GB2246576A - A process for the preparation of hydrocarbons, a process for the shutdown of a reactor for carrying out said process and a reactor to be used therefor - Google Patents

Abstract

In a process for the preparation of hydrocarbons from hydrogen and carbon monoxide and a process for the shutdown of a catalytic reactor 1 used therefor a buffer vessel 12 is used for a controlled expansion of the products from the reactor 1 in case of a shutdown situation. Depressurization takes place by sending gas to the buffer vessel 12 in order to control the discharge of gaseous reaction products and unreacted feed gas in the reactor on the one hand and controlling the gas flow to a flare via line 9a or other use on the other hand. <IMAGE>

Description

A PROCESS FOR THE PREPARATION OF HYDROCARBONS, A PROCESS FOR THE SHUTDOWN OF A REACTOR FOR CARRYING OUT SAID PROCESS AND A REACTOR TO BE USED THEREFOR.

The invention relates to a process for the preparation of hydrocarbons by - feeding a hydrogen and carbon monoxide containing feed gas into a reactor; - reacting said feed gas in said reactor at elevated temperature and pressure in the presence of a catalyst; - discharging the gaseous and/or liquid reaction products from said reactor through at least one discharge section.

Processes for the preparation of hydrocarbonaceous products by catalytic reaction of carbon monoxide and hydrogen (synthesis gas) are well known.

This reaction is highly exothermic and cooling means are used in the reactor for the removal of heat from the reaction zone. Additionally such a reactor is usually provided with means to recycle gas through the catalyst bed for equalizing the temperature in the catalyst bed.

Preferably such a reactor is also provided with means to recycle liquid hydrocarbonaceous product through the catalyst for equalizing the temperature of the catalyst bed, and further to avoid the formation of hydrocarbonaceous deposits on the catalyst.

When such a reactor is to be shutdown the supply of carbon monoxide and hydrogen is interrupted. Above and in the catalyst bed a large amount of reactant gas mixture is present, which will pass through the catalyst bed at a relatively low velocity. The reaction heat is insufficiently removed and hot spots are formed in the catalyst bed. These hot spots result in a deterioration of the performance of the catalyst.

According to GB-A-8817407.3 the abovementioned problem of overheating the catalyst may be overcome by carrying out the following steps: (i) interrupting the feed of synthesis gas; (ii) depressurizing the reactor downstream of the catalyst, and providing the reactor upstream of the catalyst with inert gas and/or hydrogen, preferably hydrogen; and (iii) cooling the catalyst to ambient conditions.

In one embodiment the dome-like space above the catalyst bed is filled with packing bodies, said bodies containing hydrogen releasable therefrom when the pressure in the reactor falls below the working pressure. Then hydrogen is automatically released. Said inert bodies may comprise an interfacial membrane permeable to hydrogen and impermeable to carbon monoxide, or said inert bodies may comprise material which absorbs hydrogen under reaction conditions and desorbs under shut-down conditions. Depressurization is not further elucidated. Normally, however, the gaseous substances in the reactor are discharged through a flare.

An object of the present invention is to improve the prior art processes by providing economical means for a controlled and fast depressurization of the reactor immediately after the occurrence of a shut-down situation. This object can be realized according to a process as mentioned in the preamble, wherein in a buffer section connected with to said discharge section a pressure is maintained at a value below that of said discharge section, said buffer section and said dis charge section being connected by at least one valve. By such a process on the one hand a decline in performance of the catalyst can be avoided as it is possible to protect the catalyst instantaneously with a hydrogen quench at the desired moment, e.g. when a trip occurs, and on the other hand a very large flare may be avoided.

Details about the conditions to be used in the process according to the invention will be described hereinbelow.

It will be self-evident that the pressure in the buffer section will be kept as low as possible during normal operation. In general, the buffer section may be maintained at a value of 1-150, preferably 5-50 bar below that of the feed gas during the normal operation of the reactor.

It is feasible to keep the pressure in the buffer section at a value of about 1 bar. In general, a value below 0.1 bar will not give particular advantage, whereas a pressure of more than 5 bar will not be necessary.

In carrying out the process of the invention the buffer section will normally not be in open communication with the discharge section. At any desired moment, e.g. in case of an emergency situation or in any instance in which the pressure in the reaction system should be decreased, the valve to the buffer section may be opened in order to expand the gaseous products being present in the reactor and discharge section. It will be appreciated that at the same time the pressure in the reactor system may be decreased by discharging gaseous products to other systems, e.g. a flare system.

The invention also relates to a process for the shutdown of a reactor system which is used for the preparation of hydrocarbons by reaction of a hydrogen and carbon monoxide containing feed gas at elevated temperature and pressure and using a catalyst, said reactor system comprising a reactor containing said catalyst, and a discharge section for gaseous and/or liquid reaction products by interruption of the feed of said feed gas, wherein after said interruption of the feed gas, gaseous products being present in said reactor and discharge section are expanded in a buffer section connected with said discharge section by at least one valve.

In general, . the expansion of the gaseous products in the buffer section gives a reduction of the pressure in the reactor of 10-90 %, preferably 20-50 %.

Consequently, the present invention relates to a process wherein by said expansion of said gaseous products in the buffer section the pressure in the reactor is reduced with 10-90, preferably 20-50%.

According to an embodiment of the reactor system of the invention said discharge section comprises a flare line being connected with said buffer section by means of at least one inlet and/or outlet valve.

More specifically, the invention relates to a reactor system suitable for carrying out the processes as described in the above, said reactor system comprising a reactor housing provided with at least one catalyst section for accommodating a catalyst, said catalyst section being in communication with inlet means for synthesis gas, with product outlet means, with means for recycling gaseous and/or liquid (reaction) products, said reactor system further comprising a buffer section for depressurizing the catalyst section.

For controlling and adjusting the pressure in the reactor system during normal operation and in shutdown situations microprocessors with appropriated software may be used.

The processes according to the invention are particularly suitable for a reactor in which synthesis gas is converted into hydrocarbons, preferably having at least 10 carbon atoms per molecule; more preferably paraffinic hydrocarbons having at least 20 carbon atoms per molecule. This reaction is known as the Fischer Tropsch synthesis. The hydrocarbons formed in this reaction can be characterized by the well-known Schultz Flory distribution. The distribution is normally described by the chain growth probability factor alpha.

Alpha has preferably a value between 0.8 and 0.95, more preferably between 0.86 and 0.94.

Normally synthesis gas is used as the feed gas for the reactor. Synthesis gas contains as major compounds hydrogen and carbon monoxide; in addition it may contain small amounts of carbon dioxide, water, nitrogen, argon and minor amounts of compounds having 1-4 carbon atoms per molecule, such as methane, methanol and ethene.

The synthesis gas is prepared in any manner known in the art, for instance by means of steam/oxgen gasification of hydrocarbonaceous material such as brown coal, anthracite, coke, crude mineral oil and fractions thereof, oil recovered from tar sand and bituminous shale. Alternatively, steam methane reforming and/or catalytic partial oxidation of a hydrocarbonaceous material with an oxygen-containing gas may be used to produce synthesis gas.

The process conditions in the reactor for the preparation of the hydrocarbons are: a temperature from 100-500 C, preferably 125-350 C, in particular 175250 C, a pressure of 1-200 bar, preferably 5-100 bar, in particular 12-50 bar and a space velocity from 20020,000 m3 (S.T.P.) gaseous feed/m3 reaction zone/hour, preferably 500-5000 m3 (S.T.P.) gaseous feed/m3 reaction zone/hour. The expression "S.T.P." as referred to hereinbefore means Standard Temperature (O C) and Pressure (1 bar abs.). The molar ratio of hydrogen to carbon monoxide is normally 0.4-4, preferably from 0.62.5, more preferably 1.0-2.1.

Catalysts often used for the present reactions comprise zeolites or one or more metals from group VIII of the Periodic Table, especially from the iron group, supported on a carrier, optionally in combination with one or more metal oxides and/or other metals as promoters. The metal oxide promoters are usually chosen from groups Ill, IIIb, IVb, and/or Vb of the Periodic Table as well as from lanthanides and/or actinides. The metal promoter may be selected from the groups VIIb and/or VIII of the Periodic Table.

Reactors for carrying out the present processes are well known in the art. In general, multitube reactors comprising about 5000 - 30,000 reactor tubes with a diameter of about 20-50 mm may be used. Reactors and processes, which will be suitable for applying the present invention, are described in EP 207547 and EP 163357.

The processes and reactors according to the present invention relate in particular to the synthesis of heavy paraffins (C5-clOO) from synthesis gas.

Normally a H2/CO ratio of about 0.9-2 is used. In case of a shutdown, e.g. when a reactor trip occurs, this ratio should be increased to a value above e.g. about 2.1 in order to protect the catalyst. In such an event the pressure in the reactor is reduced from the normal operating pressure of e.g. about 40 bar to about 7 bar (4000-700 kPa) over a preferred period 1-60 minutes, more preferably 1-10 minutes.

The invention will now be further elucidated with reference to the accompanying Figure showing a flow diagram of an embodiment of the reactor system according to the invention.

Referring to the Figure, 1 represents a reactor for the synthesis of hydrocarbons, e.g. a multitubular reactor or a reactor comprising a large fixed catalyst bed provided with wound cooling tubes, preferably helically wound cooling tubes. During normal operation synthesis gas is feeded through line 2 into reactor 1.

Liquid reaction products are discharged through line 3, optionally recycled through line 4, back into the reactor or through line 5 to other uses.

Gaseous reaction products including unreacted feed gas is discharged from reactor 1 through line 6.

Normally, this gas stream comprises steam, carbon monoxide, hydrogen and C1-C5 hydrocarbons as well as higher paraffins. In a separation step which may comprise one or more separators 7 a three-phase separation is carried out. Liquid paraffins and water are discharged through line 8 and line 8a respectively. Gas is leaving separator 7 through line 9. The gas in line 9 may be recycled through line 10 to feed line 2 or may be led through line 11 to other uses or may be flared through line 9a.

A buffer vessel 12 is connected with line 6 and line 9a by means of an inlet valve 13 and an outlet valve 14.

At the beginning of the depressurization procedure the reactor 1 is blocked in by suddenly closing valves in feed line 2 and line 6. To maintain a minimum gas flow and thus minimum heat transfer in the catalyst bed, reactor 1 has to be depressurized via its gas outlet 6 and 9a. The minimum discharge flow required decreases with the actual pressure in reactor 1, but at relatively high pressures the minimum discharge flow will exceed the capacity of the flare line 9a. For instance, when the normal pressure in the reactor is about 30-50 bar, the pressure has to be decreased to a value in the range of e.g. 20-30 bar in order to have a sufficient discharge flow a reasonable flare line capacity.So, during the first part of the depressurization period a part of the discharge flow may be stored in buffer vessel 12 by opening inlet valve 13, the remainder of the discharge flow going to the flare through flare line 9a at a constant flow rate using the maximum capacity of the flare line 9a.

During the first period the pressure in the buffer vessel 12 may increase from e.g. 1-2 bar to e.g.

20-30 bar. The buffer outlet valve 14 remains closed and the inlet valve 13 serves as the control valve for the discharge flow through line 6, and line 6a. This discharge flow is a certain function of the reactor pressure which, consequently, is a certain function of time. By making use of an external ramp signal presenting the desired pressure/time function, the decrease of the reactor pressure and indirectly the decreasing molar discharge flow are controlled.

The first period of depressurization is finished when the reactor pressure has dropped sufficiently, e.g. to a value of approximately 10-30 bar.

Then, the inlet valve 13 of buffer vessel 12 is closed.

During a second period the depressurization may be continued via the flare line 9a. The flow to the flare exceeds more and more the required minimum at the reactor outlet 6. All valves of the reaction systems remain in the same position as at the end of the first period until a reactor pressure of e.g. 3-10 bar is reached. At this pressure purge gas for purging the reaction system, e.g. hydrogen optionally containing other inert gases such as nitrogen, argon or inert gaseous hydrocarbons, is fed through line 2a into reactor 1. With this purging the third period begins. At the same time the discharge of gas through flare line 9a is controlled at a desired value, e.g. 4-7 bar. Owing to the low pressure and the high purge gas flow with respect to this pressure, a temperature runaway in the catalyst bed cannot occur anymore. The purge gas flow is far below the capacity of the flare line 9a. The reactor purge will depend on the reactor temperature.

At the end of the third period the feed of purge gas is stopped. The reactor is now still under elevated temperature and pressure and filled with reducing purge gas and relatively small quantities of reactants.

Because of the small partial pressures, reaction rates are negligible. The reactor can now be repressurized via feed line 2 to normal operating pressure. Of course, the gas stored in buffer vessel 12 may be led to line 9a at any desired moment, provided that the desired capacity is sufficient at such a moment.

Claims (10)

1. A process for the preparation of hydrocarbons by - feeding a hydrogen and carbon monoxide containing feed gas into a reactor; - reacting said feed gas in said reactor at elevated temperature and pressure in the presence of a catalyst; - discharging the gaseous and/or liquid reaction products from said reactor through at least one discharge section; characterized in that in a buffer section connected with said discharge section a pressure is maintained at a value below that of said discharge section, said buffer section and said discharge section being connected by at least one valve.

2. The process according to claim 1, characterized in that the pressure in said buffer section is maintained at a value of 1-150, preferably 5-50 bar below that of the feed gas during the normal operation of the reactor.

3. The process according to claim 1 or 2, characterized in that the pressure in said buffer section is maintained at 0.1-5 bar, preferably about 1 bar.

4. A process for the shutdown of a reactor system which is used for the preparation of hydrocarbons by reaction of a hydrogen and carbon monoxide containing feed gas at elevated temperature and pressure and using a catalyst, said reactor system comprising a reactor containing said catalyst, and a discharge section for gaseous and/or liquid reaction products by interruption of the feed of said feed gas, characterized in that after said interruption of the feed gas gaseous products being present in said reactor and discharge section are expanded into a buffer section connected with said discharge section by at least one valve.

5. The process according to claim 4, characterized in that by said expansion of said gaseous products in the buffer section the pressure in the reactor is reduced with 10-90 %, preferably 20-50 %.

6. A reactor system for carrying out the process of any of the preceding claims, comprising a reactor and a discharge section for gaseous and/or liquid reaction products, characterized in that said discharge section is connected with a buffer section by means of at least one inlet and/or outlet valve.

7. The reactor system according to claim 6, characterized in that said discharge section comprises a flare line being connected with said buffer section by means of at least one inlet and/or outlet valve.

8. A process or reactor substantially as hereinbefore described with particular reference to the accompanying drawing.

9. Hydrocarbons prepared by using a process or reactor according to any of the preceding claims.

10. Software to be used in a process or a reactor according to any of claims 1-8.