High-temperature steam reforming designs

Using the materials of construction and mechanical design procedures which are typical of the late 1970 s, a conventional high temperature steam reformer would have operating and design parameters approximately as given in Table 2 for a 10 million scf/day hydrogen plant. 

Based on the type of thermal destruction process selected, there are several different commercial designs and configurations of the reactor that have been utilized for a particular application. Some of the most commonly used technologies include rotary kilns, starved air incinerators, fluidized beds, mass-bum incinerators, electrically heated reactors, microwave reactors, plasma, and other high-temperature thermal destruction systems. Recent advances include gasification and very high temperature steam reforming. 

The subsequent steam reforming section is operated at very high temperatures 850-900 °C. The SMR catalysts themselves are already active below 400 °C, but high temperatures are necessary to drive the strongly endothermic reaction forward [8]. In industry, nickel catalysts are used in high-alloy reaction tubes, which are heated by external burners. This design is expensive and leads to heat losses, although much of the heat is recuperated. Noble metal catalysts such as sup-... 

Ce02-supported noble-metal catalysts such as Pt, Pd and Rh are of interest because of their importance in the so-called three-way converter catalysts (TWC), designed to reduce emissions of CO, NOx and uncombusted hydrocarbons in the environment and to purify vehicle-exhaust emissions. Such catalysts are also of current interest in steam reforming of methane and other hydrocarbons. Conventional practical catalysts for steam reforming consist of nickel supported on a ceramic carrier with a low surface area and are used at high temperatures of 900 C. This catalyst suffers from coke formation which suppresses the intrinsic catalyst activity. Promoters such as Mo are added to suppress coke formation. Recently, Inui etal(l991) have developed a novel Ni-based composite... 

Description Natural gas or another hydrocarbon feedstock is compressed (if required), desulfurized, mixed with steam and then converted into synthesis gas. The reforming section comprises a prereformer (optional, but gives particular benefits when the feedstock is higher hydrocarbons or naphtha), a fired tubular reformer and a secondary reformer, where process air is added. The amount of air is adjusted to obtain an H2/N2 ratio of 3.0 as required by the ammonia synthesis reaction. The tubular steam reformer is Topsoe s proprietary side-wall-fired design. After the reforming section, the synthesis gas undergoes high- and low-temperature shift conversion, carbon dioxide removal and methanation. 

After the secondary reformer of steam reforming plants the gas has to be brought down from around 1000 °C to about 350 °C for the HT shift. In earlier-generation plants two boilers were usually installed in series, with a bypass around the second to control the inlet temperature for the HTS. Common practice for a long time was to use a water-tube design. A famous example is the Kellogg bayonet-tube boiler, applied in more than 100 plants. Because of size limitations two parallel units were installed. For sufficient natural water circulation these boilers needed a steam drum at a rather high elevation and a considerable number of downcomers (feed water) and risers (steam/water mixture). 

Key features are the high reforming pressure (up to 41 bar) to save compression energy, use of Uhde s proprietary reformer design [1084] with rigid connection of the reformer tubes to the outlet header, also well proven in many installations for hydrogen and methanol service. Steam to carbon ratio is around 3 and methane slip from the secondary reformer is about 0.6 mol % (dry basis). The temperature of the mixed feed was raised to 580 °C and that of the process air to 600 °C. Shift conversion and methanation have a standard configuration, and for C02 removal BASF s aMDEA process is preferred, with the possibility of other process options, too. Synthesis is performed at about 180 bar in Uhde s proprietary converter concept with two catalyst beds in the first pressure vessel and the third catalyst bed in the second vessel. 

The most common industrial method to make ultra-pure hydrogen is by steam-methane reforming (SMR) using a catalyst at the temperature 890-950° C. The reformed gas is then subjected to a high temperature water gas shift (WGS) reaction at 300-400°C. The WGS reactor effluent typically contains 70-80% H2, 15-25% CO2, 1-3% CO, 3-6% CH4, and trace N2 (dry basis), which is fed to a PSA system at a pressure of 8-28 atm and a temperature of 20 0°C for production of an ultrapure (99.99+ mol%) hydrogen gas at the feed pressure. Various PSA systems have been designed for this purpose to produce 1-120 million cubic feet of H2 per day. 

Residual methane is present at the exit of the combustion zone. In the catalytic bed, the methane steam-reforming and the water shift reactions take place. The gas leaving the ATR reactor is in chemical equilibrium. Normally, the exit temperature is above 900-1100°C. The catalyst must withstand very severe conditions when exposed to very high temperatures and steam partial pressures. One example of an ATR catalyst is nickel supported by magnesium aluminum spinel. For compact design, the catalyst size and shape is optimized for a low pressure drop and high activity. 

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