Distillation apparatus

Apparatus for the distillation of styrene from hydrocarbon feedstock incorporating a system for injection of gaseous polymerization inhibitor.

BACKGROUND OF THE INVENTION 
The present invention relates to apparatus for the distillation of styrene 
monomer from hydrocarbon feedstocks, wherein polymerization of the styrene 
product is prevented by injection of a gaseous inhibitor. More 
particularly, the present invention relates to apparatus for distillation 
of styrene from ethylbenzene wherein a system is incorporated for 
injecting nitrogen-based gases as polymerization inhibitors. 
DESCRIPTION OF THE PRIOR ART 
It is well known in the prior art to produce styrene monomer via various, 
diverse distillation processes from hydrocarbon feedstocks. For example, 
it has long been known to subject ethylbenzene to catalytic 
dehydrogenation followed by fractional distillation to yield styrene. The 
fractionation serves to separate styrene from other reaction products and 
to yield a relatively pure product stream freed from such contaminants as 
tars and/or polymeric materials. Difficulty is encountered, however, since 
the styrene monomer has a propensity to polymerize under conditions of 
heat, and particularly those to which the styrene-containing feedstock is 
exposed in the operation of an efficient distillation process. Therefore, 
it has become widespread practice to employ various styrene polymerization 
inhibitors such as sulfur, tertiarybutylcatechol, hydroquinone, certain 
nitroso compounds, etc. Typically, these polymerization inhibitors are 
added as liquids at suitable points within the distillation train. See, 
for example, U.S. Pat. No. 3,515,647. 
Recent developments in the field of styrene polymerization inhibitors, 
especially those effective during the separation and purification of 
styrene, have provided certain effective gaseous inhibitors. These gaseous 
inhibitors, most notably nitrogen oxides and derivatives thereof, have 
proved exceedingly effective in inhibiting the polymerization of styrene 
monomer during distillation thereof. The degree of efficacy permits 
introduction of the inhibitors into the distillation system at various, 
advantageous points and also under differing conditions of operation. 
Accordingly, the need exists to provide suitable apparatus which may 
incorporate this new technology into commercial production. 
SUMMARY OF THE INVENTION 
Therefore, it is a primary object of the present invention to provide an 
apparatus for the distillation of styrene monomer which incorporates means 
for injecting gaseous inhibitor to prevent polymerization during the 
distillation process. 
It is another object of the present invention to provide an apparatus 
capable of yielding commercially significant quantities of pure styrene 
monomer in a simple, yet highly efficient manner by injecting gaseous, 
nitrogen-based inhibitors into the distillation train whereby styrene 
polymerization is precluded. 
In accomplishing the foregoing objects there has been provided, according 
to the present invention, an apparatus for the distillative purification 
and recovery of styrene monomer from a styrene-containing feedstock, which 
apparatus includes at least one distillation column for receiving the 
feedstock, means for introducing the gaseous inhibitor into the column 
whereby polymerization of the styrene monomer is prevented, and means for 
recovering said monomer. The gaseous inhibitor is introduced through flow 
means for controllably admitting the inhibitor to the distillation column. 
The single distillation column is adapted for use in conjunction with a 
crude styrene feedstock. In the event the feedstock is derived from, for 
example, the catalytic dehydrogenation of ethylbenzene, a plurality of 
distillation columns is provided. The distillation train is comprised of 
at least three columns and, peferably, four, each of which has associated 
therewith a reboiler. Gaseous inhibitor may be injected directly into the 
reboilers and/or, optionally, into the vapor space of each of the columns. 
Additionally, when a distillation train is employed, a manifold is 
provided whereby gaseous inhibitor may be selectively and controllably 
admitted to each individual column. 
Other objects, features and advantages of the invention will become 
apparent from the detailed description of preferred embodiments which 
follows, when considered together with the attached sheet of drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The distillation of styrene monomer has heretofore normally been 
accomplished under the influence of either liquid or solid polymerization 
inibitors. To the contrary, the present invention is directed to 
distillation apparatus adapted for utilization of the newly-developed 
gaseous inhibitor technology by incorporating means into the distillation 
apparatus for injecting gaseous inhibitor during the distillation process. 
Broadly, the present invention pertains to apparatus for inhibiting 
polymerization of styrene during separation and purification of the 
monomer. Separation is accomplished by distillation conducted at elevated 
temperatures and, generally, under reduced pressure, i.e., subatmospheric. 
The feed to the distillation apparatus is, in some cases, crude styrene, 
which can be defined as a mixture predominantly composed of styrene, but 
contaminated with undesirable quantities of non-volatile components, tars 
and/or polymeric material, which must be separated. It is possible to 
employ a single fractionating column or a distillation train including a 
plurality of columns. The number of columns required will vary with the 
purity of the styrene and the type of impurities contained therein and can 
readily be determined by those skilled in the art. Regardless, however, of 
the precise nature of the feed, and hence, the number of columns, the new 
gaseous inhibitors have been found to effectively preclude polymerization 
of styrene in the apparatus if means are provided for appropriately 
introducing the inhibitors thereto. 
The FIGURE of drawing diagrammatically depicts an apparatus according to 
one embodiment of the present invention. In the illustrated apparatus, the 
feed to the distillation unit is that resulting from the catalytic 
dehydrogenation of ethylbenzene. Preliminary separation of various 
constituents is carried out prior to the purification of styrene. 
Depending upon the purity of the styrene and the nature of its impurities, 
that portion of the apparatus associated with the initial, separatory 
treatments may not be required. Accordingly, the FIGURE of drawing will 
serve to illustrate either eventuality. 
Ethylbenzene is delivered to a reactor 10 wherein it is catalytically 
dehydrogenated to yield an effluent containing benzene, toluene, unreacted 
ethylbenzene, styrene, high boiling polymers, and some tar. This effluent 
is directed to a separator 12, as is conventional in the art, before being 
routed to a distillation train, generally denoted as 20. The liquid 
effluent is subjected to a first fractionation in column 22 wherein 
benzene and toluene are taken overhead. In the event the initial 
ethylbenzene feed is derived from the alkylation of benzene and ethylene, 
the overhead fraction of benzene and toluene may be recycled to the 
associated apparatus. A bottom mixture of ethylbenzene and styrene 
monomer, along with residue, is recovered and routed to a second column 
24, wherein unreacted ethylbenzene is recovered overhead and recycled to 
reactor 10. Crude styrene is recovered as a bottom product from tower 24 
and is directed to a first finish column 26. 
The overhead product from column 26 is styrene of high purity which is 
routed to storage. The apparatus may be limited solely to the first finish 
column for separation of styrene monomer. However, there is preferably 
provided a second finish column 28 which receives the bottom product from 
tower 26, which bottom product is comprised of styrene and some tar 
residue. Second finish column 28 removes the tar component that yields 
high purity styrene as an overhead product which is also directed to 
storage. 
During the distillative separation of the styrene monomer from the other, 
undesirable components, the conditions of heat within distillation train 
20 are such that there is a pronounced tendency for polymerization of the 
styrene. To prevent this highly deleterious result, gaseous inhibitor is 
injected into the various columns of the distillation train. The gaseous 
inhibitor is stored in a suitable vessel 30 which communicates with a 
manifold 32 via regulator 34. From manifold 32, the gaseous inhibitor is 
routed to each of the columns 22, 24, 26 and 28 through appropriate flow 
control apparatus. Generally, it has been determined that the distribution 
of the gaseous inhibitor between the columns 22, 24, 26 and 28 will depend 
upon the relative temperatures of operation of the columns with the amount 
increasing with higher temperatures and decreasing with lower 
temperatures. The optimum amounts may be readily determined by those 
skilled in the art. 
To suitably apportion the requisite quantity of gaseous inhibitor to the 
appropriate column, there are provided a plurality of rotameters 36, 38, 
40 and 42 which provide operative communication between the source of 
inhibitor 30 and columns 22, 24, 26 and 28, respectively. Valves 46, 48, 
50 and 52 are included to permit selective control of the distribution of 
inhibitor in the event it is desirable to modulate injection thereof to 
one or more of the columns within the distillation train. 
Reboilers 54, 56, 58 and 60 are associated with each of the distillation 
columns 22, 24, 26, and 28, respectively, as is conventional. Gaseous 
inhibitor is selectively and controllably admitted to the bottoms of each 
of the distillation columns through the associated reboiler, precise 
control of the desired quantity being achieved by appropriate manipulation 
of the respective rotameters. Should more sophisticated instrumentation 
than rotameters be desired, electronic mass flow meters and controllers, 
and the like, are readily available in the market place than may be 
employed to this end. Regardless of the manner in which flow is controlled 
and apportioned, the ultimate operating parameters of the distillation 
train, which will be individually determined via actual trials for each 
apparatus, will dictate in large part the appropriate quantity of gaseous 
inhibitor to each column. More importantly, however, the amount of gaseous 
material to be employed will depend upon the particular inhibitor taking 
into account its longevity as well as its inhibiting effect. For example, 
in the case of nitrogen oxide-type inhibitors, it is preferred to maintain 
a concentration of from between about 100 ppm and about 3,000 ppm, and 
preferably from about 100 ppm to about 1,000 ppm in the column. 
As illustrated in the FIGURE of drawing, there are provided parallel flow 
paths for injection of gaseous inhibitor into each of the columns within 
distillation train 20. Viewing column 22 and its associated apparatus as 
exemplary of each of the remaining columns, it is possible to precisely 
control the injection of gaseous inhibitor into the column to provide the 
most efficient operation thereof. Normally, the gaseous inhibitor will be 
injected into the bottom of column 22 through reboiler 54 by suitably 
opening and closing valves 62 and 64. An optional point of injection is 
provided within the vapor space of the column, the flow path therefor 
being shown in phantom lines. Moreover, it may well be desirable to inject 
a portion of the gaseous inhibitor into the bottom of column 22 via 
reboiler 54 and also inject a portion within the vapor space. This is 
easily accomplished by adusting rotameter 36 to the desired, absolute flow 
rate and then appropriately setting valves 62 and 64 to yield the desired 
division of gas. The remaining columns are provided with identical means 
to selectively and controllably admit any desired quantity of gaseous 
inhibitor to the column, apportioning same between bottoms and vapor 
spaces as desired. 
Various gaseous inhibitors may be employed for injection within the 
distillation train. Insofar as the broad thrust of the present invention 
regards the distillation of styrene, the preferred gaseous inhibitors are 
nitrogen oxides and derivatives thereof which include, as preferred, 
NO,N.sub.2 O.sub.3,NO.sub.2, and NOCl. Most favored is NO as this gas has 
been determined particularly effective in inhibiting the polymerization of 
styrene during the distillation process. This material may be supplied 
from a mobile shipping container or by onsight generation. Accordingly, 
the source of gaseous inhibitor has merely been designated, in general 
terms, by the reference numeral 40 in the drawing. 
While the present invention has now been described in terms of certain 
preferred embodiments thereof, the skilled artisan will appreciate that 
yet other modifications, changes, omissions and substitutions may be made 
without departing from the spirit of the invention. Accordingly, it is 
intended that the scope of the present invention be limited only by the 
following claims.