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Tag: Urea

Wastewater treatment in a urea plant

A detailed rain and wastewater concept is an important part of a urea plant to meet current stringent environmental standards. Wastewater can originate from the process reaction or from outside the process equipment. In order to optimise the wastewater system of a urea plant, both the amount and type of contamination need to be known. With this knowledge, non-contaminated rainwater as well as process drains can be kept separate as much as possible to minimise the amount of wastewater to be treated, saving costs and energy. Wastewater treatment concepts from thyssenkrupp Industrial Solutions and Toyo Engineering Corporation are described.

Problem No. 63: Holding reactor content during a long shutdown

Every urea plant continuously fights again corrosion. The intermediate product ammonium carbamate is extremely corrosive under synthesis conditions. The applied materials of construction require oxygen to form a protective passive layer of chromium oxides. The ammonium carbamate solution will continuously dissolve the passive layer, therefore it is vital to continuously supply oxygen, typically in the form of air, to maintain the passive layer. During blocking-in conditions of the synthesis section it is not possible to add air and the oxygen present will be consumed as a result of the passive corrosion reactions, while at the same time the passive layer dissolves in the ammonium-carbamate solution. At a certain point, the oxygen content in the solution becomes too low to assure a passive layer. At that moment active corrosion will start with much higher corrosion rates than passive corrosion. The picture on the left side shows the passive layer (blue, brown, grey surface) and the picture on the right side shows active corrosion (a shiny silver surface). It is important to realise that once active corrosion starts it cannot be stopped, adding more oxygen at this stage, for example, will not work. Active corrosion will continue, leading to the risk that the protective layer will be severely damaged. The only way to solve this situation is to drain the synthesis section and re-passivate the surfaces.

Redefining the sustainability of effluent treatment processes

Nitrogen-rich wastewaters remain a major issue for fertilizer and other industries. Saipem’s new electrochemical technology, SPELL, is an important step towards the overall objective of zero industrial pollution. A complete engineering review of the technology, its alignment with all international applicable standards, and optimisation has now been concluded and the technology is ready for deployment for the removal of ammonium nitrogen from industrial waters and wastewaters. Saipem discusses the key features of SPELL and reports on the first two industrial references.