BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to the hydrogenation of benzene
to produce cyclohexane. More particularly the invention relates to a process wherein
the hydrogenation of the benzene and separation of the product by distillation is
carried out simultaneously in a distillation column reactor.
Cyclohexane is the main precursor for the production of
nylon products and as such the demand remains strong. Cyclohexane was first obtained
by the direct fractional distillation of suitable crude petroleum refinery streams.
Now the major portion of cyclohexane is obtained from the direct hydrogenation of
benzene. Conventionally the reaction is carried out in vapor or mixed phase using
a fixed bed reaction. The reactor temperature is controlled to be between 350 to
500°F. Higher temperatures can lead to thermodynamic limitations on benzene
conversion, thermal cracking and increased by-product.
Peterson in U.S. Pat. No. 2,373,501 discloses a liquid
phase process for the hydrogenation of benzene to cyclohexane wherein a temperature
differential is maintained between the top of the catalyst bed where benzene is
fed and the outlet where substantially pure cyclohexane is withdrawn. The temperature
differential is due to the change in the exothermic heat of reaction released as
less and less benzene is converted as the concentration of benzene decreases. Specifically
the top of the catalyst bed is at a higher temperature than the lower catalyst bed.
Hydrogen is supplied counter current to the benzene/cyclohexane flow. Temperature
control coils are disposed within the reactor to maintain the temperature differential
if the exothermic heat of reaction is not sufficient or to cool the bed if too much
heat is released. Peterson recognizes that although the bulk of his reaction takes
place in the liquid phase a portion of the benzene and cyclohexane will be vaporized,
especially near the top of the reactor where the benzene concentration is highest
and conversion is highest. A reflux condenser is provided to condense the condensible
material and return it to the reactor. Thus, a substantial portion of the heat of
reaction is removed by condensation of the reactants vaporized throughout the reaction.
Peterson maintains a liquid level above the topmost catalyst bed but allows room
for vapors to escape to the condenser where the heat of reaction is removed.
Larkin, et al. in U.S. Pat. No. 5,189,233 disclose another
liquid phase process for the hydrogenation of benzene to cyclohexane. However, Larkin,
et al utilize high pressure (2500 psig) to maintain the reactants in the liquid
state. In addition Larkin, et al disclose the use of progressively more active catalyst
as the concentration of benzene decreases to control the temperature and unwanted
Hui, et al. in U.S. Pat. No. 4,731,496 disclose a gas phase
process for the hydrogenation of benzene to cyclohexane over a specific catalyst.
The catalyst reported therein is nickel supported on a mixture of titanium dioxide
and zirconium dioxide.
The hydrogenation of benzene is also useful to remove that
aromatic compound from gasoline streams. One example of this process is disclosed
by Hsieh, et al in U.S. Pat. No. 5,210,348 wherein hydrogenation of the benzene
fraction is used alone or in combination with alkylation. The hydrogenation of the
benzene is disclosed to be in a standard single pass fixed bed reactor. In some
schemes for the reduction of aromatic compounds in gasoline the ASTM D-86 90% point
is specified such that the aromatic and unsaturated cyclic and polycyclic compounds
are precluded from the gasoline blending pool. This has been termed a T-90 gasoline
stock having a desired ASTM 90% point. The resultant T-90+ bottoms which are largely
unsaturated cyclic and polycyclic compounds must be disposed of and hydrogenating
them to produce lighter more saturated compounds for the gasoline pool is an attractive
A typical problem with the hydrogenation of benzene to
cyclohexane is the competing reactions. Particularly isomerization to methyl cyclopentane
is unwanted. Additionally at higher temperatures cracking of the ring occurs producing
undesirable C5 and lighter products. U.S. Pat. No. 5,773,670 discloses
a process wherein unsaturated cyclic and polycyclic compounds (particularly benzene)
are hydrogenated. In the process disclosed therein the hydrogen and unsaturated
cyclic and polycyclic compounds are fed together as one stream below the catalyst
bed in the distillation column reactor. In addition, to achieve complete conversion
of the benzene to cyclohexane a polishing reactor was necessitated.
U.S Pat. No. 5.856,602 discloses the hydrogenation of a
selected aromatic compound contained in a naphtha stream by feeding the naphtha
stream and hydrogen to a distillation column reactor below the bed containing the
It has been found that a downflow catalytic distillation
reactor, that is, one in which the benzene containing stream is fed above the catalyst
zone provides very high conversion. The major by product in conventional reactions
is methyl cyclopentane, which is not present in the present reaction.
SUMMARY OF THE INVENTION
The present invention comprises feeding benzene to a distillation
column reactor at a point above the catalyst bed and a hydrogen stream at an effectuating
hydrogen partial pressure of at least about 0.69 kPa (0.1 psia) to less than 1380
kPa (200 psia) preferably less than 1172 kPa (170 psia) in the range of 517-1034
kPa (75 to 150 psia) to the distillation column reactor at a point below the bed
containing a hydrogenation catalyst which is a component of a distillation structure
and hydrogenating substantially all of the benzene.
The hydrogen rate must be adjusted such that it is sufficient
to support the hydrogenation reaction and replace hydrogen lost from the catalyst
but kept below that which results in flooding of the column which is understood
to be the "effectuating amount of hydrogen " as that term is used herein. Generally
the mole ratio of hydrogen to benzene in the feed to the fixed bed of the present
invention will be about 3:1 to 15:1. preferably up to about 10:1.
The term "reactive distillation" is used to describe the
concurrent reaction and fractionation in a column. For the purposes of the present
invention, the term "catalytic distillation" includes reactive distillation and
any other process of concurrent reaction and fractional distillation in a column
regardless of the designation applied thereto.
BRIEF DESCRIPTION OF THE DRAWING
The figure is a flow diagram of one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
To provide the desired degree of temperature and residence
time control a process and apparatus is provided wherein the reaction liquid is
boiling within a distillation column reactor. Overheads are withdrawn and condensed
with some of the condensate being returned to the distillation column reactor as
reflux. The advantage of the present process is that due to the continual reflux
a portion of the benzene is always condensing on the catalyst structure.
The present hydrogenation may be carried out to produce
substantially pure cyclohexane from benzene.
The hydrogenation described herein is an exothermic reaction.
In the past the temperature has been controlled by quench at strategic points within
a reactor by addition of cool hydrogen. The addition of the hydrogen also acted
to maintain a molar excess of hydrogen within the reactor to prevent coking and
other undesirable side reactions. It is believed that in the present reaction catalytic
distillation is a benefit, because all the components are boiling, whereby the temperature
of reaction is controlled by the boiling point of the mixture at the system pressure
and the reaction and distillation are occurring concurrently in the same reaction
distillation column. The heat of reaction simply creates more boil up, but no increase
in temperature at a given pressure.
The present invention carries out the method in a catalyst
packed column which can be appreciated to contain a vapor phase and some liquid
phase as in any distillation. The distillation column reactor is operated at a pressure
such that the reaction mixture is boiling in the bed of catalyst. The present process
for hydrogenating benzene operates at overhead pressure of said distillation column
reactor in the range between 0 and 2413 kPa, preferably below 1380 kPa such as 517
to 1380 kPa above atmospheric pressure (0 and 350 psig, preferably 200 or less,
such as 75 to 200 psig) and temperatures in said distillation reaction bottoms zone
in the range of 38 to 260°C (100 to 500°F) preferably 138 to 193°C
(280 to 380°F). The feed weight hourly space velocity (WHSV), which is herein
understood to mean the unit weight of feed per hour entering the reaction distillation
column per unit weight of catalyst in the catalytic distillation structures, may
vary over a very wide range within the other condition perimeters. e.g. 0.1 to 35.
In the current process the temperature is controlled by
operating the reactor at a given pressure to allow partial vaporization of the reaction
mixture. The exothermic heat of reaction is thus dissipated by the latent heat of
vaporization of the mixture. The vaporized portion is taken as overheads and the
condensible material condensed and returned to the column as reflux.
Without limiting the scope of the invention it is proposed
that the mechanism that produces the effectiveness of the present process is the
condensation of a portion of the vapors in the reaction system, which occludes sufficient
hydrogen in the condensed liquid to obtain the requisite intimate contact between
the hydrogen and the benzene in the presence of the catalyst to result in their
hydrogenation. Additionally, the vaporization of the liquid feed removes a substantial
amount of the exothermic heat of reaction. Since the liquid is at the boiling point
in the reactor, the temperature may be controlled by the pressure. An increase in
pressure increases the temperature and a decrease in pressure decreases the temperature.
The downward flowing liquid causes additional condensation
within the reactor as is normal in any distillation. The contact of the condensing
liquid within the column provides excellent mass transfer for dissolving the hydrogen
within the reaction liquid and concurrent transfer of the reaction mixture to the
catalytic sites. It is thought that this condensing mode of operation results in
the excellent conversion and selectivity of the instant process and allows operation
at the lower hydrogen partial pressures and reactor temperatures noted. A further
benefit that this reaction may gain from catalytic distillation is the washing effect
that the internal reflux provides to the catalyst thereby reducing polymer build
up and coking. Internal reflux may vary over the range of 0.2 to 20 L/D (wt. liquid
just below the catalyst bed/wt. distillate) give excellent results.
A preferred embodiment is for the production of cyclohexane
from the hydrogenation of benzene. When cyclohexane is the product, the benzene
containing feed is characterized as preferably containing at least 5 wt% benzene
up to 100 wt%. Other components are typically C5, C6 and C7
hydrocarbons. Since other unsaturated compounds may be hydrogenated, the presence
of these compounds are detrimental to the process when cyclohexane is the desire
product. Preferably other unsaturated compounds should be limited to less than 30%
of the feed. Cyclohexane is the preferred diluent, since it is the desired product.
However, other inerts such as other alkanes are acceptable, such as C5's
up to C9's.
The present process is also quite well suited for removing
the benzene from reformate streams, so that they may be used as gasoline blending
stock, by selective hydrogenation. The operation of the distillation column reactor
to maintain a desired aromatic fraction in the reaction action zone is described
in U.S. Pat. No. 5,856,602,
As described the catalytic material employed in the hydrogenation
process is in a form to serve as distillation packing. Broadly stated, the catalytic
material is a component of a distillation system functioning as both a catalyst
and distillation packing, i.e., a packing for a distillation column having both
a distillation function and a catalytic function.
The reaction system can be described as heterogenous since
the catalyst remains a distinct entity. Any suitable hydrogenation catalyst may
be used, for example Group VIII metals of the Periodic Table of Elements as the
principal catalytic component, alone or with promoters and modifiers such as palladium/gold,
palladium/silver, cobalt/zirconium, nickel preferably deposited on a support such
as alumina, fire brick, pumice, carbon, silica, or resin.
Among the metals known to catalyze the hydrogenation reaction
are platinum, rhenium, cobalt, molybdenum, nickel, tungsten and palladium. Generally,
commercial forms of catalyst use supported oxides of these metals. The oxide is
reduced to the active form either prior to use with a reducing agent or during use
by the hydrogen in the feed. These metals also catalyze other reactions, most notably
dehydrogenation at elevated temperatures. Additionally they can promote the reaction
of olefinic compounds with themselves or other olefins to produce dimers or oligomers
as residence time is increased.
The catalyst may be prepared in the form of a catalytic
distillation structure. More particularly the hydrogenation catalyst is generally
a metal supported on an alumina carrier in the form of extrudates or spheres. The
extrudates or spheres arc placed in porous containers and suitably supported in
the distillation column reactor to allow vapor flow through the bed, yet provide
a sufficient surface area for catalytic contact.
The catalyst component may take several forms. In the case
of paniculate catalytic material, generally from 60 mm to about 1 mm down through
powders, is enclosed in a porous container such as screen wire, or polymeric mesh.
The material used to make the container must be inert to the reactants and conditions
in the reaction system. The screen wire may be for example aluminum, steel, or stainless
steel. The polymer mesh may be nylon, Teflon, or the like. The mesh or threads per
inch of the material used to make the container is such that the catalyst is retained
therein and will not pass through the openings in the material. Although the catalyst
particles of about 0.15 mm size or powders may be used and particles up to about
6 mm (1/4 inch) diameter may be employed in the containers.
A preferred catalyst structure for the hydrogenation of
benzene comprises at least one plurality of flexible, semi-rigid open mesh tubular
elements filled with a particulate catalytic material (catalyst component) and sealed
at both ends, intimately associated with and supported by a wire mesh screen coiled
into a spiral having a longitudinal axis, said tubular element being arrayed at
an angle to the longitudinal axis thereby forming a bale and is described in detail
in U.S. Pat. 5.431,890. The flexible, semi-rigid open mesh tubular element filed
with a paniculate catalytic material preferably has a fastener every 1-12 inches
along the length of the tube to form a multiple link shaped catalytic distillation
structure. The links formed by the fasteners may be evenly or irregularly spaced.
Bale shaped catalytic distillation structures are formed
by placing at least one tubular element.on top of the wire mesh screen, such as
demister wire, in a diagonal array, such that when the wire mesh screen is rolled
up, the rolled structure provides a new and improved catalytic distillation structure.
Further embodiments include multiple stack arrangements of alternating wire screen
mesh and tubular elements that are rolled into a new bale shaped catalytic distillation
structure. The tubular elements on alternating layers are preferably arrayed on
the wire mesh screen in opposite directions such that their paths cross. Each tubular
element will define a spiral within the bale.
A most preferred catalyst structure for the hydrogenation
is described in U.S. Pat. No. 5,730,843. Disclosed therein is a contact structure,
useful as a distillation structure which has a rigid frame made of two substantially
vertical duplicate grids spaced apart and held rigid by a plurality of substantially
horizontal rigid members and a plurality of substantially horizontal wire mesh tubes
mounted to the grids to form a plurality of fluid pathways among the tubes. At least
a portion of the wire mesh tubes contain a particulate catalytic material. The catalyst
within the tubes provides a reaction zone where catalytic reactions may occur and
the wire mesh provides mass transfer surfaces to effect a fractional distillation.
The spacing elements provide for a variation of the catalyst density and loading
and structural integrity.
Referring now to the figure there is shown a flow diagram
showing the hydrogenation of benzene. Benzene is fed via line 1 at a point above
the catalyst bed 12 and hydrogen via line 2 at a point below the catalyst bed 12
contained in distillation column reactor 10. If desired the benzene feed may be
diluted with cyclohexane. Heat necessary for start up and to balance the process
is provided by circulating the bottoms stream 4 through reboiler 50 and return line
5. The benzene flows downward into the bed where a portion reacts with the rising
hydrogen to form a reaction mixture containing the reaction product cyclohexane,
unreacted benzene and unreacted hydrogen. The exothermic heat of reaction causes
more boil up of the reaction mixture with the vaporized portion leaving the column
as overheads via flow line 7. Unreacted hydrogen also exits with the overheads.
The gaseous overheads containing unreacted benzene, cyclohexane, lighter compounds
and hydrogen are passed through condenser 30 where substantially all of the benzene
and cyclohexane are condensed. The overheads stream is then passed to receiver/separator
40 where the gas which is mostly hydrogen is separated and the liquid collected.
The gas is removed via line 9 for recycle or use later in the process.
All of the condensed liquid is returned to the distillation
column as reflux via flow line 6 where it provides additional cooling and condensing
within the column. The bottoms, containing a small amount of benzene and cyclohexane,
are removed via flow line 4 with a portion being recirculated through reboiler 50
and flow line 5. There is no overheads liquids product taken. All of the product
is taken as bottoms via flow line 8.
The present process allows for the use of much lower hydrogen
partial pressures and somewhat lower temperatures than earlier processes to obtain
the same results of about 100% benzene conversion with 100% selectivity to cyclohexane.
A three inch diameter distillation column reactor was used.
The rigid catalyst structure was filled with 3.07 kg (6.77 pounds) of Crosfield
HTC-400 catalyst (12% nickel on Alumina) as described above and placed in the middle
of 2.7 meters (9 feet) of the reactor in the packing as described in U.S. Patent
5730843. The bottom 15 meters (50 feet) were filled with inert distillation packing.
Typical conditions are set out in Table 1 below:
Time on stream (hrs)
Pressure above atmospheric
1380kPa (200 psig)
Reaction Temp Top Cat. Bed
Bottom Cat bed
Internal reflux Rate (L/F)
Feed rate (liquid) per hour
1.4 kg (3.1 lb
2.7 kg(6.0 lb
3.7 kg (8.1 lb)
H2 Rate, gas
71 l/s (151 scfh)
71 l/s (151 scfh)
71 l/s (151 scfh)
H2/Bz mole ratio
Benzene in feed, wt%
521 kPa (75.5 psia)
535 kPa (77.6 psia)
Benzene in btms, wt
Bottoms analysis wt% cyclohexane