Low temperature gas stripping techniques

• Low temperature condensation (LTC) is an isobaric process of cooling to the temperature at which the liquid phase is formed at the given pressure. In order to separate hydrocarbon gases using LTC, they must be cooled to the given temperature. This leads to the condensation of the gas components which are then separated from the gaseous phase. It is hardly feasible to provide a distinct separation of hydrocarbon gases by means of a single-staged condensation and subsequent separation. That is why the up-to-date LTC schemes include a de-methanization/de-ethanization tower. Here the gaseous phase is taken off the last separation stage, and the liquid phase from the heat exchanger (after the heat exchange with the raw gas stream) is fed to the de-methanization/de-ethanization tower. In this case, the rectification, as a rule, is designed for the separation of the remained dissolved gas from the liquid phase.
• Low-temperature rectification (LTR) is based on the cooling of the raw gas to the temperature at which the gas system undergoes a phase transition and becomes a two-phase system, with the subsequent separation of the formed gas/liquid mixture without any pre-separation in the tray-type or packed rectifying tower. As compared with LTC, LTR makes it possible to separate hydrocarbon mixtures yielding purer individual hydrocarbons or narrower fractions.
• Low-temperature absorption (LTA) uses the difference between the gas-in-liquid solubilities at low temperatures for individual gas components. The absorption is followed by the desorption of the extracted components in the desorbers employing the full rectification scheme. The advantage of LTA over LTR can be defined as the ability to separate hydrocarbon gases at moderate temperatures. As the source of cold, propane evaporators can be used for example, which are not capable enough for LTR. But, the accuracy of the gas components separation in LTA is lower than in LTR.
• Low-temperature adsorption (LT-adsorption) uses the difference between the adsorption capabilities for individual gas components. These processes are generally employed to extract gas components with very low partial pressures, which can hardly be extracted from the gas stream by any other technique. In contrast to all the other low-temperature gas separation processes, these processes show high selectivity. At the same time they are rather costly and require efficient heat takeoff and efficient process control. Accordingly, they are only used to produce high purity products, for example to remove trace contaminants from helium.

So, all the listed low-temperature processes, due to their special features, can be used comprehensively at various gas processing stages, particularly where a wide variety of articles is produced.