plant to operate at reduced rates and/or result in premature tube failures. The use of a pre-reforming stage allows a variety of feedstocks (natural gas through to naphtha) to be used without the need for changing the installed catalysts in the plant. Pre-reforming is an adiabatic process and generally operates at temperatures between 350°C and 650°C. The feed to the pre-reformer is preheated using heat from the SMR flue gas duct. Depending on the feedstock, the pre- reformer effluent may be reheated before the gas is sent to the SMR (see Figure 1 ). Hydrocarbons heavier than methane are first steam reformed (Equation 1) . Generated CO is then methanated (Equation 2) in the presence of hydrogen. The water gas shift reaction (Equation 3) also occurs under these conditions and leads to the production of carbon dioxide (CO₂). Both the methanation reaction and the water gas shift reaction are reversible; thus, concentrations of methane, CO, CO₂, and hydrogen are dependent on equilibrium:
Naphtha
LPG
Natural gas
Distance along catalyst bed
Figure 2 Comparison of temperature profile of pre-reforming with different feeds There is a trend of pre-reformers being increasingly included in new hydrogen plant designs for one or more of the benefits listed above. Additionally, as the demand on existing hydrogen plants also changes, pre-reformers are regularly considered in revamp projects. Feedstocks A traditional pre-reformer operates as an adiabatic bed, generating a reaction temperature profile that moves through the bed as the catalyst ages. Figure 2 shows typical profiles produced when processing different feeds. “ The use of a pre-reforming stage allows a variety of feedstocks (natural gas through to naphtha) to be used without the need for changing the installed catalysts in the plant ” For a light feed that only contains a small quantity of higher hydrocarbons, the overall reaction is endothermic. This allows the cooler exit gas from the pre-reformer to be reheated in the flue-gas duct before entering the reformer tubes, thus reducing the overall heat duty of the SMR. Such an arrangement leads to fuel cost savings and a reduction in capital cost associated with a smaller SMR. The savings in fuel demand lead to further benefits, including a reduction in CO₂ and nitrogen oxides discharged to atmosphere. Further savings may also be achieved through higher preheat temperatures at the inlet to the reformer tubes than would be possible without a pre-reformer.
• CnHm + nH₂O g nCO + (n+½m)H₂ • CO + 3H₂ ⇌ CH₄ + H₂O • CO + H₂O ⇌ CO₂ + H₂
(Eq 1) (Eq 2) (Eq 3)
The process stream that exits the pre-reformer essentially contains no hydrocarbons heavier than methane. This benefits the downstream process, particularly as methane is less susceptible to forming carbon deposits in the SMR and heating coil compared to heavier hydrocarbons. Thus, a process stream that has been pre-reformed can be heated to higher temperatures prior to the SMR, or the SMR can be operated at lower S:C ratios. The advantages provided through the inclusion of a pre-reforming unit are plant specific and can include: • Fuel savings over stand-alone SMR. • Higher SMR preheat temperatures. • Lower involuntary steam production. • Increased feedstock flexibility. • Lower overall S:C ratios. • Provides additional protection for the SMR and other downstream units. • Reliable and easy operation. • Reduced capital cost for the SMR.
Refining India
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