At the outlet of steam reformers, partial oxidation reactors or coke oven gas units the syngas contains H2, CO, CO2, CH4 and water in chemical equilibrium at high temperatures in the range of 700 to 1400 °C depending on the process pressure and the mixture of feed stock and process steam or water.
By means of the CO shift conversion an important portion of the CO content in the cracked gas is used for additional hydrogen generation, which is following the chemical reaction
CO + H2O <=> H2 + CO2
This process is exothermic and is limited by the chemical equilibrium.
There are three different versions of CO shift conversion:
High temperature (HT) CO shift conversion at about 300 to 450 °C down to approx. 2.5 % CO on dry basis at the reactor outlet
Medium temperature (MT) CO shift conversion or so-called isothermal shift conversion at about 220 to 270 °C down to approx. 0.5 % CO on dry basis at the reactor outlet.
Low temperature (LT) CO shift conversion at about 180 to 250 °C down to approx. 0.2 % CO on dry basis at the reactor outlet
For every process a special catalyst will be used in a fixed bed reactor to get maximum yield of H2 product.
The use of HT CO shift conversion is state of the art in almost every hydrogen plant.
The application of the low temperature CO shift conversion is normally installed downstream of the HT shift at already reduced CO content in the feed gas.
The additional investment is considered for plants having a capacity above approx. 40,000 Nm³/h H2 product. The catalyst of the LT shift is very sensitive against sulphur, chlorine and liquid water and special attention is required during start up and plant upset conditions.
In former concepts the LT shift was important because of the downstream methanisation of CO following the CO2 removal unit, to meet the product purity at minimum H2 losses. After the implementation of the Pressure Swing Adsorption (PSA unit) for H2 purification these process steps have become obsolete.
The MT CO shift conversion as isothermal reaction can be approximated in several adiabatic reactors with intercoolers or better in an isothermal reactor with integrated steam generation for cooling of the process gas. The temperature of the shift reaction is controlled easily by setting the pressure of the generated steam.
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