(384c) Solubility of Hydrocarbons in Liquid Oxygen

Industry is large consumer of air gas, mainly oxygen, nitrogen, argon for many and varied applications: metallurgy, chemistry, petrochemistry, electronics ?

A variety of technologies has been developed to separate air into its components but the three technologies that are currently commercially significant in the field of air separation are cryogenic distillation and the noncryogenic methods of pressure swing adsorption and membrane separation. Cryogenic distillation of air is the oldest but most highly developed separation technology for the industrial production of oxygen and nitrogen simultaneously and also the most employed of these technologies. Cryogenic distillation is based on fractional distillation (at cryogenic temperatures) in which components of air mixture are separated on the basis of differences in boiling point like in distillation separation at higher temperatures.

Air, raw material, comes from the atmosphere and, before a liquefaction step, has to be removed from the components incompatible with the low temperatures, in particular carbon dioxide and water, and secondary natural pollutants or produced by various anthropic activities (industry, heating, road traffic,?), like hydrocarbons (such as propane or ethane), or ozone and nitrous protoxide.

These secondary pollutants represent only very small amounts in distillation units. However, being not very volatile, they concentrate throughout the process of distillation in the oxygen which is that of the three principal components of air having the highest boiling point. In the case of the hydrocarbons, also they can form with oxygen highly reactive mixtures. In this way, significant efforts are made by the oxygen producing companies to ensure the safety of cryogenic air separation unit. In fact, their aim is to avoid that undetected contaminants reach the vaporizer-condenser, are concentrated in liquid oxygen and form a critical mass of solid deposit which could lead to an explosion. It is clear that a complete knowledge of contaminants nature and behavior in gaseous and liquid streams of a cryogenic air separation unit is a key issue for efficient and safe operation. Solid depositions in main exchangers and vaporizers are the main concern. Carbon dioxide deposition in old plants (not equipped with a front-end purification unit) is well-known to be the cause of sub-coolers plugging and to be involved in acetylene accumulation and explosion. More recently, nitrous oxide has appeared to be another source of solid deposit even if its concentration in ambient air is a thousand times lower than carbon dioxide one. Thus, global approaches are developed by these companies to minimize and control that risk. These latter have led to the development of processes equipped with liquid oxygen filters, the development of new adsorbents in the front-end purification unit and monitoring strategies to track accurately the most volatile contaminants from the feed air to the liquid oxygen.

In the past many physical properties of several air pollutants have been studied in mixture with air in process cryogenic conditions (from 90 to 150 K). Nevertheless for some of them, like ethane and propane, properties and behavior remain badly-known because of the difficulties inherent in the handling of these reactive cryogenic mixtures. Only few solubility data of hydrocarbons in liquid oxygen have been published in scientific literature and moreover appear as much dispersed according to authors. For example, literature brings back variations of experimental results of an order of magnitude on the values of propane solubility in oxygen in cryogenic conditions.

Solubility data of hydrocarbons in cryogenic liquids, and in particular in liquid oxygen will be very useful to design safe equipment for cryogenic distillation of air into its constituents. The experimental study of the solubility of hydrocarbons in liquid oxygen is far from trivial due to explosive properties of this type of systems.

Thus, in order to work on these systems in full safety from very weak dilutions to saturation, a new apparatus was designed as the result of collaboration between L'Air Liquide and the ?Thermodynamique et des Equilibres entre Phases? laboratory. The method retained for this work is of ?static-analytic? type with micro-sampling device and on-line gas chromatographic analyses. The equilibrium cell has a very small volume and holds one ROLSITM sampler to perform on-line chromatographic analyses of the liquid phase and which allows in-situ withdrawing of reliable samples, small enough to ensure that thermodynamic equilibrium remains undisturbed inside the equilibrium cell.

As already mentioned, hydrocarbon-oxygen equilibria are not well-known but it has been assumed that their phase diagrams could be of type III according to the van Konynenburg and Scott classification like hydrocarbon-nitrogen systems. Thus, the main objective of the study is to determine the hydrocarbon concentration in liquid oxygen from very low molar fraction to the solubility limit reached when a second liquid phase rich in hydrocarbon appears.

Propane and ethane have been studied in the temperature range 90 to 130 K. Experimental data obtained will allow calibration of equations of state or other adapted thermodynamic models in order to be used in a process simulator. Thus, the more precise knowledge of these concentration limits will make it possible to improve evaluation and control of risks on existing air distillation units. Furthermore, these new data will be taken into account in new designs of air distillation plants. ants.