| 摘要 |
Difficulties associated with the operational stability of a conventional cryogenic plant for manufacturing ammonia synthesis gas from a steam rich in hydrogen and a stream of nitrogen are reduced by using three heat exchangers in place of the conventional single heat exchanger. In use, the first heat exchanger, which is preferably arranged with its cold end uppermost, is used to cool the stream of nitrogen to a temperature which: (a) if the nitrogen is above its critical pressure is between 8 and 28.degree.C above its critical temperature; or (b) if the nitrogen is below its critical pressure is between 3 and 9.degree.C above its dew point. Part of the nitrogen leaving the first heat exchanger is passed downwardly through a third heat exchanger where it is further cooled before being introduced into a nitrogen wash column where it acts as reflux. The third heat exchanger is cooled by vapour leaving the top of the nitrogen wash column. After being cooled in the first heat exchanger the stream rich in hydrogen is passed upwardly in a second heat exchanger where it is further cooled before being introduced into the lower portion of the nitrogen wash column. This further cooling is achieved by: (i) the balance of the vapour leaving the top of the nitrogen wash column; and (ii) vapourising the two phase mixture formed by expanding the liquid in the bottom of the nitrogen wash column; and (iii) adding external refrigeration if necessary. The overall layout has the advantage that the stream of nitrogen is passing downwardly in the third heat exchanger which operates in the temperature range where the density of the nitrogen increases rapidly as its temperature falls. In addition the largely liquid two phase mixture from the bottom of the nitrogen wash column vapourises as it passes downwardly through the second heat exchanger thereby reducing the problems associated with slug flow. Although three heat exchangers are used the overall capital cost of the installation is comparable with a conventional installation. This is because certain phase separators and distributors can be omitted. A modification is also described which facilitates recovery of most of the small amount of hydrogen in the liquid leaving the bottom of the nitrogen wash column. |
