Advanced gas control in gas-liquid mixing systems

Sloped inner and outer conical baffles are provided in a gas-liquid mixing system having a recirculating portion of a body of liquid, separated from a quiescent portion of the body of liquid and an overhead gas phase, precluding the collection of gas under the baffles, eliminating dead zones of gas, and minimizing the passage of gas to the overhead gas phase during normal operation, while facilitating the venting of gases during upset conditions.

FIELD OF THE INVENTION 
This invention relates to the mixing of gases and liquids. More 
particularly, it relates to the containment and disengagement or gases 
during gas-liquid mixing operations. 
DESCRIPTION OF THE PRIOR ART 
A wide variety of processes and systems are known in the art for mixing of 
gases and liquids. Thus, stirred tank reactors, including the Advanced Gas 
Reactor (AGR) system of the Litz patent, U.S. Pat. No. 4,454,077, have 
been employed for such mixing operations. 
In some instances, however, as in the oxidation of organic liquids, it is 
necessary to prevent a potentially explosive or flammable vapor-gas phase 
mixture from developing in the overhead gas phase. For this purpose, 
oxidation reactions are carried out so as to prevent the oxygen in the gas 
phase from exceeding the lower flammability limit for a particular 
operation. In other instances, it may be necessary or desirable to carry 
out a non-oxidation operation, e.g. hydrogenation, chlorination or other 
gas-liquid mixing reactions, so as to minimize the loss of gas to the 
overhead gas phase. For such purposes, a so-called Liquid Organic Reactor 
(LOR) process and system, as described and claimed in the Litz et al. 
patent, U.S. Pat. No. 4,900,480, was developed. The LOR process and system 
enables gases and liquids to be mixed, for dissolving, reacting, or other 
mixing purposes, without appreciable loss of gas to the overhead gas 
phase. 
The LOR approach, in its most common embodiment, uses a mixing impeller and 
draft tube arranged to disperse and circulate oxygen bubbles in the liquid 
phase. When used to safely react gaseous oxygen with flammable liquids, 
the bubbles comprise a mixture of said oxygen, flammable organic vapor and 
by-product gases. When the gas is dispersed as small bubbles throughout 
the liquid phase, the flammability hazard associated with the oxygen and 
organic gas mixture is mitigated by the heat capacity of the surrounding 
liquid, which absorbs the heat of reaction in the event of a bubble 
ignition, and because the flame from a single bubble can not propagate 
through the liquid phase. 
In the LOR system as described in the Litz patent, a recirculating liquid 
reaction zone is separated from, but remains in fluid communication with, 
a quiescent zone that is in contact with the overhead gas phase, typically 
as shown in FIG. 1 of the patent. A baffle between said zones serves to 
substantially prevent gas bubbles that are carried with the liquid in the 
recirculating liquid zone from disengaging the liquid because of their 
buoyancy, thus insuring that the bubbles are recirculated with the liquid 
and are efficiently consumed by reaction. Any gas bubbles that do escape 
from the recirculating liquid zone under the baffle, and pass upward 
through the quiescent zone, are collected in the gas space above the 
quiescent zone, where they are rendered non-flammable by the addition of 
inert gas to said gas space. 
In laboratory size, e.g. one gallon, LOR systems, a flat baffle is 
positioned above the draft tube so as to separate the recirculating 
portion of the body of liquid from the quiescent zone, while maintaining 
fluid communication therebetween. The baffle serves to direct unreacted 
gas into the impeller suction, where it is redispersed and recirculated by 
the mixing impeller, along with the liquid phase. High liquid flow 
velocity in the upper part of the reactor vessel and the draft 
tube/impeller suction is implicit because of the small size of the LOR 
unit, and the large diameter of the draft tube relative to the diameter of 
the vessel in typical laboratory LOR system embodiments. The high velocity 
liquid flow serves to drag the gas bubbles into the draft tube faster than 
buoyant effects cause such gas bubbles to collect on the underside of the 
baffle or pass into the quiescent zone. In addition, any gas that reaches 
the underside of the baffle is dragged by the recirculating liquid into 
the draft tube where it is dispersed upon downward flow therein before it 
has a chance to form a bubble of dangerous size. 
In larger, commercial size LOR systems, however, the typical flat baffle 
does not function as well to preclude the formation of gas pockets under 
the baffle as in said laboratory size LOR systems. Regions of low flow 
velocity, or dead zones, are often found to exist in the upper part of the 
recirculating liquid zone under the baffle, thereby decreasing the ability 
of the system to maintain the desired level of gas dispersion. Hence, the 
ability of the flow of recirculating liquid to prevent gas from collecting 
in pockets on the underside of the baffle, and the ability of the liquid 
flow to remove any gas from the underside of the baffle, once it collects 
thereunder, is appreciably diminished in larger sized LOR systems. Since 
radial flow velocity decreases with radial distance from the draft tube 
centered in the reaction vessel, and since absolute distances are a 
function of scale, the problem of dead zones under the baffle, and the 
consequent potential for forming hazardous gas pockets, increases with 
scale when a flat baffle is used to separate the recirculating portion of 
a body of liquid in an LOR system from the quiescent portion of said body 
of liquid. 
In larger LOR systems, the pumping rate is highest at the impeller tip and 
lowest near the impeller shaft. Thus, a dead zone is formed near the 
center of the draft tube in which the impeller is positioned, where the 
downward liquid flow velocity is not sufficiently high to drag gas 
downward through the draft tube. As a result, gas tends to collect on the 
underside of the baffle in the dead zone. This is undesirable because it 
can lead to the formation of a flammable bubble of dangerous size in the 
reaction vessel. 
It has been proposed to prevent such formation of large flammable gas 
bubbles under the baffle by simply employing a porous baffle that would 
enable all gas that would otherwise collect along the underside of the 
flat baffle to vent into the quiescent zone above the baffle and therefrom 
into the inert gas space above the baffle. However, such an approach would 
not be efficient, because the reactant gas, e.g. oxygen, hydrogen, 
chlorine or the like, which escapes from under the baffle would be wasted 
and, in the case of oxygen, could cause a flammable or explosive condition 
in the gas space unless a suitable amount of nitrogen or other inert gas 
is added thereto. 
Under some circumstances, however, particularly when proper gas dispersion 
in the liquid can not be maintained, the use of such a porous flat baffle 
may nevertheless be desirable or necessary because accumulated gas must be 
vented to the inert gas space above the baffle to prevent the formation of 
a flammable gas pocket in the reaction vessel. This would be the case, for 
example, under upset conditions when too much oxygen is fed to the 
reaction vessel, or in the event the impeller drive were to fail, and it 
is desirable to allow the gas to escape to the inerted overhead gas space 
because the gas, e.g. oxygen, can not be maintained as a dispersed phase 
in the body of liquid in the reaction zone. 
There is a need in the art, therefore, to provide an LOR system for 
effectively containing the gas during normal operation thereof, in such a 
way as to prevent the formation of dangerous gas bubble accumulation and 
growth in the recirculating liquid zone. Such an LOR system must continue 
to provide for efficient utilization of feed gas during normal operation, 
and should allow the gas to be safely vented from the liquid phase in the 
event that the means for maintaining desired gas dispersion is interrupted 
or lost. 
It is an object of the invention, therefore, to provide an improved LOR 
system. 
It is another object of the invention to provide an LOR system to enhance 
the dispersion of feed gas in the liquid phase under normal operating 
conditions so as to enable essentially complete reaction of gas and liquid 
to occur, and to preclude the formation of any large flammable gas 
bubbles, or gas pockets, in the reaction vessel. 
It is a further object of the invention to provide an LOR system that 
enables the feed gas to escape to an inerted overhead gas space during 
upset conditions, when it is not possible to maintain the feed gas as a 
dispersed phase in the body of liquid in a reaction vessel. 
With these and other objects in mind, the invention is hereinafter 
described in detail, the novel features of the invention being recited in 
the appended claims. 
SUMMARY OF THE INVENTION 
An LOR system is operated with sloped inner and outer conical baffles above 
the draft tube, and a recirculating liquid flow pattern, such as to direct 
the flow of liquid and dispersed gas into an impeller suction, with 
minimal collection of gas under the baffle, to eliminate the dead zone 
near the impeller shaft, and to facilitate the venting of gas to an inert 
gas space above the quiescent zone during upset conditions, while 
minimizing the passage of gas to the gas space during normal operating 
periods.

DETAILED DESCRIPTION OF THE INVENTION 
The objects of the invention are accomplished by operating an LOR system 
adapted to (1) maintain a high velocity flow across the diameter of the 
draft tube in the impeller suction zone, thus eliminating a dead zone in 
the center of the draft tube; (2) minimize the flow of gas through the gas 
vent during normal gas-liquid mixing operations therein; (3) enable the 
flow of a large volume of gas through the gas vent when the recirculating 
flow of liquid in the reaction vessel is reduced or stopped; and (4) 
eliminate dead zones in the upper portion of the reaction vessel under the 
baffle separating the recirculating portion of the body of liquid in the 
reaction vessel from the quiescence zone. The invention thus addresses 
practical operational problems encountered in the operation of the highly 
desirable LOR processes and systems, rendering the LOR technology more 
convenient and more advantageous for use in a variety of significant 
industrial gas-liquid mixing operations. 
The heart of the invention resides in the use of sloped inner and outer 
conical baffles that, together with the management of the liquid flow 
field in the impeller suction, enables such desirable benefits to be 
achieved. First, the inner conical baffle is constructed in a way such as 
to maintain a high velocity flow field across the diameter of the draft 
tube in the impeller suction zone. The slope toward the impeller shaft 
effectively eliminates the dead zone in the center of the draft tube. 
Thus, downward liquid flow, with sufficient velocity to drag gas down into 
the draft tube, is maintained across the diameter of the draft tube. 
Second, the gas vent is constructed so as to, as indicated above, minimize 
the gas flow through it during normal operation, i.e. under normal liquid 
flow conditions, while allowing a large volume of gas to pass through when 
the liquid flow is reduced or stopped. Finally, the slope of the outer 
conical baffle precludes the collection of gas underneath it. Gas that 
hits the underside of said outer baffle is directed into the impeller 
suction zone under the inner baffle by buoyant effects, thereby 
eliminating dead zones outside the draft tube. 
With reference to the embodiment of the invention illustrated in the 
drawing, a body of liquid represented by the numeral 1 contained in 
reactor vessel 2 has a major portion 3 separated from, by baffle means 
described below, but in fluid communication with, a relatively quiescent 
portion 4, which has a gas-liquid interface 5 with overhead gas space 6. 
Said major portion 3 is maintained in a recirculating flow condition by 
impeller means 7, having drive shaft 8 that extends upward through an 
inner baffle, described below, for connection with suitable driving means, 
not shown. Impeller means 7, illustrated as being positioned within hollow 
draft tube 9, is adapted to provide the indicated vertical flow pattern in 
which liquid containing dispersed gas passes downward in draft tube 9 and 
upward in the annular space 10 between draft tube 9 and vertical wall 11 
of reactor vessel 2. 
In the illustrated embodiment, vertical wall 11 of reactor vessel 2 is 
shown as extending upward from floor 12 of the vessel to a point at which 
the wall slopes upward and inward to form outer conical baffle 13. Said 
outer conical baffle 13 is connected with generally vertical wall 14 of an 
upper cone shaped portion of the reactor vessel terminating at roof 15 and 
forming an enclosed space for the relatively quiescent portion 4 of the 
body of liquid and for overhead gas space 6. Inert gas can be passed to 
said overhead gas space 6 through line 16 to assure against the creation 
of flammable conditions in said overhead gas space 6, with gases being 
vented therefrom through line 17. 
Inner conical baffle 18 is positioned above impeller means 7, and above 
draft tube 9 in the illustrated embodiment, and co-acts with outer conical 
baffle 13 to enable the desirable benefits referred to above to be 
achieved in the operation of the LOR system of the invention. Inner 
conical baffle 18 slopes downward and inward toward impeller shaft 8 from 
a point near the outer portion of the wall of reactor vessel 2, desirably 
in the region near the upper end of outer conical baffle 13. Thus, a vent 
opening 19 is provided in the space between the upper end of inner conical 
baffle 18 and the upper end of outer conical baffle 13. 
In order for a gas bubble to escape from under inner conical baffle 18, it 
must come in contact with a hole in or around said baffle, i.e. with said 
vent opening 19. Thus, the amount of gas that escapes is proportional to 
the probability that a gas bubble will hit a hole. If the gas bubble 
remains in the liquid flow field, shown by flow pattern A in the lower 
portion of reactor vessel 2, and by flow pattern B in the upper portion 
thereof, so that it flows downward into draft tube 9 upon contact with 
inner conical baffle 18, it can not escape from the recirculating body of 
liquid and pass into quiescent zone 4 and overhead gas space 6. Thus, the 
downward slope of inner conical baffle 18 is intended to maximize the flow 
velocity of liquid under said baffle, minimize the probability that a 
bubble will come into contact with a hole in or around said baffle, i.e. 
vent opening 19 under normal operating conditions, and maximize the 
probability that a bubble will come in contact with a hole, i.e. said vent 
opening 19, in the event of an upset condition where it is necessary or 
desirable to vent all or a substantial portion of the gas from under inner 
conical baffle 18. 
Under normal flow conditions, the liquid flow velocity on the underside of 
inner conical baffle 18 is directed radially inward and downward toward 
impeller shaft 8 and impeller means 7 and away from vent opening 19. This 
downward and inward liquid flow drags most gas which contacts the 
underside of inner conical baffle 18 into the downward impeller suction 
and away from vent opening 19. When the recirculating flow of liquid is 
stopped, as when the drive motor for said impeller fails, gas bubbles rise 
through major portion 3 of body of liquid 1 to the underside of inner 
conical baffle 18. The slope of said inner baffle directs the gas bubbles 
to vent opening 19, through which the gas bubbles pass to quiescent 
portion 4 of the body of liquid and to inerted overhead gas space 6. 
It will be understood that various changes and modifications can be made in 
the details of the invention without departing from the scope of the 
invention as recited in the appended claims. Thus, outer conical baffle 13 
desirably has an inwardly extending flap section 13A, conveniently 
horizontally, to facilitate the flow of liquid and dispersed gas bubbles 
away from vent opening 19. Outer conical baffle extension 13A conveniently 
extends from the upper end of outer conical baffle 13 inward to a position 
under inner conical baffle 18, preferably at the outer, upper end thereof. 
As will be seen from the drawing, such a positioning of outer conical 
baffle extension 13A serves to make vent opening 19 a slit positioned away 
from the generally inward and downward flow of liquid under said inner 
conical baffle 18, thereby minimizing the amount of gas venting through 
vent opening 19 under normal operating conditions in reactor vessel 2. 
It will be seen from the drawing that outer conical baffle 13, which is 
sloped upward and serves to eliminate dead spots outside draft tube 9 
since gas cannot collect underneath it, is conveniently the sloping 
portion of the wall of reactor vessel 2 that extends upward and inward 
from the top of vertical wall 11 of said reactor vessel 2. It should be 
noted that, while the illustrated embodiment is convenient and preferred, 
it is also within the scope of the invention for vertical wall 11 to 
extend to the upper part of reactor vessel 2, with a separate outer 
conical baffle 13 being positioned therein to create the desired upward 
and inward liquid flow path toward inner conical baffle 18 with the flow 
path of liquid on the underside of inner baffle 18 being downward and 
inward into the impeller suction above and in draft tube 9. In this 
regard, it will be appreciated that the upper portion of reactor vessel 2, 
in which quiescent portion 4 and overhead gas space 6 are contained, need 
not be smaller diameter than the lower portion of said reactor vessel 2, 
although the illustrated embodiment is convenient to construct and operate 
for the subject gas-liquid mixing purposes of the invention. 
Attention is called to the downwardly extending portion 18A of inner 
conical baffle 18. Such portion 18A, generally in the center of reactor 
vessel 2 above draft tube 9, will be seen to further direct the downwardly 
and inwardly flowing liquid under said inner conical baffle 18 downward 
into the suction of impeller means 7 for the desired downward flow of the 
recirculating portion 3 of the body of liquid through draft tube 9. Drive 
shaft 8 for impeller means 7 will be seen to extend upward through opening 
20 defined by said extension 18A of inner conical baffle 18 for connection 
to suitable drive means for impeller means 7. Due to the downward flow of 
liquid above draft tube 9 and impeller means 7, the amount of gas that 
will escape upward through opening 19 will be negligible during operation 
of the gas-liquid mixing system. 
The feed gas stream is introduced into reaction vessel 2 through conduit 
means 20 directly into the recirculating portion of the body of liquid, 
i.e. major portion 3, so that the bubbles of gas formed are maintained 
essentially in dispersed form in said major portion 3 of the body of 
liquid. For this purpose, conduit means 20 preferably extend into said 
major body of liquid in the vicinity of liquid flow pattern B near the top 
of major portion 3 of the body of liquid for enhanced mixing under the 
influence of the draft tube-impeller means configuration as enhanced by 
the inner and outer conical baffles of the invention. It is within the 
scope of the invention, however, to introduce the feed gas stream to the 
reactor vessel at any other convenient point, as in liquid flow pattern A 
near the bottom of reactor vessel, or elsewhere in the recirculating flow 
of liquid. 
It will be noted in the drawing that hollow draft tube 9 contains a 
conically flared portion 9A at the upper end thereof. This generally 
preferred, but not essential feature, serves to further facilitate the 
flow of gas bubble-liquid mixture into said hollow draft tube 9 for 
downward passage therein. Impeller means 7 positioned within said hollow 
draft tube 9 are illustrated as a simple impeller blade device adapted to 
pump liquid down through said draft tube 9 and upward in the annulus 
between said draft tube 9 and outer wall 11 of reactor vessel 2, and the 
outer conical baffle 13 portion thereof. Those skilled in the art will 
appreciate that other impeller means can be employed in the practice of 
the invention, such as commercially available axial flow helical impeller 
means for enhancing the desired liquid pumping action and the overall 
gas-liquid mixing achieved in the practice of the invention. Those skilled 
in the art will also appreciate that the draft tube-impeller means 
arrangement can be provided at different locations within reactor vessel 
2, i.e. the distance from the bottom of draft tube 9 to floor 12 of 
reactor vessel 2, so long as the desired recirculating liquid flow pattern 
is maintained. In various embodiments, flow pattern A in the lower portion 
of reactor vessel 2 may serve to create rolls cells of enhanced turbulence 
further facilitating the desired gas-liquid mixing operation. 
While downward pumping impeller means are herein described and claimed, it 
will be appreciated that upward pumping impeller means can also be 
employed for gas-liquid mixing operations, and baffle arrangements can be 
devised to accomplish the desirable results achieved using the invention. 
However, such reverse flow operation is generally less convenient and less 
suited than the invention as herein described and illustrated. 
While the invention has been illustrated by an embodiment employing a 
desirable and advantageous hollow draft tube-impeller means configuration, 
it should be noted that the incorporation of a hollow draft tube, while 
highly preferred, can be omitted in various embodiments provided that the 
desired recirculating flow condition can be maintained sufficiently 
without the preferred use of said hollow draft tube. 
Those skilled in the art will appreciate that the quiescent portion of the 
body of liquid can be of any suitable size to provide the desired 
separation of the recirculating portion of liquid from the overhead gas 
phase and to accommodate a change in liquid level in response to a change 
in volume of said body of liquid between the condition in which no gas 
bubble are in the body of liquid and the condition that exists when a 
desired gas bubble concentration is developed therein. 
The liquid flow velocity into the draft tube section of the system should 
generally be about 1.5 ft/sec. or more, preferably greater than about 2.0 
ft/sec., for the recirculating flow of liquid to carry gas bubbles into 
the impeller means without significant disengagement of the gas bubbles 
from the liquid. For a given total flow rate, the desired flow velocity 
into the draft tube-impeller suction can be adjusted by properly setting 
the clearance between the draft tube and the inner and outer conical 
baffles of the invention, i.e. by setting the baffle clearance so that the 
radial flow velocity across the top of the draft tube toward the center 
thereof is desirably greater than about 2.0 ft/sec. 
For advantageous operation of the invention, the outer diameter of inner 
conical baffle 18 should generally be from 0.75 to 2.0 times the diameter 
of the upper entrance to draft tube 9. The slope of said inner conical 
baffle 18 should generally be from about 5.degree. to about 35.degree., 
preferably about 15.degree., with respect to the horizontal. Outer conical 
baffle 13 should generally be from about 45.degree. to about 75.degree. 
with respect to the horizontal, preferably about 60.degree. with respect 
to the horizontal. 
Extension 13A of outer conical baffle 13 will be seen as a flap that 
overlaps vent opening 19 opening so that there is no uncovered area under 
inner conical baffle 18 in the recirculating flow path. Said extension 13A 
and vent opening 19 will be seen to form a vent slit, with a gas bubble 
having to move against the liquid flow field to pass through the vent 
slit. The vent opening, and, in the illustrated embodiment, the vent slit, 
should desirably be located at the highest point of inner conical baffle 
18. 
Those skilled in the art will appreciate that any baffle configuration that 
meets the criteria referred to above, i.e. that minimizes the dead zones 
previously encountered in the practice of the LOR technology and 
facilitates high liquid flow in the impeller suction zone of the system 
can be used in the practice of the invention. In general, baffles with 
smooth contours that more closely follow the streamlines of the liquid 
flow field in the impeller suction zone that is accomplished by the use of 
flat baffle surfaces would generally be desirable from a hydrodynamic 
viewpoint. However, the cost of fabricating such a more complex shape must 
be weighed against the added benefits thereof. 
It will be understood that any desired gas-liquid mixing operation in which 
the LOR approach is necessary or desirable can be benefited by the 
practice of the invention. The oxidation of an aliphatic aldehyde is an 
illustrative example of the type of reaction that can be carried out 
advantageously using an LOR system containing the desirable baffle 
arrangement herein described and claimed. The invention also provides a 
highly desirable improvement in LOR systems for use in other oxygen 
applications not involving the potential presence of flammable mixtures in 
the gas phase, and in other valuable hydrogenation, chlorination and other 
practical gas-liquid mixing operations. By overcoming various practical 
operating problems encountered in the practice of the highly desirable and 
advantageous L0R system technology, the invention provides a significant 
improvement that enhances the ability of the LOR technology to satisfy the 
need for efficient, effective and economical gas-liquid mixing operations 
for a wide variety of desirable LOR applications.