GB2280122A - Fractional distillation - Google Patents

Abstract

Fractional distillation apparatus comprises a fractional distillation column 2, and a first reboiler 4 having vertical boiling passages located at an intermediate region of the column 2. Each boiling passage of the reboiler 4 has (a) an inlet communicating with a header 32 which in turn communicates via conduit 30 with a first set 10 of liquid-vapour mass exchange trays and (b) an outlet at its bottom for a mixture of vapour and residual liquid above a collector-cum-vapour disengagement means 36 comprising a multitude of inclined vanes 38 each having an upturned end portion defining a channel 42. Each channel 42 communicates with an annular trough 44 for collecting the liquid and feeding it to trays below. <IMAGE>

Description

FRACTIONAL DISTILLATION This invention relates to a method and apparatus for fractional distillation. It is particularly concerned with the operation of distillation column in which liquid is reboiled in a region of the column intermediate sets of liquid-vapour mass transfer elements.

Fractional distillation is conventionally conducted substantially adiabatically: all the requirements of the distillation for the addition of heat are met by adding the necessary heat to the region of the column where the highest temperature obtains, and all the requirements of the column for the removal of heat are met by extracting heat for region of the column where the lowest temperature obtains.

Thus, a distillation column is typically provided at its bottom with a boiler and at its top with a condenser or other source of (liquid) reflux. It has been realised that in theory more thermodynamically efficient distillation can be achieved by providing heat to the column at one or more levels intermediate the top and bottom.

Practical realisation of this theory is particularly advantageous if the separation is energy intensive, for example, if the mixture to be separated is air. There are therefore a number of proposals in the air separation art to employ a reboiler in a distillation column at a level intermediate liquid-vapour mass transfer elements therein. One such example is the air separation plant illustrated in Figure 1 of US patent 4464 188.

Conventionally, a reboiler operates as a thermosiphon thereby requiring the liquid to be collected in a sump in which the reboiler is partially immersed. Accordingly, to use such a reboiler at an intermediate level in a distillation column causes considerable engineering problems in maintaining a suitable depth of liquid in a collection vessel and in maintaining appropriate liquid and vapour flows in the column. The solution to this engineering problem is normally to employ the intermediate reboiler in a vessel outside the column walls, to withdraw liquid at a suitable rate from a suitable level of the column to the vessel, and to return the resulting vapour to the column at an appropriate position. See for example EP-A-O 218 467. Such a solution can however be criticised on the ground that it lacks elegance, requiring as it does an additional vessel and connections to be made linking the vessel to the distillation column.

An alternative kind of reboiler to the thermosiphon one, in which alternative a falling film of liquid is boiled, has been proposed in for example US-A-4 599 097 and EP-A-O 469 780. Engineering problems analogous to those discussed above would arise if a falling film reboiler were used at an intermediate level of a distillation column, that is at a level above and below which there are liquid-vapour mass exchange elements. Indeed, there have been no proposals in the art to use a falling film boiler at an intermediate level in a distillation column. The invention for the first time enables such use to be made of a falling film reboiler.

According to the present invention there is provided fractional distillation apparatus comprising a distillation column, at least one first reboiler, within the distillation column, having generally vertical boiling passages, at least one first set of liquid-vapour mass exchange elements above the said first reboiler, at least one second set of liquid-vapour mass exchange elements below the said first reboiler, characterised in that each boiling passage has (a) an inlet for liquid at its top communicating with a first liquid distributor which in turn communicates with said first set of liquid-vapour mass transfer elements and (b) an outlet at its bottom for a mixture of vapour and residual liquid above a liquid collector-cum-vapour disengagement means comprising a plurality of open channels communicating with a second liquid distributor for distributing liquid to the said second set of liquid-vapour mass exchange elements.

The invention also provides a fractional distillation method including the steps of contacting a liquid and vapour in a distillation column on at least one first set of liquid-vapour mass exchange elements, feeding liquid from the said first set of mass exchange elements to at least one first reboiler, within the distillation column, having generally vertical boiling passages, heating the boiling passages so as to cause a part of the liquid fed to the said first reboiler to vaporize, passing residual liquid to at least one second set of liquid-vapour mass exchange elements within the distillation column and contacting the liquid with vapour thereupon, characterised in that liquid being boiled flows downwardly through the boiling passages, the liquid phase of a resultant liquid-vapour mixture issuing from the bottom of the boiling passages is collected in a collector-cum-vapour disengagement means comprising a plurality of open channels, and the collected liquid flows along the channels in a non-turbulent manner thereby permitting engaged vapour to escape therefrom.

Preferably, the collector-cum-disengagement means comprises a plurality of horizontally spaced vanes each having an upturned lower edge so as to define the channels. Preferably, the bottom of each boiling passage is aligned with a channel so that liquid issuing from the boiling passages is able to fall under gravity and to be collected in the channels.

In a typical arrangement, there are a plurality of first reboilers arranged side by side within the distillation column.

A method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram illustrating a rectification column for the separation of air in accordance with the invention; Figure 2 shows schematically on a larger scale and in more detail a part of the rectification column shown in Figure 1; Figure 3 is a schematic perspective view of part of a liquid collector associated with the intermediate reboiler shown in Figures 1 and 2; Figure 4 is a schematic perspective view of part of a heat exchanger forming part of intermediate reboiler shown in Figures 1 and 2; Figure 5 is a schematic, partial, exploded perspective view, partly cut away, of the heat exchanger shown in Figure 4; Figure 6 is a schematic cross-section taken through a condensing passage of the heat exchanger shown in Figures 4 and 5.

The drawings are not to scale.

Referring to Figure 1 of the drawings, there is shown a rectification or fractional distillation column 2 for separating air. The rectification column 2 is provided with an internal first or intermediate reboiler 4 at an intermediate level thereof and a second or main reboiler 6 located in a sump 8 at the bottom of the column 2. A first set 10 of liquid-vapour mass exchange trays 14 is located in the rectification column 2 above the intermediate reboiler 4; a second set 12 of such trays 14 is located below the first reboiler 4 but above the second reboiler 6. As is well known in the art, liquid is able to descend the column 2 from tray to tray through downcomers 16. The trays 14 are provided with apertures (not shown) so that ascending vapour can enter into mass exchange relationship with liquid on the trays 14.

The rectification column 2 has an inlet 20 at its top for liquid nitrogen reflux and an intermediate inlet 22 above the level of the reboiler 4 for oxygen-enriched liquid air.

The liquid nitrogen and oxygen-enriched liquid air may be formed by separating air in another column (not shown) operating at a higher pressure than the column 2.

Liquid flowing down the column 2 from tray 14 to tray 14 of the first set 10 enters the reboiler 4. As will be described below, the reboiler 4 is of the falling film kind and boils a part of the liquid entering it at its top. A mixture of resulting vapour and unboiled liquid passes out of the bottom of the boiling passages the intermediate reboiler 4. The vapour ascends the column 2 coming into mass exchange relationship with the descending liquid while the unboiled liquid is conducted to the second set 12 of trays 14. This liquid flows down the column from tray 14 to tray 14 of the second set and comes into mass exchange relationship with ascending vapour. Liquid flows from the bottom of the set 12 of trays 14 into the sump 8. The reboiler 6 is of the thermosiphon kind, being partially immersed in the volume of liquid that collects in the sump 8. The reboilers 4 and 6 each comprise boiling passages (not shown in Figure 1) arranged alternately with condensing passages (also not shown in Figure 1) through which a heating fluid is passed. Operation of the reboilers 4 and 6 creates the necessary upward flow of vapour through the column for the mass exchange to take place between rising vapour and descending liquid. Typically, a nitrogen product is withdrawn from the top of the column 2 through an outlet 24 and an oxygen product from the bottom of the column 2 through an outlet 26.

The arrangement for passing liquid to the top of the intermediate reboiler 4 and collecting a mixture of resulting vapour and residual liquid from the bottom thereof is shown in Figure 2. A downcomer 16 communicates with a conduit 30 terminating in a header 32 for introducing the liquid into the boiling passages of a heat exchanger 34 forming part of the intermediate reboiler 4. Liquid and vapour fall from the boiling passages under gravity and the liquid is collected in a collector 36. The collector 36 includes a multiplicity of inclined vanes 38 having upturned end portions 40 (as shown in Figure 3). Each upturned lower edge portion 40 defines a channel 42 with the main part of the vane 38 with which it is associated.

Each channel 42 communicates with an annular trough 44 having a bottom outlet 46 which in turn communicates with a conduit 48 leading to the next downcomer 16 serving the second set 12 of trays 14.

The collector 36 is spaced from the bottom of the heat exchanger 34 so as to permit vapour and liquid issuing therefrom to disengage by virtue of their different densities. Nonetheless, some vapour will remain entrained in the liquid entering the channels 42. The channels are disposed such that the bottom of each boiling passage (not shown in Figures 2 and 3) of the heat exchanger 34 is axially aligned with a respective channel 42. The channels 42 conduct liquid to the annular trough 44 and thence to the conduit 48 and the downcomer 16 associated therewith. Since the channels 42 are relatively narrow, liquid flow therealong is essentially laminar, that is to say non-turbulent. Accordingly, vapour engaged in the liquid is able to ascend to the surface of the liquid in the channels 42 and disengage therefrom. Avoiding turbulence is desirable since such turbulence would tend to hold bubbles of vapour within the liquid. If such bubbles of vapour were to be carried down to the next downcomer 16, the efficiency of the separation in the rectification column 2 would be reduced.

The heat exchanger also has vertical passages for heating fluid (not shown in Figure 2) arranged alternately with the boiling passage. The heating fluid flows from top to bottom through its passages.

The construction of the heat exchanger 34 is shown in more detail in Figures 4 to 6 of the drawings. Referring to Figures 4 and 5 of the drawings, the main body of the heat exchanger 34 comprises a set of alternate condensing and boiling passages. The passages are defined by a multiplicity of spaced apart parallel plates 118. The heat exchanger 34 also has at its top the header 32 for liquid to be boiled. The header 32 has an inlet 122 which communicates with the conduit 30 shown in Figure 2. In Figures 4 and 5, the condensing passages are indicated by reference numeral 136 and the boiling passages by the reference numeral 138.

The top of each condensing passage 136 communicates with a header 114 for gas and the bottom of each such passage 136 communicates with a header 128 for conducting away resulting condensate. The top of each boiling passage 138 communicates with the liquid header 32 while the bottom of each such passage 138 is open so that a mixture of resulting vapour and residual liquid can pass freely out of the bottom of the heat exchanger 34.

As best shown in Figure 5, each condensing passage 136 is provided with fins of heat conductive metal. The fins are provided by a corrugated sheet of metal 140 in each passage 136. Each sheet 140 is in heat conductive relationship with and joined to the respective surfaces of the respective plates 118 defining the passage 136 in which it is situated. Typically, the joining of the sheets 140 to the plates 136 is performed by vacuum brazing, a technique well known in the art. If desired, each sheet 140 may be formed with serrations (not shown in the drawings) in it so as to promote good distribution of the fluid in each condensing passage 136.

Still referring to Figure 5, the plate surfaces defining each passage 138 preferably have a porous metal coating 142 that encourages a homogeneous distribution of a film or films of boiling liquid. The porous coating may generally be of the kind described in our European patent application EP-A-303 493. Alternatively, it may, for example, be of the kind disclosed in US Reissue Patent No 30 077. It is to be appreciated that any form of enhanced boiling surface that promotes the homogeneous distribution of liquid may be used in each boiling passage of an apparatus according to the invention. The coating 142 may be of the same or a different composition from the surface to which it is applied. Typically, the coating is of aluminium, or copper, or an alloy based on one of these metals. The coating is preferably formed by depositing a mixture of particles of the desired metal and particles of a suitable plastics material (or, instead of the mixture, particles of a performed composite of metals and plastics material) onto the respective surfaces of the plates 118 to form a coating comprising particles of plastics material embedded in the material. The resulting coating is then heated to volatilise the plastics material, thereby removing it, and leaving a porous metal coating including a multitude of irregular, interconnected cavities. The average pore size of the coating is typically in the range of 15 to 50 microns and the resulting coating typically has a velocity of from 20 to 60%. Suitable plastics materials vaporise at temperatures of at least 5000C, and typically from about 500 to 6000C without leaving a carbonaceous or other reside. Preferred plastics materials are polyesters which meet the above criterion.

A typical manufacturing procedure for preparing the coating 142 is disclosed in our European patent application EP-A-303 493.

Each porous coating 142 provides a multiplicity of nucleation sites for the boiling of the liquid. Provided the liquid to be boiled is uniformly distributed at the top of each plate 118, the capillary action of the pores has the effect of helping to maintain the relatively uniform distribution of the liquid. A means which may be employed in the heat exchanger 34 for obtaining relatively uniform distribution of the liquid at the top of the plates 118 is shown in Figure 6 of the drawings.

Figure 6 shows schematically the front of a plate 118 defining one wall of a condensing passage. Figure 6 thus, in effect, shows a condensing passage with one plate 118. At the bottom of the condensing passage is a liquid collector 144 which communicates with the header 128 and which extends from one side to the other of the plate 118. In order to promote the collection of condensate from all parts of the condensing surface, the collector 144 is provided with two sets of perforate distributor fins 146 and 148 respectively. Both the sets of fins 146 and 148 take the form of corrugated strips of metal (e.g. aluminium). In the set 146, the corrugations runs a slight angle to the horizontal, while in the set 148 the corrugations run vertically. The arrangement ensures that there is a uniform distribution of liquid in the collector and thus there is a smooth flow of liquid from the condensing passage into the outlet 130 of the header 128. The sets of fins 146 and 148 are preferably permanently joined to the respective plates 118 defining the passage 136, for example, by vacuum brazing.

Located above the collector 144 are the main heat transfer fins associated with the passage 136. As described above with reference to Figure 5, the fins are defined by a corrugated sheet 140 disposed so that the corrugations run vertically. The top of the sheet 140 terminates well below the top of each associated plate 118.

A distributor chamber 150 for vapour to be condensed in the passage 136 is located above the fins 140. The distributor chamber 150 communicates with the header 114 and extends from one side to the other of the condensing passage 136. The chamber 150 is provided with two sets 152 and 154 of perforate distributor fins. Both sets 152 and 154 of fins are typically joined to respective plates 118 and each comprises strips of corrugated metal, typically aluminium. In the set 152 of fins, the corrugations run at an angle to the horizontal with the end of each corrugation remote from the header 114 being at a lower level than the opposite end. The set 154 of distributor fins have corrugations that run downwards. In operation the sets 152 and 154 of distributor fins ensure that the gaseous feed to the condensing passages 136 is uniformly distributed. Each passage 136 is provided with sealing bars 156 along its top, side and bottom edges. The sealing bars 156 are each joined to the respective surfaces of the plates 118 for example by being vacuum brazed thereto.

Each plate 118 extends above the top of its associated condensing passage.

Above the gas distributor 150 of the passage 136 there is a liquid distributor 158 which communicates with the liquid header 32 and which is able to distribute liquid to the boiling passages 138. A distributor 158 takes a form of a chamber in which extends across the respective plates 11 8 from one side to the other and which contains a perforate strip 160 of corrugated metal (of a kind sometimes referred to in the art as 'hard way fins') whose corrugations run horizontally from one side to the other of the distributor chamber 158. In operation, the strip 160 ensures a uniform distribution of liquid in the distributor chamber 158. There is an elongate horizontal slot 162 in each plate 118 bounding the chamber 158 (only one such slot 162 is shown in Figure 6) through which liquid flows out of the chamber 158 into the adjacent boiling passages (not shown in Figure 6). In operation there is thus a uniform flow of liquid into the top of each boiling passage 138 and the formation of uniform boiling films of liquid on the porous heat transfer surfaces 142 bounding the boiling passages 138 is thereby facilitated.

Further sealing or spacer bars 156 are joined (for example, by vacuum brazing) to the facing surfaces of each pair of plates 118 defining a condensing passage 136 at the top, sides and bottom of the distributor chamber 158.

Alternative arrangements can be used to distribute fluids to and from the heat exchanger 34. See for example US patent No 3 559 722.

As shown in Figure 5, sealing bars 156 are also provided at the sides and the top of each condensing passage 136. As previously described, however, each boiling passage 138 is open at its bottom to enable a mixture of vapour and residual liquid to flow therethrough.

Referring to Figure 1, in a typical example of the operation of the rectification column 2; the fluid supplied to the condensing passages of the intermediate reboiler 4 may typically be nitrogen vapour separated in another rectification column operating at a higher pressure than the column 2. A part of the resulting condensate is used as reflux at the top of the column 2 while the remainder is returned to the higher pressure column (not shown) to provide reflux therein. The fluid supplied to condensing passages of the second reboiler 6 may in this example comprise a stream of pre-cooled and purified air. The resulting liquid air may be introduced into the higher pressure column.

As shown in Figures 1 and 2, the distillation column 2 employs distillation trays as the liquid-vapour mass exchange elements. It is within the scope of the invention to use packing elements, for example structured packing elements, instead of some or all of the trays.

Claims (8)

1. Fractional distillation apparatus comprising a distillation column, at least one first reboiler, within the distillation column, having generally vertical boiling passages, at least one first set of liquid-vapour mass exchange elements above the said first reboiler, at least one second set of liquid-vapour mass exchange elements below the said first reboiler, characterised in that each boiling passage has (a) an inlet for liquid at its top communicating with a first liquid distributor which in turn communicates with said first set of liquid-vapour mass transfer elements and (b) an outlet at its bottom for a mixture of vapour and residual liquid above a liquid collector-cum-vapour disengagement means comprising a plurality of open channels communicating with a second liquid distributor for distributing liquid to the said second set of liquid-vapour mass exchange elements.

2. Fractional distillation apparatus as claimed in claim 1, in which the collector-cum-liquid disengagement means comprises a plurality of horizontally spaced vanes each having an upturned lower edge so as to define the channels.

3. Fractional distillation apparatus as claimed in claim 1 or claim 2, in which the bottom of each boiling passage is aligned with a channel so that, in use, liquid issuing from the boiling passages is able to fall under gravity and to be collected in the channels.

4. Fractional distillation apparatus as claimed in any one of the preceding claims, in which the distillation column additionally includes a second reboiler in a bottom region thereof.

5. Fractional distillation apparatus substantially as herein described with reference to the accompanying drawings.

6. A fractional distillation method including the steps of contacting a liquid and vapour in a distillation column on at last one first set of liquid-vapour mass exchange elements, feeding liquid from the said first set of mass exchange elements to at least one first reboiler, within the distillation column, having generally vertical boiling passages so as to cause a part of the liquid fed to the said first reboiler to vaporise, passing residual liquid to at least one second set of liquid-vapour mass exchange elements within the distillation column and contacting the liquid with vapour thereupon, characterised in that liquid being boiled flows downwardly through the boiling passages, the liquid phase of a resultant liquid-vapour mixture issuing from the boiling passages is collected in a collector-cum-vapour disengagement means comprising a plurality of open channels, and the collected liquid flows along the channel sin a non-turbulent manner thereby permitting engaged vapour to disengage therefrom.

7. A fractional distillation method as claimed in claim 6, in which the liquid and vapour each comprise a mixture of oxygen and nitrogen.

8. A fractional distillation method substantially as described herein with reference to the accompanying drawings.