Vapor pressure above formaldehyde solutions

Table 2. Vapor Pressure above Formaldehyde Solutions, kPa ...

These chemical reactions have a significant influence on the physical properties of the solution and on the phase equilibria. The formaldehyde, which would normally be expected preferably in the vapor phase, behaves now like a component with a similar vapor pressure as water and methanol. To create a model, the reactions listed above have to be taken into account, at least to an extent where significant amounts of the higher methylene glycols or hemiformals are formed. 

Often, a fixed composition of the effluent liquid is required for the product to be marketed. In such a case, the feed liquid rate to the absorber is set by a material balance. As an example, we will consider the absorption of formaldehyde from a gas stream to produce a 50 wt % aqueous solution. Although, HCHO is highly soluble in water, it releases 27,000 BTU/lb mol HCHO of heat when absorbed into water [16]. Because the vapor pressure of HCHO above its solutions increases with both concentration and temperature, it is necessary to withdraw and cool the liquid phase repeatedly. 

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