Apr-2013

Steam reforming natural gas containing higher hydrocarbons

A method is described for estimating reformed gas composition in a primary reformer when the feed is a natural gas containing higher hydrocarbons

VISHWAS DESHPANDE and SUBRATA SAHA
Reliance Ports & Terminals Limited

Article Summary

Higher hydrocarbons under typical reforming conditions are converted to methane. The method described in this article solves non-linear equations describing mass balances for carbon, hydrogen, oxygen and higher hydrocarbons, and calculates reformed gas composition for a modified feed to the primary reformer containing essentially methane by Mearn’s method. The procedure can assist both process engineers tackling the problem of estimating output from a primary reformer at the design stage and plant operating personnel. The solution, approach and programming technique can vary.

Estimation of the outlet gas composition of a primary reformer operating with a feed of methane- rich natural gas and steam has been presented by Hampson.^1,2 The mass balance equations here are solved to converge on equilibrium constants for the reforming and shift reactions. This involves simultaneously solving non-linear equations relating to the composition variables and equilibrium constants. The basic requirement is the set of primary reactions and values of equilibrium constants for the reforming and shift reactions. Alternatively, the equilibrium composition is obtained by minimisation of the Gibbs free energy function.³ This latter method has the added advantage that a knowledge of the set of primary reactions and the values of equilibrium constants is not necessary to determine the equilibrium composition.⁴ In another method, the primary and secondary reformers are modelled in Aspen as an RGibbs reactor with RK Soave equations of state and solved to obtain equilibrium product composition.⁵ In this article, a method is suggested to tackle higher hydrocarbons such as propane, butane and hexane, which may be present in the natural gas feed to the primary reformer. The actual operating data of a primary steam natural gas reformer are used in the demonstration of the method.

Formulation of the problem
Principal reactions governing the steam methane reforming process are:

CH4 + H2O ⇔ CO + 3 H2 …. Reforming
(H0   = + 206 kJ/mol )                    (1)
    298
CO + H2O ⇔ CO2 + H2 …..   Shift           
(H0   = -  41 kJ/ mol )                     (2)
   
The formation of CO2 in the reforming reaction could be considered as an alternative to CO:

CH4 + H2O ⇔ CO + 3 H2               (3)

However, only two out of three equations are necessary to represent the overall equilibria. As a general observation, when reforming higher saturated hydrocarbons and provided that contact time is long enough, the exit gas composition is that which approximately corresponds to the chemical equilibria involving the methane-steam and water gas shift reactions (Equations 1 and 2).

Mearns6 has suggested that, for reforming of higher hydrocarbons such as those present in light naphtha, it has been observed that under typical reforming conditions the only hydrocarbon present is methane. And it is further suggested that the higher hydrocarbons undergo a complete reaction with water according to the following equations:

CmH2n + m H2O → m CO + (m + n) H2                     
                                    (4)                                                

CmH2n + 2m H2O → m CO2 + (2m +n )H2                                                             (5)                                                                                   

The steam reforming of higher hydrocarbons will then be described by either Equation 4 or 5 together with Equations 1 and 2. Thus, higher hydrocarbons present in the feed will undergo reforming following Equations 1 through 5, and at the end of reaction the only gases present will be CH4, CO, CO2, H2 and H2O. These components are then added to the original feed, which contains lower hydrocarbons. The feed will now contain only lower hydrocarbons — namely, methane — which is the simplest hydrocarbon to reform. It is considered that equilibrium is reached in the shift reaction, but it does not for the methane steam reaction. An allowance is made for this deviation from equilibrium by assuming an approach to equilibrium, and thus both K1 and K2 values are known at the given outlet temperature.

Method of solution
The operating data of a primary reformer with natural gas steam feed are shown overleaf. The detailed composition of natural gas to the primary reformer and equilibrium constants for the reforming and shift reactions are shown in Table 1.
Operating data of a primary reformer are:
• Gas flow = 19 800 nM3/hr    •    Steam flow = 73.2 T/hr
• Inlet gas temperature = 830°K            
• Outlet gas temperature = 10 31°K
• Pressure at inlet = 3137.28 kPa                                     
• Pressure drop = 310.66 kPa
• K2 (shift reaction) = 1.229

For estimation of the steam-to-carbon ratio in the feed gas:
• Volumetric flow rate of natural gas = 19 800 nM3/hr. Considering that 1 g mole @ NTP = 22.7 Lit
• Gram moles/h of natural gas = 19 800 x 1000/22.4 = 883.929 Kmoles/hr
• Steam = 73.2 x 1000/18 = 4066.6667 Kmoles/hr
19 800 x 1000/22.4 = 883.929 Kmoles/hr
• Initial fSteam pratio = 4066.67 =           Carbon        1039.9375
3.9104

The complete composition of the feed to the primary reformer is shown in Table 2.