AMIE Section B Chemical key terms

The partial molar Gibbs free energy of a component in a mixture; it measures how the free energy of the system changes when a small amount of that component is added at constant temperature and pressure. It drives phase and chemical equilibrium.

A corrected partial pressure that replaces true pressure in equilibrium calculations for real gases, defined so that the same chemical-potential equations hold as for ideal gases. Its ratio to pressure is the fugacity coefficient.

A factor that corrects mole fraction to effective concentration (activity) in non-ideal mixtures, accounting for intermolecular interactions. A value of one denotes ideal behaviour; values away from one indicate deviation from Raoult's law.

A mixture composition at which vapour and liquid phases have identical composition and boil at constant temperature. Simple distillation cannot separate past this point, so special methods such as extractive or azeotropic distillation are needed.

The ratio of the vapour pressures (or K-values) of two components, expressing how easily they separate by distillation. Values far from one mean easy separation; values near one require many stages or alternative methods.

The ratio of liquid returned from a distillation column condenser to the distillate withdrawn as product. Raising it improves separation but increases energy and capital cost; the minimum value sets the infinite-stage limit.

A graphical technique for sizing distillation columns that steps between the equilibrium curve and operating lines to count theoretical stages for a given reflux ratio and product purity. It assumes constant molar overflow.

A dimensionless group comparing inertial to viscous forces in a flowing fluid, equal to density times velocity times length divided by viscosity. It predicts whether flow is laminar or turbulent and governs friction factor correlations.

A dimensionless coefficient relating pressure drop in a pipe to fluid kinetic energy and pipe geometry. It depends on Reynolds number and relative roughness through the Moody chart or Colebrook equation.

A dimensionless group giving the ratio of convective to conductive heat transfer across a fluid layer, equal to heat transfer coefficient times characteristic length divided by thermal conductivity. It correlates with Reynolds and Prandtl numbers.

A dimensionless property of a fluid comparing momentum diffusivity to thermal diffusivity. It links velocity and temperature boundary layers and appears in convective heat transfer correlations.

A dimensionless group comparing momentum diffusivity to mass diffusivity, used in mass transfer correlations. It is the mass transfer analogue of the Prandtl number and relates to boundary layer thicknesses.

A dimensionless group expressing the ratio of convective to diffusive mass transfer at a surface, equal to mass transfer coefficient times length divided by diffusivity. It is the mass transfer counterpart of the Nusselt number.

A correlation predicting pressure drop through a packed bed by adding viscous (laminar) and inertial (turbulent) contributions, using particle size, bed voidage and fluid properties. It is widely used in reactor and column design.

The fraction of a packed bed or particle mass that is empty space rather than solid material. It strongly affects pressure drop, holdup and residence time in packed columns and reactors.

The average time a fluid element spends inside a vessel or reactor, often estimated as volume divided by volumetric flow rate. It is a key design and control variable for conversion and product quality.

The volumetric feed rate per unit volume (or mass) of catalyst, expressed per hour. It sets the throughput capacity of a catalytic reactor and is inversely related to residence time.

The fraction of a reactant fed that has reacted by the reactor outlet. It is the primary performance measure for reactor sizing and is related to reactor volume through the rate law.

The proportionality coefficient in a kinetic rate law that captures the temperature dependence of reaction speed, usually through the Arrhenius equation. Its units depend on the reaction order.

The minimum energy barrier that reactant molecules must overcome to react. It appears in the Arrhenius equation and controls how strongly reaction rate responds to temperature change.

A substance that increases reaction rate by providing a lower-energy pathway without being consumed overall. It affects kinetics and selectivity but not the equilibrium conversion of the reaction.

A dimensionless group comparing reaction rate to transport rate (flow, diffusion or heat transfer). It indicates whether a process is reaction-limited or transport-limited and guides reactor and mixing design.

A dimensionless number comparing intrinsic reaction rate to diffusion rate inside a porous catalyst pellet. Large values mean reactants react near the surface, reducing catalyst effectiveness.

The ratio of the actual reaction rate in a porous catalyst pellet to the rate if the whole pellet were exposed to surface conditions. It falls below one when pore diffusion limits the reaction.

The appropriate average temperature driving force for heat exchangers with countercurrent or cocurrent flow, computed from terminal temperature differences. It corrects to the true mean for multipass exchangers via a geometry factor.

A combined coefficient describing total resistance to heat flow between two fluids through a wall, accounting for convection on both sides, wall conduction and fouling. It converts area and driving force into heat duty.

An added thermal resistance representing deposits that build up on heat transfer surfaces over time. It is included in exchanger design to ensure adequate duty between cleaning intervals.

The lowest reflux ratio that can achieve a specified separation with an infinite number of stages. Operating near it minimises energy but requires very tall columns, so designs use a practical multiple above it.

The highest pressure a vessel or piping system is permitted to operate at under its design code, based on wall thickness, material strength and temperature. It sets relief device settings and inspection requirements.

Extra wall thickness added to equipment during design to compensate for metal loss by corrosion over the intended service life. It is estimated from expected corrosion rates and inspection intervals.

A safety device that automatically opens to release pressure when a vessel or system exceeds a set limit, protecting equipment from overpressure. Its sizing is governed by code and credible relief scenarios.

A structured team review that systematically examines deviations from design intent in a process using guide words, to identify hazards and operability problems. It is a core technique in process safety management.

A semi-quantitative risk method that evaluates independent protection layers (such as alarms, interlocks and relief systems) against a target risk, to decide whether additional safeguards are needed.

The time required for cumulative cash inflows from a project to recover its initial investment. It is simple to compute but ignores cash flows after payback and the time value of money.

The sum of a project's future cash flows discounted to today at a chosen rate, minus the initial investment. A positive value indicates the project earns more than the required return.

A valuation method that converts future cash flows into present value using a discount rate reflecting time value and risk. It underpins net present value and internal rate of return analysis.

Localised breakdown of a passive film causing small deep cavities in a metal surface, often by chloride attack on stainless steel. It is dangerous because penetration can occur while overall weight loss stays low.

Formation of a thin, stable oxide film on a metal surface that greatly slows further corrosion. Stainless steels rely on this film, which can be damaged by chlorides or reducing conditions.

The amount of dissolved oxygen consumed by microorganisms while decomposing organic matter in a water sample over a set period, usually five days. It is a key measure of organic pollution strength.

The oxygen equivalent of organic and oxidisable matter in water, measured by chemical oxidation with a strong oxidant. It is faster than biological oxygen demand and includes non-biodegradable matter.

An aerobic wastewater treatment in which microorganisms in a mixed liquor consume organic matter and are separated in a clarifier, with sludge recycled to maintain biomass. It is widely used for secondary treatment.

A scale rating a gasoline's resistance to knocking in spark-ignition engines, referenced to isooctane and normal heptane. Higher numbers allow higher compression ratios and require appropriate refinery blending.

A refinery process that breaks large hydrocarbon molecules into smaller, more valuable ones over an acid catalyst to produce gasoline-range products. Fluid catalytic cracking is the dominant commercial form.

A refinery process that rearranges hydrocarbon structures, often converting paraffins to aromatics and branched molecules, to raise octane number or produce hydrogen and synthesis gas from methane.

The average number of monomer units combined in a polymer chain. It controls molecular weight and strongly influences mechanical strength, viscosity and processing behaviour.

The ratio of weight-average to number-average molecular weight, describing how broadly a polymer's chain lengths are distributed. Values near one indicate narrow distribution; higher values affect melt flow and toughness.

The temperature below which an amorphous polymer becomes hard and glassy and above which it is rubbery and flexible. It guides service temperature limits and processing conditions.

The substrate concentration at which an enzyme-catalysed reaction proceeds at half its maximum velocity. It indicates the enzyme's affinity for substrate; a low value means high affinity.

An enzyme attached to or entrapped in a solid support so it can be reused and easily separated from product. Immobilisation often improves stability but can reduce activity through mass transfer limits.

The mass or moles of biomass or product formed per unit of substrate consumed in a biological process. It is used to size fermenters and estimate nutrient and oxygen requirements.