PEC EPE Chemical flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Steady-state mass balance

For a control volume at steady state, accumulation is zero, so total mass entering equals total mass leaving plus any generation minus consumption within the boundary.

Gibbs free energy criterion

At constant temperature and pressure, a process is spontaneous when the change in Gibbs free energy is negative; equilibrium corresponds to zero change.

Compressibility factor Z

Z equals PV divided by nRT and measures deviation from ideal-gas behavior; Z near one indicates ideal gas, while large deviations require an equation of state.

Heat of reaction from formation

Standard heat of reaction equals the sum of formation enthalpies of products minus reactants, each multiplied by stoichiometric coefficient; elements in standard states have zero formation enthalpy.

Reynolds number

Re equals density times velocity times diameter divided by viscosity; it indicates laminar flow below about 2100 and turbulent flow above about 4000 in circular pipes.

Darcy-Weisbach equation

Pressure drop equals friction factor times length over diameter times density times velocity squared over two; friction factor depends on Reynolds number and relative roughness.

Log mean temperature difference

LMTD equals the temperature difference at one end minus the other end divided by the natural logarithm of their ratio, used for heat exchanger sizing with countercurrent or multipass correction.

NPSH available

NPSH available equals absolute suction pressure minus vapor pressure, divided by density times gravity, plus velocity head minus suction-line losses and static lift; it must exceed pump NPSH required.

Fick's first law

Molar flux of component A equals negative diffusivity times concentration gradient; the negative sign shows net diffusion from high to low concentration.

Relative volatility

Relative volatility equals the vapor-liquid equilibrium ratio of the more volatile component divided by that of the less volatile component; values above one indicate easier distillation.

Kremser equation

The Kremser equation relates absorption or stripping stage count to absorption factor, inlet and outlet solute ratios, and equilibrium; it assumes constant flow and equilibrium slope.

Langmuir isotherm

Langmuir adsorption assumes a finite number of identical sites with no multilayer coverage; loading equals maximum capacity times partial pressure divided by a constant plus partial pressure.

Arrhenius equation

Rate constant equals pre-exponential factor times exponential of negative activation energy divided by gas constant times temperature; higher temperature or lower activation energy increases rate.

CSTR design equation

For a continuous stirred-tank reactor at steady state, volume equals molar feed rate times conversion divided by the reaction rate evaluated at outlet conditions.

PFR design equation

For a plug-flow reactor, volume equals molar feed rate times the integral from zero to conversion of reciprocal reaction rate with respect to conversion.

Thiele modulus

Thiele modulus compares intrinsic reaction rate to pore diffusion rate; large values indicate strong diffusion limitation and effectiveness factor below one.

Net present value

NPV sums each cash flow divided by one plus discount rate raised to its period, then subtracts initial investment; positive NPV means the project exceeds the required return.

Six-tenths rule

The six-tenths rule estimates equipment cost at a new capacity by multiplying known cost by the capacity ratio raised to about 0.6, valid for similar equipment and modest ranges.

Degrees of freedom

Degrees of freedom equal the number of independent variables minus independent equations; a process model is exactly specified when this count is zero.

Sequential modular simulation

Sequential modular simulation solves each process unit in flow order, using recycle convergence loops; it is robust for flowsheets but can be slow with many recycles.

Lower flammable limit

The lower flammable limit is the minimum fuel concentration in air that supports combustion; below it the mixture is too lean to burn.

HAZOP guidewords

HAZOP guidewords such as no, more, less, reverse, and as well as prompt systematic deviation questions for each node, identifying causes, consequences, and safeguards.

Layer of protection analysis

LOPA estimates risk by combining initiating-event frequency with independent protection-layer probabilities; required risk reduction determines additional safeguards or safety integrity level.

Inherent safety hierarchy

Inherent safety prioritizes eliminating or minimizing hazards through chemistry, inventory, and conditions before adding engineered controls, administrative procedures, or personal protective equipment.

PID controller equation

A PID output combines proportional error, integral of error, and derivative of error, each with a tuning constant; integral removes offset and derivative anticipates change.

Dead time

Dead time is the delay between a manipulated-variable change and the measurable response; it reduces achievable controller gain and destabilizes feedback loops.

Cascade control

Cascade control uses a primary controller to set the setpoint of a faster secondary controller, rejecting disturbances in the inner loop before they affect the primary variable.

Control valve fail-safe

Fail-safe action defines whether a valve opens, closes, or stays put on loss of signal or power; it is selected from process safety consequences.

Chloride stress corrosion cracking

Chloride stress corrosion cracking affects austenitic stainless steels under tensile stress, chloride exposure, and elevated temperature; avoiding chlorides or using duplex or ferritic grades reduces risk.

Galvanic corrosion

Galvanic corrosion occurs when dissimilar metals contact in an electrolyte; the more active metal becomes anodic and corrodes faster, driven by the potential difference.

Minimum design metal temperature

MDMT is the lowest temperature at which equipment may operate without unacceptable brittle fracture risk; impact testing or material selection is required below it.

Cathodic protection

Cathodic protection makes the protected metal the cathode by supplying electrons from a sacrificial anode or impressed current, reducing corrosion rate.