The present invention relates to a vessel system with means for catalyst protection during safety interlock trips or shut-down of a reactor. It also relates to a process for shut-down of the vessel system of the invention while decreasing the risk of carbon formation in a downstream reactor.

As used herein, a "safety interlock trip" is an automat ^¬ ic stop of process in order to protect personnel, pro ^¬ cess equipment or the environment in case abnormal oper ^¬ ation jeopardizes any of these. To achieve maximum safe- ty, the trip system is typically independent of the nor ^¬ mal control system. The system is activated if a process value deviates from a pre-set value.

More specifically, the invention is based on installing an adsorption vessel on the inlet line either upstream or downstream a sulfur guard, in parallel to normal pip ^¬ ing, to be used in case of trips or shut-down. When the pressure in the system is reduced due to shut-down, then the gas upstream the adsorption vessel will pass through said vessel to increase the hydrogen concentration by retaining CH ₄ and CO, thereby decreasing the risk of carbon formation in the downstream reactor, which is preferably a methanation reactor. The filling material in the adsorption vessel can e.g. be Sylobead®.

In methanation processes, the formation of methane from carbon oxides and hydrogen proceeds quickly to equilib- rium in the presence of a catalyst and in accordance with either or both of the following reaction schemes:

CO + 3H ₂ <=> CH ₄ + H ₂ 0 (1)

C0 ₂ + 4H ₂ <=> CH ₄ + 2H ₂ 0 (2)

These reactions will be coupled to an equilibrium be ^¬ tween carbon monoxide and carbon dioxide as follows:

CO + H ₂ 0 <=> C0 ₂ + H ₂ (3)

The net reaction of methane formation, whether by reaction (1) or (2) or both, will be highly exothermic.

It is known from the field of steam reforming that cata ^¬ lysts may form carbon depending on the operating conditions and the actual catalyst formulation. Carbon may be formed on the catalyst either from methane, carbon mon ^¬ oxide or higher hydrocarbons. The formation of carbon from methane and carbon monoxide may be expressed by the following reactions:

CH ₄ <=> C(s) + 2H ₂ (4) 2CO <=> C(s) + C0 ₂ (5) CO + H ₂ <=> C(s) + H ₂ 0 (6)

The carbon formed depends on the operation conditions and the catalyst. Typically, carbon on a Ni-catalyst is in the form of so-called carbon whiskers. As mentioned, the choice of catalyst and operating conditions will de ^¬ termine whether carbon will form or not. According to the so-called principle of equilibrated gas, carbon will form if thermodynamics predict carbon formation from one or more of reactions (4) -(6) after equilibration of re ^¬ actions (l)-(3) . Means to avoid carbon formation in this case include reducing the temperature and increasing the steam content in the feed gas to the reactor. It should be pointed out that carbon may be formed as whiskers or gum, even if the principle of equilibrated gas does not predict carbon formation. This possibility depends on the actual catalyst and detailed operating conditions, and it will typically be assessed based on experimental data.

Carbon may also form from higher hydrocarbons according to a reaction similar to the above reaction (4) as given below (for ethane) :

C ₂ H ₆ <=> 2C(s) + 3H ₂

The carbon formed from higher hydrocarbons may also be in the form of whiskers, graphite or gum. It is a com- plex task to assess the risk of carbon formation from higher hydrocarbons. The risk of carbon formation in this case also depends upon the catalyst and the select ^¬ ed operating conditions. Also in this case, increasing the content of steam is one way to ensure operation out of the carbon-forming operating conditions. In some cas ^¬ es the so-called critical steam to higher hydrocarbons ratio (S/HHC) can be used as an indicator of whether or not carbon will be formed on the catalyst.

As regards prior art, GB patent application 2 223 237 A describes a shut-down process for a Fischer-Tropsch reactor and the reactor itself. More specifically it de ^¬ scribes a process for the shut-down of a reactor for the preparation of an at least partly liquid hydrocarbona- ceous product by a catalytic reaction of carbon monoxide with hydrogen at an elevated temperature and pressure and using a catalyst, said reactor being provided with cooling means and with means to recycle gas through the catalyst for temperature equalizing of the catalyst. The process comprises the steps of (1) interrupting the feed of synthesis gas, (2) depressurizing the reactor downstream of the catalyst, providing the reactor upstream of the catalyst with inert gas, and (3) cooling the cat ^¬ alyst to ambient conditions. US patent application 2008/0075986 Al concerns a fuel cell stack shut-down purge method, more specifically a method for purging hydrogen from a fuel cell stack after fuel cell shut-down. The method includes providing an air stream, providing a temporary nitrogen stream by re- moving oxygen from the air stream with an adsorbent bed and passing the nitrogen stream through the fuel cell stack. This is done to avoid a loss of carbon substrate and catalyst area, which otherwise will reduce the oper ^¬ ating voltage and ultimately limit the life time of the stack. WO 2005/101556 relates to fuel cell shut-down with steam purging, more specifically to a method and an apparatus for steam purging a solid oxide fuel cell stack. Purging the SOFC stack with steam has a physical flushing ef- feet, removing carbon monoxide containing reformate and free oxygen gas from the anode area, thereby reducing the potential for nickel oxide or nickel carbonyl for ^¬ mation . EP 0 157 480 A2 describes a process for producing ammo ^¬ nia synthesis gas from a raw gas comprising hydrogen, carbon dioxide and medium boiling point gases including nitrogen in excess of the proportion required in ammonia synthesis gas, by a pressure swing adsorption (PSA) pro- cess. A raw gas is fed to the absorbent, in which gas hydrogen and total medium boiling point gases are pre ^¬ sent in a volume ratio of 1.25-2.5, and the medium boil ^¬ ing point gases comprise nitrogen to the extent of at least 90 vol% of the total of such gases. The PSA is used during normal operation, primarily to remove carbon monoxide which is a poison to the ammonia catalyst. Me ^¬ thane is likewise removed to improve the efficiency, but it is not a poison, and the EP application has nothing to do with carbon formation.

Finally, US 2012/0077096 Al discloses a fuel cell system comprising an oxygen-removing device having an inlet fluidly connected to at least one of the reactant gas source and an outlet of the cathode gas flow field, and an outlet fluidly connected to each of an anode control valve and a cathode control valve. Methane is not re ^¬ moved, and the feed is different at shut-down (the fuel gas source is shut off) . Also here, the US application has nothing to do with carbon formation.

The vessel system according to the invention with means for catalyst protection during safety interlock trips or shut-down of a reactor comprises a reactor R,

- an inlet line i for a process gas,

- a sulfur guard S,

- at least one adsorption vessel A containing an adsorbent selective for adsorbing a purge gas, and

- an outlet line o for the product gas stream from the reactor R, wherein the adsorption vessel is installed either up ^¬ stream or downstream the sulfur guard, in parallel to the normal piping, to be used in case of safety inter ^¬ lock trips or shut-down.

Advantageously, two adsorption vessels arranged in par ^¬ allel can be used. This way, one of the vessels can be regenerated while the other is in use. Regeneration of the adsorption vessel can for instance be done by relieving the pressure to flare.

The adsorption vessel in the system of the present in ^¬ vention is preferably based on the use of pressure swing adsorption (PSA) , a technology used to separate a gas species from a mixture of gases under pressure according to the molecular characteristics of the species and its affinity for a given adsorbent material. PSA operates at near-ambient temperatures and differs significantly from e.g. cryogenic distillation techniques of gas separa ^¬ tion. Specific adsorptive materials, such as zeolites, activated carbon, molecular sieves etc, are used as a trap, preferentially adsorbing the target gas species at high pressure.

PSA processes rely on the fact that under high pressure, gases tend to be attracted to solid surfaces, or "ad ^¬ sorbed". The higher the pressure, the more gas is ad ^¬ sorbed. When the pressure is reduced, the gas is re ^¬ leased, or "desorbed". PSA processes can be used to ef ^¬ fectively separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly. If, for example, a gas mixture such as air is passed under pressure through a vessel containing an adsorbent bed of zeolite that attracts ni ^¬ trogen more strongly than it does oxygen, then part or all of the nitrogen will remain in the bed, and the gas coming out of the vessel will be enriched in oxygen. When the bed reaches the end of its capacity to adsorb nitrogen, it can be regenerated by reducing the pres ^¬ sure, thereby releasing the adsorbed nitrogen. It is then ready for another cycle of producing oxygen- enriched air.

Aside from their ability to discriminate between differ ^¬ ent gases, adsorbents for PSA systems are usually very porous materials, chosen because of their large surface areas. Typical adsorbents are activated carbon, silica gel, alumina and zeolites. Though the gas adsorbed on these surfaces may consist of a layer only one or at most a few molecules thick, surface areas of several hundred square meters per gram enable the adsorption of a significant portion of the adsorbent's weight in gas.

The vessel system according to the invention has a layout as shown on the appended figure. The inlet gas stream (i) is passed through a sulfur guard S and pro ^¬ ceeds either to a reactor R, preferably a methanation reactor, possibly via a heat exchanger and/or after mixing with some recycle gas (not shown) or to an adsorption vessel A containing an adsorbent selective for ad ^¬ sorbing a reactor purge gas. The valve VI is normally open, while the valves V2 and V3 are closed. This way the process gas stream (p) will proceed directly from the sulfur guard outlet to the reactor R. If the valve VI is closed, while the valves V2 and V3 are opened, the process gas stream will pass through the adsorption vessel A, which can be a PSA unit or any other device containing an adsorbent as used in connection with PSA units. As mentioned above, the adsorption vessel A may advantageously be placed upstream instead of downstream the sulfur guard S. This way a temperature "runaway" or temperature rise in the sulfur guard can be avoided be ^¬ cause of a formation of by-products in the stagnant low- flow gas .

The adsorption vessel may be preceded by a cooler (not shown) in order to increase the capacity of the vessel. In the system according to the invention, said process gas preferably is a synthesis gas comprising carbon ox ^¬ ides and hydrogen. Preferably the adsorption vessel in the system of the invention contains a carbon monoxide and methane selec ^¬ tive adsorbent, such as Sylobead®. A "heat-sink" can be placed upstream the adsorption vessel to lower the temperature of the gas, whereby the capacity of the adsor- bent may increase. The heat-sink may consist of an inert material, or it may be the adsorbent itself, most pref ^¬ erably with high heat capacity. During normal operation, when the adsorbent system is not in use, the heat sink and the adsorbent are cooled by the surroundings or al- ternatively cooled by an inert gas, such as nitrogen.

When the plant is tripped or shut down, gas at an ele ^¬ vated temperature is led through the heat sink and ad ^¬ sorbent system, where the heat sink adsorbs the heat be ^¬ fore cooled gas enters the adsorbent.

Sylobead® is a brand of molecular sieve and silica gel products originally developed to remove acid gas compo ^¬ nents such as CO ₂ , ¾S and other sulfur compounds from raw natural gas and recover by-products, such as conden- sates and natural gas liquids, from the gas. It is very well suited for use as a CO and CH ₄ selective adsorbent in the adsorption vessel of the reactor system of the invention . The adsorption vessel can also be used to simplify the reduction procedure, if any catalyst should need that. The reduction can be carried out by using syngas (i.e. CO + ¾) . However, that procedure is simpler if the car ^¬ bon monoxide can be removed. The reason why it is sim ^¬ pler is that CO ₂ build-up can be avoided because CO ₂ is formed when CO is used as a reducing agent.

Steam may be added to the hydrogen-rich gas downstream the adsorption vessel A to increase the cooling potential of the gas (mass/heat capacity) .

In the vessel system according to the invention, the reactor R comprises a catalytically active material which is sensitive to the gas composition contacting the cata ^¬ lytically active material. The catalytically active ma ^¬ terial is preferably a material, which is active in the methanation process.

The process according to the invention for safety interlock trips or shut-down of a vessel system comprises the following steps: a) directing a process gas through an adsorption vessel arranged in parallel to the normal piping to separate methane and carbon monoxide from the gas, thereby de ^¬ creasing the risk of carbon formation in a downstream reactor, and b) adjusting the process conditions to provide a shut ^¬ down or safety interlock trips state in the system.

The invention is illustrated further by the following examples : Example 1

This example shows the normal situation under operation of the system according to the invention with a carbon limit of around 700°C.

The individual streams in the system have the composi ^¬ tions as indicated in Table 1 :

Table 1

where "o" is the outlet gas stream after the reactor R.

Example 2

This example shows the shut-down situation under opera- tion of the system without PSA and with a carbon limit of around 535°C. The individual streams in the system have the composi ^¬ tions as indicated in Table 2 :

Table 2

Example 3

This example shows the shut-down situation under opera ^¬ tion of the system with PSA and with a carbon limit of around 885°C.

The individual streams in the system have the composi ^¬ tions as indicated in Table 3: Table 3

Stream i V2 V3 P o

Composition mole % mole % mole % mole % mole %

Ar 0.1 0.1 0.0 0.1 0.1

CH ₄ 15.0 15.0 6.0 15.2 15.2

CO 20.0 20.0 8.0 1.3 1.3 co ₂ 1.0 1.0 0.4 0.3 0.3

¾ 63.9 63.9 85.5 74.7 74.7

N ₂ 0.1 0.1 0.0 0.0 0.0

H ₂ 0 8.4 8.4