CE 10167 - CE Principles (Mass Balances) Flashcards Preview

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What’s an adiabatic system?

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.


What’s an isothermal process?

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


What is flow work?

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.


How is flow work calculated?

W = PV


What is the first Law of Thermodynamics?

Energy cannot be created or destroyed.

Input energy = output energy + energy accumulation


What's a steady and unsteady state system?

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

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


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

Steady - zero. There is no accumulation.

Unsteady - accumulation can be positive or negative.


What is an open and closed (flow) system?

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).


What energy transfers occur in closed systems?

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.


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

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


What energy balances occur on closed systems?

ΔU + ΔEk + ΔEp = Q + W


What is enthalpy?

A combination of internal energy and flow work.

H = U + PV = U + Wf


What is a 'process unit' or 'unit operation'?

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

They can be represented by process block diagrams.


What are active process units?

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


What are inactive process units?

Units which do not change flow or composition.

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


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

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.


What’s a PFD?

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.


What’s the Ostwald process?

The process for the production of nitric acid.


What’s a P & ID?

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.


What’s the Haber Bosch process?

The process for the production of ammonia.


What’s a chemical process?

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.


What’s a system and surroundings?

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.


What’s the law of conservation of mass?

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).


What are examples of system boundaries?

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.


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

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)


What does a dot above a symbol mean?

The rate of (flow of) that value.


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

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


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

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.


What’s a steady state system?

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


How does steady state differ from equilibrium?

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.