Many hydrocarbon process systems involve water-sensitive processes. For example: refrigeration systems, parts of
ethylene plants, parts of LPG plants, LNG plants etc.
The low operating temperature of the above processes, and the potential for hydrate formation in the event of water contamination, require stringent drying criteria. In addition, the actual nature of the products within some processes causes an unwanted reaction in combination with the presence of water (e.g. alkylation plants, catalytic reactors, ammonia plants etc.)
For the above reasons, following construction or a shutdown maintenance workscope, and prior to re-introduction of hydrocarbons, drying services would need to be employed due to water presence within the process systems, perhaps caused by hydrotesting, flushing and water jetting, chemical cleaning or simply an open system being subject to humid ambient air.
Dependent upon the complexity of the process system and the dewpoint requirements, various methods of drying services are available:
Liquid nitrogen is supplied with a dewpoint of less than –65°C, consequently nitrogen drying is often used for systems requiring a very low dewpoint. Liquid nitrogen is vaporised and pumped into the system via pre-determined flow paths and, as it passes over wet areas, the dry nitrogen picks up moisture until it becomes saturated. The wet nitrogen is vented-off in a safe area.
This method of air drying uses standard air compressors in combination with downstream pressure-swing absorption dryers (PSA). Individual compressors can provide up to 2,000 m3/hr of air and the PSA dryers can reduce the dewpoint of the air to lower than –40°C. This method of drying would be used where a large volume of free water remains within the system to be dried. In order to remove the free water to the vent, the system would be pressurised with dry air and depressurised very rapidly, via the vents, gradually moving the free water.