Carbon dioxide systems system leakages

Carbohydrazide itself is of very low volatility, but it decomposes at relatively low temperatures to produce volatile carbon dioxide and ammonia. In theory, the combined corrosive effects of these two materials should be negated in the condensate system, but in practice, this is not always so and both steel and copper corrosion transport problems may develop, primarily as the result of corrosion-enhancement reactions resulting from oxygen in-leakage. It is presumed, therefore, that (similar to hydrazine) some deliberate after-desuperheating line addition of CHZ is necessary if post-boiler section corrosion is to be avoided. 

The FW oxygen scavengers mentioned earlier are volatile and can therefore provide additional protection against post-boiler section corrosion induced or enhanced by oxygen in-leakage. These particular scavengers also break down under pressure to produce some level of volatile ammonia, which can neutralize any carbon dioxide found in the steam-condensate system. 

In the shift conversion step, carbon monoxide reacts with steam to form equivalent amounts of hydrogen and carbon dioxide. Upon cooling of the effluent gas, most of the unreacted steam is condensed and separated as process condensate. Modem ammonia plants utilize a two-step, in-series shifting, carried out at high and then low temperatures to increase conversion efficiency. Use of the dual-shift conversion system lowers overall plant steam requirements, and the lower CO leakage results in reduction in plant feed requirements due to more complete conversion of CO to hydrogen. Under normal operating conditions there is no emission from the shift converters. 

Pure ethylene oxide is cheaper than gas mixtures. At one time it was used undiluted, but it is no longer possible to have this practice underwritten for insurance purposes. All existing processes, whether using pure ethylene oxide plus a diluent or using a gas mixture, operate at a positive pressure to the atmosphere. Any leakage of gas from the chamber must therefore be toward dilution in the external environment rather than toward formation of an explosive mixture in the chamber. Gas mixtures with fluorinated hydrocarbons or carbon dioxide require higher operating pressures to achieve the same sterilant concentrations as diluted pure ethylene oxide systems. 

When condensate returns to a boiler, it may be contaminated by pipeline corrosion or with process materials. Likely sources of the latter include exchanger leakage and poorly installed process tie-ins. Carbon dioxide and oxygen are the chief causes of corrosion in steam and condensate systems. Section 12.4.3.3 discusses the treatment of boiler feed water to remove these contaminants, as well as hardness. 

---