PE Chemical key terms

An equation of state, PV = nRT, that relates pressure, volume, moles, and absolute temperature for a gas whose molecules occupy negligible volume and do not interact. It is accurate at low pressure and high temperature.

A dimensionless ratio, Z = PV/nRT, that shows how far a real gas departs from ideal behavior. Values near one indicate ideal behavior; charts or equations of state supply Z for nonideal conditions.

The pressure one component of a gas mixture would exert if it alone occupied the whole volume at the same temperature. Component partial pressures sum to the total pressure by Dalton's law.

A condition in which process variables do not change with time, so mass and energy entering equal what leaves and nothing accumulates inside the system. Most design calculations assume steady state.

The reactant that would be completely consumed first if the reaction went to completion, set by the stoichiometric ratio and the feed amounts. It fixes the maximum possible product and the basis for conversion.

The fraction of the limiting reactant fed that has reacted. It ranges from zero to one and links feed amounts to reactor outlet composition in material balances.

A small stream deliberately withdrawn from a recycle loop to keep inert or accumulating components from building up. Purge rate trades off lost valuable material against inert concentration.

The pressure of vapor in equilibrium with its liquid or solid at a given temperature. A liquid boils when its vapor pressure equals the surrounding pressure, and vapor pressure rises sharply with temperature.

For an ideal liquid mixture, the partial pressure of each component equals its liquid mole fraction multiplied by the pure-component vapor pressure. It gives the basis for bubble-point and dew-point calculations.

The energy absorbed or released when a substance changes phase at constant temperature, such as vaporization or fusion, without changing its temperature. It is often reported per mole or per unit mass.

A thermodynamic function that decreases for a spontaneous process at constant temperature and pressure; at equilibrium its change is zero. Its standard change relates to the equilibrium constant.

A number relating product and reactant activities at chemical equilibrium at a given temperature. Its value indicates how far a reaction proceeds and it shifts with temperature according to the heat of reaction.

A dimensionless group comparing inertial forces with viscous forces in a flowing fluid, calculated from density, velocity, diameter, and viscosity. It indicates whether flow is laminar, transitional, or turbulent.

An energy balance for steady, incompressible, frictionless flow along a streamline, relating pressure, elevation, and velocity head. It is used to estimate flow rates and pressure changes in piping.

A measure of a fluid's resistance to shear deformation, defined as shear stress divided by shear rate for Newtonian fluids. It falls with rising temperature for liquids and rises for gases.

A dimensionless coefficient in the Darcy-Weisbach equation that accounts for pressure loss from friction in a pipe. It depends on Reynolds number and on the pipe's relative roughness.

Net positive suction head: the absolute pressure head available at a pump suction above the liquid's vapor pressure. Insufficient NPSH causes cavitation, so the available value must exceed the required value.

A single coefficient combining convective film coefficients, wall conduction, and fouling resistances to relate heat duty to area and temperature difference. Its reciprocal is the total resistance.

The effective average temperature driving force in a heat exchanger, computed from the terminal temperature differences at both ends. It applies to countercurrent or cocurrent flow with constant heat capacities.

A dimensionless heat transfer group giving the ratio of convective to conductive heat transfer across a fluid layer. Correlations express it as a function of Reynolds and Prandtl numbers.

An added thermal resistance representing deposits that build up on heat transfer surfaces, reducing the overall coefficient over time. It is included in exchanger design to provide excess area.

The concentration of a sparingly soluble gas dissolved in a liquid is proportional to the gas partial pressure above the liquid. The proportionality constant depends strongly on temperature and on the solvent.

The ratio of vapor-liquid equilibrium K values of two components, showing how easily they can be separated by distillation. Values far from one mean an easier separation and fewer required stages.

A proportionality constant between a molar flux and a concentration difference driving force at a phase boundary. It lumps together diffusion and convection effects in the film.

The lowest reflux that can achieve a specified separation with an infinite number of stages; operating near it makes columns tall and expensive. Designs usually use a multiple of it.

An expression giving reaction rate as a function of reactant concentrations, usually with a rate constant and exponents called orders. It must be determined experimentally, not from stoichiometry.

An equation showing that the rate constant rises exponentially with temperature through an activation energy and a pre-exponential factor. Plotting the logarithm of the rate constant against reciprocal temperature gives a straight line.

Continuous stirred-tank reactor: a vessel with continuous feed and product in which mixing is assumed perfect, so contents are uniform and the outlet has the same composition as the reactor.

A tubular reactor model in which fluid moves as a series of unmixed slices with no axial mixing, so composition changes continuously along the length. It usually needs less volume than a CSTR for the same conversion.

The extent to which a reaction produces a desired product rather than undesired by-products, often expressed as a ratio of moles. High selectivity reduces raw material waste and separation cost.

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

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

The total cost of designing, building, and installing a plant, including equipment, piping, buildings, and engineering, but excluding working capital and operating expenses. It is depreciated over the plant's life.

A feedback controller that computes output from the sum of three actions: proportional to error, integral of error over time, and derivative of error. Tuning its three constants determines loop stability and speed.

The delay between a change in controller output and the first observed response in the measured variable, caused by transport or analyzer lag. It makes control difficult and reduces achievable loop gain.

A control strategy that measures a disturbance and adjusts the manipulated variable before the disturbance affects the process. It acts in advance and is often combined with feedback.

The lowest temperature at which a liquid gives off enough vapor to form a flammable mixture with air near its surface. It is a key flammability property used to classify liquids for storage and handling.

The minimum concentration of a flammable gas or vapor in air below which a flame cannot propagate. Concentrations below it are too lean to burn; the upper limit marks the rich boundary.

A safety device that automatically opens to vent fluid when system pressure exceeds its set point, then recloses. It protects equipment from overpressure caused by fire, blockage, or control failure.

Hazard and operability study: a systematic team review that applies guide words to process parameters at each node to identify deviations, their causes, and consequences. It generates recommendations for safeguards.