Subject:- Refrigeration and Air Conditioning

Topic:- Duct Design, Controls & Applications

Difficulty:- Medium

Carbon dioxide for manufacture of dry ice is obtained by acid treatment of limestone or by controlled combustion of coke or fermentation of organic substances.

Carbon dioxide gas can be liquefied by compression to pressure of about 60 to 70 bar and then condensed by cooling water.

This liquid is then expanded or throttled to pressure below that of triple point.

The liquid flashes directly into gas and $〖CO〗_2$ snow at the discharge of the expansion valve. The snow is then removed and mechanically compressed into cakes. This way liquid $〖CO〗_2$ is solidified to produce solid$〖CO〗_2$ or dry ice.

The basic cycle is almost identical to vapour compression cycle except that means must be provided for supplying the make-up $〖CO〗_2$ gas and for removal of the $〖CO〗_2$ snow. The layout and p-h diagram are shown in figures.

The carbon dioxide gas is compressed isentropically from point 1 to 2 to a high pressure represented by curve 1-2. It is then condensed in a condenser from point 2 to 3.

The high pressure liquid $〖CO〗_2$ is now expanded in an expansion valve from point 3 to 4, to a pressure below triple point.

The $〖CO〗_2$ at point 4 is a mixture of solid $〖CO〗_2$ which can be removed at point 5 and gaseous $〖CO〗_2$ which after removing at point 6 is mixed with make-up $〖CO〗_2$ gas at point 7.

But this simple cycle requires too large power per ton of dry ice to be economical. Hence several modifications are applied to the system to reduce cost and increase yield such as multistage compression with intercoolers. Figure shows three stage compression with flash intercooling for production of $〖CO〗_2$