Note: Descriptions are shown in the official language in which they were submitted.
94~3
a ~
The present inventlon r~lates in general to a
process for improvlng the octane rati.ng of certain petrole-
um fractions and, more particularly9 to ~ process :Eor
continuously separating normal hydrocarbons from admixture
with branched chain hydrocarbons and lsomerizing the
normals fraction to form branched ch~in hydrocarbons of
higher octane number.
High octane gasoline ha~ become cri~ically short
in supply and many processes have been devise~ to amelior-
ate this situation by making the most eficient conversion
possible of the available petroleum. His~orically~ reform-
ing, in many detailed proc~s~ v~rsions has been used to
convert low octane gasoline range fr~ctions 3 such as heavy
naphthas~ in~o higher oc~ane gasoline products. Isomeriza-
- tion, another catal~tic hydrocarbon conversion process, has
also been ~mployed. The advent of size or shape s~lective
molecular sieve adsorption separation processes has offered
~he petrole~m re~iner still another method for obtaining a
:, :
high oc~ane product by removal of ~he lower octane straight
- 20 ch~in gasoline range hydrocarbons. When used alone, ~he
catalytic and the selec~ive adsorption proceæses have had
: certain drawbacksg either in yield or conversion to high
octane product, or in the oc~ane value of its product.
number of processes have been devlsed which aggrega~e a
catal~tic process and ~ selec~lve ad~orptive process, bu~
none has achieved the succes~ul in~egration of such
proces~e~ ~o ach~eve the ef~e~ivene~8 and ~implicity of
'
~ ~,
, 9443
the novel process of the pres~nt invention.
In the isomerlzatlon process whlch comprises pro-
viding ~hree fixed adsorbent beds, eac:h havlng essenti~lly
the same adsorption capacity and void space ~nd containing
partieles comprisin~ a crystalline zeoli~ic molecular sieve
adsorbent having ~n effective pore diameter of from greater
than 4 to less than 6 ~ng~troms~ and further providing as
adsorption feed a mi~ture of normal and non~normal hydro-
carbon compound~, the normal paraf~in compon~nt of which
comprises n-pentane and n-hexane, adsorbing the n-pentane
and n-hexane from the ~aid fe~d ln the said fixed adsorbent
beds by pas~age of the feed into one end of each of xaid
beds and recovering the non~adsoxbed non~normal hydrocarbon
- eompounds from the other end of each bed, desorbing the
n-pentane and n-hexane from each of said beds by purging
with hydrogen in a dlrection countercurrent to the direction
~ of inflow of said feed, and passing the e~fluent hydroge~,
: n~pentane and n-hexane in admi~ure through a rea~tor con-
taining a zeolitic molecular ~ieve i~omerlzat~on catalyst
under isomeriz~tion condi~ions, the improvement which com-
pri~es desorbing the said ~hree bed~ in ~he manner such that
~o more than two beds are being dasorbed at any given time
~` and ~he terminal stage of desorption in one of the three
bed~ i~ contemporaneou~ wi~h the initial stage of desorp~
.
tion in anot~er of the three beds.
`'~ .
~ ~ 3 ~
~ - . . - . . . .
.
~443
The aforesaid process provLdes a hlghly ~atlsfact-
ory mean~ for dealing with a problem encountered ln separa-
ting normal from non-normal hydrocarbons by s~lectlve adsorp
tion of the normals in ixed bed selective adsorbers and
th~rea~ter desorbing the normals with a purge gas ~o produce
an eEfluent stream to be fed directly t:o ~he catal~tic isomer-
izer. The problem arises rom the fac~ tha~ thP adsorbed
hydrocarbons are not desorbed and flow out of the adsorption
bed a~ a unifonm rate. A number of factor~ are :Lnvolved
-: 10 but the overall effect 15 that the concentr&~ion of normal
hydrocarbons ln the adsorber effluent is ~ignificantly
greater in the lnitial stage~ of desorption than in ~he
ter~inal stage~, When thi9 ad~orb~r e~fluent i~ pa6~ed
.. . .. . . . . . . _ . .
direetly to the ca~alytic isomer~zer, ~he w~ight hourly
space velocity (W~ISV) o~ the normal hydrocarbons through
~he isomerizer fluctua~e~ over undesirably wide rang~
Although the WHSV can vary over ~he range of from about 1 to
10 tweight hydrocarbon per hour per weigh~ c~talyst compo~
sition~ ~or any given isomer~zation unit, the op~im~ value
wi~hin this r~nge is dic~ated by ~he other operation con-
di~ions selected. In practlce i~ i~ no~ ~easibLe to con-
stantly adjust and correl~te all o the operating ~onditions
with a con~in~ally changi~g W~SV value durillg the cour~e of
the isomeriza~ion proce~r ~hus w~e~ WHSV value~ fluctua~e,
a departur~ frQm overall optimum operation i5 inlevl~abl~ the
. , ~
~ ~ 4 ~
." ' ' . .
9~43
result wi~h ~:endant loss in yield and/or conversion rateO
In accordance with the present i.nvention, the
staged desorption of two beds so that the hydrocarbon rich
initial effluent from one bed is combined with the hydro-
carbon-poor terminal e:Efluent from the other bed greatly
reduces WHSV :Eluctîons in the isomerizer and thu~ permi~s
much improved operation.
The present procPss also minimizes a problem
ordinarily existing when the feedstock to be catalytically
isomerized comprises a mix~ure of C~ and C6 hydrocarbons.
The problem is one of temperature control in the isomerizer
and involves several facto~q. The-optimum ~emperature for
isomerizing n~hexane is in part dependent upon the particu-
lar catalyst composition employed 9 but i~ any event involves
a balance-between the degree of conver~ion of the n-hexane
to the high octane isomerlc forms and ~he degree of cracking
o th~ n-hexane to undesired products. A relatively low
temperature ~avors the productio~ of hi~h yields of low
oc~ane isomerizate per pas~ through the isomerizer~ By
increasing the tempera~ure ~lightly one can obtain a some-
what higher octane product~ bu~ in substantially lower yields,
With continually rising isomeriza~ion tempera~ures 9 the
yield o~ desired product, C5+, declines rapidl~ due to
.~ xce~sive hydrocracking eve~ though ~he octane number of
~he desired product does no~ decrease great~y.
The same con~ideratio~s apply in the eaSQ of
isomerizing n pentane. However~ the optimum i~omerization
',;
~''
. _
,~ , , .
,, - : .,:
. ~ . .
9443
temperature for n-pentane is not the same ag for n~hexane,
and when mixtures of n-pentane and n-hexane are to be
isomerized, the relatlve proportions of th~e two compound~
entering the isomerizer determine~ khe optimum temp~rature
`. for the i.somerization process.
I~ is therefore, ~or this re~son as well as
others, important that large fluctuations in the coneen-
tration of either n~penta~e or n-hexane in the feed to the
isomerizer be avoided. Such fluctuations inher~ntly result
from feeding dire~tly to the i30merizer the effluent from
a single adsorption bed used to selectively adsorb n~pentane
and n-hexane from admixture wlt~ isopen~aaes and isohexanes.
Due to the ~act tha~ n-hexane i~ more strongly adsorbed by
molecular sieve zeolite~ than 1~ n-pentane, ~he n-hexane
tends to be more concentrated in the in1uent end o~ the
adsorption bed and the n-pentane more conoen~rated at the
,
effluent end of the bed. Thus during purge desorption of
the bed to obtain a feedstock for the isomerlzer, large
changes in composition occur in the desorba~e ef~luent.
Heretofore, uniformity of this feeds~Qck was achieved by
using large holding ~a~ks in which ths entlre desorbate
from ~he adsorption bed was accumulated and admlged b~ore
. ,;
~; being passed to the isomerizer. This accumulation ~me and
.. the e~pensive apparatus i~volved is dispensed with by the
process of ~he present inven~ion. By the coordlnaked and
staged desorp~ion of two of the ad80rption beds ~ogether
at certain ~ime~ ~o that the n~hexane enr~rhed efflulent
. '
6 ~
,
9~43
from the terminal desorption stage of one adsorber is mixed
in ~he pipeline to ~he isomeri~er with the n-pentane enriched
effluent from the initial desorption ~tage of another
adsorber, a preselected optim~m isome~izer temp~rature is
continually appropriate ~or converting the feedstock to high
octane product.
The ~eed~tocks w~ich are to be up~raded with
re~pec~ to octane number in accordance wi~h the present
process are composed principally of the various isomeric
forms of satur~ted hydrocarbons having from 5 to 6 carbon
atoms inclusive. Advan~ageously ~he C5 and C6 saturated
hydxocarbon isomer~ comprise at least 40 mole per cen~ of
the feedstock wi~h n-pen~ane and n-hexane each belng at
least 10 mole per cen~ of the overall feed3~0ck.
As will be illu~trated hereinafter, the compo~i~ion
.: of ~he available feedstock will ln some measure dictate the
preferential u~e in a given process embodlments. That is,
the fresh féedstock can enter the proce~s ~hrough the i~o-
merization unit or the selectlve adsorption uni~, or both.
The location of ~he specific feedpoi~t depends upon one or
: more a~pec~s of the ~eedstock composi~ion.
In the process embodiment where~n the feedstock
ls in~roduced ln~o the syst~m through ~he ad~orp~ion unit,
the eads~ock is composed principally of he varic)us isomeric
forms of satura~ed ~ydrocarbons having rom 5 ~o 6 carbon
a~oms inclusiv~. The feedstock can contain subs~m~ial
amounts of iso~C7 molecules, but should be relatively re~
~ 7
9443
of n-C7 and higher hydrocarbons. The presence of iso-C7 in
the feed to the proces~ of thl~ lnvention i~ permisslble
since the feed flrqt en~ers the molecular sieve separation
stage from which the iso-C7 is dlrected to up~raded product
collec~ion ra~her than ~o ~he catalytic conversion stage
wherein it would be cracked and contribute to coke ormation,
Olefinically unsaturated hydrocarbons should be absent from
the feedstock ~o a practical degree since ~hey con~ume hydro~
gen during the i80merization operation and form residues in
~: 10 ~he adsorption bed. Advan~ageously, the C5 and C6 ~turated
hydrocarbon isomers compri~e at least 40 mole per cen~ of the
feeds~ock, with at least lO mole per cent being normal pen~
tane and/or normal hexane, The feed wili frequen~ly contai~
.- naphthene~, sometim~s up ~o 20 per cent. A ~eed~tock of the
ollowing c~mposition is typical:
i-C5 16.6
: n-Cs 23.2
~-C6 ~1,2
n-C6 22.2
i~C7 16.7
', n-C7 nil
In the process ~mbodiment whereln the ~eed~tock i~
in~roduced into ~he system through the isomerlzer, ~he feed-
.~ stock i9 composed princlpally of ~he various isvmerlc orms
of ~a~ur~d hydrocarbons havi~g fr~m 5 to 6 carbon atoms
inclusive. Such feedstocks ~r~ normally the result of
~'
8 -
~, . .
~ 94~3
refinery di~tillation operation~, and thus may conta-ln small
amounts of C7 and even hi~her hydrocarbon~, but these are
~requently present, if a~ all, onl~ in ~r~ce amoun~O 01~
finic hydrocarbons are advan~ageously le~s than about 4 mole
per cent in the feed~tock for the s~e ~ea~ons as 8~t forth
hereinbefore. Aromatic and cyclop~raff.in molecules have a
relatively high octane number, but are to a sub~tantial
degree cracked and/or converted lnto molecules of much
lower octane number in the i~omerizer. Accordinglyg the
feedstock should not contain more than about 15 mole per
cent combined arom&tic ~nd cyclop~r~ffinlc hydrocarbons.
Advantageously, ~he Cs and C6 non-cyclic paraffln~ comprise
~ at least 85 mole per cent of the feed~tock~ w;th at lcast
:~ 10 mole per cent being normal penkane and/or norm~l hexaneO
A feedstock of the following composition i5 typical:
Components
i-C~ 12.5
n-C5 29.5
i-C6 32.0
n-C6 24.6
i ~7 1.4
~C7 nil
In the foregoing description of the feeds~ocks suitably treated
in accordance with the present process the expre~sion "the
various isomeric forms o pen~ane and hexane" i~ intended to
denote all the branched chain form8 of the compounds ~s ~ell
as ~he straight chain forms. Also th~ prefix notations "iso"
; and t'i" are intended ~o be generic designa~ions ~f al.l branched
chain forms o the indicated com~ound.
~ _ g_
;,
~ 9443
The hydrogen stream used as the purge gas in
desorbing the adsorption bed and as the hydrogenation
ma~erial in the isomeriza~ion reactor need not be pure and
is generally composed o one or a com~ination of two or more
refinery hydrogen streams such as refoI~er hydrogen and the
like. An~ impurities pre~ent should be relatively non-
sorbable and inert toward ~he ze~lite adsorben~, ~he zeolite
catalyst and the hydrocarbon in the sy~tem. It will be
understood that light hydrocarbons containi~g from 1 to 4
carbon a~oms inclusive will appear in the recycle hydrogen
in the course of opera~ion cf ~he proce~ si~ce these low
boiling materials are produced in the cataly~lc unit. Pref-
erably, the recyc~e hydrogen s~ream i9 at lea~t 60 mole per
cent hydrogen.
The zeolitic molecular sieve employed in the
adsorption bed mus~ be capable o~ selectively adsorbing ~he
~:~ normal paraffins of the ~eeds~ock u~ing mol0cular sis~e and
configura~ion as the crlterion. Such a molecular g:Leve
should, thereore, have an apparent pore diameter o~ less
20 than about 6 Angstroms and gre~ter ~h~n about 4 Ang~trom~.
A particularly suitable zeolit'e of this type is zeolite A9
described in U.S.P. 2,883,243, which in several of its
divalent exchanged forms, no~abl~ the calci~m cation form,
i ha~ an apparent porediame~er~-o~ about 5 Angstroms~ and has
a very large capacity for adsorbing normal para~:Eins~ her
,~ .
~ui~able molecular sieve~ include zeol~e R, U.S.P.3J030,181;
zeolite T, U.S.PO 2,950,952, and ~he naturally occurring
zeoli~ic molecular sieves chabazi~e and erionite. The t~rm
"apparent pore diame~er" ~s us~d herein may be d~fined ~s
the ma~im~m critical dimension3 or the molecular species
- 10 -
,. ~. .
'
9~43
which is adsorbed by the adsorbent under normal conditlons,
The critlcal dimension i~ dain~d as the diameter of the
smallest cylinder which will acco~modate a model of the
molecule constructed using the available value~ of bond
distances, bond angles and van der Waals' radii. The appar-
ent pore diameter will always be larger than the structural
pore diameter, which can be defined as the free diameter of
the appropriate silicate ring in the structure of the ad-
sorbent~
The zeoli~ic ca~al~3~ u~ed in the isomerization
reactor can be any of the various molecular ~ievc based
catal~st compositions well known ln the art which exhibits
selective and substantial lsomerization ~ctivity und~r the
operating condltlons of the presen~ process. As a general
class, such catalysts comprise the crystalline æeoliti~
molecular sieves having an apparent pore diameter large
enough ~o ad~orb neopentane) a SiO2/A1203 molar ratLo of
grea~er than 3; le88 than 609 preferably less th~n 15,
equlvalent per cent alkali me~al ca~ion~ and having tho3
A1~4- te~rahedra not associated with alkali met~l catlons
either not associated with a~y ~etal cationj or assnciated
wlth dival~n~ or o~her polyvalent met~l ca~ions, said
zeolitic component b~ing co~kined with a hydrogenation cat-
alys~, pr~ferably a nobl2 met~l of group VIII of the
:;. P~riodic classifiration v~ ~he Elemen~s. The catalyst com~
po~ition can be us~d alon~ or e~n be combin~d wl~h a
por~u~ inorganic oxide diluent a~ a blnder materialO The
:~ hydrog~na~ion agen~ can be carri~d ~i~h~r on ~he zeolitic
:' .
- 11 ~ .
'
9443
.
component and/or on the binder. A wide variety of inorganic
.. . ...... . ., . .. _ _ _ . .. . . . . .
oxide diluent materials are knowhin the art -- some of which
exhibit hydrogenation activity per se. It will, accordingly,
be understood that the expression "an inorganic diluent
having a hydrogenation agent thercon" i9 meant to include
both diluen~s which have no hydrogena~on ac~ivity per se
and carry a separate hydrogen~tion age~t and those diluents
which are per se hydrogenation cataly~ts. Oxide~ suitable
as diluents, which of themselves exhibit hydrogenation
activity, are the oxides of the metals of Group Vl of the
Mendeleev Periodic Table of Elements. Representative of
; these metals are chromium, molybdenum and tungsten. It is
preferred, however, tha~ the diluent ma~erial possess no
pronounced catalytic activity per se, e~pec~ally cracking
ac~ivity, In all even~s, the diluent should not exhibi~ a
greater quanti~ative degree of cracking ac~ivi~y ~han~he
. zeolitic componen~ of the overall isomerizat~on catalyst
composition. Suitable oxides of this lat~er olass are the
aluminas, silicasg the oxid~s of metals of Groups III,
IV-A and IV-B of the Mend~leev Periodic Table, and cogels
20 of silica and oxides of ~he metals of tbe Groups III9 IV-A
and IY-B3 espécially al itania, thoria
and com~inations thereof. Aluminosilicate clay~ ~uch as
kaolin, a~tapulgite, sepiolite, polygarski~, ben~onite3
montmorillonite and the like w~en rend~red in a plian~
plasticNlike condltion by in~ima~e admixture wLt:h water are
also sui~able diluellt msterials, p~r~eularly w~en said
. .~,
~ ~ 12 ~
9443
clays have not been acid-washed to remove substantial
quantities of alwnina. Superior catalysts or isomeriza~
tion reactions are di~clo~ed in detail in U.S.P. 3,236,761
and U.S.P. 3,236~762. A particularly preferred catalyst
is one prepared from a zeolite Y (U.S.P. 3~130,007) having
a S102/A1203 molar ratio of about 5 by reducing the sodium
cation content to less than about 15 equivalent per cent
by ammonlum cation exchange, then introducing between about
35 and 50 equlvalent per cent of rare ear~h metal ca~ions
by ion e~change and thereafter calcini~g the zeolite to
effec~ substantial deammination. A~ a hydrogena~on com-
ponent, platinum or pa~ladi~un in an amount of abou~ 0.1
to 1.0 weight per cent c~n be placed on the zeolite by any
. . , ~
:~ conventlon~l method,
Depending on the par~icular cataly~t compos~ion
employed, the operating ~emperature of the i~omerizer is
gen~rally within the range of 240C. to 390~C, ~nd the
- pressure ig within the range of 175 to 600 psia. Although
it i5 preferable to carry out the overall adsorption separ~
~ 20 ation and isomerizatlon process under essentiall~ isobari~
: and isothermal conditionsg the effective operating condltions
_ _ _ . . , . _ _ _ _ _ , _ . . ...................... ... . . . ..
in ~he adsorption beds are somewhat bro~der in range than
, .
in the isomerlzer. Pre~ures above atmospheric in con~
~unc~ion with temperature~ in ~e r~ng2 of ~40 ~o 3gO C.
which maintain the ~eed~ock in ~he vapor s~ate are sult~
. able for oper~tlon of the ad~orbers.
:,
~ ~ 13 ~
: . , , - , -
9~43
In the present process the molar ratio of hydrvgen
to hydrocarbons having from 5 to 6 carbon atom~ inclusive
of the feed entering the isomerizer is at lea3t 2~ prefer~
ably at least 3, up to ahou~ 30. In the even~ the hydrogen
con~ent of the effluent from the adsorption u~it passing to
the isomerizer is inadequate a~ ~ny time to maintain the
minimum of this ratio range, supplamental hydrogen can be
admixed into the isomerizer feed stream from any suitable
source, but preferabl~ is a diverted ~ortion of the recycle
; 10 hydrogen stream of the same process ~yst~m~
... , , ... . ........ . _ . _ _ _ _ _ _ . . .. . ............... . .. .
: "Bed void space" for purpose~ of thi~ inventlon
is intended to mean any space ln the bed not orcupied by
solid material except ~he intracrystalline cavi~ies of the
zeolite crystals. The pore~ within any binder material
w~ich may be used to form agglomerates of the zeoli~e
crystals is con~idered to be bed void ~pacs.
Wikh respect ts the desorption step which each of
the three ad~orbent bed undergoes as a result of counter~
current purging with ~ydrogen, t:he ~erm "ini~iaï stage" i3
not intended to deno~e any particular frac~ion o ~he total
desorption period but ra~her ~hat first portion o ~he d~-
: sorption period which include~ the m~ximum eoncentration
.~
.~ of n~pentane in the bed effluent, In no even~ will the
.
"initial stage" e~ceed the first S0 per cent of b~d
:I desorption period and can be onl~ ~he first 5 per cen~ or
:` le9~ o the de~orp~ion period dependillg upon such factor~
as ~he concen~ration of normal hydrocarbons in th~ feedstock
.
~ 14 -
9443
~ 3~
being treated and the r~latlve conc~ntration of n-pentane
and n-hexane with respect to each other. Similarly
t'terminal stage" is a final portion of the desorption period
which includes the maximum concentration of n hexane in the
bed effluent and is equal in time of duration as the afore-
said "initial stage."
In the drawings:
Fig. 1 is a schematic 10w diagram illu~trating
the process embodiment of ~his invention wherein the eed-
stock is fed ini~ially to the adsorption-separation unit
and the isomerizer ef1uent i~ recycled to ~he separation
unit.
Fig. 2 is a ~chematic flow d~agram illustrating
the process embodiment wherein the feedstoc~ is fed initial-
ly to the isomerizer and the effluent therefrom is recycled
to the adsorption~saparat~on unit.
The invention is illustrated by ~he following
examples:
,
Example 1.
~, 20 A feedstock having the following composition W~5
.~ , . . - .isomerized in accordance with a preferr~d em~odiment of ~he
present process:
Feedstock Mole-% of
. ~ .
isopentane 18~2
. n-pentane 25~6
cyclopen~a~e 3.2
: : 2,2-dimethylbutane 0.73
:'. 2~3-dime~hylbutane 1.95
.: - 15 -
.~, .
.... .... . , ,. .- . .,- ..... . . ~ -. . . .
.. .. . . ..
9443
Feedstock Mole-% of
Total Feedstock
2-methylpentane 13.4
3-methylpentane 8O5
n-hexane lg.5
benzene 1.17
cyclohexane .45
methylcyclopentane 7-3
In describing the c~cllc process it ls ~o be under~tood
that the sys~em has been operating for a time suf~Eicient to
reach a stead~ ~tate uslng essentiall~ the same f~eds~ock
described above. With reerence to Flg. 1~ fresh eedstock
eaters the system through line 10 and is intermlxed with
~ .
the conden~ate fraction of i80merizer ef~lu~n~ deliv~red
through line 11 in feed drum 1~ in which ~he temperature i~
; about 90F. a~d ~he pre~sure is 1~5 psia. The mix~d feed
from ~he feed drum 12 passes through li~e 13 a~d pump 14 to
heat exchanger 15, urnace 16~ ~nd then ~hrough line 17 and
valve 18 to adsorber 19. At ~he ~ime the fe~ds~ock enters
adsorber 19, it is a~ a ~emper~ure of 700F. at a pre~ure
of 23~ psia and has a compo~i~ion a~ ollQws:
.. . ... . .
-~ Feedstock Mole~% of
: Component Total F~edstock
Hydrogen 0 43
Methane 0.37
Ethane Or 25
: Propane 0.54
i Butane 0.43
n~Butane 0.19
.. 30 i~o-pen~ane 26,41
n~pentane 24,14
cyclop~ntane 1.94
: 2,2~dimethylbuta~e 2.80
- 2 3 3-dime~hylbu~ane 2O36
. 2-methylpentane 12,62
~. 3-m~hylpe~tan~ . 8.04
..
:.
:,
,;
: ~ 16 -
`~
.~ .
.. ". . ,, . .. ~ .. . . ~ :
94~3
n-hexane 14.16
benzene .65
: cyclohexana .~7
methylcycïopentane 4.39
Absorber 19, as do adsorbers 20 and 21 respectively, contains
a fixed bed o~ 1/16" di~me~er pellets of zeoli~ic molecular
sieve Type 5A hav~ng effective pore openlngs of about 5 Ang-
strom units, The opening of valve 18 to pass in the eom~
pressed hea~ed hydrocarbon vapor initiates ~he adsorption-
fill step of the process at which po~nt the adsorber c~ntains
: essentially hydrogen gas at 700F. and 239 psia. As the ~eed
, . _ . . .
hydrocarbon vapors enter the ad~orber, the hydrogen g~s
exits the other end of the adsorber through valve 22 into
manifold conduit 23 and valve 24 to adsorber 20 ~imultane-
ously undergoing the desorp~ion ~tep of the process. Valve
35 is open and adsorber 21 i~ being simulta~eou~ly co~purged.
At this point~ it should be understood khat while adsorber
19 is in ~he s~ep of adsorp~io~-fill~ adsorber 21 is simul-
taneously in the s~ep of co~purge and adsorber 20 is in the
~tep of coun~ercurrent purge de~orption and tha~ eaoh o~ the
adsvrbers sequentially undergoes the ~teps of adso~ption-
fill, co~purge and coun~ercurrent purge desorptlon. Over
th~t period of the adsorption stroke in ~bsorber 19, wher~ln
the one bed volume of hydro~en is being ~orced from the
adsorber, the effluent ga8 stream pa~slng through valve ~2
is directed thrcugh manifold 23 and valve 2h into adsorber
20. When the product non~normal paraffins which ~orce the
hydrogen from ~he ab~orb0r 19 reach ~h~ effluent end o the
.
~ . - 17 -
. - ~ , . . .
9443
~ 3~
said adsorber 19, valv~ 22 is clo~ed and ~he product non-
normals are passed through valve 25 into collection mani~old
26 and removed from the system after the major portion of
its entralned h~drogen h~s been remov~d ~n coollng unit 27.
The adsorption stroke in adsorber 19 with the recovery of
product non~normals is discont~nued p:r~or to the time the
normal paraffin content of vold ~p~ce feeds~ock in the
adsorber exceeds the unused sorptive capacity of the bed
for the feedstock normal hydrocarbons. In all events, the
adsorp~ion stroke is tenminated prior to br~akthrough of
the normals mas~ transfer æoneO
~, A~suming a tim~ period of twel~e equal unit~
for a complete cycle of adsorption~ cocurrent purge and de-
sorp~ion for each o~ adsorber beds 1~, 20 and 21, the follow~
ing chart shows the time relationship of the three step~
undergone by each of the adsorbers in this exæmple:
Time ~nits
Adsorber
`~ 19 . Adsorpti.on ~ CoPur~e ¦ Desorp.
~; 20 ~
21 ~
Accordingly, over ~he period ~rom the beginning of the adscrp-
tion stroke in bed ~9 up to the closin~ of valve 22, the hy-
, . .
: drogen effluent from ad~orber 19 i5 employed in purge de~orb-
;~ îng adsorber 20~ To complete desorp~ion of adsorber 20, coin~
; cid~nt with the termina~io~ of ~he adsorp~ion stroka in ad~orb~
er lg3 ~ydro~e~ purge g~8 compri3ing recycle hydrogen from
cooling unit 44 ~nd as ~eded make~p hydrogen en~ering the
~ 18 -
, ~ ... . . .
9443
system throwgh line 28 is fed through line 29, blower 47,
heat exchanger 30,furnace 31, line 32 ~o manifo~d 23 and thence
through valve ~4. Coincident with the beginning of the ad-
sorption stroke in adsorber 19~ the desorption ~f adsorber
20 is well in progress and ~he effluent consis~s of a mixture
of hydrogen purge gas and desorbed normal hydrocarbons. At
the same poin~ ln time~ adsorber 21 has just completed an
adsorption stroke and has an unused adsorption capacity suf-
ficient to adsorb all of ~he ~ormal hydrocarbons presen~ in
the feedstock cont~ined in the bed void space. At this point
effluent from adsorber 20 through valve 48 is directed into
manifold 33 and through valve 34 where it supplies cocurrent
purge for adsorber 21. Cocurrent purging of adsorber 21 is
continued so long a~ ~he effluent non~no~mal hydrocarbon~
passing out of the bed through valve 35 are of the de~ired
purity. This portion of ~he pro~uct non-normal hydrocarbon~
is fed through ~anifold 26 and eollected through cooler 27
After completion of the cocurrent purge ~troke in adsorber
21, valve 35 is clo~ed and a hydrogen purge-desorption ~tre~m
is passed through valve 36 rom manifold 23 and ~he desorbed
normals and hydrogen purge gas are pas~ed through valve 34,
manifoLd 33 heat exchanger 15~ line 37 to catalytlc isomerizer
39. Because the cocurrent purge of ad~orber 21 w~s carried
out using a~ the purge gas ~he mLxture of hydrogen and normal
hydrocarbons from m~nifold 33~ a~ the be~inning of ~he
purge desorption ~troke in adsorber 21
~ ,. .
-, .
.
9~ 3
~S~ a~
the bed void space of adsorber 21 contains a greater con-
centration of normal hydrocarbons than would be the case if
essen~ially pure hydrogen were employed for the cocurrent
purge step. Moreover, had ess~ntially pure hydrogen been
used as the cocurrent purge gas, there would have resulted
some unloading of` normal hydrocarbon adsorbate from the
ingress end of the adsorber. Consequently, upon the begin
ning of the countercurrent desorption stroke in adsorber 21,
the first ef~luent therefrom, which is passed directly to
isomerizer 39, would have been quite lean with respect to
hydrocarbons. Since fluctuations in the hydrocarbon con-
centration of the feed to the lsomeriz~r 39 are undesirable 9
it is advantageous to avoid thi~ type of operation. The
present process does greatly mlnimize the~e undersirable
fluctuations by the particular cocurrent purge step de-
scribed above in which h~drocarbon adsorbate is not ~mloaded
from the ingres~ end of the adsorbcr during the cocurrent
purge. Thus the initial ~ffluent from adsorber 21 during
the purge desorption stroke is not unduly lean with respect
to hydrocarbons. The catalyst in isomerizer 39 is a
zeolite Y- pall~dium composition in which the zeolite has
;~ a molar SiO2tA1203 ratio of 5, a sodium cation population
o abou~ 10 equivalent per cen~ a rare earth ca~ion
population oi' about 43 equivalent
'
~ 20 ~
.
~3
per cent and having the remaining cation sites in the state
resulting from calcination o~ the zeolite at 550C. for 2
hours when the said sites were occupied by ammonium cations.
The composition contains 0.5 weight per cent finely divided
palladium.
With reference to the time relationship chart,
above, it is to be noted that the terminal portion of the
desorp~ion s~roke in adsorber 20 is contemporaneous with
the initial stage of the desorption in adsorber 21, and
also tha~ there is a similar overlap of desorption stroke
s~agas wi~h respect to ab~orber 21 taken with adsorber 19
and adsorber 19 taken with adsorber 20. Because the efflu-
ent from adsorber 20 during its~terminal desorption stage
is relatively lean with respec~ to desorbed hydrocarbons
and the effluent from adsor~er 21 during its initlal de-
sorption stage is relatively rich with respect to hydro-
carbons, the desorbate stream in mani~old 33 from adsorbers
20 and 21 i8 thus mNch more uniform at all times with
re~pect to hydrocarbon concentra~ion ~han would o~herwise
be ~he case.
The temperature in reactor 39 is maintained a~
abou~ 500F and the internal pres~ure is about 220 psia.
Ordinarily the operation of the system as hereinbefore
described will maintain ~he hydrogen to hydrocarbon molar
ratio in the reactor above 2~ but in the event of some
sys~em change thi~ ra~io in ~he ga~ stream en~erîng ~he
reac~or k~come~ les~ than 2 9 addi~iona~ hydrogen is added
~'''
~ 21 -
.
.. . .
9443
t~"
from the recycle hydrogen from llne 29 through valve 41,
line 42 and valve 38. The effluent from reactor 39 through
line 40 and heat exchanger 30 has the composition:
Component Mole-%
Hydrogen 68.45
Methane 8.75
Ethane 1.49
Propane lo 30
iso-Butane O47
: lO n~Butane .16
iso-Pentane 8,52
n~Pentane 4.65
Cyclopentane .00
: 2,2-dimethylbutane 1,13
~ 2 7 3-dimethylbutane.55
: 2-me~hylpent~ne 2.08
3-methylpentane 1.33
n-hexane 1.11
and after passage through valve 43, recycle separa~or 44,
to remove the bulk of hydrogen and the light hydrocarbons,
chiefly those havlng 1 to 4 carbon atoms, is recycled to
the adsorption unit through valve 45 and line 11.
.. . . . . ..
In the operation of the above-described process
embodiment~ it is not e~sential that the hydrogen be removed
~rom the isomerizer effluent in cooling uni~ 44 prior to
passage to ~he ad~orption unit comprising beds 19, 20 and
21~ If desired7 cooling unit 44 can bc by-passed by meanæ
of valve 43, line Sl and valve 45 and ~he en~ire efluent
stream from i90merizer 39 can be recycled through line 11.
When this is done, the sys~em is maintained at a substan~
tially greater operating pre~sure than in ~he above-described
embodiment in order that ~he par~ial pr~sure of the
hydrocarbon componen~ of the ~eed to ~he adsorber beds is
at an adequate level for efficient operation o~ t~he adsorbersO
22 -
.. :
:, : .: ., . ; . .
A pressure of about 700 psla in the adsorber i~ quite suit-
able for this purpo~e, but is not a critical value. In
this "high pressure" mode o~ operation, hydrogen for use
in desorption of the adsorption bed is recycled through
line 50 from cooling unit 27 in which product non-normals
- are removed from the system. Vent means 49 is utilized
to remove from the system a fraction of the hydrogen stream
from cooling unit 49 since thi~ s~ream also contains a
minor portion of light hydrocarbons not removed by the
cooling unit 27, and would otherwi3e undesirably accumulate.
A compensating amount of hydrogen is added to the system
through line 28.
` Example 2.
; A feedstoek having the following composition
was isomerized in accordance with another preferred embodi-
men~ of ~he present process:
Feedstock Mole-% of
A~
isopentane 18.23
-: 20 n-pentane 25.56
cyclopentane 3.21
. 2,2-dimethylbutane .73
2,3-dime~hylbutane l.9S
2-methylpentane 13.41
.~ 3-methylpentane 8.50
~:. n-he~ane 19.56
benzena 1.08`
` cyclohexane .45
:~ methylcyclopentane 7.32
In describing the eyclic process it is ~o be und~erstood
~hat the system has been ope~ating for a ~ime sufficien~ to
~.~
reach a steady state using eæsentially the same feedstock
.
~ - 23 -
9443
described above. With reference to Fig. 2, fresh feedstock
enters the system through line 52 and pas~es through heat
exchanger 53, line 54 9 line 55 into isomerizer. The tempera
ture in the isomerizer is about 500F and is operated at a
pressure of about 220 psia. Normal hydrocarbons desorbed
from the selective adsorption unit and the hydrogen used
in purge desorption thereof are admixed with the fresh
feedstock to the reactor from manifold 57. The catalyst in
~ isomerizer 56 is a zeolite Y- palladium somposition in
: 10 which the zeoli~e has a molar SiO2/A1203 ra~io of 5, a
sodium cation population of about 10 equivalent per cent,
a rare earth cation population of about 43 equivalent per
cent and having the remaining cation sites in the state
:~ resulting from calcination o~ the zeolite at 550C. ~or 2
hours when the said sites were occupied by ammonium cation~.
; The composition contains 0.5 weight per cent finely dlvided
palladium,
The mixed feed enterlng isomerizer 14 has the
following composition:
~;; 20 Feed Mole-% of
Total Feed
~ydrogen 59,88
Methane 11.4
E~hane 2.17
Propane 1.53
i~Bu~ane .71
~ n~Butane .12 .3
-. iso-pen~ane 4.70
n-pentane 8.49
~ 30 cyclopentane .55
; 2~2-dimethylbutanP .11
2,3~dimethylbu~ane035
- 2~m~thylpentane 2.30
''~
'
,
9443
3-mPthylpenkane 1.4b~
n-hexane 4.86
benzene .17
cyclohexane .07
methylcyclopent~ne1.19
The reactor e~luent passes through li.ne 58, heat exchanger
53 and valve 59 to separator 60j~ wherein the bulk o~ the
hydrogen and light hydrocarbons, chief.ly those containing
1 to 4 carbon atoms9 are removed through line 61, and the
remainder of the effluent is therea~ter passed through
valve 62 9 he~ter 63, line 64 containing pump 65 to adsorp-
tion bed 67 through valve 66. Adsorber 67, as do adsorbers
68 and 69 respectively, contai~s a fixed bed of l/16"
diameter pellets of zeolitic molecular ~ie~ve Type SA having
effective pore openings of about 5 Angstrom units. The
opening of valve 66 to pass in ~he compressed heated hydro~
carbon vapor inlti~es the adsorption-~ill step of ~he
process at which point the adsorber contains essentially
hydrogen gas at 700F. and 239 psia. As the feed hydro~
carbon vapors enter the adsorber, hydrogen gas from the bed
void exits the other and of ~he adsorber ~hrough valve 70
lnto manifold conduit 71 and valve 72 to adsorber 68 simul-
taneously undergoing the desorption step of the process.
Valve 73 is open and adsorber 69 is being simultaneously
~'
co~purged. At this poin~ it should be und~rstood ~ha~
.
while adsorber 67 is in the step of adsorp~ion~ill, adsorb-
er 59 is slm~ltaneously in ~he step of co~purge and adsorber
68 i8 in the step o countercurren~ purg~ desorp~ioll and
th~t each o the adsorbers sequentlally und~rgoe~ the steps
,
~ 25 ~ .
` , . . . . .... .
9443
of adsorption~fill, co-purge and countercurrent purge
desorption, Over that period of the ~Idsorption stroke in
adsorber 67, wherein the one bed vol~e of hydrogen is being
forced from the adsorber, the effluent: gas stream passing
through valve 70 is clirected through manifold 71 and valve
72 into adsorber 68. When the product non-normal paraffins
which force the hydrogen from ~he adsorber 67 reach the
e~flu~n~ end of the said adsorber 679 valve 70 is closed
and the product non-normals are passed through valv~e 74
into collection m~niold 75 and removed ~rom the system
after the major portion of its entrained hydrogen has been
removed in cooling unit 76. The adsorption stroke in
adsorber 67 with the recovery of product non~normals is
discontinued prior to the time the normal paraffin content
- of void space feedstoek in the adsorber exceeds the unused
; sorptive capacity of the bed for the feedstock normal hydro-
carbons. In all ~vents the adsorption stroke is terminated
prior to breakthrough of the normals mass transfer zone.
Assuming a time period of twel~e equal unit~ for
a complete cycle o~ adsorptlon~ cocurrent purge and desorp-
tion for each of adsorber beds 67, 68 and 69, the follGwing
chart shows the ~ime relationship of the three s~eps under~
gone by ea h of the ads~rbers in this ex~mple:
Time Units
Adsorber I l 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 1 10l 11l 12
67 Adsorp~ion IcoPurgel Desorp
__ _~ ~__
68 ~ ~ e D~
69 CoPur~el D~sor~ion Adso ption
- ~6
,
~ ..
. .
9443
Accordingly; over the period :Erom the beginning of the ad-
sorption stroke in bed 67 up to the closing of valve 70,
the hyclrogen eEflucnt Erom adsorber 67 is employed in purge
dPsorbing adsorber 68. To complete desorption of adsorber
68, coincident with the termination o the adsorption stroke
in adsorber 67, hydrogen purge gas comprising recycle hydro-
gen from cooling unit 60 via line 61 and as needecl make-up
hydrogen entering the system through line 77 is fed through
line 78, pump 799 furnace 80, line 81 to manifold 71 and
thence through valve 72. Coincident with the beginning of
the adsorption stroke in adsorber 67, the desorption of
adsorber 68 is well in progress and the effluent consists
of a mlxture of hydrogen purge gas and desorbed normal
hydrocarbons. At the same point in time, adsorber 69 has
just completed an adsorption stroke and has an unused
A, adsorption capacity sufEieient to adsorb all o~ the normal
hydrocarbons present in the feedstock contained in the bed
void space~ At this point the efflu~nt from adsorber 68
~hrough valve 82 i~ directed through maniEold 57 and valve
.' 20 83 where it serves as a cocurrent purge in adsorber 69.
The cocurrent purge of adsorber 69 is continued so long
: as the e~fluent non-normal hydrocarbons passing out of
., :
~he bed t~rough vslve 73 are of ~he desired purity. This
~,j por~ion o~ the product non-normal hydrocarbons is fed
through maniEold 75 and collected ~hro~gh cooler 76. After
, comple~ion of the cocurren~ purge stroke in adsorber 69
valve 73 is closed and a hydrogen purge-desorption stream
'' '
~ 27 -
.
:, ; . .
: . - . ,
9443
~g,~
is passed through valve 84 from manifold 71 and the desorbed
normals and hydrogen purge gas are passed through valve 83,
manifold 57 to catalytic isomerizer 56. Because the co-
current purge of ad~sorber 69 was carrLed out using as the
purge gas the mixture of hydrogen and normal hydrocarbons
~rom manifold 57, at the beginning of the purge desorption
stroke in adsorber 69 the bed void space of adsorber 69
contains a greater concantration of normal hydrocarbons
than would be the case if essentially pure hydrogen were
employed for the cocurrent purge step. Moreover, had
essentially pure hydrvgen been used as the cocurrent purge
gas, there would have resulted some unloading of normal
hydrocarbon adsorbate from ~he ingress end of the adsorber.
- Conse~uently, upon the beginning of the countercurrent
desorption stroke in adsorber 6~, the first effluent there-
from, which i~ passed directly to isomerizer 56, would have
been quite lean wi~h respect to hydrocarbons. Since
fluctuatlons in the hydrocarbon concentration of the feed to
the isomerizer 56 are und~sirable, it is advan~ageous to
avoid this type of operation. The present process does
;~ greatly ~minimize these undersirable fluctuation~ by the
par~icular cocurrent purge step described above in which
~' hydrocarbon adsorba~e is not unloaded from the ingre~s end
of the adsorber during the cocurrent purge. Thus the
initial effluent from adsorber 69 during the purge desorp-
tion stroke i3 not ~nduly lean with respec~ ~o hydrocarbons,
.
_ 28 -
'~"
: . . , : -, . . . ~
94~3
,~ f~
In the operation of the above-described process
embodiment, it is not essential that the hydrogen be
removed from the isomerizer effluent in cooling unit 60
prior to passage to the adsorption un.it comprising bed~ 67
68 and 6~. If desired, coollng ~mit 60 can be by~passed
by means of valve 59, line 85 and valve 62 and ~he entire
effluent stream from isomerizer 56 can be recycled through
heater 63, line 64 and blower 65. When this is done, the
sys~em is malntained at a substantially greater operating
pressure than in the above-described embodiment in order
~hat the partial pressure of the hydrocarbon components
of the ~eed to the adsorber bed~ is at an adequate level
for efficient operation of the adsorbers. A pressure of
abou~ 700 psia in the adsorber i~ quite suitable for this
; purpose~ but is not a critical value. In this "high
pressure" mode of operat~on, hydrogen for use in desorp~ion
of the adsorption bed is recycled through line 86 from
:: cooling unit 76 ln which product non-normals are removed
from the systQm. Vent mean~ 87 is utili~ed to remove from
the system a fraction of the hydrogen stream from cooling
: unit 76 since this ~tream a~soloontains a minor portion of
light hydrocarbons no~ r~moved by the cooling unit 76, and
would otherwise unde3irably acc~mulate. A compensating
amount of hydrogen is added to ~he sys~em through line 77.
: ' .
, .
~ - 29 -
.
: , .
