12.3 Hydrogen production technology (Figure 12.1)

Figure 12.1 Flow sheet of Hydrogen Production Technology

• •  Feed purification: Feed pretreated to remove sulphur and halogens traces. H₂S is removed using zinc oxide catalysts. Organic sulphur removed by hydrotreating and H₂S removal from the feed. Halogens are removed using adsorption technique.

•  Furnace reactor: The methane steam reforming reaction occurs in a furnace which is fed with the hydrocarbons mixed with steam. The steam to carbon ratio varies and is about 2.5 – 3.5. The furnace reactor consists of reactor tubes packed in the furnace chamber. The reactor tubes consist of the nickel oxide catalyst for carrying out the reaction.

•  Shift reactors: The furnace reactor product is sent to both high temperature and low temperature shift reactors. It is interesting to note that these reactors operate at low temperatures. Therefore, cooling is carried out for the furnace reactor product and steam is generated. Similarly, steam is generated after the high temperature shift reactor also.

•  Absorber stripper: The absorber stripper is used for CO₂ removal of the product emanating from the low temperature shift reactor.

•  Methanation reactor: Here, traces of the CO₂ and CO are converted to CH₄and H₂O using nickel catalyst and reversible reaction scheme. The product from methanation reaction consists of 97 % H₂.

•  Alternatively, in some modern refineries, the low temperature shift reactor product is fed to a pressure swing adsorption (PSA) unit which produces 99.9 % hydrogen as a main product.

•  The PSA also produces a tail gas which is used as a fuel in the furnace used in the reformer process.

Figure 12.2 Methanation Reactor

• •  PSA technology also is incapable for CO and CO₂ removal as the adsorbents are not competent enough to separate these components to a large extent. But they can effectively do separation when hydrocarbons are present but not oxides in the hydrogen rich stream. Therefore, from PSA perspective as well it is important to remove CO₂ bulk with amine scrubbing followed with methanation reaction for both CO and CO₂ conversion.

12.4 Claus sulphur recovery process technology (Figure 12.3)

Figure 12.3 Flow sheet of Claus Sulphur Recovery Process Technology

 

• •  In a furnace reactor, H₂S is partially oxidized with air to produce water and SO₂. The reaction is highly exothermic. Therefore, steam is generated using the products from the furnace reactor.

•  The remaining H₂S is then sent to a converter at about 250°C to allow the reaction between H₂S and SO₂ and produce Sulphur and water. The emanating product is at 290°C.

•  The second reactor (H₂S to SO₂) is having severe equilibrium limitations. Therefore, it is sent to two to three reactors for maximizing conversion.

•  After each converter, the product stream is cooled and sent to another reactor. Subsequently, Sulphur is removed as a product from the coolers.

•  Finally tail gas is obtained from the last converter which consists of unreacted H₂S, N₂ and O₂.

•  The tail gas requires treatment as well. This is because the gas consists of components such as H₂S, CS₂ etc. The tail gas is fed with air to another burner and converter that converts sulphur compounds to H₂S. The H₂S thus generated is separated using amine scrubbers. The H₂S thus recovered is sent as a recycle stream to the partial oxidation reactor.