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SCHOOL OF CHEMICAL AND BIOMEDICAL ENGINEERING

Nanyang Technological University

Year 1 Lab

Material/energy Balance for a Combustion Process at Steady State

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CH1702 Material/energy Balance for a Combustion Process at Steady State Objectives:

1. Formulate the material & energy balance for a steady-state combustion process; 2. Understand and calculate the air excess ratio, stoichiometry, higher heating value, lower

heating value, etc. 3. Learn the principle & the application of Gas Chromatography (GC).

Principles & Definitions: Under the atmospheric pressure, a fuel (propane gas) reacts with O2 (Air, O2: N2 = 1 :3.76 ). The process is exothermic and the stoichiometric combustion formula is:

HNOHCONOHC Δ+++→++ 2222283 8.1843)76.3(5 As the Air/fuel ratio changes, the enthalpy of combustion and the composition of the product (exhaust gases) will change accordingly. However, the principles of energy and materials balance are always valid and can be used to solve the unknowns. Theoretical & Excess Air:

Theoretical air is the quantity of air that contains theoretical oxygen (5 moles in the above example)

Percent excess air: %100)(

)()(×

−

theo

theofed

airmolesairmolesairmoles

Dry and Wet basis: composition on a wet basis denotes the component mole fractions of a gas that contains water; composition on a dry basis signifies the component mole fraction of the same gas without water. Standard heat (enthalpy) of combustion, 0

cHΔ : the heat of reaction of the substance with oxygen to yield specific products (e.g., CO2, H2O), with both reactants and products are at the reference temperature and pressure (25°C, 1 atm). Standard heat (enthalpy) of formation, 0

fHΔ : enthalpy change associated with the formation of 1 mole compound from naturally occurring elements under reference state (25°C, 1 atm ). (e.g. H2 + ½ O2 H2O + 0

fHΔ ). The heat of combustion is related to heat of formation:

∑∑ Δ−Δ=Δtsreac

fiiproducts

fiic HHHtan

000 γγ

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Higher heating value (HHV): Negative of heat of combustion when H2O is at liquid state in the product. Lower heating value (LHV): Negative of combustion when H2O is at vapor state in the product. First laws for open system at steady state: Our system is the open system at steady state, the first laws can be written as: Enthalpy changes for processes involving chemical reaction: Two methods exists for the calculation of the overall enthalpies changes, they are: (1) heat of reaction method (molecular species at 25°C, 1atm), and (2) heat of formation method (naturally occurring elemental species at 25°C, 1atm). Details can be read from the Text book of CH1004: Elementary Principles of Chemical Processes.

Equipment:

“Fireboy” burners equipped with quartz tube are used as the combustion device (chambers), propane gas as the fuel.

A MKS Mass gas flow controller is used to regulate the volumetric flow rate of the fuel gas.

Shimadzu GC 14B is used to analyze the composition of the exhaust gases (Appendix I).

A micro-syringe is used as the sampling device.

Experimental procedures

1. Start the GC (follow the procedures listed in the Lab) and wait for the base-line to be stable (The lab staff may have done this for you); 2. Turn on the fuel gas supply, switch on the mass flow controller (MFC) and check zero. 3. Set the propane flow rate, then ignite (ask lab-staff for safety precautions). 4. Wait (at least 3 mins) until the combustion reach steady state; 5. Take the sample from the exhaust gas using the micro-syringe, measure the temperature of outlet exhaust. 6. Inject the sample gas into the analytical port of the GC and start analysis; (Please note: the GC result is on dry basis) 7. Switch off the burner, the MFC and turn off the gas supply valve; 8. Do the computation/analysis and try to work on the logsheet.

→Δ+Δ+Δ=+•••••

pks EEHWQ ∑∑••••

−=Δ=in

iiout

ii HnHnHQ^^

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Reports: a) The experimental data may be shared within a group. The sample data calculation and the

analysis/discussions should be done individually, Identical reports will be equally penalized.

b) Calculate the higher heating value (HHV) and the low heating value (LHV) of propane gas; c) Calculate the composition of the exhaust gas in terms of wet basis, with the aid of result of

GC analysis; d) Calculate the net rate of heat transfer of the combustion chamber. The property Table is

given in Appendix II.

Q&A: Please discuss the following questions in your report (you may use the back of the Logsheet)

a) What is the detector used in the GC? Briefly describe its working principles. b) What are the basic assumption(s) of this experiment (e.g. the combustion process) ? c) Briefly comment on the experimental data (possible factors of errors, etc.)

Appendix I: Introduction to Gas Chromatography Introduction

Gas chromatography (GC) is a chromatographic technique used to separate compounds that are volatile. A GC consists of a flowing mobile phase, an injection port, a separation column containing the stationary phase, a detector, and a data recording system. The compounds are separated due to differences in their partitioning behavior between the mobile gas phase and the stationary phase in the column.

Instrumentation Carrier gas: The carrier gas must be chemically inert (helium in this experiment). The carrier gas system also contains a molecular sieve to remove water and other impurities.

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Sample injection port: A microsyringe is used to inject sample through a rubber septum into a flash vapouriser port at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample.

Columns: There are two general types of column, packed and capillary. Most packed columns (This experiment) are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.

Column temperature: For precise work, column temperature must be controlled to within tenths of a degree. The optimum column temperature is dependant upon the boiling point of the sample. As a rule of thumb, a temperature slightly above the average boiling point of the sample results in an elution time of 2 - 30 minutes. Minimal temperatures give good resolution, but increase elution times. If a sample has a wide boiling range, then temperature programming can be useful.

Detectors: A Thermal Conductivity Detector (TCD) is used which is universal for organic and inorganic compounds. The Flame Ionization Detector (FID) is suitable for organic compounds.

The TCD measurement is based on changes in electrical conductivity of gases. The background signal is the thermal conductivity of pure helium (carrier gas).

TCD is a non-destructive means for detecting tiny amounts of organic compounds present in the gas phase. TCD consists of passing the He stream over a wire that has a constant voltage applied. As long as only He passes over the wire, the rate of heat and consequently the electrical resistance stay constant. As an organic vapor passes over a small filament with current, the temperature, and hence the resistance of the filament will change. The change in resistance is an electrical signal, which is then amplified, conditioned, and plotted versus separation time. The result is a series of peaks plotted versus time called a chromatogram. The area under each peak can be calibrated with known standards, and the analysis can be quantitative.

Retention Time (RT): the time for a compound to travel from the injection port to the detector. The retention time is measured by the recorder as the time between the moment you press start and the time the detector sees a peak.

The actually structure and operation manual of the GC14 B can be viewed in the Lab.

Appendix II: Property Table

C3H8 O2 N2 H2O CO2 CO Standard Heat of Formation

(kJ/mol), 0^

fHΔ

-119.8(l) -103.8(g)

0 0 -285.8 (l) -241.8(g)

-393.5 -110.5

Standard Heat of Combustion

(kJ/mol), 0^

cHΔ

-2,204.0(l) -2220.0(g)

0 0 0 0 -283

Heat Capacity (kJ/mol.C) CP = a + b×T

a×103 68.032 22.59

29.61 1.158

29.00 0.2199

33.46(g) 0.6880

36.11 4.233

28.95 0.4110 b×105

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CH1702 Material/Energy Balance for a Combustion Process at Steady State (Log Sheet)

Name __________ Group ___________ Date:___________ Basic Data Room Temp____ (°C), Product Temp. _____(°C), Room Pre _____(kPa) Flow rate of propane: ________(mole/s)

The Diagram of the Combustion Process

Table 1 The enthalpy inlet-outlet Table (based GC analysis)

Reference :

Inlet Outlet GC analysis yi

in•

iH^ yi

in•

iH^ 1 2

% mol/s kJ/mol % mol/s kJ/mol N2 O2

C3H8 H2O CO2 CO

•

Q

nitro

oxy

prop

n

n

n

•

•

•

waterCO

CO

nitro

oxy

prop

nn

n

n

n

n

••

•

•

•

•

2

Tin = Tout =

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(The sample calculation and discussions can be written on the space below and the back of this page) Table 2 Energy & Material Balance (formula of calculation required)

Standard Enthalpy of Combustion (ΔHc°) =

kJ/molHigher heating value (HHV) =

kJ/molLower heating Value (LHV) =

kJ/molThe heat rejection of the

combustion chamber •

Q = kWThe percentage of air excess ratio (based on N2)

%

Sample calculation & discussion: