PEC EPE Chemical key terms

Application of conservation of mass to a chosen system boundary, accounting for streams entering and leaving, accumulation, and generation or consumption by reaction. It underpins every yield, sizing and emission calculation in plant design.

Thermodynamic property equal to enthalpy minus temperature times entropy. Its change at constant temperature and pressure tells whether a process is spontaneous, and its minimum defines the equilibrium composition of a reacting mixture.

A corrected pressure that replaces partial pressure in phase-equilibrium calculations for real gases. Its coefficient tends to one at low pressure and deviates as non-ideal behaviour increases.

Factor that converts mole fraction into effective concentration in a non-ideal liquid mixture. Values far from one signal strong deviation from ideal behaviour, typically caused by polarity or hydrogen bonding.

Highest temperature attainable when a fuel burns completely with no heat lost to the surroundings. Designers use it to select refractory and alloys and to check burner and furnace limits.

Dimensionless ratio of inertial to viscous forces in a flowing fluid. It separates laminar from turbulent flow and is the key grouping in friction factor and heat transfer correlations.

Dimensionless ratio of convective to conductive heat transport at a surface. Correlations give it as a function of Reynolds and Prandtl numbers for a particular geometry.

Effective average temperature driving force in a heat exchanger, calculated from the terminal differences of the two streams. Correction factors extend it to multipass and crossflow units.

Combined conductance for heat flow between two fluids, including both film coefficients plus wall conduction and fouling resistances. Duty equals this coefficient times area times mean temperature difference.

Absolute head at a pump suction above the liquid vapour pressure, either available from the system or required by the pump. Too little margin causes cavitation, noise and impeller damage.

Liquid returned to the top of a distillation column divided by distillate withdrawn. Raising it improves separation but increases reboiler duty; the minimum value corresponds to infinite stages.

Ratio of the corrected vapour pressures of the two key components being separated. Values close to one mean a difficult split requiring many stages and high reflux.

An ideal contacting stage in which the leaving streams reach thermodynamic equilibrium. Real trays and packing are rated against it through efficiency, allowing stage counts to be estimated.

Number of transfer units measures how difficult a separation is; height of a transfer unit is the packing depth giving one unit. Multiplying the two gives the packed height required.

Model placing all resistance to interphase mass transfer in thin stagnant films on either side of the interface, which is assumed to be at equilibrium. It defines individual and overall mass transfer coefficients.

Diffusive flux is proportional to the concentration gradient, with diffusivity as the constant of proportionality. It is the basis for describing mass transfer through films, porous solids and membranes.

Relation in which the rate constant rises exponentially with temperature through an activation energy. It lets rate data measured at one temperature be extrapolated, cautiously, to plant conditions.

Volumetric feed rate divided by catalyst volume, commonly expressed per hour. It fixes contact time in a catalytic reactor and scales laboratory kinetics to commercial beds.

Fraction of the limiting reactant fed that reacts. It says nothing about how much of the reacted material became the wanted product, so it is always read together with selectivity.

Share of the converted reactant that ends up as the desired product rather than by-products. In many processes it drives raw material cost and separation load more strongly than conversion does.

Dimensionless ratio of reaction rate to a transport rate such as flow or diffusion. Large values point to transport limitation; small values indicate kinetic control.

Gradual loss of catalytic activity from coking, poisoning, sintering or fouling. Reactor design provides spare volume, guard beds or regeneration so the run length meets turnaround plans.

Sum of future cash flows discounted at a chosen rate, minus the initial investment. A positive value means the project earns more than the hurdle rate and is normally preferred.

Equipment or plant cost varies roughly with capacity raised to the power 0.6. It is useful for early budget estimates, provided adjustments are made for materials, location and date.

Technique that finds the minimum heating and cooling a process network needs by matching hot and cold streams. It sets targets for heat exchanger network design before detailed flowsheeting.

Drawing that shows equipment, piping, instruments and control loops with standard symbols and tag numbers. It is the reference document for detailed design, hazard studies and commissioning.

Team-based study that applies guide words to process parameters at each node of a plant to find deviations, causes, consequences and safeguards. It is the standard hazard identification method in the chemical industry.

Lower and upper explosive limits are fuel concentrations in air below and above which a flame cannot propagate. They define the flammable range used for ventilation and gas detection design.

Semi-quantitative method that credits independent protection layers with a probability of failure on demand and compares total risk against a target. It converts HAZOP scenarios into safety instrumented function requirements.

Design approach that removes or reduces hazards by minimising inventories, substituting less hazardous materials, moderating conditions and simplifying the process, instead of adding controls to manage the hazard.

Self-acting device that opens to discharge fluid when pressure exceeds its set point, protecting equipment from overpressure. Sizing uses the worst credible scenario, such as fire or blocked outlet.

Discrete rating of the reliability required from a safety instrumented function, linked to its probability of failure on demand. Higher levels demand redundancy, diagnostics and shorter proof-test intervals.

Algorithm combining proportional, integral and derivative action on the error between measurement and setpoint. Tuning the three terms trades speed of response against overshoot and offset.

Delay between a change at the process input and the first detectable movement of the measurement. It restricts the controller gain that can keep a loop stable.

Arrangement where the output of a primary controller sets the setpoint of a faster secondary controller. Disturbances inside the secondary loop are corrected before they disturb the main variable.

Extra metal thickness provided so that expected wastage over the design life does not reduce the wall below its required strength. It is set from corrosion rate and inspection philosophy.

Crack growth when a susceptible alloy carries tensile stress in a specific corrosive environment, often at raised temperature. Failure can occur well below yield and is difficult to detect.

Factor below one used in pressure vessel thickness formulas to allow for welded seams and for the degree of radiographic examination applied to those seams.