Naphtha is a liquid fraction obtained from the petroleum refinery, it contains high molecular carbon and hydrogen compounds. Fertilizer and petrochemical industries use naphtha as raw material for the generation of hydrogen and CO components which in turn be used for other chemical products manufacturing. A process technology presented here briefs the hydrogen generation from naphtha.
Unit process requirements for hydrogen generation process plant
Raw naphtha contains harmful catalyst poison compound built with sulfur. During the catalytic reaction, they deactivate the catalyst active sites. So two-step naphtha sweetening process is usually required based on optimization of the catalyst life and sulfur adsorption on the packed bed of the absorber. The two process units are:
- Pre-desulfurization process
- Final desulfurization process
After sulfur removal, naphtha is vaporised and high molecular compounds are broken down with help of steam at high temperatures. Reforming unit process is used for this operation.
The outlet process stream of the reformer contains H2, carbon monoxide, CO2 and water. Carbon monoxide when reacted with steam, it gives out hydrogen and carbon dioxide. For this operation, High temperature shift reaction process is used.
Finally, a recovery unit is used to retain the escaped hydrogen along the carbon dioxide, steam and methane. A pressure swing adsorption process best suits this operation.
Process flow-sheet of hydrogen generation from naphtha
- Process description of hydrogen generation from naphtha process
Sour naphtha is pumped from surge drum to the vaporizer and heated to 340˚C in a superheater (fire heater). Makeup hydrogen recovered from PSA process is mixed with naphtha feed line. With 24.5 kg pressure of vaporised naphtha is fed to the predesulfuriser to remove the sulfur compounds in the form of hydrogen sulphide. The outlet stream of the predesulfuriser is cooled with inlet stream. Cooled naphtha is injected to the separation column to remove the hydrogen sulfide.
Sweet naphtha is pumped to the vaporizer and 390˚C is attained by the furnace and passed to reformer convection section to heat up. With 30.8 kg/cm2 pressure, the process stream is sent to final-desulfurizer to convert sulfur to H2S, Co-Mo oxide catalyst is used in the reactor.
Superheated steam with the temperature of 379˚C is mixed with 377˚C process gas stream and increased to the temperature of 520˚C in a superheater (furnace). Reformer tube filled with nickel tube is heated by the side burner and process gas along with steam is passed into them. Reforming reaction takes place and hydrogen, carbon-monoxides and carbon-dioxide are produced at the 800-860˚C temperature and 24.6 kg/cm2 pressure.
Carbon monoxide present in the reformed gas combines with steam in high-temperature shift reactor, packed with iron and chromium oxides to that enhances the water-gas shift reaction. Hydrogen production rate is further increased by this process
Finally, to recover the hydrogen, the process gas is sent to PSA (pressure swing adsorption). CH4, CO, CO2 are adsorbed on molecular sieves. Hydrogen yield is improved by recovering it from these gases.
Research conclusions regarding hydrogen generation from naphtha:
Studies on the advancement of hydro desulfurization catalyst conclude that use of chelating agents is must to improve the catalyst active phase by thermal stability.
Important factors it contributes to design a novel catalyst are
- Pore and acid distribution
- Metal and nanometer carrier interaction
Some of the new hydro-desulfurization catalyst under research that can be used for both load and unload type operations:
- Phosphide based silicon aluminum sieves
- Carbide on metallic oxides
- Nitride with silica molecular sieves