UPDA Chemical key terms

An accounting of the mass entering, leaving, accumulating in, or being generated within a system over a defined period, based on conservation of mass. It underpins sizing, yield estimates, and emission calculations.

An accounting of all energy crossing a system boundary plus internal generation, accumulation, and consumption, usually written as enthalpy in equals enthalpy out plus losses. It supports utility sizing and heat integration studies.

The amount of a stream returned to a process step divided by the amount of fresh feed or product leaving that step. It affects overall conversion, equipment size, and operating cost in separation and reaction loops.

A deliberate bleed of a stream from a recycle loop to prevent buildup of inert or undesired components. Purge rate is set as a trade-off between component accumulation and loss of valuable material.

The fraction of a limiting reactant fed to a reactor that is consumed by reaction. It is a primary performance measure and links reactor design to downstream separation duty.

The fraction of reacted material that is converted into a desired product rather than undesired by-products. High selectivity reduces raw-material waste and downstream purification load.

The amount of desired product formed per amount of reactant fed or consumed, expressed as a fraction or mass ratio. It combines conversion and selectivity into a single measure of process efficiency.

The volumetric feed rate divided by reactor or catalyst volume, usually expressed per hour. It indicates how much feed a given reactor volume processes and is used to scale catalytic reactors.

The average time a fluid element spends inside a vessel or reactor, equal to volume divided by volumetric flow for ideal cases. It governs the extent of reaction and mixing in continuous equipment.

An ideal flow pattern in which fluid moves through a vessel as ordered, non-mixing layers with uniform velocity and no axial dispersion. It gives the highest conversion per volume for positive-order reactions.

Axial dispersion or recirculation that causes fluid leaving a region to mix with fluid entering it, reducing the driving force for reaction or separation. It is often characterised by a dispersion number or tracer response.

A thermodynamic property combining internal energy with the product of pressure and volume, convenient for constant-pressure processes. Changes in enthalpy give the heat absorbed or released in most process equipment.

The heat absorbed or released when a substance changes phase at constant temperature and pressure, without a change in temperature. It dominates duties in reboilers, condensers, and evaporators.

The amount of heat required to raise the temperature of a unit amount of material by one degree. It converts temperature changes into enthalpy changes for sensible heating and cooling calculations.

The temperature at which the first vapour bubble forms from a liquid mixture at a given pressure. It defines the lower bound of the two-phase region and is used in distillation and flash calculations.

The temperature at which the first liquid droplet condenses from a vapour mixture at a given pressure. It marks the upper bound of the two-phase region and is important for condensation and drying duties.

The ratio of the vapour-liquid equilibrium constants of two components, indicating how easily they can be separated by distillation. Values near one imply many stages and high reflux.

The condition in which a liquid and its vapour coexist with no net transfer of components between phases. The resulting composition relationships are the basis for distillation, absorption, and flash design.

A dimensionless ratio of inertial to viscous forces in a flowing fluid, used to classify flow as laminar, transitional, or turbulent. It sets friction factor and heat and mass transfer correlations.

A dimensionless coefficient relating pressure drop in a pipe to fluid velocity, density, pipe diameter, and length. It depends on Reynolds number and pipe roughness and drives pump sizing.

Net positive suction head, the margin of absolute pressure above a liquid's vapour pressure available at a pump suction. Insufficient margin causes cavitation, vibration, and impeller damage.

The reduction in pressure as fluid flows through a pipe, fitting, valve, or packed bed, caused by friction and flow obstructions. It determines pumping power and equipment operating limits.

A combined coefficient describing total resistance to heat flow across a wall, including convection on both sides and conduction through the material. It is used with area and temperature difference to size exchangers.

The effective average temperature driving force in a heat exchanger, calculated from the hot and cold stream terminal temperatures. It is used with the overall coefficient to determine required area.

The ratio of actual heat transferred to the maximum possible heat transfer for given inlet conditions and flow rates. It allows rating exchangers without separately computing the temperature difference.

An additional thermal resistance allowance added to account for deposits building up on heat transfer surfaces over time. It results in oversized exchangers and higher capital and cleaning costs.

An idealised equilibrium contact stage in which the leaving streams reach equilibrium. Real columns require more actual stages than theoretical stages, with the gap described by stage efficiency.

The ratio of liquid returned from the top of a distillation column to the product withdrawn. It trades operating cost against the number of stages needed for a given separation.

The superficial gas velocity at which drag on a bed of particles just balances their weight, marking the onset of fluidised behaviour. It sets the lower operating limit for fluidised-bed equipment.

A dimensionless group comparing the rate of chemical reaction to the rate of a transport process such as flow, mixing, or diffusion. It indicates whether a system is reaction- or transport-limited.

A coefficient relating the molar flux of a component to its concentration driving force at a phase boundary. It is central to sizing absorbers, strippers, dryers, and extraction equipment.

A feedback controller that combines proportional, integral, and derivative actions on the error between setpoint and measurement. Tuning its three parameters balances speed of response against stability and offset.

The delay between a change in a manipulated variable and the first detectable response in the measured variable, caused by transport lag or analyser delay. It limits achievable controller gain and stability.

A final control element that throttles flow in response to a controller signal, typically described by its flow characteristic and sizing coefficient. Its rangeability and authority strongly affect loop performance.

The gradual rise in concentration of non-reacting components within a recycle loop when they are not removed. It dilutes reactants and can force shutdown unless a purge is provided.

The time required for cumulative cash inflows from a project to recover its initial investment. It is simple to compute but ignores cash flows after the cutoff and the time value of money.

The sum of a project's future cash flows discounted to the present at a chosen rate, minus the initial investment. A positive value indicates the project adds value at that discount rate.

A valuation method that converts future cash flows into present value using a discount rate reflecting time and risk. It underpins investment appraisal and is often used to derive internal rate of return.

A structured team review that systematically applies guide words to process parameters to identify deviations, their causes, and consequences. It generates recommendations to reduce risk in a design.

A semi-quantitative method that evaluates independent protection layers against a target risk to determine required risk reduction. It is commonly used to set safety integrity levels for instrumented functions.

An independent control system designed to bring a process to a safe state when predetermined limits are exceeded. Its reliability is specified by a safety integrity level.

The minimum concentration of fuel vapour in air below which a flame will not propagate at specified conditions. It defines the lean boundary for flammable atmospheres and ventilation requirements.

The lowest temperature at which a liquid gives off enough vapour to form a flammable mixture with air near its surface. It is a key classification property for storage and handling hazards.