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.
1. 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.
2. Method of claim 1 wherein said desiccant salts comprise calcium
chloride and lithium chloride in a weight ratio of from 0-100% to
3. Method of claim 1 wherein said solid desiccant salts are in
a bed of tablets.
4. Method of claim 1 wherein said solution is in a sump area.
5. Method of claim 1 wherein said heat of hydration is transferred
to said solution through a heat exchange fluid.
6. Method of claim 1 wherein said heat of hydration is transferred
to said solution through metal elements.
7. Method of claim 1 including agitating said solution.
8. 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.
9. Method of claim 8 wherein said agitator is moved by the force
of said flowing gas.
10. Method of claim 8 wherein said agitator is moved by force of
11. 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.
12. Apparatus of claim 11 wherein said means for transferring heat
of hydration comprises at least one metal element for conducting
heat of hydration.
13. Apparatus of claim 11 wherein said means for transferring heat
of hydration comprises a vessel jacket for containing a heat exchange
14. Apparatus of claim 13 wherein said vessel jacket contains a
heat exchange fluid comprising potassium formate.
15. 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
16. Apparatus of claim 15 wherein said means for transferring heat
includes means for containing a solution comprising potassium formate.
17. Apparatus of claim 15 including means for agitating said used
18. Apparatus of claim 17 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.
19. Apparatus of claim 15 wherein said means for transferring heat
comprises metal rods.
20. Apparatus of claim 15 wherein said means for transferring heat
comprises a vessel jacket containing a heat transfer fluid.
 This application is based on and incorporates in its entirety
Provisional Application Ser. No. 60/192306 filed Mar. 27 2000
and claims the benefit of its filing date.
 This invention relates to gas drying. More particularly,
it relates to inhibiting the formation and deposition of solids
from used desiccant solutions.
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 the sump.
 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 materials.
 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.2O 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.2O, CaCl.sub.2.2H.sub.2O, and CaCl.sub.2.4H.sub.2O.
It is known that CaCl.sub.2.H.sub.2O and CaCl.sub.2.2H.sub.2O have
higher heats of solution than either anhydrous or the other hydrates
 Lithium chloride is also useful as a desiccant in our invention.
LiCl, LiCl.H.sub.2O, and LiCl.2H.sub.2O 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 beat of solution.
 Following are certain heats of solution, in kg.-cal./g.mole,
as taken from a standard chemical engineer's handbook:
1 CaCl.sub.2 +4.9 CaCl.sub.2 .multidot. H.sub.2O +12.3 CaCl.sub.2
.multidot. 2 H.sub.2O +12.5 CaCl.sub.2 .multidot. 4 H.sub.2O +2.4
LiCl +8.66 LiCl .multidot. H.sub.2O +4.45 LiCl .multidot. 2 H.sub.2O
 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.