Direct heating of drying air with natural gas in the preparation of milk powder

Abstract

The question is considered whether in the production of milk powder direct heating of the drying air can be used as an alternative to indirect heating. Direct gas firing appears to result in substantial savings in the cost of energy and in capital expenditure. As compared with indirect heating, direct gas firing leads to the milk powder showing a slight increase in nitrate content (on average 8–14 mg/kg) and a marked increase in nitrite content (on average 0.4‐3 mg/kg). This level of contamination is expected to diminish when a “low NOx” burner is used. Whereas in preliminary research into the presence of volatile N‐nitrosamines a level slightly above 1 μm N‐nitrosodimethylamine (NDMA)/kg powder was found, more recent experiments showed a maximum of 0.3 g NDMA/kg. There seemed to be no significant relation between the nitrosamine content of the product and the method of heating the drying air.

PERFORMANCE of HEAT RECOVERY SYSTEM FOR A SPRAY DRYER

“…Improvements can also be made in the design and operation of the spray dryer, including the use of direct air heating (Jansen and Elgersma 1985;Shebler 1977), optimization of process parameters (Cook and Dumont 1988;Gronlund 1984), foaming the feed solution (Romero-Ferrer et al 1986; Okazaki and Crosby 1984) and establishing good operational practices (Baker and Bahu 1983).…”

Analysis and simulation of the drying-air heating system of a Brazilian powdered milk plant

“…Indirect heating systems, comprising high-efficiency finned-tube exchangers and saturated steam, are normally employed, though units with hot oil as the heating fluid can also be found. As an alternative, air can be heated by direct contact with combustion gases from a burner (Jansen and Elgersma, 1985;Kelly et al, 1989). Despite the substantially lower capital investment and the high thermal efficiency associated with direct heating, it is not recommended due to the likely contamination of powdered milk brought about by the nitrogen oxides from the combustion gases, which can result in the formation of cancerous chemicals, such as nitrosamines (Caric, 1994).…”