In industrial production, jacketed reactors are widely used in chemical, pharmaceutical, food and other industries for processes such as mixing, chemical reaction and concentration. Heat transfer media, seemingly ordinary liquids or gases, undertake the critical task of transferring heat in and out of the reactor. They directly determine the reaction rate, reaction selectivity and final product quality. This article introduces the commonly used heat transfer media for jacketed reactors.

Ⅰ. Heating Media

1. Saturated Steam — The Most Common Heating Medium

Saturated steam is the preferred option when heating reaction systems below 180°C. Its widespread popularity stems from the enormous latent heat released during condensation into water. Each kilogram of steam releases approximately 2,200 kilojoules of heat, far exceeding the sensible heat released by hot water of the same mass. Consequently, steam heating enables more compact equipment layout and faster thermal response.

It is essential to understand the corresponding relationship between steam pressure and temperature:

· Gauge pressure of 0.1 MPa corresponds to around 120°C

· 0.5 MPa corresponds to around 158°C

· 1.0 MPa corresponds to around 184°C

Higher temperatures require higher-pressure steam, which places strict demands on the pressure resistance of the reactor jacket. This is where honeycomb jacket reactors demonstrate unique advantages. Their reinforced structural design allows the application of high-pressure steam to achieve high-efficiency heating.

2. Heat Transfer Oil — The Premier Choice for HighTemperature Conditions

When reaction temperatures need to exceed 180°C or even reach above 300°C, saturated steam is no longer feasible due to excessive pressure requirements, and heat transfer oil becomes the ideal alternative. Heat transfer oils are specially formulated synthetic or mineral oils featuring high boiling points, excellent thermal stability and low vapor pressure.

Common types include mineral-based oils (such as alkylbenzene and alkylnaphthalene) and synthetic oils (such as biphenyldiphenyl oxide blends, commercially known as Dowtherm A). The biphenyldiphenyl oxide mixture can operate stably for long-term service below 400°C with relatively low vapor pressure, making it the mainstream medium for high-temperature heating.

Attention should be paid to thermal cracking and coking risks under high-temperature operation. Regular testing and replacement are necessary, and a nitrogen sealing system is recommended to prevent oxidative deterioration.

3. Hot Water — A Gentle Heating Option

Hot water is ideal for processes with low temperature requirements (generally below 95°C) and high sensitivity to temperature fluctuations, such as biological fermentation and enzyme-catalyzed reactions. It provides mild heating performance and precise temperature control without the drastic condensation shock caused by steam. Equipped with accurate temperature control valves and circulating pumps, hot water systems can achieve a temperature control accuracy of ±0.5°C or higher.

Ⅱ. Cooling Media

1. Cooling Water — The Most Economical Cooling Source

Circulating cooling water is the most cost-effective solution for cooling reaction systems down to ambient temperature (20°C–30°C). Industrial cooling water is generally supplied by cooling towers, with fluctuating temperatures across seasons — higher in summer and lower in winter. Such seasonal variations must be fully considered in process design. For highly exothermic reactions, cooling water serves as the primary cooling barrier.

2. Chilled Water / Brine — Cooling Below Zero

When cooling is required between 0°C and -20°C, ethylene glycol aqueous solution or calcium chloride brine is adopted as the low-temperature cooling medium. A 30%–50% volume concentration of ethylene glycol solution features a freezing point of -15°C to -30°C and low metal corrosion, making it the most widely used low-temperature medium in pharmaceutical and fine chemical industries. Calcium chloride brine can achieve even lower temperatures (below -40°C), yet it is highly corrosive and requires upgraded equipment materials.

3. Organic Refrigerants — Solutions for UltraLow Temperature Conditions

Conventional brine cannot meet ultra-low temperature demands of -40°C or even -80°C. In such cases, organic silicone oil, ethanol, acetone low-temperature baths or direct liquid nitrogen evaporation cooling are applied. These media maintain appropriate fluidity without solidification at extremely low temperatures. For instance, dimethyl silicone oil remains flowable at -50°C and is a common option for deep cooling.

Ⅲ. Special Media for Extreme Working Conditions

1. Molten Salt — Medium for UltraHigh Temperature Limits

When reaction temperatures exceed 400°C, ordinary heat transfer oils begin to decompose, and molten salt becomes the optimal choice. The most widely used formula is a ternary mixture of potassium nitrate, sodium nitrite and sodium nitrate (commercially named Hitec), with a stable operating range of 150°C to 550°C. Molten salt delivers outstanding thermal stability and a high heat transfer coefficient. Its main drawback is complicated reheating procedures once it solidifies.

2. Switchable Heating and Cooling Media

Many batch reactions require a complete temperature cycle: heating → heat preservation → cooling. Traditional processes often rely on two independent jacket systems or manual medium switching. Modern designs adopt reversing valve groups and dual-temperature control systems to realize rapid switching between heating media and cooling media inside a single jacket, greatly shortening auxiliary operating time and improving production efficiency. The low internal volume of honeycomb jackets enables faster and more sensitive medium switching.

In actual production, the selection of heat transfer media is not a simple one-to-one match, but a comprehensive decision based on multiple factors:

· Temperature range: Ethylene glycol or brine for conditions below -20°C; steam for 20°C to 180°C; heat transfer oil for 180°C to 350°C; molten salt for temperatures above 350°C.

· Temperature control accuracy: Media with moderate heat capacity, paired with precision control valves, are preferred for high-precision temperature regulation, avoiding media with excessive thermal inertia.

· Operational safety: Certain heat transfer oils are flammable at high temperatures and require nitrogen protection; steam systems carry scalding risks; molten salt leakage may lead to severe safety hazards.

The heat transfer medium system determines the overall operational performance of jacketed reactors. From conventional saturated steam and high-performance synthetic heat transfer oil, to ordinary cooling water and professional ultra-low-temperature refrigerants, every medium plays an irreplaceable role within its applicable temperature range.

Driven by the growing demands for process intensification and precision manufacturing, heat transfer medium technology continues to evolve. New products such as high-thermal-stability synthetic oil, low-toxic eco-friendly refrigerants and intelligent medium switching systems are constantly being developed and applied in modern industrial production.