PSA process with reaction for reversible reactions

A process is disclosed for the production of a high octane product from a feed mixture comprising C.sub.5 -C.sub.6 normal paraffins in which an equilibrium reaction to produce mono and dimethyl branched paraffins is achieved by conducting the reaction and the product separation in a pressure swing adsorption and reaction zone containing a uniformly distributed adsorbent for the selective adsorption of normal paraffins and a catalyst for the equilibrium conversion of normal paraffins to mono and dimethyl branched paraffins. More specifically, the process achieves the isomerization of the normal paraffins by the reaction of the normal paraffins in the presence of hydrogen with the simultaneous removal of the mono and dimethyl branched paraffin product at the same temperature and pressure. In one embodiment, the passing of the feed mixture to the bed is terminated and the bed is purged with one of the reactants which in turn further reacts to displace heavier paraffin and enhance the overall product octane. The advantage of the present invention over the conventional process is the higher octane of the product produced and that this higher octane can be achieved at lower severity since the product is removed from the reaction zone as soon as it is produced. The lower operating severity provides longer catalyst life, and reduces the amount of heavy paraffins lost to side reactions such as cracking.

FIELD OF THE INVENTION 
This invention pertains to the field of chemical reaction combined with 
adsorptive separation. More particularly, the invention relates to the 
combination of pressure swing adsorption processes with reversible 
chemical reactions. Most particularly this invention relates to the field 
of isomerization of paraffins by a combination of a pressure swing 
adsorption process and reaction for the production of high octane gasoline 
components. 
BACKGROUND OF THE INVENTION 
Theoretical models of adsorptive reactors which combine multi-bed pressure 
swing adsorption and chemical reaction have been studied for some limited 
types of reversible and irreversible reactions. A paper entitled, 
"Combined Reaction and Separation in Pressure Swing Processes," by E. 
Alpay et al. and presented at the International Symposium on Chemical 
Reaction Engineering, Sep. 25-28, 1994, Baltimore, Md., describes the 
advantages of such a system for a dissociation reaction producing two 
components where one of the products is the only adsorbing component. 
Another paper entitled, "A Theoretical Investigation of Pressure Swing 
Reaction," by N. F. Kirkby and J.E.P. Morgan, and published in the 
TRANSACTIONS OF INDUSTRIAL CHEMICAL ENGINEERING, Vol. 72, Part A, July 
1994, explores a simplified model of pressure swing reaction applied to a 
non-adsorbable reactant undergoing an irreversible reaction to produce an 
adsorbable product. These studies indicate that combinations of pressure 
swing adsorption and reaction have some advantages over conventional 
reaction systems. The conclusions reached by these studies suggest that 
the results of the combined PSA and reaction, process are significantly 
different from conventional PSA technology and the steps of the cyclic 
operation are dependent upon the many variables that relate to the 
relative adsorption of the products and reactants and the degree to which 
equilibrium reactions are affected by the adsorption of the reactants and 
the products of the reaction. To date there have been few commercial 
applications of combined pressure swing adsorption and reaction where the 
reactants and the products can be non-adsorbable, less-readily adsorbable 
and more-readily adsorbable. 
U.S. Pat. No. 4,968,722 to Westerterp discloses a process for producing 
methanol by reacting carbon monoxide and hydrogen wherein these reactants 
in the gas phase are introduced into a reaction zone comprising one or 
more fixed catalyst beds and a liquid absorbent. The liquid absorbent 
selectively absorbs substantially all of the methanol produced. The liquid 
absorbent is subsequently pumped out of the reactor and flashed to recover 
the product methanol. In an earlier patent, U.S. Pat. No. 4,731,387, 
Westerterp discloses a methanol reaction zone containing a fixed bed of 
coarse catalyst particles having interstices between them and passing a 
fine particle solid adsorbent downwardly through the interstices to adsorb 
substantially all of the methanol product. U.S. Pat. No. 5,254,368 to 
Kadlec et al. discloses the integral coupling of reaction with a 
single-bed rapid cycle pressure swing adsorber to provide better 
separation and more efficient, irreversible reactions wherein the reactant 
is adsorbed, and those wherein the reactant is not adsorbed. Kadlec et al. 
describe the use of a single-bed pressure-periodic process for a 
two-reactant CO oxidation process for automobile pollution control. Kadlec 
et al. further teach a sequence of operation of the single-bed process 
which includes a delay step following the introduction of the feed gas and 
prior to an exhaust step, such that during the delay step the pressure 
within the single-bed adsorber is permitted to equalize as a continuous 
stream of product is removed. 
Alkane isomerization processes are widely used by refiners to convert 
normal C.sub.4 and C.sub.5 alkanes and normal and mono-methyl-branched 
C.sub.6 alkanes into more valuable branched akanes. The 
multi-methyl-branched C.sub.6 alkanes have a higher octane number and are 
used as gasoline blending components to boost the octane number of the 
gasoline. The mono-methyl-branched C.sub.4 and C.sub.5 alkanes may also be 
used as intermediates, after dehydrogenation, for such oxygenate products 
as methyl tertiary butyl ether, ethyl tertiary butyl ether, and tertiary 
amyl methyl ether. 
Typically, commercial isomerization processes have had at least a two-stage 
design; the first stage is a fixed bed reactor, and the second stage is a 
separation unit. See, for example, U.S. Pat. Nos. 5,146,037 and 5,245,102. 
The isomerization that takes place in the fixed bed reactor is limited by 
thermodynamic equilibrium, which results in the reactor effluent 
containing a substantial amount of unconverted alkanes. The separation 
unit, which is usual either an adsorption or a fractionation unit, is used 
to separate the unconverted alkanes from the isomerized product alkanes. 
The unconverted alkanes are generally recycled to the fixed bed reactor. 
With this type of design, the recycle stream is usually substantial, and 
methods of increasing the yield of highly branched alkanes are in demand. 
One commercial process for the isomerization of C5/C6 paraffins is 
disclosed in U.S. Pat. No. 4,210,771, hereby incorporated by reference, 
which integrates pressure swing adsorption with a separate fixed bed 
isomerization reactor. 
U.S. Pat. No. 4,783,574 disclosed a fixed bed reactor containing two 
sub-beds of adsorbent at opposite ends of the reactor and one sub-bed of 
catalyst in the center of the reactor. The feed was introduced near the 
catalyst sub-bed, and a desorbent was introduced at one end of the 
reactor. The isomerization was catalyzed and unconsumed reactants were 
adsorbed on the adsorbent sub-bed downstream of the catalyst sub-bed in 
the direction of the desorbent flow. Then the desorbent flow was reversed 
by introducing the desorbent from the opposite end of the reactor to 
desorb the unconsumed reactants and carry them back to the catalyst 
sub-bed. 
Pressure swing adsorption (PSA) provides an efficient and economical means 
for separating a multi-component gas stream containing at least two gases 
having different adsorption characteristics. The more strongly adsorbable 
gas can be an impurity which is removed from the less strongly adsorbable 
gas which is taken off as product, or the more strongly adsorbable gas can 
be the desired product which is separated from the less strongly 
adsorbable gas. For example, it may be desired to remove impurities such 
as carbon monoxide and light hydrocarbons from a hydrogen-containing feed 
stream to produce a purified (99+%) hydrogen stream for use in a 
downstream catalytic process where these impurities could adversely affect 
the catalyst or the reaction. On the other hand, it may be desired to 
recover more strongly adsorbable gases, such as ethane, from a feedstream 
to produce an ethane-rich product. 
In pressure swing adsorption, a multi-component gas stream is typically fed 
to at least one of a plurality of adsorption zones at an elevated pressure 
effective to adsorb at least one component, while at least one other 
component passes through. At a defined time, the feedstream to the 
adsorber is terminated and the adsorption zone is depressurized by one or 
more cocurrent depressurization steps wherein pressure is reduced to a 
defined level which permits the separated, less strongly adsorbed 
component or components remaining in the adsorption zone to be drawn off 
without significant concentration of the more strongly adsorbed 
components. Then, the adsorption zone is depressurized by a countercurrent 
depressurization step wherein the pressure on the adsorption zone is 
further reduced by withdrawing desorbed gas countercurrently to the 
direction of the feedstream. Finally, the adsorption zone is purged and 
repressurized. The combined gas stream produced during the countercurrent 
depressurization step and the purge step is typically referred to as the 
tail gas stream. The final stage of repressurization is typically 
performed by introducing a slipstream of product gas comprising the 
lightest gas component produced during the adsorption step. This final 
stage of repressurization is often referred to as product repressurzation. 
In multi-zone systems there are typically additional steps, and those noted 
above may be done in stages. U.S. Pat. Nos. 3,176,444 issued to Kiyonaga, 
3,986,849 issued to Fuderer et al., and 3,430,418 and 3,703,068 both 
issued to Wagner, among others, describe multi-zone, adiabatic pressure 
swing adsorption systems employing both cocurrent and countercurrent 
depressurization. The disclosures of these patents are incorporated by 
reference in their entireties. 
Various classes of adsorbents are known to be suitable for use in PSA 
systems, the selection of which is dependent upon the feedstream 
components and other factors generally known to those skilled in the art. 
In general, suitable adsorbents include molecular sieves, silica gel, 
activated carbon, and activated alumina. When PSA processes are used to 
purify hydrogen-containing streams, the hydrogen is essentially not 
adsorbed on the adsorbent. 
Improved processes are sought for the combination of pressure swings 
adsorption and reaction for reversible reactions. 
Improved processes are sought for the isomerization of alkanes to form high 
octane products. 
Processes are sought which extend the equlibrium conversion in providing a 
high octane product to proceed with a greater conversion per pass and with 
a high yield of a high octane product. In addition, processes are sought 
which substantially reduce the recycle of unreacted components. 
SUMMARY OF THE INVENTION 
Many commercial processes for the production of chemicals involve the 
integration of chemical reaction and separation--such as distillation, 
adsorption, and condensation, but not in the same zone. Conventionally, 
these reactions are carried out in the vapor phase at high pressure and 
the products of the reaction are separated at lower pressure by pressure 
swing adsorption techniques. For example, if the reaction involves the 
production of a readily-adsorbable product from at least one reactant 
which is non-adsorbable, the non-adsorbable reactant is separated at low 
pressure following the reaction and retered to the reactor. Generally the 
conversion of the reactant in the reactor is low, requiring large amounts 
of the reactant to be recompressed to the higher reaction pressure and 
returned to the reactor as a recycle gas. This recycle operation is costly 
in terms of capital and operating costs. When the reaction and the PSA 
process can be combined, the overall process can be improved considerably 
to improve the conversion of the reaction and to decrease the amount of 
recycle. 
The purpose of this invention is to provide a process for the continuous 
isomerization of an alkane to produce an isomerized product through 
contacting the alkane with a pressure swing adsorption and reaction zone 
wherein a catalyst for isomerization and an adsorbent for the alkanes is 
integrated to improve the overall conversion and yield of high octane 
product. The alkane may be n-pentane and the isomerized product 
2-methylbutane or 2,2-dimethylpropane, the alkane may have from 6 up to 
about 8 carbon atoms with no more than one methyl branch and the 
isomerized product having the same number of carbon atoms and at least two 
methyl branches, or the reactant may be a mixture of the foregoing alkanes 
with the corresponding isomerized products being formed. The advantage of 
the present invention over the conventional process is the higher octane 
of the product and that this higher octane can be achieved at lower 
severity since the product is removed from the reaction zone as soon as it 
is produced. The lower operating severity provides longer catalyst life, 
and reduces the amount of heavy paraffins lost to side reactions such as 
cracking. 
In one embodiment, the present invention is a process for the isomerization 
of a C.sub.5 /C.sub.6 hydrocarbon feed mixture. The C.sub.5 /C.sub.6 
hydrocarbon feed mixture comprises C.sub.5 and C.sub.6 normal paraffin 
components as reactants. The process produces a high octane product 
comprising mono and dimethyl branched paraffins. The process comprises the 
series of steps that follow. The feed mixture is passed at reaction 
conditions including a reactor temperature and a reactor pressure in the 
presence of hydrogen to carry out at least one reversible isomerization 
reaction in a fixed bed of a pressure swing adsorption and reaction zone. 
The fixed bed contains a physical mixture of a selective adsorbent and a 
catalyst. The selective adsorbent is selective for the adsorption of at 
least a portion of the mono methyl branched paraffins and the normal 
paraffins. The catalyst is selective for the isomerization of the feed 
mixture to produce mono and dimethyl branched paraffins. A first effluent 
stream comprising hydrogen and the high octane product is withdrawn. The 
fixed bed is countercurrently depressurized and a desorption effluent 
comprising normal paraffins is withdrawn. The fixed bed is repressurized 
with a repressurization stream comprising hydrogen. The above series of 
steps is repeated to provide a continuous process. 
In another embodiment, the invention relates to a process for the 
isomerization of a C.sub.5 /C.sub.6 hydrocarbon feed mixture. The feed 
mixture comprises normal paraffin components as reactants which, upon 
isomerization, will produce a high octane product comprising mono and 
dimethyl branched paraffins. The process comprises the series of steps 
that follow. The feed mixture at reaction conditions including a reactor 
temperature and a reactor pressure in the presence of hydrogen is passed 
to a fixed bed of a pressure swing adsorption and reaction zone to carry 
out at least one reversible reaction. The fixed bed contains a homogeneous 
mixture of a selective adsorbent and a catalyst. The selective adsorbent 
is selective for the adsorption of at least a portion of the mono methyl 
branched paraffins and the normal paraffins. The catalyst is selective for 
the isomerization of the feed mixtures to produce mono and dimethyl 
branched paraffins. The reactants are isomerized and a first effluent 
stream comprising hydrogen and the high octane product is withdrawn. The 
passing of the feed mixture to the fixed bed is terminated and the fixed 
bed is countercurrently purged with a first purge seen comprising normal 
pentane. The normal pentane produces additional isomerization of the 
normal paraffin components and a second effluent stream comprising mono 
methyl branched paraffins is withdrawn. The fixed bed is countercurrently 
depressurized to a desorption pressure and a desorption effluent stream 
comprising normal paraffins is withdrawn. The fixed bed is repressurized 
with a repressurization stream comprising hydrogen. The above steps are 
repeated to provide a continuous process.

DETAILED DESCRIPTION OF THE INVENTION 
Typical light naphtha streams available in petroleum refineries are poor in 
highly branched paraffins such as 2,2 dimethyl butane and 2,3 dimethyl 
butane but rich in normal paraffins and mono-branched paraffins. The 
reactions to form the highly branched paraffins occur as below: 
EQU nC6&lt;=&gt;2,mp 3,mp&lt;=&gt;2,2dmb, 2,3dmb nC5&lt;=&gt;iC5 
The present invention provides a process in which 2,mp and 3,mp and nC6 are 
retained in a catalyst bed and 2,2 dmb and 2,3 dmb are rejected which 
permits the reaction to be carried to completion. Similarly, it was found 
that when n-pentane was retained and iC5 was not retained as strongly, 
then the reaction of nC5 to iC5 can be carried substantially over 
equilibrium. This can be accomplished by using a bed containing a physical 
mixture of catalyst and adsorbent particles, the catalyst being either 
tungstated zirconia, sulfated zirconia or Pt/mordenite. The adsorbent can 
be such as Ca--Sr--X or Silicalite or Ferrierite. The adsorbent 
preferentially adsorbs normal and mono-branched paraffins over dimethyl 
paraffins. 
Integration of the pressure swing adsorption separation process with a, 
reversible chemical reaction results in a combination of two 
unsteady-state phenomena. By the proper combination of catalyst, 
adsorbent, reaction and adsorption rates, and adsorbents having varying 
degrees of selectivity for the reactants and products, a processing cycle 
can be developed to improve the conversion of the reactants compared to a 
conventional steady-state reaction and separation system. Reversible 
reaction systems--particularly those systems wherein one reaction is 
favored at a high pressure, and the reverse reaction is favored at a lower 
pressure, and where one or more of the reactants may be more-readily or 
less-readily adsorbable than another--are particularly preferred systems 
for pressure swing adsorption and reaction processes. Such systems are 
more preferred if the rates of reaction are similar to the rate of 
adsorption and desorption of the reactants and products for the range of 
temperatures and pressures over which the process is operated. In a simple 
system a feedsteam comprising a non-adsorbable reactant is passed to a PSA 
reaction zone to produce a more-readily adsorbable product by reaction at 
high pressure. If an excess of reactant is present in the feedstream, an 
effluent stream comprising the non-adsorbable reactant will be withdrawn. 
Upon depressurization, the more-readily adsorbable product would be 
desorbed and withdrawn as a product stream. Most commercial processes are 
not this simple and often require at least a second, less-readily reactant 
adsorbable reactant to form the product of the reaction. In addition, 
there will be co-products, some of which will be non-adsorbable and some 
will be more-readily adsorbable. The non-adsorbable co-products will be 
withdrawn with the effluent during a reaction/adsorption step, while a 
more-readily adsorbable co-product will be recovered with the product and 
require further separation. 
In a reaction system comprising both an adsorbent selective for the 
adsorption of the readily-adsorbed product and the more-readily adsorbed 
co-product, the equilibrium reversible reaction can be made to favor the 
production of the product by removing the product and the co-product from 
the reaction zone as soon as they are produced while the non-adsorbable 
reactant is withdrawn. When the reaction takes place in a fixed adsorbent 
bed, mass transfer zones of each species of less-readily, readily-, and 
more-readily adsorbable components are formed within the fixed bed. As the 
reaction proceeds during a reaction/adsorption step, the reactant mass 
transfer zones lead the product mass transfer zones. It was discovered 
that when one of the reactants is less-readily adsorbable, the conversion 
can be improved by the addition of a high pressure purge step following 
the adsorption/reaction step wherein the non-adsorbable reactant is 
employed to cocurrently purge the fixed bed. Surprisingly, it was 
discovered that when the reactant employed in the purge step also reacts 
to produce a less-readily adsorbable quantity may be withdrawn which 
further reduces the amount of recycle in the process. During the high 
pressure purge step, the less-readily adsorbable reactant is converted to 
product while the unreacted reactant continues to be withdrawn from the 
bed. This technique for purging at high pressure to drive the reaction 
toward completion may be applied to reactions wherein the co-product is 
recovered with the product on depressurization or wherein the co-product 
is non-adsorbable and is recovered with the intermediate effluent at high 
pressure. 
During the adsorption/reaction step, the reaction may be exothermic and 
produce heat, or the reaction may be endothermic and consume heat. It is 
preferred to maintain the reaction temperature near isothermal conditions 
within the bed. Thus, provisions are made to add or remove heat from the 
bed as the reaction proceeds. Such provisions may include the use of heat 
exchange coils or tubes, or the use of a diluent such as nitrogen, 
methane, and mixtures thereof, in the reaction mixture to maintain the 
temperature change or the difference between the reaction and adsorption 
temperatures in the bed to less than 20.degree. C., and preferably to 
maintain the temperature change in the bed to less than 15.degree. C. 
The bed may be depressurized in a countercurrent manner, i.e., in a 
direction opposite to the flow of the synthesis gas during the previous 
adsorption/reaction step. The desorption effluent is recovered from the 
feed end of the bed. The bed also may be depressurized in a cocurrent 
manner, i.e., in the same direction and the adsorption/reaction step, 
wherein the desorption effluent is withdrawn from the effluent end of the 
bed. The isomerization reaction requires the reaction to be carried out in 
the presence of hydrogen. Preferably, the molar ratio of hydrogen to 
hydrocarbon will be between about 10:1 and about 1:10 and more preferably, 
the ratio of hydrogen to hydrocarbon will range from about 3:1 to about 
1:3. Preferably, the reaction and adsorption conditions in the pressure 
swing adsorption and reaction zone will range from a reaction temperature 
from about 150.degree. C. to about 250.degree. C., and more preferably 
will range from about 150.degree. C. to about 225.degree. C. The reaction 
may be carried out over a wide range of pressure which retains the 
reactants and products in the vapor state, preferably, the reaction 
pressure will range from about 450 kPa to about 1150 kPa. 
A wide variety of solid catalysts and adsorbents are available, and each 
isomerization application may require a different combination of solids. 
The solid or mixture of solids acting as a catalyst may be any of the 
commonly used isomerization catalysts including, but not limited to, 
platinum on mordenite, aluminum chloride on alumina, and platinum on 
sulfated or tungstated metal oxides such as zirconia. See generally, 
Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M., 
Eckroth, D., Eds.; John Wiley & Sons: New York, Vol. 11 p.664, Vol 12 pp. 
911 and 922, and Vol 15 p. 651. Depending upon the composition of the 
feed, several different catalysts may be combined in order to accomplish 
the catalysis function. The preferred catalyst is platinum on tungstated 
zirconia, see, for example, WO 95/03121, U.S. Pat. No. 5,113,034, U.S. 
Pat. No. 5,422,327, U.S. Pat. No. 4,663,304, U.S. Pat. No. 5,489,733 and 
U.S. Pat. No. 5,420,092. The most preferred catalyst contains from about 
7.5 to about 12.5 weight percent tungstate on zirconia with from about 
0.25 to about 0.5 weight percent platinum. 
The adsorbent solid or mixture of solids are selected to either have a pore 
size capable of admitting alkane reactants but not the isomerized 
products, or an affinity for alkanes with no or low branching. Examples of 
suitable adsorbents include, but are not limited to, silicalite, 
ferrierite, Ca-A zeolite, MAPO-31, SAPO-31, SAPO-11, EU-1, ZSM-12, SAPO-5, 
Y-82 faujasite, Erionite, zeolite beta exchanged with sodium, lithium, 
potassium, barium, calcium, strontium or combinations thereof, faujasite 
such as X zeolite exchanged with calcium and strontium, mordenite 
exchanged with sodium, lithium, potassium, barium, calcium, strontium, or 
combinations thereof. Depending upon the composition of the feed, several 
different adsorbents may be combined in order to accomplish the separation 
function. For example, when the feed contains C.sub.6 to C.sub.8 almanes, 
a portion of the adsorbent should retain both normal and 
mono-methyl-branched alkanes, so that they are retained in the bed until 
they are isomerized to form multi-methyl-branched isomerized products. 
Preferred adsorbents capable of retaining both normal and 
mono-methyl-branched alkanes are silicalite and X zeolite exchanged with 
calcium and strontium. When the feed contains n-butane or n-pentane, a 
portion of the adsorbent should retain only normal alkales, since the 
C.sub.4 and C.sub.5 mono-methyl-branched alkanes are isomerized products 
which are collected. 
A preferred adsorbent capable of retaining only normal alkanes is Ca-A 
zeolite. The adsorbents may be combined in different volume ratios 
depending upon the composition of the feed. As an illustration, in an 
embodiment where the adsorbent is a mixture of Ca-A zeolite and another 
adsorbent and the feed contains n-butane or n-pentane, the greater the 
concentration of n-butane or n-pentane present in the feed increases, the 
greater the required concentration of Ca-A zeolite in the adsorbent 
mixture, or in an embodiment where the adsorbent is a mixture of X zeolite 
exchanged with calcium and strontium and another adsorbent and the feed 
contains C.sub.6 to C.sub.8 alkanes, the greater the concentration of 
C.sub.6 to C.sub.8 alkanes present in the feed, the greater required the 
concentration of X zeolite exchanged with calcium and strontium in the 
adsorbent mixture. 
The feed introduced to the pressure swing adsorption and reaction system 
contains at least one alkane which is to undergo catalytic isomerization 
to form at least one isomerized product. Examples of suitable alkanes 
include: normal pentane, 2-methylbutane, normal hexane, 2-methylpentane, 
3-methylpentane. Preferably the feed contains normal pentane, normal 
hexane, 2-methylpentane, and 3-methylpentane. The feed is usually derived 
from other refinery processes and may contain cyclic alkanes, olefinic 
hydrocarbons, aromatic hydrocarbons, and other hydrocarbons. The feed may 
also be the effluent of a fixed bed isomerization unit where alkane 
reactants and the corresponding isomerized products are present in amounts 
determined by the conversion in the fixed bed which is limited by 
thermodynamic equilibrium. The liquid hourly space velocity of the 
feedstream is typically from about 0.05 to about 5. 
Examples of the isomerized products include: 2-methylpropane, 
2-methylbutane, 2,2-dimethylpropane, 2,3-dimethylbutane, 
2,2-dimethylbutane, 2,2-dimethylpentane, 3,3-dimethylpentane, 
2,3-dimethylpentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane, 
2,2-dimethylhexane, 3,3-dimethylhexane, 2,3-dimethylhexane, 
3,4-dimethylhexane,2,4-dimethylhexane,2,5-dimethylhexane,2,2,3-trimethylpe 
ntane, 2,3,3-trimethylpentane, 2,3,4-trimethylpentane, and 
2,2,4-trimethylpentante. Preferably the high octane product of the present 
invention for the isomerization of C5/C6 paraffin hydrocarbons comprises 
mono methyl branched paraffins, such as 2-methylpentane and 
3-methylpentane, and dimethyl branched paraffins such as 2,2-dimethyl 
butane and 2,3-dimethyl butane. The octane quality of the high octane 
product of the present invention will range from about 90 to about 105 
research octane, and when the process is operated to produce an 
intermediate product, the octane of this second effluent stream ranges 
from about 74 to about 93 research octane. 
Operating conditions will depend upon the catalyst and adsorbent selected. 
Preferred operating temperatures for the process are about 100.degree. C. 
to about 500.degree. C., most preferably the operating temperature will 
range from about 150.degree. to about 250.degree. C. Preferably the 
reaction pressure ranges from about 4 kPa to about 2100 kPa, and most 
preferably the reaction pressure ranges between about 450 kPa (65 psia) 
and about 1500 kPa (200 psia). Preferably the desorption pressure ranges 
between about 70 kPa, (10 psia) and about 360 kPa (45 psia), and more 
preferably the desorption pressure ranges between about atmospheric 
pressure and about 360 kPa (45 psia). As outlined above, the process 
conditions are set so that all the streams are in the gas phase. 
Without intending any limitation on the scope of the present invention and 
as merely illustrative, this invention is explained below in specific 
terms as applied to one specific embodiment of the invention, the 
continuous isomerization of normal pentane and n-hexane, 2-methylpentane, 
and 3-methylpentane to form 2,2-dimethylbutane and 2,3-dimethylbutane 
using a mixture of X zeolite exchanged with one or more alkaline earth 
metals such as calcium and strontium, and platinum on tungstated zirconia 
in a 4:5 volume ratio to effect catalysis of the isomerization and the 
separation of the products and reactants through adsorption. Preferably, 
the fixed bed pressure swing adsorption and reaction zone of the present 
invention contains a physical mixture of the catalyst and the adsorbent. 
More preferably, the fixed bed pressure swing adsorption and reaction zone 
of the present invention contains a homogeneous mixture of catalyst and 
adsorbent. The catalyst and adsorbent may be present as separate 
particles, or the adsorbent and catalyst may be combined into a single 
particle comprising both the catalyst and adsorbent. The single particle 
may comprise layers of catalyst and adsorbent such as layered particle 
having a core of adsorbent and an outer layer of catalyst. 
The pressure swing adsorption process is an essentially adiabatic process 
for separating a multi-component fluid containing at least one selectively 
adsorbable component. The PSA process of the invention relates to 
conventional PSA processing in which each bed of an adsorption zone 
undergoes, on a cyclic basis, high pressure adsorption, optional cocurrent 
depressurization to intermediate pressure level(s) with release of void 
space gas from the effluent end of the bed, depressurization to lower 
desorption pressure with the release of desorbed gas from the feed end of 
the bed, with or without purge of the bed, and repressurization to higher 
adsorption pressure. The adsorption zone is then countercurrently or 
cocurrently depresserzed to a desorption pressure that is at or above 
atmospheric pressure with the more adsorbable component(s) being 
discharged from the feed end thereof as a product. In the multi-bed 
adsorption systems to which the invention is directed, the displacement or 
purge gas used for each bed is obtained by diverting a portion of the gas 
released from that or another bed in the system during the adsorption 
steps. Repressurization of the bed is obtained by introducing a portion of 
the purge gas or by introducing the synthesis gas at the adsorption 
pressure. 
Those skilled in the art will appreciate that the high pressure adsorption 
step of the PSA process comprises introducing the PSA feedstream to the 
feed end of the adsorbent bed at a high adsorption pressure. The less 
readily adsorbable component(s) passes through the bed and is discharged 
from the effluent or produce end thereof. Adsorption fronts comprising the 
more adsorbable component(s) are established in the bed with the fronts 
likewise moving through the bed from the feed end toward the product end 
thereof. When the feedstream contains a less readily adsorbable component 
and a more readily adsorbable component, a leading adsorption front of the 
more readily adsorbable component will be established and will move 
through the bed in the direction of the product or discharge end thereof. 
DETAILED DESCRIPTION OF THE DRAWINGS 
The further description of the process of this invention is presented with 
reference to the attached schematics, FIG. 1 and FIG. 2. The figures 
represent preferred arrangements of the invention and are not intended to 
be a limitation on the generally broad scope of the invention as set forth 
in the claims. Of necessity, some miscellaneous appurtenances including 
valves, pumps, separators, heat exchangers, etc. have been eliminated. 
Only those vessels and lines necessary for complete and clear 
understanding of the process of the present invention are illustrated. 
Referring to FIG. 1, a schematic diagram of a process flow diagram of the 
present invention is illustrated. The operation of the pressure swing 
adsorption and reaction process illustrated in FIG. 1 can be employed on a 
continuous or periodic operation. For continuous operation, at least 3 
fixed beds are preferred to provide a continuous flow of high octane 
product and intermediate octane product. A C.sub.5 /C.sub.6 hydrocabon 
feed mixture is passed in line 1 to lines 3 and 5, valve V14 and line 7 to 
the feed end of a pressure swing adsorption with reaction (PSAR) vessel 
101 which contains a homogeneous mixture of a selective adsorbent and an 
isomerization catalyst. The C.sub.5 /C.sub.6 hydrocarbon feed mixture is 
passed to the PSAR vessel 101 at a reaction temperature and a reaction 
pressure. Initially the PSAR vessel may be preloaded with hydrogen at the 
reaction pressure or hydrogen may be introduced with the feed mixture. As 
the feed mixture enters the PSAR vessel, the isomerization reaction 
proceeds. The normal C.sub.6 paraffins first react to produce 
methylcyclopentanes which react further to produce dimethyl butanes. The 
hydrogen and the dimethyl butanes which are non-adsorbable are recovered 
in a first effluent stream 9 which exits the PSAR vessel 101 via line 9, 
valve V4, and lines 11, 13, and 15 to a first condenser 105 and via line 
17 to a first separator 106. The first effluent stream 15 is cooled in 
condenser 105 to at least partially condense a first hydrocarbon phase 
which is recovered as a high octane product in line 19. A high pressure 
vent stream, comprising hydrogen is recovered from the first separator 106 
in line 21. The passing of the feed mixture 7 to the PSAR vessel 101 is 
continued until a point prior to the breakthrough of methylcyclopentane 
and normal hexane. At this point, the feed mixture flow to the PSAR vessel 
101 is terminated and the PSAR vessel 101 is countercurrently 
depressurized to a desorption pressure to produce a desorption effluent 
29. The desorption effluent is passed via line 29, valve V16, line 31, and 
line 39 to a second condenser 107 which cools the desorption effluent to 
provide a cooled desorption effluent 41 and the cooled desorption effluent 
is passed via line 41 to a second separator 108. In the second separator 
108, a second vent stream 45 comprising hydrogen at desorption pressure 
and an intermediate octane stream 43 are produced. At least a portion of 
the intermediate octane stream 43 is reed via line 49', valve V26 and line 
51 to be admixed with the feed mixture 1 to improve the overall 
conversion. A portion of the intermediate octane stream which comprises 
methylpentanes and normal hexane may be recovered as an intermediate 
octane product in line 49. The second vent stream 45 may be recompressed 
in compressor 109 to provide a compressed vent stream 47 at the reaction 
pressure and the compressed vent stream 47 may be combined with the high 
pressure vent stream 21 to produce a hydrogen purge stream 23'. At the 
completion of the countercurrent depressurization step, the PSAR vessel 
101 is repressurized by introducing the hydrogen purge stream 23' via line 
23, line 23', line 25, valve V2 and line 27. The above steps of reaction 
with adsorption, countercurrent depressurization, and repressurization are 
repeated for PSAR vessel 101. 
The process illustrated in FIG. 1 is operated in a continuous fashion by 
alternating the operation of PSAR vessels 101, 103, and 105 in a cyclic 
manner to provide a continuous flow of product streams from the process. 
For example, when PSAR vessel 101 is switched from the reaction/adsorption 
step to the countercurrent depressurization step, the feed mixture is 
passed to PSAR vessel 103 via lines 1, 3, and 53, valve V18 and line 57. 
The first effluent is removed from PSAR vessel 103 via line 65, valve V8, 
line 67, line 13, and line 15 to the first condenser 105 and the first 
separator 106 for recovery of the high octane product. During the 
countercurrent depressurization step, the desorption effluent from PSAR 
vessel 103 is passed through line 33', valve V20, line 33, and line 39 to 
the second condenser 107 and via line 41 to the second separator 108. 
During the repressurization step, the hydrogen purge stream 23' is passed 
to PSAR vessel 103 via line 23', line 23, line 61, valve V6, and line 63. 
Similarly, PSAR vessel 105 in the reaction/adsorption step is charged with 
the feed mixture 1 via line 53, line 58, valve V22, and line 59. The first 
effluent stream is withdrawn from PSAR vessel 105 via line 73, valve V12, 
line 75 to line 15 for high octane product recovery from separator 106. 
During the countercurrent desorption step, the desorption effluent is 
withdrawn in line 37 and is passed to valve V24, line 35, and line 39 to 
intermediate product recovery. During repressurization PSAR vessel 105 is 
repressurized by passing the hydrogen purge stream 23' via line 69, valve 
V10, and line 71 to PSAR vessel 105. 
Referring to FIG. 2, a C.sub.5 /C.sub.6 hydrocarbon feedstream 101 is 
admixed with a recycle stream 161 and a combined feedstream 101' is passed 
to a splitter zone 200. Splitter zone 200 which employs conventional 
fractionation to split the combined feedstream into a light feedstream 102 
comprising normal pentane and a heavy feedstream 103 comprising normal 
hexane. Preferably, the light feedsteam comprises greater than about 60% 
normal pentane, and more preferably, the light feedsteam comprises more 
than about 80% normal pentane, and most preferably, the light feed stream 
comprises more than about 90% normal pentane. In a reaction/adsorption 
step, the heavy feedstream 103 is passed at a reaction temperature and a 
reaction pressure as a vapor via lines 103 and 104, valve V37 and line 105 
to PSAR vessel 201 to produce a first effluent stream 106 which is passed 
via valve V20', line 107, line 108, line 109, and line 157 to a first 
condenser 205 to provide a cooled effluent stream 110. The cooled effluent 
stream 110 is passed to a first separator 206 to provide a first vent 
stream 112 comprising hydrogen and a high octane product stream 111 
comprising dimethyl butanes. The passing of the heavy feedstream to PSAR 
vessel 201 is terminated prior to the breakthrough of normal hexane and 
the PSAR vessel 201 is first purged in a first purge step at the reaction 
pressure with a portion of the light feedstream 102 which is introduced 
via line 102, line 118, valve V22' and line 119 and a purge effluent 
stream 121 comprising isopentane and methylpentanes is withdrawn. As the 
normal pentane in the light feedstream is introduced to the PSAR vessel 
201, it is believed that the normal pentane displaces the adsorbed normal 
hexane and as these normal pentane proceeds further, a portion of the 
normal pentane is converted to isopentane. Because the isopentane is a 
weaker desorbent than normal pentane, the isopentane now displaces mostly 
the methylpentanes and not as much of the normal hexane thereby producing 
a high pressure purge effluent comprising isopentane and methylpentanes. 
The high pressure purge step is terminated prior to the breakthrough of 
normal pentane. The high pressure purge effluent is withdrawn in line 121 
and passed via valve V38, line 122, line 130, line 135, and line 137 to a 
second condenser 208 to provide a condensed high pressure purge effluent 
138. The condensed high pressure purge effluent 138 is passed to a second 
separator 207 to provide an intermediate product 140 and a second vent 
stream 140' comprising hydrogen. The second vent stream 140' is combined 
with the first vent stream 112 via line 140' and line 163'. At the 
completion of the first purge step with normal pentane, the PSAR vessel 
201 is countercurrently depressurized to produce a low pressure purge 
effluent stream 123 which is passed via lie 123, valve V40, line 124, line 
131, and line 132 to the third condenser 209 to provide a condensed low 
pressure purge effluent 132'. The condensed low pressure purge effluent 
132' is passed to a third separator 211 to provide a third vent stream 162 
and a heavy hydrocarbon stream 160. At least a portion of the heavy 
hydrocarbon same 160 comprising normal hexane is passed to pump 215 to 
return the portion of the heavy hydrocarbon stream to be admixed with the 
feedstream 101 as the recycle stream. At the completion of the 
countercurrent depressurization step, the PSAR vessel 201 is returned to 
the reaction pressure by repressurization with the combined vent stream 
112' comprising hydrogen. Although this repressurization can be carried 
out in a cocurrent or countercurrent fashion, it is preferred to 
repressurize in a countercurrent direction to move any remaining normal 
paraffins in the PSAR vessel 201 toward the feed end of the vessel prior 
to the reintroduction of the feedsteam. The process comprising the steps 
of reaction/adsorption, high pressure purging, depressurization, and 
repressurization is repeated for PSAR vessels 201, 202, 203, and 204 on a 
cyclic basis to provide a continuous process. Each of the above steps is 
implemented sequentially for each of the above 4 PSAR vessels to provide a 
continuous flow of high octane and intermediate products. 
As illustrated above with respect to PSAR 201, each of the remaining 3 PSAR 
vessels cycles through the steps of the process. For example, PSAR vessel 
202 in the reaction/adsorption step is charged with the heavy fee am 103 
via line 125, line 126, valve V41, and line 126'. The first effluent 
stream is withdrawn in line 149' and passed to valve V26', line 149, line 
108, line 109, and line 157 to recover the high octane product 111 and the 
first vent stream 112. During the first purge, or high pressure purge 
step, the light feedstream 102 is passed to PSAR vessel 202 via line 102, 
line 146, line 147, valve V28 and line 147'. The high pressure purge 
effluent stream is withdrawn from PSAR vessel 202 in line 127', valve V42, 
line 127, line 130, line 135, and line 137 to intermediate product 
recovery from separator 207. During the countercurrent depressurization 
step, the desorption effluent 128' is passed via line 128', valve 143, 
line 128, line 131, and line 132 to the third condenser 209 and separator 
211 for the recovery of the heavy hydrocarbon stream 160. In the 
repressurization mode, the combined vent gas stream 112' is passed to the 
PSAR vessel 202 via lines 113, 114, and 148, valve V30 and line 148'. 
Similarly, for the operation of the operation of the process with respect 
to PSAR vessel 203, the heavy feedstream is introduced during the 
reaction/adsorption step via line 103, line 125, line 125', line 141, 
valve V44 and line 141' and the first effluent stream is withdrawn via 
line 150', valve V31, line 150, line 109, and line 157 to the recovery of 
the high octane product 111. In the high pressure purge step, the light 
feedstream 102 is passed via line 102, line 146, line 151, line 152 and 
line 152' and the high pressure purge effluent stream is withdrawn in line 
142' and passed to intermediate product recovery via valve V45, line 134, 
line 135, and line 137. When PSAR vessel 203 is countercurrently 
depressurized, the desorption effluent is withdrawn via line 143', valve 
V46, line 143, line 133, and line 132 for the recovery and recycle of the 
heavy hydrocarbon stream comprising normal paraffins such as normal 
hexane. PSAR vessel 203 is repressurized by passing the combined vent gas 
stream 112' to line 113, line 153, valve V33, and line 153'. 
The operation of the process with respect to PSAR vessel 204 is similar to 
that of PSAR vessel 203. During the reaction/adsorption step, the heavy 
feedstream 103 is passed via lines 125, 125', and 144, valve V47 and line 
144' and the first effluent stream is withdrawn to high octane product 
recovery via line 156', valve V34, line 156 and line 157. During the high 
pressure purge step, the light feedstream 102 is introduced via lines 102, 
146, 151, and 155, valve V35 and line 155', and the high pressure purge 
effluent is withdrawn to intermediate product recovery via line 136', 
valve V48, line 136, and line 137. During the countercurrent 
depressurization step, the desorption effluent is withdrawn and passed to 
the recovery of the heavy hydrocarbon stream 160 via line 145', valve V49, 
line 145 and line 132. During the repressurization step, the combined vent 
gas stream 112' is passed to the PSAR vessel 204 via line 158, valve V36, 
and line 158'. The PSAR vessels may be cycled in any order with each PSAR 
vessel in a different mode of the process. It is preferred that initially 
at least one of PSAR vessels be pressurized to a reaction pressure with a 
hydrogen-containing gas. 
It will further be understood that various changes and modifications can be 
made in the details of the PSA zone as herein described and illustrated 
above without departing from the scope of the invention as set forth in 
the appended claims. In addition the number of beds employed may be varied 
depending upon the circumstances and results desired in any given 
application. Accordingly, the individuals PSA steps described, as well as 
conventional variations thereof, are generally known by those skilled in 
the art and need not be further described herein. It will be further 
understood that PSA systems necessarily incorporate various conduits, 
valves, and other control features to accomplish the necessary switching 
of adsorbent beds from one step to the next, in appropriate sequence as in 
conventional PSA operations. 
The following examples are provided to illustrate the present invention and 
are not intended to limit the scope of the claims that follow. 
EXAMPLES 
Example I 
A series of adsorption screening tests for the reactants, intermediate 
products, and final products of the C.sub.5 /C.sub.6 isomerization 
reactions. These tests were carried out in a modified BEI adsorption 
apparatus. The apparatus measures adsorption by sensing changes in 
pressure and temperature inside a reference volume which is attached to an 
adsorption vessel containing the adsorbent sample. The adsorbent sample 
was maintained at a steady temperature by the action of a temperature 
controlled heated chamber. The reference volume can be isolated from the 
adsorbent vessel by means of an isolation valve. The reference volume may 
also be connected or isolated from a vapor source by means of another 
isolation valve, and finally the absolute pressure of the reference volume 
may be controlled by means of a high vacuum pump which is also connected 
to the reference volume by means of a third isolation valve. Adsorption 
measurements were made by first evacuating the reference volume and the 
adsorbent sample vessel to a pressure of approximately 5.times.10.sup.-6 
torr while heating the adsorbent and adsorbent vessel to an activation 
temperature of approximately 300.degree. C. The temperature of activation 
was controlled and monitored. After the adsorbent sample has been dried at 
300.degree. C. for 4 hours, its volume and weight are measured and it is 
loaded in the sample chamber. Vacuum is applied to the system at 
250.degree. C. for several hours until no increase in pressure is 
detected. At this time, the temperature was adjusted to 225.degree. C. for 
collection of isotherm data. After activation, the sample was isolated 
from the reference volume. The reference volume also evacuated to 
5.times.10.sup.-6 torr was also isolated from the vacuum pump and was 
charged to a vapor pressure of about 5 torr with purge sample hydrocarbon 
vapor. The pressure of the sample hydrocarbon vapor was monitored by a 
pressure transducer. Once stable readings were obtained on both the 
pressure and temperature within the reference volume, the isolation valve 
separating the adsorbent sample from the reference volume was opened and 
the pressure and temperature of the system were monitored until they 
stabilized, i.e., changing no more than by 1 part in 10.sup.6 within one 
minute. Adsorption isotherms were obtained by repeating the isolation, 
charging and equilibration of the reference volume with the adsorbent 
vessel until a pre-determined loading level or pressure level were 
obtained. Adsorbent loadings of each of the following hydrocarbons: a) 
normal pentane, b) isopentane, c) normal hexane, d) 2-methylpentane, and 
e) 2,2-dimethyl butane on silicalite were determined. 
The adsorption isotherms at 225.degree. C. over silicalite adsorbent for 
the above list of hydrocarbons is shown in FIG. 3. From the loadings shown 
in the range of about 30 psia to about 60 psia, the 2,2-dimethyl butane 
exhibited the lowest loading; the 2-methyl pentane showed an intermediate 
loading and the normal pentane showed the highest loading. All loadings 
were expressed in grams per 100 grams of adsorbent. These results support 
the conclusion that at reaction conditions, the highest octane materials 
(e) and (d) are least adsorbable, while the normal C.sub.5 and C.sub.6 
hydrocarbons, the primary reactants, are most adsorbable. In all cases, 
the silicalite adsorbs normal and monomethyl paraffins over dimethyl 
paraffins. Thus, it is possible to retain the reactants and the 
intermediate products in a catalyst bed of a pressure swing adsorption and 
reaction system while rejecting the highest octane product. In this 
manner, the reaction can be carried to completion. Furthermore, the data 
suggest that the reaction of normal pentane to isopentane can also be 
carried out beyond equilibrium. 
Example II 
Isomerization reactions were carried out in a stainless steal tubular 
reactor approximately 45 cm in length and having an inside diameter of 
about 8 mm. The reactor was loaded with about 17 grams of a 
platinum-mordenite catalyst bound with alumina. Normal hexane (99% pure) 
was charged, at a liquid hourly space velocity of about 1 and at a rate of 
about 20 ml/hour, to the reactor which was maintained a temperature of 
225.degree. C. Hydrogen (99.9% pure) was passed to the reactor such that 
the hydrogen to hydrocarbon ratio was about 1. The reaction was carried 
out at two reaction pressures: 450 kPa (65 psia) and about 1150 kPa (165 
psia). The results as shown in Table 1. The conversion and the production 
of higher octane products favored the lower pressure operation. 
TABLE 1 
______________________________________ 
NORMAL HEXANE ISOMERIZATION 
PRESSURE 
COMPONENT 450 kPa 1150 kPa 
______________________________________ 
2,2-dimethyl butane 
18.09 15.06 
2,3-dimethyl butane 
9.59 9.76 
2-methylpentane 33.34 34.31 
3-methylpentane 21.26 21.71 
normal hexane 17.10 18.91 
Conversion, % 82.9 81.1 
______________________________________ 
Example III 
The performance of an isomerization process for the conversion of a C.sub.5 
/C.sub.6 paraffin feedstock in a conventionally integrated reaction and 
adsorption process is best characterized by the Total Isomerization 
Process disclosed in U.S. Pat. No. 4,210,771. In such a process the octane 
of the product produced will be about 87 research octane and have an 
overall recovery of about 97 percent. In comparison, the process of the 
present invention will produce a high octane product having a research 
octane of about 92 with about the same overall recovery. The advantage of 
the present invention over the conventional process is the higher octane 
of the product and that this higher octane can be achieved at lower 
severity since the product is removed from the reaction zone as soon as it 
is produced. The lower operating severity provides longer catalyst life, 
and reduces the amount of heavy paraffins lost to side reactions such as 
cracking.