PE Chemical flashcards

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

Reynolds number

Re = ρvD/μ for pipe flow; values below about 2100 indicate laminar flow and above about 4000 indicate turbulent flow.

Ideal gas law

PV = nRT relates pressure, volume, moles, and absolute temperature; at 1 atm and 0°C one mole occupies 22.414 L.

Gibbs phase rule

F = C - P + 2 gives the number of intensive degrees of freedom for a nonreactive system with C components and P phases at equilibrium.

Material balance equation

Accumulation equals input minus output plus generation minus consumption; at steady state with no reaction, total mass in equals total mass out.

Heat of reaction

Calculated from standard heats of formation as ΔH_rxn = Σ ν_i ΔH_f,i, using negative coefficients for reactants and positive coefficients for products.

Excess air

Percent excess air equals actual air supplied minus theoretical air, divided by theoretical air, multiplied by 100; it helps ensure complete combustion.

Carnot efficiency

Maximum thermal efficiency of a heat engine operating between hot and cold reservoirs is η = 1 - T_C/T_H, with both temperatures absolute.

Equilibrium constant

K = exp(-ΔG°/RT) relates standard Gibbs free energy change to equilibrium; K greater than one favors products at standard conditions.

Compressibility factor

Z = PV/(nRT) measures deviation from ideal-gas behavior; Z equals one for an ideal gas and is near one at low pressure.

Bernoulli equation

For steady, incompressible, frictionless flow, P/ρ + v²/2 + gz is constant along a streamline; it balances pressure, kinetic, and potential energy.

Darcy-Weisbach equation

Friction pressure drop in a pipe is ΔP = f(L/D)(ρv²/2), where f is the Darcy friction factor and L and D are pipe length and diameter.

Pump hydraulic power

Useful pump power equals ρgQH, where ρ is density, Q is volumetric flow, g is gravity, and H is pump head; divide by efficiency for shaft power.

Fourier conduction law

Heat conduction rate is q = -kA dT/dx, where k is thermal conductivity, A is area, and dT/dx is the temperature gradient.

Overall heat transfer coefficient

U combines convection and conduction resistances; 1/U equals the sum of inside film, wall, fouling, and outside film resistances on a consistent area basis.

Biot number

Bi = hL/k compares internal conduction resistance to external convection resistance; lumped capacitance analysis is valid when Bi is less than about 0.1.

Fick's first law

Diffusive molar flux is J_A = -D_AB dC_A/dx, where D_AB is the diffusivity and dC_A/dx is the concentration gradient in the direction of diffusion.

Sherwood number

Sh = k_c L/D relates the mass-transfer coefficient k_c, characteristic length L, and diffusivity D; it is the mass-transfer analog of the Nusselt number.

Relative volatility

α = (y_A/x_A)/(y_B/x_B) measures separation ease in distillation; larger α means fewer stages are needed for a given separation.

Arrhenius equation

Rate constant k = A exp(-E_a/RT), where A is the pre-exponential factor, E_a is activation energy, R is the gas constant, and T is absolute temperature.

CSTR design equation

For steady-state reaction, reactor volume V = F_A0 X / (-r_A), where F_A0 is feed molar flow, X is conversion, and -r_A is reaction rate.

PFR design equation

For plug flow, V = F_A0 ∫ dX/(-r_A) from zero to X; the integral accounts for rate changing with conversion along the reactor.

Six-tenths rule

Capital cost scales with capacity as C2 = C1(S2/S1)^0.6; it estimates equipment cost at a different size using a known cost and capacity.

Net present value

NPV sums discounted future cash flows minus initial investment; a positive NPV means the project earns more than the discount rate.

Payback period

Payback period is the time required for cumulative undiscounted cash inflows to equal the initial investment; it ignores cash flows after payback.

PID controller equation

Controller output u = K_c e + (K_c/τ_I)∫e dt + K_c τ_D de/dt, where e is error, K_c is gain, and τ_I and τ_D are integral and derivative times.

Integral control action

Integral action adds output proportional to the time integral of error; it eliminates steady-state offset but can reduce stability and cause windup.

Ultimate gain

Ultimate gain is the proportional gain at which a closed-loop system with integral and derivative action off first sustains continuous oscillation; it is used in Ziegler-Nichols tuning.

Lower flammability limit

The LFL is the minimum fuel concentration in air that sustains combustion; below it the mixture is too lean to burn.

HAZOP guide words

HAZOP systematically applies guide words such as no, more, less, and reverse to process parameters to identify deviations and their consequences.

Inherent safety hierarchy

Inherent safety prioritizes minimizing hazardous material, substituting safer materials, moderating conditions, and simplifying designs rather than adding control layers.

Recycle and purge

Recycle returns unconverted material to increase overall conversion, while a purge removes inert or accumulating species to prevent buildup in the loop.

Height of transfer unit

Packed-column height equals HTU times NTU; HTU represents the height for one transfer unit and depends on flow, packing, and mass-transfer coefficient.