EPA-R2-72-073
DECEMBER 1972 Environmental Protection Technology Series
Solvent Extraction Status Report
National Environmental Research Center
Office of Research and Monitoring
U.S. Environmental Protection Agency
Corvallis, Oregon 97330
-------
RESEARCH REPORTING SERIES
Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL
PROTECTION TECHNOLOGY series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and
methodology to repair or prevent environmental
degradation from point and non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards.
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402
Price 76 cents Domestic postpaid or 50 cents GPO Bookstore
-------
EPA - R2-72-073
December 1972
SOLVENT EXTRACTION STATUS REPORT
Luther F. Mayhue
National Petrochemical Wastes Research Program
Robert S. Kerr Environmental Research Laboratory
P. 0. Box 1198
Ada, Oklahoma 74820
Project 12020 EWZ
Program Element 1B2036
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
CORVALLIS, OREGON 97330
-------
ABSTRACT
The history, basic principles, process application, laboratory approach,
and grant program concerning solvent (liquid-liquid) extraction was
investigated in relation to its application to industrial effluent
waste water systems. A search of the literature reveals that little
consideration has been given to solvent extraction as a feasible waste
treatment method. Various aspects of solvent extraction technology are
presented along with a number of industrial wastes which should be
considered for treatment. One of the areas of greatest need for research
concerning industrial waste water treatment is in the removal of re-
fractory, and taste and odor causing compounds. Application of solvent
extraction to waste systems containing low concentrations (0.05%) may
be feasible on a "swap out" basis or in conjunction with biological,
adsorption, or incineration treatments as a pretreatment step. Applica-
tion of solvent extraction to waste systems for recovery of salable
products to offset cost of treatment should be studied for feasibility.
Accomplishments and plans regarding industrial research projects are
presented.
iii
-------
CONTENTS
Section Page
I Conclusions 1
II Recommendations 3
III Introduction 5
IV History of Solvent Extraction 7
V Solvent Extraction - Basic Considerations 9
VI Solvent Extraction Process Application 13
VII Solvent Extraction Equipment 17
VIII Solvent Extraction As Applied to Waste Water Renovation 25
IX Laboratory Bench Scale Approach. 27
X Pilot Plant Approach 29
XI Research Program 33
XII Grant Program 35
XIII References 37
-------
FIGURES
PAGE
1 Mixer-Settlers 19
2 Sieve or Perforated Plate Column 20
3 Spray Towers (Columns) 21
4 Baffle Plate Column 22
5 Packed Column 23
6 Rotating Disc Contactor 24
vi
-------
TABLES
No.
Process Wastes (27)
Page
31
vii
-------
SECTION I
CONCLUSIONS
1. The opportunity and technology are available for successful
application of solvent extraction to industrial waste water treatment.
2. The industrial climate is favorable to sound pollution abatement
or waste treatment processes.
3. One of the areas of greatest need for research concerning industrial
waste water treatment is in the removal of refractory and taste and odor
causing compounds.
4. Application of solvent extraction to waste systems containing low
concentrations (0.05%) may be feasible on a "swap out" basis or in
conjunction with biological, adsorption, or incineration treatments as
a pretreatment step.
5. Application of solvent extraction to waste systems for recovery of
salable products to offset cost of treatment should be studied for
feasibility.
-------
SECTION II
RECOMMENDATIONS
1. Additional research should be conducted for the investigation of
water renovation by solvent extraction as applied to reduction of halo-
genated hydrocarbons and other refractory compounds which affect potable
water supplies and aquatic and marine life with respect to tainting and
odors or toxicological manifestations either directly or through bio-
logical magnification processes.
2. A two year investigation (Industrial Pollution of the Lower Missis-
sippi River in Louisiana., EPA Region VI, Surveillance and Analysis Div-
ision, Dallas, Texas, April 1972) into the deterioration of water quality
in the Lower Mississippi was completed by the Environmental Protection
Agency's Lower Mississippi River Basin Field Station during mid 1971.
This study relates industrial pollution to river water quality. In view
of the findings in this study, an extended and reinforced program of EPA
research into advanced waste treatment technology for the protection
(and rehabilitation) of the Lower Mississippi River, delta, marsh, and
tideland areas from contamination by industrial wastes should be expedited.
3. Research should be undertaken on a definitive basis for the survey of
industrial manufacturing process wastes from the organic chemical industry
included in the listing of 50 volume chemicals to determine possibilities
for application of solvent extraction. This project should be directed
by an individual knowledgeable in. extraction technology.
4. A continuing in-house study program by the Environmental Protection
Agency's research staff should be initiated for conducting solvent
extraction research studies as related to waste water renovation. This
program would minimize the normal funding mechanism as well as support-
ing manpower requirements.
-------
SECTION III
INTRODUCTION
Because of the water quality standards established by the various states,
the Federal Government, pollution abatement legislation, and policies of
the Federal Government, the industrial community has become increasingly
aware of its responsibility to promote the conservation of the environ-
ment in which its plants operate. Technical studies of liquid waste
treatment, as well as solid wastes and air emission studies, have been
initiated to minimize detrimental impact on the environment. In the
forefront among these studies has been an inquiring look at secondary
waste treatment as traditionally viewed: The stabilization or conver-
sion of organic matter in liquid wastes by processes of biosynthesis
and respiration (1).
Since biological treatment has been applied successfully in municipal
type waste processes for many years, theological course of events led
to an examination of this treatment method for possible expansion of the
techniques to industrial wastes of increasing complexity. This treat-
ment application, as well as application of other processes, has been
the object of broad scale investigation. In many instances, biochemical
treatment has been found to be unsuitable in its adaptation to industrial
wastes, due to the toxic or refractory nature of many of the organic and
inorganic compounds common to most present day plant effluent streams.
Initial limitations of this time honored biochemical process to treat-
ment of refractory chemicals has led to consideration and investigation
of fields of physical and physicochemical separation processes. A
number of processes have been found which, under controlled circumstances,
will separate or change chemicals not easily changed or destroyed by
biological treatment. A good many of these processes fall into the
separation category.
Solvent extraction is one such process which, alone or in combination
with other processes, has promise for removal of refractory compounds in
either high or low concentrations.
-------
SECTION IV
HISTORY OF SOLVENT EXTRACTION
Nature rarely provides a raw material in pure form and few processes
produce pure products. Because of this thoughtless provision of nature
and the ineptitude of chemical synthesis, physical separation has become
the most important single industrial process. The petroleum, chemical,
and petrochemical industries, in general, have large investments in
separation equipment and spend even more in operation. The more compli-
cated our industrial economy becomes, the more different are the materials
required in relatively pure form for feed stock, and the more finished
from by-products and impurities.
In definition, separations considered here are those separations where
mixtures are divided into pure compounds or at least into two or more
fractions having different compositions by processes sometimes classified
as "diffusional or mass transfer" operations (2). Common among these
processes are fractional distillation, fractional crystallization, sol-
vent extraction, adsorption, absorption, evaporation, etc.
Of these separation processes, solvent extraction has had, by far, a
more marked impact on civilization than any of the other separation
processes. In domestic and public service, the practice of bathing
and similar washing, cleaning, and a great part of food preparation
operations can be placed in the category of solvent extraction. This
same process of solvent extraction has had its impact on the petroleum
refining and chemical community as well. Solvent extraction may be
defined broadly as a separation process in which two or more immiscible
or partially immiscible fluids are brought into contact for the transfer
of one or more components (3) from one fluid to the other.
Prior to about 1933, solvent extraction, as related to petrochemical
industrial processing, was known only in the laboratory—effective
methods of recovering solvents had not as yet been developed. One
possible exception is the classical Benzene-Caustic-Dephenolization
Process by Pott and Hilgenstock (4)—although this extraction process
was used in coke oven plants.
After solvent recovery adaptation to chemical processing and industrial
acceptance, information in the literature by 1949 or 1950 was still
meager enough to summarize all of it regarding liquid-liquid extraction
through definition of equipment alone (5). Industrial use was still
generally limited to systems unsuited to distillation (6)—materials of
very close volatility, such as the separation of aromatics from ali-
phatics in a particular petroleum cut. In the United States, among the
first uses of solvent extraction in the early years was the separation
of a general group of offensive materials or hydrocarbons from oil,
thereby saving in the cost of acid treatment. The extraction processes
gave higher yields of superior products with salable reject materials,
whereas the acid sludge from sulfuric acid treatment was (is) a nuisance
and a disposal problem.
-------
Just as vacuum distillation permitted the processing of black crude oils
for lubricants, so the solvent processes tended to erase the old crude
oil marketing system by which only a few crude oils were considered
satisfactory for lubricant manufacture. By solvent methods, the original
properties of the oil were "changed" so that a uniform grade of oil could
be manufactured from a wide variety of crudes (7). From the removal of
the dark resinous materials from lube stocks, diesel fuel, and solvents
to the removal of unsaturated and aromatic hydrocarbons from kerosene
and the removal of mercaptans from gasoline, solvent extraction has pro-
gressed to present-day technology. Today, making use of preferential
solubility of various components between two immiscible or partially
immiscible solvents, extraction competes with distillation, crystalli-
zation, absorption, and chemical reaction in quality, if not in quantity.
Present applications of extraction include refining of petroleum, chemi-
cals, nuclear fuels processing, vitamin and antibiotic purification, and
refining of vegetable oils. Close boiling and freezing points, other
unsuitable physical properties, and such considerations as heat sensi-
tivity justify the use of extraction in many separation processes. In
fact, the necessary separations may be very difficult or even impossible
to achieve by other processes. The refining-petrochemical industries
are unquestionably the largest users of solvent extraction. They use
the greatest variety of extraction processes and equipment and treat a
larger quantity of feed material than all other industries combined.
Nevertheless, few extraction processes involve separation from water,
and almost none involve water as the major (bulk) phase.
Solvent extraction should be particularly attractive when applied to
relatively concentrated wastes segregated from the common waste water
system. Thus, particularly objectionable wastes may be eliminated
before contamination of the entire disposal system. In addition, if the
contaminate is a product of commercial value and in reasonable concen-
tration, extraction will offer the possibility of reduced unit treatment
costs and/or additional profit through sale of recovered products.
Therefore, since solvent extraction is used by practically every branch
of the refining-petrochemical industry; and since little has been done
on its application to waste water treatment, its application to indus-
trial wastes is a natural step. Just as solvent extraction was instru-
mental in elevating the naphthenic base crudes to the position of a
paraffinic sophisticate, so may solvent extraction also be instrumental,
through advanced waste treatment, in reaching the ultimate objective of
waste water renovation for direct reuse (8, 9)—thereby solving both
problems of water supply protection and waste water treatment simultaneously.
Solvent extraction will be one element in a closed-loop system of water
recycle which will insure enough water for all in places of high demand.
It is estimated that the United States will need one trillion gallons of
water each day by the year 2000 when the population reaches 300 million (10).
-------
SECTION V
SOLVENT EXTRACTION - BASIC CONSIDERATIONS
Fundamentally, solvent (liquid) extraction is a method of separating the
components of a solution. The separation of a mixture by extraction
requires that the constituents have different relative solubilities in two
immiscible, or only partially miscible, liquid solvents. The ratio of the
concentrations of a particular dissolved substance (solute) in two coexist-
ing liquid phases at equilibrium is constant and is called the distribution
coefficient (6) Dj « Yi/X-^. YI and Xi are the concentrations of i (i -
some component) in the conjugate phases. Conventionally, the Y phase is
the one in which the key solute is preferentially soluble, thus giving a
distribution coefficient of more than unity for this component. When the
separation of two components in a mixture is under consideration, the ease
of separation is conveniently measured by the separation factor, a, which
is the ratio of the distribution coefficients of the two components
between the two solvents (note the equivalence to relative volatility in
distillation): a.±j = Di/Dj. If the components form ideal solutions in
each phase, the distribution coefficient and separation factor will be
constant and independent of the actual concentrations of the solutes.
This rarely occurs in practice, and D and a can be expected to be functions
of absolute concentration. In addition, the distribution coefficient for
a particular component between two solvents is usually a function of the
nature of the other components present and their concentrations (6).
It may be seen, therefore, that all separations, such as distillation,
extraction, etc., are basically analogous. Industrial application of new
separation processes to old separations, or of familiar separation pro-
cesses to new separations, becomes much easier to visualize when the
inherent similarities are realized. Certain criteria must be used and
certain limitations are always present regardless of the separation pro-
cess applied. Nevertheless, there are no different problems between the
"difficult" and "easy" separation processes, only the emphasis is different.
For instance, in easy separations attention is directed toward controlling
minor impurities while the main separation is treated somewhat incidentally.
In difficult separations, attention is directed toward the mechanism of
the separation transfer; minor impurity problems are there but their
solution is secondary to that of the primary separation.
In every case, two phases are passed countercurrent to each other. Means
are provided to bring the phases into equilibrium with each other while
simultaneously the countercurrent flow continuously separates phases
approaching equilibrium. Finally, the concentration of one component
relative to another component of the mixture to be separated is less in
one "equilibrium" phase than in the other.
The means used to perform these fundamentally similar operations differ
enough to give rise to separate and distinct treatments in textbooks
and handbooks. These distinctions arise in part from the development
stages of the several separation methods. Only in comparatively recent
-------
times have enough fundamental data been accumulated to explain the
method of operation of these processes, and the fund of data is still
inadequate. In early development, the practical knowledge was obtained
by bitter experience, and results were expressed empirically and reflected
the individual peculiarities of the processes rather than the inherent
similarities. Today, with a greater fund of information, the similari-
ties among separation processes should be stressed so that useful infor-
mation learned on one process can be used to assist in designing and
operating another process. This will not only allow the more effective
use and application of both general and specific knowledge to all opera-
tions but the real differences between the various separation processes
could be given special attention for their better understanding (2).
Many times in practice there is a choice among methods when separations
are to be made. These include mechanical, physical, and chemical. Sol-
vent extraction can be divided into two broad categories according to
the origins of the differential solubility. In some cases, it arises
from purely physical differences between the solutes, such as polarity,
and the influence of these on the concentration dependence of chemical
potential. In others, the differential solubility can be traced to a
definite chemical reaction between one or more of the solutes and one
of the solvents (6).
The maximum separation that can be achieved between two solutes in a
single equilibrium step of the two phases is governed by the separation
factor, a, and the relative amounts of the two phases used, the phase
ratio. Combination of overall mass and component balances with the
distribution coefficients allows the compositions of the phases at
equilibrium to be computed. If the separation achieved is inadequate,
it can be increased by either changing the phase ratio or, more usually,
by addition of further contacting stages.
It is important to distinguish two classes of the solvent extraction
system: (1) those in which the two phases are completely immiscible or
in which the relative miscibility of the two phases is constant and
independent of the solute concentration and (2) those in which the
relative miscibility of the two phases vary with the solute concen-
trations. Systems in which the differential solubility derives from
complex formation between solute and solvent normally belong to the
former group, whereas a variation of solvent miscibility with solute
concentration is a characteristic of systems depending only on physical
differences for the differential solubility.
The solvent extraction contactor may be considered to be divided into
two parts, the extraction section and the scrubbing section (6). Aqueous
solutions (original solutions) are usually introduced somewhere near the
center of the multistage contactor. The second solution (usually called
"the solvent," since it is the one with a preferential affinity for the
key component) is introduced at one end of the extraction section through
which it flows countercurrent to the feed. The extraction section is
10
-------
so-called since it is here that the required solute is extracted. The
solute plus the solvent is called the extract. The residual stream
leaving the contactor is depleted in this solute and is known as the
raffinate. The number of equilibrium stages required in the extraction
section depends on the phase flow ratio and the distribution coefficient
of the extracted solute. However specific a solvent may be for a pari-
ticular solute, it will inevitably extract a certain amount of the other
solutes that are present, since distribution coefficients are rarely zero.
The purpose of the scrubbing section is to provide a means to wash the
extract (leaving the extraction section) with the original solvent in
which undesired solutes will be preferentially soluble. The number of
equilibrium stages required in the scrubbing section is therefore a
function of the distribution coefficients of the components other than
the material desired to be extracted in conjunction with the phase flow
ratio. It does not depend on the distribution of the desired solute. A
complete design therefore demands a knowledge of the distribution data
for each constituent of a mixture over the full range of concentrations
to be anticipated. Needless to say, this is rarely available and a com-
promise usually has to be reached by considering the key components in
the mixture. The key components are, in general, the two constituents
between which the separation is to be made. When two phases that are not
in equilibrium are brought together, the rate of transfer of solute
between them depends on the extent to which the concentrations of the
solute in the two phases differ from equilibrium values as fixed by the
distribution coefficient. Actually, the above information is related to
the absorption or stripping-factor method originally developed for
absorber and stripper calculations and applied to multistage extraction (11).
Now, like all other separations problems, extraction has been programed
for computer application in a convenient analytical manner. The method
involved is an adaptation of the Theiele-Gedes method for multicomponent
distillation.
Having achieved the extraction, the desired solute is dissolved in the
solvent, and the final requirement is the separation of the two. This is
a very important step as it will most often dominate the economics of
the overall process. The method adapted for solvent recovery depends on
the system involved, but two broad categories can again be distinguished
according to whether the original extraction involved interaction between
the solvent and solutes or depended only on physical effects. In the
latter case, solvent recovery is by physical means, of which distillation
is by far the most common. If the extraction has involved chemical inter-
action, then solvent recovery demands reversal of this reaction. This is
usually achieved by some form of chemical conditioning. Thus, it is seen
that only a minor part of the total equipment in a solvent extraction
plant is devoted to the extraction operation. An elaborate equipment
installation is required to "distill" the solvent from the extract and
raffinate solutions to separate the solvent from the finished product
and, finally, to recover the solvent and to purify it as well as to
purify the solute, when necessary.
11
-------
SECTION VI
SOLVENT EXTRACTION PROCESS APPLICATION
Despite the apparent disadvantages of solvent recovery, it is obvious
that liquid extraction provides a less expensive overall process for
applications where other methods of separation are "border line or fail
altogether." The separation of the components of a solution by extraction
may be accomplished in a number of ways, depending on the nature of the
solvent system and the physical arrangement of the apparatus used. Some
typical areas (12) where usefulness has been demonstrated are as follows:
A. Separation of close boiling liquids.
B. Separation of liquids of poor relative volatility.
C. As a substitute for vacuum distillation.
D. As a substitute for evaporation.
E. As a substitute for fractional crystallization.
F. Separation of heat sensitive materials.
G. Separation of mixtures that form azeotropes.
H. Separation according to chemical type where boiling points
overlap.
I. As a substitute for more expensive chemical methods, e.g.,
the separation of uranium from vanadium in ore-leach liquors.
After a review of the above areas of solvent extraction application,
attention may be directed to the considerations concerning solvent
selection. In choosing a solvent for an extraction process, several
principles may be used as a guide. Some of these are frequently con-
flicting, and no single solvent is likely to possess every desirable
characteristic. At any rate, the following factors should be considered
to insure the best solvent performance. These factors are listed as to
relative importance to aid in compromise.
1. Selectivity: Selectivity will be the first property studied in
relation to the process. It refers to the ability of the solvent to
extract one component (class) of a solution in preference to another.
The ideal solvent, from this standpoint, would dissolve all of one com-
ponent and none of the accompanying components.
2. Distribution Coefficient: The distribution coefficient is the
ratio of concentrations of a solute in equilibrium phases and is a direct
measure of selectivity; the ideal solvent would produce a distribution
13
-------
coefficient which would numerically approach infinity. The selectivity
and distribution coefficient may be altered and improved in some instances
by altering the pH of the solution or by buffer addition.
3. Capacity: Ordinarily, a solvent should have the capacity to
dissolve relatively large quantities of the desired solute. The distri-
bution coefficient should be large, but so should the capacity, other-
wise the solvent would be uneconomical to use.
4. Solvent Solubility: The desired solvent should have a high
degree of insolubility in relation to innocuous solutes. This produces
a high selectivity ratio.
5. Recoverability: The solvent should be removed (recovered) from
the products in all extraction processes. This is important from the
standpoint of product contamination as well as solvent reuse. The entire
problem of solvent recovery is of such importance to the economic success
of the extraction process that it must be considered separately.
6. Density: The difference in densitites of the the contacting
phases throughout the operating range should be as great as possible. This
will not only promote minimum disengaging rates between the immiscible
layers but will also minimize equipment sizes.
7. Interfacial Tension: Little is known about the real effects
of interfacial tension as related to solvent extraction. In general,
interfacial tension between two immiscible liquid phases should be as
high as possible to promote liquid disengagement. Low interfacial ten-
sion will promote the formation of stable emulsions. Data from the
literature are of no practical help in evaluating an extraction system
since only data for pure substances may be obtained. From a practical
view, a shake-out test of the liquids to be contacted will show much
toward settling characteristics.
8. Chemical Reactivity and Stability: The solvent should ordinarily
be stable in contact with the feed solution. Chemical reaction is undesir-
able since product yield is reduced, solvent recovery is made more diffi-
cult, and solvent loss may be high. On the other hand, some extraction
processes employ chemical reaction as the only possible solution for
separation. However, the reaction must be readily reversed so as to mini-
mize the cost of solvent recovery. A plus on the chemical reaction side
usually results through an increase in the distribution coefficient. A
minus may possibly result from the formation of corrosive products.
9. Corrosiveness: Ideally, the solvent should contribute no more
severe corrosion difficulties than those ordinarily associated with the
feed stream.
14
-------
10. Viscosity: Low viscosity solvents promote the following
beneficial factors*-low power requirements, rapid extraction, quick
settling, and high heat and mass transfer rates.
11. Vapor Pressure: Although low vapor pressure is desirable from
the standpoint of the extraction operation itself, as well as from the
handling and storage of the feed and product streams; the exception to
these benefits should be made to promote the ease of solvent recovery.
Vapor pressures should be such as to result in big differences in
relative volatility between the liquid phases, since relative volatil-
ity is a direct measure of the ease of separation of components in a
mixture by distillation (separation). Low freezing point, flammability,
toxicity, and cost are additional solvent attributes to be considered
in the process. In order to have a favorable separation, the following
primary properties (as listed above) must be properly assessed: selec-
tivity, interfacial tension, density, and chemical reactivity. The
remaining factors (secondary) should be considered from a technical view-
point since all will contribute to facility of design.
15
-------
SECTION VII
SOLVENT EXTRACTION EQUIPMENT
Equipment used in liquid-liquid extraction is quite varied, but
fortunately it can be classified according to construction and/or
operational characteristics into two classificationsf-stagewise con-
tactors and differential contactors (5).
Stagewise contacting represents a method of performing extraction and
many types of equipment fall into this category. This type of contact-
ing is characterized by one or more stages of mixing followed by settling,
and each stage approaches one theoretical (ideal) stage. Differential
contact is a very general term and for all practical purposes includes
all other types of extractors. The distinguishing feature of differen-
tial contact is its incomplete separation of the two phases following
mixing. Some of the dispersed phase will be carried along by movement
of the continuous phase (back mixing). The net effect is a loss in
efficiency.
The list below is a sampling of commonly used types; a more extensive
list is found in References 5, 21, and 22. Classification and diagrams
of selected equipment follow:
1. Stagewise Contactors:
1.1 Mixer-Settlers (horizontal or vertical)
1.2 Nonmechanical (no agitation) Plate Columns
1.2.1 Perforated Plate Column
1.2.2 Vertical Plate Column
2. Differential Contact Extractors:
2.1 Nonmechanical Gravity Separated Columns
2.1.1 Spray Column
2.1.2 Baffle Plate Column
2.1.3 Packed Column
2.2 Mechanical Gravity-Separated Columns
2.2.1 Rotary Disc Contactor
2.2.2 Pulsed Column
2.3 Mechanical Centrifugally-Separated Contactors
2.3.1 Podbielniak Extractor
2.3.2 Luwesta Extractor.
17
-------
Selection of a particular extractor for a separation is still based
largely upon experience. In general, it is necessary to establish the
desired and/or possible solute recovery (the chemical and physical
properties of the system being known) for specified flow rates. The
cost of installation, maintenance, and operation must be estimated for
a given extractor that most nearly meets all requirements.
18
-------
Extract
Mixing
Mixing
_Light Phase
(Solvent 4, Solute)
Raw Feed
Settling (Vertical or horizontal)
O
Fresh Solvent
Mixing
Raffinate
Heavy Phase
(Raw Feed - Solute)
Figure 1.
MIXER-SETTLERS
The mixer-settler is one of the oldest examples of extraction
equipment. The settlers may be arranged in cascade to take advantage
of gravity flow of the heavy phase. Mixing may be obtained by flow
mixers, nozzles, orifices, injectors, etc. Most extractors are some
variation of the mixer-settler.
19
-------
Extract
Raw Feed
Fresh Solvent
Light Phase
(Solvent + Solute)
Perforated Plate
Downcomer
Baffle
Heavy Phase
(Raw feed - Solute)
Figure 2.
SIEVE OR PERFORATED PLATE COLUMN
The sieve or perforated trayed column is in reality a series of
spray columns. The above example shows layout so that the heavy phase
(continuous) will flow downward through downcomers and across each
tray while the light (dispersed) phase will flow upward through
perforations in each tray. Extraction is promoted by repeated dispersion
of one liquid phase through the other. There is repeated dispersion
through, and settling between trays. It will be obvious that down-
comer orientation determines which phase is dispersed, e.g., by
inverting the downcomers the heavy liquid will be the dispersed phase
in Figure 2 above.
20
-------
Light Phase
Heavy Phase
Heavy Phase
"Light Phase
Figure 3.
SPRAY TOWERS (COLUMNS)
Spray towers are the most simple of all extraction equipment but
are generally the least efficient due to axial channeling. An
improved version of the spray tower is that of Elgin (13), U.S. Patent
2,364,892 (1944), shown in Figure 3.
21
-------
V -- , I/-
— — — — ^p.
«X~
r
: . • '• ^_
A 0
Heavy P
Light P
Heavy P
Light P
Section of Baffle Plate Column
Figure 4.
BAFFLE PLATE COLUMN
The baffle plate column is actually a combination of the old "side
to side pan" column, used in the early refining era, and the spray
column. Another configuration is the old "disc and donut" tray
arrangement composed of alternate flat rings and discs. The principle
advantage of the baffle tower over the spray tower is the increase in
residence (contact) time and a decrease in channeling.
22
-------
Light Phase
Heavy Phase
Packing
Light Phase
Heavy Phase
Figure 5.
PAfiRBB COLUMN
The packed column is another improved version of the old spray
tower and properly designed promotes high mass transfer rates. The
packing may consist of ceramics, metal, plastics, etc., of various
shapes and sizes.
23
-------
Power
Heavy Phase
Light Phase
Light Phase
_Rotating Disc
Donut Annular Ring
Shaft
Heavy Phase
Figure 6.
ROTATING DISC CONTACTOR
The rotating disc contactor is a disc and donut column. The discs
are attached to a rotating shaft, usually centrally located in the
column. Each disc is separated from other discs by annular flat rings
fastened to the shell.
24
-------
SECTION VIII
SOLVENT EXTRACTION AS APPLIED TO WASTE WATER RENOVATION
In the development of a new extraction process, such as that applicable
to waste water renovation, it will .not be possible to rely on equilibrium
data obtained from the literature based on fairly pure materials. Often
the chemicals are technical grade solvents contaminated with minor con-
stituents which may be quite unpredictable as to their effect. Too, the
waste streams to be studied will not usually be simple solutions of two
or three components or solutes but will contain varying amounts of sub-
stances whose effect in an extraction process cannot be predicted.
Indeed, there will generally be traces of unsuspected chemicals present
which may not be found until their separation or concentration by the
process. The waste contaminates to be separated will, in most instances,
be varied and complex. Attempts to obtain detailed distribution data,
though desirable, will be costly. Such will be especially true when the
distribution of constituents are synergistic and interdependent. Calcu-
lation procedures will be very difficult if not impossible. Distribution
curves will be unpredictable in the dilute concentration ranges and cannot
be extrapolated. Such are some of the problems peculiar to investigation
of the solvent extraction of contaminates from industrial waste effluents
ranging in the parts per million concentration. Little work has been
done in the separation of organics from waste water by solvent extraction,
except that attendant with other separating work (14). Some extraction
work patterned after desalination techniques has been done on sewage
secondary effluents to separate a "purified" water from brine containing
organic and inorganic compounds (15).
One pretreatment installation is known to exist at a plant manufacturing
complex halogenated phenolic compounds. Counter-current flow solvent
extraction was utilized to extract unreacted feedstocks from the unit
process waste water. The extraction solvent utilized was the same com-
pound as the carrier solvent for feedstock thereby allowing direct recycle
to the process input. Basic purpose of the installation was to reduce the
load of halogenated phenolics to a level acceptable by a municipal secon-
dary waste water treatment plant (16).
Lewis and Martin (17) report on an electrostatic extraction process to
cemove phenols from cat cracker distillates. This process is said to
reduce phenol content in the distillate water by 90 percent or to a level
of 30 ppm (plus) in the waste stream. Beychok (18) reports on improve-
ment of the distribution coefficient in this type of process by the
addition of pyridine and quinoline. Weinberger and Stephan (19) report
in "Technology of Advanced Waste Treatment," as of May 1967 that no
extraction processes were under study by FWPCA. Further, these authors
report that biochemical oxygen demand (BOD) and coliform bacteria are no
longer the only measures of wastes discharged. Other types of materials
are of growing concern—refractory organics, radionuclides, nutrients,
viruses, inorganic salts, and heat. Pollutional effects of these materials
25
-------
may include accelerated eutrophication (over-fertilization); interference
with water treatment or industrial use of the water; tainting of fish flesh;
fish kills, or other damage to stream biota; taste, odor, color, or foam
in water supplies; and potentially toxic or carcinogenic damage to man.
Only in the last 10 years has any concerted effort been directed to the
exploring of treatment approaches other than those in the municipal waste
treatment field. Very few new concepts in waste treatment have been
developed and placed in operation. The fact that "appropriate" treatment
would restore a waste stream to any desired level of quality makes waste
treatment the most important and potentially the most fruitful area for
"breakthrough" in an intensified research. Physical separation principals,
in particular solvent extraction, deserve much increased attention as
supplements to biological treatment methods, etc. Solvent or liquid
extraction could be more readily adapted to the new concept of treatment
on a demand basis in order to achieve a desired level of pollution abate-
ment at a variable cost. This concept should be fully explored, especially
since extraction makes available a physicochemical process with a possible
variable degree flexibility. This approach would harmonize with the
objective in advanced waste treatment research to develop the minimum
cost system of physical, chemical, and biological treatment to achieve
the degree of pollution control that may be required for any specific
situation.
Advanced waste treatment research will follow the normal process develop-
ment sequence from desk top and laboratory exploratory studies through
feasibility determinations at bench level to engineering studies in field
scale pilot plants for larger scale process evaluation. The objective of
the field scale pilot plant is to establish the performance and cost of
an advanced waste treatment system on a scale of operation sufficiently
large that potential municipal and industrial users of the system may
select it with assurance. This type of assurance of performance and cost
of a new water pollution control technology will greatly expedite its
introduction into practice and is, in fact, necessary before application
of a new technology can be expected.
It will be desirable to carry out multi-stage extractions in the
laboratory to provide information suggested above. For complex mix-
tures, conditions should approximate as closely as possible those antici-
pated for industrial use with respect to operating and design parameters.
Counter-current multi-stage extraction seems to be suggested here. True
counter-current processes are necessarily continuous; consequently,
either of two procedures could be followed: (1) batch simulation of the
counter-current multi-stage process or (2) continuous extraction in a
miniature bench-scale extractor of known stages. Both procedures may be
used, but miniature extractors are frequently inflexible due to limi-
tation in number of stages and difficult to feed continuously in a steady
state fashion on a small scale. Batch simulation, therefore, is most
often used with subsequent miniature pilot scale to obviate the use of
a continuous bench extractor.
26
-------
SECTION IX
LABORATORY BENCH SCALE APPROACH
The selection of the best solvent for a given separation is the first
problem to be met in the design of an extraction process. Several cri-
teria are to be considered in this selection such as availability,
enhancement of relative volatility, etc. The technique of gas-liquid
chromatography for screening solvents has shown some promise in this
field (20). Bench simulation is usually carried out with ordinary
laboratory separatory funnels according to Jantzen (21),and involves
the simple shake-settle method of approach. This procedure assumes that
each extraction, or "shake-out," is an equilibrium extraction or an
approach to ideal-stage extraction. For this purpose, equilibrium
between the phases may be reached with hand shaking. Generally, 50 simple
inversions during a 1.5-minute period of a separatory funnel containing
an extraction system of ordinary viscosity results in equilibrium
between the phases. More violent shaking may lead to false equilibrium.
The rate of attainment of equilibrium should be checked to account for
possible chemical reaction between solvent and solute.
To insure that transfer of liquids repeatedly from funnel to funnel is
representative; sufficiently large batches must be used. Small batches
produce errors due to the inevitable losses incurred through improper
drainage from the funnel. Larger batches are necessary when liquids of
high viscosity are used. Settling must be complete and, after phase
withdrawal, additional settling should be allowed with subsequent drain-
age of additional accumulation.
Repeated introduction of feed mixture and solvent will produce the same
effect as a steady-state continuous process, but the approach to this
condition is asymptotic. The liquid batches should be in the same ratio
as the rates of flow to be simulated in the continuous cascade.
There are published procedures (21) for laboratory simulation of various
modifications of the solvent extraction process; i.e., counter-current
extraction, counter-current extraction with reflux, fractional extraction,
and miniature continuous extractors.
27
-------
SECTION X
PILOT PLANT APPROACH
Pilot plant extractions are done on a larger scale than those thus far
described. They have as objectives:
1. To demonstrate feasibility.
2. To establish operating and design characteristics
3. To determine solvent characteristics
4. To check feed stock and product characteristics
5. To determine long-term accumulation of undesirable
constituents in the solvent
6. To provide product for evaluation.
In general, small units will give desired data for product evaluation and
economics of installation and operation. Some extractor types are more
reliable than others, therefore, the extractor size should be correlated
with stream hydraulic characteristics. Scale-up procedures are more
reliable for mixer-settlers and mechanically agitated tower extractors;
therefore, these types of equipment can be expected to be more efficient
when scaled up to plant equipment.
Materials of construction should be evaluated early in the research study
like those for large scale by testing in a simulated plant atmosphere,
particularly in the case of aqueous systems. In many cases, corrosion
will be important, but operating characteristics such as the wetting or
dispersion of the liquid must also be studied.
Start-up of the extractor must be according to a set procedure to mini-
mize problems such as flooding and for assurance that steady-state has
been attained before samples are taken for analysis. The time for steady-
state is variable depending upon the type of extractors used. Tower
extractors require greater time due to the appreciable volumes of liquids
maintained in the upper and lower sections. The time to reach steady-
state may be calculated through knowledge of the system, the solvent
involved, solvent/feed ratio, and the distribution coefficient for the
solute.
Sampling is of prime importance and should be done during the start-up
period to determine the approach to steady-state before data are accepted
for computing performance. During the course of the study, care must be
taken to sample at points along the extractor length to obtain data to
determine the column characteristics apart from the limiting conditions
at the column ends.
29
-------
Demonstration of an extraction process for renovation of a waste water
should be sufficient in time to establish a basis for economic evaluation.
During such time, attention should be given to solvent losses, deteriora-
tion and breakdown, solubility characteristics, etc. Since solvent losses
from a small scale unit generally constitute a significant fraction of
the throughput, accurate records are essential to the economics of the
extraction process.
High concentrations of solutes increase the chances for successful
efficient wastewater renovation processes. To exemplify systems which
offer promise for treatment by solvent extraction, the following list
(Table I) was taken from The Cost of Clean Water and Its Economic Impact,
Volume IV (22). The report was compiled by Cyrus W. Rice and Company and
entitled "Projected Wastewater Treatment Costs in the Organic Chemicals
Industry." No attempt was made to include all wastes reported therein;
only those with exceptionally strong waste loads or unusual composition
were considered. The table shows processes by product, the annual pro-
duction volume per unit of product, and strength in pounds per ton or
mg/1 of COD.
The list is suggested as a guide to current possibilities and should'not
be construed as that expected from the continual growth of the petroleum-
petrochemical industry. Data in the table include several processes
which produce organic waste loads that exceed 0.1 percent. Of all the
examples cited, none would contain a single solute as a pollutant. Cal-
culations, therefore, cannot be made to reflect the recovery and value
of solutes in these systems, but they exemplify likely candidates for
solvent extraction. Each process would have to be considered separately
and the economics judged from the results obtained. In most instances,
the value of the clean water product would exceed that of any chemical
product recovered. The reduction in quantity of wastes discharged would
be the deciding factor as to the reuse of the treated water.
30
-------
TABLE 1
PROCESS WASTES (27)
PRODUCT
Acetaldehyde
Acetic Acid and Anhydride
Acrylates
Benzene
Butadiene
Cyclohexane
Vinyl Chloride and
Polyvinylchloride
Ethylene Oxide
Phenol
Chlorinated Hydrocarbons
Formaldehyde
Polystyrene
U.S. CAPACITY
POUNDS /YEAR
1.8 x
le 2.0 x
0.5 x
7.7 x
3.2 x
3.3 x
5.0 x
4.5 x
2.0 x
, 1.8 x
5.0 x
1.9 x
9
10
9
10
9
10
9
10
9
10
9
10
9
10
9
10
9
10
9
10
9
10
9
10
WASTE
STREAM
VOLUME
Gal /Ton
1200
1000
1500
—
100
200-2000
2000
1650
1500
—
100
600
ORGANIC
CONTENT
rng/1
10,000
30,000
10-20,000
50-200
250-375
50-200
1200-1500
1400
13,200
—
1-5000
1400
31
-------
SECTION XI
RESEARCH PROGRAM
The direction of time and funds toward utilization of solvent extraction
as an industrial process for waste water renovation has been limited.
It is expected that the effort must increase.substantially. Many pro-
cesses produce wastes suitable for solvent extraction waste water treat-
ment. The history of solvent extraction has shown the logic of directing
extraction research toward those systems which defy "conventional" treat-
ment and/or those which are likely to produce salable products. Accord-
ingly, future research efforts must be directed toward industrial process
wastes containing substances which are refractory to biological treatment,
such as halogenated hydrocarbons. Such compounds are not only refractory
but are known causes of taste and odors in municipal water supplies in
very small concentrations.
33
-------
SECTION XII
GRANT PROGRAM
EPA's interest in utilization of solvent extraction has resulted in three
research proposals to apply the technology to various industrial waste
systems.
1. Project EEQ 12020 - Treatment of Waste Waters Resulting from
the Production of Polyhydrdic Organic Compounds-- The waste system con-
sidered contained 8-12 percent sodium chloride, 0.2 percent glycols in
water. Diisopropylamine and methyl diethyl amine were used for the
extraction of the glycols. Results showed a high selectivity for the
glycol in the solvent phase at the salt saturation stage of the extraction.
The selectivity was much lower at higher temperatures simulating the top
of the extractor but still favored the solvent phase.
Stagewise calculations showed that five theoretical stages were
required. At solvent-to-feed ratio of 5.7, the saturated raffinate con-
tained 0.08 percent glycol. Product water contained 0.25 percent glycol
and 2.5 percent salt. Although better separation could be obtained with
reflux, the high solvent-to-feed ratio makes the process uneconomical
compared to other types of treatment. Project time and funding constraints
did not permit further work concerning liquid extraction.
2. Project GLN 12020 - Extraction of Chemical Pollutants from
Aqueous Industrial Streams with Volatile Solvents - This is a recently
funded grant (September 1970) to develop a solvent extraction system
which uses liquified gases (carbon dioxide, propane, etc.) as the solvent.
The system will involve contacting waste water with liquified gaseous
solvent, separation of the raffinate, and flashing the gases followed by
recovery of the pollutant solutes. After obtaining sufficient physico-
chemical data on various organic solutes, a bench-scale continuous-flow
demonstration unit will be constructed for evaluation of petroleum
refining and petrochemical plant waste streams.
Applications to be considered are:
A. Phenolic waste waters
B. Aromatics nitration wastes
C. Aromatics extraction process waste water
D. Acetic acid manufacture waste water
E. Ethanolamine recovery system waste water
F. Synthetic resin manufacture waste water.
35
-------
3. Project R800947 - Extraction or Destruction of Chemical Pol-
lutants from Aqueous Industrial Waste Streams ~ This project was funded
as a federal grant to investigate, recommend methods and procedures, and
make preliminary designs for advanced waste treatment processes for the
reduction of contaminates in petrochemical plant waste effluent streams,
equivalent to or exceeding second stage biological treatment processes.
The waste products to be reduced will include halogenated hydrocarbons
and other refractory compounds which effect potable water supplies,
aquatic and marine life with respect to taste, tainting, and odor. The
primary objectives of this proposed research project are: (1) to accom-
plish the necessary bench work to quantitatively evaluate how selected
petrochemical waste streams can be treated economically by solvent
extraction; (2) to obtain distribution, performance, and other data
necessary for preliminary design of in-plant continuous flow pilot plants
for both solvent extraction and solvent recovery systems; (3) to make
preliminary designs of continuous flow in-plant pilot plants for the
above processes; (4) to determine and recommend the most suitable types
of equipment for the above processes and prepare cost estimates and
economics for the full-scale industrial process units. The basic pro-
gram will encompass necessary analytical and laboratory studies to
determine solvents and performance data applicable for the removal of
waste contaminates from selected waste streams to be made available by
five cooperating petrochemical industrial complexes.
The specific aim of the project will be to achieve a practical and work-
able foundation from which techniques will be made available for the
reduction of refractory chemicals in industrial waste effluent streams.
Emphasis will be placed on reducing chemicals affecting potable water
supplies, aquatic and marine life with respect to tainting and odor.
Since halogenated hydrocarbons are refractory to biological treatment,
control by solvent extraction may be feasible as a single process or in
combination with other methods of treatment. The proposal is premised
for joint participation between Federal, State, and industry. Various
industries which produce halogenated hydrocarbons have made commitments
to participate in the project and to apply solvent extraction technology,
along with other waste treatment processes, to determine the most feasible
method of treatment to reduce the concentrations of the offending chem-
icals .
The objectives of the project are to demonstrate waste treatment pro-
cedures for the reduction of certain petrochemical plant waste products
including refractory halogenated hydrocarbons which affect potable water
supplies and aquatic and marine life with respect to taste, odor, and
tainting.
Beyond these immediate objectives, however, is the concern over the
possible health hazards of these chemicals to the people consuming the
processed water.
36
-------
SECTION XIII
REFERENCES
1. McKinney, R.E., "Biological Oxidation of Organic Matter," Advances
in Biological Treatment. (Edited by Eckenfelder, W. W. Jr.; McCabe,
J.) Pergamon Press, New York, 1963.
2. Hachmuth, Karl H., "Industrial Viewpoints on Separation Processes,"
Chemical Engineering Progress. 48, No. 10, 11, 12, 1952.
3. Elgin, J. C.; Wynkoop, R.; and Lane, J. A., "Solvent Extraction and
Dialysis," Chemical Engineers Handbook. McGraw-Hill, 1963.
4. Wurm, H. J., "The Treatment of Phenolic Wastes," 23rd Industrial
Waste Conference 1968. Part II, Purdue University.
5. Oberg, A. G., and Jones, S. C., "Liquid-Liquid Extraction," Chemical
Engineering. 1963.
6. Hanson, C., "Solvent Extraction," Chemical Engineering, 1963.
7. Nelson, W. L., "Solvent Treating and Extraction Processes," Petroleum
Refinery Engineering. Third Edition, McGraw-Hill, 1949.
8. Lacy, W. J., "The Industrial Water Pollution Control R&D Program,"
National Association of Corrosion Engineers. 26th National Con-
ference . Philadelphia, Pennsylvania, March 2-6, 1970.
9. Rey, G., "Plan for the Future—Industrial Waste Water Reuse," Draft
Report.
10. Cywin, A., "Water Resources in the Year 2070," A.S.C.E. National
Water Resources Engineering Meeting, 1970.
11. Friday, J. R., PhD Thesis, Purdue University, Lafayette, Indiana, 1963,
12. Treybal, R. E., "Liquid Extraction, Second Edition," McGraw-Hill, 1963,
13. Elgin, J. C., U. S. Patent 2,364,892 (1944).
14. Zeitoun, M. A., and Davison, R. R., "Clean Water By Extraction,"
Chemical Engineering Progress. 60, No. 12, 1964.
15. Zeitoun, M. A.; Davison, R. R.; White, F. B.; Hood, D. W., "Solvent
Extraction of Sewage Secondary Effluents Heterogeneous Equilibrium
of Organics and Inorganics," Division of Water and Waste Chemistry.
147th ACS Meeting. April 5-10, 1964.
16. Wood, M. L., Private Telephone Communication.
37
-------
17. Lewis, W. L., and Martin, W. L., "Eemoval Phenols from Waste Water,"
Hydrocarbon Processing. Gulf Publishing Company, 46, No. 2, 1967.
18. Beychok, M. R., "Refinery and Petrochemical Effluents," Aqueous
Wastes from Petroleum and Petrochemical Plants. John Wiley & Sons,
1967.
19. Weinberger, L. W., and Stephen, D. G., "Technology of Advanced Waste
Treatment," International Conference on Water for Peace, Washington.
D. C.. May 23-31, 1967.
20. Tassios, D., "GLC Screens Extraction Solvents," Newark College of
Engineering, Newark, New Jersey.
21. Jantzen, E., "Das Fractoniente Distillieren Das Fractionierte
Verteilen," Dechema Monographien V.. 48, Verdaz Chemi, Berlin, 1932.
22. U.S. Federal Water Pollution Control Administration. The Cost of
Clean Water and Its Economic Impact. Vol IV, "Projected Wastewater
Treatment Costs in the Organic Chemicals Industry," by Cyrus Wm. Rice
and Co., 1969.
23. Akell, R. B., "Extraction Equipment Available in the U.S.," Chemical
Engineering Progress. 62 No. 9, 1966.
24. Reman, G. H., "Extraction Equipment Outside the U.S.," Chemical
Engineering Progress, 62 No. 9, 1966.
38
-------
INPUT TRANSACTION FORM
•••••^•••••^•••i
4. Title
SOLVENT EXTRACTION STATUS REPORT
.6.
•8.-
7. Author(s)
9. Organization
Environmental Protection Agency
Robert S. Kerr Water Research Center
10. Project Mo.
12020 SSZ
II, Contract/Grant No.
&v
tiafc Qreaaizstion
•• ..;•{., ;.:.\7-..,, -,:
15. Supplementary Notes
Environmental Protection Agency report
number EPA-R2-72-073, December 1972
16. Abstract
The history, basic principles, process application, laboratory approach, and grant
program concerning solvent (liquid-liquid) extraction was investigated in relation
to its application to industrial effluent waste water systems. A search of the
literature reveals that little consideration has been given to solvent extraction
as a feasible waste treatment method. Various aspects of solvent extraction tech-
nology are presented along with a number of industrial wastes which should be
considered for treatment. One of the areas of greatest need for research concern-
ing industrial waste water treatment is in the removal of refractory, and taste and
odor causing compounds. Application of solvent extraction to waste systems con-
taining low concentrations (0.05%) may be feasible on a "swap out" basis or in
conjunction with biological, adsorption, or incineration treatments as a pretreat-
ment step. Application of solvent extraction to waste systems for recovery of
salable products to offset cost of treatment should be studied for feasibility.
Accomplishments and plans regarding industrial research projects are presented.
17a. Descriptors
Solvent extraction, Separation techniques, Desalination processes, Solubility,
Solvents, Waste water treatment, Tertiary treatment, Water reuse, Water allocation,
Immiscibility.
17b. Identifiers
Solvent solubility, Solvent selectivity, Liquid extraction contactors.
17c. COWRR Field & Group
IS. Availability
Kftjrcfi
' SkcitntyCisss.
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, D. C. 2O24O
I institution EPA - Robert S. Kerr Water Research Center
Luther F. Mavhue
WRSIC 102 (REV. JUNE 1971)
------- |