Supercritical Fluid: Fluid over its critical temperature and pressure, exhibiting good solvent power ; in most applications, carbon dioxide is used pure or added with a co-solvent ; the main interest of supercritical fluids is related to their “ tunable ” properties, that can be changed easily by monitoring pressure and temperature : good solvent power at high densities (temperature near critical temperature and pressure much over critical pressure) to very low solvent power at low densities (temperature near or higher critical temperature and pressure lower critical pressure).
Carbon dioxide and other fluids
Carbon dioxide : CO2 is a very attractive supercritical fluid for many reasons :
-- very cheap and abundant in pure form (food grade) worldwide ;
-- non flammable and not toxic ;
-- environment-friendly, as non polluting gas and as most of CO2 is manufactured from waste streams (mainly fertilizer plants gaseous effluents) ;
-- critical temperature at 31°C, permitting operations at near-ambient temperature, avoiding product alteration ;
-- critical pressure at 74 bar, leading to “ acceptable ” operation pressure, generally between 100 and 350 bar.
However, carbon dioxide always behaves as a “ non-polar ” solvent that selectively dissolves the lipids that are water-insoluble compounds like vegetal oils, butter, fats, hydrocarbons….Carbon dioxide does not dissolve the hydrophilic compounds like sugars and proteins, and mineral species like salts, metals,…
Co-solvent : Organic solvent added to the main fluid (generally carbon dioxide) to modify its solvent power vis-à-vis “ polar ” molecules as the fluid itself is only able to dissolve “ nonpolar ” molecules ; generally, the co-solvent is chosen between short-chain alcohols, esters or ketones. For obvious reasons, ethanol is preferred in many cases as it is abundant and cheap in pure forms (food grade, pharmacopoeia grade), not environment hazardous and not very toxic (as it is well known ! ).
Carbon dioxide is available either in commercial cylinders or spheres of limited volumes, or in bulk from tanks containing between 4 and 30 tonnes of liquefied gas at a pressure of 18 bar and at a temperature around –18°C. These tanks are directly fed from cryogenic trucks as shown on the picture below. The price is very sensitive to the quantity delivered as transportation is the main cost because carbon dioxide is available at very low price in various chemical plants (mainly ammonia plants).
As most SCF plants require liquid carbon dioxide under a pressure of 45 bar or so, the liquefied gas must be compress form the storage pressure (18 bar) to 50 bar through a pump. For applications in the food and pharmaceutical industries, it is mandatory to use a “dry” pump to avoid any contamination by pump lubricant.
Moreover, great care must be taken when designing the feed line of liquefied gas to the plant(s) in order to avoid any over-pressure when the flow is stopped and the temperature increases. As detailed below, no section must be without over-pressure protection (relief valve, burst disk,..) as dilatation of the fluid may cause dramatic accidents.
No need to say that drastic precaution must be enforced to avoid the leakage of the whole tank inside a building in case of any problem on one operating plant.