Cryogenic Rectification

One of the most important milestones in the history of industrial air separation was the introduction of the soolled double column for the distiUative separation of air into its components under cryogenic conditions. Even today this principle is stiU applied in numerous variations in most of the cryogenic air separators. A frequently applied process is going to be introduced in the foUowrng by example. It is built this way or similarly by aU commercial vendors of air separation units. Elementary concepts of process technique will be applied to explain the key features of the air separator, such as concentration profiles in the columns, argon production or refrigeration. A separate section describes so-called internal 

1 Process with Air Booster and Medium-Pressure Turbine for the Recovery of Compressed Oxygen, Nitrogen and Aigon 

The process (Fig. 2.3A) is designed for delivering products as described in Table 2.5. 

A plant based on such a process with a production capacity of 45 000 m h of oxygen typically looks like the one shown in Fig. 2.3B. Compressed gaseous oxygen is often supplied exclusively to a bulk consumer, for example a steel mill. 

The planf s liquid production is stored in tanks. During a plant stop this Hquid is evaporated to guarantee the continuous gas supply of the bulk consumer and it is also distributed by trucks to smaller consumers in the local area. 

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Table 2.2 Application range of membrane separation, pressure swing adsorption and cryogenic rectification.

Kr/Xe-fine-purification plants are available in numerous designs, combining the separation techniques cryogenic rectification, catalysis, adsorption, chemisorption or membrane separation. They may be operated continuously or, due to the small capacities, in batch mode. 

In the following a process will be described that purifies krypton and xenon by means of cryogenic rectification in five columns. 

Rectification is accomplished in room temperature devices simply by inserting an element into an ac circuit that will offer a large impedance to current flowing in one direction, and a relatively small impedance to current in the other direction. In cryogenic devices, this can be done by superconductor switching. 

The rectification occurs at low temperatures between 80 K and 100 K. Therefore air must be cooled down prior to rectification and the gaseous products must be warmed up afterwards. This process is associated with cryogenic losses which have to be compensated by performing additional work on the system. Losses... 

The separation of close boiling components in a mixture has been accomplished commercially by cascading several evaporations and condensations of such mixtures in a device known as a distillation or rectification column. There are two types of columns, namely, packed columns and plate columns. The former are vertical cylinders filled with a variety of packings that provide a large surface area per unit volume to promote maximum contact between the downward liquid flow and the upward vapor flow. Since this type of column can encounter poorer vapor-liquid contact than the plate column, it is only seldomly used in cryogenic separation systems. 

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