Get a copy of ASME Boiler and Pressure Vessel Code, Section VIII, Div 1 and you can determine the optimum material selection. If you are unfamiliar or don't even know this code exists, find an organization in pressure vessel design that will help you.
SA 516/70(impact tested to the design material temeprature) is common MOC for demethanizer.There are plenty of such columns designed with this material, subject to acceptable design & process conditions.
For the design temperature mentioned above the other good option could be SA 537 Cl-1 or Cl-2 steel.
Material selection depends on a lot of other factors also:-
-sour service requirements
-corrosion rates
-overpressurizing conditions which may lower the temperature below -51F
-Other design conditions as applicable
Generally good EPC companies would have the required engineering experiences on such equipments. As advised by "metengr" you need to consult them.
