Process for separating saturated fatty acids from each other

A process for separating a first saturated fatty acid from a second saturated fatty acid contained in a feed mixture comprising the acids, the chain length of the first being at least two carbon atoms greater than that of the second. The process comprises contacting the feed mixture at adsorption conditions comprising a crystalline silica having a silica to alumina mole ratio of at least 12, thereby selectively adsorbing the first saturated fatty acid. The remainder of the feed mixture is then removed from the adsorbent, and the first acid recovered from the adsorbent by desorption at desorption conditions with a desorbent liquid soluble in the feed mixture and having a polarity index of at least 3.5.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The field of art to which this invention pertains is the solid bed 
adsorptive separation of fatty acids. More specifically the invention 
relates to a process for separating saturated fatty acids which process 
employs an adsorbent comprising particular polymers which selectively 
adsorb one fatty acid from a feed mixture containing more than one fatty 
acid. 
2. Background Information 
It is known in the separation art that certain crystalline aluminosilicates 
can be used to separate certain esters of fatty acids from mixtures 
thereof. For example, in U.S. Pat. Nos. 4,048,205, 4,049,688 and 
4,066,677, there are claimed processes for the separation of esters of 
fatty acids of various degrees of unsaturation from mixtures of esters of 
saturated and unsaturated fatty acids. These processes use adsorbents 
comprising an X or a Y zeolite containing a selected cation at the 
exchangeable cationic sites. 
The use of crystalline silica for the separation of a fatty acid from a 
rosin acid is disclosed in U.S. Pat. No. 4,404,145 to Cleary et al. That 
patent also teaches the use of a displacement fluid having a minimum 
desired polarity index, i.e., at least 3.5. The hypothesis stated in U.S. 
Pat. No. 4,404,145 to Cleary et al. as to the unique suitability of its 
process for its claimed separation was that the silicalite pores were of a 
size and shape that enabled the silicalite to function as a molecular 
sieve, i.e., accept the molecules of fatty acids into its channels or 
internal structure, while rejecting the molecules of rosin acids. 
The adsorptive separation of saturated fatty acids of different chain 
lengths from each other was disclosed in U.S. Pat. No. 4,353,839 to Cleary 
et al. The adsorbent used in the process of that patent was a hydrophobic 
insoluble crosslinked polystyrene polymer, and the desorbent a mixture of 
dimethylformamide and water. 
U.S. Pat. No. 4,444,986 to Dessau discloses the use of a high silica 
zeolite for the separation of compounds of the same homologous series, 
including acid substituents of hydrocarbons, with the separation occurring 
in the presence of a solvent, including a polar solvent. 
The present invention is based on the discovery that crystalline silica is 
highly suitable as an adsorbent for the separation process of this 
invention in that it exhibits relative selectivity for a long chain 
saturated fatty acid with respect to a shorter chain saturated fatty acid 
when used with an appropriate desorbent. 
SUMMARY OF THE INVENTION 
In brief summary my invention is, in one embodiment, a process for 
separating a first saturated fatty acid from a mixture comprising the 
first saturated fatty acid and a second saturated fatty acid, the chain 
length of the first being at least two carbon atoms greater than that of 
the second. The process comprises contacting at adsorption conditions that 
mixture with an adsorbent comprising a crystalline silica having a silica 
to alumina mole ratio of at least 12, thereby selectively adsorbing the 
first saturated fatty acid. The remainder of the feed mixture is removed 
from the adsorbent, and the first saturated fatty acid recovered from the 
adsorbent by desorption, at desorption conditions, with a desorbent liquid 
soluble in the feed mixture and having a polarity index of at least 3.5. 
Other embodiments of our invention encompass details about flow schemes, 
feed mixtures, adsorbents, desorbent materials and operating conditions, 
all of which are hereinafter disclosed in the following discussion of each 
of the facets of the present invention. 
DESCRIPTION OF THE INVENTION 
At the outset the definitions of various terms used throughout the 
specification will be useful in making clear the operation, objects and 
advantages of the process. 
A "feed mixture" is a mixture containing one or more extract components and 
one or more raffinate components to be separated by the process. The term 
"feed stream" indicates a stream of a feed mixture which passes to the 
adsorbent used in the process. 
An "extract component" is a compound or type of compound that is more 
selectively adsorbed by the adsorbent while a "raffinate component" is a 
compound or type of compound that is less selectively adsorbed. In this 
process a first fatty acid is an extract component and a second fatty acid 
is a raffinate component. The term "desorbent material" shall mean 
generally a material capable of desorbing an extract component. The term 
"desorbent stream" or "desorbent input stream" indicates the stream 
through which desorbent material passes to the adsorbent. The term 
"raffinate stream" or "raffinate output stream" means a stream through 
which a raffinate component is removed from the adsorbent. The composition 
of the raffinate stream can vary from essentially 100% desorbent material 
to essentially 100% raffinate components. The term "extract stream" or 
"extract output stream" shall mean a stream through which an extract 
material which has been desorbed by a desorbent material is removed from 
the adsorbent. The composition of the extract stream likewise can vary 
from essentially 100% desorbent material to essentially 100% extract 
components. At least a portion of the extract stream and preferably at 
least a portion of the raffinate stream from the separation process are 
passed to separation means, typically fractionators, where at least a 
portion of desorbent material is separated to produce an extract product 
and a raffinate product. The terms "extract product" and "raffinate 
product" mean products produced by the process containing, respectively, 
an extract component and a raffinate component in higher concentrations 
than those found in the extract stream and the raffinate stream. Although 
it is possible by the process of this invention to produce a high purity, 
first or second fatty acid product (or both) at high recoveries, it will 
be appreciated that an extract component is never completely adsorbed by 
the adsorbent, nor is a raffinate component completely non-adsorbed by the 
adsorbent. Therefore, varying amounts of a raffinate component can appear 
in the extract stream and likewise, varying amounts of an extract 
component can appear in the raffinate stream. The extract and raffinate 
streams then are further distinguished from each other and from the feed 
mixture by the ratio of the concentrations of an extract component and a 
raffinate component appearing in the particular stream. More specifically, 
the ratio of the concentration of the first saturated fatty acid to that 
of the less selectively adsorbed second saturated fatty acid will be 
lowest in the raffinate stream, next highest in the feed mixture, and the 
highest in the extract stream. Likewise, the ratio of the concentration of 
the less selectively adsorbed second saturated fatty acid to that of the 
more selectively adsorbed first saturated fatty acid will be highest in 
the raffinate stream, next highest in the feed mixture, and the lowest in 
the extract stream. 
The term "selective pore volume" of the adsorbent is defined as the volume 
of the adsorbent which selectively adsorbs an extract component from the 
feed mixture. The term "non-selective void volume" of the adsorbent is the 
volume of the adsorbent which does not selectively retain an extract 
component from the feed mixture. This volume includes the cavities of the 
adsorbent which contain no adsorptive sites and the interstitial void 
spaces between adsorbent particles. The selective pore volume and the 
non-selective void volume are generally expressed in volumetric quantities 
and are of importance in determining the proper flow rates of fluid 
required to be passed into various operational zones for efficient 
operations to take place for a given quantity of adsorbent in simulated 
moving bed embodiments of this process. 
Before considering feed mixtures which can be charged to the process of our 
invention, brief reference is first made to the terminology and to the 
general production of fatty acids. The fatty acids are a large group of 
aliphatic monocarboxylic acids, many of which occur as glycerides (esters 
of glycerol) in natural fats and oils. Although the term "fatty acids" has 
been restricted by some to the saturated acids of the acetic acid series, 
both normal and branched chain, it is now generally used, and is so used 
herein, to include also related unsaturated acids, certain substituted 
acids, and even aliphatic acids containing alicyclic substituents. The 
naturally occurring fatty acids with a few exceptions are higher straight 
chain unsubstituted acids containing an even number of carbon atoms. The 
unsaturated fatty acids can be divided, on the basis of the number of 
double bonds in the hydrocarbon chain, into monoethanoid, diethanoid, 
triethanoid, etc. (or monoethylenic, etc.). Thus the term "unsaturated 
fatty acid" is a generic term for a fatty acid having at least one double 
bond, and the term "polyethanoid fatty acid" means a fatty acid having 
more than one double bond per molecule. Fatty acids are typically prepared 
from glyceride fats or oils by one of several "splitting" or hydrolytic 
processes. In all cases the hydrolysis reaction may be summarized as the 
reaction of a fat or oil with water to yield fatty acids plus glycerol. In 
modern fatty acid plants this process is carried out by continuous high 
pressure, high temperature hydrolysis of the fat. Starting materials most 
commonly used for the production of fatty acids include coconut oil, palm 
oil, inedible animal fats, and the commonly used vegetable oils, soybean 
oil, cottonseed oil and corn oil. The composition of the fatty acids 
obtained from the "splitter" is dependent on the fat or oil from which 
they were made. As detailed data for the fatty acid composition of fats 
have accumulated over a wide range of material, it has become more and 
more apparent that natural fats tend to align themselves, by their 
component acids, in groups according to their biological origin. Moreover, 
it has become clear that the fats of the simplest and most primitive 
organisms are usually made up from a very complex mixture of fatty acids 
whereas as biological development has proceeded, the chief component acids 
of the fats of the higher organisms have become fewer in number. In the 
animal kingdom this change in type is remarkably consistent and 
culminates, in the fats of the higher land animals, in fats in which 
oleic, palmitic and stearic acids are the only major components. All fats 
of aquatic origin contain a wide range of combined fatty acids, mainly of 
the unsaturated series. On passing from fats of aquatic to those of land 
animals there is also a marked simplification in the composition of the 
mixed fatty acids; most of the unsaturated acids, except oleic acid, 
disappear. The final result is that in most of the higher land animals the 
major component acids of the fats are restricted to oleic, palmitic and 
stearic and moreover, that about 60-65% of the acids belong to the 
C.sub.18 series, saturated or unsaturated. 
Lauric (C.sub.12 o) and myristic (C.sub.14 o) acids are obtained in 
admixture from palm oil. These acids may be used as ingredients in 
perfumes. Individual acids rather than the mixture, however, may be 
desirable so as to tailor the properties of the perfume to exactly what is 
required. Thus, the separations obtained by the process of the present 
invention would be particularly useful in the perfume industry. 
Fractionation of saturated fatty acids according to molecular weight is 
sometimes accomplished in fractional distillation. There is somewhat of a 
difference in the volatility of any two fatty acids of different chain 
length and in practice, the utility of fractional distillation is enhanced 
by the absence of odd-membered acids in the natural fats, so that 2 carbon 
atoms is nearly always the minimum difference in chain length of the fatty 
acids present in a mixture. Fractionating columns in such operation are 
sometimes capable of producing fatty acids of 95% purity or better from 
the viewpoint of chain length depending on the chain length in question. 
It is not possible, however, to separate certain saturated fatty acids 
from each other by commercial fractional distillation, particularly 
stearic acid from palmitic acid which have carbon atom chain lengths of 18 
and 16, respectively, or lauric from myristic acid which have chain 
lengths of 12 and 14, respectively. 
Our process is directed to separating certain mixtures of saturated fatty 
acids. An example of a typical feed mixture is known as U.S. 
pharmaceutical grade "stearic acid", which in fact is about a 50--50 
mixture of stearic and palmitic acids. A mixture of lauric and myristic 
acids is contained in coconut oil. Feed mixtures which can be charged to 
our process may contain, in addition to fatty acids, a diluent material 
that is not adsorbed by the adsorbent and which is preferably separable 
from the extract and raffinate output streams by fractional distillation. 
When a diluent is employed, the concentration of diluent in the mixture of 
diluent and fatty acids may be from a few vol. % up to about 90 vol. %. 
Desorbent materials used in various prior art adsorptive separation 
processes vary depending upon such factors as the type of operation 
employed. In the swing bed system in which the selectively adsorbed feed 
component is removed from the adsorbent by a purge stream, desorbent 
selection is not as critical and desorbent materials comprising gaseous 
hydrocarbons such as methane, ethane, etc., or other types of gases such 
as nitrogen or hydrogen may be used at elevated temperatures or reduced 
pressures or both to effectively purge the adsorbed feed component from 
the adsorbent. However, in adsorptive separation processes which are 
generally operated continuously at substantially constant pressures and 
temperatures to insure liquid phase, the desorbent material must be 
judiciously selected to satisfy many criteria. First the desorbent 
material should displace an extract component from the adsorbent with 
reasonable mass flow rates without itself being so strongly adsorbed as to 
unduly prevent an extract component from displacing the desorbent material 
in a following adsorption cycle. Expressed in terms of the selectivity 
(hereinafter discussed in more detail), it is preferred that the adsorbent 
be more selective for all of the extract components with respect to a 
raffinate component than it is for the desorbent material with respect to 
a raffinate component. Secondly, desorbent materials must be compatible 
with the particular adsorbent and the particular feed mixture. More 
specifically, they must not reduce or destroy the critical selectivity of 
the adsorbent for an extract component with respect to a raffinate 
component. Desorbent materials should additionally be substances which are 
easily separable from the feed mixture that is passed into the process. 
Both the raffinate stream and the extract stream are removed from the 
adsorbent in admixture with desorbent material and without a method of 
separating at least a portion of the desorbent material, the purity of the 
extract product and the raffinate product would not be very high, nor 
would the desorbent material be available for reuse in the process. It is 
therefore contemplated that any desorbent material used in this process 
will preferably have a substantially different average boiling point than 
that of the feed mixture to allow separation of at least a portion of 
desorbent material from feed components in the extract and raffinate 
streams by simple fractional distillation, thereby permitting reuse of 
desorbent material in the process. The term "substantially different" as 
used herein shall mean that the difference between the average boiling 
points between the desorbent material and the feed mixture shall be at 
least about 5.degree. C. The boiling range of the desorbent material may 
be higher or lower than that of the feed mixture. Finally, desorbent 
materials should also be materials which are readily available and 
therefore reasonable in cost. In the preferred isothermal, isobaric, 
liquid-phase operation of the process of this invention, it has been 
found, as will be discussed at length hereinbelow, displacement fluids 
comprising a diluent soluble in the feed mixture and having a polarity 
index of at least 3.5 to be effective when the conditions at which the 
retention and displacement is carried out is from about 20.degree. C. to 
about 200.degree. C. with pressure sufficient to maintain liquid phase. 
When the feedstock is tallow, the preferred conditions are about 
120.degree. C. to about 150.degree. C. with pressure sufficient to 
maintain liquid phase. 
The prior art has also recognized that certain characteristics of 
adsorbents are highly desirable, if not absolutely necessary, to the 
successful operation of a selective adsorption process. Such 
characteristics are equally important to this process. Among such 
characteristics are: (1) adsorptive capacity for some volume of an extract 
component per volume of adsorbent; (2) the selective adsorption of an 
extract component with respect to a raffinate component and the desorbent 
material; and (3) sufficiently fast rates of adsorption and desorption of 
an extract component to and from the adsorbent. Capacity of the adsorbent 
for adsorbing a specific volume of an extract component is, of course, a 
necessity; without such capacity the adsorbent is useless for adsorptive 
separation. Furthermore, the higher the adsorbent's capacity for an 
extract component the better is the adsorbent. Increased capacity of a 
particular adsorbent makes it possible to reduce the amount of adsorbent 
needed to separate an extract component of known concentration contained 
in a particular charge rate of feed mixture. A reduction in the amount of 
adsorbent required for a specific adsorptive separation reduces the cost 
of the separation process. It is important that the good initial capacity 
of the adsorbent be maintained during actual use in the separation process 
over some economically desirable life. The second necessary adsorbent 
characteristic is the ability of the adsorbent to separate components of 
the feed; or, in other words, that the adsorbent possess adsorptive 
selectivity, (B), for one component as compared to another component. 
Relative selectivity can be expressed not only for one feed component as 
compared to another but can also be expressed between any feed mixture 
component and the desorbent material. The selectivity, (B), as used 
throughout this specification is defined as the ratio of the two 
components of the adsorbed phase over the ratio of the same two components 
in the unadsorbed phase at equilibrium conditions. Relative selectivity is 
shown as Equation 1 below: 
##EQU1## 
where C and D are two components of the feed represented in volume percent 
and the subscripts A and U represent the adsorbed and unadsorbed phases 
respectively. The equilibrium conditions were determined when the feed 
passing over a bed of adsorbent did not change composition after 
contacting the bed of adsorbent. In other words, there was no net transfer 
of material occurring between the unadsorbed and adsorbed phases. Where 
selectivity of two components approaches 1.0 there is no preferential 
adsorption of one component by the adsorbent with respect to the other; 
they are both adsorbed (or non-adsorbed) to about the same degree with 
respect to each other. As the (B) becomes less than or greater than 1.0 
there is a preferential adsorption by the adsorbent for one component with 
respect to the other. When comparing the selectivity by the adsorbent of 
one component C over component D, a (B) larger than 1.0 indicates 
preferential adsorption of component C within the adsorbent. A (B) less 
than 1.0 would indicate that component D is preferentially adsorbed 
leaving an unadsorbed phase richer in component C and an adsorbed phase 
richer in component D. Ideally desorbent materials should have a 
selectivity equal to about 1 or slightly less than 1 with respect to all 
extract components so that all of the extract components can be desorbed 
as a class with reasonable flow rates of desorbent material and so that 
extract components can displace desorbent material in a subsequent 
adsorption step. While separation of an extract component from a raffinate 
component is theoretically possible when the selectivity of the adsorbent 
for the extract component with respect to the raffinate component is 
greater than 1, it is preferred that such selectivity approach a value of 
2. Like relative volatility, the higher the selectivity the easier the 
separation is to perform. Higher selectivities permit a smaller amount of 
adsorbent to be used. The third important characteristic is the rate of 
exchange of the extract component of the feed mixture material or, in 
other words, the relative rate of desorption of the extract component. 
This characteristic relates directly to the amount of desorbent material 
that must be employed in the process to recover the extract component from 
the adsorbent; faster rates of exchange reduce the amount of desorbent 
material needed to remove the extract component and therefore permit a 
reduction in the operating cost of the process. With faster rates of 
exchange, less desorbent material has to be pumped through the process and 
separated from the extract stream for reuse in the process. 
The adsorbent to be used in the process of this invention comprises 
crystalline silica having a silica/alumina mole ratio of at least 12. One 
such crystalline silica is known as silicalite which has a silica/alumina 
mole ratio of infinity, i.e., it contains no alumina. Silicalite is a 
hydrophobic crystalline silica molecular sieve. Silicalite is disclosed 
and claimed in U.S. Pat. Nos. 4,061,724 and 4,104,294 to Grose et al., 
incorporated herein by reference. Due to its aluminum-free structure, 
silicalite does not show ion-exchange behavior, and is hydrophobic and 
organophilic. Low alumina crystalline silica is uniquely suitable for the 
separation process of this invention for the reason that it exhibits 
relative selectivity for the longer chain saturated fatty acids, 
presumably because of varying degrees of electro-chemical attraction 
between the crystalline silica and different saturated fatty acids. This 
is in contradistinction to the process of aforementioned U.S. Pat. No. 
4,404,145 in which the effectiveness of crystalline silica is based on the 
hypothesis that its pores are of a size and shape that enable it to 
function as a molecular sieve, i.e., accept the molecules of saturated 
fatty acids (which are relatively flexible) into its channels or internal 
structure, while rejecting the molecules of unsaturated fatty acids (which 
are relatively rigid), the separation from which was the concern of that 
patent. A more detailed discussion of silicalite may be found in the 
article, "Silicalite, A New Hydrophobic Crystalline Silica Molecular 
Sieve"; Nature, Vol. 271, Feb. 9, 1978, incorporated herein by reference. 
Examples of other crystalline silicas suitable for use in the present 
invention are those having the trademark designation "ZSM" and 
silica/alumina mole ratios of at least 12. The ZSM adsorbents are as 
described in U.S. Pat. Nos. 4,309,281 and 4,444,986 to Dessau, 
incorporated herein by reference. The latter Dessau patent does make 
certain broad statements that the ZSM type zeolite may be employed to 
selectively sorb higher molecular weight organic compounds in the same 
homologous series and that the sorption may take place in the presence of 
a polar solvent, (although no preference is stated, and a non-polar 
solvent is exemplified) but is completely silent as to how desorption is 
to be effected, other than by stating "by conventional desorbing 
techniques such as stripping." In order to have a viable process, 
desorption is as important a criteria, if not more important, than 
adsorption. The present invention addresses the complete separation scheme 
necessary for a viable process, including the use of very specific 
desorbents as will be discussed hereinbelow. 
Typically, adsorbents used in separative processes contain the crystalline 
material dispersed in an amorphous material or inorganic matrix, 
particularly an amorphous material having channels and cavities therein 
which enable liquid access to the crystalline silica. The binder aids in 
forming or agglomerating the crystalline particles of the crystalline 
silica which otherwise would comprise a fine powder. The silica molecular 
sieve may thus be in the form of particles such as extrudates, aggregates, 
tablets, macrospheres or granules having a desired particle range, 
preferably from about 16 to 60 mesh (Standard U.S. Mesh). Colloidal 
amorphous silica is an ideal binder for crystalline silica in that like 
the crystalline silica itself this binder exhibits no reactivity for the 
free fatty acids. The preferred silica is marketed by DuPont Company under 
the trademark "Ludox." The crystalline silica powder is dispersed in the 
Ludox which is then gelled and treated so as to substantially eliminate 
hydroxyl groups, such as by thermal treatment in the presence of oxygen at 
a temperature from about 450.degree. C. to about 1000.degree. C. for a 
minimum period from about 3 hours to about 48 hours. The crystalline 
silica should be present ih the silica matrix in amounts ranging from 
about 75 wt. % to about 98 wt. % crystalline silica based on volatile free 
composition. 
It has been observed that even crystalline silica may be ineffective in 
separating fatty acids from each other. It is hypothesized that 
hydrogen-bonded dimerization reactions occur in which there is an 
alignment between the molecules of the fatty acids. These dimerization 
reactions may be represented by the formula: 
EQU FA+FA.revreaction.(FAFA) 
where FA stands for fatty acids. The dimers would preclude separation of 
the fatty acids by blocking access to the adsorbent or reducing the 
selectivity. This hindrance to separation caused by the presence of dimers 
does not appear to be a significant problem in the aforementioned process 
for separation of esters of fatty acids. 
It has been discovered that the above dimerization reactions may be 
minimized if the desorbent is properly selected. There are liquids which 
exhibit the property of minimizing dimerization. The measure of this 
property was found to be the polarity index of the liquid. Polarity index 
is as described in the article, "Classification of the Solvent Properties 
of Common Liquids"; Snyder, L. J. Chromatography, 92, 223 (1974), 
incorporated herein by reference. The minimum polarity index of the 
desorbent required for the process of the present invention is 3.5. 
Polarity indexes for certain selected diluents are as follows: 
______________________________________ 
Solvent Polarity Index 
______________________________________ 
Isooctane -0.4 
n-Hexane 0.0 
Toluene 2.3 
p-Xylene 2.4 
Benzene 3.0 
Methylethylketone 4.5 
Acetone 5.4 
3-Pentanone (estimated) 
4.4 
______________________________________ 
The adsorbent may be employed in the form of a dense compact fixed bed 
which is alternatively contacted with the feed mixture and desorbent 
materials. In the simplest embodiment of the invention the adsorbent is 
employed in the form of a single static bed in which case the process is 
only semi-continuous. In another embodiment a set of two or more static 
beds may be employed in fixed bed contacting with appropriate valving so 
that the feed mixture is passed through one or more adsorbent beds while 
the desorbent materials can be passed through one or more of the other 
beds in the set. The flow of feed mixture and desorbent materials may be 
either up or down through the desorbent. Any of the conventional apparatus 
employed in static bed fluid-solid contacting may be used. 
Moving bed or simulated moving bed flow systems, however, have a much 
greater separation efficiency than fixed bed systems and are therefore 
preferred. In the moving bed or simulated moving bed processes, the 
retention and displacement operations are continuously taking place which 
allows both continuous production of an extract and a raffinate stream and 
the continual use of feed and displacement fluid streams. One preferred 
embodiment of this process utilizes what is known in the art as the 
simulated moving bed countercurrent flow system. The operating principles 
and sequence of such a flow system are described in U.S. Pat. No. 
2,985,589 incorporated herein by reference. In such a system, it is the 
progressive movement of multiple liquid access points down a molecular 
sieve chamber that simulates the upward movement of molecular sieve 
contained in the chamber. Reference can also be made to a paper entitled 
"Continuous Adsorptive Processing--A New Separation Technique" by D. B. 
Broughton presented at the 34th Annual Meeting of the Society of Chemical 
Engineers at Tokyo, Japan on Apr. 2, 1969, both references incorporated 
herein by reference, for further explanation of the simulated moving bed 
countercurrent process flow scheme. 
Another embodiment of a simulated moving bed flow system suitable for use 
in the process of the present invention is the co-current high efficiency 
simulated moving bed process disclosed in our assignee's U.S. Pat. No. 
4,402,832, incorporated by reference herein in its entirety. 
The size of the units which can utilize the process of this invention can 
vary anywhere from those of pilot plant scale (see for example our 
assignee's U.S. Pat. No. 3,706,812, incorporated herein by reference) to 
those of commercial scale and can range in flow rates from as little as a 
few cc an hour up to many thousands of gallons per hour. 
It is contemplated with any flow scheme used to carry out the present 
invention that at least a portion of the extract output stream will pass 
into a separation means wherein at least a portion of the desorbent can be 
separated to produce an extract product containing a reduced concentration 
of desorbent. Preferably, but not necessary to the operation of the 
process, at least a portion of the raffinate output stream will also be 
passed to a separation means wherein at least a portion of the desorbent 
can be separated to produce a desorbent stream which can be reused in the 
process and a raffinate product containing a reduced concentration of 
desorbent. The separation means will typically be a fractionation column, 
the design and operation of which is well known to the separation art. 
Although both liquid and vapor phase operations can be used in many 
adsorptive separation processes, liquid-phase operation is preferred for 
this process because of the lower temperature requirements and because of 
the higher yields of extract product that can be obtained with 
liquid-phase operation over those obtained with vapor-phase operation. 
Desorption conditions will thus include, as hereinbefore mentioned, a 
pressure sufficient to maintain a liquid phase. Adsorption conditions may 
include, as a matter of convenience, the same range of temperatures and 
pressures as used for desorption conditions. 
A dynamic testing apparatus is employed to test various adsorbents with a 
particular feed mixture and desorbent material to measure the adsorbent 
characteristics of adsorptive capacity, selectivity and exchange rate. The 
apparatus consists of an adsorbent chamber comprising a helical column of 
approximately 70 cc volume having inlet and outlet portions at opposite 
ends of the chamber. The chamber is contained within a temperature control 
means and, in addition, pressure control equipment is used to operate the 
chamber at a constant predetermined pressure. Quantitative and qualitative 
analytical equipment such as refractometers, polarimeters and 
chromatographs can be attached to the outlet line of the chamber and used 
to detect quantitatively or determine qualitatively one or more components 
in the effluent stream leaving the adsorbent chamber. A pulse test, 
performed using this apparatus and the following general procedure, is 
used to determine selectivities and other data for various adsorbent 
systems. The adsorbent is filled to equilibrium with a particular 
desorbent material by passing the desorbent material through the adsorbent 
chamber. At a convenient time, a pulse of feed containing known 
concentrations of a tracer and of a particular extract component or of a 
raffinate component or both, all diluted in desorbent, is injected for a 
duration of several minutes. Desorbent flow is resumed, and the tracer and 
the extract component or the raffinate component (or both) are eluted as 
in a liquid-solid chromatographic operation. The effluent can be analyzed 
onstream or alternatively effluent samples can be collected periodically 
and later analyzed separately by analytical equipment and traces of the 
envelopes of corresponding component peaks developed. 
From information derived from the test adsorbent, performance can be rated 
in terms of void volume, retention volume for an extract or a raffinate 
component, selectivity for one component with respect to the other, and 
the rate of desorption of an extract component by the desorbent. The 
retention volume of an extract or a raffinate component may be 
characterized by the distance between the center of the peak envelope of 
an extract or a raffinate component and the peak envelope of the tracer 
component or some other known reference point. It is expressed in terms of 
the volume in cubic centimeters of desorbent pumped during this time 
interval represented by the distance between the peak envelopes. 
Selectivity, (B), for an extract component with respect to a raffinate 
component may be characterized by the ratio of the distance between the 
center of the extract component peak envelope and the tracer peak envelope 
(or other reference point) to the corresponding distance between the 
center of the raffinate component peak envelope and the tracer peak 
envelope. The rate of exchange of an extract component with the desorbent 
can generally be characterized by the width of the peak envelopes at half 
intensity. The narrower the peak width, the faster the desorption rate. 
The desorption rate can also be characterized by the distance between the 
center of the tracer peak envelope and the disappearance of an extract 
component which has just been desorbed. This distance is again the volume 
of desorbent pumped during this time interval.

The following working example is presented to illustrate the process of the 
present invention and is not intended to unduly restrict the scope and 
spirit of the claims attached hereto. 
EXAMPLE 
This example presents the results of using Ludox bound silicalite for 
separating myristic acid from about a 50--50 mixture of myristic and 
lauric acids diluted in desorbent in a volume ratio of desorbent to acid 
mixture of 10:1. The desorbent used was 100% 3-pentanone. 
Data was obtained using the pulse test apparatus and procedure previously 
described at a temperature of 120.degree. C. Specifically, the adsorbent 
was placed in a 70 cc helical coiled column and the following sequence of 
operations was used. Desorbent material was continuously run downflow 
through the column containing the adsorbent at a flow rate of 1.2 ml/min. 
At a convenient time, the flow of desorbent material was stopped, and a 5 
cc sample of feed mixture was injected into the column via a sample loop 
and the flow of desorbent material was resumed. Samples of the effluent 
were automatically collected in an automatic sample collector and later 
analyzed by chromatographic analysis. 
The FIGURE is a graphical presentation of the results of the pulse tests. 
The FIGURE shows that myristic acid is more strongly adsorbed than lauric 
acid, particularly for the desorbent used. Furthermore, the separation 
achieved for this combination was substantial and clearly of commercial 
feasibility.