Registered Chemical Engineer key terms

The ratio of actual heat transfer in an exchanger to the maximum possible transfer for given inlet conditions and flow rates. It is a function of the number of transfer units and the capacity ratio, allowing sizing without trial-and-error.

A temperature driving force used to size heat exchangers, calculated as the difference of the two terminal temperature differences divided by the natural logarithm of their ratio. It applies to countercurrent or corrected multipass arrangements.

A combined conductance that lumps convective film resistances, wall conduction, and fouling into a single value, so that heat duty equals this coefficient times area times the temperature driving force.

An added thermal resistance that accounts for deposits building up on a heat transfer surface over time, reducing performance and forcing designers to specify extra area for continued operation.

A dimensionless ratio of inertial to viscous forces in a flowing fluid. It distinguishes laminar from turbulent flow and sets which friction factor or heat transfer correlation applies.

A dimensionless coefficient relating pressure drop in a pipe to the square of velocity, pipe length, and diameter. It depends on Reynolds number and relative roughness, and is read from a Moody chart.

A dimensionless ratio of convective to conductive heat transfer across a fluid layer. Empirical correlations express it as a function of Reynolds and Prandtl numbers for forced convection.

A dimensionless property ratio of momentum diffusivity to thermal diffusivity. It characterizes how velocity and temperature profiles develop in a flowing fluid and appears in convection correlations.

A dimensionless factor multiplying velocity head to account for pressure loss through fittings, valves, bends, and entrance or exit effects, separate from straight-pipe friction.

An energy balance along a streamline for steady, incompressible, frictionless flow, relating pressure, elevation, and velocity head. With a friction loss term it becomes the mechanical energy balance used in piping design.

The absolute pressure margin above a liquid's vapor pressure at a pump suction. Available NPSH must exceed required NPSH to prevent cavitation, which damages impellers and erodes performance.

Formation and collapse of vapor bubbles in a liquid where local pressure falls below vapor pressure. In pumps and valves it causes noise, vibration, and progressive surface damage.

Scaling relationships showing that flow varies with impeller speed, head with speed squared, and power with speed cubed for a geometrically similar pump. They guide speed control and impeller trimming.

A plot of head required by a piping system versus flow rate, combining static head and friction losses. The intersection with a pump curve defines the operating point.

The lowest reflux at which a specified separation remains achievable with an infinite number of stages. It bounds economic design, since operating reflux is set as a multiple of this value.

A graphical stage-by-stage procedure for binary distillation that steps between the equilibrium curve and operating lines to count theoretical stages at a chosen reflux ratio.

The ratio of vapor-liquid equilibrium constants of two components, expressing how easily they separate. Values near one require many stages or alternative separation methods.

The theoretical stage count at total reflux, estimated from the Fenske equation using relative volatility and product compositions. It represents a lower bound on stages for a given separation.

The ratio of actual composition change achieved on a real tray to the change attainable at equilibrium. It converts theoretical stages into actual trays for equipment sizing.

The depth of packing that produces one theoretical stage of separation. Dividing required stages by HETP gives packed height, which is sensitive to flow rates and maldistribution.

A solid-liquid separation operation in which a soluble component is dissolved out of a solid matrix by contact with a solvent. Design uses equilibrium and stage-wise mass balances.

A separation in which a solute transfers from one immiscible liquid phase to another by contact with a solvent. Stage counts come from equilibrium and operating data.

A material balance line on an equilibrium diagram that relates passing-stream compositions in a stage-wise separation. Its slope reflects the flow ratio or solvent rate chosen.

The minimum energy barrier molecules must overcome for a chemical reaction to proceed. It is obtained from the slope of an Arrhenius plot of rate constant versus inverse temperature.

An expression relating the rate constant to temperature through a pre-exponential factor and activation energy, usually written in exponential form and linearized by taking logarithms.

A proportionality factor in a rate law that depends on temperature and catalyst but not on reactant concentrations. Its units depend on the overall reaction order.

The exponent on a species concentration in an empirical rate law, or the sum of exponents for overall order. It is determined experimentally and need not match stoichiometric coefficients.

The time required for reactant concentration to fall to half its initial value. For first-order kinetics it is independent of starting concentration and equals the natural logarithm of two divided by the rate constant.

A dimensionless group comparing reaction rate to convective transport rate. Large values indicate reaction-limited conversion, small values indicate transport-limited behavior in a reactor.

The volumetric feed rate divided by reactor or catalyst volume, usually per hour. It expresses how many reactor volumes of feed pass through per unit time and scales reactor sizing.

The probability distribution of times that fluid elements spend inside a vessel, characterized by tracer tests. It reveals bypassing, dead zones, and departure from ideal plug flow or perfect mixing.

A rule stating that a system at equilibrium shifts to oppose an imposed change in temperature, pressure, or concentration. It guides choices of operating conditions in equilibrium-limited reactions.

A thermodynamic quantity relating product and reactant activities at equilibrium. Its temperature dependence follows the van't Hoff relation and determines the maximum achievable conversion.

A dimensionless parameter comparing reaction rate to diffusion rate inside a porous catalyst pellet. Large values mean reactants are consumed before reaching the interior, lowering catalyst utilization.

The ratio of actual reaction rate in a porous catalyst pellet to the rate if the entire interior were exposed to surface conditions. It falls as diffusion resistance grows.

An accounting of mass entering, leaving, accumulating in, or being generated within a system boundary. It is the fundamental tool for flowsheet calculations and process specification.

An accounting of energy crossing a system boundary plus internal generation or consumption, usually written as enthalpy changes for steady flow. It supports heat duty and utility estimation.

The enthalpy change accompanying a reaction at specified conditions, negative for exothermic and positive for endothermic reactions. It sets reactor cooling or heating duty.

The temperature increase in an insulated reactor if all reaction heat raised the reacting mixture, calculated from heat of reaction divided by the mixture heat capacity per mole converted.

The amount of unreacted material returned to a reactor divided by fresh feed. It raises overall conversion but increases separation and utility costs, so it is optimized economically.