thfrecovery, biotransformation
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Recovery of Tetrahydrofuran (THF)
Product Information
DuPont Terathane
®
Products
Foreword
Reclamation of solvent from waste air or liquid
streams for reuse is often essential to the economic
success of an industrial process. This operation is
widely known as solvent recovery and is the subject
of this publication.
Tetrahydrofuran can be recovered safely and
efficiently on an industrial scale by conventional
methods from operations such as topcoating,
printing, film casting, and chemical processes in
which it is used as a reaction medium. It may be
recovered alone or in mixtures with other solvents.
Overall recovery efficiencies at plants where THF
is being recovered range from 85 to 97%. THF loss
during adsorption, drying, and separation varies
with the number of solvents involved, but seldom
exceeds 3% in a properly designed and operated
recovery system. The greatest factor influencing
the efficiency of recovery from air streams is the
efficiency of vapor collection, which in turn is
strongly influenced by the degree of enclosure
permitted by the operation.
The physical properties of tetrahydrofuran that are
generally of importance in the design of recovery
systems are listed in
Table 1
. Detailed information
is given on the recovery of THF from air streams,
separation of THF from its water azeotrope, and
separation of THF from solvent mixtures.
Table 1
Physical Properties of Tetrahydrofuran*
Boiling Point at 760 mmHg,
°
C (
°
F)
66 (151)
Specific Gravity, 20/4
°
C (68/39
°
F)
0.888
Weight, lb/gal at 20
°
C (68
°
F)
7.4
20
D
Refractive Index, n
1.4073
Latent Heat of Vaporization,
cal/g, 66
°
C (151
°
F)
95
Flash Point (Tag Closed Cup),
°
C (
°
F)
–20 (–4)
Flammability Limits, vol% in air,
25
°
C (77
°
F), Lower
2
Upper
11.8
Miscibility: water; esters, ketones;
alcohols; diethyl ether; aliphatic,
aromatic, and chlorinated hydrocarbons
¥
* These properties are drawn from various DuPont and other
sources. DuPont does not make any express or implied
warranty that future production will demonstrate or
continue to possess these typical properties.
economical operation. Adsorption and absorption
systems for the recovery of THF vapors are out-
lined in the following section.
Adsorption
Tetrahydrofuran vapor may be recovered in com-
mercially available fixed-bed adsorption units
charged with gas-adsorbent grade of activated
carbon.
1,2
The vapor-air mixture is generally drawn
by a fan through a filter to remove all solid par-
ticles, then cooled and blown through a bed of
activated carbon that adsorbs the vapor and allows
the stripped air to pass through. Typical commer-
cial units provide two adsorbers connected in
parallel to permit continuous solvent recovery.
When one unit becomes saturated with solvent,
the vapor-air flow is switched to the other adsorber.
Solvent is removed from the saturated unit by
passing low pressure steam through the carbon bed
countercurrent to the direction of vapor-air flow.
The steam revaporizes the solvent and carries it
from the adsorber. A flow sheet for the recovery
of THF vapor from air by adsorption is shown in
Figure 1
.
Recovery of Tetrahydrofuran
from Mixtures with Air
Tetrahydrofuran can be recovered safely and
efficiently from lean solvent vapor-air mixtures by
ordinary solvent recovery procedures such as
adsorption and absorption. Adsorption on activated
carbon is the most widely used method because of
its versatility and efficiency. Absorption is seldom
used but can be employed where installation of
carbon beds is not feasible. Recovery of THF by
condensation is not generally applicable due to the
high solvent vapor concentrations required for
Figure 1.
Recovery of Tetrahydrofuran by Carbon Adsorption
VENT RETURN
CONDEN
SERS
EXHAUST
AIR
THF-WATER
AZEOTROPE
ADSORBER NO. 1
ACTIVATED
CARBON
STRIPPER
STILL
VAPOR-
LADEN AIR
L.P.
ST
EAM
ADSORBER NO. 2
FILTER
& COOLER
BLOWER
& MOTOR
L.P.
STEAM
EFFLUENT
TO WASTE
The concentration of THF vapor in exhaust air
streams from drying ovens is normally maintained
below 1% by volume to avoid approaching the
explosive range, 2.0 to 11.8% by volume in air.
Because of this low concentration and the substan-
tial volumes of vapor-air mixture resulting from
large-scale operations, three or more adsorbers are
frequently required. In such cases, the vapor-air
mixture is passed through two adsorbers in parallel
while the third is being steamed. Series operation is
employed when maximum recovery efficiency is
desired but requires a minimum of three and often
four adsorption units. To improve efficiency
further, adsorption systems are often equipped with
automatic sequence controls that operate on a fixed
cycle or a cycle that varies to conform with solvent
loading. Regardless of the number of units used,
they must be large enough to prevent excessive gas
velocity and provide adequate dissipation of heat of
adsorption.
Tetrahydrofuran recovery efficiencies of 98 to 99%
may be expected from activated carbon adsorbers.
The adsorptive capacity for THF will vary from one
brand of activated carbon to another, but 0.35 lb
THF/lb carbon is an approximate average equilib-
rium value. Commercial design is often based on
half the equilibrium value, because “breakthrough”
normally occurs at about that level.
The amount of steam required for regeneration
depends on the amount of solvent in the carbon and
the physical characteristics of the carbon itself.
Under normal conditions, about 3 lb steam/1 lb
THF is required, yielding a stripper still feed
containing 25% THF. The stripper still removes
gross water from the mixture, to give an azeotropic
distillate containing 5.3% water. The vapor-liquid
equilibrium curve for the THF-water system at
atmospheric pressure is presented in
Figure 2
.
It is apparent from the curve that the azeotrope may
be easily obtained in spite of wide variations in feed
composition.
Absorption
Tetrahydrofuran may be recovered from an air
stream by absorption in water where adsorption is
impractical. However, economics generally restrict
consideration of absorption to large volume recov-
ery units, as the THF vapor concentration is
very low.
The absorption system depicted in
Figure 3
can be
used to recover THF from air streams.
In this system, the solvent vapor-air mixture is
filtered, cooled, and blown upward through a
column where the THF is absorbed by water
flowing countercurrent to the direction of gas flow.
2
Figure 2.
Vapor/Liquid Equilibrium
for separating THF from its water azeotrope have
been developed, and several are in commercial use.
The operating principles and factors to be consid-
ered in design of these systems are discussed
individually in the following sections.
1.0
0.8
Brine Extraction
Tetrahydrofuran, like many other organic solvents,
may be dried by extracting the water with a satu-
rated calcium chloride brine solution.
Figure 4
illustrates the operation of this system.
The THF-water azeotrope is pumped into the
bottom of the extraction column where it flows
upward, countercurrent to the saturated brine
solution. The brine removes most of the water from
the azeotrope and flows out of the bottom of the
extractor to an evaporator, where it is concentrated
for reuse. The water removed by the evaporator
contains a small amount of THF and is recycled to
the recovery system stripper still.
The water content of the THF stream leaving the
top of the extractor is about 1.8% by weight when
proper contact between the two streams has been
maintained. Final drying is obtained by distilling
this stream to yield dry THF and the azeotrope,
which is recycled to the extractor. The water
content of THF dried by this method can be re-
duced below 0.1% when a batch still is used but is
more commonly about 0.5%, depending on the
efficiency of the still.
Brine extraction is a relatively foolproof method for
separating THF from its water azeotrope and is
economical, provided a high degree of dryness is
0.6
0.4
0.2
Vapor-Liquid Equilibria
THF-Water
760 mmHg
0
0
1.0
0.2
0.4
0.6
0.8
Mole Fraction THF in Liquid
The absorber effluent is then stripped of gross
water by distillation to yield the THF-water
azeotrope. The water is recycled to the absorber
to minimize solvent loss.
Separation of Tetrahydrofuran
from Its Water Azeotrope
Both the adsorption and absorption methods of
recovering THF yield an azeotrope containing
about 5.3% water, from which THF cannot be
separated by simple distillation. Because most
operations require dry THF, a number of methods
Figure 3.
Recovery of Tetrahydrofuran by Water Absorption
EXHAUST
AIR
CONDENSER
COOLER
THF-WATER
AZEOTROPE
ABSORBER
SEWER
STRIPPER
STILL
L.P. STEAM
PREHEATER
VAPOR-
LADEN AIR
FILTER
& COOLER
BLOWER
& MOTOR
3
Figure 4.
Brine Extraction
CONDENSER
CONDENSER
RECYCLE TO
STRIPPER STILL
L.P.
STEAM
BATCH OR
CONTINUOUS
STILL
BRINE
EVAPORATOR
EXTRACTION
COLUMN
L.P.
STEAM
THF-WATER
AZEOTROPE
DRY THF
COOLER
not required. It is useful where moderate to large
quantities of THF must be dried and is readily
adaptable to batch or continuous operation. Solvent
loss should not exceed 1% in the system described,
but will be significantly larger if the recycle to the
stripper still or evaporator is eliminated. The brine
extraction method is particularly useful when
mixtures containing several solvents must be dried
simultaneously.
water is continuously removed from the base of the
pressure still. While pressure distillation is capable
of producing drier solvents than the other common
drying methods, the degree of dryness obtained is
determined by design of the pressure still.
Pressure distillation is the simplest method for
separating THF from its water azeotrope and is also
the most economical where THF is the only solvent
Pressure Distillation
The shift in composition of an azeotrope that occurs
when pressure is changed may be used to separate
THF from its azeotrope with water.* The water
concentration in this azeotrope increases from 5.3%
to about 12% as the pressure is increased from
atmospheric to 100 psig. This shift is shown
graphically by the vapor-liquid equilibrium diagram
in
Figure 5
.
The system shown in
Figure 6
employs this
principle to dry THF without use of extraneous
chemicals or solvents.
THF-water azeotrope from the stripper still of the
recovery system is pumped under pressure through
a preheater directly to the pressure still. The
overhead stream from the pressure still is the THF-
water azeotrope at 100 psig, which is recycled to
the stripper still. Dry THF containing 0.05% or less
Figure 5.
Vapor/Liquid Equilibria
1.0
Vapor-Liquid Equilibria
THF-Water
100 psig
0.8
0.6
0.4
0.2
0
0
0.2
0.8
1.0
0.4
0.6
Mole Fraction THF in Liquid
* Can. Patent 546,591 (1957) assigned to E. I. duPont de Nemours &
Co.
4
Figure 6.
Pressure Distillation
COOLER
RECYCLE TO
STRIPPER STILL
CONDENSER
L.P.
STEAM
THF-WATER
AZEOTROPE
PRESSURE
STILL
COOLER
DRY THF
CALANDRIA
H.P. STEAM
to be dried, other equipment does not exist, or
substantial quantities of THF must be dried. These
factors usually indicate the desirability of continu-
ous operation. Multisolvent mixtures may be dried
by this method, but the advantages of simplicity
and economy are lost. Solvent loss should be
negligible, provided all equipment is tightly sealed
and the condenser-cooler combination is efficient.
Dry THF is obtained from the organic layer by
distilling off pentane and water. This distillation is
usually carried out at a pressure of 15 psig to
reduce cooling water requirements. The pentane-
water condensate forms two layers, which may
be separated by decantation and recycled. THF
containing less than 0.1% by weight water may
be recovered by this system.
With minor modifications, existing solvent recov-
ery equipment may often be adapted for THF
drying by pentane extraction. However, it must be
recognized that pentane is a low-boiling, low-flash
solvent that requires careful attention to equipment
design and safe handling procedures. The nature of
the pentane extraction method generally restricts its
use to continuous processing of large volumes of
THF. Recycle of minor streams as shown should
result in negligible THF loss.
Solvent Extraction
Tetrahydrofuran may be effectively dried by adding
certain water-immiscible solvents to the THF-water
azeotrope.* Pentane is one of the best solvents for
this purpose because of its low price and high
efficiency. A THF drying system employing
pentane is illustrated in
Figure 7
.
The THF-water azeotrope from the stripper still is
continuously mixed in a vessel with an equal
volume of pentane. This causes formation of two
phases: an organic phase containing pentane, most
of the THF, and a small amount of water; and an
aqueous phase that contains some THF and traces
of pentane. The exact composition of each phase
may be predicted from the THF-pentane-water
ternary miscibility diagram shown in
Figure 8
.
The two-phase mixture is pumped to a decanter,
where the aqueous layer is separated and recycled
to the stripper still for recovery of dissolved THF.
Other Techniques
Several methods that employ solids have been
developed for separating THF from its water
azeotrope. These are particularly useful if small to
moderate quantities of THF must be processed or if
a high degree of dryness is required. They include
the following:
• Dehydration with caustic soda
• Dehydration with calcium chloride
• Liquid phase adsorption
* U.S. Patent 2,790,813 (1957), Bente, P. F. (assigned to E. I. du Pont
de Nemours & Company)
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