Jul-2013

Optimised hydrogen production by steam reforming: part 2

Emerging technologies for hydrogen production from steam reforming are modelled

Kedar V PATWARDHAN, Sanke RAJYALAKSHMI and P BALARAMAKRISHNA
Larsen & Toubro

Article Summary

Stringent emission control legislations and a well-informed market have compelled refiners to build highly complex refineries for producing cleaner and more efficient fuels. With crude slates getting heavier and sour, the demand for hydrogen is rapidly increasing and refiners are looking into new reforming technologies and schemes for optimum hydrogen production. In a previous article (PTQ, Q1 2012), we identified and optimised process parameters that affect the energy performance in hydrogen plants with conventional steam methane reforming (SMR) technology. Autothermal reforming (ATR) and gas heating reforming (GHR) are among the new technologies available at present, and future developments are focused on economical and environmentally friendly plant operations in a sustainable manner. Process schemes such as GHR in series or parallel combination with SMR and ATR are gaining prominence, as these are expected to have lower capital and operating costs and plot space requirements, as well as a reduction in flue gas emissions.

In the present study, a mathematical model is developed for a greater understanding of the functioning of GHR. A simulation model is then developed to optimise the performance of a SMR/ATR + GHR combination and the results are validated with reference data. The model is used to study the effect of operating parameters such as S/C and O2/C ratios, the extent of feed preheating and the natural gas feed split on specific energy consumption and export steam production. Performance comparisons indicate that a saving in natural gas consumption and a reduction in flue gas emissions of 6-7% is possible with new reforming technologies and process schemes when compared to a conventional SMR configuration for refinery hydrogen generation.

Increased hydrogen demand
In the last decade, the worldwide refining industry has been impacted by several trends that have increased hydrogen demand significantly. First, in aggregate, crude oil has been getting heavier and contains more sulphur and nitrogen; second, a decreasing heavy fuel oil demand requires more bottoms upgrading; and, third, increasingly stringent environmental regulations require cleaner transportation fuels production. These factors have led to a substantial increase in hydrogen consumption in a refinery complex. Generally, complex refineries source 30-60% of their total hydrogen requirements from on-purpose hydrogen capacity. Overall, approximately 95% of the on-purpose hydrogen is supplied by steam reforming of light hydrocarbons.

The hydrocarbons such as natural gas (mainly methane) up to naphtha and refinery off-gas can be converted into hydrogen by either steam reforming technology or through partial oxidation and a combination thereof. The future advances in reforming technology are focused primarily on reducing energy consumption and stack flue gas emissions. This article evaluates various reforming technology options available on an industrial scale, compares the performance parameters based on a simulation model and suggests an optimal configuration.

Autothermal reforming
The ATR unit is a refractory-lined pressure vessel containing a burner, a combustion chamber and a catalyst bed. The hydrocarbon feedstock is mixed with steam and pure oxygen, enriched air and airat the top of the reactor. In the combustion chamber, partial oxidation reactions take place and the generated heat is utilised within for endothermic steam reforming reactions. In the lower section of the reactor (loaded with reforming catalyst), the steam reforming and shift conversion reactions occur as the gas passes through the fixed bed, generating a gas mixture of H2 and CO. A general schematic of ATR is shown in Figure 1.

Gas-heated reformer
GHR uses the heat available in the process gas at the reformer exit for steam reforming in a heat exchanger type of reactor. This scheme is available in series or parallel combination with SMR or ATR. In a parallel combination with the available heat, up to 20% of the feed can be split and taken to GHR. Normally, export steam production is minimum in these configurations.

Process scheme: GHR with SMR
Since the outlet temperature of GHR is less than the SMR outlet temperature, the methane slip is higher in GHR. The higher methane slip can be counteracted by adjusting the steam-to-carbon (S/C) ratio and the inlet temperature to GHR. This option is available in the case of a parallel arrangement only, as it allows for adjustment of the operating parameters to obtain the perfect balance between the size of GHR and combined outlet composition.

Process scheme: GHR with ATR
The purpose is to increase the ratio between steam reforming and partial oxidation so that the synthesis gas will have a higher hydrogen-to-carbon monoxide (H2/CO) ratio than ATR alone. This will also result in a reduced high-cost oxygen consumption and load on the shift section. In a series arrangement, all gas passes through the steam reforming unit and then through ATR. This will mean that the reforming catalyst may set a lower limit for the S/C ratio. In a parallel arrangement, the two reformers are fed independently, giving the freedom to optimise the S/C ratio individually. However, the gas heated reformer must operate at a higher temperature than in a series arrangement in order to obtain a low methane concentration in the synthesis gas. The GHR arrangement in parallel with SMR/ATR is shown in Figure 2.

A comparison between various reforming schemes is shown in Table 1.

Basis of simulation
Model development and process simulation are carried out on the following basis:
• Impurities in the feed are efficiently removed in the feed treatment stage and hence do not influence reforming reactions
• The calorific value of natural gas available at the battery limit is 11 211 kcal/Nm3 with a methane content of 72% and a pressure of 37.5 bar
• The hydrogen product flow rate requirement is 100 kNm3/hr with a purity of >99.5%
• The kinetics of the reforming and shift reactions is taken from the open literature and is not specifically applicable to any particular catalyst
• The final purification is achieved by pressure swing adsorption operating with a working efficiency of 89%
• A HT shift and LT shift reactor configuration is considered for the water gas shift reaction.