Process for the production of dehydrogenated hydrocarbons

A process for the dehydrogenation of a dehydrogenatable hydrocarbon by (1) contacting the dehydrogenatable hydrocarbon with a liquid alkali metal in a dehydrogenation zone to produce a stream containing a dehydrogenated hydrocarbon and an unconverted dehydrogenatable hydrocarbon, and an alkali metal hydride; (2) heating the alkali metal hydride to produce a heated liquid alkali metal and hydrogen; (3) recycling the heated liquid alkali metal to the dehydrogenation zone; (4) contacting the stream containing dehydrogenated hydrocarbon and unconverted dehydrogenatable hydrocarbon with a selective adsorbent to produce a stream containing dehydrogenated hydrocarbon and a stream containing an unconverted hydrogenatable hydrocarbon; (5) recycling the stream of the unconverted dehydrogenatable hydrocarbon to the dehydrogenation zone; and (6) recovering the stream containing dehydrogenated hydrocarbon.

FIELD OF THE INVENTION 
The field of art to which this invention pertains is the production of 
dehydrogenated hydrocarbons by the dehydrogenation of dehydrogenatable 
hydrocarbons in a dehydrogenation zone. This invention relates more 
specifically to a process for the dehydrogenation of a dehydrogenatable 
hydrocarbon by contacting the dehydrogenatable hydrocarbon with a liquid 
comprising an alkali metal in a dehydrogenation zone to produce a 
hydrocarbon stream containing dehydrogenated hydrocarbons and an alkali 
metal hydride. The resulting alkali metal hydride is heated to produce a 
heated liquid alkali metal and hydrogen. The heated liquid alkali metal is 
recycled to the dehydrogenation zone to provide heat. The hydrocarbon 
stream containing dehydrogenated hydrocarbons is contacted in a selective 
adsorbent zone with an adsorbent to produce a stream containing 
dehydrogenated hydrocarbons and a stream containing unconverted 
dehydrogenatable hydrocarbons which is recycled to the dehydrogenation 
zone. 
There is a steadily increasing demand for technology which is capable of 
producing olefins from dehydrogenatable hydrocarbons containing from 2 to 
about 18 carbons atoms. Dehydrogenating hydrocarbons is an important 
commercial hydrocarbon conversion process because of the great demand for 
dehydrogenated hydrocarbons for the manufacture of various chemical 
products such as detergents, high octane motor fuels, pharmaceutical 
products, plastics, synthetic rubbers, polymerization and other products 
well known to those skilled in the art. Processes for the dehydrogenation 
of light acyclic hydrocarbons are well known to those skilled in the 
hydrocarbon conversion arts. 
INFORMATION DISCLOSURE 
In U.S. Pat. No. 4,675,465 (Fanelli et al.), a process is disclosed for 
dehydrogenating reactants wherein a reactant comprising a hydrocarbon is 
exposed to a solid admixture of a platinum on alumina catalyst for 
dehydrogenation and a material to remove at least one hydrogen atom from 
the hydrocarbon and form a material hydride. The material is selected from 
the group of metals, alloys and intermetallic compounds having a negative 
free energy of formation for a hydrided product. The '465 patent fails to 
disclose the contacting of a dehydrogenatable hydrocarbon with a liquid 
comprising an alkali metal in a dehydrogenation zone to produce a 
dehydrogenated hydrocarbon and an alkali metal hydride. 
Other prior art processes for the dehydrogenation of paraffins suffered 
under several disadvantages including poor olefin product yields and poor 
catalyst life caused by the relatively high catalyst inlet temperature 
required to supply the essential heat of reaction and the relatively high 
cost of the required multi-stage reactors and their attendant 
interheaters. 
BRIEF SUMMARY OF THE INVENTION 
The invention provides a process for the dehydrogenation of a 
dehydrogenatable hydrocarbon by contacting the dehydrogenatable 
hydrocarbon with a liquid comprising an alkali metal in a dehydrogenation 
zone to produce a hydrocarbon stream containing a dehydrogenated 
hydrocarbon and an alkali metal hydride. At least a portion of the 
resulting alkali metal hydride is heated to produce a heated liquid alkali 
metal and hydrogen. At least a portion of the heated liquid alkali metal 
is recycled to the dehydrogenation zone to provide heat. The hydrocarbon 
stream containing dehydrogenated hydrocarbons is contacted in a selective 
adsorbent zone with an adsorbent to produce a stream containing 
dehydrogenated hydrocarbons and a stream containing unconverted 
dehydrogenatable hydrocarbons which is recycled to the dehydrogenation 
zone. The present invention provides a convenient and economical process 
for the production of olefinic hydrocarbons. Important elements of the 
process are the facile removal of hydrogen from the dehydrogenation zone 
which minimizes chemical equilibrium constraints and simplifies the 
recovery of the resulting olefinic hydrocarbons and the supply of heat to 
the dehydrogenation zone without the need to heat the dehydrogenatable 
hydrocarbon reactants to reaction temperature prior to entering the 
reaction zone. In addition, unconverted dehydrogenatable hydrocarbons are 
recovered and recycled to the dehydrogenation zone. 
One embodiment of the present invention may be characterized as a process 
for the dehydrogenation of a dehydrogenatable hydrocarbon which process 
comprises: (a) contacting the dehydrogenatable hydrocarbon with a liquid 
comprising an alkali metal in a dehydrogenation zone at dehydrogenation 
conditions to produce a stream comprising a dehydrogenated hydrocarbon and 
an unconverted dehydrogenatable hydrocarbon, and an alkali metal hydride; 
(b) removing and heating at least a portion of the alkali metal hydride 
from the dehydrogenation zone to produce a heated liquid alkali metal and 
hydrogen; (c) recycling at least a portion of the heated liquid alkali 
metal to the dehydrogenation zone in step (a); (d) contacting at least a 
portion of the stream comprising dehydrogenated hydrocarbon and 
unconverted dehydrogenatable hydrocarbon with a selective adsorbent in a 
selective adsorbent zone to selectively adsorb dehydrogenated hydrocarbon 
and produce a stream comprising an unconverted dehydrogenatable 
hydrocarbon and to desorb the selective adsorbent and produce a stream 
comprising an unconverted dehydrogenatable hydrocarbon; (e) recycling at 
least a portion of the stream comprising unconverted dehydrogenatable 
hydrocarbon recovered in step (d) to the dehydrogenation zone in step (a); 
and (f) recovering the stream comprising the dehydrogenated hydrocarbon.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a process for the dehydrogenation of a 
dehydrogenatable hydrocarbon. The dehydrogenatable hydrocarbon is 
contacted with a liquid containing an alkali metal in a dehydrogenation 
zone at dehydrogenation conditions to produce a stream containing 
dehydrogenated hydrocarbon and unconverted dehydrogenatable hydrocarbon, 
and an alkali metal hydride. The stream containing dehydrogenated 
hydrocarbon and unconverted dehydrogenatable hydrocarbon is preferably 
removed from the dehydrogenation zone in gaseous phase and the alkali 
metal hydride is removed from the dehydrogenation zone in a liquid phase. 
The alkali metal hydride is subsequently heated to remove hydrogen, 
thereby providing heated metal which may be recycled to serve as a 
hydrogen sponge and to provide heat for the endothermic dehydrogenation 
reaction in the dehydrogenation zone. The stream containing dehydrogenated 
hydrocarbon and unconverted dehydrogenatable hydrocarbon is removed from 
the dehydrogenation zone and contacted with a selective adsorbent in a 
selective adsorbent zone to produce a stream containing dehydrogenated 
hydrocarbon and a stream containing an unconverted dehydrogenatable 
hydrocarbon. 
Paraffin dehydrogenation is an endothermic reaction and the heat of 
reaction for the formation of a mono-olefin is approximately 30 
kilocalories/gram mol for a feed that may vary from C.sub.2 (ethane) to 
C.sub.18 paraffins. Therefore, when olefins are produced from paraffins, 
the heat of reaction must be supplied from an external source. 
In accordance with the present invention, the dehydrogenatable hydrocarbon 
charge stock may contain from 2 carbons to about 18 carbon atoms. 
Representative members of this class are ethane, propane, butane, pentane, 
hexane, heptane and mixtures thereof. A particularly important class of 
charge stocks include propane, butane, pentane and mixtures thereof and 
which are readily prepared by the fractionation of relatively low boiling 
point hydrocarbon fractions. 
The dehydrogenatable hydrocarbon feedstock is introduced into a 
dehydrogenation zone and contacted with a liquid comprising an alkali 
metal at dehydrogenation conditions to produce a dehydrogenated 
hydrocarbon and an alkali metal hydride. In a preferred embodiment, the 
contacting is performed by bubbling a gaseous dehydrogenatable hydrocarbon 
through a liquid phase comprising an alkali metal. In another preferred 
embodiment, the contacting is performed by intimately admixing a liquid 
dehydrogenatable hydrocarbon with a liquid phase comprising an alkali 
metal. Preferred alkali metals are lithium and sodium. When the reacted 
hydrocarbons are in the gaseous phase, the gaseous phase is removed from a 
vapor-liquid separation zone and when the reacted hydrocarbons are in the 
liquid phase, the liquid hydrocarbon phase is separated from the alkali 
metal liquid phase in a liquid--liquid separator. Preferred 
dehydrogenation conditions include a pressure from atmospheric to about 
500 psig (3447 kPa gauge), a temperature from about 392.degree. F. 
(200.degree. C.) to about 1310.degree. F. (700.degree. C.), and a 
hydrocarbon to alkali metal mol ratio from about 1 to about 20. 
A resulting hydrocarbon stream containing dehydrogenated hydrocarbons and 
unconverted dehydrogenatable hydrocarbon is removed from the 
dehydrogenation zone and recovered. In accordance with the present 
invention, the resulting hydrocarbon stream is separated to recover the 
olefin hydrocarbons and to produce a stream of unreacted hydrocarbons 
which may then be recycled to the dehydrogenation zone to produce 
additional olefin hydrocarbons. 
A liquid stream containing alkali metal hydride is removed from the 
dehydrogenation zone and is heated to produce a heated liquid alkali metal 
stream and hydrogen. In order to regenerate the alkali metal hydride 
stream, it is preferably heated in a heating zone to a temperature in the 
range from about 752.degree. F. (400.degree. C.) to about 1562.degree. F. 
(850.degree. C.). The circulation rate of the heated liquid alkali metal 
stream is preferably selected to ensure that the required heat is 
subsequently supplied to the dehydrogenation zone to maintain the desired 
dehydrogenation reaction temperature. 
In accordance with the present invention, the alkali metal may be selected 
from the group consisting of lithium, sodium, potassium, rubidium, cesium, 
and admixtures thereof. In one embodiment of the present invention, the 
circulating liquid stream containing alkali metal and/or alkali metal 
hydride may be transferred to and from the dehydrogenation zone and the 
heating zone by means of pumps, gravity or thermal siphon. 
A resulting hydrocarbon stream containing dehydrogenated hydrocarbons and 
unconverted dehydrogenatable hydrocarbon is removed from the 
dehydrogenation zone and contacted with a selective adsorbent in a 
selective adsorbent zone to produce a stream containing a dehydrogenated 
hydrocarbon and a stream containing an unconverted dehydrogenatable 
hydrocarbon. 
Adsorptive separation requires the sequential performance of three basic 
steps. The adsorbent must first be brought into contact with a feed stream 
at adsorption-promoting conditions. This adsorption step should continue 
for a time sufficient to allow the adsorbent to collect a near equilibrium 
amount of the preferentially adsorbed dehydrogenated hydrocarbon. The 
second basic step is the contacting of the adsorbent bearing both 
dehydrogenated hydrocarbon and dehydrogenatable hydrocarbon with a 
material which displaces the latter from the adsorbent. The second step is 
performed in a manner which results in the adsorbent containing 
significant quantities of only the dehydrogenated hydrocarbon and the 
material used to displace the dehydrogenatable compounds. 
The third basic step is the desorption of the dehydrogenated hydrocarbon 
from the adsorbent. This may be performed by changing the conditions of 
temperature and pressure, but preferably it is performed by contacting the 
adsorbent with a desorbent stream. The desorbent stream contains a 
chemical compound capable of displacing or desorbing the dehydrogenated 
hydrocarbon from the adsorbent to thereby release the dehydrogenated 
hydrocarbon and prepare the adsorbent for another adsorption step. 
The contacting of the adsorbent with either the adsorbent feed stream or 
the desorbent stream leaves the interstitial void spaces between the 
adsorbent particles filled with the components of these particular 
streams. When the next contacting step begins, this residual liquid is 
admixed into the entering liquid. This results in the effluent streams 
removed from the adsorbent bed being mixtures of compounds from the two or 
more streams which are passed into the adsorbent bed. In the present 
invention, two such effluent streams are produced. They comprise a mixture 
of the desorbent and the dehydrogenated hydrocarbon and a mixture of the 
desorbent with dehydrogenatable hydrocarbons. In order to obtain a high 
purity product stream of the dehydrogenatable hydrocarbon which is 
suitable for recycle to the dehydrogenation zone, it is necessary to 
fractionate the stream to recover the desorbent and produce the recycle 
stream. In order to obtain a high purity product stream of the 
dehydrogenated hydrocarbon and to recover the desorbent, it is also 
necessary to fractionate the mixture. The two effluent streams are 
therefore fractionated in two separate fractionation columns referred to 
as the raffinate column and the extract column. 
The sequential adsorption and desorption steps of an adsorptive separating 
procedure may be performed using a fixed bed of adsorbent having fixed 
inlet and outlet points at opposite ends of the adsorbent bed. However, it 
is preferred to use a simulated moving bed of adsorbent. These benefits 
include the continuous production of a high purity recycle stream and 
product stream. Preferably, the countercurrent flow of the bed of solid 
adsorbent and the various entering liquid streams, such as the feed and 
desorbent streams, is simulated. 
Two separate actions are involved in this simulation. The first of these is 
the maintenance of a net fluid flow through the bed of adsorbent in a 
direction opposite to the direction of simulated movement of the 
adsorbent. This is performed through the use of a pump operatively 
connected in a manner to achieve this circulation along the length of the 
entire bed of adsorbent. The second action involved in simulating the 
movement of the adsorbent is the periodic actual movement of the location 
of the various zones, such as the adsorption zone, along the length of the 
bed of adsorbent. This actual movement of the location of the various 
zones is performed gradually in a unidirectional pattern by periodically 
advancing the points at which the entering streams enter the adsorbent bed 
and the points at which the effluent streams are withdrawn from the 
adsorbent bed. It is only the locations of the zones as defined by the 
respective feed and withdrawal points along the bed of adsorbent which are 
changed. The adsorbent bed itself is fixed and does not move. 
The bed of adsorbent may be contained in one or more separate 
interconnected vessels. At a large number of points along the length of 
the bed of adsorbent, the appropriate openings and conduits are provided 
to allow the addition or withdrawal of liquid. At each of these points, 
there is preferably provided a constriction of the cross-section of the 
bed of adsorbent by a liquid distributor-collector. These 
distributor-collectors serve to aid in the establishment and maintenance 
of plug flow of the fluids along the length of the bed of adsorbent. The 
two points at which any one stream enters and the corresponding effluent 
stream leaves the bed of adsorbent are separated from each other by at 
least two or more potential fluid feed or withdrawal points which are not 
being used. For instance, the feed stream may enter the adsorption zone at 
one point and flow past nine potential withdrawal points and through nine 
distributor-collectors before reaching the point at which it is withdrawn 
from the adsorbent bed as the raffinate stream. 
The gradual and incremental movement of the adsorption zone is achieved by 
periodically advancing the actual points of liquid addition or withdrawal 
to the next available potential point. That is, in each advance of the 
adsorption zone, the boundaries marking the beginning and the end of each 
zone will move by the relatively uniform distance between two adjacent 
potential points of liquid addition or withdrawal. The majority of the 
zone is unaffected and remains intact since the zone extends past several 
of these fluid transfer points. 
The switching of the fluid flows at these many different locations may be 
achieved by a multiple-valve manifold or by the use of a multiple-port 
rotary valve. A central digital controller is preferably used to regulate 
the operation of the rotary valve or manifold. Further details on the 
operation of a simulated moving bed of adsorbent and the preferred rotary 
valves may be obtained from U.S. Pat. Nos. 2,985,589; 3,201,491; 
3,291,726; 3,732,325; 3,040,777; 3,422,848; 3,192,954; 2,957,485; 
3,131,232; 3,268,604 and 3,268,605. 
The present process may be practiced by using any suitable type of 
commercially operable and practical selective adsorbent. A preferred 
adsorbent comprises a selective zeolite commonly referred to as a 
molecular sieve. The preferred zeolites comprise synthetic crystalline 
aluminosilicates. Since the pure zeolites are relatively soft and powdery, 
the commercially used molecular sieves comprise a binder such as clay or 
alumina to produce a stronger and more attrition-resistant adsorbent 
particle. The adsorbent particles preferably have a size range of about 20 
to about 40 mesh. 
The particular adsorbent selected and utilized in the present invention 
will depend on the particular hydrocarbonaceous materials which it is 
desired to separate. The selective adsorption of olefinic hydrocarbons 
from saturated hydrocarbons may be performed using a copper-exchanged Type 
Y zeolite as described in U.S. Pat. No. 3,720,604. 
Although adsorptive separation can be operated with both vapor-phase and 
liquid-phase conditions, the use of liquid-phase conditions is preferred. 
Adsorption-promoting conditions therefore include a pressure sufficient to 
maintain all of the compounds present in the adsorbent bed as liquids. A 
pressure from atmospheric to about 50 atmospheres may be employed with the 
pressure preferably being between 1 and about 32 atmospheres gauge. 
Suitable operating temperatures range from 40.degree. C. to about 
250.degree. C. 
In accordance with the present invention, any suitable desorbent may be 
selected and utilized which is capable of desorbing the dehydrogenated 
hydrocarbon (extract component) from the bed of the adsorbent. It is 
preferred that the desorbent has a boiling point or boiling range such 
that the dehydrogenated hydrocarbon may be separated and recovered from 
the desorbent via fractionation. Preferred desorbents may be selected from 
the group consisting of hexene, octene and decene. 
DETAILED DESCRIPTION OF THE DRAWING 
With reference now to the drawing, a dehydrogenatable hydrocarbon feed 
stream is introduced via conduit 1 and is admixed with a recycle 
dehydrogenatable hydrocarbon stream supplied via conduit 13 and the 
resulting admixture is introduced into dehydrogenation zone 3 via conduit 
2 and contacted with a heated liquid stream containing alkali metal which 
is introduced via conduit 8 into dehydrogenation zone 3. A resulting 
gaseous hydrocarbon stream containing olefin hydrocarbons is removed from 
dehydrogenation zone 3 via conduit 10 and introduced into adsorption zone 
11. A resulting dehydrogenated hydrocarbon stream is removed from 
adsorption zone 11 via conduit 12 and recovered. A dehydrogenatable 
hydrocarbon stream is recovered from adsorption zone 11 via conduit 13 and 
recycled as described hereinabove. A liquid stream containing alkali metal 
hydride is removed from dehydrogenation zone 3 via conduit 4 and 
introduced into heat exchanger 5. A heated effluent from heat exchanger 5 
is transported via conduit 6 and introduced into vapor-liquid separator 7. 
A gaseous stream containing molecular hydrogen is removed from 
vapor-liquid separator 7 via conduit 9. A liquid stream containing alkali 
metal is removed from vapor-liquid separator 7 via conduit 8 and 
introduced into dehydrogenation zone 3 as described hereinabove. 
The process of the present invention is further demonstrated by the 
following illustrative embodiment. This illustrative embodiment is, 
however, not presented to unduly limit the process of this invention, but 
to further illustrate the advantages of the hereinabove-described 
embodiment. The following data were not obtained by the actual performance 
of the present invention, but are considered prospective and reasonably 
illustrative of the expected performance of the invention. 
ILLUSTRATIVE EMBODIMENT 
A fresh feed stream of pure isobutane in an amount of 100 mass units per 
hour is combined with an unconverted isobutane recycle stream in an amount 
of 2000 mass units per hour and the resulting admixture is bubbled through 
a liquid containing lithium at a temperature of 932.degree. F. 
(500.degree. C.) and a pressure of 50 psig (345 kPa gauge) in a 
dehydrogenation zone. A gaseous effluent having the characteristics 
presented in Table 1 is continuously withdrawn from the dehydrogenation 
zone and introduced into an adsorptive separation zone containing a 
copper-exchanged Type Y zeolite. A stream of isobutane in an amount of 
2000 mass units per hour is recovered from the adsorptive separation zone 
and recycled as described hereinabove. An isobutylene stream in an amount 
of 100 mass units per hour is recovered from the adsorptive separation 
zone. A liquid stream containing lithium hydride is removed from the 
dehydrogenation zone and heated to produce hydrogen and a liquid stream 
containing lithium which is recycled to the dehydrogenation zone to 
provide the heat of reaction and hydrogen sponge function. 
TABLE 1 
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DEHYDROGENATION ZONE EFFLUENT ANALYSIS 
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Isobutane, weight percent 
94.5 
Propane, weight percent 
0.5 
Isobutylene, weight percent 
4.5 
Hydrogen, weight percent 
0 
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The foregoing description, drawing and illustrative embodiment clearly 
illustrate the advantages encompassed by the method of the present 
invention and the benefits to be afforded with the use thereof.