Reaction calorimeter

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An example of a reaction Calorimeter

A reaction calorimeter is an instrument that enables the energy being released or absorbed by a reaction to be measured. The majority of events taking place within a reactor release or absorb energy, so if that can be measured it allows a view of what is taking place to be gained.

[edit] Applications

Safety

When considering scaling up a reaction to large scale from lab scale, it is important to understand how much heat is released. At a small scale heat released may not cause a concern, however when scaling up, that heat can build up and be extremely dangerous.

Crystallisation

Crystallising a reaction product from solution is a highly cost effective purification technique. It is therefore valuable to be able to measure how effectively crystallisation is taking place in order to be able to optimise it. The heat absorbed by the process can be a useful measure.

Kinetics

The energy being released by any process in the form of heat is directly proportional to the rate of reaction and hence reaction calorimetry (as a time resolved measurement technique) can be used to study kinetics.

Process Development

The use of reaction calorimetry in process development has been historically limited due to the cost implications of these devices however calorimetry is a very fast and easy way to fully understand the reactions which are conducted as part of a chemical process.

[edit] Reaction Calorimetry Techniques

Heat Flow

Heat flow calorimetry works by looking at the heat flowing across the reactor wall and quantifying this in relation to the other energy flows within the reactor.

Q = U.A.LMTD

U = Heat transfer coefficient

A = Heat transfer area

LMTD - Log Mean Temperature difference between reactor contents and Jacket Fluid

N.B. U.A is usually measured as a combined variable (rather than separately) by using a calibration heater. This calibration needs to be done at the beginning and end of a heat-flow experiment and during the process if U.A changes significantly.

RTCal™ - Calorimetry in Real Time

RTCal™ is a patented measuring technique which is based on heat flux sensors that are located on the wall of the reactor vessels. The sensors measure heat across the reactor wall directly and thus, the measurement is independent of temperature, the properties or the behavior of the reaction mass. Heat flow as well as heat transfer information are obtained immediately without any calibrations during the experiment.

Heat Balance

Heat balance calorimetry uses the temperature of the recirculating fluid within the reactor jacket to measure the heat loss from the system.

Q = m_f.Cp_f.ΔT

m_f - mass of coolant fluid

Cp_f - heat capacity of coolant fluid

ΔT - heat loss or gain from coolant fluid (usually T_out - T_in)

Power Compensation

Power compensation uses a heater placed within the vessel to maintain a constant temperature. The energy supplied to this heater can be varied as reactions require and the calorimetry signal is purely derived from this electrical power. This technology was developed by HEL LTD.

Q = IV or I - I₀

I - current supplied to heater

V - voltage supplied to heater

I₀ - current supplied to heater at equilibrium (assuming constant voltage / resistance)

Constant Flux

Constant flux calorimetry (or COFLUX as it is often termed) is derived from heat balance calorimetry and uses specialist control mechanisms to maintain a constant heat flow (or flux) across the vessel wall.

The equation used within COFLUX calorimetry is the same as in heat balance.