Temperature control of jacketed reactors

Temperature control of jacketed reactors is the regulation of process temperature by circulating a heating or cooling medium through the jacket surrounding a reactor vessel. The jacket acts as a heat-transfer surface between the circulating fluid and the material inside the reactor.

Jacket temperature control is used in laboratory, pilot-scale and industrial processes where the reactor contents must be heated, cooled or maintained at a controlled temperature. The design of the system depends on the process temperature range, heat load, reactor size, jacket area, circulation flow rate and allowable jacket pressure.

A jacketed reactor has a space around part or all of the vessel wall through which a heat-transfer fluid can circulate. Heat passes through the reactor wall without direct contact between the heat-transfer fluid and the process material.

During heating, the circulating fluid is maintained at a temperature above the process temperature. During cooling, the circulating fluid is maintained below the process temperature.

The jacket therefore operates as a type of heat exchanger.

A basic temperature-control loop may include:

• a jacketed reactor;
• a heating or cooling source;
• a circulation pump;
• a temperature sensor;
• a temperature controller;
• piping, valves and hoses;
• an expansion or buffer vessel where required.

Heating and cooling may be supplied directly from plant utilities, or by a separate recirculating temperature-control unit.

The heat-transfer fluid must be suitable for the operating temperature range and for the materials used in the reactor jacket, pump, piping and seals.

Common fluids include:

Water has a high specific heat capacity and is commonly used for moderate-temperature heating and cooling. Its useful temperature range is limited by freezing, boiling and the operating pressure of the system.

Mixtures of water and ethylene glycol or propylene glycol are commonly used when cooling below the normal freezing point of water is required.

Increasing the glycol concentration lowers the freezing point, but also changes viscosity and heat-transfer performance. At low temperatures, viscosity becomes particularly important because it affects circulation flow and pump pressure.

The glycol concentration should therefore be selected according to the required minimum temperature rather than simply using the highest possible concentration.

Thermal oils and other synthetic heat-transfer fluids can be used where operating temperatures are outside the practical range of water-based fluids.

Fluid selection should consider:

• minimum and maximum operating temperature;
• viscosity;
• specific heat capacity;
• thermal stability;
• compatibility with seals and piping;
• toxicity;
• flammability;
• fluid expansion;
• maintenance requirements.

The heat-transfer fluid manufacturer's operating limits should be followed.

A first estimate of the energy required to change the temperature of a process material can be made from:

Q = m × Cp × ΔT

where:

• Q is the heat required;
• m is the mass of the material;
• Cp is the specific heat capacity;
• ΔT is the required temperature change.

If the required heating or cooling time is known, the average thermal power can be estimated from:

P = Q / t

where t is the heating or cooling time.

This calculation only considers the process material. A practical system may also need to account for:

• heating or cooling of the reactor vessel;
• heat generated or absorbed by the reaction;
• heat loss or gain from the surroundings;
• evaporation or condensation;
• heat introduced by mixing;
• piping and equipment heat capacity.

For an exothermic reaction, the cooling system must be able to remove both the sensible heat and the heat generated by the reaction.

The heat that can actually pass through the jacket is also limited by the heat-transfer area and overall heat-transfer coefficient. A simplified relationship is:

q = U × A × ΔT

where U is the overall heat-transfer coefficient and A is the available heat-transfer area.

Agitation inside the reactor can improve heat transfer by reducing temperature gradients near the vessel wall.

Heating or cooling capacity alone does not determine the performance of a jacket temperature-control system. The heat-transfer fluid must also circulate through the jacket at a suitable flow rate.

Flow is affected by:

• jacket geometry;
• pipe and hose diameter;
• hose length;
• number of valves, bends and fittings;
• fluid viscosity;
• operating temperature;
• pump performance.

Low-temperature fluids may become considerably more viscous. This can increase pressure drop and reduce circulation flow.

The circulation pump must provide sufficient flow without exceeding the allowable pressure of the reactor jacket.

This is particularly important for glass reactors and other jackets with relatively low allowable pressure. Pump pressure, static head and pressure drop in the return line should all be considered.

Several arrangements can be used.

Steam, hot water, cooling water or another plant utility can be supplied directly to the reactor jacket.

This arrangement can be simple and suitable where only a limited temperature range is required. Separate heating and cooling utilities may require valves to switch between services.

A recirculating temperature-control unit uses a pump to continuously circulate a heat-transfer fluid between the unit and the reactor jacket.

The unit adds or removes heat from the circulating fluid according to the required process temperature.

Some systems use one heat-transfer fluid for both heating and cooling. This avoids repeatedly replacing the fluid in the jacket when changing between heating and cooling.

A recirculating system may contain:

• electric heater;
• refrigeration circuit or cooling heat exchanger;
• circulation pump;
• expansion volume;
• temperature sensors;
• controller;
• safety devices.

The selected unit must be evaluated at the actual operating temperature. Cooling capacity, fluid viscosity and pump performance can change significantly across the operating range.

The location of the temperature sensor affects system response.

Possible control variables include:

• reactor contents temperature;
• jacket inlet temperature;
• jacket outlet temperature;
• temperature-control-unit outlet temperature.

For many batch processes, the temperature of the reactor contents is the main process variable.

A simple controller can adjust heating or cooling according to the difference between the process temperature and the set point.

More demanding processes may use cascade control. In a cascade arrangement, the reactor temperature controller provides a set point to a faster jacket or circulating-fluid temperature controller.

This can improve response where the jacket temperature changes much faster than the bulk reactor temperature.

Controller settings must also consider the difference between heating and cooling response. Excessive controller action can cause temperature overshoot or oscillation.

Common problems in jacket temperature-control systems include:

• insufficient cooling capacity at the required low temperature;
• insufficient circulation flow;
• excessive pressure in the reactor jacket;
• heat-transfer fluid with unsuitable low-temperature viscosity;
• piping or hoses that are too small;
• long circulation lines with high pressure drop;
• air trapped in the circulation system;
• poor temperature sensor location;
• incorrect controller settings;
• contamination or degradation of the heat-transfer fluid;
• thermal shock caused by rapid temperature changes.

For glass reactors, rapid changes in jacket temperature should be avoided where they may produce excessive thermal stress. The temperature and pressure limits specified by the reactor manufacturer should be followed.

Temperature control can represent a significant part of the energy demand of a reactor system.

Energy use can be reduced by:

• insulating the reactor and circulation piping;
• reducing unnecessary pipe length;
• selecting an appropriate heating and cooling capacity;
• avoiding unnecessarily low cooling temperatures or high heating temperatures;
• maintaining clean heat-transfer surfaces;
• maintaining the heat-transfer fluid in usable condition;
• recovering process heat where practical.

Leaks of glycol or thermal oil should be prevented and collected. Fluid toxicity and local disposal requirements should be considered when selecting and maintaining a heat-transfer fluid.

Where contamination of food, pharmaceutical or other sensitive products is possible, the toxicity and suitability of the heat-transfer fluid require additional consideration.

Jacketed reactor temperature control is used in processes such as:

• chemical synthesis;
• crystallization;
• polymerization;
• fermentation;
• botanical extraction;
• solvent evaporation;
• material preparation;
• laboratory and pilot-scale process development.

The temperature-control requirements can vary considerably between these applications. Reactor volume alone is therefore not sufficient for selecting a heating or cooling system.

• Heat exchangers
• Chiller ECM
• Open Source Medical Laboratory Bioreactor
• Energy efficiency

• HJCHEM — How to Choose the Right Circulator for a Glass Reactor
• Spirax Sarco — Heating with Coils and Jackets
• Eastman — Therminol Heat Transfer System Design Resources
• Dow — DOWCAL 200 Heat Transfer Fluid
• Emerson — Improve Batch Reactor Temperature Control