Heat-pack reaction

What kind of energy transfer occurs during the exothermic heat-pack reaction Heat produced by the reaction flows from the heat pack (the system) to yom cold hands (part of the surroundings). 

The sign of the enthalpy of reaction Recall the heat-pack reaction of iron with oxygen. 

According to the equation, the reactants in this exothermic reaction lose heat. Therefore, //products < reactants- When //.eactants is subtracted from the smaller //products negative value for is obtained. Enthalpy changes for exothermic reactions are always negative. The equation for the heat-pack reaction and its enthalpy change are usually written hke this. 

In order to analyse the packed bed process, it is necessary to consider both heat transfer from the solid to the gas and reaction heat which may be transmitted to the gas. The composition of the gas, and hence its physical properties, are determined by the rate of reduction, which in turn depends on each layer of the packed bed, and on the degree of reduction which has already occurred. In the reduction of haematite, there are three stages in the reduction, corresponding to the formation of Fe304 and FeO before the metal is formed. The thermal data for the reduction processes can be approximated by the respective heats of reduction by H2 and CO gases. Taking 1000°C as a typical mean temperature, the mean value for the heats of reaction per 2 gram-atom of iron are... 

The process. A typical process for phthalic anhydride starts with mixing hor q-xylene vapor with excess preheated air in a ratio of 20 1 air to o-xylene by weight. The gaseous mixture is then fed to a reactor consisting of tubes packed with vanadium pentoxide catalyst on a silica gel. The reaction takes place at 800—1000°F. Like most oxidation reactions, this one is exothermic, and the heat of reaction must be removed from the tubes to maintain the reaction temperature. 

One common application of thermochemistry in everyday life is the use of hot and cold packs. A variety of these are used to treat injuries and provide warmth. One type of pack used to provide heat utilizes the exothermic oxidation of iron to produce heat. The reaction can be represented as... 

A hot pack is activated when someone initiates the reaction by some physical action such as shaking or breaking a seal on the pack. The actual hot pack reaction involves several other chemicals, but the primary reaction is the oxidation of iron. The iron in the hot pack is a fine powder to increase the efficiency of the reaction. As expected, cold packs depend on some type of endothermic process. One common cold pack is based on the mixing of ammonia nitrate and water. In this case, the heat of... 

Steam reforming reactors have the supported nickel catalyst packed in tubes and the endothermic heat of reaction supplied from a... 

Vinyl acetate reactors that have a supported Pd/Au catalyst packed in 25-mm (0.082-ft) ID tubes and exothermic heat of reaction removed by generating steam on the shell side. The feed contains ethylene, oxygen, and acetic acid in the vapor phase at 150 to 175°C (302 to 347°F). 

The multitubular fixed-bed reactor (Fig. IB) constitutes the oldest and still predominant representative of this class. The catalyst packing is located in the individual tubes of the tube bundle. The heat-transfer medium is circulated around the tube bundle and through an external heat exchanger, in which the heat of reaction is supplied or removed ( Fig. 16). Whereas with endothermic reactions circulating gas can be used as heat transfer medium, for strongly exothermic reactions exclusively liquid or boiling heat transfer media are used. Only in this way can the catalyst temperature (c.g., in the case of partial oxidations) be held in the narrow temperature range necessary for selective reaction control. 

This reaction is highly endothermic, and the heat of reaction is supplied by firing natural gas and purge gas in a fire box external to these tubes. The heat is then transferred through the tube wall into the catalyst packed reaction zone. 

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