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When an object or fluid is at a different temperature than its surroundings or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium.

Heat is transferred in three ways: conduction, convection, and radiation. Heat can be transferred only if a temperature difference exists, and then only in the direction of decreasing temperature. Beyond this, the mechanisms and laws governing each of these ways are quite different.

Reactor design uses information, knowledge, and experience from a variety of areas —— thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass transfer, and economics.

Thermodynamics gives two important pieces of information needed in design, the heat liberated or absorbed during reaction and the maximum possible extent of reaction. Chemical reactions are invariably accompanied by the liberation or absorption of heat, the magnitude of which must be known for proper design.

Under appropriate conditions feed materials may be transformed into new and different materials which constitute different chemical species.

Chemical kinetics searches for the factors that influence the rate of reaction. It measures this rate and proposes explanations for the values found.

The aim of designing a reactor is to produce a specified product at a given rate from known reactants.

Chemical reactors may be divided into two main categories, homogeneous and heterogeneous. In homogeneous reactors only one phase, usually a gas or a liquid, is present. If more than one reactant is involved, provision must of course be made for mixing them together to form a homogenous whole.

In tubular reactors, heat is removed either by providing cooling tubes running parallel inside the reactor, or through external heat exchangers.

Types of catalysis

Catalysts can be either heterogeneous or homogeneous, depending on whether a catalyst exists in the same phase as the substrate.

Heterogeneous catalysts are those which act in different phases than the reactants. Most heterogeneous catalysts are solids that act on substrates in a liquid or gaseous reaction mixture. Diverse mechanisms for reactions on surfaces are known, depending on how the adsorption takes place. These catalysts are typically "supported", which means that the catalyst is dispersed on a second material that enhances the effectiveness or minimizes their cost. Sometimes the support is merely a surface upon which the catalyst is spread to increase the surface area. More often, the support and the catalyst interact, affecting the catalytic reaction.

The second law of thermodynamics explains the tendency and limit of a process, such as the change of temperature, pressure, and chemical potential for an isolated system. The law is used to answer questions like whether a process can happen, and what tendency it happens toward. Consequently, the law is usually stated in physical terms of impossible processes. The second law can be expressed in many specific ways, but the first formulation is credited to the German scientist Rudolf Clausius. In classical thermodynamics, the second law is a basic postulate applicable to any system involving measurable heat transfer, while it is a consequence of unitarity in quantum theory in statistical thermodynamics. In classical thermodynamics, the second law defines the concept of thermodynamic entropy, while in statistical mechanics entropy is defined from information theory, known as the Shannon entropy. From the state of thermodynamic equilibrium, the principle of the entropy increase is deduced, and the phenomenon of irreversibility in nature can be explained.

The third law of thermodynamics is a statistical law of nature about entropy. The entropy of a perfect crystal approaches zero as temperature approaches absolute zero. For other, The third law of thermodynamics is a statistical law of nature about entropy. The entropy materials, the residual entropy is not necessarily zero, although it is always zero for perfect crystals in which there is only one possible ground state.

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