Reduction of nitrile contaminants by selective hydrogenation

A process for selectively reducing nitrile contaminants in fluids such as water, methanol or hydrocarbon streams containing mono olefins and which contain minor amounts of contaminants comprising nitriles in the presence of hydrogen and a supported cobalt catalyst. In the olefin stream the nitrile contaminants are substantially reduced without substantial reduction of the mono olefins.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the selective hydrogenation of 
contaminants contained in a light refinery stream. 
2. Related Information 
Dienes and nitriles are known catalyst poisons in processes using acid 
catalysts. Many important processes in the petroleum industry require acid 
catalysts. The production of gasoline octane enhancers such as methyl 
tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME) or 
catalyzed alkylation processes require acid catalysts. 
Mixed refinery streams often contain a broad spectrum of olefinic 
compounds. This is especially true of products from either catalytic 
cracking or thermal cracking processes. These olefinic compounds comprise 
ethylene, acetylene, propylene, propadiene, methylacetylene, butenes, 
butadiene, etc. Many of these compounds are valuable, especially as feed 
stocks for chemical products. Ethylene, especially is recovered. 
Additionally, propylene and the butenes are valuable. However, the olefins 
having more than one double bond and the acetylenic compounds (having a 
triple bond) have lesser uses and are detrimental to many of the chemical 
processes in which the single double bond compounds are used, for example 
polymerization. 
Refinery streams are usually separated by fractional distillation, and 
because they often contain compounds that are very close in boiling 
points, such separations are not precise. A C.sub.5 stream, for instance, 
may contain C.sub.4 's and up to C.sub.8 's. These components may be 
saturated (alkanes), unsaturated (mono-olefins), or poly-unsaturated 
(diolefins). Additionally, the components may be any or all of the various 
isomers of the individual compounds. 
Several of the minor components (diolefins) in the feed will react slowly 
with oxygen during storage to produce "gum" and other undesirable 
materials. However, these components also react very rapidly in the TAME 
process to form a yellow, foul smelling gummy material. Thus it is seen to 
be desirable to remove these components whether the "light naphtha" cut is 
to be used only for gasoline blending by itself or as feed to a TAME 
process. 
Diene contaminants can be removed by selective hydrogenation in the 
presence of olefins. The most recommended catalyst being palladium on a 
support, sometimes with promoters. 
Hydrogenation is the reaction of hydrogen with a carbon-carbon multiple 
bond to "saturate" the compound. This reaction has long been known and is 
usually done at superatmospheric pressures and moderate temperatures using 
an excess of hydrogen over a metal catalyst. Among the metals known to 
catalyze the hydrogenation reaction are platinum, rhodium, 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. 
Selective hydrogenation of hydrocarbon compounds has been known for quite 
some time. Peterson, et al "The Selective Hydrogenation of Pyrolysis 
Gasoline" presented to the Petroleum Division of the American Chemical 
Society in September of 1962, discusses the selective hydrogenation of 
C.sub.4 and higher diolefins. Boitiaux, et al in "Newest Hydrogenation 
Catalyst", Hydrocarbon Processing, March 1985, presents an overview of 
various uses of hydrogenation catalysts, including selective 
hydrogenation, utilizing a proprietary bimetallic hydrogenation catalyst. 
The known method of removal of nitriles from hydrocarbon feeds involves a 
water wash of the hydrocarbon feed. This requires a number of stages, 
depending on the relative solubility of the nitrile in water versus 
hydrocarbon. These additional stages provide additional production costs. 
It is particularly difficult to remove propionitrile by water wash. The 
additional stages required for water washing increase complexity and 
production costs. Although there is a substantial body of art relating to 
the hydrogenation of nitriles to produce amines or other amino compounds, 
there is no suggestion as to the fate of olefinic compounds during those 
processes. 
U.S. Pat. No. 2,449,036 teaches the hydrogenation of nitriles to primary 
amines using nickel or cobalt catalysts providing the reduction is in the 
presence of a strong aqueous basic solution in ethyl alcohol. 
U.S. Pat. No. 3,565,957 discloses the reaction of nitrilotriacetonitrile 
with hydrogen and a large amount of ammonia in the presence of a catalyst, 
chosen from a group consisting of nickel, cobalt, and rhodium. 
U.S. Pat. No. 4,186,146 discloses the hydrogenation of aromatic nitriles to 
the corresponding aminomethylbenzene derivatives in a solvent system 
containing water, ammonia, and water miscible ether solvents using a 
cobalt or nickel catalyst. 
U.S. Pat. No. 4,235,821 discloses the hydrogenation of aliphatic nitriles 
in a solvent system of water, ammonia, and water miscible ethers using a 
ruthenium catalyst. 
U.S. Pat. No. 4,739,120 teaches the hydrogenation of an organic nitrile 
group to a primary aminomethyl group in the presence of a rhodium 
catalyst, a basic substance, and a two-phase solvent system comprising an 
immiscible organic solvent and water. 
U.S. Pat. No. 5,075,506 describes a method for producing secondary amines 
from fatty nitriles, with ammonia and hydrogen over a cobalt catalyst 
promoted with zirconium. The catalyst may be supported on kieselguhr or 
other support. A second stage using the same catalyst but without ammonia, 
may be used to increase the proportion of secondary amines. 
It is an advantage of the present hydrogenation process to selectively 
hydrogenate contaminants with little if any saturation of the olefins. The 
absence of hydrogenation of olefins is an unexpected benefit, since cobalt 
may used as a hydrogenation catalyst for olefins. A particular feature of 
the present process is that nitriles are hydrogenated to amines, which can 
be removed easily by water wash, when compared to nitriles, due to fact 
that the low molecular wt. amines are very soluble in water. 
SUMMARY OF THE INVENTION 
Briefly, the present invention is the removal of minor amounts of nitrile 
contaminants from a fluid material by treatment with hydrogen in the 
presence of a cobalt catalyst. 
In one embodiment the present invention is a process for the treatment of 
olefin containing hydrocarbon streams comprises feeding a light naphtha 
cut containing mono olefins and minor amounts of contaminants comprising 
nitriles in the presence of hydrogen and a cobalt catalyst to reduce the 
contaminants without substantial reduction of the mono olefins. In a 
further embodiment of the present invention the hydrogenated stream is 
water washed to remove the amine products of the nitrile hydrogenation. 
In another embodiment nitrile contaminants are removed from streams 
comprising methanol, water and particularly methanol admixed with water. 
DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS 
Unexpectedly the nitrile contaminant is selectively and substantially 
removed, while the olefin concentration is substantially unchanged. The 
nitriles as contaminants are usually present in amounts of about 1 to 5000 
ppm which can be substantially reduced or essentially eliminated by the 
present process. 
The catalyst comprise a supported cobalt hydrogenation-dehydrogenation 
catalyst, such as cobalt oxide or cobalt metal which may comprise from 
about 1 to 70% of the catalyst. A zirconium promoter may be present as a 
compound, such as zirconium oxide. The cobalt comprises 30 to 70 wt %, 
preferably about 40 to 60 wt % of the catalyst. The zirconium comprises 
about 1 to 5 wt. %, preferably about 2 to 3 wt. % of the catalyst. The 
supports include alumina, silica, titania, kieselguhr (also called 
diatomaceous earth, diatomite and infusorial) and the like. 
The reaction is preferably carried out at mild temperatures and elevated 
pressures. The process may generally operate at a temperature in the range 
of from about 30.degree. to 200.degree. C., more preferably about 
60.degree. to 80.degree. C. The pressure may range from about 50 to 5000 
psig, preferably from about 200 to 300 psig. The hydrogen is maintained at 
an excess of that utilized in the process. The rate of hydrogen addition 
is such as to maintain an excess of hydrogen in the process. The residence 
time may be expressed as the liquid hourly space velocity (LHSV) which may 
be in the range of 1-12. 
The reaction may be carried out in any suitable reactor, including a fixed 
bed straight pass reactor, positioned horizontally or vertically, with an 
up or down flow. A catalytic distillation reactor may also be used. 
Suitable feeds for the present process are light refinery streams generally 
comprising predominately hydrocarbons having up to 9 carbon atoms, e.g., 
C.sub.3 to C.sub.8 cuts, usually 4 to 8 linear carbon atoms, including 
both alkane and alkenes with the impurities noted above. A preferred feed 
is a C.sub.5 cut. The olefins may comprise from about 5 to 95% of the 
hydrocarbon stream, but generally comprises about 10 to 60%. 
The C.sub.5 's in the feed are contained in a single "light naphtha" cut 
which contains everything from C.sub.5 's through C.sub.8 's and higher. 
This mixture can easily contain 150 to 200 components and thus 
identification and separation of the products are difficult. 
The diene contaminants may comprise several percent of the feed and may 
require prior or subsequent treatment to reduce them to very low levels. 
Dienes may be detrimental to the catalyst and are preferably removed 
before the present process is applied to a feed. For a similar reason 
mercaptans and other sulfur compounds are preferably removed before 
carrying out the present process. However, the nitriles are usually 
present in only a few parts per million, e.g. up to 100 parts per million 
and their removal is almost complete, or at least to levels which 
substantially inhibit the poisoning of resin type catalysts. 
Apparatus and General Process 
The data that are the subject of this investigation were obtained from an 
insulated and thermosttated 2'.times.3/8" i.d. (30 ml. catalyst capacity) 
copper tube plug-flow reactor. Reaction temperature control was maintained 
by a steam jacket held at 610.54 torr vacuum (2.89 psia., 149.46 torr) for 
140.degree. F. reactions, 576.62 torr (3.72 psia., 192.37 torr) for 
150.degree. reactions, and 514.87 torr (4.74 psia., 245.13 torr) for 
160.degree. F. reactions. (Matheson Gas Products Model 3491 Vacuum 
Regulator, Precision Scientific, Inc. Vacuum Pump, Model 31 D-25; 
0.88L/min. pumping speed). Temperatures were attained utilizing a bayonet 
reboiler (Gaumer Co. Model 1P1N5RI, 1", 750 W, 120 VAC Stainless Steel 
Screw Plug Heater) immersed in the steam jacket water well. Thermostatic 
conditions were maintained by variable transformer (Staco Type 3 PN1010, 
120 VAC) control. An Omega Engineering thermocouple thermometer (Model 
DP41-TC- A-S2) was used to monitor temperature from two regions: one 
auxiliary bead-type thermocouple (K-type) strapped onto the steam jacket 
at the reactor exit (steam jacket front) and one grounded (Omega 
Engineering Model KQIN-18; 1/8", K-type thermocouple) immersed into the 
water well at the reactor entrance (steam jacket reflux) and in contact 
with the steam. The reactor tube was operated "down-flow". 
Mixed gas-liquid phase products were maintained in the liquid state by 
collection in a sample cylinder that was filled from the bottom to top 
through a tube inserted at the cylinder top and extending to the bottom of 
the cylinder. Samples were removed into evacuated sample cylinders from 
the bottom of the loop collection cylinder. A back-pressure regulator (Go, 
Inc. Model P/N102765) was placed at the exit of the collection system with 
a cracking pressure of about 150 psig. 
Data were tabulated in the form of gas chromatographic integrations 
(Perkin-Elmer Model 8400 Capillary Column G.C. with a Supelco Separation 
Technologies Petrocol DH150 150 meter capillary column) and standardized 
weight percentages from the appropriate response factors. Temperature 
programming was reemployed as follows: isothermal at 0.degree. C. for 24 
min., followed by a 10.degree. C./min. ramp rate to 250.degree. C. where 
it was held for 24 min. Gas/liquid sample injections onto the GC column 
were made via a l.mu.L. loop sample valve connected directly to the 
injector port. 
Additional items include three metering pumps that were used to control the 
flow of reactants into the reactor tube. Two pumps were used separately 
and also in conjunction to select a wide range of liquid hourly space 
(volume flow) velocities (Milton-Roy LDC Division Model 2369-89; Duplex; 
max. 920 mL./hr. flowrate, Milton-Roy LDC Division Model 396-31 Simplex; 
160 ml./hr. max. flowrate). A separate pump was used to pump different 
reactants, i.e., pure methanol, into the same reactor tube (Eldex 
Laboratories Model A-60-VS fitted with both motor speed and plunger stroke 
controls; 180 ml/hr. max. flow). 
Feed tank pressures were maintained at about 50-75 psig, well below the 
pressure in the reactor tube and connecting 1/8" stainless steel tubing 
and fittings, in order to permit efficient differential pressure pump 
operation. Reactor system pressures were maintained by a back-pressure 
regulator (Mity-Mite Model S-91LW, 25-400 psig). Approximately 200-220 
psig N.sub.2 (gauge: 0-400 psig) was applied at the back-pressure 
regulator (the reactor exit). There was no observable pressure gradient 
over the length of the reactor tube. Additionally, a metering pump (Eldex 
Laboratories, Inc. Model VS-60; 0.05-3.0 mL/hr., controlled both by 
variable motor speed and manual micrometer-controlled piston stroke) was 
employed to add methanol to the TAME feed material for catalyst 
regeneration (see below). Hydrogen flow rate, for the hydrodenitrogenation 
reaction, was controlled by a gas flow controller (Matheson Gas Products 
Flowmeter/Controller Model 8270; 0-500 sccm; calibrated for nitrogen gas). 
Initial reactions were typically carried out under one of two hydrogen 
flow rates, 100 and 200 sccm, and at one of two temperatures, 125.degree. 
F. and 212.degree. F. These reactions were accomplished by heating the 
reactor tube to the reaction temperature, setting the hydrogen flow rate, 
and then pumping hydrocarbon feed into the reactor tube. 
Catalyst 
Each catalyst sample was pretreated by outgassing in a catalyst surface 
area apparatus at 250.degree. C. in vacuo (10.sup.-5 torr) for 18 hours, 
then the catalyst metal was chemically reduced by exposure to hydrogen at 
250.degree. C. for 18 hours (atmospheric pressure; approx. 760 torr). The 
sample was then allowed to cool to the ambient temperature in vacuo 
(approx. 10.sup.-4 to 10.sup.-5 torr). A standard volume was evacuated, 
filled with CO, and the pressure was then recorded. The catalyst was 
heated to 200.degree. C. in vacuo in order to desorb the chemisorbed CO. 
Hydrogen chemisorption was accomplished by exposing the catalyst to 
hydrogen, then heating to 200.degree. C., and then allowing it to cool to 
the ambient temperature in vacuo. The standard volume was evacuated and 
then filled with hydrogen up to atmospheric pressure. The catalyst was 
then exposed to the hydrogen. The sample volume containing the catalyst 
and hydrogen was heated to 100.degree. C. and allowed to remain at this 
temperature for 1 hour. The sample was then allowed to cool to the ambient 
temperature and the pressure was recorded. 
Propionitrile (PN) and acetonitrile (ACN) are the most common contaminants.