Steam reformers for hydrogen and synthesis gas
Modern steam methane reformer design is a complex process that needs to be carefully optimised in terms of capital and operating costs
NORM PELLETIER and GOUTAM SHAHANI
Selas Fluid Processing Corporation (Now Linde Engineering North America)
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The popular press and scientific literature has been rife with references to the discovery of vast quantities of shale gas in North America. This resource has become economically recoverable due to advances in horizontal drilling and well fracturing or fracking. The advent of shale gas has been referred to as a game-changer, which has the potential to make North America energy independent and the refining and petrochemical sectors more cost competitive.
Many refinery operations and petrochemical processes require large quantities of hydrogen (H2) and synthesis gas (carbon monoxide [CO] and H2, also known as syngas). The dominant method of producing H2 and syngas is steam reforming of light and medium hydrocarbons. This technology will become even more popular in the future with the advent of relatively cheap natural gas.
The historic and future supply, demand and pricing for US natural gas in a recent Department of Energy study is shown in Figure 1. This graph shows total natural gas production in the US, where shale gas production will increase from 5 trillion cu ft in 2010 to 13.6 trillion cu ft in 2035, representing an annual increase of 4.1% per annum. This growth in natural gas production will make the US a net exporter in the early part of next decade. The price of natural gas is expected to increase to ~$7.4/MMbtu (2010 dollars) in 2035 based on supply, demand and incremental cost of production. It is expected that natural gas will be relatively cheap compared to other fuels on an energy equivalent basis. Based on these projections, it is expected that the dominant method of producing H2 and syngas is set to remain steam methane reforming.
Petrochemical and refining
The refining and petrochemical sectors consume large quantities of H2, CO and syngas, which are collectively known as HYCO. Hydrogen is used for refining crude oil, which is increasingly becoming heavier and more sour. Also, ever more stringent environmental regulations will drive H2 demand. In addition, H2, CO and syngas are used in various chemical synthesis reactions. A summary of the major industrial gases applications in refining and petrochemicals is shown in Table 1.
As noted previously, the dominant method of producing H2, CO and syngas is steam reforming of natural gas. H2, CO and syngas plants require a hydrocarbon feedstock such as natural gas or naphtha and utilities such as water, electricity and nitrogen. The outputs are H2, CO, syngas and varying amounts of steam. Given that these plants are designed to last decades, it is very important to understand the current and future costs of feedstocks. This is essential to make the appropriate trade-off between capital cost and feedstock consumption.
In particular, consumers of tonnage quantities of industrial gases have to make informed investment decisions in an uncertain economic environment. Plant owners/operators need to examine carefully their HYCO needs in order to develop the most cost-effective plant configuration for a steam reformer. This can be done by partnering with an experienced engineering company that owns a complete technology portfolio for producing H2, CO and syngas. Ideally, the engineering and manufacturing companies have to work together as a single team to identify the most efficient and economic plant design. This includes an in-depth assessment of capital and operating costs, taking plant reliability and process safety into account in order to deliver an optimal solution. It is important to consider all the industrial gases as well as steam and utility needs at a given manufacturing complex, in conjunction with available feedstock and wastes to develop the best possible long-term solution. All of these industrial gas requirements have to be examined over a long-term horizon in a holistic manner.
Steam reforming of hydrocarbon feedstock to produce H2, CO and syngas has been the dominant process for producing H2, CO and syngas for over 80 years. A simplified block flow diagram of the steam methane reforming process showing how the reformer fits into the overall plant is depicted in Figure 2.
Conversion of steam/hydrocarbon mixtures is carried out in catalyst-filled tubes heated from the outside for the production of syngases rich in H2 and CO. Feedstock can be methane, ethane, LPG or naphtha. The basic design criteria for reformers are burner design and configuration, heat recovery from the hot flue gases and furnace control. A variety of design options with burners arranged in the ceiling or at the sidewalls of the furnace can be considered. The blue-shaded steam reforming block in Figure 2 represents the furnace’s radiant section, where the tubes receive most of the input heat by radiation. In the convection section, heat is recovered in both the process streams and steam system by convective heat transfer. Some other considerations are:
• Desired operating temperature and pressure
• Amount of export steam
• Feed flexibility for natural gas, LPG and heavy naphtha
• Burners for low NOx generation and low noise generation
• Selective catalytic reduction for NOx control, if required
• Flue gas heat recovery by preheating feed, water and generating steam
• Fully automatic reformer using advanced control technology.
For large installations, the top-fired design combines both process and physical benefits to minimise the total cost of ownership. The top-fired design allows either modularised or stick-built construction, taking the relative differences in shop assembly versus field construction costs into account. Furthermore, this configuration results in a compact firebox, leading to fewer burners relative to the number of tubes. The smaller surface area per unit volume when compared with sidewall-fired reformers minimises heat loss and reduces construction time. The main advantages of a top-fired design are.
• Easy access to burners
• Simplified combustion control with fewer burners
• Heat recovery flexibility (omega, horizontal, vertical orientations available)
• Maximum reforming efficiency
• Highest heat flux due to co-current flow.
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