2 replies to this topic

[XBOX999]

Please help me solve this question I have midterm after two days.
1. Jet fuel, also known as isooctane (C8H18), is combusted in a turbine engine. Carbon monoxide is not present in the exhaust gases and 90% of the isooctane is converted to CO2. The fuel is combusted in 20% excess air. Calculate the composition of the exhaust gases, based on the two following methods: (1) using the extent of reaction, and (2) a balance on the atomic elements.

[Andres Valencia Michaud]

Please help me solve this question I have midterm after two days.
1. Jet fuel, also known as isooctane (C8H18), is combusted in a turbine engine. Carbon monoxide is not present in the exhaust gases and 90% of the isooctane is converted to CO2. The fuel is combusted in 20% excess air. Calculate the composition of the exhaust gases, based on the two following methods: (1) using the extent of reaction, and (2) a balance on the atomic elements.

I'm gonna reply this because it can be seen you are a first year student, otherwise you would do this in no-time! This forum is NOT a place where students post their homework for others to do it, but I don't like to leave people without an answer...

FIRST: I'm not going to give you the results. I'm gonna give you the way to solve it.
SECOND: If you didn't know how to solve this particular problem, then be very much worried for your exam in two days. Go study.

SOLUTION:
Method 1

a) First solve the stoichiometry of the reaction. It says there's no CO present, meaning a full combustion (in gaseous state).

C8H18 + 25/2 O2 --> 8 CO2 + 9 H2O

Set a CALCULATION BASE, for example: feed is 1 mole of isooctane.

c) Calculate the oxygen needed for the fuel to be fully burnt, in this case, if you use 1 mole of isooctane, you'll need 25/2 moles of oxygen, but the problem says "there is an excess of 20% air" so, not only you'll need to calculate the amount of oxygen really fed, but also how much NITROGEN comes with it.

d) Solve the mass balance. You have 1 mole of isooctane, but only 90% of it will be consumed, so in the flue gases you'll have: oxygen, nitrogen, water, carbon dioxide AND isooctane.

e) Be smart... the problem says "calculate the composition of the exhaust gases..." you have two options: either you use a mole basis or a mass basis, but when we talk about gases we always use mole basis as this also represents the volume basis. DONE!!

Method 2:
This is just an atom balance... there's not much to talk about. The results are not the same because here you'll find how much carbon, oxygen (elemental), nitrogen and hydrogen is present.

I hope this helps you. Then again, don't ask in the forum to solve your problems, because it's your duty to study to become an engineer. Ask for DIRECTIONS!!

[XBOX999]

Thank you. That helps me a lot.