Chemical Engineering

85 flashcards by MentisLibrary

Mass and energy balances, fluid flow, heat transfer, distillation, and reactor design. The unit operations and first principles behind every process flow diagram you will ever read.

What does it mean for an equation to be dimensionally homogeneous?
Every additive term has the same dimensions, and quantities can only be added, subtracted or equated if their units match. Arguments of exp, ln, sin and similar functions must be dimensionless.
How do you convert a quantity between unit systems?
Multiply by conversion factors written as ratios equal to 1, arranged so the unwanted units cancel. Chain as many factors as needed.
Convert between the four common temperature scales.
Kelvin and Rankine are absolute; Celsius and Fahrenheit are relative. Degree sizes: 1 K = 1 C degree and 1 R = 1 F degree.
Distinguish gauge pressure from absolute pressure.
Gauge pressure is measured relative to local atmospheric pressure; absolute pressure is measured from a perfect vacuum. Thermodynamic equations require absolute pressure.
How are mass flow rate and volumetric flow rate related?
Through density. Volumetric flow depends on temperature and pressure, so mass or molar flow is the more stable basis for a balance.
What is specific gravity and how does it relate to density?
Specific gravity is the ratio of a substance's density to a reference density, normally liquid water at 4 C (1000 kg/m^3). It is dimensionless.
Convert mass fractions to mole fractions in a mixture.
Take a convenient basis mass, divide each component mass by its molar mass to get moles, then divide each by the total moles.
How do you compute the average molecular weight of a mixture?
On a mole basis it is the mole-fraction-weighted sum of molar masses. On a mass basis the reciprocal is the mass-fraction-weighted sum of reciprocals.
State the general material balance equation.
For any species over any control volume: input minus output plus generation minus consumption equals accumulation. Generation and consumption are zero for non-reactive species and for total mass.
How do you carry out a degrees of freedom analysis?
Count unknown variables, subtract the number of independent equations (balances, given relations, specified compositions summing to one). Zero degrees of freedom means the problem is exactly solvable.
Why is a recycle stream used in a process?
To return unconverted reactant to the reactor, raising overall conversion and reducing raw material cost, and sometimes to moderate reactor temperature or maintain flow.
What is a purge stream and what happens without one?
A small stream bled from a recycle loop to remove inerts or byproducts. Without it, species that enter but never leave accumulate without bound until the loop composition is ruined.
What does a bypass stream accomplish?
It routes part of a feed around a unit and remixes it downstream, giving fine control of the outlet condition without changing the unit itself.
Distinguish single-pass conversion from overall conversion.
Single-pass conversion uses the reactor inlet and outlet, so it includes recycled material. Overall conversion uses the fresh feed into the process and the net product leaving it.
Define the extent of reaction.
A single variable that tracks how far a reaction has progressed, common to every species. Moles of each species change by their stoichiometric coefficient times the extent.
How is the limiting reactant identified?
Divide each reactant's fed moles by its stoichiometric coefficient. The smallest ratio is the limiting reactant, the one that runs out first.
How is percent excess of a reactant calculated?
Compare the amount fed against the amount stoichiometrically required to consume the limiting reactant completely.
Define fractional conversion of a reactant.
The fraction of the reactant fed that has reacted.
Distinguish yield from selectivity in a multiple reaction system.
Yield compares desired product actually formed against what would form if the limiting reactant fully reacted to that product. Selectivity compares desired product to undesired product formed.
When is an atomic species balance preferable to a molecular balance?
When reactions occur, especially several unknown ones. Atoms of C, H, O and N are conserved regardless of stoichiometry, so no generation or consumption terms appear.
Write the first law for a closed system.
The change in internal energy equals heat added minus work done by the system.
Write the steady state open system energy balance.
Heat added minus shaft work equals the change in enthalpy plus kinetic and potential energy between outlet and inlet.
Why is enthalpy defined as H = U + PV?
Because flow streams carry both internal energy and the flow work needed to push them across the boundary. Bundling the two makes open system balances simple.
How is sensible heat calculated?
Integrate heat capacity over the temperature change. If Cp is roughly constant, the integral reduces to a product.
How do Cp and Cv relate for an ideal gas?
They differ by the gas constant on a molar basis, because constant pressure heating also does expansion work.
What is latent heat and why does temperature stay constant during a phase change?
Latent heat is the enthalpy absorbed or released during a phase change at constant temperature and pressure. The energy goes into breaking intermolecular attractions rather than into raising kinetic energy.
State Hess's law and its practical value.
The enthalpy change of a reaction is independent of the path taken, so reactions can be added, reversed and scaled to build an unmeasured heat of reaction from measured ones.
How is a standard heat of reaction computed from heats of formation?
Sum the formation enthalpies of the products, each times its stoichiometric coefficient, and subtract the same sum for reactants.
What is the adiabatic flame temperature?
The temperature reached if all the heat released by complete combustion goes into heating the product gases, with no heat loss and no work.
State the ideal gas law and the conditions under which it holds.
Pressure times volume equals moles times gas constant times absolute temperature. It applies at low pressure and high temperature, where molecules are far apart and intermolecular forces are weak.