Title: Process for effecting mass transfer between a liquid phase and a gaseous phase
Abstract: A process for effecting mass transfer between a liquid phase and a gaseous phase in a filled-type column having an external shell which accommodates at least one filler-containing basket wherethrough the phases are caused to flow in countercurrent relationship. The process includes the step of feeding the gaseous phase to the at least one basket through a gas-permeable surface thereof which is larger than the basket cross-section, preferably in a prevailing radial flow direction.
Patent Number: 7,014,177 Issued on 03/21/2006 to Filippi
| Inventors:
|
Filippi; Ermanno (Castagnola, CH)
|
| Assignee:
|
Ammonia Casale S.A. (Lugano-Besso, CH)
|
| Appl. No.:
|
230058 |
| Filed:
|
August 29, 2002 |
| Current U.S. Class: |
261/148; 261/149; 261/94; 261/96; 96/290; 96/296 |
| Current Intern'l Class: |
B01F 3/04 (20060101) |
| Field of Search: |
261/94,96,128,136,148,149
96/290,294,296,FOR.112,FOR.123
95/211,221,227,228
|
References Cited [Referenced By]
U.S. Patent Documents
| 1085366 | Jan., 1914 | Stuart.
| |
| 1798307 | Mar., 1931 | Cooper.
| |
| 2332224 | Oct., 1943 | Heath et al.
| |
| 2475855 | Jul., 1949 | Peters.
| |
| 3445343 | May., 1969 | Popov.
| |
| 3759496 | Sep., 1973 | Teller.
| |
| 4177044 | Dec., 1979 | Jernqvist.
| |
| 4256674 | Mar., 1981 | Shafranovsky et al.
| |
| 4307063 | Dec., 1981 | O'Brien.
| |
| 6183541 | Feb., 2001 | Compton.
| |
| 6221133 | Apr., 2001 | Moore.
| |
| Foreign Patent Documents |
| 1 243 143 | Jun., 1967 | DE.
| |
| 0 386 692 | Sep., 1990 | EP.
| |
| 2 056 163 | Mar., 1996 | RU.
| |
Primary Examiner: Bushey; Scott
Attorney, Agent or Firm: Sughrue Mion, PLLC
Parent Case Text
This is a divisional of application Ser. No. 09/645,313 filed Aug. 25, 2000
now U.S. Pat. No. 6,467,758, which is a Continuation Application of PCT Application
No. PCT/EP99/01275 filed Feb. 23, 1999; the above noted prior applications are
all hereby incorporated by reference.
Claims
What is claimed is:
1. Filled-type column for mass transfer between a liquid phase and a gaseous
phase, comprising:
an external shell (
2), substantially cylindrical in shape;
a plurality of annular baskets (
3a-
3e) for containing
the filler (
4) which are disposed coaxially within said shell (
2)
and includes opposite respectively inner and outer gas-permeable cylindrical walls
(
9,
10), each basket (
3a-
3e) being through-penetrated
by said liquid phase in a substantially axial flow direction;
a first free space (
11) defined between an inner wall (
2a)
of the shell (
2) and said outer wall (
10) of the basket (
3a-
3e);
a second free space (
12) defined inwardly of said inner wall (
9)
of the basket (
3a-
3e);
respective means (
5,
6) for feeding said liquid phase and said gaseous
phase into said column;
means (
9a,
10a,
14,
15) for causing
at least a major portion of said gaseous phase to flow through each basket (
3a-
3e),
from said first free space (
11) to said second free space (
12), or
the other way round, wherein a portion (
9a,
10a) of
said basket outlet wall (
10) and/or said basket inner wall (
9), at
the top end of each basket (
3a-
3e), is gas-impermeable;
respective means (
7,
8) for extracting said liquid phase and said
gaseous phase from said column.
2. Column according to claim 1, characterized in that each basket is divided
into a plurality of contiguously superimposed zones (
3a-
3e),
said means (
9a,
10a,
14,
15) for causing
at least a major portion of said gaseous phase to flow through the basket (
3)
from said first free space (
11) to said second free space (
12), or
the other way round, being arranged at each of said zones (
3a-
3e).
3. Column according to claim 1, characterized in that said gas-impermeable portion
(
9a,
10a) is of different length for the basket outer
(
10) and inner (
9) walls.
4. Column according to claim 1, characterized in that said gas-impermeable portion
(
9a,
10a) stretches over 5% to 30% of the length of
said outer (
10) and/or inner (
9) walls of the basket (
3a-
3e).
5. Column according to claim 1, characterized in that each basket (
3a-
3e),
comprises gas-impermeable closure means (
14,
15) for said free spaces
(
11,
12) disposed at said first and said second free space (
11,
12).
6. Column according to claim 5, characterized in that said closure means (
14,
15)
comprise an annular gas-impermeable baffle (
14) disposed at a bottom end
of said first free space (
11), and a circular gas-impermeable baffle (
15)
disposed at a top end of said second free space (
12), or an annular gas-impermeable
baffle (
14) disposed at a top end of said first free space (
11),
and a circular gas-impermeable baffle (
15) disposed at a bottom end of said
second free space (
12).
7. Column according to claim 1, characterized in that said plurality of annular
baskets (
3a-
3e) for containing the filler (
4)
are superimposed and disposed coaxially within said shell (
2), said baskets
(
3a-
3e) being contiguous with one another and/or different
in length.
8. Column according to claim 2, characterized in that it comprises at least one
heat exchanger (
16) arranged in said second free space (
12) to provide
an indirect heat exchange with at least some of the gaseous phase flowing from
one zone (
3a-
3d) to the next (
3b-
3e)
of said at least one basket (
3), respectively from one basket (
3a-
3d)
to the next (
3b-
3e).
9. Column according to claim 1, characterized in that each basket (
3a-
3e)
is open-ended at the top.
10. Column according to claim 1, characterized in that said baskets (
3a-
3c)
have a perforated bottom (
13).
11. Column according to claim 10, characterized in that said perforated bottom
(
13) is suitably sized to deflect the gaseous phase in radial direction,
respectively crosswise direction, and/or to collect and re-distribute the liquid
phase between successive baskets (
3a-
3c).
Description
FIELD OF THE INVENTION
This invention relates to a process for effecting mass transfer between a liquid
phase and a gaseous phase in a filled-type column wherethrough said phases are
caused to flow in a countercurrent relationship.
In the description given below and in the following claims, the term: "filled-type
column", is understood to mean—in general—an apparatus having, disposed
on its interior, a plurality of variously shaped and sized elements (filler) at
whose surfaces a liquid phase and a gaseous phase are caused to contact each other
in order to effect mass transfer; apparatus of this type are widely employed in
chemical plants, e.g. for decomposing, absorbing, distilling, and scrubbing chemicals.
The invention also relates to a filled-type column for implementing the above
process, and to a method of retrofitting a pre-existing column for conversion into
a filled-type column according to the invention.
In the description given below and in the following claims, the term: "retrofitting",
is understood to mean the in-situ modification of a pre-existing column of the
filled or plates type to improve its performance and, for example, to increase
its capacity and/or its efficiency in mass transfer between the liquid phase and
the gas phase, as well as to reduce its energy consumption.
As is known, in the field there is increasingly felt the need of providing processes
easy to implement, which can effect mass transfer between a liquid phase and a
gaseous phase in a simple and efficient way, at low investment and operating costs
and with a low energy consumption.
PRIOR ART
In order to meet the above requirement, processes for effecting mass transfer
have been proposed in the field wherein a liquid phase and a gaseous phase are
caused to flow through a filled-type column in a downward and an upward (substantially
axial) direction, respectively.
While being simple to implement, such prior processes are beset with problems
arising mainly from the large pressure drop experienced by the gas phase in going
through the filler.
The filler is, in fact, disposed inside substantially cylindrical columns having
a large height-to-diameter ratio intended to ensure an interphase contact of sufficient
duration for the phases to enhance the mass transfer.
Consequently, in flowing through the filler, the gas phase undergoes
a significant reduction in pressure (pressure drop), which places limitations on
the amount of gas that can be fed into the column, thus curtailing the column capacity.
By reason of this pressure drop, the flow rate of the gas phase fed into the
column
must be held below predetermined values, the overtaking of which would result in
the appearance of an undesirable "flooding" phenomenon, whereby the column becomes
flooded with liquid phase which is held back in its downward movement by frictional
drag from the gas phase. It will be appreciated that in this condition the column
would no longer be operable, due to the mass transfer dropping to virtually nil.
In other words, the large pressure drop of the gaseous phase in going through
the filler is a limiting factor of the gas phase flow rate through the column designed
to implement the above prior art processes, and hinders an effective mass transfer
between the phases.
To counteract the pressure drop in the gaseous phase, and thus to increase the
capacity of filled-type columns according to the prior art, some special fillers
having a high void ratio have been proposed, wherethrough the gaseous phase stream
undergoes no large pressure drops.
While such fillers allow the flow rate of the gas phase being fed into the
column to be increased somewhat, they have been unsuccessful in providing a fully
satisfactory mass transfer between the phases, because the pressure drop of the
gas phase through the column is still quite significant, as are the operational
constraints due to the aforementioned flooding phenomenon.
Columns of the so-called plates type, i.e. provided with a plurality of perforated
plates fitted horizontally inside the column, have been proposed alternatively
to the filled-type columns.
In this case, the process for effecting mass transfer between the liquid and
gaseous
phases comprises mixing the phases together in the plates, the plates being typically
designed to enhance the mutual contact of the phases.
Plates-type columns have been found suitable for low liquid flow-rate
applications, but are liable to become flooded, disallowing any further effective
mass transfer between the liquid and gas phases.
It is on account of the above disadvantages that prior art processes for effecting
mass transfer between a liquid phase and a gaseous phase have provided unsatisfactory
performance heretofore, both in terms of overall efficiency of the transfer between
the phases, energy consumption, and operating and investment costs of the columns
used to implement such processes (which are, as said before, beset with several limitations).
All this, despite this technology having been utilized in a variety of chemical
applications for years and the aforementioned need being increasingly felt in the field.
SUMMARY OF THE INVENTION
The problem underlying the present invention is that of providing a process for
effecting mass transfer between a liquid phase and a gaseous phase, which process
affords a highly efficient transfer between the phases in a simple and effective
way, at low investment and operating costs and with low energy consumption.
This problem is solved, according to the invention, by a process as indicated
above for effecting mass transfer between a liquid phase and a gaseous phase within
a filled-type column which comprises an external shell accommodating at least one
filler-containing basket wherethrough said phases are caused to flow in countercurrent,
which process is characterized in that it comprises the step of feeding said gaseous
phase to said at least one basket through a gas-permeable basket surface which
is larger than the basket cross-section.
In this way, by causing the gaseous phase to flow through a permeation surface
of said at least one basket which is advantageously made larger than the basket
cross-section, a corresponding reduction is obtained in the pressure drop of said
phase flowing through the filler, which allows the gas flow rate to be increased
and—at the same time—to operate at slower velocities than prior art
processes, thereby significantly enhancing the mass transfer between the phases.
The benefits in terms of improved efficiency of mass transfer are the more substantial,
the larger the permeation surface presented to the gaseous phase.
In this respect, the above problem is solved, in a particular effective way,
by
a process for effecting mass transfer between a liquid phase and a gaseous phase
within a filled-type column which comprises an external shell accommodating at
least one filler-containing basket whose cross-section is smaller than the cross-section
of said shell, said process comprising the steps of:
- feeding said liquid phase and gaseous phase into said filled-type column;
- causing the liquid phase to flow through said at least one basket in
a substantially axial direction;
- causing the gaseous phase to flow through said at least one basket in
a prevailing radial direction;
- extracting said liquid phase and gaseous phase from said filled-type column.
With the process of this invention, and particularly by causing the gaseous
phase to flow through the filler in a prevailing radial direction—rather
than in an axial direction as taught by prior art processes—the permeation
or passage surface can be drastically expanded to positively enhance in a simple
and effective way the mass transfer between the phases and, hence, to significantly
increase the capacity of the column designed to implement such a process over a
column of comparable size operating according the aforementioned prior art processes.
Stated otherwise, for a given capacity, the column implementing the process
of this invention can by made substantially smaller than a prior art column.
This arises mainly from that the pressure drop undergone by the gas phase in
flowing radially through the filler can be considered trivial compared to the pressure
drop involved in going through the filler in the axial direction, so that the process
of this invention can advantageously be carried out at higher gas flow rates than
in the prior art, before such undesirable phenomena as flooding occur, thereby
enhancing the mass transfer between the liquid and the gaseous phase.
In particular, the process of this invention can virtually eliminate—in
an extremely simple and effective way—the constraints of pressure drop and
limited flow-rate of the gas phase through the column, allowing to operate at desired
volumes and velocities of the gas and liquid flows for optimum utilization of the
exchange surface area of the filler for effecting mass transfer.
Advantageously, thanks to this process, a highly efficient mass transfer
between the phases can now be achieved using a filled-type column which performs
most efficiently, and involves low investment and operating costs and at low energy consumption.
The fact that the mass transfer between the phases can be significantly improved
by flowing the gas phase in a prevailing radial direction through the filler, is
in sharp contradiction with the constant prior art teaching that a crossflow of
the gaseous phase relative to the liquid phase is less advantageous, from the standpoint
of the mass transfer, than an axial flow of the phases in true countercurrent relationship.
In addition, since the conditions and the chemical/physical processes which are
responsible for the mass transfer between the phases can vary substantially with
the flow type, the benefits of the process according to the present invention were
not at all predictable a priori.
It was only the research work carried out by the Applicant that unexpectedly
brought
out how, by having the gaseous phase flow in a prevailing radial direction, in
countercurrent to the substantially axial flow of the liquid phase, the driving
force responsible for the mass transfer between the phases could act much more
efficiently than in a true countercurrent flow, and thus the effectiveness and
intensity of the mass transfer could be advantageously augmented.
Particularly satisfactory results have been obtained by causing the
gas phase to flow through said at least one basket in substantially radial, substantially
axial-radial, or substantially crosswise directions.
Preferably, the prevailing radial flow of the gaseous phase through said
at least one basket is of an either centrifugal, or centripetal, or alternating
centrifugal and centripetal type.
In a particularly advantageous embodiment of the invention, the liquid phase
and
the gas phase are caused to flow in said at least one basket through a plurality
of contiguously superimposed zones, with the liquid phase and the gas phase being
fed to an upper zone and a lower zone, respectively, of said at least one basket,
and extracted from a lower zone and an upper zone thereof, respectively.
By dividing the filler into a plurality of zones to be traversed by the gas phase
in a prevailing radial direction, it has been obtained an increase in the radial
component of the gas flow, with the consequence that it is possible to further
augment the flow rate of the gas phase and thus improve the mass transfer between
the phases and with them, the capacity of the column used to implement this process.
Alternatively, the same result can be obtained by causing the liquid
phase and gas phase to flow in a substantially axial, respectively prevailing radial
direction through a plurality of superimposed filler-containing baskets.
Advantageously, according to the last-mentioned embodiment, the process
according to the invention further comprises the step of:
- collecting and re-distributing said liquid phase between successive baskets.
In this way, it is possible to maintain the whole filler constantly swept by
the
liquid phase, thus avoiding the latter from flowing down along preferential paths
which swept only local zones of the filler, resulting in a much reduced effective
surface area being available for the mass transfer.
Preferably, the gas phase is caused to flow radially through contiguous
zones of said at least one basket or through successive baskets, in opposite directions,
thereby to provide a zigzag flowpath for the gas phase within the column, which
proves advantageous from the constructional standpoint.
For implementing the above process, the invention advantageously provides a filled-type
column for effecting mass transfer between a liquid phase and a gaseous phase,
which comprises:
- an external shell;
- at least one basket for containing the filler extending inside said
shell, said at least one basket being through-penetrated by said phases in countercurrent relationship;
- respective means for feeding said liquid phase and said gaseous phase
into said column;
- respective means for extracting said liquid phase and said gaseous phase
from said column; the column being characterized in that said at least one basket
is provided with a gas-permeable surface, for passing the gaseous phase, which
is larger than its cross-section.
Preferably, the column according to the invention is characterized in
that said at least one basket has a smaller cross-section than the shell cross-section
and has opposite sidewalls which are gas-permeable, and in that it further comprises
means for causing said gaseous phase to flow through said at least one basket in
a prevailing radial flow.
According to a preferred embodiment, the column advantageously comprises:
- an external shell, substantially cylindrical in shape;
- at least one annular basket for containing the filler which is disposed
coaxially within said shell and includes opposite respectively inner and outer
gas-permeable cylindrical walls, said at least one basket being through-penetrated
by said liquid phase in a substantially axial flow direction;
- a first free space defined between an inner wall of the shell and said
outer wall of the basket;
- a second free space defined inwardly of said inner wall of the basket;
- respective means for feeding said liquid phase and said gaseous phase
into said column;
- means for causing at least a major portion of said gaseous phase to
flow through said at least one basket, from said first free space to said second
free space, or the other way round;
- respective means for extracting said liquid phase and said gaseous phase
from said column.
According to a further preferred embodiment, the column advantageously comprises:
- an external shell;
- at least one basket for containing the filler, disposed inside said
shell coaxially therewith and provided with opposite sidewalls, preferably plane,
which are gas-permeable, said at least one basket being through-penetrated by said
liquid phase in a substantially axial flow direction;
- first and second free spaces, located opposite to each other and defined
between an inner wall of said shell and said sidewalls of the basket;
- respective means for feeding said liquid phase and said gaseous phase
into said column;
- means for causing at least a major portion of said gaseous phase to
flow through said at least one basket, from said first free space to said second
free space or the other way round;
- respective means for extracting said liquid phase and said gaseous phase
from said column.
According to a further aspect, the invention provides a method of retrofitting
a column for effecting mass transfer between a liquid phase and a gaseous phase,
being of either the filled or the plates type, which method is characterized in
that it comprises the step of:
- providing, inside said column, at least one basket for containing a
filler and having a gas-permeable surface, for passing the gaseous phase, which
is larger than its cross-section.
Thanks to the above method of retrofitting an existing column, a process for
effecting mass transfer between a liquid phase and a gaseous phase can be obtained
which allows to provide a high rate of transfer between the phases in a simple
and effective manner, at low investment and operating costs, and with low energy consumption.
Further features and advantages of this invention will become apparent from
the following description of an embodiment of the process according to the invention,
given by way of non-limitative example with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a longitudinal section view of a first embodiment of a filled-type
column, as constructed either from an original design or by modifying a traditional
column, to implement the process according to the invention;
FIG. 2 is a longitudinal section view of the column shown in FIG. 1, taken along
line II—II in FIG. 1;
FIG. 3 is a longitudinal section view of a second embodiment of a filled-type
column, as constructed either from an original design or by modifying a traditional
column, to implement the process according to the invention;
FIG. 4 is a longitudinal section view of the column shown in FIG. 3, taken along
line IV—IV in FIG. 3;
FIG. 5 is a longitudinal section view of a third embodiment of a filled-type
column, as constructed either from an original design or by modifying a traditional
column, to implement the process according to the invention;
FIG. 6 is a longitudinal section view of the column shown in FIG. 5, taken along
line VI—VI in FIG. 5.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Referring to FIGS. 1-6, generally shown at
1 is a filled-type column
for effecting mass transfer between a liquid phase and a gaseous phase, according
to the invention.
The column
1 comprises an external shell
2, at least one basket
3 extended inside the shell
2 to receive a filler
4, respective
means
5 and
6 for feeding a liquid phase and a gaseous phase into
the column
1, and respective means
7 and
8 for extracting
the liquid phase and gaseous phase from the column
1.
Means
5 and
7 for feeding, respectively extracting, the liquid
phase to/from the column
1 generally comprise conduits, nozzle-type dispensers
or collecting chambers of known types to the person skilled in the art, and accordingly,
no further discussed in detail herein.
Similarly, means
6 and
8 for feeding, respectively extracting,
the gaseous phase to/from the column
1 generally comprise gas inlets and
outlets of known types to the person skilled in the art, and accordingly, no further
discussed in detail herein.
In the examples of FIGS. 1,
3 and
5, the liquid phase is fed by
the means
5 to a top end of the column
1, and is extracted by the
means
7 from a bottom end thereof. Thus, the liquid phase will flow through
the column
1 in a manner known per se, i.e. in a substantially axial direction.
The gaseous phase is fed by the means
6 to a bottom end of the column
1 and extracted from a top end thereof by the means
8.
The liquid phase and gaseous phase could, however, be fed in and extracted at
intermediate locations on the column
1, as is usually done in traditional
filled-type columns.
In other words, means for feeding, respectively extracting, the liquid phase
and/or
the gaseous phase may be provided anywhere along the shell
2, in order to
maintain constant operational conditions within the column or to improve the mass
transfer between the phases.
According to the present invention, said at least one basket
3 is
advantageously provided with a gas-permeable surface for the passage of the gas
phase through the filler
4, which is larger than its cross-section.
For the purpose, the basket
3 is given a smaller cross-section than the
cross-section of the shell
2, and has opposite sidewalls
9 and
10
which are gas-permeable.
In addition, the column
1 advantageously includes appropriate means, to
be described, adapted to cause the gas phase to flow through the basket
3
in a prevailing radial direction.
In this way, the gas phase is caused to flow through the column
1—contrary
to the constant teaching of the prior art—in countercurrent to the liquid
phase along a prevailing radial flow direction.
The gas-permeable surface of the sidewalls
9 and
10 is advantageously
provided larger than the cross-section of the basket
3.
According to the embodiment shown in FIG. 1, the filled-type column
1
of this invention comprises an external shell
2 which is substantially cylindrical
in shape, and at least one annular basket
3a-
3e for
containing the filler
4 which is mounted inside the shell
2 coaxially
therewith and has opposite gas-permeable cylindrical inner and outer walls,
9
and
10 respectively.
A first free space
11 is formed between an inner wall
2a of
the shell and the outer wall
10 of the basket, and a second free space
12
is formed within the inner wall
9 of the basket.
Advantageously, the column
1 further comprises means
9a,
10a, 14,
15, for causing at least a major portion
of the gas phase to flow through said at least one basket
3a-
3e,
from the first free space
11 to the second free space
12 or the
other way round.
In the example of FIG. 1, the column
1 contains five annular baskets
3a-
3e
for containing the filler
4, which are superimposed and disposed coaxially
within the shell
2.
These baskets are flown by the liquid phase from above downward of the column
1 with substantially axial direction, and by the gas phase from beneath
upward with substantially axial-radial direction, as indicated by arrows Fg.
The number of baskets
3 inside the shell
2 may vary to suit the
size of the column
1 and the rate of mass transfer sought. A number of baskets
in the one to twenty range should prove adequate to meet most requirements.
To promote the mass transfer between the phases and ensure optimum operating
conditions
throughout the column
1, the baskets
3a-
3e for
containing the filler
4 may advantageously be provided in different lengths,
e.g. with the lowermost basket
3a being the longest and the other
baskets decreasing gradually in length to the uppermost basket
3e which
would be the shortest.
The baskets
3a-
3e shown in FIG. 1 are laid contiguously,
and have each a perforated bottom
13 which can be permeated by both the
liquid phase and the gaseous phase.
Advantageously, the perforated bottoms
13 of the baskets
3a-
3e
are suitably sized to deflect the gas phase radially and/or to collect and
re-distribute the liquid phase between the baskets.
The presence of the perforated bottoms
13 between successive contiguous
baskets
3a-
3e produces a controlled increase of predetermined
entity in pressure drop of the gas phase flowing through the basket which allows
at least some of that phase to be deflected radially, and the outflowing liquid
phase from one basket to be optionally collected and re-distributed to the next basket.
By having the liquid phase collected and re-distributed between successive baskets
3a-
3e, optimum conditions can be ensured for the mass
transfer between the phases, with the entire surface area of the filler
4
being utilized and preventing the formation of preferential flowpaths for the liquid
phase going through the filler.
The portion of the gas phase which has been deflected radially out of one basket
3a-
3d is then re-introduced into the next basket
3b-
3e,
again in radial flow direction.
Preferably, the radial flow of the gas phase through the baskets
3a-
3e
is either of centrifugal or centripetal type.
Particularly satisfactory results have been obtained using an alternating
centripetal and centrifugal flow (or vice versa), as indicated by the arrows Fg
in FIG. 1. That is, the gaseous phase goes radially through successive baskets
3a-
3e in opposite directions.
The baskets
3a-
3e, or just some of them, may be disposed
inside the column
1 at suitable distances from one another to define respective
cylindrical spaces between successive baskets.
These spaces may advantageously accommodate means (not shown) of collecting
and re-distributing the liquid phase from one basket to the next, which means are
conventional and no further described herein.
Alternatively to having a plurality of baskets, the column
1
according to the present invention could advantageously have a single basket
3
divided into a plurality of zones (indicated in FIG. 1 by the same references
3a-
3e
as the baskets), contiguous one above the other. This arrangement is to be
regarded as substantially equivalent to the basket arrangement shown in FIG. 1.
According to this embodiment, no perforated bottoms
13 are provided,
and the filler
4 inside the basket
3 forms a continuous single body.
According to a particularly advantageous aspect of the invention, the outer
wall
10, or the inner wall
9, of at least one basket
3a-
3e
comprises a portion
10a, respectively
9a, which
is gas-impermeable and of a predetermined length along a top end thereof.
In the example of FIG. 1, both the outer wall
10 and the inner wall
9
of each basket
3a-
3c advantageously comprise the gas-impermeable
portion
10a and
9a, respectively.
Where, instead of the baskets
3a-
3c, the column
1 comprises a single basket
3 divided into a plurality of zones
3a-
3c
as described above, the gas-impermeable portions
9a and
10a
of predetermined length of the inner wall
9 and/or of the outer wall
10, respectively, of the basket
3 would be defined along a top end
of each zone.
By providing a gas-impermeable top portion on at least one of the, preferably
both, sidewalls of the baskets or zones
3a-
3c, a major
portion of the gas phase stream flowing from bottom to top through the filler
4
can be deflected radially in a simple and effective manner, allowing the gas phase
to flow through the column
1 in a prevailing radial direction.
In fact, the unperforated portion of the sidewalls
9 and/or
10
produces
a controlled increase (of predetermined entity) in the pressure drop of the gas
phase which advantageously impedes the axial flow through the filler
4,
thereby deflecting the gas phase stream radially.
The proportion of the gas phase that goes radially, respectively axially, through
the filler
4 is dictated by the lengths or extents of the gas-impermeable
portions
9a and/or
10a, because the pressure drop undergone
by the gas phase would vary correspondingly with such portions. By increasing the
length of the gas-impermeable portion, the resistance to the axial flow and, hence,
the radial flow component of the gas phase is increased.
As shown in FIG. 1, it is only a minor portion of the gas phase that goes through
the filler
4 in a substantially axial flow, while the major portion thereof
follows substantially radial flowpaths, resulting in a mixed flow being established,
i.e. axial-radial.
Where both sidewalls
9 and
10 of the basket(s)
3a-
3e
include gas-impermeable portions
9a and
9b, the
length of each portion may advantageously be different for the inner wall
9
and the outer wall
10, so as to augment or diminish the radial component
of the gas phase flow.
Particularly satisfactory results have been obtained with the gas-impermeable
portion
9a, 10a being 5% to 30% of the length of the
inner wall
9 and/or outer wall
10 of the basket
3a-
3e.
Preferably, the length of the gas-impermeable portion will be in the
range of 10% to 25%.
According to a further particularly advantageous aspect of this invention,
the column
1 includes gas-impermeable means
14 and
15 for
closing the free spaces
11 and
12 arranged at the first and at the
second free space
11 and
12 respectively.
According to the example of FIG. 1, in at least one basket
3a-
3e,
these closure means advantageously comprise an annular baffle
14 which
is gas-impermeable and disposed at a top end of the first free space
11,
and a circular baffle
15 which is gas-impermeable and disposed at a bottom
end of the second free space
12.
In this way, the gaseous phase can be caused to pass from one basket
3a-
3d
to the next basket
3b-
3e in an alternating centripetal
and centrifugal flow of prevailing radial direction, while preventing the gas phase
from bypassing one or more of the baskets, which would adversely affect the overall
efficiency of the mass transfer taking place between the phases.
In fact, when at least a major portion of the gas phase is caused to flow radially
through the filler
4, it matters that the gas exiting a given basket be
directed to the next basket in such a manner that it cannot flow out and sweep
across other baskets in the column or even escape from the column.
Where the gaseous phase is to move from one basket
3a14
3d to the next
3b-
3e in a mainly radial
flow of the alternating centrifugal and centripetal type, the gas-impermeable annular
baffle
14 would be disposed at a bottom end of the first free space
11,
and the gas-impermeable circular baffle
15 would be disposed at a top end
of the second free space
12.
In the example of FIG. 1, with the column
1 being traversed by a substantially
axial-radial flow of the gaseous phase, the baskets
3a-
3e
are left open at their top ends, rather than being closed by a gas-impermeable cover.
This greatly facilitates the axial traversing of the same by the liquid phase,
and particularly in the upper basket
3e, facilitates the feeding
and distributing of this phase on the filler
4 therein contained.
In FIG. 1, the numeral
16 generally indicates a heat exchanger disposed
in the second free space
12 in the basket
3c, for subjecting
the gaseous phase which leaves the basket
3c with radial flow and
is directed to the next basket
3d, to an indirect heat exchange with
a heating or cooling fluid, such as water.
In particular, the gaseous phase is caused to flow through the heat exchanger
16 on the shell side, with the heating or cooling fluid on the tube side.
For simplicity, the means for feeding and extracting the heating or cooling fluid
to/from the heat exchanger
16 have been omitted from FIG. 1 because known
per se.
Depending on individual requirements, one or more heat exchangers
16
providing indirect heat exchange for at least some of the gaseous phase flowing
between successive baskets
3a-
3e may be arranged in
the free space
12.
By providing a heat exchanger
16 in the column
1, the scrubbing,
absorbing distilling and decomposing processes for which the column is intended
can all be improved.
As mentioned above, all the features, except the gas permeable bottom
13,
described in relation to the baskets
3a-
3e also apply
to the particular embodiment, not shown, of this invention wherein the column
1
comprises a single basket
3 divided into a plurality of zones
3a-
3e.
FIG. 3 shows an embodiment of a column
1 for implementing the process
according to the present invention, which differs from the example of FIG. 1 mainly
in that the gaseous phase is caused to flow through the filler
4 in a substantially
radial direction, rather than axial-radial.
In this figure, the parts of the column
1 which are structurally and functionally
equivalent to those shown in FIG. 1, carry the same reference numbers and will
be not further described.
Advantageously, the gaseous phase is caused to flow in substantially
radial direction by closing the top end of the basket(s)
3 loaded with the
filler
4 disposed inside the column
1.
For the purpose, at least one basket
3 has a gas-impermeable cover
17
mounted at its top end.
In the example of FIG. 3, the baskets
3a and
3b are
each provided with a cover
17 such that not even a minor portion of the
gaseous phase will be allowed to go through the filler
4 in an axial flow direction.
Thanks to the gas-impermeable cover
17, the gaseous phase is forced
to flow radially through the filler
4, and is prevented from exiting the
baskets
3a-
3b in an axial direction.
In this case, the gas-impermeable portion
9a, respectively
10a,
of the inner and outer sidewalls
9 and
10 of the baskets
3a-
3b,
additionally to deflecting the gaseous phase flow in a radial direction, advantageously
prevents the baskets
3a-
3b from being undesirably bypassed
by at least a portion of said phase which might leak, through any gap between the
cover
17 and the filler
4, directly from the first free space to
the second,
11 respectively
12, or the other way round, affecting
the overall efficiency of the mass transfer between the phases.
Preferably, the gaseous phase will flow, as indicated by the arrows Fg
in FIG. 3, radially outwards through the first basket
3a and radially
inwards through the second basket
3b.
The radial direction of the gaseous phase may be selected to suit the column
1 construction, and may be purely centripetal or purely centrifugal, or
centripetal and centrifugal (or vice versa). Similar to the example shown in FIG.
1, the advantages resulting by selecting a particular direction for the gaseous
phase flow do concern the construction of the column
1, but are of trivial
importance to the efficiency of the mass transfer between the phases.
Furthermore, a combined process could be provided with some of the baskets
3 being traversed by the gaseous phase in substantially radial flow direction,
and some others in axial-radial direction.
In this case, not all of the baskets
3a-
3b would
be
fitted with a gas-impermeable cover
17.
The number of baskets
3a-
3b shown in FIG. 3 and their
different lengths are merely illustrative, and may vary according to necessity
as previously described in relation to FIG. 1.
It should be noted that the means
5 for feeding the liquid phase into
the
column
1 locates between the cover
17 of the upper basket
3b
and the filler
4.
According to this embodiment, the liquid phase flowing out of the upper
basket
3b is picked up—by means not shown because known per
se—and re-distributed to the lower basket
3a, for example,
by a nozzle type dispenser
18, itself disposed between a respective cover
17 and the filler
4.
Advantageously, the baskets
3a and
3b locate
adjacent to each other, with the cover
17 of the lower basket
3a
forming the gas-impermeable bottom of the upper basket
3b. A
column of compact and simple construction is thus obtained. However, a plurality
of baskets
3 spaced apart inside the column
1 could be provided instead.
Although not shown in FIG. 3, the free space
12 may advantageously
accommodate one or more heat exchangers for an indirect heat exchange between the
gaseous phase flowing from one basket to the next and a heating or cooling fluid,
as described in relation to FIG. 1.
According to a further embodiment of this invention depicted in FIG. 5,
the column
1 includes at least one basket
3a-
3c
for containing the filler
4 and being disposed coaxially within the
shell
2 and having opposite sidewalls
19,
20 which are gas-permeable
and preferably plane.
Defined between the inner wall
2a of the shell
2 and
the sidewalls
19 and
20 of the baskets
3a-
3c
are first and second free spaces
21 and
22, opposite to each other.
Advantageously, appropriate means (
19a, 20a,
23) are provided for causing at least a major portion of the gaseous
phase to flow into the column
1 through at least one basket
3a-
3c,
from the first free space
21 to the second free space, or the other
way round.
In FIG. 5, the parts of the column
1 which are structurally and functionally
equivalent to those shown in the previous figures are indicated by the same reference
numbers and will be no further described.
According to this embodiment of the invention, the liquid phase is caused
to flow through the filler
4 from above downward in a substantially axial
direction, whereas the gaseous phase advantageously flows through the filler
4
in a substantially crosswise direction from bottom to top.
In other words, the gaseous phase is moved from one side of the basket
3a-
3c
to the other in an essentially oblique flow wherein the horizontal component prevails.
Preferably, as shown in FIGS. 5 and 6, the shell
2 is substantially
cylindrical in shape, and the cross-sectional shape of the free spaces
21
and
22 is a circular sector. In this way, the space available inside the
column
1 can be best utilized, and roomy baskets
3a-
3c
for containing the filler
4, as well as sufficiently broad free spaces
21 and
22 to allow the gaseous phase to flow through without undue
frictional drag, can be provided.
Similarly as the previous examples, here again at least one of the sidewalls
19 and
20, preferably both, of the basket
3a-
3c
comprises a gas-impermeable portion
19a, respectively
20a,
of predetermined length at a top end thereof, which is effective to deflect
the gaseous phase flow horizontally.
In addition, the column
1 advantageously includes gas-impermeable means
23 for closing the free spaces
21 and
22, disposed at the
first and at the second free space
21 and
22.
For the purpose, the closure means comprise a gas-impermeable baffle
23
located at a top end of the first free space
21, and a gas-impermeable baffle
23 located at a bottom end of the second free space
22.
In an alternative embodiment, not shown, the gas-impermeable baffles
23
are located at a bottom end of the first free space
21 and at a top end
of the second free space
22, respectively.
In the example of FIG. 5, these baffles
23 have a circular-sector shape.
The baffles
23 are advantageously effective to induce a crossflow of the
gaseous phase through the filler
4 from the first free space
21 to
the second
22 and/or the other way round, while preventing the gaseous phase
from undesirably bypassing the baskets
3a-
3c.
As concerns the length of the gas-impermeable portion
19a and
20a
of the sidewalls
19 and
20, as well as the number, the arrangement
and the length of the baskets
3a-
3c inside the column
1, the same characteristics and advantages apply as have been discussed
above in relation to the previous figures.
Just as example, it is noted that in column
1 of FIG. 5 three catalytic
baskets
3a-
3c are coaxially arranged, loaded with the
filler
4, superimposed and contiguous with one another, with the top basket
3c being longer than the underlying baskets
3a-
3b.
A perforated bottom
13 is provided in each basket
3a-
3c
for passing the liquid phase and a minor portion of the gaseous phase between
successive baskets. In addition, the baskets
3a-
3c are
open ended at the top.
As a result, in the example of FIG. 5, the gaseous phase is advantageously allowed
to flow through the baskets
3a-
3c in a mixed crosswise
and axial direction.
Alternatively, in an embodiment not shown in the drawings, the baskets
3a and
3c may be fitted with respective gas-impermeable
covers at their top ends, so that a true crosswise flow of the gaseous phase can
be provided through the filler
4.
Also within the scope of this invention is a column
1 comprising some
of the baskets flown by the gaseous phase in a substantially crosswise direction
and others of the baskets flown in a mixed crosswise and axial direction.
The baskets
3a-
3c may advantageously be disposed
in a mutually spaced-apart relationship inside the column
1. Also, suitable
means may be provided between successive baskets
3a-
3c
to collect and re-distribute the liquid phase.
In the example of FIG. 5, these means comprise the perforated bottoms
13
provided at the bottom ends of the baskets
3a-
3c, which
is suitably sized to deflect the gaseous phase flow crosswise and/or to collect
and redistribute the liquid phase flowing between successive baskets.
Alternatively, the column
1 could advantageously comprise,
instead of the baskets
3a-
3c, a single basket
3
divided into a plurality of contiguously superimposed zones, not shown, which can
be likened in every respect to the above-described baskets
3a-
3c
and have the same characteristics and advantages. Of course, no perforated
bottoms
13 or gas-impermeable covers would be provided between such contiguous zones.
A column
1 could also be arranged to incorporate a plurality of baskets
3a-
3c, with at least one of the baskets being divided
into the aforesaid plurality of zones.
According to an embodiment, not shown but particularly advantageous, of
this invention, a substantially radial or crosswise flow of the gaseous phase can
be provided through a column
1 of the type described in relation to FIG.
3 or
5, by replacing the gas-impermeable covers with gas-permeable perforated
covers, and advantageously exploiting the hydrostatic liquid head that forms at
such perforated covers to stop the gaseous phase from flowing axially through the basket.
In other words, the liquid phase—which is caused to flow from above downward
through the column
1—upon leaving one basket, collects at the perforated
cover of the next basket, and before being re-distributed and going through this
next basket, forms a horizontal liquid layer that cannot be penetrated by the gaseous
phase, so that the gaseous phase will be obliged to flow in a crosswise or radial direction.
The present invention is not limited by a specific type of filler
4, and
any filler types may be used inside the column
1, including texturized or
random types. Advantageously, fillers
4 of different type may be interposed
between baskets or between zones of one basket, so that the gaseous phase flow
rate, and the mass transfer between the phases flowing through the column
1,
can be controlled or modified.
The filler
4 may be made of inert material with respect to the flowing
liquid and gaseous phases or it can be a suitable catalyst, or mixtures thereof.
In case a catalyst is used in the column
1, then chemical reactions may
take place in addition to the mass transfer between the phases.
As brought out by the embodiments described in relation to FIGS. 1-6, according
to the process of the present invention for effecting mass transfer between a liquid
phase and a gaseous phase inside a filled-type column
1 comprising an external
shell
2 which accommodates at least one basket
3 for containing the
filler
4 wherethrough such phases are caused to flow in countercurrent,
the gaseous phase is advantageously fed into said at least one basket
3
through a gas-permeable surface (
9,
10,
19,
20) thereof
which is larger than the basket cross-section.
Preferably, this process for effecting mass transfer between a liquid
phase and a gaseous phase is carried out within a filled-type column
1 comprising
an external shell
2 which accommodates at least one basket
3 for
containing the filler
4 and having a smaller cross-section than the cross-section
of the shell
2, and comprises the steps of feeding (means
5 and
6)
the liquid and gaseous phases into the filled-type column
1, causing the
liquid phase to flow through said at least one basket
3 in a substantially
axial direction, causing the gaseous phase to flow through said at least one basket
3 in a prevailing radial direction (means
9a, 10a,
14,
14,
19a, 20a, 23), and
extracting (means
7 and
8) the liquid and gaseous phases from the
filled-type column
1.
As previously described, a reduction in the pressure drop of the gaseous phase
flowing through the filler
4 can be provided by the present process, whereby
optimum flow rates and velocities can be achieved for that phase (and for the liquid
phase) effective to augment the mass transfer between the liquid phase and the
gaseous phase.
Additionally thereto, it has been found unexpectedly that by causing
the gaseous phase to flow mainly radially through the filler
4, superior
efficiency can be achieved for the mass transfer since the driving force which
is responsible for such transfer is positively acted upon.
The operating conditions (such as pressure and temperature) of the present process
may be changed within broad limits to suit the types of fluids to be processed
and any chemical operations to be carried out.
The present invention can also be advantageously applied in columns having both
plates and filler containing baskets
The column shown in FIGS. 1,
3 and
5 may be a brand new one, or
alternatively be—according to an advantageous and preferred aspect of this
invention—a pre-existing column of the filled or plates type retrofitted
for carrying out mass transfer between a liquid phase and a gaseous phase.
Advantageously, the method for retrofitting such a column would comprise
the step of providing within the same at least a basket
3 for containing
a filler
4 which has a gas-permeable surface, for passing the gaseous phase,
which is larger than its cross-section.
In particular, the retrofitting method according to this invention is characterized
in that it comprises the steps of providing, inside the pre-existing column, at
least one basket
3 for containing the filler
4 which has a smaller
cross-section than the column cross-section and has opposite gas-permeable sidewalls
9,
10,
19,
20, and of providing, inside the column,
means
9a, 10<
