CE 10167 - CE Principles (Mass Balances)

Question 1

What’s an adiabatic system?

Answer 1

A process that does not involve the transfer of heat or matter into or out of a system, so that Q = 0.

A system that occurs without transfer of heat or mass of substances between a thermodynamic system and its surroundings.
In an adiabatic process, energy is transferred to the surroundings only as work.

Question 2

What’s an isothermal process?

Answer 2

A change of a system, in which the temperature remains constant: ΔT = 0.

Question 3

What is flow work?

Answer 3

Flow work/energy is the energy needed to push the fluid into or out of the boundaries of a control volume if mass flow is involved.

Flow work is necessary for maintaining a continuous flow through a control volume.

Question 4

How is flow work calculated?

Answer 4

W = PV

Question 5

What is the first Law of Thermodynamics?

Answer 5

Energy cannot be created or destroyed.

Input energy = output energy + energy accumulation

Question 6

What’s a steady and unsteady state system?

Answer 6

Steady - quantities of variables or conditions are NOT changing with time.

Unsteady - quantities of variables or conditions ARE changing with time.

Question 7

What is the accumulation term for steady and unsteady state systems?

Answer 7

Steady - zero. There is no accumulation.

Unsteady - accumulation can be positive or negative.

Question 8

What is an open and closed (flow) system?

Answer 8

Open - there are input and output streams. Mass can cross.

Closed - there is no mass crossing. (Energy or momentum may still be able to flow).

Question 9

What energy transfers occur in closed systems?

Answer 9

Heat (Q) / energy that flows due to a temperature difference. Heat is defined as positive when it is transferred to the system from surroundings.

Work (Ws) which flows due to a driving force (e.g. torque).
Work is defined as positive when it is transferred to the system from surroundings.

Question 10

What is sensible energy (of internal energy, U)?

Answer 10

The total of vibrational, rotational and kinetic energies of molecules.

Question 11

What energy balances occur on closed systems?

Answer 11

ΔU + ΔEk + ΔEp = Q + W

Question 12

What is enthalpy?

Answer 12

A combination of internal energy and flow work.

H = U + PV = U + Wf

Question 13

What is a ‘process unit’ or ‘unit operation’?

Answer 13

Any operation that alters flow, composition, temperature or pressure of any stream.

They can be represented by process block diagrams.

Question 14

What are active process units?

Answer 14

Units which change flow rates or compositions e.g. separators, reactors, splitters and mixers.

Question 15

What are inactive process units?

Answer 15

Units which do not change flow or composition.

They may change pressure or temperature e.g. pumps, compressors and heat exchangers without phase change.

Question 16

What is the difference between a process block diagram, a process flow diagram and a piping and instrumentation diagram?

Answer 16

PBD - simplest representation. Blocks are used to represent stages or pieces of equipment.

PFD - has more detail and shows all major pieces of equipment and flow streams.

P&ID - most detail and also includes details of equipment, piping, valves, materials and fittings.

Question 17

What’s a PFD?

Answer 17

Process flow diagram

It provides more detail than a block diagram and is a standard method for documenting engineering designs.

The diagram shows the arrangement and interconnection of all major pieces of equipment and all flow streams.

The equipment is represented by symbols or icons that look like the actual equipment.

Question 18

What’s the Ostwald process?

Answer 18

The process for the production of nitric acid.

Question 19

What’s a P & ID?

Answer 19

A piping and instrumentation diagram.

It provides greater detail than a PFD and is a standard method for documenting engineering designs.

It includes engineering details of equipment, instrumentation, piping, valves and fittings.

Piping size, material specification, process instrumentation and control lines are all shown.

Question 20

What’s the Haber Bosch process?

Answer 20

The process for the production of ammonia.

Question 21

What’s a chemical process?

Answer 21

A series of steps and operations where starting materials are converted into desired products via specific, interconnected processes and streams.

It’s often represented on a process flow stream.

Question 22

What’s a system and surroundings?

Answer 22

A process or part of process identified for investigation.

The surroundings are everything else (plus other parts of a process).

The system boundary separates the system from surroundings.

Question 23

What’s the law of conservation of mass?

Answer 23

Mass can neither be created nor destroyed but it can be changed in form. The total mass must be conserved however the mass of any component, e.g. benzene, may be changed in a chemical reaction (transformation).

Total mass must be conserved but it’s chemical, biological or physical form can change (e.g. water to steam).

Question 24

What are examples of system boundaries?

Answer 24

Walls of a fermenter
Site boundary of an oil refinery
Walls of a distillation column
Part of a pharmaceutical manufacturing plant
The human body 
The whole world

The system boundary separates the system from surroundings.

Question 25

What does the law of conservation of mass suggest / what is its mathematical representation?

Answer 25

Any mass that goes into a system must either come out of the system elsewhere, get used up or regenerated by the system or remain in the system and accumulate.

(Rate of mass in + rate of mass generated) = (rate of mass out + rate of mass consumed + rate of mass accumulated)

Question 26

What does a dot above a symbol mean?

Answer 26

The rate of (flow of) that value.

Question 27

How are volumetric flow rates (Q or Vdot) written?

Answer 27

They can be measured directly using flowmeters and they are especially useful for gases.

Volumetric flowrates are not conserved due to
compressibility of gases. Therefore if we are given a volumetric flow rate we should change it into a mass (or mole) flow rate before applying the balance equations.

Volumetric flowrates also do not lend themselves to splitting into components e.g. miscible liquids, solutions or gases in a tank. So, the mass flow rate in the outlet is commonly written as:

∑ m dot out = ρ mix * Q mix

Question 28

Why is using molar flow rates (N or n dot) useful?

Answer 28

Using moles instead of mass allows you to write material balances in terms of reaction conversion and stoichiometry. Unlike mass, total moles are not conserved.

For example, consider the reaction between hydrogen and oxygen gases to form water:
H2 + 0.5O2 -> H2O

This reaction consumes 1.5 moles of reactants for every mole of products produced, and therefore the total number of moles entering the reactor will be more than the number leaving it.

Question 29

What’s a steady state system?

Answer 29

A system which does not accumulate a substance.

None of the variables (e.g. concentration, temperature, etc.) in the process system vary with time

m dot = 0

Question 30

How does steady state differ from equilibrium?

Answer 30

Steady state remains constant over time but requires continual work whereas equilibrium is stable over time but requires no energy or work to maintain the condition.

Question 31

What’s an unsteady state system?

Answer 31

One or more of the variables in the process system vary with time.

m dot doesn’t equal 0.

Question 32

What is the method for writing / setting up mass balances?

Answer 32

1. Draw a diagram that clearly shows the system and system boundaries.
2. Select a consistent set of units (e.g. use kg/s throughout).
3. Set the basis for the calculations and state it clearly (e.g. 10kg/s methane vapour).
4. State all necessary assumptions & approximations (e.g. steady state, isothermal, well-mixed etc.)
5. Write stoichiometric equations, when reactions are involved.
6. Write mass balance equations (e.g. : m1 = m2 + m3).
7. Solve the balances and check (that they make sense).

Question 33

How can mass balances for multicomponent systems be written?

Answer 33

[Rate of mass in through system boundary +
Rate of mass generated within system] =
[Rate of mass out through system boundary +
Rate of mass consumed within system +
Rate of mass accumulation

Overall mass balance at steady state: ∑m in + ∑ m out

Question 34

What is molarity?

Answer 34

A major unit of concentration: mole / L (moles per litre).

Molarity can change with temperature so molarity should always be given at a specific temperature. Molarity of gaseous mixtures can also change with pressure (therefore not usually used for gases).

Question 35

What are mole fractions?

Answer 35

A unit of concentration which allows you to work out the molar flow rate of any component from total flow rate.

It is always a value between 0 and 1.

Question 36

What is a batch process?

Answer 36

A process used to process a fixed amount of material in a certain period of time.

The system is closed - mass doesn’t enter or leave.

It is unsteady state - parameters such as conc’, temp’, pressure etc. may change with time.

Question 37

What is the system and state of a batch process.

Answer 37

Closed system - mass doesn’t enter or leave.

Unsteady state - parameters such as conc’, temp’, pressure etc. may change with time. Info is collected over a period of time.

Question 38

What is a semi-batch process?

Answer 38

A process that either allows input or output of mass, but not both.

Mass can accumulate or deplete, therefore they are open systems and unsteady state.

Question 39

What is the state and system of semi-batch processes?

Answer 39

Open and unsteady state.

A semi-batch process allows either input of mass or output of mass, but not both.

Question 40

What’s a fed-batch process?

Answer 40

A process that allows input of mass but not output.

They’re therefore open and unsteady.

Question 41

What’s a continuous process?

Answer 41

One which allows mass to flow in and out without interruption.

It is the only process that can operate at steady state (if inflow = outflow)

It’s an open system.

Question 42

What must be known for a material balance with chemical reactions occurring (multi-component)?

Answer 42

The reaction stoichiometry (chemical equation) must be known and it must be balanced.

It is best to work in molar units since chemical reactions (and their stoichiometry) are balanced in molar quantities.

A stoichiometric table is ideal since it contains all the information so can deal with all of the following:

• incomplete conversions of reactants
• excess of reactants
• presence of inerts

Question 43

How are material balances applied to multiple unit operations?

Answer 43

They must be applied to individual subsystems and/or to the overall system.

In general, you can write an independent material balance equation for each component present in each unit or subsystem except for splitters.
For splitters only one independent mass balance can be written regardless of the number of components involved

Question 44

What is recycling (in a process) and how is it beneficial?

Answer 44

Recycling is the act of taking one stream in a process and reusing it in an earlier part of the process rather than discarding it.

• Recycling allows a wider range of separations (e.g. distillation)
• Recycling, in combination with some sort of separation process, an increased overall conversion of an equilibrium reaction can be achieved (e.g. separator)
• It could recover expensive catalysts and reagents
• It can decrease the amount of equipment needed to get a process to meet specifications and consumer demand.
• It reduces waste

The biggest difference between recycle and non-recycle systems is that the extra splitting and recombination points must be taken into account, and the properties of the streams change from before to after these points. We need to take it into account by performing a mass balance on the recombination point and one on the splitting point.

Question 45

What are the recombination and splitting points?

Answer 45

Splitting - the part where process feed is split into products and materials to be recycled

Recombination - where the materials being recycled are fed into the original feed (before the process has occurred)

Question 46

What’s a bypass?

Answer 46

A stream that skips one or more stages of the process and goes directly to another downstream stage.

It can be used to control the composition of a final exit stream of a unit by mixing the bypass stream and the unit exit stream in suitable proportions to obtain the desired final composition.

Question 47

What’s a purge?

Answer 47

An outlet which enables the accumulating gasses useless to the process to be removed.

It is the removal of the accumulation of inert or unwanted materials/impurities.

Question 48

What happens in a mixer, splitter and separator?

Answer 48

Mixer - two or more entering streams of different compositions are combined.

Splitter - two or more streams exit, all of which have the same composition.

Separator - two or more streams exit, which can be of different compositions.

Question 49

What’s accumulation?

Answer 49

The rate of change with time of the amount of material under study in the
chosen system.

For mass, it will be dm/dt where m is the mass of material in the system at
time t.

For a component/species, therefore, it could be d(cV)/dt
where c (kg m-3, for example) is the
concentration of the component in a system volume of V (m3).

Accumulation can be either positive or negative.

Question 50

What are the 3 ways to describe problem solvability?

DOF - degrees of freedom

Answer 50

Well-defined: the problem has a finite (not necessarily unique) set of solutions.

Over-determined: (aka over-specified) you have too much information and it’s either redundant or inconsistent.

Under-determined: (underspecified) you don’t have enough info to solve all your unknowns.

Question 51

How are degrees of freedom (DOF) analysed for mass balances in a single process system?

Answer 51

1) Determine the number of unknowns in the process. In a material balance calculation, masses and concentrations are the most common.
2) Subtract the number of equations you can write on the process. This can include mass balances, energy balances, equilibrium relationships (in a reactor), relations between concentrations, and any equations derived from additional information about the process.
3) The number you are left with is the degrees of freedom of the process.

Question 52

What do the values of DOF (degrees of freedom) mean?

Answer 52

If the degrees of freedom are negative, the unit operation is over-specified.

If it is positive, the operation is underspecified.

If it is zero then the unit operation is well-defined, meaning that it is theoretically possible to solve for the unknowns with a finite set of solutions.

Question 53

How are degrees of freedom (DOF) analysed for mass balances in a multiple process system?

Answer 53

1) Label a process flowchart completely with all the relevant unknowns.
2) Perform a degree of freedom analysis on each unit operation, as described above.
3) Add the degrees of freedom for each of the operations.
4) Subtract the number of variables in intermediate streams, i.e. streams between two unit operations. This is because each of these was counted twice, once for the operation it leaves and once for the one it enters.

The number you are left with is the process degrees of freedom, and this is what will tell you if the process as a whole is over-specified, under-specified, or well-defined.

Question 54

What’s Gaussian elimination?

Answer 54

An algorithm for solving systems of linear equations.