluid at the vortex plate is practically absent.

 Maintaining the required fluid level on the vortex plate is carried out dynamically. As a result of rotation in the peripheral region, there is an increased fluid pressure. It is in this area that drain channels are located and it is here that area 22 exists with liquid without bubbles. It is from this area that the "clean" liquid is drained onto the lower plate. Moreover, the surface tension forces in narrow channels (capillary effect), designed to drain the liquid, prevent the penetration of steam through them, and they also prevent the plate from completely emptying while reducing the steam load on it (reducing the flow rate of steam in the column).

 Against the background of the increased contact area of the two phases and the multiply enhanced effects of concentration equalization inside each phase, as well as the absence of inter-tray entrainment of the liquid, the heat and mass transfer processes on the vortex plate are intensified to such an extent that the efficiency of the plate is practically indistinguishable from 100%.

 As a result of such a powerful heat and mass transfer effect, it is possible to significantly reduce the thickness of the liquid layer on the plate and reduce its total resistance. The experiments showed that a decrease in the total resistance of the plate to 8-15 mm water column practically does not affect the efficiency of the proposed device.

 Taking into account the reduced two-phase layer and the almost completely absent foam layer in the experimental distillation columns, it was possible to arrange vortex plates with a pitch of only 35-40 mm. Such a significant reduction in the step between the plates and taking into account the increase in their efficiency leads to a 5-10-fold reduction in the height of the column itself.

 The operation of the contact device is influenced by liquid and vapor loading, as well as the nature of the interacting phases. However, taking these factors into account, it is only with certain sizes of the vortex plate itself and certain ratios between the sizes of its structural elements that the operation of this device is described above. We will justify this without going into a detailed formula description of those complex processes that occur on a plate.

или запишем иначе:

где V_г - модуль вектора скорости выхода пара (газа) из наклонных однонаправленных каналов 7 (см. фиг.5);
R_г - средневзвешенный радиус выхода пара (газа) из наклонных однонаправленных каналов 7, причем этот радиус находится приблизительно в центре просечек 4, т.е. R_г≈(R-1/2•L) (см. фиг.1, 3 и 4);
γ - средний угол между основанием 1 тарелки и вектором скорости V_г (см. фиг.5).The average angular velocity of rotation of the liquid ω _W on the plate completely determines the effect of the vortex interaction of phases and is proportional to:

or write it differently:

where V _g is the module of the vector of the steam (gas) exit velocity from the inclined unidirectional channels 7 (see Fig. 5);
R _g is the weighted average radius of the exit of steam (gas) from the inclined unidirectional channels 7, and this radius is approximately in the center of the slots 4, i.e. R _g ≈ (R-1/2 • L) (see figures 1, 3 and 4);
γ is the average angle between the base 1 of the plate and the velocity vector V _g (see figure 5).

что и заявляется.The vapor velocity (Vg) is a fairly conservative quantity and cannot vary widely, this is due to the fact that its increase leads to an increase in the pressure drop across the plate. With this in mind, the value of this velocity must be selected in the range V _g = 3 ... 6 m / s. There is also a theoretical limitation of the average vapor velocity V _gk = 0.6 ... 1.4 m / s in the full cross-section (area S _k ) of the column. This is due to the fact that there is some balance between the forces holding the liquid film on the inner surface of the column (surface tension) and gas-dynamic resistance of V _rk flowing only through the Earth gravity (g = 9,81 m / c ²⁾ of the liquid film. Given that the total cross section of the column is equal to the area of the plate itself S _t = S _k , in accordance with Bernoulli's law, the ratio of these speeds is almost inversely proportional to the ratio of the passage areas, therefore:

as stated.

The relative length of the grooves L / R can theoretically vary in the range from 0 to 1. However, with its increase (L / R tends to 1), the radius of the center of the grooves R _g decreases and, in accordance with the formula {1}, the angular velocity of rotation of the fluid ω _w decreases. With a decrease in the relative length of the cuts (L / R tends to zero), while observing the relation {3}, the angle γ rapidly increases, which leads to the degeneration of the inclined unidirectional channels 7 into ordinary holes (sieve plates), and the “turbine” effect of twisting the steam (see Fig. 12) in accordance with the formulas {1 and 2} completely disappears. In both cases, due to lower rotating liquid ω _w effect of vortex mixing of the two phases described above, completely disappears. The experiments showed that the optimal relative length of the cuts should be in the range L / R = 0.3 ... 0.6.

As for the channels for draining the liquid, in order to dynamically maintain the optimum height of the vapor-liquid layer on the plate, it is necessary to maintain the ratio of the areas of all channels for draining the liquid with respect to the entire area of the plate - S _w / S _t = 0.01 ... 0.07. The choice of a specific value of this ratio in any case depends on the properties, the interacting phases and the constructive implementation of these channels (see Figs. 8, 9 and 10) and is determined empirically. However, this ratio should be realized constructively with the smallest possible hydraulic radius of the channels. This means that the entire liquid cross-sectional area cannot be replaced with one round hole with an equivalent area. In the ultimate best case (for a liquid not containing suspended particles) this area may be an annular gap 11 around the entire perimeter of the plate with an equivalent area (as in figure 10). In order to avoid clogging of these channels, the restriction may be only the maximum sizes of particles in the liquid.

The last, most important constructive parameter of the vortex plate is its absolute size - the radius of the plate R. The above-described operation of the vortex plate with those powerful effects of phase interaction is realized only at a sufficiently high speed of rotation of the liquid ω _w = 1000 ... 4000 rpm. Given the limited speed V _g = 3 ... 6 m / s (even with other optimal dimensions of the vortex plate), such a revolution of the liquid on the plate can be achieved only if its radius does not exceed 50 mm. On the other hand, with a decrease in the radius of the plate less than 10 mm, it is practically impossible to realize a “turbine” steam output through the inclined unidirectional channels 7, since the latter degenerate into ordinary holes.

 Thus, for each pair of interacting gas + liquid phases, their properties and the ratio of their flow rates, there is an optimal vortex plate on which the maximum efficiency of their interaction (maximum efficiency) is realized at the lowest resistance.

Claims (5)

1. A contact device for heat and mass transfer apparatus, containing a vortex plate, along the periphery of which there are perforations located around the circumference, and a drain device for draining the liquid from the upper plate to the lower one, characterized in that the vortex plate is made of 0.1-0.0 mm thick sheet metal foil 5 mm and has a radius of 10-50 mm, the drainage device contains a number of peripherally located drainage channels, and the notches are made so that their edges are bent up and down with the formation of inclined unidirectional channels als for the passage of gas or vapor phase mixture and impart rotational movement on the plate only in one direction, the dimensions of embossments, channels passage section areas for the passage of gas or vapor and the overflow channels are associated with the size of plates following relationships:
L / R = 0.3-0.6;
S _g / S _t = 0.1-0.4;
S _w / S _t = 0.01-0.07;
where R is the radius of the vortex plate, mm;
L is the length of the cuts (the length of the channels for the passage of gases), mm;
S _t - the area of the vortex plate (S _t = πR ² , π = 3,14), mm ² ;
S _g - the total area of the passage sections of the channels for the passage of gas or steam, mm ² ;
S _W - the total area of the passage sections of the channels for draining the liquid, mm ² .  2. The contact device according to claim 1, characterized in that around the periphery of the vortex plate are bent perpendicular to the base of the bead for mounting the plate to the inner surface of the heat and mass transfer apparatus.  3. The contact device according to claim 2, characterized in that the drain channels are made in the base of the plate in the form of holes with bent down edges and placed in such a way that they adjoin the inner surface of the sides of the plate.  4. The contact device according to claim 2, characterized in that the drain channels are formed by concave portions of the peripheral sides of the plate and the inner surface of the heat and mass transfer apparatus adjacent to them.  5. The contact device according to claim 2, characterized in that the vortex plate is mounted with respect to the internal surface of the heat and mass transfer apparatus and the drain channels are formed by the internal surface of the heat and mass transfer apparatus and the outer surface of the peripheral sides of the plate.

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