Steam Methane Reforming
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, alumina, magnesium aluminate, or calcium aluminate titanate. Temperature of the outlet gas is between 800 and 870°C and outlet pressures are usually between 300 and 350 psig. The outlet gas composition correlates a 0°C to 25°C temperature approach to steam-reforming equilibrium. Temperatures of the flue gases exiting the convection section of the furnace reach 980-1040°C.
Greater efficiency is achieved by reclaiming this heat and using it to heat other process such as the hydrocarbon feed before sulfur removal, the feed mixture entering the radiant section of the furnace, the combustion air for the radiant section burners, and for heating or superheating steam. The primary reformer can achieve up to a 95% conversion of CH4, and this step results in a hydrogen concentration of about 76. 7%. After exiting the primary reformer, the gases enter the shift conversion section for the gas shift reaction step which will convert the CO into CO2 and hydrogen.
The reaction begins in the high temperature shift (HTS) reactor at about 370° at the inlet on a chromium-promoted iron oxide catalyst. The gasses are converted and exit the HTS to be cooled to 200-215°C before being sent to the low temperature shift (LTS) converter to complete the water gas shift reaction over a copper-zinc oxide catalyst supported on alumina. This step is completed at as low a temperature as possible in order to operate at the most favorable equilibrium constants. The gas is then cooled and the heat recovered for other use.
The gas shift reaction step produces gas with a H2 concentration of about 77%. The last steps for low purity hydrogen include the CO2 scrubber and methanation. The CO2 is scrubbed out by hot potassium carbonate or one of several other processes. The gases are then reheated to 315°C and passed over a methanation catalyst of nickel on silica to convert the remaining carbon dioxides to methane. CO and CO2 are hydrogenated to CH4 on this catalyst before the outlet gasses are cooled and any entrained water is separated.
The result is a hydrogen product with a concentration of about 98. 2% H2. When high purity hydrogen is needed the shifted gas can be purified by pressure-swing adsorption (PSA) instead of the CO2 scrubbing and methanation. Pressure-swing adsorption purification separates the hydrogen gas from the other, larger, molecules by selective adsorption through the use of a molecular sieve. Because hydrogen has a very weak affinity for adsorption, the PSA process produces very pure hydrogen and, with increased adsorption stages, at recoveries up to 90%.
This process operates at room temperature in a pressurization-depressurization cycle. The desorption process is endothermic and the adsorption process is exothermic. The adsorption bed is then depressurized and purged with pure hydrogen to accomplish regeneration. The PSA system concentrates to 99% H2 and offers safer, more reliable hydrogen production with efficient heat recovery and reduced production costs in exchange for increased feed requirements and a larger reformer furnace. Large scale production of hydrogen does have environmental impacts.
The generation of synthesis gas is the primary area requiring environmental controls. Different processes require different controls, with the methane steam reforming process being the most environmentally friendly. Environmental concerns regarding hydrogen manufacturing can be minimized by using natural gas reforming and recovering hydrogen as a by-product. Concerns for coal feedstocks stem from potential particulate emissions and require careful handling of condensate streams, ash, and slag.
Concerns over partial oxidation of heavy liquid hydrocarbons are eased by scrubbing the soot from the raw synthesis gas for recycling or recovery. The potential pollutants produced are less than with coal gasification although the sulfur and condensate treatments are generally similar. The cleanest of the synthesis gas generations is reforming of natural gas or naphtha. The low levels of sulfur in most natural gasses can be removed in a fixed-bed adsorption system. Higher levels of sulfur are treated in solvent adsorption-stripping systems for acid gas removal.
Naphtha’s organic sulfur compounds are usually hydro treated and stripped as hydrogen sulfide before residual sulfur is removed in a fixed-bed system similar to use for natural gas before reforming. Process condensate is treated by steam stripping, a process that has become more environmentally conscious in recent years. Hydrogen gas also has health and safety factors that must be considered. While hydrogen gas itself is not toxic, it can cause suffocation by the exclusion of air. Its extreme flammability in oxygen or air presents the largest danger when working with liquid and gaseous hydrogen.
Hydrogen is difficult to detect if released, partly due to it being odorless, colorless, and burning with a nearly invisible flame. Hydrogen can be explosive in mixtures with fluorine and has much wider detonation and flammability limits when mixed with air than either gasoline or methane. There are mandatory regulations governing the safe distribution, handling, and use of both liquid and gaseous hydrogen. The steam methane reforming process for producing chemical hydrogen is the most common manufacturing process for hydrogen and is vital to the petroleum refining industry.
Steam methane reforming desulfurizes light hydrocarbon feeds and converts them into synthesis gas which is then purified by CO2 scrubbing and methanation or through pressure-swing adsorption purification depending on the level of purity desired. Natural gas or naphtha reform is the most environmentally friendly and cost effective of the processes available to produce hydrogen. The health and safety concerns for hydrogen are mostly due to its flammability in oxygen or air combined with difficulties detecting a hydrogen spill.
Large scale hydrogen production can have environmental impacts, but when federal and local regulations are followed the environmental effects are minimized.