Abstrict Supercritical CO.sub.2 is dried by use of a desiccant composition
comprising glycerine and bis-hydroxymethyl dioxane, especially a
by-product composition recovered from an effluent stream from the
production of epoxy resins.
Claims What is claimed is:
1. A process for reducing the moisture content of substantially
supercritical moist carbon dioxide which comprises contacting moist
carbon dioxide at substantially supercritical conditions of temperature
and pressure of from 70.degree. F. to 150.degree. F. and 1071 psia
to 2400 psia with a desiccant composition comprising glycerine containing
5%w to about 20% by weight of at least one bis-hydroxymethyl dioxane
based on the total weight of the composition and recovering substantially
supercritical carbon dioxide having reduced moisture content.
2. A process according to claim 1 wherein the concentration of
bis-hydroxymethyl dioxane is from about 10% w to about 20% w based
on the total weight of the composition.
3. A process according to claim 2 wherein the concentration of
bis-hydroxymethyl dioxane is from about 12% w to about 16% w based
on the total weight of the composition.
4. A process for the reduction of desiccant loss in the drying
of moist substantially supercritical carbon dioxide which comprises
contacting the moist substantially supercritical carbon dioxide
at substantially supercritical conditions of from 70.degree. F.
to 150.degree. F. and 1071 psia to 2400 psia with a desiccant composition
comprising glycerine containing 5%w to about 20% weight of bis-hydroxymethyl
dioxane based on the total weight of the composition and recovering
substantially supercritical carbon dioxide having reduced moisture
content.
5. A process according to claim 4 wherein the concentration of
bis-hydroxymethyl dioxane is from about 10% w to about 20% w based
on the total weight of the composition.
6. A process according to claim 5 wherein the concentration of
bis-hydroxymethyl dioxane is from about 12% w to about 16% w based
on the total weight of the composition.
Description FIELD OF THE INVENTION
The present invention relates to a desiccant composition and the
use thereof for drying supercritical carbon dioxide.
BACKGROUND OF THE INVENTION
Supercritical (fluid) carbon dioxide (CO.sub.2) is different from
conventional liquids and gases in that it is a dense fluid which
is compressible like a gas but unlike a gas is pumpable like a liquid.
It is desirable to pipeline CO.sub.2 as a supercritical single phase
fluid, for ultimate use, e.g., as an injection fluid in tertiary
oil recovery, and to have a dry fluid carbon dioxide to prevent
corrosion during the transport and utilization of supercritical
carbon dioxide as an injection fluid.
Generally, CO.sub.2 produced from wells is depressurized, demoisturized
in the gaseous state and repressurized at the field location for
pipelining in its supercritical state. This requires (1) sizable
on-site equipment for depressurization, demoisturizing, pressurizing
and solvent handling and (2) high energy demands. In the case of
supercritical CO.sub.2 most of the conventional desiccants used
for gases don't work well because supercritical CO.sub.2 is a very
good solvent, and itself undesirably dissolves many potential treating
solvents. Also, moisture removal is not possible unless the solvent
can form a separate non-miscible phase.
U.S. Pat. No. 4478612 describes a process for the desiccation
of supercritical CO.sub.2 using glycerol. However, commercial glycerol
is expensive and some of the glycerol is lost because of solubility
of glycerol in the supercritical CO.sub.2. In the above patent,
it is noted that the glycerol recycled from the regenerator is 99%
pure and the same is said for the prior use of triethylene glycol.
Clearly, use of such high grade desiccants can be expensive and
impractical at field locations.
Surprisingly, it has been found that certain compositions of glycerine
with certain dioxane derivatives are good desiccant compositions
for the removal of moisture from supercritical CO.sub.2 overcoming
disadvantages in the use of commercial glycerol. Moreover, the new
drying agents for supercritical CO.sub.2 can be obtained in an inexpensive
manner as the by-products of other processes, thus, reducing the
cost of desiccant but yet providing better desiccant properties
for the dehydration of supercritical CO.sub.2 than various highly
pure reagents of the prior art.
SUMMARY OF THE INVENTION
The present invention is directed to a novel desiccant composition
and the use thereof for drying substantially supercritical carbon
dioxide. More specifically, the present invention is directed to
a process for reducing the moisture content of substantially supercritical
moist carbon dioxide which comprises contacting moist carbon dioxide
at substantially supercritical conditions of temperature and pressure
with a desiccant composition comprising glycerine containing a minor
amount of up to about 20% by weight, of at least one bis-hydroxymethyl
dioxane based on the total weight of the composition, and recovering
substantially supercritical carbon dioxide having a reduced moisture
content.
The use of the desiccant composition of the present invention to
remove moisture from moist supercritical CO.sub.2 reduces corrosion
in the subsequent pipelining and use of supercritical CO.sub.2
and reduces the cost of using commercial glycerol and the loss thereof
because of solubility of glycerol in the supercritical CO.sub.2.
Also, because the solvent compositions of the present invention
have a lower viscosity than commercial glycerol, the mass transfer
is improved and it is easier to pump the desiccant. The solvent
compositions of the present invention also have a lower freezing
point than pure commercial glycerol and this can be useful in dehydration
at remote locations and in the transport of the solvent composition
in cold weather conditions.
Bis-hydroxymethyl dioxane is a dimer of glycerol (commonly referred
to as diglycerol) and includes the four isomer forms, i.e., cis-25-bis-dihydroxymethyl
dioxane, trans-25-bis-dihydroxymethyl dioxane, cis-26-bis-dihydroxymethyl
dioxane and trans-26-bis-dihydroxymethyl dioxane. These isomers,
which are not present to any degree in pure commercial glycerol,
are useful in the desiccant composition of the present invention
because their presence surprisingly alters the properties of the
desiccant in terms of properties which are practically important,
such as (1) a lower solubility in CO.sub.2 so that less of the desiccant
composition is lost in the attempts to dehydrate CO.sub.2 ; (2)
a lower freezing point of the novel desiccant of the invention,
which aids in the handling and use in cold weather; and (3) a lower
viscosity of the novel desiccant composition of the invention, which
aids in mass transfer during the dehydration and facilitates pumping.
Such changes in properties were not predictable from the prior art
teachings.
Carbon dioxide is commonly produced from subterranean reservoirs
near or above its critical point conditions of temperature and pressure
of 87.9.degree. F. and 1071 psia. For transmission, the temperature
and pressure of CO.sub.2 are maintained near or above its critical
point and CO.sub.2 is conveniently dehydrated in substantially its
supercritical form, that is near or above the critical point of
temperature and pressure. Preferably, it is dehydrated at from about
70.degree. F. to about 120.degree. F., and a pressures from about
1071 psia to about 2200 psia, and preferably from about 1200 psia
to about 2000 psia. Higher temperature up to about 150.degree. F.
and pressures up to about 2400 psia can be used, if desired.
DETAILED DESCRIPTION OF THE INVENTION
The novel desiccant composition for reducing the moisture content
of supercritical CO.sub.2 comprises glycerine and a minor amount
of at least one bis-hydroxymethyl dioxane. While the desiccant composition
can be synthetically prepared by forming an admixture of the desired
chemical composition of ingredients, this may be expensive except
for the ability it gives to tailor the desiccant composition to
the particular stream of moist supercritical CO.sub.2 to be dehydrated.
However, convenient and inexpensive sources of the desiccant composition
of the present invention are compositions of glycerine and bis-hydroxymethyl
dioxane, which are present in the effluent stream from the production
of epoxy resins. Methods of obtaining the compositions of the present
invention from the effluent streams from the production of epoxy
resins are known in the art, such as (1) the process of U.S. Pat.
No. 4560812 directed to the recovery of glycerine from saline
waters resulting from the manufacture of epoxy resins by a multi-step
process, including the use of electrodialysis, and (2) the process
of U.S. Pat. No. 4599178 directed to the recovery of glycerine
from saline water from the manufacture of epoxy resins by a multi-step
process, including extraction with pentanol. The disclosures of
which U.S. patents are incorporated herein by reference. The resulting
glycerine containing the diglycerols of the invention is dehydrated
to obtain a a salt-free product comprising about 80 to 95% glycerine
and about 5 to about 20% of the diglycerols of the invention.
The moist supercritical (fluid) CO.sub.2 is conveniently treated.
e.g., at field locations by contacting the supercritical fluid CO.sub.2
near or above its critical point of temperature and pressure with
the desiccant composition of the present invention. The moisture
loaded desiccant composition is removed from the contacting zone
and subjected to desorption or stripping or the like to unload the
moisture providing lean desiccant composition for recycle to the
contacting zone. Those skilled in the art of gas purification will
appreciate that there should be sufficient trays in the various
contacting and regeneration zones or units for the effects of the
process of the invention to be fully utilized and appreciated. The
terms "zones" or "units", as employed in the
present application, include, where suitable, the use of segmented
equipment operated in series, or the division of one unit or zone
into multiple units or zones because of size constraint, etc. For
example, a dehydration column can comprise two separate columns
in which the solution from the lower portion of the first column
can be introduced into the upper portion of the second column, the
supercritical CO.sub.2 from the upper portion of the first column
being fed into the lower portion of the second column. Parallel
operation of units or zones, is, of course, well within the scope
of the present invention.
As will be understood by those skilled in the art, the desiccant
compositions of the invention can contain other conventional materials
or additives used in the treating of moist supercritical CO.sub.2
for transport or oil recovery use, including corrosion inhibitors,
hydrate inhibitor and the like. The amount of moisture to be removed
from the moist supercritical CO.sub.2 is, of course, a matter of
individual choice depending on such factors as expense of materials,
logistics of the field location and ultimate use of the dried supercritical
CO.sub.2.
In one embodiment of the processes according to the present invention,
the concentration of bis-hydroxymethyl dioxane is from about 5%w
to about 20%w or conveniently from about 10%w to about 20%w based
on the total weight of the composition and, preferably, the concentration
of bis-hydroxymethyl dioxane is from about 12%w to about 16%w based
on the total weight of the composition.
In a further embodiment, the present invention is directed to a
process for the reduction of desiccant loss in the drying of moist
substantially supercritical carbon dioxide which comprises contacting
the moist substantially supercritical carbon dioxide with a desiccant
composition comprising glycerine containing up to about 20% by weight
of bis-hydroxymethyl dioxane based on the total weight of the composition
and recovering substantially supercritical carbon dioxide having
reduced moisture concentration.
A process for lowering the viscosity of glycerine desiccant in
the drying of moist substantially supercritical carbon dioxide which
comprises contacting the moist substantially supercritical carbon
dioxide with a desiccant composition comprising glycerine containing
up to about 20% by weight of bis-hydroxymethyl dioxane based on
the total weight of the composition and recovering substantially
supercritical carbon dioxide having reduced moisture concentration.
In another embodiment, the present invention is directed to a process
of reducing the freezing point of glycerine desiccant used in the
drying of moist substantially supercritical critical carbon dioxide
which comprises contacting the substantially supercritical carbon
dioxide with a desiccant composition comprising glycerine containing
up to about 20% by weight of bis-hydroxymethyl dioxane based on
the total weight of the composition and recovering substantially
supercritical carbon dioxide having reduced moisture content.
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