B48BA Exam Questions v1

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HERIOT-WATT UNIVERSITY

School of Engineering & Physical Sciences _______________________

Chemical Engineering

_______________________

Process Industries C Course Code: B48BA1

Monday, 5th December 2011 16:30-18:30

Location: Sports Hall 1

Information Section

Answer FOUR questions, including at least one from SECTION B.

Numbers in brackets indicate the marks allocated.

Candidates are expected to make reasonable assumptions

where necessary

Where a distribution of marks within a question is shown, this should not be taken to be definitive but is for guidance only

©HERIOT-WATT UNIVERSITY……………………………………………………………June 2014 v1

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SECTION A 1. The Antoine equation relates vapour pressure to temperature and can be stated as:

( )CTBAPLog+

−=*10

Where P* = vapour pressure in mm Hg

T = temperature in °C A, B and C are constants for a particular substance.

Atmospheric pressure is taken as 760 mm Hg The values for A, B and C for two unknown substances are given below:

A B C Component X 8.112 1592.864 226.184 Component Y 8.379 1788.020 227.438

a) Determine the boiling point of each pure component at atmospheric pressure

(8) b) Using four intermediate temperatures, construct a T-x-y diagram for the

system at atmospheric pressure (14)

c) What assumptions must you make to carry out parts (a) and (b)? (3)

2. A feed of 1600 kg/day to an ammonium sulphate concentration process contains

0.1 kg (NH4)2SO4/kg solution. The solution is aqueous. The feed is mixed with a recycle stream and passed into an evaporator where some of the water (only) is removed. The evaporator product contains 60%w/w (NH4)2SO4 and this passes to a crystalliser. Crystals from the crystalliser contain 5%w/w water and the remaining liquor, which contains 35% w/w (NH4)2SO4, is recycled back to the original feed stream to the Evaporator. a) Draw a schematic diagram of the layout of the system

(5) b) Find all mass flow rates and stream compositions

(20) Cont’d..


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3. (a) A manometer is open to the atmosphere at one end and the other is connected to a pipe, as shown in the diagram below. The manometer contains oil, of a density 1400 kg m-3. Calculate the pressure in the pipe.

(8) (b)

(i) Show how the Bernoulli equation can be used to estimate the volumetric flow rate of liquid draining from an open tank through a hole in the side, at the bottom. Assuming that the liquid in the tank is water (density 1000 kg m-3) calculate the velocity of the liquid draining out of the tank, assuming a depth of 5 m of water in the tank

(12) (ii) If the tank diameter is 5 m and the hole diameter is 10 cm,

calculate how long the tank would take to drain, based on the liquid velocity in (i) above. Why would the tank take much longer to drain in reality?

(5)

Cont’d..


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4. A furnace is constructed with an insulating wall, consisting of 0.2m of firebrick, 0.1m of insulating brick and 0.2m of standard building brick in series. Given that the inside temperature of the furnace is 1200K and the outside temperature is 330K and that the thermal conductivities of each of materials are as shown below, calculate:

a) the heat loss per unit area through the walls of the furnace

(10) b) the temperatures at the junction of both the firebrick and insulating brick,

and insulating brick and building brick (10)

The furnace building’s dimensions are 3m by 2m with a height of 2m. Ignoring any heat loss to the ground and assuming the roof is of the same construction as the walls and that differences between internal and external surface areas are negligible. Thermal conductivity values k, below, are are in W/m.K

c) calculate total heat loss from the furnace

(5)

Cont’d..


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SECTION B 5. Gases can be studied using Thermodynamics or Kinetic Gas Theory. Both

contribute important aspects to our understanding.

a) Imagine two ideal-gas systems that are identical, except system 1 contains twice the number of molecules compared to system 2. Use Thermodynamics to determine the pressure ratio.

(5)

b) Justify your result in part (a) using your understanding of the particulate nature of gases.

(5)

c) We always associate a hotter gas with faster moving gas molecules.

Show this by showing that is always positive.

(In Kinetic Gas Theory the pressure is given as .)

(15) 6. A chemical reactor of spherical shape (radius: r=3m, volume: V=4/3 πr3) is filled

to 1/2 of its height with a liquid reaction mixture. The reactor has a vent that ensures that the pressure inside the reactor is equal to the environmental pressure of 0.1MPa at all times.

a) Due to a fault the vent is closed and the temperature inside the reactor

increases from 40°C to 80°C. Calculate the resulting pressure inside the reactor.

(5)

b) At these elevated temperatures it must be expected that one of the components of the reaction mixture decomposes. This decomposition results in the release of 500kg of methane (Mmethane=16g mol-1). Calculate the final reactor pressure in this case.

(10)

c) By integration of the isothermal compressibility

show that a gas with must satisfy Boyle’s law.

(10)

END OF PAPER ©HERIOT-WATT UNIVERSITY……………………………………………………………June 2014 v1

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HERIOT-WATT UNIVERSITY

SCHOOL OF ENGINEERING & PHYSICAL SCIENCES

Chemical Engineering

____________________________________________________________________

B48BA1

PROCESS INDUSTRIES C

Semester 1 – 2012-2013 ____________________________________________________________________

Wednesday, 5 December, 2012

Time: 13:30 – 15:30

Duration: 2 hours

Information Section

Answer FOUR questions, including at least one from SECTION B.

Numbers in brackets indicate the marks allocated.

Candidates are expected to make reasonable assumptions

where necessary

Where a distribution of marks within a question is shown, this should not be taken to be definitive but is for guidance only


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Section A.

1. a) A 100 m3 tank at 300ºC contains a gaseous mixture with 12 kg of hydrogen and

56 kg of nitrogen. (Gas constant R = 8.314 kJ kmol-1K-1)

i. Calculate the number of moles of each component. (3)

ii. Calculate the mole fraction of each component. (2)

iii. Estimate the total pressure of the gas in the tank and the partial pressure of each component.

(5)

b) Haber process is an industrial process to manufacture ammonia gas using H2 and N2 with iron as a catalyst.

i. Write a balance equation representing the Haber process (3)

ii. If the process only records a 15% conversion under the above conditions, what is the number of mole and mass of NH3 generated?

(6) iii. Given that the heat of reaction ΔH = -92.22 kJ mol-1, why is the

Haber process carried out at such a high temperature (300ºC) and at a pressure estimated in (a.ii)?

(4) iv. What is the function of the catalyst?

(2)


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Cont’d.. 2.

A sample of magnetic nanoparticles (MNP) has the chemical formula of Fe3O4 (density = 5.0 g cm-3).

a) Calculate the specific surface area in m2 g-1(surface area of a nanoparticle per unit mass) of this magnetic nanoparticle sample if the particle diameter is also 20 nm. Also, calculate how many iron atoms are there in ONE nanoparticle.

(Atomic mass: Fe = 56, O = 16)

(10)

b) To prepare Fe3O4 nanoparticles, a mixed aqueous solution of Fe2+ and Fe3+ at a 1:2 ratio is reacted with a base, e.g. NaOH, at 80°C.

i. Write down the balanced equation for this reaction. (4)

ii. If a target mass of 1 kg of magnetic nanoparticles is required, what is the minimum mass of NaOH required?

(6) iii. Draw a “flow chart” diagram for this process, assuming the Fe2+

and Fe3+ are fed as a mixed aqueous solution. Which reactant should be in excess if we want to ensure a high conversion?

(5) ©HERIOT-WATT UNIVERSITY……………………………………………………………June 2014 v1

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Cont’d.. 3. A stream of “low wine” with 20% ethanol and 80% water is fed into a continuous distillation column at a flow of 200 kg min-1. There are two product streams from the column with the following compositions: Top stream: 60% ethanol, 40% water Bottom stream: 5% ethanol, 95% water Density of the streams at 20°C: 5% ethanol = 0.997 kg dm-3 20% ethanol = 0.995 kg dm-3 60% ethanol = 0.988 kg dm-3

a) Draw a “flow chart” diagram to represent this distillation system (3)

b) Calculate the mass flow rates of the two product streams (5)

c) Calculate the mole fractions of ethanol and water in all three streams (9)

d) Find out the volumetric flow rate of all three streams (6)

e) If the pipe has a diameter of 10 cm, calculate the velocity of flow for the top product streams.

(2)


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Cont’d..

4.

a) Give the equation for the Gibbs phase rule with a brief description of each term. Explain the term “degrees of freedom”.

(3) b) Use the Gibbs phase rule to determine the number of degrees of freedom in the

following systems:

i. A close system with dry ice and gaseous CO2 ii. A saturated aqueous solution of copper(II) sulphate (CuSO4) with

CuSO4 crystals suspended in solution. iii. A vapour-liquid mixture of methanol, ethanol, and propanol.

(6)

c) The figure below shows a water heater. If the water flows in to the system at a flow rate of 1L/min at 20°C and the hot water flow out at a temperature of 90°C. (Specific heat capacity of water = 4.2 J g-1 K-1)

i. Calculate the enthalpy required to heat the water (in kJ kg-1) (3)

ii. A minimum power input for the heater (in kW) (3)

iii. Calculate the heat loss due to conduction if the heater is made of “steel” (thickness = 3 mm), and compare this figure with a heater made of “stainless steel” (also thickness = 3 mm), consider the boiler tank as a flat surface for this calculation. Which material will you use if you are asked to construct a water heater? Give reasons for your choice in terms of energy efficiency. Given that the conductivity (k) of steel is 45 W m-1 K-1 and that of stainless steel is 16 W m-1 K-1. Assuming that the whole heater is at a constant temperature of 90°C and the outside temperature is 20°C.


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(10)

Cont’d..

Section B Some useful equations and constants

Average kinetic energy per molecule from Kinetic Gas Theory: Ekin =m2

v2

Internal energy of a monatomic molecule from Thermodynamics: u1 = Etherm =32

kT

Pressure from Kinetic Gas Theory: P =13

NV

mv2

MCO2=44g/mol, MN2=28g/mol, MO2=32g/mol, MAr=18g/mol, MKr=36g/mol 5. The dangers of drinking. 5 guys are in and airtight elevator at 0.1MPa and 19.85°C. The gas volume in the elevator is 3m3. It is filled with air, which we assume to consist of 70% N2 and 30% O2 given as volume fractions. The guys bought some beer for the evening. They all bought 0.5L bottles. Due to some mechanical fault the elevator suddenly stops, causing all the carrier bags to rupture. The 100 bottles fall to the floor and break. The beer releases 7g of CO2 per liter of beer.


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a) Calculate the partial pressures and partial volumes of all gases in the elevator after all the CO2 is released. Assume that all gases can be treated as ideal gases.

(16) b) At a mole fraction above 5% CO2 causes confusion and unconsciousness. Are

the guys in danger? (4)

At 20°C the internal energy per molecule of N2 is (5/2)kT. c) For N2 at 20°C calculate the molar heat capacity at constant volume

cV =dudT

V

where u is the molar internal energy. Provide your answer in

J/(mol K). (5)

Cont’d..


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6. Thermodynamics and Kinetic Gas Theory. Consider 1L of air at 20°C. We assume that air consist of 70% N2 and 30% O2 given as volume fractions. We also assume that air can be treated as an ideal gas.

a) How many molecules does 1L of air contain? (4)

Air also contains small amounts of noble gases, such as Argon and Krypton. These gases are monatomic.

b) For the noble gases Argon and Krypton in air calculate v2 as an estimate of the velocities of the gas atoms.

(9)

Dalton’s law can be directly obtained from Kinetic Gas Theory.

c) What is Dalton’s law and how does it result from Kinetic Gas Theory? (6)

d) The molar heat capacity at constant pressure is given as cP =dhdT

P

where h is

the molar enthalpy. The Enthalpy is defined as H =U + PV where U is the internal energy. Calculate the molar heat capacity at constant pressure for Argon.

(6)

END OF PAPER ©HERIOT-WATT UNIVERSITY……………………………………………………………June 2014 v1

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B48BA Exam Questions v1