ChE Board key terms

A single physical or chemical processing step, such as distillation, filtration, or drying, that transforms material without changing its chemical identity. Design problems build whole processes by linking these steps.

A processing step in which a chemical reaction converts reactants into products, such as nitration or polymerization. Distinguished from physical separation steps in process synthesis.

An accounting of material entering, leaving, and accumulating in a system, written as input equals output plus accumulation. The foundation for sizing equipment and checking data consistency.

An accounting of heat and work crossing a system boundary plus internal energy change, usually written as enthalpy in minus enthalpy out equals heat added minus work done.

An assumed quantity, such as 100 kg or 1 mol, chosen to make a material balance solvable. Results are later scaled to the actual throughput.

Counting unknown variables and independent equations for each process unit to confirm whether a material or energy balance is solvable, under-specified, or over-specified.

The fraction of a limiting reactant fed that actually reacts in a reactor. Used with selectivity to describe reactor performance and to size recycle streams.

The ratio of desired product formed to undesired product formed, or to reactant consumed, in a reacting system. High selectivity reduces downstream separation cost.

A stream returned from downstream back to a reactor or earlier unit to recover unreacted material or improve yield. Requires iterative or algebraic solution of balances.

A small stream bled from a recycle loop to prevent accumulation of inert components or impurities. Its size is set so inerts leave at the rate they enter.

A stream routed around a unit, blending with that unit's outlet. Used to control composition or temperature and must be included in balance equations.

The thermodynamic property combining internal energy with pressure-volume work, convenient for constant-pressure processes. Heat duties of exchangers are computed as enthalpy changes.

The energy required to raise a substance's temperature by one degree at constant pressure or volume. Needed to compute sensible heat changes of process streams.

The energy absorbed or released during a phase change at constant temperature, such as vaporization or fusion. Central to condenser and reboiler duty calculations.

The pressure exerted by a vapor in equilibrium with its liquid at a given temperature. Determines boiling point and drives distillation and flashing calculations.

A model stating that the partial pressure of each component equals its liquid mole fraction times its pure-component vapor pressure. Valid for ideal mixtures and low pressures.

The ratio of vapor pressures or equilibrium ratios of two components, indicating how easily they separate by distillation. Values near one make separation difficult.

The ratio of liquid returned to the top of a distillation column to distillate withdrawn. It trades operating cost against required number of theoretical stages.

The reflux ratio at which an infinite number of stages would be needed for a specified separation. Actual columns operate at some multiple above it.

A graphical technique for estimating theoretical stages and reflux requirements using an equilibrium curve, operating lines, and a feed line on a mole-fraction diagram.

A dimensionless group equal to inertial forces divided by viscous forces, used to classify flow as laminar or turbulent and to select friction-factor correlations.

A dimensionless coefficient relating wall shear stress to kinetic energy in pipe flow, depending on Reynolds number and roughness. Used to compute pressure drop.

A relation expressing pressure drop or head loss in a pipe as a function of friction factor, length, diameter, and velocity head. Standard for pipe sizing.

Net positive suction head, the margin of absolute pressure above vapor pressure available at a pump suction. Insufficient margin causes cavitation and impeller damage.

A manufacturer's plot of head developed versus flow rate for a pump. The operating point is where it crosses the system curve.

A plot of the head required to move fluid through a piping system versus flow rate, rising with flow due to friction. Intersection with the pump curve gives the duty point.

A combined coefficient describing total resistance to heat flow across a wall, including convection on both sides and conduction through the metal. Used in exchanger sizing.

The effective average temperature driving force in a heat exchanger, computed from terminal temperature differences. Used with the overall coefficient in design equations.

An added thermal resistance representing deposits on heat transfer surfaces over time. It reduces the effective overall coefficient and requires excess area.

A heat exchanger analysis approach using effectiveness as a function of number of transfer units and capacity ratio, convenient when outlet temperatures are unknown.

A multiplier applied to countercurrent log mean temperature difference to account for multipass or crossflow arrangements. Values far below one signal poor design.

An expression relating reaction rate to concentrations or partial pressures of species and a rate constant, with orders determined experimentally rather than from stoichiometry.

A relation giving the temperature dependence of a rate constant through an activation energy and pre-exponential factor. Used to extrapolate or predict reactor behavior.

The minimum energy barrier reactants must overcome for a reaction to proceed. Determines temperature sensitivity of the rate constant.

Reactor volume divided by volumetric feed rate at specified conditions, giving the time to process one reactor volume. Used to size continuous reactors.

A continuous stirred-tank reactor modeled as perfectly mixed, so outlet composition equals contents. Requires larger volume than plug flow for positive-order reactions.

A plug flow reactor modeled as no axial mixing, with composition changing along the length. Gives the smallest volume for many positive-order reactions.

A rule stating that a system at equilibrium shifts to partially offset a change in temperature, pressure, or concentration. Guides operating condition choices for reversible reactions.

A ratio of product to reactant activities at equilibrium, dependent on temperature. Determines the maximum attainable conversion for a reversible reaction.