GB2157307A - A reactor for quench catalytic cracking of crude oil and residua - Google Patents

Abstract

A new process for catalytic hydrocracking of heavy oils is presented. The energy required for endothermic chemical change is derived from an electromagnetic field. In this new technique only the catalyst itself achieves high temperatures. Thus carbon formation by pyrolysis in the fluid phase is avoided.

Description

SPECIFICATION A reactor for quench catalytic cracking of crude oil and residua This invention relates to a chemical reactor to process heavy fuel oils to to chemical feedstocks and distillate fuels.

In the recent past the petrochemical industry has relied on natural gas, associated light liquid hydrocarbons (LNG) and light distillates (naphtha) to provide the basic feedstocks for chemical synthesis. Little use has been made of vacuum distillates of crude oil or the bottom products of the atmospheric still, despite an increasingly attractive cost differential. The main technical problems involved are concerned with retaining adequate catalytic activity (heavy oils contain signifiant quantities of sulphur compounds) and preventing the onset of pyrolysis forming an accumulation of carbon residues.

Conventional processing of crude oil yields a range of distillates and a range of fuel oils in approximately equal mass ratio. It is expected that the demand for distillates will continue to expand but residual fuel consumption will fall due to competition and replacement by natural gas, coal and nuclear power. The market reflects this trend with a growing differential in price between crude oil and heavy fuel oil. As a consequence further fuel oil processing is now taking place in order to manufacture higher quality distillate products.

All these processes are endothermic and can be classified in terms of carbon-rejection or hydrogen addition to the fuel oils. Fluidised catalytic cracking is a well-established example of a carbon-rejection route. Coke is continuously deposited on the catalyst and removed in a separate gasifier. Hydrotreating has, however, gained more recent acceptance because of its flexible operation which allows production of a wider range of high-quality distillates. The operation is critically dependent on the availability of adundant and cheap hydrogen.

In the reactor described below the required hydrogen is generated in situ from the steam supplied with the oil feestock. Unlike other process routes catalytic activity is unaffected by the sulphur content of the oil. Moreover, the steam-carbon ratio need be only slightly greater than the initial value predicted on thermodynamic criteria for coke formation. These beneficial effects result from selective heating of the catalyst using an electromagnetic field for energy transfer rather than a conventional furnance.

The arrangement of the reactor is shown schematically in Fig. 1. Catalyst pellets composed as Raschig rings 1.5 cm in length and diameter are supported on a perforated distributor plate in the reactor vessel. When operating as a two-phase reactor for hydrogen or synthesis gas production hydrocarbon vapour and steam are mixed below this plate. Sufficient steam is provided to exceed the critical steam-carbon atom ratio at which coke deposition can be expected on thermodynamic grounds. The reactor works well with a conventional nickel on silica-alumina catalyst but other formulations have also been successful.

The catalyst is held at the centre of an electromagnetic field generated by a radio frequency oscillator operating at 20 MHz but other frequencies can be used. It is able to absorb sufficient power to achieve a working temperature up to 1 00 C. The salient feature is that only the catalyst is heated to this temperature not the reactor vessel nor its fluid contents. The surface temperature of the catalyst bed is monitored by optical pyrometry using a fibre optic probe. A digital output from the pyrometer is used for feedback control on the power supply.

Synthesis gas, hydrogen and SNG have been made from light petroleum distillates for over two decades. The new reactor was commissioned for such a system where the equilibrium products are CH4, H2, CO, CO2 and H2O. The actual composition can be calculated on the basis of thermodynamic equilibrium. It can be varied as required by changing the steam-hydrocarbon ratio and reactor temperature. Experimental data are shown in Fig. 2 for a naphtha feedstock. On a weight basis the product gas contains 13.5% hydrogen and 84.5% carbon dioxide.

The olefins are intermediate species produced in the cracking of hydrocarbon and have a potential commercial market. In an attempt to quench such intermediates to form the main commercial product the reactor was subsequently converted to run with a feed of liquid hydrocarbons.

For olefinic production a three-phase reactor is established by feeding the hydrocarbon liquids at ambient temperature directly to the catalyst bed above the distributor plate. Heavy residua fractions need some preheating to be reasonably fluid. By establishing an electromagnetic field around the reactor bed energy is supplied directly to the catalyst which is selectively heated to the reaction temperature of circa 800 C. Heat transfer takes place from the catalyst to the oil which distils at about.400"C. A vapour film forms around the hot catalyst protecting the surface to some extent from deactivation and fouling. Within this boundary film the hydrocarbon vapours are hydrocracked and reaction intermediates quenched by the liquid phase, are degassed and removed from the reflux condenser.In this way, the selective production of light olefins such as ethylene, propylene and butenes has been achieved for a wide range of oil feedstocks including crude oil itself and the vacuum-distillation residue.

A range of heavy fuel oils were chosen from the distillation of a Kuwait sou crude. These ncluded the crude oil itself, gas oil-the bottom product of the atmospheric distillation (227 to 333"C boiling range) and wax distillate the top product of the vacuum distillation (248 to 517"C boiling range). Conversion was also attempted with the atmospheric residua (IBP 268'C) 3nd the deasphalted vacuum residua (IBP 444"C).

The oils were charged as liquid feed to the reactor and were vaporized or converted from the iquid phase which boiled at around 400"C. Oils are transparent to radio frequency electromag ietic radiation; the energy flux fo vaporization and conversion came from heat released from the surface of the catalyst which could absorb energy from the electromagnetic field and reach temperatures up to 11 00,C.

The temperature difference between the boiling oil and catalyst surface far exceeds the critical ialue for nucleate boiling so film boiling is rapidly established. Thus, a three-phase reactor has veen created with the solid catalyst separated from the liquid phase by a layer of hydrocarbon vapour. The heat flux generated within the catalyst provides energy for thermal and catalytic racking of the hydrocarbons and is also absorbed by the oil to maintain the hydrocarbon vapour film.

Steam is admitted to the reactor through a distributor plate which supports the boiling oil. Its introduction induces mixing in the reactor so promoting quiescent boiling and removal of reactor products. It also participates in the reaction by yielding hydrogen and forming carbon dioxide.

Less than 5% of the steam is involved directly in chemical reaction. The remainder provides sufficient reactant excess to prevent the formation of carbon and also reduce the boiling point of the oil as in steam distillation.

Table 1 illustrates typical product distributions from a wide range of experiments at different temperatures, flowrates and feed compositions.

Table 1 : Three Phase Reactor : Steam Reforming and Cracking of Heavy Oils Variation of Product Yields with Different Feed Oils Reactor : Cocurrent Upflow Mixed Bed Oil Feed Rate : 14 kg oilXkg Catalyst : Ni on Silica Alumina catalyst/hr Reaction Temp : 8000C Contact Time s 0.25 sec Pressure : Atmospheric Wax crude Atmospheric Deasphalted Feed Oil :Gas Oil Distillate Oil Residue Vacuum Residue Steam/Carbon Ratio : 3.1 3.1 3.2 3.2 3.2 Boiling Range 227 248 - IBP 268 iHP 444 : : - 333 - 517 Vapour Composition w/w * Hydrogen 4.0 3.7 3.9 3.8 3.7 Methane 4.9 3.5 4.0 3.3 3.1 Ethylene 17.3 14.3 13.7 12.9 8.2 Ethane 9.2 9.2 9.8 7.7 5.6 Propylene 12.9 11.0 11.6 11.8 9.0 Propane 4.8 4.5 6.1 4.3 3.8 C4 Fraction 5.1 9.0 6.7 5.1 6.2 Fraction above C4 4.0 7.7 7.8 5.7 11.5 Carbon Monoxide 8.9 7.3 9.0 8.3 8.6 Carbon Dioxide 24.5 25.3 23.5 32.6 35.9 Hydrogen Sulphide 4.4 4.5 4.0 4.5 4.4 Gasiffication Ratio, wit$: 20.0 18.0 15.5 10.5 8.9 Reactor products were separated in a condenser and the emitting vapour analysed by gasliquid chromatography. The vapour product represented about 20% gasification of the distillate fuel oils per pass and 10% gasification of the residua per pass. Gasification of the crude was intermediate between these two. Ethylene and propylene represented about one-quarter of the product vapour, slightly less in the case of the deasphalted residue. Hydrogen production was about one-third of the expected equilibrium product. One-quarter of the gasified hydrocarbon is rejected carbon dioxide. The variation of gas composition with temperature is plotted in Fig. 3.

Claims (10)

1. A new reactor is claimed for endothermic catalytic reactions. Energy requirements are supplied by the irradiation of the catalyst in an electromagnetic field.

2. For endothermic reactions the effective use of the intrinsic surface of the catalyst is poor.

This is not true of the irradiated catalyst because heat is dissipated throughout the catalyst volume.

3. Energy is supplied directly to the seat of chemical chance so avoiding costly ancillary equipment such as preheaters and furnaces.

4. The reactor works successfully close to the thermodynamic miniumum of steam to hydrocarbon to avoid carbon formation.

5. The two phase reactor will generate hydrogen, synthesis gas or substitute natural gas. It will work successfully fuel oils considerably cheaper than naphtha.

6. The three phase reactor can produce olefins from a wide range of oil feedstocks including crude oil itself. Film boiling established on the surface of the catalyst provides a successful quench reactor for the production of cracking intermediates such as olefins.

7. When operated at a mild temperature circa 500,C the reactor will yield light distillates from heavy fuel oils.

8. The reactor can be used from claim 7 in place of the reboiler in a crude distillation column.

9. Claim 8 provides for selective hydrocracking and upgrading in value of heavy oils.

10. The reactor can provide a new means for desulpherization of fuel oils by hydrocracking the organic sulphur compounds to hydrogen sulphides.