Randy Moore, Siemens Energy, USA, outlines the benefits of retrofitting ammonia plant compressors with modern dry gas seals.
As global demand for ammonia grows, producers are increasingly looking for ways to boost capacity and extend the life of their existing facilities. Steam turbines and compressors have become a logical target of modernisation initiatives, largely due to the significant impact they have on overall plant efficiency, production capacity, reliability, and operating costs (OPEX).
Particularly in older plants, the large, multi-stage compressors used to supply air to the secondary reformer can become a production bottleneck. This typically occurs in summer months when ambient temperature is high and air density is reduced. Retrofitting these machines with modern dry gas seal (DGS) technology provides a path to increase throughput by reducing leakage and improving volumetric efficiency – without requiring full equipment replacement or large capital expenditures (CAPEX).
DGS technology can also be applied to existing syngas, recycle, and refrigeration compressors to improve efficiency and eliminate the risk of oil entering the process gas, which can damage downstream catalysts and result in costly replacement and downtime.
Understanding compressor dynamics
The secondary reformer follows the primary reformer in the Haber-Bosch ammonia production process. Its main purpose is to introduce process air to supply nitrogen for ammonia synthesis, partially combust residual methane, hydrogen, and CO with oxygen from the air, and raise the gas temperature to complete methane reforming over a downstream catalyst.
Like the primary reformer, the secondary reformer operates at high pressure (typically 20 - 40 bar). To inject air into the environment, it must be compressed to a pressure higher than that of the reformer. Large, multi-stage centrifugal compressors are preferred for this application. With these machines, volumetric flow is primarily determined by rotational speed and impeller design. However, the mass flow (which is what matters for the chemical reactions in an ammonia plant) depends on the density of the air.
Consequently, in summer months when ambient temperature is high, it is not uncommon for ammonia production rates to decline, as there is reduced air mass flow through the compressor. To compensate, some plants will bring in temporary compressors (often diesel or gas-driven). However, this increases operating expenses (e.g., cost of fuel, rental fees, maintenance, etc.) and Scope 1 emissions. It can also be challenging from a logistical standpoint.
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