Carbon dioxide freeze-drying

We call solid carbon dioxide (CO2) dry ice . To the eye, it looks just like normal ice, although it sometimes appears to smoke see below. Carbon dioxide is a gas at room temperature and only solidifies (at atmospheric pressure) if the temperature drops to about —78 °C or less, so we make dry ice by cooling gaseous CO2 below its freezing temperature. We call it dry ice because, unlike normal ice made with water, warming it above its melting temperature leaves no puddle of liquid, because the CO2 converts directly to a gas. We say it sublimes. 

This process, frequently called lyophilisation, is necessary when water is to be removed from solutions containing heat-labile materials so that conventional distillation, even under reduced pressure, would cause extensive losses by decomposition. Examples are to be found in the removal of water from aqueous solutions of enzymes, polysaccharides, peptides, etc. In principle the aqueous solution is frozen in a suitable solid carbon dioxide freezing mixture (see Section 2.12), and the ice is sublimed off to leave a dry residue.42... 

Carbon dioxide freezes at -109° F (-78° C), turning it into dry ice. Dry ice gives the appearance of fog or smoke when exposed to warmer temperatures. 

In general, the process involved in producing solid carbon dioxide is as follows First, cold liquid carbon dioxide is piped into either a special hydraulic press or an extruder. As the liquid boils and evaporates, the vapors are vented or pumped off. The remaining liquid cools until it finally freezes into solid carbon dioxide particles (dry ice snow). After the vapor pressure has been reduced to atmospheric pressure by pumping or bleeding the vapors away, the solid carbon dioxide particles are pressed into a block or extruded through dies. 

The transport of perishables by air does not require mechanical refrigeration, as low temperatures prevail at the heights flown. Fresh vegetables and flowers need to be protected from freezing, and produce will usually be in insulated containers. A feature of this traffic is the prompt and speedy handling at the airports. Coldrooms are provided at some airports to store produce immediately before and after transit. Solid carbon dioxide ( dry ice ) is used for shortterm cooling of airline passenger meals. 

Amination (11) and solution carbonation (8) reactions were carried out as described previously. For solid-state carbonations, a benzene solution of poly(styryl)lithium was freeze-dried on the vacuum line followed by introduction of high-purity, gaseous carbon dioxide (Air Products, 99.99% pure). Analysis and characterization of polymeric amines (11) and carboxylic acids (8) were performed as described previously. Benzoyl derivatives of the aminated polystyrenes were prepared in toluene/pyridine (2/1. v/v) mixtures with benzoyl chloride (Aldrich, 99%). 

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