Calcium dipropionate, also known as calcium propionate (empirical formula C6H10CaO4; CAS No. 4075-81-4) is employed worldwide on an industrial scale both as a food additive (E 282) and in the formulation of feed additives. Typically, an aqueous suspension of calcium hydroxide, either produced directly from calcium oxide through hydration through suspension of calcium hydroxide in water, is neutralized with propionic acid. After complete neutralization, the calcium dipropionate produced will dissolve in hot water and can be subjected to a clear filtration to remove impurities, followed by concentration. The separation of calcium dipropionate from the aqueous solution is achieved either through evaporation with subsequent separation of the crystals developed in a normal solid-liquid separation systems, or through spray-drying. This technology results in high purity products, but is quite expensive, not least because of the low water solubility of calcium dipropionate, the high energy requirement during the processing and the necessity of highly complex production units.
A great number of the known methods for the production of alkali and alkaline earth propionates are based on neutralizing alkali and/or alkaline earth hydroxides with propionic acid in the aqueous phase. The methods are carried out without pressure or under elevated pressure, with either anhydrous or hydrated propionic acid. DE 32 15 752 A1/EP 0 093 317 A1/U.S. Pat. No. 4,700,000 describes such a method in an exemplary way. The resulting reaction solutions are typically purified by clear filtration, again due to the moderate solubility of calcium dipropionate under elevated pressure and temperature. The separation and further processing of the produced alkali and/or alkaline earth propionates is carried out using different separation methods. In the case of calcium dipropionate, a significant amount of the currently produced product is produced by a spray-drying method from the purified calcium dipropionate solution, or from a calcium dipropionate suspension. Furthermore, the concentration by evaporation of the reaction solution and the crystallization of calcium dipropionate from the saturated solution with subsequent separation by centrifugation is very important to the currently used technologies. Further, the direct reaction of calcium oxide and/or calcium hydroxide with propionic acid in reactors without added water is also carried out. This reaction takes place exclusively in the solid phase. Problems in the removal of the reaction heat lead to local overheating, decomposition and, as a result, to color changes (graying) of the calcium dipropionate as well as to losses of propionic acid due to the azeotropes produced when the reaction is still incomplete. These problems arise from the inhomogeneous reaction control and are due to the exothermic nature of the neutralization. The quality differences that result therefrom in the commercially available calcium dipropionate products are the reason why such solid-state methods are not widely practiced.
EP 1 072 581 A2 describes a method for the production of calcium propionate, and a device for performing the process. What is described is a method for the production of calcium propionate powder using a convection dryer in which a one-part pumpable suspension of calcium propionate in water with a calcium propionate content equal to or above 30% (w/w) is supplied to a convection dryer, as well as a device for performing this process, comprising a reactor, a loop, a dispersion machine, and optionally a storage tank with a dryer.
U.S. Pat. No. 6,986,909 B2 describes an attempt to solve the problem of poor heat dissipation by applying the propionic acid on an inert inorganic carrier, which should enable a more favorable heat distribution in the subsequent reaction by adding calcium hydroxide. The inert carrier, however, remains in the end product, which would make it suitable only for application in feedstuffs.