Jan-2012

Optimised hydrogen production by steam reforming: part I

Modelling optimisation of process and design parameters for minimising natural gas consumption in hydrogen production by steam reforming

Sanke Rajyalakshmi, Kedar Patwardhan and P V Balaramakrishna
Larsen and Toubro

Article Summary

Hydrogen is a particularly important feedstock in refineries because of stringent environmental legislation for producing low-sulphur gasoline and diesel fuels. The refinery hydrogen requirement is fulfilled through the route of steam methane reforming (SMR) of natural gas. SMR is a highly energy-intensive technology and this energy requirement can be minimised by combining various reforming techniques. In this article, process optimisation of the hydrogen plant is carried out to minimise natural gas consumption by considering operating parameters such as the steam-to-carbon ratio, various reactor configurations, methane slip, choice of product purification step, and natural gas composition in a conventional SMR process.

In a second article to follow, new process schemes incorporating an autothermal reformer (ATR) and a heat exchange reformer (in series and parallel combination with the ATR) offered by licensors are evaluated and compared with the conventional process. It has been observed that the new schemes, from the point of view of natural gas consumption, are more beneficial compared with the conventional process but at a cost of lower steam production from the hydrogen plant. Hence, where the natural gas price is substantial and dominates the plant operating cost, the new process scheme should be utilised.     

Along with increased hydrogen consumption for deeper hydrotreating, additional hydrogen is needed for processing heavier and higher sulphur crude slates. In many refineries, hydroprocessing capacity and the associated hydrogen network limits refinery throughput and operating margins. Furthermore, higher hydrogen purities within the refinery network are becoming more important to boost hydrotreater capacity, achieve product value improvements and lengthen catalyst life cycles. Improved hydrogen utilisation and expanded or new sources for refinery hydrogen and hydrogen purity optimisation are now required to meet the needs of the future market for transportation fuels and the drive towards higher refinery profitability.  

A variety of process technologies can be used for hydrogen production, including steam reforming, cracking, gasification and electrolysis. The choice depends on the scale of production required and the cost of available feedstocks. 

For large-scale production, the steam reforming of natural gas has become the preferred solution. In some cases, partial oxidation has also been used, particularly where heavy oil is available at low cost. However, oxygen is then required and the capital cost of producing oxygen plant makes partial oxidation expensive. On the other hand, the steam reforming of natural gas offers an efficient, economical and widely used process for hydrogen production, and provides near- and mid-term energy security and environmental benefits. The efficiency of the steam reforming process is about 65% to 75%, among the highest of current commercially available production methods.

The SMR process is divided into sections: feedstock purification for the removal of sulphur and other impurities; steam reforming for synthesis gas generation; shift conversion/carbon monoxide removal; and hydrogen purification. A general block diagram with natural gas as the feedstock is shown in Figure 1.

Natural gas feed is preheated in coils in the waste heat section of the reformer, and sulphur is removed over a zinc oxide catalyst. Process steam is added, and the mixture of natural gas and steam is further preheated before entering the tubular reformer. Here, conversion to equilibrium of hydrocarbons to hydrogen, carbon monoxide and carbon dioxide takes place over a nickel-based reforming catalyst. The gas exits the reformer and is cooled by steam production before entering the shift converter. Over the shift catalyst more hydrogen is produced by converting carbon monoxide and steam to carbon dioxide and hydrogen. The shifted gas is cooled further to an ambient temperature before entering the pressure swing adsorption (PSA) unit. High-purity hydrogen product is obtained, and the off-gas from the PSA unit is used in the reformer as fuel supplemented with natural gas fuel. Combustion air for the tubular reformer burners can be preheated in coils in the reformer waste heat section. Part of the steam produced in the hydrogen plant is used as process steam; the excess steam is exported.

The objective of this study is to analyse the process parameters of the present configuration and to identify optimum conditions to enhance the process performance. Different process layouts with high, medium- and low-temperature shift reactors and also with different purification processes (PSA and methanator) are studied and compared.

Identification and optimisation of process parameters
The study focuses on the optimisation of process/design parameters for the minimisation of natural gas consumption. The study is particularly useful in countries such as India, where natural gas prices dominate operating cost. The most important parameters to influence natural gas consumption are:
• Methane slip
• Steam-to-carbon ratio
• Natural gas composition
• Shift converter configuration
• Choice of hydrogen purification step.

The following assumptions were made while carrying out the analysis:
• Impurities in feed are efficiently removed in the feed purification stage. The total volume of reactors in the purification stage does not change within the variation of operating parameters under study  
• Hydrogen production is fixed at 90 000 Nm3/h for any variation in the operating parameter 
• PSA efficiency is 89%
• The kinetics of the reaction are taken from the open literature and are not specifically applicable to any particular catalyst.

Methane slip
The steam methane reforming reaction is strongly endothermic and is therefore favoured by a higher temperature. Typical reformer outlet temperatures fall in the range 810–900°C. As the temperature is increased, the hydrogen yield increases, which is observed as a reduction in the methane concentration in the reformer effluent, known as methane slip. The higher the yield, the less the amount of feedstock that will be consumed.