12.5 Technical questions
1. How good is conversion in the steam methane reforming reaction ?
Ans: By maintaining good process conditions, the conversion of the methane steam reforming reaction can be achieved to about 60 – 80 % in a single furnace reactor. Therefore, a single furnace reactor is sufficient to achieve the desired conversion.
2. Why methanation cannot be avoided in the hydrogen production process ?
Ans: The trace gases CO and CO2 are reactive towards the catalysts due to the presence of oxygen molecule. Therefore, H2 sent to various other units should be bereft of the CO and CO2 which can poison the catalyst due to the reactive oxygen molecule.
3. Why CO2 bulk removal is targeted in the hydrogen production process ?
Ans: CO2 bulk production if avoided will affect the product quality. The methanation catalyst is not competent to fully convert CO2 back to methane due to the reversibility of the reaction. Therefore, CO2 removal is required.
4. What oppurtunities exist for heat integration in the hydrogen production process ?
Ans: The feed to the process needs pre-heating therefore, is a cold stream. Now this stream can be integrated with the furnace itself where the combustion gases can be used to pre-heat the stream. Other than this the furnace product, high and low temperature shift reactor products are also available. The choice of heat integration and steam generation at various sites in the process is a complex optimization problem and has to be addressed by solving separate problems to finally arrive at the one optimal solution. Mostly, the furnace combustion gases will be integrated for energy recovery with the feed stream and steam generation is effected from the high and low temperature shift reactors.
5. Explain how using three reactors in series will enhance equilibrium conversion of the second conversion reaction i.e, H2S to S in Claus process ?
Ans: Equilibrium conversion is a function of temperature and pressure. There are two types of equilibrium reactions namely exothermic and endothermic reactions. For exothermic reactions, the conversion increases with decreasing temperature. Therefore, after first converter, the product is cooled and sulphur is also removed. The reduction in temperature and elimination of sulphur moves the process conditions to a shift equilibrium to the right hand side. Therefore, series of chemical reactors followed with product cooling will enhance equilibrium conversion. By the by, the same concept is also used in hot and cold shot reactors. The only difference between this arrangement and the shot reactor arrangement is that there direct heat transfer by quenching or heating is allowed where as in the shown flow sheet, indirect heat transfer is facilitated only.
6. Why burner and converter are required in the tail gas processing unit ?
Ans: The burner enhances the temperature of the stream and produce H2 . The produced H2 reacts with sulphur compounds to thereby produce H2S.
References:
1. Gary J.H. , Handwerk G.E., Petroleum Refining: Technology and Economics, Taylor & Francis, 2005
2. Jones D.S.J., Elements of Petroleum Processing, John Wiley & Sons, 1995