CA1063926A - Multiple cross-flow contacting system - Google Patents
Multiple cross-flow contacting systemInfo
- Publication number
- CA1063926A CA1063926A CA230,821A CA230821A CA1063926A CA 1063926 A CA1063926 A CA 1063926A CA 230821 A CA230821 A CA 230821A CA 1063926 A CA1063926 A CA 1063926A
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- Prior art keywords
- liquid
- gas
- vapour
- streams
- mass transfer
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-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D3/00—Distillation or related exchange processes in which liquids are contacted with gaseous media, e.g. stripping
- B01D3/14—Fractional distillation or use of a fractionation or rectification column
- B01D3/16—Fractionating columns in which vapour bubbles through liquid
- B01D3/24—Fractionating columns in which vapour bubbles through liquid with sloping plates or elements mounted stepwise
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/14—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by absorption
- B01D53/18—Absorbing units; Liquid distributors therefor
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- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Engineering & Computer Science (AREA)
- Analytical Chemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Vaporization, Distillation, Condensation, Sublimation, And Cold Traps (AREA)
- Physical Or Chemical Processes And Apparatus (AREA)
Abstract
A B S T R A C T
There is provided an improved process and apparatus for mass trans-fer between a liquid and a gas which process comprises providing a series of x contacting passes or stages, each comprising n contacting zones in each of which zones the liquid and the gas or vapor are brought into intimate con-tact with each other, the liquid being fed into the system in n separate streams each of which follows its own discrete-essentially vertical path through a corresponding one of said n zones through all of said x stages, sequentially from 1 to x thereof, and the gas or vapor being fed essentially horizontally through each of said stages, sequentially from x to 1 thereof, such that, in each of said stages, said gas or vapor is brought into intimate contact sequentially from n to 1 thereof, with each of said separate liquid streams in said corresponding one of said contacting stages, and wherein x and n are greater than one. The invention provides a higher theoretical separation efficiency at each stage than is possible in conventional app-aratus.
There is provided an improved process and apparatus for mass trans-fer between a liquid and a gas which process comprises providing a series of x contacting passes or stages, each comprising n contacting zones in each of which zones the liquid and the gas or vapor are brought into intimate con-tact with each other, the liquid being fed into the system in n separate streams each of which follows its own discrete-essentially vertical path through a corresponding one of said n zones through all of said x stages, sequentially from 1 to x thereof, and the gas or vapor being fed essentially horizontally through each of said stages, sequentially from x to 1 thereof, such that, in each of said stages, said gas or vapor is brought into intimate contact sequentially from n to 1 thereof, with each of said separate liquid streams in said corresponding one of said contacting stages, and wherein x and n are greater than one. The invention provides a higher theoretical separation efficiency at each stage than is possible in conventional app-aratus.
Description
1o~3~26 This invention relates to an apparatus and method in which two fluid streams are brought into intimate contact for the purpose of progress-ively increasing or decreasing the concentration of a particular comQonent or components in one stream or the otherO The invention is useful in the common commercial operations of distillation and absorptionO
It is common in these operations to use vertical enclosed cylindri-cal columns or towers containing either a plurality of horizontal trays ("tray columns"), for stepwise contacting of the two streams, or containing a bed of suitab~e packing material ("packed columns"), for continuous contact-ing. The two types of columns have a number of advantages and disadvantages relative to each other, but in addition, each contains certain inherent theoretical limitations on efficiencyO
me present invention provides contacting apparatus leading to im-proved theoretical efficiency.
; The advantages of the apparatus of the present application can be more clearly illustrated by reference to one of the common operations; viz., distillation, considering, first, the principles of operation of convention-al tray-type columnsO
me most common type of tray column is the "cross-flow" type in - 20 which the direction of liquid flow alternates from plate to plate, passing - from one plate to the next by gravity through vertical enclosed conduits or "downcomers"O Another type, much less commonly used, is the "parallea-flow"
type, in wh~ch the direction of liquid flow is the same on all plates, the . liquid passing downward from plate to plate through sloping downcomersO In - either type, the vapours rise essentially vertically, entering each tr~ay through a multitude of ports or devices of a wide variety of designs such that efficient contacting is achieved with a minimum of pressure loss in the vapour streamO
In separation operations involving continuously flowing contacting streams, it is usually beneficial to minimize the degree of internal mixing 10~39Z6 wi~lin the streams 90 that a concentration gradient is established within each stream when the system is at equilibriumO Thus, for tray-type distilla-tion columns it has been shown by Lewis (Lewis, WoKo Industrial ~ Engineering Chemistry 28 NoO 4, 399 (1936)) that maximum efficiency is obtainable only when the trays are operated in parallel-flow configuration, as described above, and when there is no mixing within the liquid or vapour streams in the hor-izontal direction ("horizontal mixing~)0 Under any given conditions of oper-ation, cross-flow trays have a lower maximum theoretical efficiency than parallel-flow trays, the efficiency of the former being maximum when there is no horizontal mixing within the liquid but complete mixing within the vapour between trays. Actually, when the vapour is completely mixed it makes no difference whether the liquid flow pattern is cross- or parallel-flowO It is largely for this reason that cross-flow trays are much more common than the more expensive parallel-flow type, since it is difficult to avoid con-siderable mixing within the vapour streams in commercial columns~
Furthermore, in processes in which both the desired and the undesir-ed components can be transferred freely from one stream to the other, it is well known that the potential efficiency of separation is increased the clos-er together are the concentrations of the two streams at any given point of contact since this n~n;~;zes the diluting effect of one stream on the other.
This principle is applicable to distillation since all components in the vapour are transferrable to the liquid phase by condensation, the extent of transfer depending on the degree of approach to equilibrium between the va-pour and the liquid, m us, there are three basic theoretical objectives for efficient mass transfer in distillation processes in general:
(à) mat the exit vapour from any given contact zone be as close as possible to equilibrium with the liquid at that point, (b) That there be a mLnimum of mixing within the individual phases, -. 10639Z6 (c) That the vapour and liquid entering any given contact zone be of as nearly equal composition as possible.
It will be evident from the foregoing discussion that there is no theoretical barrier to the attainment of objectives (a) and (b) in conven-tional tray equipment, the degree of approach to the theoretical ideal being limited only by the degree of refinement of the design and the method of oper-ation of the equipmentO
Conventional equipment cannot, however, usually be operated in such a way as to achieve objective (c) because the flow configurations and/or the operating conditions dictate a certain concentration difference between the mixing streamsO
For absorption operations i.e., the removal of a soluble compon-ent from an inert carrier gas by a solvent - the basic principles of oper-ation of tray columns and packed columns are similar to those for distill-ation except that the carrier gas is not usually significantly soluble in the absorbing mediumO The driving force governing the rate of absorption is the , difference between the concentration of the soluble component in the gas phase and the concentration in the gas which would exist if the gas were allowed to reach eq~ ibrium with liquid at the concentration of the liquid stream at that pointO Thus, while objectives (a) and (b), described above, apply to absorption as well as to distillation, objective (c) is not appli-cable to absorptionO me corresponding objective in ~bsorption is that the driving force be as high as possible. If the gas and liquid leaving a given contact zone are nearly at equilibrium with each other, then at the next contact zone this driving force can be increased only to the degree that the liquid concentration can be varied from one zone to the next~ In tray col-umns, the liquid concentration gradient from tray to tray is limited to the extent that the relative rates of flow of the two streams and the residence time an~ degree of mixing on each tray are limitedO In packed colum~s of conventional design, stepwise changes in the liquid concentration are not _3__ ~063926 possible and the average driving force is thus relatively low, necessitating a rather deep contacting bed.
Conventional gas absorption equipment, therefore, has limitations in respect to the provision of a large driving force.
The present application describes a scheme in which the fluid streams are contacted in such a way as to permit a closer approach to ideal contacting conditions thAn is possible in equipment of conventional design, in the following respects: (a) A smaller difference in concentration between the mixing streams, in the case of distillation, or (b) A greater driving force for mass transfer, in the case of absorption.
One aspect of the present invention provides a method of effecting mass transfer of one or more components between a liquid and a gas or vapour by intimate contacting of the fluids, which method comprises: (a) feeding a flow of liquid into the top of a vertically disposed mass transfer column and dividing the flow of liquid into a plurality of separate liquid streams; (b) causing each separate liquid stream to follow a sequentially interrupted sub-stantially vertically downward path through a plurality of spaced superposed contact zones, the superpositioning of the contact zones being maintained throughout the column so that the relative orientation of the liquid streams with respect to the contact zones remains the same at all levels; and (c) pass-ing the gas or vapour generally upwardly through the mass transfer column in a generally helical path so as to successively contact each separate liquid stream at each of said contact zones to effect mass transfer between said liquid and said gas or vapour.
Another aspect of the invention provides an apparatus for effect-ing mass transfer of one or more components between a liquid and a gas or va-pour by intimate contacting of the fluids, which apparatus comprises: (a) outer and inner co-axial substantially vertical columns defining an annular space therebetween; (b) continuous baffle means in the annular space including a plurality of mutually superposed substantially horizontal baffle portions joined to each other by sloping ramps, whereby to define a generally helical path through the annular space for the passage of gas or vapour upwardly through said space; (c) a plurality of spaced liquid-permeable areas on each . . .
10~39'~6 said horlzontal barrle portion, ~;ach said liquid-permeable area on a given bafrle portion bein substantially vertically aligned ~rlth the respective liquid-permeable area in the baffle portions above and below, whereby to facilitate vertically do~n~.ard travel of a plurality of separate liquid streams through the respective vertically aligned liquid-permeable areas so as to contact said upwardly travelling gas or vapour in a plurality of contact zones; (d) weir means on either side of each liquid-permeable area to prevent mixing bet~een ad~acent liquid streams and to provide a sufficient head of liquid at each liquid-permeable area to substantially prevent gas or vapour ~e~ i from passing through the liquid-permeable areas, and~means by which the various fluids may be fed into or withdrawn from the apparatus at different levels.
In dra~ings which illustrate embodiments of the invention, Figure 1 is a schematic representation of the flow configuration according to the present invention.
Figure 2 is a sectional perspective of one embodiment.
Figure 3 i5 a graph showing the theoretical stage-to-stage differ-ences in concentrations of mixing streams in the flow configuration of the present invention as compared with those of conventional parallel-flow trays for the distillation example.
10~39Z6 Figure 4 is a graph showing the theoretical stage-to-stage exit liquid concentrations in the flow configuration of the present invention as conpared to those of conventional parallel-flow trays for the distillation exampleO
Referring to Figure 1, the scheme consi9ts of a number of passes or stages in each of which the gas or vapor stream flows in an essentially horizontal direction perpendicular to the vertically downflowing liquid, and is brought into intimate contact with the liquid stream in a number of contacting zones before moving on to the next stageO The number of contact-ing zones in each pass is carefully chosen so as to gi~e optimum overall mass transfer efficiency for a particular application, as established by empirical data and/or theoretical studiesO The liquid stream is distributed more or less equally between the zones and each stream is isolated from the adjacent liquid streams so that at equilibrium, a diffenence in concentra-tion between adjacent streams existsO
This flow configuration provides a system in which, if employed for a distillation operation, there is nottheoretical reason why the contacting streams entering any given contact zone must necessarily have appreciably different concentrations; while, if employed for absorption a large change :-in gas concentration is possible in each zone because of the differences in concentration of the liquid stream from zone to zoneO In either operation, an additional advantage is that the perpendicular flow configuration results in a lower pressure loss per unit bed thickness, for a given gas velocity and liquid irrigation rate, than in counter-flow packed columns.
One possible embodiment of this invention, therefore, would be to use one or more packed o~ss-flow contacting devices of conventional type for each stage (as commonly used in gas scrubbing applications), with inter-connecting ducting for the gas stream and with either pumped or gravity inter-stage liquid flow~
An alternative embodiment which would usually be more economical for difficult sna~9 transfer operations, or for other than ambient tesnperature and pressure conditions, is illustrated in Figure 2.
This consists of an outer vertical cylindrical column 10 and a coaxial inner cylinder 200 In the annular space 80 therebetween, is a plurality of horizontal equally spaced ba~fles 30, each pair of which is ~-a ~,oS
interconnected by sloping bafflc3 40O Each baffle 30 is provided with a certain number of perforated areas S0, these areas being arranged in vertical alignment with corresponding perforated areas in the baffles 30 above and belowO Further, each of said perforated areas 50 on a given baffle 30 is partially separated from adjacent perforated areas S0, and from the inter-~ ~o connecting bafflc 40 sloping downwardly from that baffle 30, by vertical dams 600 In operation, the liquid feed stream i9 divided more or less equal-ly among the perforated areas 50 on the top baffle 30, and thence each stream 70 follows a separate essentially vertical path downwardly through the annular space 80, each such path being defined by a set of vertically aligned perforated areas 50 and the dams 60 adjacent to each set. me dams 60 allow an accumulation of liquid above each of the perforated areas : 50 of sufficient height to prevent the gas or vapour stream from passing through the perforations.
m e upward flowing gas or vapour passes horizontally through the annular space between each adjacent pair of baffles 30 and then up the slo-~ oS
ping passage contained between the adjacent sloping ba~fle~ 40 to the next higher horizontal passO Hence, with each horizontal pass, the gas or vapour is brought into contact with each of the downwardly flowing streams 70.
me space through which the downwardly flowing liquid falls could advantageously be filled wi~h a suitable packing material to increase the contacting efficiency and enhance the establishment of a vertical concen-tration gradient in the liquid stream~ Also~ the presence of a packing material of the same or a different type or suitable baffling, between adj-acent contacting zones would be advantageous for further minimizing vertical mixing withi~ the gas stream, and for entrainment removalO
__7_ The central core 20 might advantageously be hollow and made to serve as a conduit for the exiting gas stream, as illustrated, to eliminate the need for external piping.
The baffles and packing material would, of course, need to fit sufficiently tight against the outer and inner surfaces to prevent apprec-iable leakage of either fluid stream, using conventional sealing means as in existing types of equipment.
Figures 3 and 4 give a graphical comparison in theoretical terms of the improved performance of the system of the present invention over a conventional parallel-flow tray system. The Figures are based on a hypothe-tical distillation problem. For simplicity, it will be assumed that we are dealing with an "ideal" binary system, i.e. a two-component system in which the mole fraction x of the less volatile component in the liquid is related to its mole fraction y in the vapour at equilibrium by the equation X l_y_;
where a is a constant, referred to as the "relative volatility".
The closer the value a is to unity, the greater will be the difficulty of separation. For systems such as light water - heavy water, for example, its value is about 1.05.
In our example, the following conditions will be assumed:
(1) a = 1.05
It is common in these operations to use vertical enclosed cylindri-cal columns or towers containing either a plurality of horizontal trays ("tray columns"), for stepwise contacting of the two streams, or containing a bed of suitab~e packing material ("packed columns"), for continuous contact-ing. The two types of columns have a number of advantages and disadvantages relative to each other, but in addition, each contains certain inherent theoretical limitations on efficiencyO
me present invention provides contacting apparatus leading to im-proved theoretical efficiency.
; The advantages of the apparatus of the present application can be more clearly illustrated by reference to one of the common operations; viz., distillation, considering, first, the principles of operation of convention-al tray-type columnsO
me most common type of tray column is the "cross-flow" type in - 20 which the direction of liquid flow alternates from plate to plate, passing - from one plate to the next by gravity through vertical enclosed conduits or "downcomers"O Another type, much less commonly used, is the "parallea-flow"
type, in wh~ch the direction of liquid flow is the same on all plates, the . liquid passing downward from plate to plate through sloping downcomersO In - either type, the vapours rise essentially vertically, entering each tr~ay through a multitude of ports or devices of a wide variety of designs such that efficient contacting is achieved with a minimum of pressure loss in the vapour streamO
In separation operations involving continuously flowing contacting streams, it is usually beneficial to minimize the degree of internal mixing 10~39Z6 wi~lin the streams 90 that a concentration gradient is established within each stream when the system is at equilibriumO Thus, for tray-type distilla-tion columns it has been shown by Lewis (Lewis, WoKo Industrial ~ Engineering Chemistry 28 NoO 4, 399 (1936)) that maximum efficiency is obtainable only when the trays are operated in parallel-flow configuration, as described above, and when there is no mixing within the liquid or vapour streams in the hor-izontal direction ("horizontal mixing~)0 Under any given conditions of oper-ation, cross-flow trays have a lower maximum theoretical efficiency than parallel-flow trays, the efficiency of the former being maximum when there is no horizontal mixing within the liquid but complete mixing within the vapour between trays. Actually, when the vapour is completely mixed it makes no difference whether the liquid flow pattern is cross- or parallel-flowO It is largely for this reason that cross-flow trays are much more common than the more expensive parallel-flow type, since it is difficult to avoid con-siderable mixing within the vapour streams in commercial columns~
Furthermore, in processes in which both the desired and the undesir-ed components can be transferred freely from one stream to the other, it is well known that the potential efficiency of separation is increased the clos-er together are the concentrations of the two streams at any given point of contact since this n~n;~;zes the diluting effect of one stream on the other.
This principle is applicable to distillation since all components in the vapour are transferrable to the liquid phase by condensation, the extent of transfer depending on the degree of approach to equilibrium between the va-pour and the liquid, m us, there are three basic theoretical objectives for efficient mass transfer in distillation processes in general:
(à) mat the exit vapour from any given contact zone be as close as possible to equilibrium with the liquid at that point, (b) That there be a mLnimum of mixing within the individual phases, -. 10639Z6 (c) That the vapour and liquid entering any given contact zone be of as nearly equal composition as possible.
It will be evident from the foregoing discussion that there is no theoretical barrier to the attainment of objectives (a) and (b) in conven-tional tray equipment, the degree of approach to the theoretical ideal being limited only by the degree of refinement of the design and the method of oper-ation of the equipmentO
Conventional equipment cannot, however, usually be operated in such a way as to achieve objective (c) because the flow configurations and/or the operating conditions dictate a certain concentration difference between the mixing streamsO
For absorption operations i.e., the removal of a soluble compon-ent from an inert carrier gas by a solvent - the basic principles of oper-ation of tray columns and packed columns are similar to those for distill-ation except that the carrier gas is not usually significantly soluble in the absorbing mediumO The driving force governing the rate of absorption is the , difference between the concentration of the soluble component in the gas phase and the concentration in the gas which would exist if the gas were allowed to reach eq~ ibrium with liquid at the concentration of the liquid stream at that pointO Thus, while objectives (a) and (b), described above, apply to absorption as well as to distillation, objective (c) is not appli-cable to absorptionO me corresponding objective in ~bsorption is that the driving force be as high as possible. If the gas and liquid leaving a given contact zone are nearly at equilibrium with each other, then at the next contact zone this driving force can be increased only to the degree that the liquid concentration can be varied from one zone to the next~ In tray col-umns, the liquid concentration gradient from tray to tray is limited to the extent that the relative rates of flow of the two streams and the residence time an~ degree of mixing on each tray are limitedO In packed colum~s of conventional design, stepwise changes in the liquid concentration are not _3__ ~063926 possible and the average driving force is thus relatively low, necessitating a rather deep contacting bed.
Conventional gas absorption equipment, therefore, has limitations in respect to the provision of a large driving force.
The present application describes a scheme in which the fluid streams are contacted in such a way as to permit a closer approach to ideal contacting conditions thAn is possible in equipment of conventional design, in the following respects: (a) A smaller difference in concentration between the mixing streams, in the case of distillation, or (b) A greater driving force for mass transfer, in the case of absorption.
One aspect of the present invention provides a method of effecting mass transfer of one or more components between a liquid and a gas or vapour by intimate contacting of the fluids, which method comprises: (a) feeding a flow of liquid into the top of a vertically disposed mass transfer column and dividing the flow of liquid into a plurality of separate liquid streams; (b) causing each separate liquid stream to follow a sequentially interrupted sub-stantially vertically downward path through a plurality of spaced superposed contact zones, the superpositioning of the contact zones being maintained throughout the column so that the relative orientation of the liquid streams with respect to the contact zones remains the same at all levels; and (c) pass-ing the gas or vapour generally upwardly through the mass transfer column in a generally helical path so as to successively contact each separate liquid stream at each of said contact zones to effect mass transfer between said liquid and said gas or vapour.
Another aspect of the invention provides an apparatus for effect-ing mass transfer of one or more components between a liquid and a gas or va-pour by intimate contacting of the fluids, which apparatus comprises: (a) outer and inner co-axial substantially vertical columns defining an annular space therebetween; (b) continuous baffle means in the annular space including a plurality of mutually superposed substantially horizontal baffle portions joined to each other by sloping ramps, whereby to define a generally helical path through the annular space for the passage of gas or vapour upwardly through said space; (c) a plurality of spaced liquid-permeable areas on each . . .
10~39'~6 said horlzontal barrle portion, ~;ach said liquid-permeable area on a given bafrle portion bein substantially vertically aligned ~rlth the respective liquid-permeable area in the baffle portions above and below, whereby to facilitate vertically do~n~.ard travel of a plurality of separate liquid streams through the respective vertically aligned liquid-permeable areas so as to contact said upwardly travelling gas or vapour in a plurality of contact zones; (d) weir means on either side of each liquid-permeable area to prevent mixing bet~een ad~acent liquid streams and to provide a sufficient head of liquid at each liquid-permeable area to substantially prevent gas or vapour ~e~ i from passing through the liquid-permeable areas, and~means by which the various fluids may be fed into or withdrawn from the apparatus at different levels.
In dra~ings which illustrate embodiments of the invention, Figure 1 is a schematic representation of the flow configuration according to the present invention.
Figure 2 is a sectional perspective of one embodiment.
Figure 3 i5 a graph showing the theoretical stage-to-stage differ-ences in concentrations of mixing streams in the flow configuration of the present invention as compared with those of conventional parallel-flow trays for the distillation example.
10~39Z6 Figure 4 is a graph showing the theoretical stage-to-stage exit liquid concentrations in the flow configuration of the present invention as conpared to those of conventional parallel-flow trays for the distillation exampleO
Referring to Figure 1, the scheme consi9ts of a number of passes or stages in each of which the gas or vapor stream flows in an essentially horizontal direction perpendicular to the vertically downflowing liquid, and is brought into intimate contact with the liquid stream in a number of contacting zones before moving on to the next stageO The number of contact-ing zones in each pass is carefully chosen so as to gi~e optimum overall mass transfer efficiency for a particular application, as established by empirical data and/or theoretical studiesO The liquid stream is distributed more or less equally between the zones and each stream is isolated from the adjacent liquid streams so that at equilibrium, a diffenence in concentra-tion between adjacent streams existsO
This flow configuration provides a system in which, if employed for a distillation operation, there is nottheoretical reason why the contacting streams entering any given contact zone must necessarily have appreciably different concentrations; while, if employed for absorption a large change :-in gas concentration is possible in each zone because of the differences in concentration of the liquid stream from zone to zoneO In either operation, an additional advantage is that the perpendicular flow configuration results in a lower pressure loss per unit bed thickness, for a given gas velocity and liquid irrigation rate, than in counter-flow packed columns.
One possible embodiment of this invention, therefore, would be to use one or more packed o~ss-flow contacting devices of conventional type for each stage (as commonly used in gas scrubbing applications), with inter-connecting ducting for the gas stream and with either pumped or gravity inter-stage liquid flow~
An alternative embodiment which would usually be more economical for difficult sna~9 transfer operations, or for other than ambient tesnperature and pressure conditions, is illustrated in Figure 2.
This consists of an outer vertical cylindrical column 10 and a coaxial inner cylinder 200 In the annular space 80 therebetween, is a plurality of horizontal equally spaced ba~fles 30, each pair of which is ~-a ~,oS
interconnected by sloping bafflc3 40O Each baffle 30 is provided with a certain number of perforated areas S0, these areas being arranged in vertical alignment with corresponding perforated areas in the baffles 30 above and belowO Further, each of said perforated areas 50 on a given baffle 30 is partially separated from adjacent perforated areas S0, and from the inter-~ ~o connecting bafflc 40 sloping downwardly from that baffle 30, by vertical dams 600 In operation, the liquid feed stream i9 divided more or less equal-ly among the perforated areas 50 on the top baffle 30, and thence each stream 70 follows a separate essentially vertical path downwardly through the annular space 80, each such path being defined by a set of vertically aligned perforated areas 50 and the dams 60 adjacent to each set. me dams 60 allow an accumulation of liquid above each of the perforated areas : 50 of sufficient height to prevent the gas or vapour stream from passing through the perforations.
m e upward flowing gas or vapour passes horizontally through the annular space between each adjacent pair of baffles 30 and then up the slo-~ oS
ping passage contained between the adjacent sloping ba~fle~ 40 to the next higher horizontal passO Hence, with each horizontal pass, the gas or vapour is brought into contact with each of the downwardly flowing streams 70.
me space through which the downwardly flowing liquid falls could advantageously be filled wi~h a suitable packing material to increase the contacting efficiency and enhance the establishment of a vertical concen-tration gradient in the liquid stream~ Also~ the presence of a packing material of the same or a different type or suitable baffling, between adj-acent contacting zones would be advantageous for further minimizing vertical mixing withi~ the gas stream, and for entrainment removalO
__7_ The central core 20 might advantageously be hollow and made to serve as a conduit for the exiting gas stream, as illustrated, to eliminate the need for external piping.
The baffles and packing material would, of course, need to fit sufficiently tight against the outer and inner surfaces to prevent apprec-iable leakage of either fluid stream, using conventional sealing means as in existing types of equipment.
Figures 3 and 4 give a graphical comparison in theoretical terms of the improved performance of the system of the present invention over a conventional parallel-flow tray system. The Figures are based on a hypothe-tical distillation problem. For simplicity, it will be assumed that we are dealing with an "ideal" binary system, i.e. a two-component system in which the mole fraction x of the less volatile component in the liquid is related to its mole fraction y in the vapour at equilibrium by the equation X l_y_;
where a is a constant, referred to as the "relative volatility".
The closer the value a is to unity, the greater will be the difficulty of separation. For systems such as light water - heavy water, for example, its value is about 1.05.
In our example, the following conditions will be assumed:
(1) a = 1.05
(2) Concentration of overhead stream = 0.001 (mole fraction of less volatile component)
(3) Reflux ratio (ratio of reflux stream to overhead stream) =
0.99
0.99
(4) Reflux stream split into nine equal portions
(5) Molar flow rate of all streams unchanged in passing through contact zones
(6) The average vapour leaving any given contacting zone is in equilibrium with the average liquid composition in that zone.
(7) Steady-state conditions.
From assumption (6), if we designate the average liquid concen-tration zone n by the symbol x and the average vapour concentration leaving that zone by Yn, x 1--y _ n = a n n or, n a 11rn+ (a~1) For the uppermost zone (zone 1), Y = Y1 = x~ = 0.001 since the liquid feed is of the - same composition as the exit vapour.
Thus, x1 1.05 = 0.00104995 1/0.001 + 0.05 The exit liquid concentration x1 is therefore 0.001 + (0.00104995 - 0.001) (2) = 0.0010999 The inlet vapour concentration Y2 is then calculated from the material balance relationship.
Y2 Y1 + -99/9(x1 - xO) = 0-00101099 The calculation was continued step by step for 14 stages through 9 zones in each stage.
For comparison, tray by tray concentrations were calculated for the same separation problem using a parallel-flow tray column with no mixing which,.as noted above, offers the maximum efficiency theoretically attainable with tray columns.
The percentage differences between the concentrations of the mixing streams at all points of contact in both cases are plotted in Figure 3, each tray of the tray column being considered as one stage. It _9_ can be seen that the approach to exact equality of stream concentrations is closer with the scheme of this invention than with conventional trays under ideal (non-mixing) conditions.
The average exit liquid concentrations from each stage in the proposed scheme are plotted in Figure 4 together with the exit concent-rations for parallel-flow trays as above calculated. This shows the relative performances to be as follows:
Exit liquidNo. of stages withNo. of conven-Incremental no. of conc. proposed scheme tional traYsconventional trays 1263.8 1 2.5 lo 1922.6 2 7.2 4.7 3049.7 3 12.7 5.5 4786.7 4 18.3 5.6 7455.3 5 23-8 5 5 11524.1 6 29.5 5.7 17709.3 7 35 5-5 Thus below the first couple of stages, one stage in the proposed scheme is equivalent to about 5.55 ideal parallel-flow trays. The overall tray efficiency of the latter (Eo~ in Lewis~ terminology) is calculated to be 1.82 in the above example. One stage in the proposed scheme, therefore, has a theoretical overall tray efficiency Eo of 10.1 in this example.
From assumption (6), if we designate the average liquid concen-tration zone n by the symbol x and the average vapour concentration leaving that zone by Yn, x 1--y _ n = a n n or, n a 11rn+ (a~1) For the uppermost zone (zone 1), Y = Y1 = x~ = 0.001 since the liquid feed is of the - same composition as the exit vapour.
Thus, x1 1.05 = 0.00104995 1/0.001 + 0.05 The exit liquid concentration x1 is therefore 0.001 + (0.00104995 - 0.001) (2) = 0.0010999 The inlet vapour concentration Y2 is then calculated from the material balance relationship.
Y2 Y1 + -99/9(x1 - xO) = 0-00101099 The calculation was continued step by step for 14 stages through 9 zones in each stage.
For comparison, tray by tray concentrations were calculated for the same separation problem using a parallel-flow tray column with no mixing which,.as noted above, offers the maximum efficiency theoretically attainable with tray columns.
The percentage differences between the concentrations of the mixing streams at all points of contact in both cases are plotted in Figure 3, each tray of the tray column being considered as one stage. It _9_ can be seen that the approach to exact equality of stream concentrations is closer with the scheme of this invention than with conventional trays under ideal (non-mixing) conditions.
The average exit liquid concentrations from each stage in the proposed scheme are plotted in Figure 4 together with the exit concent-rations for parallel-flow trays as above calculated. This shows the relative performances to be as follows:
Exit liquidNo. of stages withNo. of conven-Incremental no. of conc. proposed scheme tional traYsconventional trays 1263.8 1 2.5 lo 1922.6 2 7.2 4.7 3049.7 3 12.7 5.5 4786.7 4 18.3 5.6 7455.3 5 23-8 5 5 11524.1 6 29.5 5.7 17709.3 7 35 5-5 Thus below the first couple of stages, one stage in the proposed scheme is equivalent to about 5.55 ideal parallel-flow trays. The overall tray efficiency of the latter (Eo~ in Lewis~ terminology) is calculated to be 1.82 in the above example. One stage in the proposed scheme, therefore, has a theoretical overall tray efficiency Eo of 10.1 in this example.
Claims (8)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of effecting mass transfer of one or more components be-tween a liquid and a gas or vapour by intimate contacting of the fluids, which method comprises: (a) feeding a flow of liquid into the top of a vertically disposed mass transfer column and dividing the flow of liquid into a plurality of separate liquid streams; (b) causing each separate liquid stream to follow a sequentially interrupted substantially vertically downward path through a plurality of spaced superposed contact zones, the superpositioning of the contact zones being maintained throughout the column so that the relative ori-entation of the liquid streams with respect to the contact zones remains the same at all levels; and (c) passing the gas or vapour generally upwardly through the mass transfer column in a generally helical path so as to success-ively contact each separate liquid stream at each of said contact zones to effect mass transfer between said liquid and said gas or vapour.
2. A method according to claim 1, wherein the number and the flow rates of said liquid streams are chosen such that optimum mass transfer efficiency is obtained for a given system.
3. A method according to claim 1 or 2, wherein the flow rates of the separate liquid streams are essentially equal.
4. An apparatus for effecting mass transfer of one or more components between a liquid and a gas or vapour by intimate contacting of the fluids, which apparatus comprises: (a) outer and inner co-axial substantially verti-cal columns defining an annular space therebetween; (b) continuous baffle means in the annular space including a plurality of mutually superposed sub-stantially horizontal baffle portions joined to each other by sloping ramps, whereby to define a generally helical path through the annular space for the passage of gas or vapour upwardly through said space; (c) a plurality of spaced liquid-permeable areas on each said horizontal baffle portion, each said liquid-permeable area on a given baffle portion being substantially vertically aligned with the respective liquid-permeable area in the baffle portions above and below, whereby to facilitate vertically downward travel of a plur-ality of separate liquid streams through the respective vertically aligned liquid-permeable areas so as to contact said upwardly travelling gas or vapour in a plurality of contact zones; (d) weir means on either side of each liquid-permeable area to prevent mixing between adjacent liquid streams and to provide a sufficient head of liquid at each liquid-permeable area to substantially prevent gas or vapour from passing through the liquid-permeable areas, and (e) means by which the various fluids may be fed into or withdrawn from the apparatus at different levels.
5. An apparatus according to claim 4, wherein the liquid-permeable areas on said horizontal baffle portions comprise perforated areas.
6. An apparatus according to claim 4 or 5, wherein the permeability capacity of each said liquid permeable area is substantially equal, whereby the flow rates of the separate liquid streams are essentially equal.
7. An apparatus according to claim 4 or 5, wherein each of said con-tact zones is packed with conventional mass transfer packing material.
8. An apparatus according to claim 4 or 5, wherein the inner column functions as a conduit for the exiting gas or vapour.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA230,821A CA1063926A (en) | 1975-07-04 | 1975-07-04 | Multiple cross-flow contacting system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA230,821A CA1063926A (en) | 1975-07-04 | 1975-07-04 | Multiple cross-flow contacting system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1063926A true CA1063926A (en) | 1979-10-09 |
Family
ID=4103536
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA230,821A Expired CA1063926A (en) | 1975-07-04 | 1975-07-04 | Multiple cross-flow contacting system |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1063926A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6431528B1 (en) * | 1999-10-07 | 2002-08-13 | Hisao Kojima | Apparatus for removing impurities in liquid |
-
1975
- 1975-07-04 CA CA230,821A patent/CA1063926A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6431528B1 (en) * | 1999-10-07 | 2002-08-13 | Hisao Kojima | Apparatus for removing impurities in liquid |
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