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A gas contains 75.0 wt% methane, 10.0% ethane, 5.0% ethylene, and the balance water. (a) Calculate the molar composition of this gas on both awet and a dry basis and the ratio (mol H2O/ mol dry gas). (b) If100kg/hofthisfuelistobeburnedwith30%excessair,whatistherequiredairfeedrate(kmol/ h)? How would the answer change if the combustion were only 75% complete?

1 Answer

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Answer:

a) molar composition of this gas on both a wet and a dry basis are

5.76 moles and 5.20 moles respectively.

Ratio of moles of water to the moles of dry gas =0.108 moles

b) Total air required = 68.51 kmoles/h

So, if combustion is 75% complete; then it is termed as incomplete combustion which require the same amount the same amount of air but varying product will be produced.

Step-by-step explanation:

Let assume we have 100 g of mixture of gas:

Given that :

Mass of methane =75 g

Mass of ethane = 10 g

Mass of ethylene = 5 g

∴ Mass of the balanced water: 100 g - (75 g + 10 g + 5 g)

Their molar composition can be calculated as follows:

Molar mass of methane
CH_4}= 16 g/mol

Molar mass of ethane
C_2H_6= 30 g/mol

Molar mass of ethylene
C_2H_4 = 28 g/mol

Molar mass of water
H_2O=18g/mol

number of moles =
(mass)/(molar mass)

Their molar composition can be calculated as follows:


n_(CH_4)= (75)/(16)


n_(CH_4)= 4.69 moles


n_(C_2H_6) = (10)/(30)


n_(C_2H_6) = 0.33 moles


n_(C_2H_4) = (5)/(28)


n_(C_2H_4) = 0.18 moles


n_(H_2O)= (10)/(18)


n_(H_2O)= 0.56 moles

Total moles of gases for wet basis = (4.69 + 0.33 + 0.18 + 0.56) moles

= 5.76 moles

Total moles of gas for dry basis = (5.76 - 0.56)moles

= 5.20 moles

Ratio of moles of water to the moles of dry gas =
(n_(H_2O))/(n_(drygas))

=
(0.56)/(5.2)

= 0.108 moles

b) If 100 kg/h of this fuel is burned with 30% excess air(combustion); then we have the following equations:


CH_4 + 2O_2_((g)) ------> CO_2_((g)) +2H_2O

4.69 2× 4.69

moles moles


C_2H_6+ (7)/(2)O_2_((g)) ------> 2CO_2_((g)) + 3H_2O

0.33 3.5 × 0.33

moles moles


C_2H_4+3O_2_((g)) ----->2CO_2+2H_2O

0.18 3× 0.18

moles moles

Mass flow rate = 100 kg/h

Their Molar Flow rate is as follows;


CH_4 = 4.69 k moles/h\\C_2H_6 = 0.33 k moles/h\\C_2H_4=0.18kmoles/h

Total moles of
O_2 required = (2 × 4.69) + (3.5 × 0.33) + (3 × 0.18) k moles

= 11.075 k moles.

In 1 mole air = 0.21 moles
O_2

Thus, moles of air required =
(1)/(0.21)*11.075

= 52.7 k mole

30% excess air = 0.3 × 52.7 k moles

= 15.81 k moles

Total air required = (52.7 + 15.81 ) k moles/h

= 68.51 k moles/h

So, if combustion is 75% complete; then it is termed as incomplete combustion which require the same amount the same amount of air but varying product will be produced.

User Juanillo
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