Multi-bed mass transfer column with mobile packing

A packed mass transfer tower for establishing intimate gas-liquid contact and efficient mass transfer that is less susceptible to gas or liquid channeling, gas surging and uneven distribution of gas or liquid across the cross-sectional area of the tower. In one embodiment, the packed tower includes a packing arrangement including a loosely packed bed of mobile packing material, and an adjacent, tightly packed bed of mobile packing material. The relatively tightly packed bed of mobile packing material acts as a gas distributor, as well as being a material that causes good gas-liquid contact, to provide an even flow of gas across essentially the entire cross-sectional area of the loosely packed bed of mobile packing material. In another embodiment, mobile packing elements and fixed packing elements are intermixed between the same spaced fluid-pervious grids. In the second embodiment, the fixed packing elements hug the lower pervious grid to evenly distribute gas and liquid to the mobile packing elements that are aerated above the fixed packing elements during mass transfer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to packed tower or packed column structures used for 
intimate contact between a gas and a liquid to effect mass transfer 
therebetween. More particularly, this invention relates to a specific 
packing arrangement for a packed tower that includes a combination of a 
bed of mobile packing material arranged relatively loosely between fluid 
pervious spaced grids and a second bed of mobile packing material arranged 
relatively densely or tightly between more closely spaced grids adjacent 
thereto, to provide a more even distribution of gas through the loosely 
packed mobile bed and prevent the mobile packing material, liquid and gas 
streams from being unevenly distributed. The packing arrangement is 
particularly useful for gas-liquid mass transfer in relatively large 
absorption towers having a cross-sectional area more than about 3 
ft.sup.2, where gas channeling, and surging of gas and packing material is 
most prevalent. In a second embodiment, mobile packing elements and fixed 
packing elements are intermixed to form a dual bed containing a lower bed 
of fixed packing elements for even gas distribution to an upper mobile bed 
of aerated packing elements, both beds disposed between the same spaced 
grids. This invention is particularly useful in gas-liquid contacting 
processes where the packed tower or column is prone to undergo plugging 
due to the presence or formation of solid material within the tower. 
2Related Technology 
It is well known that packed towers or columns are often useful for 
efficient gas-liquid contact to provide interactions, reactions and other 
mass transfer operations between gas and liquid. However, in some such 
operations the fluid passing through the packed bed contains suspended 
solid particles that tend to accumulate on the packing, eventually 
resulting in a reduction in gas-liquid volumetric flow rates and, in 
extreme cases, plugging of the tower. 
Such plugging problems occur, for example, when a packed tower is used with 
liquids or gases that contain particulate material, or when particulates 
are formed within the tower as a result of a chemical interaction, 
reaction or the like, that precipitates solids as a result of mass 
transfer between the liquid and the gas. For example, in flue gas 
scrubbing that utilizes a liquid stream that includes an aqueous solution 
or suspension of limestone, or the like, to remove sulfur dioxide 
contained in a gas stream, a calcium sulfate precipitate is formed in the 
tower, and the resulting aqueous slurry causes plugging problems. Another 
example is a gas-liquid interphase oxidation process for removal of 
hydrogen sulfide from a gas stream that uses an oxidizing solution to 
effect oxidation of hydrogen sulfide to form elemental sulfur particles. 
Such particles may cause plugging of packed towers, requiring periodic 
shutdown of the process for a difficult and time consuming cleaning of the 
tower. 
Some prior processes have used packed towers for contact of hydrogen 
sulfide (H.sub.2 S)-containing gases with an aqueous iron-chelate solution 
or a so-called "Stretford"-type alkaline vanadium ion-containing wash 
solution that converts H.sub.2 S to elemental sulfur (S.degree.). The 
elemental sulfur formed is entrained in the liquid mixture, and removed 
from the liquid, e.g. by settling or filtration. In this type of H.sub.2 S 
removal process, some of the sulfur settles on the packing material, 
thereby lowering gas and liquid flow rates and, ultimately, the sulfur 
plugs the tower. Mobile packing beds have been used in packed towers for 
sulfur removal from H.sub.2 S-containing gas streams but suffer from 
uneven gas distribution and channeling problems, particularly in larger 
cross-sectional area towers, resulting in decreased efficiency. 
It is well known that two of the most important characteristics necessary 
for effective and efficient gas-liquid mass transfer in packed towers are 
1) the tower must contain adequate passages for both the liquid and the 
gas streams without excessive liquid holdup or pressure drop; and 2) the 
tower must provide good contact between liquid and gas. The requirement of 
good contact between liquid and gas is the most difficult to meet, 
especially in large towers. Ideally, the liquid, once distributed over the 
top of the packing, flows in thin films over all the packing surface 
completely down the tower. Actually, the films tend to grow thicker in 
some places and thinner in others, so that the liquid collects into small 
rivulets and flows along localized paths through the packing. Especially 
at low liquid rates, and in the larger towers, some of the packing surface 
may be dry or covered with a stagnant film of liquid. This effect is known 
as "channeling" and is the main reason for poor performance in large 
packed towers. Similarly, a countercurrently flowing gas stream, 
especially in larger towers, sometimes tends to follow localized paths 
instead of being distributed across the entire cross-section of the tower, 
with a consequent increase in velocity through these localized paths, and 
this effect is known as "surging". 
The use of mobile packing beds tends to decrease or eliminate packing 
material fouling adds to the "channeling" and "surging" problems inherent 
in packed towers of relatively large cross-sectional area due to the 
tendency of the relatively light mobile packing material to collect along 
the sides and other localized areas of the tower, further decreasing the 
desired cross-sectional uniformity of gas and liquid flow. This reduces 
the overall efficiency in relatively large mobile bed gas-liquid contact 
devices. This problem has been recognized for many years but, to date, no 
adequate solution to the channeling and surging problems has been 
achieved. Others have provided vertical partitions in the columns between 
grids, to provide a plurality of partitioned packing beds between adjacent 
grids, thereby decreasing the cross-sectional area of each separate 
packing bed. This partitioning has aided the gas and liquid flow 
uniformity, but not completely satisfactorily, and has created other 
problems, such as the attendant difficulty in loading and unloading 
packing material and making sure that loading is done uniformly within 
each separate compartment. 
The present invention solves this gas-liquid flow problem within mobile bed 
packed towers, without vertically partitioning the mobile beds, by 
including a relatively tightly packed, relatively short mobile bed of 
packing material directly above or below normally packed mobile beds in a 
gas-liquid mass transfer tower. In a second embodiment of this invention, 
the bed of packing material includes both 1) mobile packing elements 
capable of being aerated within the packing bed during mass transfer, and 
2) heavier, fixed packing elements that remain against a lower grid of the 
packing bed during mass transfer to evenly distribute the gas stream 
through the mobile packing elements. 
SUMMARY OF THE INVENTION 
In brief, the present invention is directed to a packed mass transfer tower 
for establishing intimate gas-liquid contact and efficient mass transfer 
that is less susceptible to gas or liquid channeling, gas surging and 
uneven distribution of gas or liquid across the cross-sectional area of 
the tower. In one embodiment, the packed tower includes a packing 
arrangement for a mobile bed mass transfer stage including a loosely 
packed bed of mobile packing material, such as hollow spheres, packed in 
the usual arrangement e.g. such that when static, the height of the mobile 
packing material is at most 75% of the height of the spacing between upper 
and lower grids that define the stage of the tower. Disposed directly 
adjacent to the loosely packed bed of mobile packing material, and sharing 
a common grid therewith, is a relatively tightly packed bed of mobile 
packing material that acts as a gas distributor, as well as being a 
material that causes good gas-liquid contact, to provide an even flow of 
gas across essentially the entire cross-sectional area of the loosely 
packed bed of mobile packing material. The mobile packing material forming 
the tightly packed mobile bed is packed tightly enough to prevent any 
substantial vertical movement of the mobile packing elements, e.g. such 
that when static, it occupies a height of no less than 90% of the height 
of the tightly packed mobile bed stage of the tower, defined as the tower 
stage height between upper and lower pervious grids that the tightly 
packed bed of mobile packing material is disposed between. 
The loose packing arrangement of at least one mobile packing bed allows the 
bed to function in its usual manner in achieving good gas-liquid contact 
and efficient mass transfer, while enabling the material to self clean due 
to the constant agitated state of, and resulting contact between, the 
spheres. In accordance with an important feature of the present invention, 
the adjacent, tightly packed mobile bed is sufficiently loose to permit 
constant agitation, and therefore self-cleaning, but is packed tightly 
enough to provide an even distribution of gas to the associated loosely 
packed mobile bed to maintain a relatively even distribution of packing 
material in the associated upper bed, and to minimize gas channeling, 
surging and sulfur fouling. 
In the second embodiment of the present invention, packing elements 
disposed between two spaced grids includes a combination of 1) relatively 
light (e.g. specific gravity less than 1.0) mobile packing elements, 
preferably spherical in shape, that are aerated upwardly during normal gas 
flow rates through the tower and 2) relatively heavy fixed packing 
elements that do not move substantially vertically under normal gas flow 
rates through the tower. The fixed packing elements also are preferably 
spherical in shape, and preferably of the same size as the mobile packing 
elements, e.g. about 1 to about 10 centimeters in diameter, preferably 
about 2 to about 5 centimeters in diameter. The combined mobile and fixed 
packing elements achieve essentially the same results as in the first 
embodiment of the present invention. The fixed or heavier packing elements 
hug the lower pervious grid of the packed bed to evenly distribute gas and 
liquid to the aerated mobile packing elements, to achieve efficient mass 
transfer without significant channeling or surging problems. 
Accordingly, an aspect of the present invention is to provide a new and 
improved mass transfer tower and method of achieving efficient gas-liquid 
contact. 
Another aspect of the present invention is to provide a method of 
continuously contacting gas and liquid in a bed of mobile packing elements 
while essentially eliminating channeling and surging problems to maintain 
a uniform flow distribution of gas and liquid across the cross-sectional 
area of the bed of packing elements. 
Another aspect of the present invention is to provide a mass transfer tower 
that includes some packing elements capable of substantial vertical 
displacement during mass transfer between countercurrently flowing gas and 
liquid streams, and some packing elements that are held relatively closely 
together during mass transfer to provide both mass transfer efficiency and 
even gas and liquid distribution over the cross-sectional area of the 
packed bed. 
Still another aspect of the present invention is to provide a gas-liquid 
contact tower including a bed of relatively light, mobile packing elements 
as well as relatively heavy, vertically fixed packing elements to achieve 
a bed of both fixed and mobile packing elements wherein the fixed elements 
act as a gas distributor to evenly distribute gas to the agitated and 
aerated mobile packing elements. 
A further aspect of the present invention is to provide an improved method 
of removing hydrogen sulfide from a hydrogen sulfide-containing gas stream 
with a liquid phase oxidation solution wherein hydrogen sulfide is 
converted to elemental sulfur, using a mobile and relatively fixed packing 
element arrangement within a packed tower. 
Another aspect of the present invention is to provide an improved packed 
tower for efficient gas-liquid contact that has a reduced rate of packed 
tower plugging, while maintaining excellent gas-liquid contact and high 
solid particle removal efficiency. 
Other aspects and advantages will be apparent to those skilled in the art 
from the following detailed description of the preferred embodiment, taken 
in conjunction with the drawings and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
For purposes of illustration only, and not by way of limitation, the 
invention is described with specific reference to a liquid phase oxidation 
process for the removal of hydrogen sulfide from a gas stream. In a 
process of this type, e.g., the so-called Stretford process, or the 
process marketed under the trademark LO-CAT.RTM. by ARI Technologies, Inc. 
of Palatine, Ill., a hydrogen sulfide-containing gas is contacted in a 
gas-liquid contactor or tower with a suitable aqueous treating solution, 
e.g., an oxidizing solution containing a polyvalent metal in its higher 
valence state. In the contactor, hydrogen sulfide is oxidized to elemental 
sulfur in the form of solid particles that are entrained (e.g., suspended 
or slurried) in the treating solution, and the polyvalent metal is reduced 
to its lower valence state. The solution is subsequently regenerated by 
contact with air or another oxygen-containing gas to convert the reduced 
polyvalent metal to its higher valence state for use in the contactor. The 
sulfur formed in the contactor is recovered from the aqueous treating 
solution by flotation, settling, filtration, or any other suitable 
separation technique. 
When using a packed tower as a contactor, countercurrent contacting is 
usually preferred but cocurrent contacting can be used under some 
circumstances. In a countercurrent system, the hydrogen sulfide-containing 
gas is passed upwardly through the packed bed(s) and the aqueous treating 
solution is passed downwardly through the packed bed(s) in intimate 
contact with the upwardly flowing gas stream. The sulfur particles formed 
by the oxidation of hydrogen sulfide tend to accumulate on the packing or 
on the packing support grid resulting in eventual plugging of the tower. 
In most instances, the plugged tower must be periodically taken out of 
service for removal and washing of the packing and for cleaning of the 
support grid. Consequently, in the past it has usually been necessary to 
provide two or more packed towers so that at least one tower is available 
for continuous operation of the process while the other tower is being 
serviced. 
It will be understood that the apparatus and process configurations shown 
in the drawings are illustrative only and that the inventive process and 
apparatus can be used with the Stretford or various LO-CAT.RTM. processes 
familiar to those skilled in the art, such as those disclosed in U.S. Pat. 
Nos. 4,011,304.; 4,125,597,; 4,189,462; 4,218,342; 4,374,104; 4,455,287; 
4,622,212; and 4,865,819. These patents illustrate the known chemistry for 
H.sub.2 S gas conversion to elemental sulfur and the combinations of 
oxidizing/reducing metals and chelating agents capable of retaining metals 
in solution over a wide range of pH and other process conditions. Although 
iron and vanadium are the polyvalent metals which are most commonly used, 
chelating agent solutions of other metals including copper, platinum, 
tungsten, nickel, mercury, tin, lead, and the like also can be used. 
FIG. 1 is an illustration of one manner of utilizing the packed tower of 
the present invention in the LO-CAT.RTM. sulfur removal process, generally 
designated by reference numeral 10, but the packed towers of the present 
invention also can be used with Stretford and other processes. 
A feed gas stream containing hydrogen sulfide is introduced through conduit 
12 to a venturi mixer 14 where the gas is intimately pre-mixed with 
regenerated chelated iron solution supplied to the mixer 14 through 
conduit 16. The outlet from the mixer 14 communicates with a lower portion 
of a vertical multi-bed packed scrubbing tower, generally designated by 
reference numeral 18. In the embodiment of the invention illustrated in 
FIG. 2, packing support grids 20 are mounted in the tower 18, as shown, to 
define a volume for a loosely packed bed of mobile packing elements, 
generally designated 22, and an adjacent more tightly packed bed of mobile 
packing elements, generally designated 24, is disposed between a lower 
surface of grid 20 and an adjacent grid 21. The packing material forming 
the loosely and tightly packed beds 22 and 24 can be formed of the same or 
different materials, preferably spherical packing elements 22a and 24a, 
respectively. One or more loosely packed beds 22 of discrete mobile 
packing elements 22a is supported on an associated grid 20, and one or 
more tightly packed beds 24 of discrete mobile packing elements 24a is 
supported on an adjacent lower grid 21, such that the mobile packed bed 22 
is relatively loosely packed, and packed bed 24 is relatively tightly 
packed to provide the advantages of the present invention, as described in 
more detail hereinafter. Regenerated treating solution is supplied to an 
upper portion of the tower 18 through conduit 26 and a spray nozzle 28. An 
effluent gas conduit 30 extends from the upper portion of the tower 18 
above the spray nozzle 28. 
In the embodiment of the invention illustrated in FIG. 3, a packed tower, 
generally designated 18a, is operated in the same way as tower 18 of FIG. 
2, except that tower 18a includes fixed packing elements that are 
intermixed with the mobile packing elements 22a to establish a lower bed 
31 of fixed packing elements 31a disposed below the mobile bed 22 of 
mobile packing elements 22a--both packed beds 22 and 31 disposed between a 
single pair of spaced fluid-pervious grids 20. During operation of the 
mass transfer tower 18a, as illustrated in FIG. 3, the bed of heavier, 
fixed packing configuration against the lower grid 20, while the mobile 
packing elements 22a are aerated vertically upwardly to achieve efficient 
mass transfer between the gas and liquid through the tower 18a. The mobile 
packing elements 22a preferably are spherical and may be hollow or solid, 
but should be light enough (e.g. a specific gravity less than 1.0, and 
preferably less than about 0.5) to be aerated upwardly from the fixed bed 
31 during normal flow rates of gas and liquid in the tower 18a. The fixed 
packing elements 31a also are preferably spherical and are heavy enough 
(e.g. a specific gravity greater than 1.0) so that they are not greatly 
aerated during normal flow rates of liquid and gas through the tower 189. 
The feed gas passes upwardly through the packed beds 24 or 31 and 22, and 
treating solution from the spray nozzle 28 passes downwardly through the 
beds 22 and 24 or 31 in countercurrent contact with the upwardly flowing 
gas stream. Although countercurrent gas-liquid contact in the tower 18 is 
preferred, the invention is not limited to the use of countercurrent flow. 
Further, the tightly packed bed 24 can be disposed above, instead of 
below, the loosely packed bed 22 to aid in achieving more even 
distribution of gas and liquid. The treated gas, which is substantially 
free of hydrogen sulfide, is removed from the upper portion of the tower 
18 through conduit 30. A liquid slurry of treating solution and suspended 
sulfur particles accumulates in the lower portion of the tower 18, as 
indicated by the upper liquid level 32, and the slurry is withdrawn at a 
controlled rate through conduit 34. 
The used treating solution containing reduced metallic ions, e.g., reduced 
forms of iron or vanadium ions, is regenerated by passing the slurry from 
conduit 34 into an upper portion of an oxidizer vessel generally 
designated 36 where the slurry accumulates, as shown by the upper liquid 
level 38. Air is supplied to the oxidizer 36 by means of a blower 40 
having an inlet conduit 42 associated with gas filter 44, and an outlet 
conduit 46 connected to a sparger ring 48 located in the oxidizer vessel 
36 substantially below the liquid level 38. Air from the sparger ring 48 
bubbles upwardly through the used treating solution and oxidizes the 
metallic ions to their higher valence state. The air, having a slightly 
diminished oxygen content, is vented from an upper portion of the oxidizer 
vessel 36 through conduit 50. The regenerated treating solution is 
withdrawn from the oxidizer vessel 36, below the sparger ring 48 through a 
suction conduit 52 by means of recirculating pump 54 and is supplied to 
the tower 18 through a discharge conduit 56 connected to tower inlet 
conduits 26 and 16. 
In the oxidizer vessel 36, a major portion of the sulfur settles to a 
cone-shaped bottom portion 58 so that the treating solution withdrawn 
through conduit 52 for recirculation to the tower 18 contains only a minor 
amount of sulfur. A dense slurry of sulfur particles is supplied from the 
bottom portion 58 of oxidizer vessel 36 through conduit 60 to a sulfur 
filter 62. The sulfur-free filtrate is withdrawn from the filter through 
conduit 64 and pump 66 and pumped through conduit 68 to an upper portion 
of the oxidizer vessel 36. 
The contacting of the feed gas with the treating solution in the tower 18 
may be carried out at ambient temperature and pressure conditions, but 
temperatures of from about 5.degree. C. to about 65.degree. C. and 
pressures ranging from subatmospheric to 100 atmospheric or greater can be 
used. A pH ranging from about 6 to about 13, particularly from about 6 to 
about 10.5, is preferably maintained by adding alkaline material as 
required. The redox potential of the solution may be used as a measure of 
catalyst activity as reflected by the ratio of ferric to ferrous ions in 
solution. Maintaining a redox potential of from about -50 to about -300, 
as measured by a calomel electrode, is desirable. 
FIG. 2 shows, schematically, the structure of a preferred embodiment of the 
gas-liquid contact scrubbing tower 12, having a bed 22 of mobile packing 
material 22a with sufficient space for vertical movement of the mobile 
packing elements within each compartment or stage, and an adjacent more 
tightly packed bed 24 of mobile packing material 24a, supported below by 
grid 21 and having an upper grid 20 that is the same as the lower grid 20 
of the upwardly adjacent mobile bed 22. Each loosely packed bed 22 of 
mobile packing material 22a and its adjacent tightly packed bed 24 of 
mobile packing material 24a is supported in the tower 18 on lower 
fluid-pervious grids 20 and 21, respectively. The particular design or 
structure of the pervious grids 20 and 21 is not critical to the practice 
of the invention, but the purpose of the grids is to support and separate 
the respective mobile packing elements into their respective zones or 
stages of the tower 18 while allowing relatively large total open area in 
the grids to permit the passage of gas being treated and aqueous liquid 
for mass transfer therebetween in the tower 18. The details of a 
particularly preferred grid support structure useful in the liquid-gas 
contact tower of this invention are described in Dowd, et al. U.S. Pat. 
No. 4,865,819, the disclosure of which is incorporated herein by 
reference. 
To achieve the full advantage of the present invention, particularly for 
mass transfer in a process where solids are formed within the tower 18, 
the mobile packing elements 22a and 24a or 31a are spherical, having a 
diameter of about 1 centimeter to about 10 centimeters preferably about 2 
centimeters to about 5 centimeters in diameter, so that the agitation and 
aeration of the packing elements achieves self-cleaning of the packing 
elements 22a and 21a without requiring shut down of the tower for packing 
material cleaning. The fixed heavier packing elements 31a of FIG. 3 also 
are preferably spherical to eliminate any sharp corners and crevices 
between adjacent elements 31a, and between packing elements 31a and the 
lower grid 20, so that solids are less likely to accumulate and plug the 
tower 18a. However, the invention is not limited to a particular shape of 
mobile packing material 22 or 22a or fixed packing elements 31a. Mobile 
packing elements can be known forms of polypropylene or other plastic 
beads, of a size and density selected to be retained upon the grid 
structure 20 and 21, and a sufficiently low specific gravity, e.g. less 
than 1.0, to be easily mobilized within the gas-liquid contact apparatus 
18. An example of a commercially available packing material which may be 
used as elements 22a, 24a and 31a is hollow plastic spheres manufactured 
by EURO-MATIC, LTD. of London, England. 
The multiple mobile packing beds arranged in the gas-liquid contact tower 
18 in accordance with the FIG. 2 embodiment, and the intermixed mobile and 
fixed packing elements of the FIG. 3 embodiment of the present invention 
alleviate problems associated with low sulfur removal efficiencies in 
prior known mobile packing bed contact towers, and provide good gas 
distribution to maintain even fluidization throughout the mobile packing 
beds. With the arrangement of both embodiments of the invention, as 
described, a sufficient number of stages of self-cleaning, mobile packing 
elements are provided to convert unexpectedly high levels of the hydrogen 
sulfide, from the gas being treated, as elemental sulfur particles 
retained in the aqueous medium. In addition, the tightly packed bed 24 of 
mobile packing material 24a acts as an excellent gas distributor 
correcting any maldistribution of the gas for contact with the liquid 
solution which may occur in the mobile beds 22. 
The foregoing detailed description is given for clearness of understanding 
only, and no unnecessary limitations should be understood therefrom, as 
modifications within the scope of the invention will be apparent to those 
skilled in the art. 
It should be understood that the present disclosure has been made only by 
way of preferred embodiment and that numerous changes in details or 
construction, combination and arrangement of parts can be resorted to 
without departing from the spirit and scope of the invention as hereunder 
claimed.