Fractionation for a C.sub.6 paraffin isomerization process

A paraffin isomerization zone is described wherein an improvement to the process comprises using a preflash tower in connection with stabilizer and de-isohexanizer towers to efficiently separate the isomerization zone effluent into valuable components having higher octane values and a recycle material which can be reintroduced along with a fresh feed into the isomerization zone reactor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to C.sub.6 paraffin isomerization in which a 
preflash tower is used to initially separate liquid product from the 
isomerization zone. 
2. General Background 
Paraffin isomerization processes are generally used in modern refineries to 
enhance the octane value of paraffinic and cycloparaffinic hydrocarbons 
having from 4 to about 7 carbon atoms. The product from the isomerization 
zone is normally fed to a fractionation section where selected C.sub.5 and 
C.sub.6 paraffins and cycloparaffins which have increased octane are 
concentrated and sent to the refinery gasoline pool. The less desirable or 
low octane liquids are generally recycled to the isomerization zone. A 
small portion of the heaviest materials produced in the isomerization zone 
is generally removed from the process as a slip stream, and light gaseous 
materials are sent to the refinery fuel gas system. 
The typical fractionation scheme for an isomerization zone includes a high 
pressure separation zone which separates the effluent from the 
isomerization zone into light gases and an isomerization zone liquid 
effluent. The latter is recovered and sent to a fractionation section. 
In cases where there is no recycle to the isomerization zone (once through 
operations) the liquid effluent from the separation zone is passed into a 
stabilizer tower which separates fuel gas from liquid isomerization zone 
product which has enhanced octane value and can be used in the gasoline 
pool. 
In many isomerization operations, some of the low octane effluent from the 
isomerization zone is separated from valuable isomerization zone product 
and recycled to the isomerization zone for additional conversion. These 
operations are generally known as recycle operations. The fractionation 
section design in these conventional operations can vary, but in most 
instances when a new isomerization unit is being built, the fractionation 
section will comprise two separate towers. 
In conventional two tower recycle operations in which no preflash tower is 
used (see FIG. 2), the first tower is a stabilizer tower into which all 
the liquid effluent from the separation zone is passed and separated into 
fuel gas comprising C.sub.3 and lighter hydrocarbons and a stabilizer 
tower bottoms stream which contains essentially all of the remaining 
liquid effluent from the separation zone. The liquid feed rate to the 
stabilizer tower in the above operation is much larger than the liquid 
feed rate to this tower when operated in once-through operations, since 
recycle material is contained in feed. 
The bottoms stream from the stabilizer tower is then fed into a 
de-isohexanizer tower for separation into the following three streams: the 
lightest stream is generally the isomerization zone product having 
enhanced octane value which is recovered from the overhead section of the 
de-isohexanizer tower and sent to the refinery gasoline pool; a side 
stream is drawn off comprising recycle material which comprises C.sub.6 
and some C.sub.7 hydrocarbons; and a bottoms stream generally comprising 
the heaviest components produced in the isomerization zone is generally 
purged from the system to avoid buildup of these heavier materials. 
When a refiner designs a grass-root paraffinic isomerization zone for 
once-through operations or recycle operations, he has much flexibility in 
reactor designs along with the sizing of the various towers used in the 
fractionation section of the isomerization zone process. 
In instances in which the refiner desires to alternately operate an 
isomerization zone in either the once-through or recycle modes, various 
compromises must be made in the fractionation section in order to provide 
adequate sizing of both stabilizer and de-isohexanizer towers for these 
two different operations. In the once-through operation, since there is no 
recycle of unreacted material to the isomerization zone reactor the 
stabilizer tower will receive a lower feed rate of liquid effluent when 
compared to the feed rate of liquid effluent it would receive during 
recycle operation. In many instances these compromises will result in less 
than ideal separations or design of tower reboilers or heat exchangers. 
Costs will not be optimized because the stabilizer tower must be designed 
for a wide range of flow rates and operating conditions. 
The present invention offers an improvement to the refiner having a 
paraffin isomerization process designed to operate alternately in either 
the once-through or recycle mode, and which is either being built as a 
grass-roots project or is a revamp using some existing equipment. In the 
present invention a process improvement results from the use of a preflash 
tower during recycle operations to make a gross separation of the 
separation zone liquid effluent into preflash overhead fraction which 
contains substantial quantities of isomerization zone product and a 
preflash bottoms fraction. The latter fraction contains much of the 
unconverted C.sub.5, C.sub.6 and C.sub.7 materials which boil at higher 
temperatures than the isomerization zone product, some isomerization zone 
product and a heavy stream comprising C.sub.6 + materials. This stream is 
passed into the de-isohexanizer tower for further separation into three 
major streams. 
The three streams are a de-isohexanizer tower overhead fraction which 
contains concentrated quantities of isomerization zone product which were 
not separated from the preflash tower bottoms fraction, a middle boiling 
stream comprising C.sub.5 and C.sub.6 recycle materials which are returned 
to the isomerization zone for further conversion, and a heavy stream 
comprising C.sub.6 + materials which are removed from the process to avoid 
buildup within the processing loop. 
The de-isohexanizer zone overhead fraction is combined with the preflash 
overhead fraction and passed into the stabilization tower. These two 
streams having been initially sent to the preflash tower now have a 
reduced quantity of heavier materials resulting in a lower quantity of 
feed material for the stabilizer tower to process. In the stabilization 
tower a separation of fuel gas from isomerization product takes place. 
By using the preflash tower during recycle operations, the feed rates to 
both the stabilization tower and the de-isohexanizer tower are reduced 
since the preflash tower performs an initial liquid-liquid separation on 
the separation zone liquid. The stabilizer tower, can therefore, be 
designed for lower feed rates. 
The reduced size of the stabilizer tower still allows its use in 
once-through operations, since in once-through operations a reduced 
quantity of liquid effluent from the isomerization zone is passed into the 
fractionation section. In once-through operations, the liquid effluent 
from the separation zone passes directly to the stabilizer tower for 
separation into fuel gas and isomerization zone product. 
SUMMARY 
The present invention can be summarized as an improved isomerization 
process in which liquid effluent from an isomerization zone is passed 
through a preflash tower to separate the effluent into an overhead 
fraction comprising dimethylbutane, pentanes, and lighter materials and a 
preflash bottoms fraction comprising dimethylbutane, pentanes, and heavier 
materials prior to these fractions passing into stabilizer and 
de-isohexanizer towers. 
It is the object of the present invention to provide an improved paraffin 
isomerization process wherein the fractionation section of the process 
utilizes a preflash tower to reduce the liquid loads to downstream 
stabilizer and de-isohexanizer towers so they can be designed in a more 
economical fashion. 
It is another object of the present invention to provide an improved 
process utilizing existing equipment when revamping an existing processing 
unit to a paraffin isomerization zone by using a preflash tower in 
situations where either a stabilization tower or a de-isohexanizer tower 
already exist and in which either of these towers is of too small a design 
to handle the liquids which would normally be passed to either of these 
towers in the absence of the use of the improved invention.

Shown in the drawing is stabilizer tower 1, preflash tower 2, and 
de-isohexanizer tower 3. These three fractionation towers are combined in 
a manner to allow the separation of liquid effluent derived from a 
paraffin isomerization zone into fuel gas, isomerization zone product, and 
other streams. 
A liquid isomerization zone fresh feed stock which comprises C.sub.5 
through C.sub.6 paraffins is combined with recycle stream 11 which is 
derived from the de-isohexanizer tower. These two liquid streams, combined 
with recycled hydrogen, pass into a state-of-the-art paraffin 
isomerization zone where the lower octane paraffins are converted to 
higher octance value materials. The effluent leaving the isomerization 
zone reactor is passed into a high pressure separator, or separation zone, 
for removal of light gases from liquid effluent. The resulting liquid 
effluent is then passed through line 4 into the preflash tower. 
The liquid effluent passing through line 4 and into the preflash tower as 
preflashed tower feed generally will be a full range boiling material, 
boiling within a range of from about 80.degree. F. to 170.degree. F. or 
more and will contain large quantities of isopentanes, normal pentanes, 
dimethylbutanes, methylpentanes and C.sub.6 normal and cycloparaffins. 
In the preflash tower a gross separation of C.sub.5 and C.sub.6 paraffins 
occur. The preflash overhead fraction comprises dimethylbutanes, pentanes 
and lighter materials while the preflash bottoms fraction comprises 
dimethylbutanes, pentanes, and heavier materials. 
The preflash tower bottoms fraction passes through line 7 into the 
de-isohexanizer tower 3 where it can be separated into three fractions--a 
de-isohexanizer overhead fraction, a side stream referred to as recycled 
material and a de-isohexanizer bottoms fraction generally comprising the 
heaviest materials contained in the feed to this tower. 
The function of the de-isohexanizer tower is to recover as much as possible 
of the remaining isomerization zone product from the preflash tower 
bottoms fraction. Additionally, this tower recovers a recycle material 
which is combined with the fresh feed and returned to the isomerization 
zone for additional conversion to more valuable, higher octane materials. 
The de-isohexanizer tower produces a heavy stream as a bottoms fraction, 
which can be removed from the process. It comprises heavy materials which 
can build up within the process loop if not removed from the process. 
The de-isohexanizer overhead fraction passes through line 6 and can be 
combined with the preflash tower overhead fraction passing through line 5 
and eventually passed through line 8 into the stabilizer tower 1. Some of 
either the materials passing through lines 5 or 6 may be removed from the 
process for other uses. These streams may also be added as separate 
streams to the stabilizer tower. It is not critical whether these overhead 
streams from the preflash and de-isohexanizer towers are combined or added 
separately to the stabilizer tower. 
In the stabilizer tower a separation occurs on the materials flowing 
through line 8. Fuel gas removed from the overhead of the stabilizer tower 
comprises essentially C.sub.3 and C.sub.4 gaseous hydrocarbons. The 
preflash tower bottoms product comprises isomerization zone product and 
contains a full range of C.sub.5 and C.sub.6 hydrocarbons having enhanced 
octane values. Specifically this material will contain large quantities of 
isopentanes, dimethylbutanes, methylpentanes and other C.sub.6 
hydrocarbons. This material can add enhanced octane values to a normal 
refiner's gasoline pool and is the primary and most valuable product 
recovered from the fractionation zone described above when used in 
connnection with the paraffin isomerization zone process. 
Shown in FIG. 2 is a conventional recycle flow scheme employing stabilizer 
and de-isohexanizer towers in which no preflash tower is used. 
Liquid effluent from the separation zone passes through line 23 into 
stabilizer tower 21 where fuel gas is removed via line 25 and a stabilizer 
bottoms fraction then passes through line 24 into the de-isohexanizer 
tower 22. 
In tower 22, three streams are recovered. An isomerization zone product is 
recovered as overhead stream 26, a recycle stream 27 is recovered and 
returned to the isomerization zone, and a heavy stream is recovered from 
line 28. 
In this case the stabilizer tower needs to be designed to have a larger 
quantity of feed passing into it through line 23, since there is no 
preflash tower to make the gross separation of C.sub.5 and C.sub.6 
materials as in the scheme shown in FIG. 1. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
In a broad embodiment, the present invention comprises: 
A process for isomerizaton of a C.sub.5 to C.sub.6 feedstock comprising 
paraffins in which the feedstock is passed into an isomerization zone at 
reaction conditions to effect production of an effluent, containing an 
isomerization product having an increased octane value, wherein the 
effluent is passed into a separation zone for removal of light gases from 
liquid effluent, the liquid effluent is fractionated into (1) a fuel gas 
stream, (2) isomerization zone product comprising iso C.sub.5 and iso 
C.sub.6 paraffins, (3) a recycle stream comprising C.sub.6 paraffins and 
cycloparaffins having higher boiling points than the isomerization zone 
product, a part of which is passed into the isomerization zone as recycle 
material and (4) a heavy stream comprising C.sub.6 and C.sub.7 and higher 
paraffins having a boiling range higher than recycle material, wherein an 
improvement comprises: (A) passing at least a portion of liquid effluent 
into a preflash tower for separation into a preflash overhead fraction 
comprising dimethylbutane, pentanes and lighter materials and a preflash 
bottoms fraction comprising dimethylbutane, pentanes and heavier 
materials, (B) passing at least a portion of the preflash bottoms fraction 
into a de-isohexanizer tower to separate said bottoms into a (i) 
de-isohexanizer overhead fraction comprising dimethylbutane, pentanes and 
lighter materials carried over into the di-isohexanizer column from the 
preflash tower, (ii) said recycle stream, and (iii) said heavy stream, (C) 
passing at least a portion of the preflash overhead fraction and the 
de-isohexanizer overhead fraction into a stabilizer tower to separate 
these fractions into (i) a fuel gas stream comprising C.sub.4 and lighter 
hydrocarbons and (ii) said isomerization zone product. 
The preflash tower, stabilization tower, and the de-isohexanizer towers as 
contemplated in the present invention can be of any normally acceptable 
design. In particular, these towers are typically fractionation towers 
which can include requisite amounts of trays and packing, if required, to 
enable the separations as described in the claims to take place. 
In order to economically construct and operate the claimed process it is 
desirable that the three towers be designed to have a minimal excess 
capacity. Accordingly, the diameters and tower lengths and tray 
efficiencies of these units should be designed so as to provide maximum 
separation for each of the separation steps required with a minimum use of 
energy in all cases. 
Typically, the fractionation towers will have control schemes including 
reflux and temperature controllers to allow the particular tower to be 
operated in an acceptable manner. The towers will also have sufficient 
feed and effluent exchangers when required, reboiler capacity, cooling 
capacity and reasonably quick response time to allow acceptable and safety 
controlled operations. 
The isomerization zone is typically a paraffin isomerization zone well 
known to those in the art. This zone will generally contain a catalyst 
containing a platinum group metal and/or other metals and often times it 
is promoted by chloride or chloride addition to enhance the acidity of the 
catalyst within the isomerization zone. 
The main purpose of the paraffin isomerization zone is to isomerize the 
C.sub.5 through C.sub.6 paraffins which are fed to this zone increasing 
their octane value. 
Typical paraffin isomerization zone reaction conditions will operate at 
liquid hourly space velocities of anywhere from less than 1 to 4 or more, 
reaction temperatures anywhere from 150.degree. F. up to 550.degree. F. or 
more depending on the type of catalyst used in the zone, reactor pressures 
can vary anywhere from 100 to 1000 psig or more. Some isomerization 
reaction zones operate at lower temperatures to take advantage of 
equilibrium considerations but at the expense of conversion. 
Isomerization zone reaction conditions also can include recycle of C.sub.5 
and C.sub.6 paraffins which have not been converted to more valuable high 
octane materials and recycle and make-up hydrogen which are normally used. 
The boiling ranges given for the various streams are generally the nominal 
initial and end boiling points and are not meant to unduly restrict the 
type of materials characterized by the boiling range. The particular 
materials characterized by boiling range will generally have most or the 
majority of their weight boiling within the stated boiling range. In a 
preferred instance, 75 percent by weight or more of the particular 
material will boil in this stated range. 
In many instances, especially the lighter fractions and the heavier 
fractions, initial boiling point or end boiling point tails of substantial 
temperature ranges can occur. These tails can extend the boiling range by 
as much as 100.degree. F. or more even though the materials causing the 
tail may be present in very small concentrations. This is most readily 
apparent when describing the boiling range of the de-isohexanizier heavy 
stream. 
The isomerization zone feedstock generally will comprise C.sub.5 and 
C.sub.6 paraffins (normal iso and cycloparaffins) and will have a boiling 
range of anywhere from 80.degree. F. to about 170.degree. F. This stream 
generally represents a fresh feed component and a recycle stream and is 
generally passed to the isomerization zone with these two materials 
combined along with make-up hydrogen and recycle gas. 
The isomerization zone liquid recycle material which is a portion or all of 
the recycle stream obtained from the de-isohexanizer tower generally boils 
in the range from about 140.degree. up to about 160.degree. F. and will 
comprise C.sub.6 hydrocarbons such as dimethylbutanes, methylpentanes, 
normal hexanes, methylcyclopentanes and cyclohexanes. A typical recycle 
stream will contain about 60 percent of C.sub.6 paraffins and as much as 
25 percent cyclohexanes with the remaining materials being lighter weight 
components. 
The hydrogen make-up which is also mixed with the feedstock passing into 
the isomerization zone will generally have at least 75 percent by volume 
gaseous hydrogen. The hydrogen recycle stream which is recovered from the 
initial separation of effluent leaving the isomerization zone will 
generally be of lower quality hydrogen. The isomerization zone effluent is 
passed into a separation zone for the first gross removal of high pressure 
gas materials from the remaining liquid. The light gas removed from the 
separation zone will generally comprise C.sub.3 and lighter hydrocarbons 
and hydrogen. 
The remaining liquid effluent that leaves the separation zone is the 
feedstock for the preflash tower. In most refining operations essentially 
all the liquid recovered from the separation zone will be passed into the 
preflash tower unless the refiner desires to add additional materials or 
remove a part of the liquid effluent recovered from the separation zone 
for other purposes. 
The liquid effluent (preflash tower feedstock) generally comprises C.sub.5 
and C.sub.6 paraffins and boils in the general range of from about 
80.degree. F. to about 170.degree. F. This material will contain iso, 
normal and cyclopentanes along with dimethylbutanes, methylpentanes, 
normal hexanes, methylcyclopentanes and cyclohexanes in various 
concentrations. The concentrations of these materials are dictated in part 
by the extent to which equilibrium is reached in the isomerization 
reaction zone. 
A typical liquid effluent passed into the preflash tower as feed will 
contain about 27 percent by weight of iso, normal and cyclopentanes and as 
much as 68 percent by weight of C.sub.6 hydrocarbons. 
In the preflash tower a gross separation takes place between a 
predominantly C.sub.5 and a predominantly C.sub.6 fraction of the feed to 
this tower. 
The preflash overhead fraction will generally boil in the range of from 
about 80.degree. F. to about 120.degree. F. and will contain some C.sub.3 
and some C.sub.4 normal gaseous materials along with iso, normal and 
cyclopentanes along with C.sub.6 hydrocarbons such as dimethylbutane, 
methylpentane, normal hexane, methylcyclopentane and cyclohexane in 
various concentrations. The typical preflash tower overhead fraction will 
contain as much as 40 weight percent of C.sub.5 hydrocarbons, and as much 
as 10 weight percent C.sub.4 and lighter materials. The remaining 
components are C.sub.6 hydrocarbons, such as dimethylbutanes, 
methylpentanes, normal hexane, methylcyclopentane and cyclohexane. 
Since the preflash tower is designed only to provide a gross separation of 
C.sub.5 and C.sub.6 components, the preflash bottoms fraction leaving the 
preflash tower will also contain some quantities of the components 
contained in the preflash overhead fraction along with additional heavier 
materials. This fraction will generally boil in a higher range of from 
about 80.degree. F. to about 170.degree. F. 
A portion or all of the preflash bottoms fraction is passed into the 
de-isohexanizer tower for separation into three streams. In this tower a 
more efficient separation takes place between a di-isohexanizer overhead 
fraction which will contain C.sub.5 and C.sub.6 hydrocarbons which 
represent a valuable component of the isomerization zone product. The 
de-isohexanizer overhead fraction will boil anywhere from about 80.degree. 
F. to about 150.degree. F. 
A side draw from the de-isohexanizer tower is the recycle stream, all or a 
part of which can be recycled to the isomerization zone with feed for 
additional conversion into a valuable isomerization zone product. This 
stream will generally boil in a range of from about 120.degree. F. to 
about 160.degree. F. and will contain essentially all C.sub.6 
hydrocarbons. These materials will generally represent dimethylbutane, 
methylpentane, normal hexane, methylcyclopentane and cyclohexane. 
Depending upon the operation of this tower, the initial and end boiling 
points of this stream can vary. If for instance the refiner desires to 
take more of the recycle stream to the overhead fraction the recycle 
stream will contain higher boiling materials. Conversely, if the refiner 
desires to take more of the recycle stream to the bottom of the 
de-isohexanizer tower, it will be a material having a lighter hydrocarbon 
composition and will not boil in as high as stated temperature range. 
The de-isohexanizer heavy stream is a bottom stream which is removed from 
the de-isohexanizer tower. This material generally will boil in a range of 
anywhere from 140.degree. F. to 170.degree. F. or more and will contain 
C.sub.6 hydrocarbons in addition to C.sub.7 + hydrocarbons. These 
materials will generally comprise methylpentane, normal hexane, 
methylcyclopentane, cyclohexane and any hydrocarbons having 7 carbon atoms 
or more in their chain or ring structure. 
The feed to the stabilizer tower generally comprises the preflash overhead 
fraction combined with the de-isohexanizer tower overhead fraction and in 
most cases will include all these materials. 
The stabilizer tower effects a removal of fuel gas components which 
generally comprise C.sub.3 and C.sub.4 materials from isomerization zone 
product. The fuel gas is removed as an overhead fraction of the stabilizer 
tower and the isomerization zone product is removed as a bottoms fraction 
from the stabilizer tower. The isomerization zone product generally will 
boil in a range of from about 80.degree. F. to about 160.degree. F. and 
comprises C.sub.5 and C.sub.6 hydrocarbons. It will contain such material 
isopentane, normal pentane, cyclopentane, dimethylbutane, methylpentane 
and normal C.sub.6 hydrocarbons. The normal pentanes and hexanes are 
generally in fairly low concentrations in this stream, since the purpose 
of the isomerization zone is to reduce these relatively low octance 
materials by their conversion to higher octane materials. 
A typical isomerization zone product can contain 32 weight percent of 
isopentane and up to 53 percent or more of iso C.sub.6 hydrocarbons with 
the remaining material being heavier or lighter materials. 
When substantially all of a stream is passed to a part of the fractionation 
section, only minor quantities are divereted to uses other than as 
described and claimed herein. In some cases small portions of the various 
streams can themselves be used as products. 
EXAMPLE I 
In this Example a fractionation section for a paraffin isomerization 
reaction zone arranged, as described in FIG. 1, was simulated to 
illustrate one embodiment of the invention. A commercial unit having this 
design is successfully operating. 
The preflash towner had an inside diameter of 11 feet and an overall length 
(tangent to tangent) of approximately 30 feet. It contained slotted ring 
packing and had five theoretical trays. The preflash feed after heat 
exchange entered the tower at a temperature of 176.degree. F., the 
preflash tower overhead fraction, prior to heat exchange, left the tower 
at a temperature of 272.degree. F., and the preflash tower bottoms 
fraction, prior to heat exchange, left the tower at a temperature of 
313.degree. F. This tower was operated at a pressure of 170 psig at the 
overhead fraction exit. 
The de-isohexanizer tower had an inside diameter of 12.5 feed and an 
overall length (tangent to tangent) of 115 feet. It contained sixty trays 
which were of a valve tray design. The feed to this tower, from the 
preflash tower, after heat exchange entered the tower at a temperature of 
217.degree. F., the de-isohexanizer tower overhead left the tower, prior 
to heat exchange, at a temperature of 195.degree. F., the recycle stream 
left the tower, prior to heat exchange, at a temperature of 240.degree. 
F., and the heavy stream left this tower prior to heat exchange, at a 
temperature of 258.degree. F. This tower was operated at a pressure of 35 
psig at the overhead fraction exit. 
The stabilizer tower had an inside diameter of seven feet and an overall 
length (tangent to tangent) of 70.25 feet. It contained 24 trays which 
were of a valve tray design. The feed to this tower, after heat exchange, 
comprised the preflash and de-isohexanizer tower overhead fractions and 
was at a temperature of 270.degree. F., the fuel gas stream leaving the 
stabilizer, before heat exchange, was at a temperature of 168.degree. F., 
and the isomerization zone product leaving the stabilizer tower as a 
bottoms fraction, before heat exchange, was at a temperature of 
284.degree. F. This tower operated at an overhead pressure of 140 psig. 
Table I below shows the various components passing through the 
fractionation section. 
TABLE I 
__________________________________________________________________________ 
Stream No. 
4 5 6 7 9 10 11 12 
Description 
Liquid 
PFT DIHT PFT Fuel 
IZ DIHT DIHT 
wt. % Effluent 
Overhead 
Overhead 
Bottoms 
Gas Product 
Recycle Stream 
Heavy 
__________________________________________________________________________ 
Stream 
C.sub.4 - 4.6 14.9 0.8 0.6 93.7 1.4 -- -- 
i C.sub.5 20.1 30.8 30.2 15.9 5.9 32.0 
-- -- 
n C.sub.5 5.7 7.3 9.6 5.0 0.4 9.1 -- -- 
Cyclo C.sub.5 
1.3 1.2 2.3 1.3 -- 1.9 0.3 -- 
2,2 Dimethyl butane 
24.8 19.9 37.6 26.7 -- 31.8 
15.6 2.0 
2,3 Dimethyl butane 
6.4 4.2 5.1 7.3 -- 5.0 10.2 4.0 
2 Methyl pentane 
17.4 11.1 10.8 20.0 -- 11.6 
31.2 15.1 
3 Methyl pentane 
9.7 5.7 3.3 11.3 -- 4.6 20.5 15.7 
n C.sub.6 5.8 3.1 0.4 7.0 -- 1.7 13.8 20.4 
Methyl cyclo pentane 
1.9 0.9 -- 2.3 -- 0.5 4.4 10.2 
Cyclo C.sub.6 
1.9 0.7 -- 2.4 -- 0.3 3.5 25.8 
Benzene + 0.4 0.1 -- 0.4 -- -- 0.5 6.7 
Relative mass 
44.8 12.8 16.8 32.0 1.8 27.8 
14.2 1.0 
flow rate 
volumetic 40,873 
12,268 
15,318 
28,605 
-- 25,415 
12,437 838 
flow rate, B/D 
__________________________________________________________________________ 
Where: 
PFT = preflash tower 
DIHT = deisohexanizer tower 
IZ = isomerization zone 
The stabilizer tower in this Example had a total feed rate of 27,586 
barrels/day of liquid (12,268 barrels/day of preflash tower overhead plus 
15,318 barrels/day of de-isohexanizer tower overhead). The flow rate of 
isomerization zone tower product recovered from this tower was 25,415 
barrels/day. The total liquid feed rate to the preflash tower in this 
Example was 40,873 barrels/day. 
EXAMPLE II 
In this Example a fractionation section for a paraffin isomerization zone 
was simulated in a once-through operation. 
Liquid effluent from the separation zone was passed directly into the same 
stabilizer tower described in Example I which separated a fuel gas 
overhead from a bottoms fraction which represented isomerization zone 
product. 
The de-isohexanizer tower was not used since no recycle to the 
isomerization zone was used. 
A commercial unit was operated having this design. 
Table II below shows the various components passing through the 
fractionation section. 
TABLE II 
______________________________________ 
Stream 
Description Stabilizer Fuel Isomerization 
wt. % Feed Gas Zone Product 
______________________________________ 
C.sub.4 - 5.2 96.6 1.4 
i C.sub.5 29.6 3.2 30.8 
n C.sub.5 7.9 0.2 8.2 
Cyclo C.sub.5 1.8 -- 1.9 
2,2 Dimethyl 20.0 -- 20.8 
butane 
2,3 Dimethyl 5.2 -- 5.4 
butane 
2 Methyl pentane 
14.0 -- 14.6 
3 Methyl pentane 
7.8 -- 8.1 
n C.sub.6 4.7 -- 4.9 
Methyl cyclo 1.7 -- 1.7 
pentane 
Cyclo C.sub.6 1.7 -- 1.7 
Benzene + 0.4 -- 0.5 
Relative Mass 24.1 1.0 23.1 
flow ratio 
Volumetic flow rate 
27,996 26,433 
B/D 
______________________________________ 
The stabilizer tower in this Example had a feed rate of 27,996 barrels/day 
of liquid. About 26,433 barrels/day of isomerization zone product was 
recovered from the stabilizer tower. 
EXAMPLE III 
In this Example a conventional fractionation section was simulated for 
recycle operation. This operation did not use a preflash tower and 
illustrates operations which might be used in situations where a refiner 
is designing a fractionation section as a grass-roots project. 
The flow scheme for this Example is shown in FIG. 2. Table III shows the 
stream compositions simulated for this Example. The liquid effluent fed to 
the stabilizer tower in this Example was assumed to have the same 
composition as the liquid effluent in Example I and the same volumetric 
flow rates were also used. 
TABLE III 
______________________________________ 
Stream No. 
27 28 
Descrip- 23 25 26 DIHT DIHT 
tion Liquid Fuel IZ Recycle 
Heavy 
wt. % Effluent Gas Product 
Stream Stream 
______________________________________ 
C.sub.4 - 4.6 93.7 1.4 -- -- 
i C.sub.5 20.1 5.9 32.0 -- -- 
n C.sub.5 5.7 0.4 9.1 -- -- 
Cyclo C.sub.5 
1.3 -- 1.9 0.3 -- 
2,2 Dimethyl 
24.8 -- 31.8 15.6 2.0 
butane 
2,3 Dimethyl 
6.4 -- 5.0 10.2 4.0 
butane 
2 Methyl 17.4 -- 11.6 31.2 15.1 
penta 
3 Methyl 9.7 -- 4.6 20.5 15.7 
pentane 
n C.sub.6 5.8 -- 1.7 13.8 20.4 
Methyl cyclo 
1.9 -- 0.5 4.4 10.2 
pentane 
Cyclo C.sub.6 
1.9 -- 0.3 3.5 25.8 
Benzene + 0.4 -- -- 0.5 6.7 
Relative mass 
44.8 1.8 27.8 14.2 1.0 
flow rate 
Volumetric 
40,873 -- 25,415 12,437 838 
flow rate 
B/D 
______________________________________ 
Where: 
PFT = preflash tower 
DIHT = deisohexanizer tower 
IZ = isomerization zone 
As can be seen from Examples I, and II use of a preflash tower allows the 
same stabilizer tower to be used for both once-through (Example II) and 
recycle operations (Example I). In the case of once-through operation, 
27,996 barrels/day of liquid feed is passed to the stabilizer tower and in 
the case of recycle operations, 27,586 barrels/day of liquid feed is 
passed to the stabilizer tower. 
If recycle operations were to be performed without the use of a preflash 
tower (as is shown in Example III) a larger stabilizer tower would be 
needed since its liquid feed rate would be about 40,873 barrels/day. 
Designing a single stabilizer tower to handle these two disparate flow 
rates would require compromises in tower performance for each flow rate.