Technology
French process separates aromatic isomers Organometallic enters into reactions that enable aromatic isomers to be separated using distillation Separation of aromatics in petroleum reforming or cracking streams has been a necessary if difficult practice. It could now become a bit easier, however, with a new process developed by Dr. Georges Gau, a French scientist working at the Centre National de la Recherche Scientifique (CNRS), Nancy (C&EN, May 3, page 27). The process operates by artificially
increasing relative volatility between isomers. In operation, a reaction mixture containing an organometallic (organosodium) compound is used to metalate a mixture of aromatics. The metalation is selective. Consequently, the mixture gets richer in certain nonmetalated aromatics. A simple distillation, Dr. Gau tells C&EN's André Pruniaux in Nancy, is then all that's necessary to separate the nonmetalated aromatics. The technique is particularly suited for separating p-xylene from m-xylene, and for separating ethylbenzene from p-xylene, Dr. Gau says. Lab stage. However, the technique is still at a laboratory stage and there are currently no plans for a pilot plant. "There should not be any problem in running a full-scale test," Dr.
Gau says, since the technique requires only classical distillation columns. The process is being offered for commercialization by Agence Nationale de Valorisation de la Recherche (ANVAR). During recent years, various techniques have been developed for separating aromatics contained in petroleum reforming or cracking streams. In some cases, the problem is to separate an aromatic from a nonaromatic (toluene from heptane, for example). In others, the problem is to isolate one aromatic isomer from another (p-xylene from m-xylene is an example). One new technique for separating an aromatic from a nonaromatic—liquid surfactant membranes—has emerged from the laboratories of Esso
Metalation a way to separate aromatics p-Xylene from m-xylene CH33
CH 3
CH,
I —C-Na
k, = 125 Metalated cumene
. |
.. |
, 4
P-Xylene
, |
CH 2 Na
CH3— C — H Metalated p-xylene
Cumene
CH 3 I C H 3 - C — Na
CH
3
CH 33 I -C-H
CH,Na
k =1160 Metalated cumene
+
~^"
m-Xylene
Cumene
|
ι
Metalated Γ m-xylene L
CH 2 Na
I IJ
CH,Na
k, = 9.2
ESSÎl^JI
CH,-C
H I
+ m"Xylene I ^ A C H +
Metalated Γ m-xylene L
p-Xylene
I] IJ
Ethylbenzene from p-xylene CH.,
— Na
CH 2 Na
C2H5 k. = 5.4
Metalated Γ Π ethylbenzene I J
. +
r v . P-Xylene ^
,i |J
Ethylbenzene!
M
+-
Metalated | p-xylene CH,
30
C&EN JUNE 14, 1971
Research and Engineering (C&EN, Oct. a reaction mixture containing 1 mole 5, 1970, page 36). For separating iso- of metalated cumene dissolved in mers, Japan Gas-Chemical's new proc- about 10 moles of cumene, the ratio ess using a mixture of hydrogen flu- of nonmetalated p-xylene to nonmetoride and boron trifluoride began op- alated m-xylene is about 3. erating in 1968. And in 1969, UniverThe equilibrium reactions can be sal Oil Products introduced its Parex carried out in a distillation unit. Orprocess, which uses a proprietary ad- ganometallic compounds are not volsorbent. According to Dr. Gau, the atile. Therefore, p-xylene and m-xymetalation-distillation process can be lene are the only components (with used both for separating aromatics cumene) present in the vapor phase. from nonaromatics and for separating Normally, the relative volatility of aromatic isomers. p-xylene to m-xylene is very near 1, Two operations. The process fea- which makes them difficult to septures two basic operations: first, the arate by normal distillation. With the production of carbanions, then the new process, however, the ratio of preaction of these ions with a mixture xylene to m-xylene in the vapor phase of aromatics and fractional distilla- is about 3. Consequently, Dr. Gau tion of the aromatics. Carbanions are notes, the separation factor of free pobtained by preparing the organome- xylene from free m-xylene—an artifitallic compound of cumene with cial relative volatility—is also 3. sodium. This compound, in which a The distillation unit includes two sodium atom replaces a hydrogen columns. p-Xylene is recovered from atom, is dissolved in excess cumene the top of the first. The bottoms of containing a chelating polyamine the first column are fractionally dis(such as tetramethyl-l,2-cyclohexane- tilled in a second column, where mdiamine). The mixture is extremely xylene is recovered. reactive and forms, at ambient temFew plates. p-Xylene recovered perature, various organometallic com- from the first column, using fewer pounds with the aromatics. than 20 plates, is 99.5% pure. If a re"The chemistry of organosodium action mixture containing 2 moles of compounds is very little known," Dr. metalated cumene dissolved in excess Gau comments. Most of the research cumene is used, the ratio of p-xylene on organometallic compounds has to m-xylene in the vapor phase is been undertaken with magnesium and about 9.2. With this artificial relative lithium, he explains. After studying volatility, fewer than 10 plates are various organosodium compounds, needed to obtain pure p-xylene, Dr. the French scientist finally selected Gau notes. the easy-to-prepare and inexpensive The chelating polyamine contained compound of cumene and sodium. in the reaction mixture decomposes Dr. Gau has focused on separation at high temperature, so fractional disof p-xylene from m-xylene. He first tillation should be carried out under dissolves 1 mole of metalated cumene partial vacuum—about 60 mm. Hg. In in 10 moles of cumene with 1 mole of any case, Dr. Gau says, temperature a chelating polyamine. The mixture should not exceed 100° C. is a very reactive ionized product. At The reaction mixture can be used room temperature, it reacts immedi- indefinitely. However, some fresh ately with the xylene mixture to give metalated organic and some fresh metalated p-xylene and metalated m- amine should be added continuously xylene. to maintain a high activity level. Constants different. Three equilib- Consumption of amine shouldn't be riums are thus achieved between cu- more than one part per thousand, Dr. mene, p-xylene, m-xylene, and their Gau estimates. metalated compounds. However the The new process isn't limited to kinetic constants are quite different, separating xylene isomers. A very and m-xylene reacts more with met- promising application, Dr. Gau says, alated cumene than does p-xylene. is separation of ethylbenzene from pThe equlibrium constant between met- xylene. alated cumene and m-xylene is 1160, For a feedstock containing 1 mole whereas it is only 125 between metal- of p-xylene and 1 mole of ethylbenated cumene and p-xylene. Further- zene, and with a reaction mixture conmore, metalated p-xylene reacts with taining 1 mole of metalated cumene m-xylene to give p-xylene and metal- dissolved in excess cumene, the equiated m-xylene, with an equilibrium librium constant between p-xylene constant of 9.2. and ethylbenzene is 5.4. The ratio of The metal exchange reactions pro- ethylbenzene to p-xylene in the vapor vide a mixture enriched in free p-xy- phase is 1.8. Ethylbenzene can be relene. Dr. Gau estimates that with covered with any specified purity at a feedstock containing 1 mole of p-xy- the top of the first distillation column, lene and 1 mole of m-xylene, and with and p-xylene in the second.
Citric acid used in S02 recovery When market analysts survey companies most likely to profit from burgeoning pollution-abatement activities, they usually concentrate on equipment makers and consulting firms. They might now broaden their sights to include citric acid producers, whose business would soar if the sulfur dioxide recovery system that the U.S. Bureau of Mines' chemical engineers have devised proves to be anything nearly as good as test results indicate. "We think we're far ahead in sulfur dioxide removal technology and much lower in operating costs," says D'Arcy George of BuMines' Salt Lake City laboratory, where the system is under development. Mr. George tells C&EN that it is possible to remove as much as 99% of the sulfur dioxide from copper ore smelter flue gas. And the elemental sulfur recovered in the operation is a remarkable 99.99% pure. In addition to trapping sulfur dioxide generated in ore smelting operations, the technique may also be applied to cleansing stack gases from fossil fuel power plants. It might also cleanse off-gas from such industrial processes as the Claus route to making sulfur from hydrogen sulfide. Complex. Key to the BuMines system is interaction between sulfur dioxide and citrate ions to form a bisulfite-citrate complex. This complex, in turn, interacts with hydrogen sulfide, yielding elemental sulfur and releasing citrate ions for re-use. Reaction rates are rapid, yields are high, and the amount of citrate lost in the cycle is quite small, Joe B. Rosenbaum, research director at the BuMines laboratory, told the American Institute of Mining, Metallurgical and Petroleum Engineers last week in Washington, D.C. For their part, citric acid makers view the development with guarded optimism. "It's too early to tell yet just where all this will lead," cautions William G. Ray, sales manager with the technical chemical department of Pfizer, Inc., the world's largest citric acid maker. "We're holding off on market projections until technological development of the process is further refined." It would be easy to let one's imagination take flight and to become dazzled by the numbers involved. In Mr. Rosenbaum's opinion, for example, a single large coal- or oil-fired public utility station could call for an JUNE 14, 1971 C&EN
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