In a gas dryer for use in a gas transmission line in which a bed
of desiccant tablets is suspended in the flow of gas, making an
aqueous solution of the desiccant salts from the moisture taken
from the gas, the solution is inhibited from generating solids and
precipitates in a sump area by conducting the heat of hydration
from the bed area to the sump area.
What is claimed is:
1. Method of inhibiting the formation of solids in a solution of
desiccant salts obtained by drying flowing gas comprising moving
an agitator in said solution wherein said agitator is moved by the
force of said gas.
2. Method of inhibiting the formation of solids in a solution of
desiccant salts obtained by drying gas by contact of said gas with
solid desiccant salts, comprising recovering heat of hydration generated
by the removal of moisture from said gas during contact with said
solid desiccant salts, and transferring the heat of hydration generated
by said contact to said solution.
3. Method of claim 2 wherein said desiccant salts comprise calcium
chloride and lithium chloride in a weight ratio of from 0-100% to
4. Method of claim 2 wherein said solid desiccant salts are in
a bed of tablets.
5. Method of claim 2 wherein said solution is in a sump area.
6. Method of claim 2 wherein said heat of hydration is transferred
to said solution through a heat exchange fluid.
7. Method of claim 2 wherein said heat of hydration is transferred
to said solution through metal elements.
8. Method of claim 2 including agitating said solution.
9. Apparatus for drying gas in a gas transmission line comprising
a vessel including a chamber for holding a bed of solid desiccant,
and means for transferring heat of hydration from said bed of solid
desiccant to a sump for a used desiccant salt solution.
10. Apparatus of claim 9 wherein said means for transferring heat
of hydration comprises at least one metal element for conducting
heat of hydration.
11. Apparatus of claim 9 wherein said means for transferring heat
of hydration comprises a vessel jacket for containing a heat exchange
12. Apparatus of claim 11 wherein said vessel jacket contains a
heat exchange fluid comprising potassium formate.
13. Apparatus for drying gas in a gas transmission line comprising
a vessel including a perforated plate for holding a bed of desiccant
tablets, a sump area for holding used desiccant solution, and means
for transferring heat from said bed of desiccant tablets to said
14. Apparatus of claim 13 wherein said means for transferring heat
includes means for containing a solution comprising potassium formate.
15. Apparatus of claim 13 including means for agitating said used
16. Apparatus of claim 15 wherein said means for agitating said
used desiccant solution comprises at least one mixer blade powered
by the flow of gas from said gas transmission line.
17. Apparatus of claim 13 wherein said means for transferring heat
comprises metal rods.
18. Apparatus of claim 13 wherein said means for transferring heat
comprises a vessel jacket containing a heat transfer fluid.
This invention relates to gas drying. More particularly, it relates
to inhibiting the formation and deposition of solids from used desiccant
BACKGROUND OF THE INVENTION
Moisture is commonly removed from natural gas in gas transmission
lines by passing the gas through a gas dryer which holds a bed of
desiccant tablets. As the desiccant tablets pick up moisture, a
solution is formed of the desiccant salts. The solution drains into
a sump area from which it is removed, usually by periodic draining.
A problem exists with such dryers and desiccants, in that the generally
highly concentrated solutions of desiccant salts which drain into
the sump tend to crystallize or otherwise come out of solution,
and remain in the sump when the solution is drained or otherwise
removed. In the quiescent sump, the solidified salts resist redissolving,
and tend to build up over time, occupying sump volume and necessitating
frequent manual removal. Gas transmission lines and dryers are placed
and used throughout a wide variety of climatic and seasonal conditions,
and, although the problem of solids formation is probably more acute
in locations where temperatures are lowest, it presents itself even
in warmer climates because the salts will come out of solution in
the cooler or colder evening and night temperatures.
A possible remedy for the problem would be to provide a heater
for the sump solution, but this is not economically desirable.
SUMMARY OF THE INVENTION
We have invented a method of inhibiting solids formation in solutions
of used desiccant salts which comprises conveying the heat of hydration
from the desiccant bed to the solution of used desiccant salts in
a sump. The heat of hydration may be transported in any feasible
manner, but we prefer to use either a heat exchange fluid in an
enclosure, or a heat conductor in the form of rods. Optionally,
we may agitate the solution in the sump. For example, the force
of the gas flow in the dryer may be used to turn mixing blades in
We use the term "heat of hydration" herein to include
heat of solution.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified view of a typical prior art conventional
gas dryer for a gas transmission line, showing the placement of
the desiccant tablets, the sump area, and a typical buildup of solids
which is the problem we attack.
FIG. 2 is a simplified view of a gas dryer equipped with a jacket
of our invention for containing a heat exchange fluid for transferring
the heat of solution of the desiccant salts from the tablet bed
to the sump.
FIG. 3 is a simplified view of a gas dryer equipped with heat conducting
rods of our invention, for conducting heat from the desiccant tablet
bed to the sump area.
FIG. 4 is a simplified view of the placement of a mixer to be turned
by the flow of gas in the gas dryer.
DETAILED DESCRIPTION OF THE INVENTION
The invention will be described with respect to a dryer for natural
gas in a natural gas transmission line, but it should be understood
that it is applicable to any type of gas drying where solid drying
salts are used to remove moisture from a gas, forming a solution
which then is accumulated in a holding area or vessel, where it
normally would tend to form precipitates, crystals, or other solid
Referring now to FIG. 1 natural gas in line 1 enters the dryer
vessel 2 at a point below perforated plate 3 which holds a bed of
desiccant tablets 4. The gas flows upward through the bed of desiccant
tablets 4 which remove moisture, and leaves the vessel through
outlet 5 for further transmission or use. The moisture from the
gas accumulates on the tablets 4 forming drops 7 of desiccant salt
solution which fall into sump area 6. Solution 8 in sump area 6
is generally highly concentrated, and will form solids 9 which accumulate
in the sump area 6. Drain 10 designed to remove the solution in
sump area 6 is ineffective at removing the solids 9 which may
adhere to the walls of sump area 6.
In FIG. 2 the vessel 2 is surrounded by a jacket 11 containing
a heat exchange fluid 12. No moving parts are necessary. Jacket
11 and heat exchange fluid 12 extend to a level high enough on the
bed of desiccant tablets 4 to assure that contact is made with the
portion of the bed of desiccant tablets 4 which generates the most
heat of hydration. The heat of hydration generated by removal of
moisture from the gas is thus transferred through the wall of vessel
2 and into heat exchange fluid 12. The jacket 11 and heat exchange
fluid 12 extend downward to surround the sump area 6 and thus the
heated heat exchange fluid will contact the wall of sump area 6
heating or warming it, and the heat is thereby transferred into
solution 8 in sump area 6. Solution 8 is thus maintained at a temperature
higher than it would otherwise be, thereby inhibiting crystal formation,
precipitation, and other solids deposition likely to occur, particularly
when the ambient temperature falls.
FIG. 3 shows heat conducting rods 13 installed to collect the heat
of hydration in the bed of tablets 4 and conduct it downwards to
solution 8 in sump area 6 thus accomplishing essentially the same
objective as the jacket 11 and heat exchange fluid 12 in FIG. 2.
Again, the solution 8 is maintained at a temperature higher than
it would otherwise be, thus inhibiting the formation of solids.
Looking now at FIG. 4 bracket holds a rod 15 having a propeller
16 on its upper end and mixing blades 17 on its lower end. Rod 15
may rotate in bracket 14 and is caused to turn by the upward flow
of gas in vessel 2; the rotation of propeller 16 causes the rotation
of mixing blades 17 placed at a level so as to agitate solution
8 in sump area 6. Agitation of solution 8 inhibits the formation
of solids regardless of whether heat has been transferred to the
sump area 6 from the heat of hydration in the bed of tablets 4
but we prefer to use the propeller and mixing blades together with
one or the other of the devices of FIG. 2 or FIG. 3.
Bearing in mind that the illustrations are a preferred embodiment,
it should be understood that the perforated plate is only one means
for holding the bed of solid desiccant, and that other devices may
be used for forming a chamber for the bed and retaining the tableted
or otherwise solid desiccants, and collecting the solution of desiccant
salts which will inevitably be formed when moisture is removed from
the gas. Likewise our invention is not limited to the transmission
of natural gas but may be used in air drying and the drying of other
The usual manner of operating a gas dryer such as the one illustrated
is to place desiccant tablets on the perforated plate to a depth
sufficient to remove a significant portion of the moisture in the
gas at its usual flow rate. As the tablets are consumed, the bed
depth will recede. Usually, the tablets will be replenished when
the bed depth reaches a level where drying is of borderline effect.
Most of the dehydration of the gas takes place in the bottom few
inches of the bed, since the newly introduced gas contains its highest
concentration of water at that point. Thus, most of the heat of
hydration is generated in the lower end of the bed, but of course
this may vary with the design of the vessel, the flow rate of the
gas, the usual moisture content of the gas, and other factors. Where
CaCl.sub.2.6H.sub.2 O is present, the heat of solution is negative,
however. Accordingly, where calcium chloride is an important part
of the desiccant, we prefer to assure that heat is removed from
the upper regions of the desiccant bed as well as the lower, because
the upper regions will contain a higher ratio of CaCl.sub.2 CaCl.sub.2.H.sub.2
O, CaCl.sub.2.2H.sub.2 O, and CaCl.sub.2.4H.sub.2 O. It is known
that CaCl.sub.2.H.sub.2 O and CaCl.sub.2.2H.sub.2 O have higher
heats of solution than either anhydrous or the other hydrates of
Lithium chloride is also useful as a desiccant in our invention.
LiCl, LiCl.H.sub.2 O, and LiCl.2H.sub.2 O all have positive heats
of solution. Other inorganic salts may also be used, but mixtures
of calcium chloride and lithium chloride, in any ratio, are preferred
variations for the desiccant tablets, although we may also use either
by itself, and our invention applies to the use of any desiccant
tablets or other solids which generate a heat of solution.
Following are certain heats of solution, in kg.-cal./g.mole, as
taken from a standard chemical engineer's handbook:
CaCl.sub.2 +4.9 CaCl.sub.2 .multidot. H.sub.2 O +12.3 CaCl.sub.2
.multidot. 2 H.sub.2 O +12.5 CaCl.sub.2 .multidot. 4 H.sub.2 O +2.4
LiCl +8.66 LiCl .multidot. H.sub.2 O +4.45 LiCl .multidot. 2H.sub.2
Persons skilled in the art will appreciate that the most efficient
use of our invention will call for reaching the lowest part of the
sump area--that is, transferring at least some of the heat of hydration
to the lowest part--so the solution in the sump will be warmed even
when the solution level is relatively low, i.e. immediately after
the sump area has been drained.
Accordingly, then, our invention is a method of inhibiting the
formation of solids in a solution of desiccant salts obtained by
drying gas, comprising removing heat of hydration (we use this term
to include the heat of solution) during the gas drying and transferring
the heat of hydration to the solution.
We may use any suitable heat exchange fluid for the variation of
FIG. 2. An example of a suitable heat exchange fluid is a 50:50
solution of ethylene glycol and water. We prefer to use heat exchange
fluids containing at least some potassium formate. Solutions of
0.5% to 75% by weight of potassium formate may be used.
For the variation of FIG. 3 we may use any suitable metal or other
heat conducting rod, vein, or other device for connecting the lower
portion of the tablet bed with the sump area.
In connection with FIG. 4 it should be understood that the flow
rate of the gas in the vessel may vary and that its velocity may
range from 0.1 to 10 feet per second. The force generated by the
rotation of the propeller should overcome the viscosity of the solution,
and accordingly the practitioner may wish to adapt the device of
FIG. 4 to the peculiarities of his particular system. Ideally the
device of FIG. 4 will provide continuous agitation sufficient to
inhibit crystal and other solid formation under the full range of
conditions in the particular vessel, including variations in temperature,
gas flow, composition of the used solution, and concentration of
the desiccant salts in the used solution. Preferably the entire
propeller and mixing blade construction will be of materials resistant
to corrosion and scale deposition. We do not intend to be limited
to the illustrated method of agitation; any suitable means for agitating
may be used. In an appropriate situation, for example, wind power
may be used to agitate the solution in a sump, and of course an
ordinary electric motor could also provide agitation force.