Process for the production of mono-olefins

A process for the production of mono-olefins from a paraffin-containing hydrocarbon feed having at least two carbon atoms which comprises a first step of partially combusting a mixture of the hydrocarbon feed and a molecular oxygen-containing gas in contact with a catalyst capable of supporting combustion beyond the normal fuel rich limit of flammability, the first step being carried out under a total pressure of greater than 5 bar absolute and at a temperature of greater than 650.degree. C.; and a second step of cooling the mono-olefinic products to 600.degree. C. or less within less than 50 milliseconds of formation.

The present invention relates to a process for the production of 
mono-olefins from a hydrocarbon feed. 
A known commercial route to the production of olefins is via steam cracking 
of paraffinic hydrocarbons. Steam cracking involves the pyrolysis of the 
hydrocarbons and in general, the conditions which favour maximum 
conversion and maximum olefin production are (1) a highly saturated feed, 
(2) a high furnace outlet temperature and (3) low hydrocarbon partial 
pressure. In particular, the process must be carried out under low 
hydrocarbon partial pressure, typically less than one atmosphere. 
Possibly, the simplest reaction in the aforementioned process is the 
cracking of ethane: 
EQU C.sub.2 H.sub.6 .fwdarw.C.sub.2 H.sub.4 +H.sub.2 
Considering this reaction, it is of course apparent that the number of 
moles of product will exceed the number of moles of reactant. Thus, if the 
partial pressure of the paraffinic hydrocarbon is increased, the reaction 
is likely to favour hydrogenation over the cracking reaction. Conventional 
understanding thus indicates that if elevated pressure is used in this 
process, conversion and selectivity to olefins will be low. 
Indeed, this teaching is disclosed in various published papers and 
textbooks including "Mono-olefins--Chemistry and Technology" by F Asinger, 
Pergammon Press, 1968, pp 62-63, 91, 121 and 125; Chem Systems Report No 
83-6, September 1984, and Chem Systems Report No 89S8, March 1991. The 
aforementioned disclosures indicate that low hydrocarbon partial pressure 
is essential to suppress secondary reactions of the olefinic products thus 
maximising the yield of olefins. 
Olefins can also be prepared by cracking a paraffinic feed wherein the heat 
required for pyrolysis is provided by the partial combustion of the 
feedstock and not by the conventional tubular fired heaters. This process 
for the production of olefins is described in our published European 
patent application No 0332289. The process can be described as 
"autothermal cracking" of paraffins and will be referred to as such 
hereafter. 
The autothermal cracking process provides the advantage over conventional 
steam cracking in that the reactor is simpler, there is less soot 
formation and the once through yield of olefins can be improved. As found 
in steam cracking, maximum yield of olefins is obtained if the process is 
carried out under low pressure, typically 1 atmosphere or less. The use of 
elevated pressure in the autothermal cracking process results in products 
which are richer in methane and carbon monoxide. 
Surprisingly, we have now found that high olefin yields can be obtained in 
the autothermal cracking of hydrocarbons at elevated pressure provided the 
products are rapidly cooled. 
Accordingly, the present invention is a process for the production of 
mono-olefins from a paraffin-containing hydrocarbon feed having at least 
two carbon atoms, the process comprising 
(A) a first step of partially combusting a mixture of the hydrocarbon feed 
and a molecular oxygen-containing gas in contact with a catalyst capable 
of supporting combustion beyond the normal fuel rich limit of 
flammability, said first step carried out under a total pressure of 
greater than 5 bar absolute and at a temperature of greater than 
650.degree. C., and 
(B) a second step of cooling the mono-olefinic products to 600.degree. C. 
or less within less than 50 milliseconds of formation. 
The conversion of the hydrocarbons to mono-olefins can be successfully 
achieved with high yields by carrying out the process at elevated pressure 
which is, of course, contrary to conventional wisdom. By employing a rapid 
cooling step, the olefinic products are preserved without a significant 
loss to the yield. 
The cooling step slows down the rates of reactions in the gaseous product 
stream thus preventing further reactions taking place. The time between 
formation of the olefinic products and cooling, hereafter referred to as 
the residence time, is very short, typically less than 50 milliseconds. 
A short residence time of less than 50 ms is essential for high pressure 
autothermal cracking to preserve the olefinic products. In sharp contrast, 
the magnitude of the residence time at atmospheric pressure is of less 
significance, with operation at short residence time optional. Indeed, a 
longer residence time in excess of 100 ms is preferable at low pressure to 
maximise conversion and ethylene yield. 
Additionally, the use of elevated pressure provides the advantage that 
smaller sized equipment is required with the elimination of compression 
stages in the downstream processing train. These benefits lead to a more 
compact, more efficient process and a reduction in overall capital costs. 
The hydrocarbon feed may be suitably ethane, propane, butane or 
paraffin-containing hydrocarbons such as naphtha, gas oil, vacuum gas oil 
or mixtures thereof. Additional feed components may be included, if so 
desired. Suitably, methane, nitrogen, carbon monoxide, carbon dioxide, 
steam or hydrogen may be co-fed into the reactant stream. It is preferred, 
although not essential, to co-feed hydrogen into the reactant stream. By 
doing so, the yields of, and selectivities to, the desired products may be 
improved. The formation of carbon monoxide and carbon dioxide may also be 
reduced. 
The hydrocarbon feed is mixed with a molecular oxygen-containing gas. 
Suitably, the gas is oxygen, optionally diluted with an inert gas such as 
nitrogen. It is preferred to pre-mix the oxygen containing gas and the 
paraffinic feed prior to contact with the catalyst. 
The composition of the hydrocarbon/molecular oxygen-containing gas mixture 
is suitably from 5 to 13.5 times the stoichiometric ratio of hydrocarbon 
to oxygen containing gas for complete combustion to carbon dioxide and 
water. The preferred composition is from 5 to 9 times the stoichiometric 
ratio of hydrocarbon to oxygen containing gas. 
A catalyst capable of supporting combustion is employed in the present 
process. The principal role of the catalyst is to stabilise partial 
combustion of the gaseous mixture which may not otherwise be flammable. 
Suitably, the catalyst is a supported platinum group metal. Preferably, the 
metal is either platinum or palladium or a mixture thereof. Although a 
wide range of support materials are available, it is preferred to use 
alumina as the support. The support material may be in the form of 
spheres, other granular shapes or ceramic foams. Preferably, the form is a 
monolith which is a continuous multichannel ceramic structure, frequently 
of a honeycomb appearance. 
A preferred support for the catalyst is a gamma alumina coated lithium 
aluminium silicate foam. The support is loaded with a mixture of platinum 
and palladium by conventional methods well known to those skilled in the 
art. The resulting compound is then heat treated to 1200.degree. C. before 
use in the process of the present invention. 
The catalyst may be used as a fixed bed or as a solids recirculating bed 
e.g. a fluid or spouted bed. It is preferred to use the catalyst in a 
fixed bed mainly because problems with attrition, which are mainly 
encountered in moving bed operations, may be avoided. 
The process is carried out at a temperature greater than 650.degree. C. 
e.g. suitably greater than 750.degree., preferably greater than 
800.degree. C. The upper temperature limit may suitably be up to 
1200.degree. C., preferably up to 1100.degree. C. 
It is preferred, although not essential, to pre-heat the feed gas and the 
oxygen containing gas to suitably 200.degree.-500.degree. C., preferably 
200.degree.-300.degree. C. The gases may be separately pre-heated or 
pre-heated following mixing. 
Preferably, the gaseous feed mixture is introduced into the reaction 
chamber under a gas hourly space velocity of greater than 80,000 hr.sup.-1 
in order to minimise the formation of carbon monoxide and carbon dioxide. 
Preferably, the gas hourly space velocity exceeds 200,000 hr.sup.-1 
especially greater than 1,000,000 hr.sup.-1. For the purposes of the 
present invention, gas hourly space velocity is defined as: 
##EQU1## 
It is essential to the present process, that the reaction takes place under 
elevated pressure. A total pressure of greater than 5 bar absolute is 
employed. 
The cooling step will of course prevent degradation of, and/or further 
reactions between, the olefinic products and may suitably be carried out 
using rapid heat exchangers of the type familiar in steam cracking 
technology. Also possible, either additionally or instead of these 
indirect heat exchangers, a direct quench may suitably be employed. 
Suitable quenching fluids include water. 
A hydrocarbon quenching fluid may also be used to reduce the product 
temperature. At the aforementioned temperature and pressure, some of the 
hydrocarbon fluid may be cracked to provide additional olefinic products 
in the effluent stream. The use of elevated pressure advantageously 
accelerates the rate of pyrolysis of such quenching fluids and leads to an 
increase in olefin yield. Such hydrocarbon fluids are generally referred 
to as reactive quenching fluids. Suitably, the reactive quench may be a 
naphtha compound. Optionally, a second quenching fluid, such as water, may 
be employed. 
It will of course be understood that the amount of quenching fluid which 
may be usefully employed will depend upon the temperature of the effluent 
stream. 
The products of the present invention include ethene, propene, butene and 
pentene, higher olefins and alkanes. In addition to these products, small 
amounts of methane, acetylenes, aromatics, water, hydrogen, carbon dioxide 
and carbon monoxide may be produced. It is of course understood that the 
composition of the product stream will depend upon the feedstock.

The process of the invention will now be further illustrated by reference 
to the following Examples. 
EXAMPLE 1 
Preparation of Ceramic Foam Catalysts 
The lithium aluminium silicate foam support was obtained precoated with 
gamma alumina from Morgan Matroc plc with a porosity of 30 ppi. The foam 
was washed with a platinum/palladium solution of tetraammine metal 
chloride salts, drawn through the support by vacuum, dried, and finally 
calcined to 1200.degree. C. for 12 hours. The impregnation of the foam was 
controlled by monitoring the volume of solution absorbed by the foam to 
give a loading of 0.25 wt % in the final catalyst. 
EXAMPLE 2 
The Pt/Pd loaded ceramic foam catalyst (approximately 15 mm 
diameter.times.30 mm length) was placed at the bottom of a quartz reactor 
consisting of a feed section 70 mm in length, 5 mm in diameter and reactor 
section 15 mm in diameter and 80 mm in length. The reactor was connected 
to a gas feed system, insulated and fitted into a pressure jacket. A water 
quench probe was located approximately 80 mm down stream of the reactor. 
Propane, hydrogen, nitrogen and oxygen were pre-heated to 200.degree. C. to 
effect autothermal operation, where the exothermic heat of combustion 
raised the heat required to pyrolyse propane. The reaction was carried out 
at 800.degree.-1000.degree. C. and under elevated pressure, typically 10 
or 11 bar absolute. The products were cooled within 20 milliseconds of 
formation. Details of the composition of the feed, the flow rates and the 
results obtained are given in Table 1. 
EXAMPLE 3 
In accordance with the current invention, a sulphur contaminated naphtha 
hydrocarbon feed was processed in the reactor as described in Example 2. 
Benzene content in the hydrocarbon feed was 2.7 wt % with a toluene 
content of 1 wt %. To obtain catalytic light up, ethane was initially fed 
to the reactor (0.9 liters/minute) with hydrogen (0.64 liters/minute) and 
nitrogen (0.18 liters/minute), with the feed preheated to 200.degree. C. 
Nitrogen was added as an internal standard for subsequent product analysis 
by gas chromatography and is not required for operation of the process of 
the present invention. A pressure of 5 bars was established prior to 
admittance of oxygen (0.6 liters/minute). The temperature was seen to rise 
to the nominal operating temperature 900.degree. C. The ethane feed was 
then gradually substituted by the sulphur contaminated hydrocarbon feed. 
The reaction was carried out at 800.degree.-1000.degree. C., and under a 
pressure of 5.1 bar absolute. The products were quenched within 20 
milliseconds of formation. Details of the feed composition, flow rates and 
the results obtained are given in Table 2. 
Comparative Example 1 
Example 1 was repeated but with a longer residence time of 220 
milliseconds. Details of the composition of the feed, the flow rates and 
the results obtained are give in Table 3. It can be seen that the yield 
of, and selectivity to ethylene are reduced when the products are not 
cooled within 50 milliseconds of formation. 
Comparative Example 2 
Into a 30 mm diameter quartz reactor as used in Example 1, was placed a 
catalyst in the form of previously calcined Pt/Pd gamma alumina spheres (2 
mm diameter), supported on a silica sintered disk. The preparation of this 
catalyst is detailed in EP-A-0332289. Propane, hydrogen, oxygen and 
nitrogen were passed over the catalyst under atmospheric pressure in the 
molar proportions, and under the conditions, as shown in Table 4. It is 
evident from these results that high conversion and high selectivity to 
ethylene is possible under atmospheric pressure provided the residence 
time is relatively high. 
TABLE 1 
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GHSV Total Flow Total Pressure 
Residence Time 
Conversion 
Selectivity (% C mol) 
(.times. 10.sup.6 hr.sup.-1) 
(nl/min) 
C.sub.3 H.sub.8 /O.sub.2 
H.sub.2 /O.sub.2 
(bara) (ms) (% mol) 
C.sub.2 H.sub.4 
C.sub.2 H.sub.2 
CH.sub.4 
CO CO.sub.2 
Coke 
__________________________________________________________________________ 
3.0 266 1.77 0.94 
10.5 20 97.9 36.5 
0.4 26.1 
19.3 
2.1 
0 
3.1 278 1.96 0.94 
10.2 20 95.4 36.6 
0.1 24.2 
17.4 
1.7 
0 
3.2 280 1.90 1.06 
11.0 20 95.6 37.0 
0.5 22.0 
13.2 
2.1 
0 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Total Yield (wt % C) 
GHSV Flow &gt;C.sub.6 
(.times. 10.sup.6 hr.sup.-1) 
(nl/min) 
H.sub.2 /O.sub.2 
Naphtha/O.sub.2 
CH.sub.4 
CO CO.sub.2 
C.sub.2 H.sub.4 
C.sub.2 H.sub.2 
C.sub.2 H.sub.6 
C.sub.3 H.sub.6 
C.sub.3 H.sub.8 
C.sub.4 
C.sub.5 
C.sub.6 H.sub.6 
C.sub.6 H.sub.7 
Liquid 
__________________________________________________________________________ 
0.99 2.09 1.12 
0.64 12.77 
18.43 
1.93 
26.97 
1.06 
3.34 
9.22 
0.53 
4.72 
3.51 
6.31 
2.78 
8.45 
0.99 2.09 1.12 
0.64 13.12 
19.71 
1.50 
25.70 
1.08 
3.18 
7.27 
0.43 
2.90 
3.87 
5.33 
3.49 
12.49 
0.99 2.09 1.12 
0.64 13.21 
19.30 
1.47 
25.71 
1.08 
3.03 
7.57 
0.43 
3.39 
5.07 
7.93 
0.85 
10.95 
0.99 2.09 1.12 
0.64 12.08 
18.52 
1.28 
25.37 
0.80 
3.34 
8.84 
0.49 
4.16 
2.26 
6.78 
4.30 
11.78 
__________________________________________________________________________ 
TABLE 3 
__________________________________________________________________________ 
GHSV Total Flow Total Pressure 
Residence Time 
Conversion 
Selectivity (% C mol) 
(.times. 10.sup.6 hr.sup.-1) 
(nl/min) 
C.sub.3 H.sub.8 /O.sub.2 
H.sub.2 /O.sub.2 
(bara) (ms) (% mol) 
C.sub.2 H.sub.4 
C.sub.2 H.sub.2 
CH.sub.4 
CO CO.sub.2 
Coke 
__________________________________________________________________________ 
3.0 268 1.72 0.91 
10.5 220 99.6 11.1 
0.1 38.9 
19.0 
7.2 
16.0 
3.1 278 1.96 0.94 
11.8 220 99.2 8.8 
0.1 36.9 
24.8 
3.7 
17.0 
__________________________________________________________________________ 
TABLE 4 
__________________________________________________________________________ 
GHSV Total Flow Total Pressure 
Residence Time 
Conversion 
Selectivity (% C mol) 
(.times. 10.sup.6 hr.sup.-1) 
(nl/min) 
C.sub.3 H.sub.8 /O.sub.2 
H.sub.2 /O.sub.2 
(bara) (ms) (% mol) 
C.sub.2 H.sub.4 
C.sub.2 H.sub.2 
CH.sub.4 
CO CO.sub.2 
__________________________________________________________________________ 
0.3 26 1.99 1.22 
1 30 75.8 39.9 
0.7 16.1 
9.7 2.3 
0.3 26 1.98 1.22 
1 60 86.9 43.2 
1.0 18.4 
9.9 2.0 
0.3 26 2.01 1.20 
1 90 91.0 43.8 
1.4 19.1 
10.2 1.8 
0.3 26 1.94 1.20 
1 120 92.4 44.9 
1.3 19.7 
10.6 1.8 
__________________________________________________________________________