EP0102112B1

EP0102112B1 - Process for the hydrotreating of a heavy oil

Info

Publication number: EP0102112B1
Application number: EP19830201140
Authority: EP
Inventors: Leonardus Johanna Van Aubel
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1982-08-26
Filing date: 1983-08-01
Publication date: 1988-12-14
Also published as: JPS5958090A; EP0102112A3; DE3378691D1; EP0102112A2; CA1230571A

Description

The invention relates to a process for the hydrotreating of a heavy oil by leading the heavy oil and hydrogen at elevated temperature and pressure cocurrently in downward direction through a reactor which contains at least one bed of a solid catalyst.
In the context of this specification and claims the term hydrotreatment is used for conversion processes in which heavy oils are converted in the presence of hydrogen. These conversion processes comprise in particular demetallization, desulphurization, denitrogenation, asphaltene conversion and hydrocracking.
The term heavy oils is used in this specification and claims for mixtures of hydrocarbons which are at least for the greater part in the liquid phase at the conditions of temperature and pressure prevailing in the reactor during normal operation of the hydrotreating process. As examples of heavy oils may be mentioned crude mineral oils, topped mineral oils, residues of atmospheric or vacuum distillation of mineral oils, deasphalted residual oils, asphalts, shale oils, oils obtained from tar sands.
Hydrotreatment of heavy oils is conventionally carried out by passing the oil together with hydrogen (in this specification the word hydrogen stands for pure hydrogen as well as for hydrogen-containing gases) in downward direction through a reactor which contains at least one bed of a solid catalyst. The heavy oil (also called the feed) trickles around the catalyst particles at the surface of which the reaction with hydrogen takes place. Because these reactions are exothermic, sufficient cooling capacity must be present during operation to control the temperature so as to avoid the development of undesired high temperatures which may lead to deactivation of the catalyst, coke formation, plugging of the catalyst bed and exposing the reactor walls to higher temperatures than those for which they are designed. This cooling capacity (also called heat-sink) is provided by the hydrogen containing gas (which may include recycle gas) and the heavy oil flowing through the reactor.
Apart from controlling the temperature rise over all individual beds in the reactor the temperature at the outlet of each bed has to be reduced to the desired temperature at the inlet of the next bed. This cooling is in many cases accomplished by injection between the catalyst beds of fresh hydrogen and/or, hydrogen-containing recycle gas at a temperature lower than the temperature prevailing in the reactor.
However it may happen that the forwarding of heavy oil or the recycle of gas to the reactors is interrupted by malfunctioning of equipment. In particular the heavy oil flow may be hampered or completely disrupted. Since the heavy oil present in the reactors will continue to react, the heat sink as provided by the gas flow of its own might become insufficient then and temperatures could increase to undesirable heights (so-called "temperature runaway"). In order to avoid such a temperature runaway in case e.g. the supply of heavy oil to the reactor is decreased or interrupted, the hydrotreatment process has to be brought to a standstill by depressurizing the reactor and discontinuing the heating of the gas and oil supply.
It would be attractive to have provisions which enable avoidance of temperature runaway in the first catalyst bed under all circumstances, even during an interruption of heavy oil or hydrogen supply to that first catalyst bed. It is possible to provide sufficient heat sink in the first catalyst bed-even in case of interruption of feed suply-by continuous injection of a larger amount of hydrogen and/or hydrogen containing recycle gas into the reactor upstream of the first catalyst bed than what is normally applied for undisturbed feed flow. However, for such a purpose gas compressors with extra high capacity would be needed, which is very unattractive for technological and economical reasons.
It has now been found that sufficient heat sink for the first catalyst bed can also be achieved by injection of a hydrocarbon mixture instead of part of a hydrogen containing gas into the reactor upstream of the uppermost (also called first) catalyst bed.
Accordingly the present invention relates to a process for the hydrotreating of a heavy oil by leading the heavy oil and hydrogen at elevated temperature and pressure cocurrently in downward direction through a reactor which contains at least one bed of a solid catalyst, in which process also a hydrocarbon mixture, which is at least for the greater part in the gaseous phase at the conditions prevailing in the reactor, is introduced into the reactor at a point upstream of the uppermost bed of solid catalyst.
The hydrocarbon mixture which is at least for the greater part in the gaseous phase at the conditions prevailing in the reactor is very suitably brought at reactor pressure in the liquid state with the aid of a pump. In this way the need for using gas compressors with a high capacity is overcome. Part of this hydrocarbon mixture may evaporate between the said pump and its entrance into the reactor owing to heating or heat exchange with other streams.
The said hydrocarbon mixture will act as a heat sink in the first reactor bed due to its heat capacity and heat of evaporation, and it may replace part of the hydrogen-containing gas in this respect during normal operation. It is preferred that such an amount of said hydrocarbon mixture is introduced into the reactor that temperature runaway in the uppermost catalyst bed does not occur when the supply of heavy oil or hydrogen is interrupted.
Although the presence of (part of the) said hydrocarbon mixture is needed when malfunctioning occurs, it is of advantage and preferred to introduce continuously said hydrocarbon mixture into the reactor, in order to avoid any risk of malfunctioning of instrument or equipment which might occur in the absence of this mixture.
It is of advantage to use independent means (e.g. separate liquid pumps with different energy sources) for the introduction into the reactor of the said hydrocarbon mixture and the feed respectively. In that way the supply of the said hydrocarbon mixture is ascertained, even if the feed pump malfunctions or falls out completely. It is of course possible to mix the feed and the said hydrocarbon mixture downstream of the feed pump and injecting the mixture thus obtained upstream of the first catalyst bed, provided the said hydrocarbon mixture is transported with the aid of a separate pump with a different energy source.
The hydrocarbon mixture which is at least for the greater part in the gaseous phase at the conditions prevailing in the reactor and which is introduced into the reactor at a point upstream of the uppermost bed of solid catalyst very conveniently consists of a fraction of the reactor effluent.
In general the effluent of the reactor which consists of hydrotreated heavy oil and a hydrogen-containing gas is separated in high temperature ("hot") separators and low temperature ("cold") separators consecutively, yielding gaseous and liquid products. Liquid product from the cold separators (which consists of condensable compounds of the gaseous product from the hot separators) is very suitable to be used as the said hydrocarbon mixture.
The amount of said hydrocarbon mixture, preferably liquid product from the cold separators, which is to be introduced in order to have available sufficient cooling capacity to avoid temperature runaway in the uppermost catalyst bed under all circumstances, even in case the feed supply or the hydrogen supply is interrupted, will depend on the type of feed, the type and degree of feed conversion to be achieved during normal operation, the reaction conditions and the catalyst. For each specific case the minimum amount of the said hydrocarbon mixture which is to be introduced into the reactor must be determined by experiments on a small scale and/or calculations.
In most cases it is also desirable to introduce fresh hydrogen or fresh hydrogen-containing gas and recycle gas (which consists for the greater part of molecular hydrogen) into the reactor between the catalyst beds in order to reduce the inlet temperature of the next bed and thereby avoiding temperature runaways in subsequent catalyst beds. Part or all of these gases may be replaced by the said hydrocarbon mixture, which can be introduced in the liquid phase between the catalyst beds. In general about 10%-85% vol. of recycle gas can be replaced by a hydrocarbon mixture. Preference is given to the use of hydrocarbon mixtures of which about 70%wt. is in the vapour phase at the conditions prevailing in the reactor.
Because of the much larger molecular weight of the vaporized part of the hydrocarbon mixture relative to the recycle gas, the total volume flow of the gas flow through the catalyst bed(s) is reduced markedly. The part of the hydrocarbon mixture which is still in the liquid phase in the reactor has an advantageous viscosity reducing effect on the heavy oil. The reduction in pressure drop achieved by these effects is very pronounced. Consequently in the process according to the invention use can be made of recycle gas compressors with a lower capacity (in terms of gas rate) and a lower differential pressure, as compared with a process in which as a heat sink use is made of feed and hydrogen exclusively.
The composition of the catalyst will be adapted to the reaction desired. In general supported catalysts will be used, the supports very conveniently being amorphous refractory oxides (or mixtures thereof) of elements of Group II, III and IV of the Periodic Table of Elements e.g. magnesia, silica, alumina, zirconia, silica-alumina, silica-zirconia. Supports consisting of crystalline materials, such as zeolites may also be used.
One or more metals (and/or compounds thereof) with hydrogenating activity are very suitably present onto the supports, in particular metals of Group VB, VIB, VIIB and/or VIII of the Periodic Table of Elements. For example, in case hydrodesulphurization is the most desired reaction to take place, catalysts which contain compounds of cobalt and/or nickel together with compounds of molybdenum and/or tungsten on alumina as a support are very suitable. In case hydrodemetallization is the most desired reaction, catalyts based on silica as a support and containing only compounds of molybdenum, or a combination of compounds of nickel and vanadium, respectively, are very convenient.
The catalyst particles present in the beds may have any suitable form, e.g. powders, spheres, pellets, cylindrical extrudates, multilobed extrudates, rings and the like. Cylindrical extrudates with a diameter from 0.5 to 2.5 mm are very suitable in general.
The reaction conditions prevailing in the reactor will be adapted to the hydrotreating reaction desired. In general, temperatures from 300-450°C, total pressures from 25-300 bar, hydrogen partial pressures from 25-250 bar, and space velocities of 0.1-10 kg feed per kg catalyst per hour will be very suitable.

Example

Four hydrotreatment experiments are carried out with a short residue of a Middle East crude as feed. This feed is passed in all cases in downward direction through two reactors in series, each of which contains three beds of catalyst.
The catalyst (in the form of extrudates with 0.8 mm diameter) consists of an alumina support onto which nickel oxide and molybdenum oxide have been applied; the catalyst is sulphided before use.
The feed of fresh hydrogen containing gas (95% vol. pure hydrogen, 5% vol. methane) is 225 nm³/ton feed. The off-gas of the reactors is (after removal of H₂S) recycled and introduced into the reactor upstream of the first catalyst bed. The gases are led over the catalyst concurrently with the feed. The reactor pressures are adapted so as to have an average hydrogen partial pessure of 150 bar in all cases.
In experiments 2 and 4 half of the recycle gas is replaced by a hydrocarbon mixture of which about 70%wt. is in the vapour phase at the conditions prevailing in the reactors. This hydrocarbon mixture is brought to reactor pressure in the liquid phase and introduced into the first reactor upstream of the first catalyst bed after heating.
In experiments 1 (a comparative experiment) and 2 (experiment according to the invention) the average reactor temperature is 380°C. The amount of recycle gas in experiment 1 as well as the amount of recycle gas together with the amount of hydrocarbon mixture in experiment 2 are sufficient to avoid temperature runaway in case the feed flow is interrupted. From the table it can be seen that the pressure drop (Ap) in experiment 1 (73 bar) is much higher than that in experiment 2 (14 bar). In order to overcome the pressure drop the pressure at the inlet of the first reactor must be higher in experiment 1 than in experiment 2. Accordingly the equipment of experiment 1 must be designed to withstand higher pressures than that of experiment 2, which of course is unattractive from an economical point of view. Moreover, the gas compressor for the recycle gas can be much smaller in experiment 2 than in experiment 1.
In experiments 3 (comparative) and 4 (according to the invention), in which the average reactor temperature is 390°C, the amount of recycle gas and the amounts of recycle gas plus hydrocarbon mixture, respectively, are not sufficient to avoid temperature runaway in case of feed flow interruption. The advantages of experiment 4 in comparison with experiment 3 as far as pressure drop (Ap) and recycle gas compressor capacity are concerned are similar to those of experiment 2 in comparison with experiment 1. Temperature runaway can be avoided when operating the reactors at 390°C by increasing the hydrocarbon mixture/feed ratio to 1.83. The temperature rise in the first bed with feed flow amounts to 26°C; in the absence of feed flow to 46°C.

Claims

1. A process for the hydrotreating of a heavy oil by passing the heavy oil and hydrogen at elevated temperature and pressure cocurrently in downward direction through a reactor which contains at least one bed of a solid catalyst, in which process also a hydrocarbon mixture, which is at least for the greater part in the gaseous phase at the conditions prevailing in the reactor, is introduced into the reactor at a point upstream of the uppermost bed of solid catalyst.

2. A process according to Claim 1, in which the said hydrocarbon mixture is brought at reactor pressure in the liquid state with the aid of a pump.

3. A process according to Claim 1 or 2, in which the said hydrocarbon mixture is introduced continuously.

4. A process according to any one of the preceding claims in which the said hydrocarbon mixture is a fraction of the reactor effluent.

5. A process according to any one of the preceding claims, in which the said hydrocarbon mixture is obtained as liquid product from a low temperature separator of the reactor effluent.

6. A process according to any one of the preceding claims, in which the said hydrocarbon mixture is introduced into the reactor by means independent of the means of introduction of the heavy oil to be hydrotreated.

7. A process according to any one of the preceding claims in which the amount of said hydrocarbon mixture introduced into the reactor is such that temperature runaway in the uppermost catalyst bed does not occur in case the supply of heavy oil or the hydrogen supply is interrupted.

8. A process according to any of the preceding claims in which the catalyst comprises one or more of the metals of Group VB, VIB, VIIB and/or VIII of the Periodic Table of Elements and/or compounds thereof, supported on an amorphous refractory oxide of elements of Group II, III and IV of the Periodic Table of Elements.

9. A process according to any one of the preceding claims which is carried out at a temperature of 300-450⁰C, a total pressure of 25-300 bar, a partial hydrogen pressure of 25-250 bar and a space velocity of 0.1-10 kg feed per kg catalyst per hour.