Synthesis gas methanation process and apparatus
In a synthesis gas methanation process, at least one first fraction of synthesis gas to treat is fed, together with steam, to a shift reactor where a shift reaction occurs; the gas flow produced in the shift reactor is then fed to a first methanation reactor where a methanation reaction occurs and then to further second methanation reactors in series, where further methanation reactions, performed with the addition of fresh synthesis gas which has not been subjected to the shift reaction.
1. A method comprising:
providing steam and a first fraction of synthesis gas to a shift reactor, wherein a shift reaction occurs to produce a shift reaction gas;
providing the shift reaction gas to a first methanation reactor, wherein a methanation reaction occurs to produce a first methanation reaction gas;
providing the first methanation reaction gas to at least one second methanation reactor;
providing a flow of synthesis gas that has not been through the shift reactor to the second methanation reactor, wherein a methanation reaction occurs to produce a second methanation reaction gas;
removing the second methanation reaction gas from each of the second methanation reactors through an outlet, wherein the flowrate of the synthesis gas provided to each second methanation reactor is adjusted as a function of the temperature of the second methanation reaction gas flow detected at the outlet of the same methanation reactor; and
adjusting the amount of steam provided to the shift reactor as a function of the temperature of the first methanation reaction gas flow detected at an outlet of the first methanation reactor.
2. The method according to claim 1 , further comprising removing the shift reaction gas from an outlet of the first methanation reactor, wherein substantially all of the shift reaction gas that was removed from the outlet of the first methanation reactor is provided to at least one of the second methanation reactors.
3. The method according to claim 1 , wherein the steam provided to the shift reactor includes a first amount sufficient to cause the shift reaction to occur and a second amount to control the exothermicity of the methanation reaction in the first methanation reactor.
4. The method according to claim 1 , wherein the shift reaction reduces the CO content of the shift reaction gas and produces an amount of CO 2 that acts, in the first and second methanation reactors, as a reagent and as an inert for containing the exothermicity of the methanation reactions, together with a H 2 O residual of the shift reaction.
5. The method according to claim 1 , wherein the shift reaction is performed in excess of H 2 O.
6. The method according to claim 1 , further comprising cooling at least one of the first fraction of synthesis gas provided to the shift reactor and the synthesis gas provided to one or more second methanation reactors.
7. The method according to claim 6 , wherein the cooling the first fraction of synthesis gas produces at least part of the steam provided to the shift reactor.
8. A synthesis gas methanation apparatus comprising:
a shift reactor connected in fluid communication to a primary line of a gas feed circuit and to a steam feed circuit, the shift reactor being configured to receive synthesis gas via the primary line, to receive steam via the steam feed circuit, and to perform a shift reaction to produce a shift reaction gas;
a first methanation reactor arranged downstream along the primary line from and in fluid communication with the shift reactor, the first methanation reactor being configured to receive the shift reaction gas and to produce a first methanation reaction gas;
one or more second methanation reactors, each arranged along the primary line and coupled in fluid communication in series from one another downstream of the first methanation reactor, with one of the second methanation reactors being coupled in fluid communication with the first methanation reactor; and
one or more auxiliary lines coupled to the gas feed circuit and to respective second methanation reactors, the gas feed circuit being configured to provide the respective second methanation reactors with synthesis gas that do not pass through the shift reactor, the auxiliary lines include respective flowrate controls that are configured to be adjusted by a control system, the flowrate being adjusted as a function of temperature values detected at an outlet of each second methanation reactor;
a control system operatively connected to the gas feed circuit and to the steam feed circuit; and
temperature controllers associated with respective flow controls and configured to detect the temperature of the gas flow circulating in the gas feed circuit in predetermined positions along the primary line, wherein the temperature controllers detect the temperature of gas flowing at an outlet of the first methanation reactor and causes the flow control to adjust the flowrate of steam that is provided to the shift reactor.
9. The apparatus according to claim 8 , comprising heat exchangers arranged along the primary line upstream at least one of the shift reactor and a respective methanation reactor, the heat exchangers being configured to cool the gas provided to the respective reactor.
10. The apparatus according to claim 9 , wherein the heat exchangers are connected to the steam feed circuit and produce at least part of the steam provided to the shift reactor.