Steam Methane Reforming|
A Hydrogen Manufacturing Process|
S. D. Duplichan|
Hydrogen is an important chemical in petroleum refining and is manufactured most commonly in the United States by steam methane reforming. |
Hydrogen as an industrial chemical is used in petroleum refining and in the synthesis of ammonia and methanol. The two largest industries consuming hydrogen in the United States are petroleum refining and the synthesis of ammonia. The hydrogen needed for refinery operations is produced through either manufacturing or by-product recovery. The largest portion (77%) of industrial hydrogen produced in the United States is manufactured by steam reforming of natural gas. Hydrogen is also produced by steam reforming of naphtha, partial oxidation of oil, coal gasification, and water electrolysis, but these processes together produce a relatively small amount compared to steam reforming of methane. This is, in part, due to steam reforming having the highest thermal efficiency and lowest net production cost of the available processes for producing hydrogen.
Steam reforming involves converting light hydrocarbon feeds into synthesis gas by a reaction with steam over a catalyst in a reformer furnace. Before entering the steam reformer, the hydrocarbon feeds must be desulfurized by processes tailored to the amount of sulfur to be removed. The mixture of gas and process steam is then introduced into the primary reformer with a nickel–based catalyst where a reversible reaction takes place. The water gas shift reaction step then converts the resulting CO to CO2 and hydrogen. After cooling, the CO2 is scrubbed out of the process and remaining carbon oxides are converted to methane through the use of a methanation catalyst. This produces a typical product of 98.2% hydrogen. If a higher purity hydrogen is desired, the shifted gas can be purified by pressure-swing adsorption (PSA) instead of CO2 scrubbing and methanation and will result in a purity greater than 99% pure hydrogen. There are environmental concerns related to these processes, and much attention is given to minimizing the environmental impact of hydrogen manufacturing. Attention must be also be paid to the health and safety factors of hydrogen production, and regulations followed for each.
The first step in the steam methane reforming process is feed preparation. The light hydrocarbon feeds used range from natural gas to straight run naphthas, all of which contain sulfur that must be removed before they enter the steam reformer. If the hydrocarbon feed contains small amounts of sulfur, the first desulfurization step consists of converting organic sulfur compounds to H2S by passing the feed at about 300-400°C over a Co-Mo catalyst in the presence of 2-5% H2. The next step reduces the sulfur level to less than 0.1 ppmwt by adsorption of H2S over ZnO catalyst. If the feed contains several hundred ppm sulfur or higher, bulk removal of H2S uses solvents such as monoethanolamine prior to the ZnO desulfurization step. The effluent from the Co-Mo reactor must be cooled for bulk removal and reheated for the ZnO purification in this case. Once the feeds have been desulfurized the resulting gas and process steam mixture moves on the reaction section of the process.
The reaction of the hydrocarbon feeds and steam over a nickel-based catalyst to produce synthesis gas takes place in a primary reformer furnace. The primary reformer furnace is a direct-fired chamber containing high nickel-alloy tubes arranged in single or multiple rows. The tube alloys are selected according to operating pressure and temperature specifications. The reaction process of hydrogen production is usually operated at 800-870°C and 300-400psig. The catalyst is made up of 5-25% nickel as NiO and usually contains potassium to inhibit coke formation from the use of feedstocks such as LPG and naphtha. The NiO is supported on calcium aluminate,...