Question

Which of the following is used as refrigerant in cryogenic freezing?

• A. Nitrogen
• B. Carbon dioxide
• C. Liquid Nitrogen and liquid Carbon dioxide
• D. Oxygen

Hint:

Cryogenic Freezing: Uses EXPENDABLE REFRIGERANTS = LIQUID NITROGEN ($-196^\circ\text{C}$) and LIQUID CARBON DIOXIDE ($-78.5^\circ\text{C}$).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Cryogenic freezing thermodynamics and industrial expendable refrigerants: cryogenic food freezing utilizes direct-contact expendable refrigerants—specifically Liquid Nitrogen ($ ext{LN}_2, ext{boiling point } -195.8^\circ ext{C}$) and Liquid Carbon Dioxide ($ ext{LCO}_2, ext{sublimation point } -78.5^\circ ext{C}$).
Key Formula or Approach:
\[ \text{Cryogenic Refrigerants: } \begin{cases} \mathbf{\text{Liquid Nitrogen (LN}_2\text{)}} \& T_{\text{boil}} = -195.8^\circ\text{C} \quad (h_{fg} = 199\text{ kJ/kg}) \mathbf{\text{Liquid Carbon Dioxide (LCO}_2\text{)}} \& T_{\text{sub}} = -78.5^\circ\text{C} \quad (h_{fg} = 571\text{ kJ/kg}) \end{cases} \]

Step 2: Detailed Explanation:
In food refrigeration engineering and ultra-rapid cryogenic freezing:
- Cryogenic Freezing involves direct exposure of food products (ice cream novelties, individual quick frozen [IQF] cheese cubes, dairy desserts) to expendable cryogenic refrigerants that change phase in direct contact with the food:
1. Liquid Nitrogen ($\text{LN}_2$): Boiling point of $-195.8^\circ ext{C$} at atmospheric pressure; sprayed directly in cryogenic tunnel freezers to achieve instantaneous crust freezing without cellular structural damage.
2. Liquid Carbon Dioxide ($\text{LCO}_2$): Stored under pressure ($2\text{ MPa}$ at $-18^\circ ext{C}$); when injected into freezers, it instantly expands into solid $\text{CO}_2$ snow and cold vapor at $-78.5^\circ ext{C$}.
- Therefore, Liquid Nitrogen and liquid Carbon dioxide (C) are the two universally used refrigerants in cryogenic freezing.

Step 3: Final Answer:
Therefore, Liquid Nitrogen and liquid Carbon dioxide are used as refrigerants in cryogenic freezing, corresponding to option (C).