Material and Energy Balances: cram pack

Formula sheets, the key method steps for every topic and 8 practice problems, with full solutions at the back.

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1Formulas and methods

Mass balance

Steady state
in = out
Total balance
ṁ1 + ṁ2 = ṁ3
Component balance
ṁ1x1 + ṁ2x2 = ṁ3x3
Evaporator
F·xF = P·xP, W = F − P

Energy balance

Sensible heat
Q = ṁ·cp·ΔT
Latent heat
Q = ṁ·hfg
Mixing
ΣṁcpTin = ΣṁcpTout
Water
cp ≈ 4.18 kJ/(kg·K)

Ideal gas law

Ideal gas law
pV = nRT
Gas constant
R = 8.314 J/(mol·K)
Combined gas law
p1V1/T1 = p2V2/T2
Moles
n = m/M
Temperature
T[K] = T[°C] + 273.15

Stoichiometry

Moles
n = m/M
Concentration
c = n/V
Mole ratio
use the coefficients of the balanced equation
Limiting reactant
the one that runs out first
Percent yield
actual / theoretical × 100%

Method

  1. Write and balance the chemical equation.
  2. Convert the given mass to moles: n = m/M.
  3. Use the coefficients as the mole ratio to find the moles of the substance you want.
  4. Convert back to mass with m = n·M. With two reactants, find the limiting reactant first.

2Practice problems

1.Mass balanceMedium
MixerStream 1850 kg/h · 5% saltStream 2ṁ₂ = ? · 50% saltProduct45% salt

How much of stream 2 must be added each hour to stream 1 so that the blended product contains 45% salt? Find ṁ2.

ṁ1 = 850 kg/hx1 = 5 %x2 = 50 %x3 = 45 %
ṁ2 = kg/h
2.Mass balanceHard
EvaporatorFeed F1800 kg/h · 19% saltProduct PP = ? · 35%Vapour W (pure water)0% salt

An evaporator concentrates a salt solution by boiling off pure water. Find the product flow P in kg/h.

F = 1800 kg/hxF = 19 %xP = 35 %
P = kg/h
3.Energy balanceMedium
hot sidecold sideethanol · 120 °C70 °Cwater · 20 °Ct = ? °C

Hot ethanol cools water in a counter-flow heat exchanger with no heat loss. Find the water outlet temperature tout.

ṁh = 2.5 kg/scp,h = 2.44 kJ/(kg·K)Th: 120 → 70 °Cṁc = 1.5 kg/scp,c = 4.18 kJ/(kg·K)tin = 20 °C
tout = °C
4.Energy balanceHard
hot sidecold sideglycol mix · 130 °C80 °Cwater · 15 °C45 °C · ṁ = ?10% lost

Hot glycol mix heats water in an exchanger that loses 10% of the heat released by the hot stream to the surroundings. Find the water flow rate ṁc needed.

ṁh = 2.5 kg/scp,h = 3.6 kJ/(kg·K)Th: 130 → 80 °Ct: 15 → 45 °Ccp,c = 4.18 kJ/(kg·K)loss = 10 %
ṁc = kg/s
5.Ideal gas lawMedium
220 g of ArV = 10 LT = 55 °CP = ?

A tank of 10 L contains 220 g of Ar at 55 °C. Treat it as an ideal gas and find the pressure P.

m = 220 gM = 39.95 g/molV = 10 LT = 55 °CR = 8.314 J/(mol·K)
P = kPa
6.Ideal gas lawHard
O₂, M = 32 g/molP = 850 kPaT = 130 °Cρ = ?

Find the density ρ of O₂ at 850 kPa and 130 °C, treating it as an ideal gas.

M = 32 g/molP = 850 kPaT = 130 °CR = 8.314 J/(mol·K)
ρ = kg/m³
7.StoichiometryMedium
m(X)n(X)n(AlCl3​)m(AlCl3​)÷ M(X)× 2/ν× M(AlCl3​)

20 g of Al and 32 g of Cl2 react according to 2Al + 3Cl2 → 2AlCl3. What mass m of AlCl3 can form?

M(Al) = 26.98M(Cl2) = 70.9M(AlCl3) = 133.33 g/mol
m = g
8.StoichiometryHard
m(H2​)n(H2​)n(H2​O)m(H2​O)÷ M(H2​)× 2/2× M(H2​O)

In the reaction 2H2 + O2 → 2H2O, 8 g of H2 reacts completely (the other reactants are in excess) and 44.33 g of H2O is collected. Find the percent yield Y.

M(H2) = 2.016 g/molM(H2O) = 18.02 g/molactual: 44.33 g
Y = %

3Solutions

1.Mass balance

Answer: ṁ2 = 6800 kg/h

Steady-state mass balance: in = out for the total and for each component. Blending problems combine both.

Balances

Total: ṁ3 = ṁ1 + ṁ2
salt: ṁ1x1 + ṁ2x2 = (ṁ1 + ṁ2)x3

Solve for ṁ2

ṁ2 = ṁ1(x1 − x3)/(x3 − x2) = 850·(5 − 45)/(45 − 50) = 6800 kg/h

Answer

ṁ2 = 6800 kg/h
2.Mass balance

Answer: P = 977.1 kg/h

Tie component: a component that passes through unchanged in one stream (here the solute) links the flows directly. Then the total balance gives the rest.

Solute balance

F·xF = P·xP → P = 1800·19/35 = 977.1 kg/h

Answer

P = 977.1 kg/h
3.Energy balance

Answer: tout = 68.64 °C

Exchanger energy balance: with no losses, ṁhcp,h(Th,in − Th,out) = ṁccp,c(tout − tin).

Heat released by the hot stream

Q = 2.5·2.44·(120 − 70) = 305 kW

Heat picked up by the water

tout = tin + Q/(ṁccp,c) = 20 + 305/(1.5·4.18) = 68.64 °C

Answer

tout = 68.64 °C
4.Energy balance

Answer: ṁc = 3.23 kg/s

Energy balance with losses: heat released = heat absorbed + heat lost.

Heat released

Qh = 2.5·3.6·(130 − 80) = 450 kW

Heat that reaches the water

Qc = (1 − 0.1)·450 = 405 kW

Water flow rate

ṁc = Qc/(cp,c·Δt) = 405/(4.18·30) = 3.23 kg/s

Answer

ṁc = 3.23 kg/s
5.Ideal gas law

Answer: P = 1502 kPa

Ideal gas law: PV = nRT, with n = m/M.

Moles

n = m/M = 220/39.95 = 5.507 mol
T = 55 + 273.15 = 328.1 K

Pressure

P = nRT/V = 5.507·8.314·328.1/0.01 = 1502 kPa

Answer

P = 1502 kPa
6.Ideal gas law

Answer: ρ = 8.115 kg/m³

Ideal gas density: ρ = PM/(RT).

Rearrange the gas law

PV = (m/M)RT → ρ = m/V = PM/(RT)
T = 130 + 273.15 = 403.1 K

Density

ρ = 850·32/(8.314·403.1) = 8.115 kg/m³

Answer

ρ = 8.115 kg/m³
7.Stoichiometry

Answer: m = 40.12 g

Stoichiometry: a balanced equation counts particles, not grams. Convert mass to moles with n = m/M, use the coefficients of the equation as the mole ratio, then convert back with m = n·M. With two reactants, the limiting reactant is the one that runs out first and fixes how much product forms.

Moles of each reactant

n(Al) = 20/26.98 = 0.7413 mol
n(Cl2) = 32/70.9 = 0.4513 mol

Limiting reactant

0.7413/2 = 0.3706, 0.4513/3 = 0.1504 → Cl2

Mass of product

n(AlCl3) = 0.1504·2 = 0.3009 mol
m = 0.3009·133.33 = 40.12 g

Answer

m = 40.12 g
8.Stoichiometry

Answer: Y = 61.99 %

Stoichiometry: a balanced equation counts particles, not grams. Convert mass to moles with n = m/M, use the coefficients of the equation as the mole ratio, then convert back with m = n·M. With two reactants, the limiting reactant is the one that runs out first and fixes how much product forms.

Theoretical yield

n(H2) = 8/2.016 = 3.968 mol
m = 3.968·2/2·18.02 = 71.51 g

Percent yield

Y = 44.33/71.51·100 = 61.99%

Answer

Y = 61.99%

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