GATE CH key terms

A single physical processing step, such as distillation, filtration, or drying, that changes a stream's composition, temperature, or form without altering its chemical identity.

A processing step in which chemical bonds are broken or formed, so the species present change; examples include nitration, polymerization, and catalytic cracking.

Movement of one or more species through a mixture driven by a concentration gradient, described by Fick's law and quantified by a mass-transfer coefficient.

A proportionality constant relating heat flux to a temperature difference between a surface and a bulk fluid, with units of W m^-2 K^-1.

A single coefficient combining convective, conductive, and fouling resistances in series so that total heat duty equals U times area times driving force.

The effective average temperature driving force in a heat exchanger, computed as the difference of terminal differences divided by the natural log of their ratio.

A heat exchanger design approach comparing actual duty to the thermodynamic maximum, using effectiveness, number of transfer units, and capacity ratio.

An added thermal resistance accounting for deposits on heat-transfer surfaces, which reduces the achievable overall coefficient over time and must be included in design.

A dimensionless ratio of convective to conductive heat transfer across a fluid layer; for fully developed laminar pipe flow it approaches a constant value near 3.66.

A dimensionless ratio of inertial to viscous forces that sets the flow regime, used to decide laminar or turbulent behaviour and to correlate friction factor and heat transfer.

A dimensionless quantity relating pressure drop in a conduit to velocity head; the Darcy form gives pressure drop as f times length over diameter times dynamic pressure.

A correlation for pressure drop through a packed bed that adds a viscous (laminar) term proportional to superficial velocity and an inertial (turbulent) term proportional to its square.

The ratio of the vapour pressures or K-values of two components, indicating how easily they can be separated by distillation; values near one require many stages.

A graphical stage-by-stage design technique for binary distillation that steps between the equilibrium curve and operating lines to count theoretical stages.

The reflux rate at which the required number of equilibrium stages becomes infinite for a specified separation; practical designs operate at a multiple of it.

A relation giving the minimum number of theoretical stages at total reflux from the relative volatility and the specified light and heavy key mole fractions.

A material-balance line on a stagewise separation diagram linking passing stream compositions between stages; its slope reflects the reflux or solvent-to-feed ratio.

Number of transfer units, a dimensionless measure of how hard a separation is; it equals the integral of composition change divided by the local driving force.

Height of a transfer unit, the packed height required for one transfer unit; column packing height is the product of HTU and NTU.

A mass-transfer model assuming a stagnant film of fixed thickness next to an interface, giving a coefficient equal to diffusivity divided by film thickness.

A mass-transfer model in which eddies expose fresh fluid to the interface for short contact times, yielding a coefficient proportional to the square root of diffusivity.

A rate constant expressing molar flux per unit interfacial area per unit concentration driving force, commonly correlated through dimensionless groups such as Sherwood number.

The fraction of a limiting reactant fed that has reacted, computed as moles reacted divided by moles fed to the reactor.

The fraction of reacted limiting reactant that ends up in the desired product, distinguishing good conversion from useful product formation.

Moles of desired product formed per mole of limiting reactant fed, equal to the product of conversion and selectivity when defined on that basis.

A single variable measuring how far a reaction has proceeded, such that each species' mole change equals its stoichiometric coefficient times the extent change.

The ratio of product to reactant activities at equilibrium; its temperature dependence is governed by the standard enthalpy of reaction through the van't Hoff relation.

A system at equilibrium shifts to oppose a disturbance in temperature, pressure, or concentration, guiding choices of operating conditions in reactor and separation design.

An empirical relation giving the rate constant as a pre-exponential factor times the exponential of minus activation energy over gas constant times temperature.

The minimum energy barrier reactants must overcome for a reaction to proceed; its magnitude sets how strongly rate responds to temperature change.

An expression relating reaction rate to species concentrations and temperature, with exponents that must be determined experimentally rather than from stoichiometry.

A dimensionless group comparing reaction rate to convective or diffusive transport rate; large values indicate transport limitation and possible mass-transfer control.

A dimensionless ratio of reaction rate to diffusion rate inside a porous catalyst pellet; large values cause steep internal concentration gradients.

The ratio of actual reaction rate in a porous catalyst pellet to the rate if the entire interior were exposed to surface conditions; below one when diffusion limits.

A vessel filled with solid particles through which fluid flows, used for catalysis, absorption, or filtration; pressure drop follows the Ergun equation.

Suspension of solid particles by an upward fluid flow once the drag balances particle weight, giving a bed that behaves like a boiling liquid.

The superficial fluid velocity at which bed pressure drop equals the particle weight per unit area, marking the onset of fluidization.

The steady settling speed of a particle when drag balances buoyant weight, used in gravity settling and elutriation calculations.

A non-Newtonian material that stays rigid until stress exceeds a yield value, after which it flows with a constant apparent viscosity.

In process control, a manual or automatic path that routes a signal or stream around an instrument or unit, often used during startup or failure.

A feedback controller whose output sums proportional, integral, and derivative actions on the error, tuned to balance speed, offset, and stability.

A scheme that measures the controlled variable and adjusts a manipulated variable based on the deviation from setpoint, correcting disturbances after they appear.

The delay between a change in manipulated variable and the first observed response in the controlled variable, which degrades achievable control loop stability.

An accounting of mass entering, leaving, accumulating in, and being generated within a system, forming the basis for process design calculations.

An accounting of energy entering, leaving, accumulating in, and being generated within a system, usually written as enthalpy changes plus heat and work terms.

Returning part of a product or unreacted stream to an earlier point in the process to raise overall conversion or recover material.

A small stream deliberately removed from a recycle loop to prevent accumulation of inert or undesired components, traded off against loss of valuable material.