Note: Descriptions are shown in the official language in which they were submitted.
3;26(~
This invention relates to the regeneration of adsorbents
and more particularly to a process for desorbing adsorbates from
polymeric adsorbents by dissolving the ads~rba~e in an inert solvent
maintained in a supercritical condition.
In the purification and removal of impurities from fluid
streams in many industrial processes an adsorbent is used to ad-
sorb the impurities from the fluid stream. Adsorbents may also
be used to separate components in a process and to isolate trace
impurities for quantitative analysis.
Thus, for example, small amounts of organics, both ali-
phatic and aromatic, have been removed by being adsorbed on ac-
tivated carbon in the treatment of wastewaters from industrial
processes. color bodies are adsorbed in the process of sugar
refining and impurities are removed from vinyl chloride streams
through adsorption. In petroleum cracking processes the high
surface area catalytic materials such as alumina, silica or like
materials, with or without such metals as nickel, cobalt, molyb-
denuM or tungsten deposited thereon, become contaminated b~
impurities which are adsorbed on them and, in some cases, chem-
ically reacted. In all such cases, the adsorbates must be period-
ically removed from the adsorbents.
~ number of inorganic adsorbents have been well known
for some time and they may generally be defined as solid phase
inorganic materials having very high surface area-to-weight ratios
; and exhibiting the ability to concentrate adsorbates on their
surfaces. Among the more commonly used inorganic adsorbents are
activated carbon, alumina, silica, and silicates. ~'-
The use of such inorganic adsorbents has normally in-
cluded one or more steps to effect their regeneration, i.e.,
the removal of all or a part of the adsorbate which has adhered
~.
. .
3Z606
-
to the surface of the adsorbent. If the adsorbate is a volatile
material, such regeneration may be accomplished by heating the
adsorbent to volatilize off the ads~rbate or by creating a vacuum
around the adsorbent. Volatilization with heating may be accom-
panied by reaction with some added reactant, e.g., oxygen to
oxidize adsorbed organic materials. It is, of course, apparent that
the less volatile adsorbates require higher temperatures to re-
move them in this manner and such temperatures may contribute
to the gradual thermal degradation of the adsorbent. Moreover,
any reactant added, such as oxygen, may chemically degrade such an
adsorbent as activated carbon, causing loss of usable capacity.
Such losses require that the adsorbent be periodically replaced.
Since the average lifetime of activated carbon is 10 to 30 regen-
erations, the loss necessitated by adsorbent replacement becomes a
significant fraction of the total operating cost. The use of a vac-
uum to remove adsorbates from an adsorbent requires the equipment
necessary to generate the required degree of evacuation and it is a
technique which is limited to only certain classes of adsorbates,
namely those which exhibit appreciable vapor pressure at temperatures
below their decomposition point. That is, many low volatility
solids ànd liquids will decompose before their vapor pressure
becomes appreciable. Finally, the use of high temperatures for
adsorbent regeneration requires a relatively high expenditure of
energy
Although activated carbon, as well as various inorganic
adsorbents are still widely use~ for many purposes, the develop-
ment of synthetic polymeric adsorbents in recent years has extended
the use of adsorbents in industrial processes to a much wider
range of applications than heretofore associated with activated
carbon. In some instances, pol~meric adsorben~s have replaced
-3-
~ ~3~Z~06
activated carbon, silica, alumina and the like. One of the
primary reas~ns for t~e rapidly expanding ~se of polymeric ad-
sorbents lies in the fact that liquids may be used to remove
the adsorbate from the polymeric adsorbent ~hrough the mechanism
of solvation or reaction. Since this liquid removal is normally
carried out under ambient conditions, many of the disadvantages
inherent in the regeneration of activated carbon, for exa~ple, can
be eliminated.
In regeneratinq the polymeric adsorbents, an organic
solvent such as methanol or isopropanol may be used. If the ad-
sorbate is a weak acid, a base may be used to react with it
to remove it; and, if the adsorbate is a weak base, an acid
may be used as a reactant. Finally, where adsorption is from an
ionic solution water may be used; and, where the adsorbate is a
volatile material, hot water or stream may be used.
By far, the most widely used technique for polymeric
adsorbent regeneration is solvent extractionO After loading the
adsorbate to the breakthrough point with the adsorbed species,
an appropriate organic solvent is passed through the polymeric
adsorbent bed to dissolve and extract the adsorbate. The cost of
solvents used for the regeneration of the polymeric adsorbates re-
quiresthat a hi~h percentage of the solvent be recovered. More-
over, many such solvents, whether in bulk or in small quantities,
cannot be disposed of without raising serious pollution problems.
In recovering and purifying such solvents for reuse, operational
factors are encountered which add considerably to the cost of such
recovery.
- In solvent re~eneration ~he solvent is used to displace
water (or other liquid fro~ which the impurity is removed) from the
adsorbate bed. This means that a solvent-water mixture is obtained
.
.. . .. : . . . :. .
æ6~
which must be separated in the solvent recovery process. Since
s~me ~f the more comm~n and inexpensive solvents which are most
effective for the regeneration of the polymeric adsorbents form
azeotropes with water, such azeotropes must be dealt with in sol-
vent recovery. In the distillation of a mixture which forms an
azeotrope one column is used to recover one component and the azeo-
trope. The azeotrope must then be sent to a second column operatin~
at either higher or lower pressures in order to recover the other
component in a purified form. Each of such columns requires a
large number of theoretical plates. It is therefore apparent that
althou~h the use of a solvent for the adsorbed species in the re-
generation of a polymeric adsorbent involves no new art, it presents
a serious economic problem. Indeed, the severity of the solvent
recovery problem often rules out the use of synthetic polymeric
resin adsorption unless the unpurified regenerating-solvent stream
can be recycled or otherwise used economically in a contiguous pro-
cess.
It would therefore be desirable to havè a process by
which adsorbates can effectively be removed or extracted from poly-
meric adsorbents efficiently and in which ~he solvent can be puri-
fied for recycling more economically than presently possible.
It is therefore a primary object of this invention to ;
provide an improved process for regenerating polymeric adsorbents.
It is another object to provide a process of the character des-
cribed based on the dissolution of adsorbates which makes possible
the efficient and economical recovery of the solvent used and, ~;
if desired, of the adsorbate. Still another object is to provide
such a process which is applicable to a wide range of polymeric
adsorbent-adsorbate combinations. A still further object is to pro
vide a process of the character ~escribed in which inevitable sol- '
- : . . ~
3Z~6
vent losses do not contribute a~ditional pollution problems.
Anot~e~ principaI object of this inventi~n is to pro-
vide improved process and apparatus for wastewater purification using a
polymeric adsorbent to remove organic impurities and an inert
solvent in the form of a supercritical fluid to desorb adsorbates
from the absorbent to regenerate it.
other objects of the invention will in part be obvious
and will in part be apparent hereinafter.
In the process of this invention polymeric adsorbents
are regenerated by desorbing adsorbates from them by dissolving
the adsorbates in a chemically inert solvent in the form of a
supercritical fluid. The adsorbent, with the adsorbate adhered
thereto, is contacted with a suitable supercritical fluid and
then the supercritical fluid containing the dissolved adsorbate
is subjected to a physical treatment which renders it a nonsol-
vent for at least a portion of the adsorbate, thus separating
the adsorbate and the supercritical fluid into two phases. Sub-
sequent to the separation of the adsorbate from the supercritical
fluid, the supercritical fluid, which may be described as being
in a nonsolvent state, is subjected to another physical treatment to
return it to the condition in which it is a solvent for the
adsorbate so that it may be recycled. Thus it may be said to
be in a solvent state prior to xecycling. The physical treatments
may constitute eithex alterin~ the temperature, the pressure or
both of the supercritical fluid. As an optional step, the adsor-
bate may be reacted with a reactant while dissolved in or mixed with
the supercritical fluid.
In the following detailed discussion the term l'non-
solvent state" is applied to the supercritical fluid to indicate
that it has a relatively low solubility for one or more adsor-
60~i
_.
bates, and the term "solvent state" is applied to the supercriticalfluid to indicate that it has a relatively high solubility for
one or more adsorbates. Thus these terms are not used in the
absolute sense, but in the relative sense.
In accordance with a specific embodiment, a process for
desorbing an adsorbate from a synthetic organic polymeric
adsorbent is characterized by contacting said synthetic organic
polymeric adsorbent with said adsorbate adsorbed thereon with a
supercritical fluid which is a solvent for said adsorbate to desorb
said adsorbate from said synthetic organic polymeric adsorbent and ~ -
to dissolve it in said supercritical fluid thereby to render said
adsorbent capable of adsorbing an additional quantity of said
adsorbate, the temperature of said supercritical fluid ranging
between about 1.01 and 1.3 times the critical temperature in
deg~ees K of said fluid and the pressure being above the critical ~ `
pressure of said fluid.
In accordance with a further embodiment of the
invention, there is provided, in a process for treating wastewater
in which at least one organic material is adsorbed on a synthetic
organic polymeric adsorbent and said synthetic organic polymeric
adsorbent is periodically regenerated by desorbing said organic~
material therefrom, the improvement comprising regenerating said
synthetic organic polymeric adsorbent by contacting said adsorbent
with said organic material with a supercritical fluid which is a
solvent for said organic material to desorb said organic material
from said adsorbent and to dissolve it in said supercritical fluid
thereby to rend~r said adsorbent capable of adsorbing an additional
quantity o~ said organic material, the temperature of said super-
critical fluid ranging between about 1.01 and 1.3 times the
critical temperature in degrees K of said fluid and the pressure ~-
being above the critical pressure of said fluid.
:,
~ ~ ~ 7 ~
. , . . . . . . . . ~ . . , -: .
6EJI~;
In accordance with a further embodiment oE the
invention, a process for treating water to remove at least one
organic material therefrom, comprises the steps of (a)
contacting a synthetic organic polymeric adsorbent with a stream
of water containing said organic material under conditions such
that said organic material is adsorbed as an adsorbate on said
synthetic organic polymeric adsorbent; (b) then contacting said ~ .
synthetic organic polymeric adsorbent with said organic material
adsorbed thereon with a supercritical fluid which is a solvent `
for said organic material to desorb said organic material and to
dissolve it in sàid supercritical fluid, thereby to render said ~:
adsorbent capable of adsorbing an additional quantity of said
organic material, said supercritical,fluid during said contacting
being at a temperature between about 1.01 and about 1.3 times its
critical temperature in dqgrees K and at a pressure above its
critical pressure; (c) separating said supercritical fluid .
with said dissolved organic material from said adsorbent; (d) . :
~ subjecting said supercritical fluid containing said organic -
: material dissolved therein to physical treatment which renders
said fluid a nonsolvent for said organic material thereby to
form a two-phase system comprising said fluid in a nonsolvent s~ate
and said organic material; (e) separating the resulting two-phase ~.
system into non-solvent state fluid and organic material; and (f)
subjecting said nonsolvent state fluid subs:equent to said separat- ~ :~
ing step to a physical treatment which converts it to a solvent
state supercritical fluid making'it a solvent for said organic ~ .
material.
The invention accordingly comprises the several steps -~
. and the relation of one or more of such steps with respect to
each of the others thereof, which will be exemplified in the
proces;s hereinafter disclosed, and the scope of the invention will
60~;
:`
be indicated in the claims.
For a fuller understanding of the nature and objects
of the invention, reference should be had to the following detailed
description taken in connection with the accompanying drawings in
which:
Fig. 1 is a graph of naphthalene solubility in carbon
dioxide over a temperature range from 35C to 55C for selected
pressures;
Fig. 2 is a flow diagram illustrating one embodiment
of the process and apparatus for regenerating adsorbents according
to this invention
Fig. 3 illustrates in diagrammatic fashion a modifica-
tion of the process and apparatus of FigO 2 to include the reaction
of the adsorbate with a chemical reàctant while dissolved in the
supercritical fluid;
Fig. 4 is a flow diagram illustrating another embodiment
` of the process and apparatus of this invention;
Fig. 5 is a flow diagram of a wastewater treatment
system incorporating the process and apparatus of this invention
for regenerating a polymeric adsorbent used therein, and
Fig. 6 is a modification of the apparatus of Fig. 5.
The commercially available polymeric adsorbents may be
described as hard, insoluble, high surface area, porous polymers.
- 7b -
.
~Lal82fj~6
Typically, they are provided in spherical form with a nominal
mesh size of about 16 to 50. They are available in a variety
of polarities and surface characteristics ~hus making it possible
to use them as adsorbents in a wide range of applications. For
example, the polymeric adsorbents may be polymers of styrene,
copolymers of styrene and divinylbenzene, or polymers containin~ acry-
lic esters, trimethylolpropane trimethacrylate or trimethylolpropane
dimethacrylate. (See for example R. M. Simpson, "The Separation of
Organic Chemicals from water" presented at the Third Symposiu~
of the Institute of Advanced Sanitation Research, International
on April 13, 1972, wherein exemplary chemical structures for
polymeric adsorbents are given. See also German Offenlegungsschrift
1,943,807.)
The polymeric adsorbents have found many varied appli-
cations in wastewater treatments. For example, they have been
used to decolorize kraft pulp mill bleaching effluent and dye
wastes and to remove pesticides from waste streams, alkylbenzene
sulfonate or linear alkyl sulfonate-type surfactants from waste-
waters and explosives such as TNT and DNT from effluent streams.
These polymeric adsorbents have also been used in analysis procedures
for determining tracè amounts(as little as parts ber billion) of
organic contaminants in water, in chemical processing and in iso-
lating enzymes and proteins as well as other biological materials
such as Vitamin B-12, tetracycline, oxytetracycline and oleandomycin.
Exemplary of the pesticides which can be removed by
adsorption on a polymeric adsorbent from a waste stream are Lindane,
DDT and Malathion and pesticides ingredients such as endrin,
heptachlor and other chlorinated hydrocarbon intermediates.
Exemplary of the organics which may be removed from
a water stream using polymeric adsorbents are those listed in
-~
-8-
: '
., ,; ~, . .. ,. , ,, ., . , . . ~ , :
3Z6~)6
Table 1 as repor~ed by Junk et al, Journal of Chromatography,
99 745-762 (1974~. The resins used were two different polystyrenes
characterized as having 42% and 51% helium porosity, surface areas
of 330 and 750 m2/gram, average pore diameters of 90 and 50 A,
s~eletal densities of 1.08 and 1.09 grams/cc, (respectively) and
nominal mesh size of 20 to 50. (Sold as X~D-2 and XAD-~ by Rohm
and Haas Company).
Table 1
Organics Removable ~rom A Water Stream
By Adsorption on Polymeric Adsorbents
Alcohols
Hexyl
2-Ethylhexanol
2-Octanol
Decyl
Dodecyl
Benzyl
Cinnamyl
2-Phenoxyethanol
Aldehydes and ketones
2,6-Dimethyl-4-heptanOne
2-VndeCanone ~'
Acetophenone
Benzophenone
Benzil
Benzaldehyde
Salicylaldehyde
Esters
Benzyl acetate
Dimethoxyethyl phthalate
Dimethyl phthalate
Diethyl phthalate
Dibutyl phthalate
Di-2-ethylhexyl phthalate
Diethyl fumarate
Dibutyl fumarate
Di-2-ethylhexyl fumarate
Diethyl malonate
Methyl benzoate
Methyl decanoate
Methyl octanoate
; Methyl palmitate
Methyl salicylate
Methyl methacrylate
Polynuclear aromatics
Naphthalene : .
2-Methylnaphthalene
l-Methylnaphthalene
. ,.
- _ 9 _ .
,. . . .. . . . . . ..
~8~
Table 1 (Continued)
Biphenyl
Fluorene -
Anthracene
Acenaphthene
Tetrahydronaphthalene
Al~yl benzenes
Ethylbenzene
Cumene
p-Cymene
Acids (acidified)
Octaonic
Decanoic
Palmitic
Oleic
Benzoic
Phenols
Phenol
o-Cresol
3,5-Xylenol
o-Chlorophenol
p-Chlorophenol
2,4,6-Trichlorophenol
l-Naphthol
Ethers
Hexyl
Benzyl
Anisole
2-Methoxynaphthalene
Phenyl
Halogen compounds
Benzyl chloride
Chlorobenzene
lodobenzene
o-Dichlorobenzene
m-Dichlorobenzene
1,2,4,5-Tetrachlorobenzene
~-o-Dichlorotoluene
m-Chlorotoluene
2,4-Dichlorotoluene
1,2,4-TrichIorobenzene
Nitrogene compounds
Hexadecylamine
Nitrobenzene
Indole
o-Nitrotoluene
N-Methylaniline .
Benzothiazole .
Quinoline .
Isoquinoline
~enzonitrile
Benzoxazole
: . . '
.
-10 - ,
.
,
'
... .. . . .: . .. . . .. .
o~
As noted above, the polymeric adsorbents are regenerated
by dissolving of~ the adsorbate when the adsorbent bed has reached
a predetermined point of saturation, normally referred to as the
breakthrough point and defined as that point when the stream dis-
charged from the bed contains a preset level of the adsorbate.
As also previously noted, this removal of the adsorbate has pre-
viously been accomplished by using an organic liquid solvent, such
as methanol or isopropanol, under ambient temperature and pres-
sure, and has included a costly solvent recovery procedure.
According to the process of this invention, a super-
critical fluid is used for adsorbent regeneration, a process which
re~uires no phase changes of the solvent and hence requires mini-
mum e~penditure of energy. This process also provides the possi-
bility of attaining efficient recovery of the adsorbate if this
is desired.
It is a well-known phenomenon that when certain gases
are subjected to a specified pressure and maintained above a cer-
tain temperature they reach a supercritical state. Broadly, this
supercritical state as the term is used herein may be defined as
~0 the region of temperature and pressure above the critical temp-
erature and critical pressure of the compound. Although the term
"supercritical" is applied generally to all temperatures above
the critical state, it will be used hereinafter to refer ~o a gas
which is at a pressure above its critical pressure and at a temP-
erature up to about 1.3 times critical temperature in K. In most
cases, however, it will be preferable to use temperatures no
greater than about 1.1 times critical temperature in K.
Supercritical fluids have for some time been recognized
as solvents for a number of different types of ma~erials, among
30 which may be listed aliphatic and aromatic hydrocarbons; organo- ~
:'. .
"
~o~o~
metallics such as metal alkyls and alcoholates, silicones and
boroalkyls; organic esters of in~rganic acids such as sulfuric
and phosphoric; and organosilicons. (See for example Chem. Rev.
44, 477-513 (1949); Tran. ~araday Soc. 49, 1401-1405 (1953),
and AIChE Journal 1, 20-25 (1355)). Generally, gases of diferent
chemical nature but with similar physical properties b~have
similarly as solvents in the supercritical state. (See for
example, "The Principles of Gas Extraction" by P.F.M. Paul and
W.S. Wise, Mills and Boon Ltd., London, 1971).
Supercritical fluids have been used to separate organic
mixtures (U.S. Patent 3,843,824, British Patents 1,057,911,
1,111,422, 1,346,134 and 1,400,098, and French Patents 1,512,060,
and 1,512,061) and to extract volatiles from coal (U.S. Patent
2,664,390 and U.S. Defensive Publication T 861,044) and oil from
shale (U.S. Defensive Publication T861,027).
Among those gases which may be converted to supercritical
fluids at temperatures and pressures commonly used industrially
are hydrocarbons such as methane, ethane, propane, butane, pentane,
hexane, ethylene, and propylene; halogenated hydrocarbons such
as hexachloroethane and other haloethanes and halomethanes; and
inorganics such as carbon dioxide, ammonia, sulfur dioxide, ni-
trous oxide, hydrogen chloride and hydrogen sulfide. Suitable
mixtures of these gases may also be used. In the process of
this invention, carbon dioxide has been found to be particularly
suitable for removing organic adsorbates from polymeric adsor-
bents. The critical temperature of carbon dioxide is 304.2K
(31.0C) and critical pressure is 72.9 atmospheres. The temper-
atures of its most advantageous condition of use for the practice
of this invention is between about 1.01 and 1.1 times the critical
temperature or between about 308K and 335K (34C and 62C).
-12-
.:
,' '
~ . . . : ., :.: - - . , ... . .. . : .
This temperature range is not far above ambient temperatures.
Moreover, carbon dioxide has several ~ther advantages among which
are the fact that it is nonpolluting to the atmosphere and that
it is inexpensive.
The critical temperatures and pressures for some se-
lected fluids are listed below in Table 2:
Table 2
Critical Properties for Selected Fluids
Fluid T ,K PC,At~
C2 304.2 72.9
3 405.5 111.3
H20 647.6 226.8
~lethanol 513.7 78.9
Ethanol 516.6 63.0
Isopropanol 508.5 47.0
Ethane 305.6 48.3
Nitrous oxide 309.7 71.4
n-Propane 370.0 42.0
n-Butane 42S.2 37.5
n-Pentane 469.8 33.3
n-Hexane 507.4 29.6
n-Heptane 540.2 27.0
2,3-Dimethylbutane 500.0 31.0 - -
Benzene 562.1 48.3
Dichlorodifluoromethane 384.9 39.4
Dichlorofluoromethane 451.7 51.0 `
Trichlorofluoromethane 469.8 41.7
Dichlorotetrafluoro- 419.3 35.5 -~
ethane
Chlorotrifluoromethane 302.0 39.0
3Q Ethylene 282.9 50.9
13-
, . . . . . . . .
illD82606
In choosing a supercritical fluid for the regeneration
of a polymeric adsorbent containing one or more organic species
adsorbed thereon, the supercritical f~uid must be a solvent for
the species to be removed and it must be a fluid which does not
react with the surface of the adsorbent.
For any one adsorbate/supercritical fluid system, the
phyQical changes to which the supercritical fluid must be subject-
ed to render it first a solvent for the adsorbate and then a non~
solvent for it will, of course, depend upon the solubility of the
adsorbate in the supercritical fluid under different ~emperatures
and pressures. Exemplary of such a system is naphthalene/super-
critical carbon dioxide. Fig. 1 is a graph of naphthalene solubil-
ity in carbon dioxide over a temperature xange from 35C to 55C
for selected pressures (taken from Yu. V. Tsekhanskaya, M.B. Iom-
tev and E.V. Mushkina, Zh. Fiz. Xhim., 38, 2166 (1964)). This
graph of Fig. 1 shows that dissolved organic compounds, such as
naphthalene, can be separated from the supercritical carbon diox-
ide solvent by either: (a) temperature-cycling at constant pres-
sure (e.g., at 300 atmospheres, absorbing at 55C and precipita-
ting at 35C; or at 80 atmospheres absorbing at 35C and precipi-
tating at 45C); (b) pressure-cycling at constant temperature
(e.g., at 55~C, absorbing at 300 atmospheres and precipitating at
80 atmospheres); or (c) a combination of temperature/pxessure
cycling.
The method and apparatus of this invention are illustra-
ted in somewha~ diagrammatic form in Fig. 2 using carbon dioxide
as the supercritical fluid to remove hydrocarbon impurities adsorb-
ed on a polymeric adsorbent. In the apparatus of Fig. 2 it will
be assumed that spent polymeric adsorbent in the form of a trans-
portable finely divided particulate ~aterial is derived from an ad-
~orbent storage tank ~see Fig. 5) and introduced at essentially am-
bient tempPrature and pressure into desorption column 10 (serving
-14_
as fluid contacting means) by way of line 11, the in-flow of
spent polymeric adsorbent being c~ntr~lled by high-pressure on-off
valve 12. The desorption column 10 must be constructed to be able
to withstand the highest pressure to whic~l the supercritical fluid
is raised. Thus,for example,desorption column 10 may be any
suitably designed pressure vessel. ~lthough the process shown
in Fi~. 2 is a batch process, it is, of course, within the
scope of this invention to use a continuous process if desired.
In such a batch process as this the spent polymeric adsorbent
is charged to desorp~ion column 10 in batches, followed by
pressurization and then desorption bv the circulation of super-
critical fluid therethrough.
Desorption column 10 also has a regenerated adsorbent
discharge line 13 with high-pressure valve 14; and a gas purge
inlet line 15 with high-pressure valve 16 and gas purqe discharge
line 17 with high-pressure valve 18~
The supercritical fluid for regeneration, e.g., carbon
dioxide at 300 atmospheres pressure and 35C is stored in a storage ~`
tank 20 and is periodically introduced into desorption column 10
through high-pressure line 21 having a hish pressure fluid flo~
control valve 22. Subsequent to its contacting the polymeric
adsorbent in column 10, the supercritical fluid having adsorbate,
e.g., organic materials, dissolved therein is withdrawn from column
10 through high-pressure line 23, the fluid flow through which
is controlled by high-pressure on-off valve 24.
In the embodiment of this invention illustrated in Fig. 2
the physical treatment to which ~he supercri~ical fluid is sub-
jected to render it a nonsolvent for ~he adsorbates is that of
decreasing its pressure. This is done by expanding the super-
critical fluid in a suitable expander, e.g., a ~urbo expander 25
-15-
- . :, , . . ., . ~ . , ~
606
or a pressure let-down valve. In this example usi~g carbon diox-
ide at 3D0 atmospheres and 35C, suc~ expansion will typically
reduce the pressure to about 80 atmospheres. Such expansion
may reduce the temperature as in the case of the example illus-
trated in Fig. 2. In any event/ it ~a~ be necessary to adjust
the temperature of the fluid so that it has minimum 501ubility
for the adsorbates prior to recycling it. Thus Fig. 2 illustrates
heat exchangers 26 and 38 in the flu-d lines for effecting tem-
perature adjustment.
The decrease in pressure experienced by the super-
critical fluid brought about through expansion in expander 25 and
any variation in temperature brought about in heat exchanger 26
renders the supercritical fluid a nonsolvent for the adsorbate.
Thus the adsorbate appears as a separate phase and there resu]ts
a two-phase fluid which is taken to phase separator 29. This
sep rator may be, for example, a cyclone separator or a holding
tank. The adsorbate is withdrawn from phase separa~or 29 throu~h
adsorbate drawoff line 30 having valve 31 and the supercritical fluid
is discharged from the separator 29 throush discharge line 32 and
valve 33. In the case of this particular example of carbon dioxide,
the temperature of the carbon dioxide is raised to about 35C in
heat exchanger 26. However, since the pressure is only about 00
atmospheres, it is necessary to compress it to return it to that
condition under which it is a solvent for the adsorbate.
This is done by compressing it in a compressor 35 which brings
it back up to 300 atmospheres pressure and about 95Co Since ~his
temperature is considerably above the desired 35C for supercritical
carbon dioxide in this example, ~he supercritical fluid is used
in heat exchanger 26 to heat the fluid discharged from expander ~`
25. Thus ~he one side of heat exchanger 26 is part of supercritical
-16-
:
b ~ . . . .
~Z606
flow line 37 connecting compressor 35 with storage tank 20. In
this example usin~ carbon dioxide, the temperature of the super-
critical fluid as it leaves heat exchanger 26 may still be above
the desired 35~C so that a second heat exchanger 38 is provided
to effect heat exchange between the supercritical fluid and an
externally-supplied coolant, e.g., water, flowing in coils 40.
Thus the supercritical fluid reaches stora~e tank 20 in a solvent
condition to be recycled.
Although the supercritical fluid may be used as a
solvent for the adsorbates at temperatures and pressures above
its supercritical temperature and pressure, it is generally
preferable to maintain its temperature no greater than about
1.1 times the critical temperature in ~ of ~he fluid and even more ;
preferable in some cases to maintain its temperature during solva-
tion no greater than about lO~K above the critical temperature.
The maximum pressure will be determined by the high-pressure
capabilities of the equipment used. Generally, the higher pres-
sures will be preferable to enhance the solubility of the adsorbate. ~ ``
In choosing the physical conditions to render the supercritical
fluid a nonsolvent for the adsorbate it will generally be desir-
able to alter the pressure and/or temperature of the supercritical
fluid no more than is necessary to separate out the adsorbate.
It may be desirable in some cases to alter the chemical
nature, and hence physical proper~ies, of the adsorbate sub~
sequent to its removal from the adsorbent. ~'his may be done by
reacting the adsorbate with a suitable reactant while it is
dissolved in or mixed with the supercritical fluid. Thus in
Fig. 3, in which the same reference numerals are used to identify
the same components of Fig. 2, there is shown a reaction chamber 45
into which a reactant for the adsorbate (e.g., oxygen to react
with organic materials) is introduced through line 46 and valve 47.
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~108~606
The supercritical fluid is then withdrawn through line 23a, which
is an extension of line 23, and valve 48. If, as shown in Fig. 3,
the reactant is introduced into the supercritical fluid while it
is in its solvent state, then reaction chamber 45 and all the lines
and valves in them must be capable of handling high pressures.
Alternatively, the reaction chamber may be associated with line
28 in the same manner as shown in Fi~. 3 for line 23 such that
the reaction may be carried out in the two-phase system wherein
the supercritical fluid is in its nonsolvent state. Any reactant
used for the adsorbate must, of course, not be a reactant for
the supercritical fluid in its state in which the reactant is
introduced.
Fig. 4 illustrates another embodiment o~ the process
and apparatus for treating the supercritical fluid to render it ...
a nonsolvent for the adsorbate and then to recon~ert it as a
solvent for recycling. In the embodiment of Fig. 4 these treating
steps are limited to changing the temperature of the supercritical
fluid to.effect phase separations and it is assumed for the sake
of illustration that the adsorbate constitutes at least two dis-
tinct chemical species which exhibit different solubilities in
the supercritical fluid. In Fig. 4 like reference numerals refer
to like components in Fig. 2.
The apparatus of Fig. 4 pro~ides for a multiple-step
s~paration brought about through two successive increases (or
decreases) in temperature~ Although two steps are illustrated, any
suitable number may be employed. Thus the supercritical fluid
containing all of the adsorbate dissolved therein is carried through .
line 23 into a first heat exchanger 50 where it is heated (or
cooled) through indirect heat exchange with an externally-supnlied ..
heat transfer fluid in line 51 to a first higher (or lower) tem-
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perature which is sufficient to separate out a first fraction
of the adsorbate. Thus, in e~fect, the supercritical fluid is con-
verted to a nonsolvent state for this first fraction of adsorbate
but remains in a solvent state for any succeeding fractions of
the adsorbate. Through line 52 this first two-phase liquid is
taken to a first phase separator 53 from which the first adsorbate
fraction is discharged through line 54 and valve 55. Then the
:
supercritical fluid containing the second adsorbate fraction
dissolved therein is directed through line 56 into a second heat
exchanger 57 where it is heated (or cooled) further by an exter-
nally-supplied heat transfer fluid in line 58. This heating sep-
erates out the second adsorbate fraction and the resulting two-phase
fluid is carried by line 59 into a second phase separator 60 from
which the second adsorbate fraction is ~ischarged throush line
61 and valve 62. The high-temperature supercritical fluid is taken
by line 63 to a heat exchanger where it is cooled (or heated)
by indirect heat exchanye with a coolant (or heating fluid) exter-
nally supplied through line 65. The supercritical fluid at the
desired temperature and pressure (i.e., in the solvent state for
all adsorbates) is then transferred through line 66 to storage
tank 20.
The use of temperature alone to render the supercritical
fluid a nonsolvent for the adsorbate requires that high~pressure
equipment be used throughout. Thus the embodiment of Fig. 4 is
more desirable in those cases where supercritical pressures are
relatively low and where the solubility of the adsor~ate is relatively
sensitive to temperature.
It is, o~ course, within the scope of this invention
to eliminate the second heat transfer step shown in Fig. 4 and
perform only one separation step through cllanging the temperature.
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It is also within the scope of this invention to carry out the
depressurizing of the s~percritical fluid as shown in Pig 2 in
more than one stage, thus effecting the separation of more than
one adsorbate. It is also within the scope of this invention to
incorporate a reaction step as illustrated in Fig. 3 into the pro-
cess of Fig. 4.
The incorporation of the adsorbent regeneration process
of this invention into a wastewater purification system such as
one of those detailed above and usin~ a polymeric adsorbent is
illustrated diagrammatically in Pig. 5. The apparatus of Fig. 2
is employed; and since like reference numerals have been used to
describe like components the description of the circulation of the
supercritical fluid need not be repeated.
Fi~. 5 illustrates the use of two alternating adsorption
columns 70 and 71 which are cycled so that while one is in use
the other may be regenerated. This is, of course, a well-known
arrangement and any suitable number of a~sorbent columns may be
used in parallel as well as in series. The wastewater to be pur-
ified is introduced through line 72 into column 70 or 71, depend-
ing upon whether valve 73 or 74 is open. Assume that column 70
is on stream, valve 73 will be open and valve 74 closed~ Column
70 is packed with the appropriate polymeric adsorbent to adsorb
impurities and the treated water is discharged through line 75 ;;
and valve 76. Since column 71 is off stream and is being readied
~or use, valve 77 in water discharge line 78 is closed. ~nder
these conditions, spent polymeric adsorbent line 79 and its assoc~
iated valve 80 of column 70 are closed; while spent polymeric
adsorbent line 81 and its associated valve 82 of column 71 are
open to enable pump 83 to transfer spent adsorbent with its `
adsorbate adhered to its surface to the spent adsorbent drain and
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~O~Z6~6
feed tank 84 from where it is introduced periodically into con-
tacting c~lumn lO for regeneration as described above in con-
junction with the discussion of Fig. 2. Slurry pumps or water
eductors to function as pump 83 are well known.
Normally the polymeric a~sorbent will not be dried
prior to desorption since water can be removed by the solvent
supercritical carbon dioxide and subse~uently separated from it by
appropriate variations in temperature and/or pressure. However, it
may be desirable in some cases to remove residual water from the
adsorbent prior to regeneration with supercritical fluid. If so,
prior to the extraction of adsorbate fro~ the adsorbent in contact-
ing column 10, valves 22 and 24 are closed and valves 16 and 18
are opened to permit a drying gas, e.g., hot air, to pass over the
spent adsorbent to remove residual water. Then carbon dioxide
at atmospheric pressure is passed up through the dried spent ad-
sorbent to remove any air remaining in the pores of the spent ad-
sorbent. Valves 16 and 18 are then closed and valves 22 and 24
are opened; and regeneration is effected as previously described.
Then with valves 12, 22, 23, 15 and 18 closed, valve 14 is opened
and the regenerated polymeric adsorbent is transferred by line 13
into adsorbent storage tank 85 from which it may be transferred b~
pump 86 into the off-stream column 71. Thus the regenerated ad-
sorbent is taken through line 87, controlled by ~alve 88, into
column 71; and when column 70 is off stream the regenerated ad-
; sorbent is introduced into it through line 89, controlled by valve 90.
Fig. 6 illustrates a modification of the apparatus shownin Fig. 5. In the arrangement of ~ig. 6, separate desorption
column 10, along with the conduit means for transferring spent ad-
sorbent into and withdrawing regenerate~ absorbent therefrom is
3~ eliminated. Instead, desorption is accomplished directly in columns
' ~ ''
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.
6~
70 and 71, in which case these columns are vessels capable of
withstanding the pressure required to maintain the supercritical
fluid supplied from storage tank 20 in its solvent state. Fluid
conduit 23, through which the supercritical fluid containing the
adsorbate is alternately withdrawn from columns 70 and 71, is
branched into conduits 23a and 23b having valves 24a and 24b,
respectively. Likewise, fluid conduit 21, throu~h which supercriti-
cal fluid is supplied alternately to columns 70 and 71, is branched
into conduits 21a and 21b having valves 22a and 22b, respectively.
The process of this invention may be further described
with reference to ~he schematic of Fig. 5, using phenol ~nd 2,4,-
6-trichlorophenol as the adsorbates, a polymeric resin as the ad-
sorbent and supercritical carbon dioxide as the solvent. Exemplary
of such a resin is a polystyrene characterized as having a porosity
volume of 51~, a true wet density of 1.02 grams/cc, a surface area
of 780m2/gram, an average pore diameter of 50A, a skeletal density
of 1.08 grams/cc and a nominal mesh size of 20 to 50. At a water
flow rate of 0.5 gallons per minute per cubic foot this poly-
. . .
meric adsorbent adsorbs about 0.78 pounds per cubic foo~ of phenol
with zero percent leakage from a concentration of 250 ppm in the
water feed. This adsorbent also adsorbs up 12 pounds per cubic
foot of trichlorophenol with.zero percent leakage at the same flo~
rate. Regeneration of the adsorbent can be carried out by using 'f
supercritical carbon dioxide under the conditions described above
in connection with the discussion of Pig. 2 when the breakthrough
point in adsorption is detected. The supercritical carbon~dloxide
can be used at 35C and 300 atmospheres to dissolve the phenol and
trichlorophenol from the polymeric adsorbent and it can then be
converted to the nonsolvent state for the adsorbates by reducing
.
~ 30 the pressure to 80 atmospheres and/or increasing the temperature to
.
22-
08~06
45C. The presence of phenol in the carbon dioxide may be detected
through ultraviolet detection means. The resultinq reactivated
adsorbent can then again be contacted with additional aqueous
phenol/trichorophenol solution.
By using a supercritical fluid to dissolve off the
adsorbates from a polvmeric adsorbent, the adsorbent is not subjected
to any appreciable thermal or chemical degradation and the ad-
sorbed species may be recovered if ~esired. Moreover, it is
possible to use such supercritical fluids as carbon dioxide, ethane
or ethylene which require temperatures and pressures well within
the capabilities of existing equipment. Finally, these fluids
tand particularly carbon dioxide) are inexpensive, a fact which
contributes materially to the economics of industrial processes
and wastewater purification.
Organic impurities in was~ewater in trace amounts may
be detected and amounts as low as parts per billion may be measured.
Subsequent to the removal of the impurities from a water stream
by adsorption on a polymeric adsorbent they are dissolved in a
supercritical fluid in accordance with the process of this invention.
Since complete separation of the adsorbate impurities from the
supercritical fluid is readily accomplished without effec~ing any
chemical or physical change in the adsorbate, well-known analytical
techniques may then be employed to determine precisely the amounts
of the impurities in a given sample. ~`
It will thus be seen that the ob~ects set forth above, ~-
among those made apparent from the preceding description, are
èfficiently attained and, since certain changes may be made in
carrying out the above process without departing from the scope
of the invention, it is intended that all matter contained in the
3Q above description or shown in the acco~panying drawings shall be
interpreted as illustrative and not in a limiting sense.
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