. 

Cryogenic air separation is the preferred technology for producing very high purity oxygen and nitrogen. It is most cost effective technology for high production rate plants. All liquefied industrial gas production plants utilize cryogenic technology to produce liquid products.

.

The energy required to operate a cryogenic plant depends on the product mix and required product purities. It requires more power to produce oxygen than nitrogen, and more than twice as much power to produce these products as liquids rather than as gases.

.

All cryogenic air separations consist of a similar series of steps. Variations reflect the desired product mix (or mixes) and the priorities/ evaluation criteria of the user. Some designs minimize capital cost, some minimize energy usage, some maximize product recovery, and some allow greater operating flexibility.

.

The cryogenic air separation flow diagram shown below illustrates (in a generic fashion) many of the important steps in producing nitrogen, oxygen and argon as both gas and liquid products. It does not represent any particular plant.

Steps in Cryogenic Air Separation:

Plants which produce high purity oxygen require more distillation stages and the removal of argon from a point in the low pressure column where its concentration is highest. The removed argon is usually processed in an additional “side-draw” crude argon distillation column that is integrated with the low pressure column. Crude argon may be vented, further processed on site, or collected as liquid and shipped to a remote “argon refinery”. The choice depends upon the quantity of argon available and economic analysis of the various alternatives.

.

Pure argon is typically produced from crude argon by a multi-step process. The traditional approach is removal of the two to three percent oxygen present in the crude argon in a “de-oxo” unit. These small units chemically combine the oxygen with hydrogen in a catalyst-containing vessel. The resultant water is easily removed (after cooling) in a molecular sieve drier. The oxygen-free argon stream is further processed in a “pure argon” distillation column to remove residual nitrogen and uncombined hydrogen.

.

Advances in packed-column distillation technology have created a second argon production option, totally cryogenic argon recovery that uses a very tall (but small diameter) distillation column to make the difficult argon/ oxygen separation. The amount of argon that can be produced by a plant is limited by the amount of oxygen processed in the distillation system; plus a number of other variables that affect the recovery percentage. These include the amount of oxygen produced as liquid and the steadiness of plant operating conditions. Due to the naturally-occurring ratio of gases in air, argon production will be less than 4.4% of the oxygen feed rate (by volume) or 5.5% by weight.

.

The cold gaseous products and waste streams that emerge from the air separation columns are routed back through the front end heat exchangers. As they are warmed to near-ambient temperature, they chill the incoming air. As noted previously, the heat exchange between feed and product streams minimizes the net refrigeration load on the plant and, therefore, energy consumption.

.

http://www.uigi.com/cryodist.html

Commercially, distillation has a number of uses. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from sea water. Air is distilled to separate its components – notably oxygen, nitrogen and argon – for industrial use. Distillation of fermented solutions has been used since ancient times to produce distilled beverages with higher alcohol content..

.

The application of distillation can roughly be divided in four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinals (herbal distillate) and food processing. The latter two are distinct from the former two, in that in the distillation is not used as a true purification method, but more to transfer all volatiles from the source materials to the distillate.

The main difference between laboratory scale distillation and industrial distillation is that laboratory scale distillation is often performed batch-wise, whereas industrial distillation often occurs continuously. In batch distillation, the composition of the source material, the vapors of the distilling compounds and the distillate change during the distillation. In batch distillation, a still is charged (supplied) with a batch of feed mixture, which is then separated into its component fractions which are collected sequentially from most volatile to less volatile, with the bottoms (remaining least or non-volatile fraction) removed at the end. The still can then be recharged and the process repeated.

In continuous distillation, the source materials, vapors and distillate are kept at a constant composition by carefully replenishing the source material and removing fractions from both vapor and liquid in the system. This results in a better control of the separation process.