水煤气变换反应1780

水煤气变换反应由意大利物理学家费利切·丰塔纳于1780年发现。 但直到很久以后，人们才认识到该反应的工业价值。20世纪初之前，氢气是通过高压蒸汽与铁反应生成氧化铁和氢气而制取的。随着需要氢气的工业工艺的发展，人们需要一种更经济、更高效的制氢方法。为了解决这个问题，人们将水煤气变换反应与煤气化相结合来制氢。

The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:

CO + H2O ⇌ CO2 + H2

The water gas shift reaction was discovered by Italian physicist Felice Fontana in 1780. It was not until much later that the industrial value of this reaction was realized. Before the early 20th century, hydrogen was obtained by reacting steam under high pressure with iron to produce iron oxide and hydrogen. With the development of industrial processes that required hydrogen, such as the Haber–Bosch ammonia synthesis, a less expensive and more efficient method of hydrogen production was needed. As a resolution to this problem, the WGSR was combined with the gasification of coal to produce hydrogen.

Applications

The WGSR is a highly valuable industrial reaction that is used in the manufacture of ammonia, hydrocarbons, methanol, and hydrogen. Its most important application is in conjunction with the conversion of carbon monoxide from steam reforming of methane or other hydrocarbons in the production of hydrogen. In the Fischer–Tropsch process, the WGSR is one of the most important reactions used to balance the H2/CO ratio. It provides a source of hydrogen at the expense of carbon monoxide, which is important for the production of high purity hydrogen for use in ammonia synthesis.

The water–gas shift reaction may be an undesired side reaction in processes involving water and carbon monoxide, e.g. the rhodium-based Monsanto process.

Reaction conditions

The equilibrium of this reaction shows a significant temperature dependence and the equilibrium constant decreases with an increase in temperature, that is, higher hydrogen formation is observed at lower temperatures.

Temperature dependence

With increasing temperature, the reaction rate increases, but hydrogen production becomes less favorable thermodynamically since the water gas shift reaction is moderately exothermic; this shift in chemical equilibrium can be explained according to Le Chatelier's principle. Over the temperature range of 600–2000 K, the equilibrium constant for the WGSR has the following relationship:

Practical concerns

In order to take advantage of both the thermodynamics and kinetics of the reaction, the industrial scale water gas shift reaction is conducted in multiple adiabatic stages consisting of a high temperature shift (HTS) followed by a low temperature shift (LTS) with intersystem cooling. The initial HTS takes advantage of the high reaction rates, but results in incomplete conversion of carbon monoxide. A subsequent low temperature shift reactor lowers the carbon monoxide content to <1%. Commercial HTS catalysts are based on iron oxide–chromium oxide and the LTS catalyst is a copper-based. The copper catalyst is susceptible to poisoning by sulfur. Sulfur compounds are removed prior to the LTS reactor by a guard bed.

Mechanism

The WGSR has been extensively studied for over a hundred years. The kinetically relevant mechanism depends on the catalyst composition and the temperature. Two mechanisms have been proposed: an associative Langmuir–Hinshelwood mechanism and a redox mechanism. The redox mechanism is generally regarded as kinetically relevant during the high-temperature WGSR (> 350 °C) over the industrial iron-chromia catalyst. Historically, there has been much more controversy surrounding the mechanism at low temperatures. Recent experimental studies confirm that the associative carboxyl mechanism is the predominant low temperature pathway on metal-oxide-supported transition metal catalysts.

Reverse water–gas shift

In the conversion of carbon dioxide to useful chemicals, fuels, and materials, the water–gas shift reaction is used to produce carbon monoxide from hydrogen and carbon dioxide. This is often called the reverse water–gas shift reaction (RWGS), and is generally differentiated from the water-gas shift process by the use-cases. Catalyst development for the RWGS reaction are researched in academia, with companies such as Dimensional Energy leading catalyst commercial efforts in a process that converts carbon dioxide to products such as sustainable aviation fuel via RWGS in tandem with the Fischer Tropsch reaction.

Water gas is defined as a fuel gas consisting mainly of carbon monoxide (CO) and hydrogen (H2).