Cycle ammonia synthesis

By-product power does not give enough power to match the demand for many processes such as ammonia synthesis, and designs have historically incorporated condensing turbines for incremental power with heat rejection to cooling water. A more effective response is use of the gas turbine combined cycle shown by Figures 5c and 6c. 

The efficiency and costs of the complete power production cycle or well to wheel cycle dictate the technology choices. Industrial hydrogen or ammonia synthesis are relatively high-value products compared with electricity and automotive fuel. 

The present treatment of the conventional process will be based on the process diagram presented in Figure 1, which represents a steam reformer coupled with an ammonia synthesis plant [6], This is one of the two cases, which were considered in the project. The other was the use of the produced hydrogen as fuel in combined cycle gas turbines. In this chapter, the steam reforming part will be treated only, but some comments on the ammonia plant will be made, in view of the composition of the product stream leaving the steam reformer. 

Forced Composition Cycling Experiments in a Fixed-Bed Ammonia Synthesis Reactor... 

The effect of feed composition cycling on the time-average rate and temperature profile was explored in the region of integral conversion in a laboratory fixed bed ammonia synthesis reactor. Experiments were carried out at 400°C and 2.38 MPa over 40/50 US mesh catalyst particles. The effect of various cycling parameters, such as cycle-period, cycle-split, and the mean composition, on the improvement in time-average rate over the steady state were investigated. 

Figure 6. Steady-state (curve 1) and cycle-average (curve 2) reaction rate versus feed composition. Experimental data for ammonia synthesis reaction by Jain, Silveston and Hudgins adapted from Chemical Reaction Engineering, ACS Symposium Ser, 1982, 196, 97-107.

A.K. Jain. P.L. Silveston. and R.R. Hudgins, Forced composition cycling experiments in a fixed-bed ammonia synthesis reactor. Chem. Reaction. Eng. 9 97 (1982). 

A very important modification of the Poly-bed and Gemini process cycles described above can be used to directly produce an ammonia synthesis gas (a N2 and H2 mixture in the molar ratio of -1 3) from SMROG as feed to the PSA systems. 

The proportions of the two reforming reactions and shift conversion are so controlled that the gas mixture obtained contains nitrogen and hydrogen in the mole ratio (volume ratio) of 1 3. However, this mixture still contains 20-30% carbon dioxide resulting from the shift conversion reaction and traces of unconverted carbon monoxide. Carbon dioxide can yield carbonates and carbamates in the ammonia synthesis cycle, which are undesirable because they can deposit in the piping. In addition oxygen, and any of its compounds such as carbon monoxide, water, etc., are also ammonia catalyst poisons [13]. Consequently they must be removed. 

All catalysts, operated either in laboratory or conmiercially, are deactivated during their use. Deactivation is very important in commercial operation because it influences the choice of the operational conditions and fixes the cycle length between regenerations and the total life of the catalyst. Some catalysts remain active for a decade (catalysts for oxidation of SO2 and for ammonia synthesis) whereas others must be regenerated after a few minutes of operation (catalysts for fluidized bed hydrocarbon cracking). 

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