Reflux calorimeter

Alternatively the reaction calorimeter can be fitted with a reflux condenser, ora modified Soxhlet assembly can be used. Both of these approaches are described below. It is difficult to obtain accurate calorimetric data with such reflux calorimeters if ... 

The reflux calorimeter measures the energy flows into and out of the reacting system. It is relatively simple to measure the energy entering the system using a conventional calorimeter. The energy leaving the system can be calculated from the rate of formation of the condensate, and the temperature of the condensate return if this is below the saturation temperature. If we assume that the condenser carries the whole reflux heat load, accurate measurement of the flowrate and temperature rise of the condenser coolant allow us to calculate the heat load due to reflux. 

The major advantage of this type of calorimeter is that the heat balance principle can easily be applied to the reflux condenser as well, which enables a simpler investigation of processes under reflux conditions. Another advantage is its independence of the heat transfer coefficient at the reactor wall. 

Usually, isothermal calorimeters are used to measure heat flow in batch and semi-batch reactions. They can also measure the total heat generated by the reaction. With careful design, the calorimeter can simulate process variables such as addition rate, agitation, distillation and reflux. They are particularly useful for measuring the accumulation of unreacted materials in semi-batch reactions. Reaction conditions can be selected to minimize such accumulations. 

Use of medium-scale heat flow calorimeter for separate measurement of reaction heat removed via reaction vessel walls and via reflux condenser system, under fully realistic processing conditions, with data processing of the results is reported [2], More details are given elsewhere [3], A new computer controlled reaction calorimeter is described which has been developed for the laboratory study of all process aspects on 0.5-2 1 scale. It provides precise data on reaction kinetics, thermochemistry, and heat transfer. Its features are exemplified by a study of the (exothermic) nitration of benzaldehyde [4], A more recent review of reaction safety calorimetry gives some comment on possibly deceptive results. [5],... 

A typical reaction calorimeter consists of a jacketed reactor, addition device, temperature transducer(s) and calibration heaters. There are a number of devices within Dow ranging from the commercially available Mettler RC-1 (1-2 L volume) to smaller, in-house reactors (10-50 ml). While each of these devices has their unique attributes (e.g., in-situ spectrometry, quick turn-around, ability to reflux, etc.), all of the calorimeters will produce a signal of heat flow vs. time. The heat flow is usually produced in response to the addition of a reagent or an increase in temperature. Volume of gas or pressure generated may also be measured. 

A series of tricyanomethyl compounds were prepared in refluxing acetonitrile hy alkylating potassium tricyanometha-nide with alkyl iodides, allyl, propargyl, and benzyl bromides. Yields of 20-57% were obtained for mono- and difunctional halides with a reflux time of 72 hours. The heats of combustion of these tricyanomethyl compounds as well as of two polycyano compounds were measured using a Dickenson-type calorimeter, and heats of formation were calculated with a precision of approximately 1.0%. From Pitzers values for C—C and C—H bond energies, that of the tricyanomethyl moiety is calculated to be about 810 kcal./mole, and the tricyanomethyl group is less stable than expected from comparison with AH°f of propylcyanide. 

The three types of isothermal heat flow calorimeters described above can be used to measure heat flow in semi-batch reactions, where one or more reactants are charged to the reactor and the other reactants are added at controlled rates throughout the reaction. With careful design the heat flow calorimeters can simulate process variables such as feed rate, stirring, distillation and reflux . 

Calorimeters can be adapted to run reactions under reflux and it is now possible to gain data just below, at and just above the boiling point of a reaction mixture. 

There are several experimental methods for obtaining calorimetry data for systems under reflux. The simplest is to carry out the reaction in a Dewar calorimeter or a reaction calorimeter, at the reflux temperature but under sufficient back pressure to just stop the mixture boiling. This ensures that heat is not lost by vaporization. Any heat generated is then measured in the normal way. 

Figure 4.23 Reflux heat flow calorimeter results from the hydrolysis ...

Steel, C.H. and Nolan, P.F., 1989, The design and operation of a reflux heat flow calorimeter for studying reactions at boiling, Int Symp on Runaway Reactions, 198-231 (CCPS, AlChE. USA). 

In Fig. 4.40 a stirred liquid bench-scale calorimeter is displayed. It closely duplicates laboratory reaction setups. The information aboutheat evolved or absorbed is extracted from the temperature difference between the liquid return (Tj) and the reactor (T ). This difference is calibrated with electric heat pulses to match the observed effect at the end of a chemical reaction. In a typical example, 10 W heat input gives a 1.0 K temperature difference between Tj and Tr. The sample sizes may vary from 0.3 to 2.5 liters. The overall sensitivity is about 0.5 W. The calorimeter can be operated between 250 and 475 K. Heat loss corrections must be made for the stirrer and the reflux unit. The block diagram in Fig. 4.40 gives an overview of the data handling. 

Isothermal phase-change calorimeters based on liquid + vapour equilibria have been used for the determination of energies of polymerization. The monomer was sealed into an ampoule immersed in the calorimetric liquid e.g. carbon tetrachloride, benzene, toluene) contained in a tube, which was suspended from a balance and surrounded by vapour of the same refluxing liquid. As the polymerization proceeded the heat liberated by the reaction caused liquid to vaporize from the tube and was measured by the consequent loss in mass. ... 

Two bench-scale heat flow calorimeters (with a Scunple size of O.3-2.5 litres) have been described a design of Hub Q3, particularly suitable for work under reflux conditions, and the instrument presented in this paper, which is a single sample active heat flow calorimeter, using the heat flow control method 31). ... 

A reflux condenser (condenser-kettle embedded in an intermediate thermostat) within a base thermostat is combined via a thermally insulated pipe with the measuring kettle of a calorimeter for a discontinuous reaction (Fig. 2.35). Provided that in the measuring kettle evaporable components are generated by a not-too-fast chemical conversion, between the measuring kettle and the condenser there exists with regard to the evaporable components a quasi-continual equilibrium. ... 

Fig. 2.35 Compact calorimeter and reflux condenser for discontinuous, isothermal reaction...

To conduct the kinetic investigaliOTi, the measuring kettle of the calorimeter is filled with a batch of sulphide, Na2Mo04, H2O2 and xylene because the industrial production is run under conditions of cooling by reflux. The batch is brought to reaction temperature and H2O2 is injected simultaneously at the start of the reaction. 

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