CE 10167 - CE Principles (Chemical Reaction Engineering) Flashcards

1
Q

How can the conversion of a limiting reactant be calculated/written?

A

X = moles ‘A’ reacted/ moles ‘A’ fed

Where X is conversion (no units)

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2
Q

How can conversion of substance ‘A’ in a reaction be written?

A

X = ( N (Ao) - N (A) ) / N (Ao)

Where: X is conversion N (Ao) is initial moles of substance A

N(A) is the final moles / output moles of substance A. This can be written for flow rate and concentration too.

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3
Q

What does the yield of a reaction show?

A

Yield, Y, shows how much of a desired product was formed.

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4
Q

What is selectivity (of a reaction)?

A

How much desired product was formed in ratio to the undesired product.

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5
Q

What are reaction kinetics?

A

The measurement of how quickly reactions (in reactors) occur, and how long reactants should remain in the reactor.

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6
Q

What is space time (aka holding time or mean residence time)?

A

The time necessary to process one reactor’s volume of fluid based on a particular set of entrance conditions.

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7
Q

How is space time calculated?

A

τ = V / v 0 (= s) Where: τ is space time V is reactor volume v 0 is volumetric flow rate

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8
Q

What is space velocity?

A

The reciprocal of space time. SV = 1/τ = v0/V (=s^-1) However, since the flow rates of the fluids are generally very large in industrial systems, space velocity is used in terms of hours.

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9
Q

How can rate of consumption of species A be calculated?

A

-rA = moles of A consumed / (volume * time) -rA = ΔN(A) / (V*Δt) [Since n = cV] rA = -(ΔC (A) / Δt) This is an average rate obtained by taking the change in concentration over a time period, which is an approximation of the average reaction rate in that time interval.

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10
Q

What formulae are used to calculate the average and instantaneous rates of reaction?

A

Average rate is obtained by taking the change in concentration over a time period: rate A = -(ΔC (A) / Δt) Instantaneous rate of reaction is defined as the change in concentration of an infinitely small time interval, which expressed as the limit or derivative expression: rate A = - lim Δt→0 (ΔC (A) / Δt)

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11
Q

How is the instantaneous rate of reaction calculated?

A

Instantaneous rate of reaction is defined as the change in concentration of an infinitely small time interval, which expressed as the limit or derivative expression: rate A = - lim Δt→0 (ΔC (A) / Δt) Taking the limit as Δt approaches to zero, gives the differential: r(A) = -dC(A)/dt Using moles instead of ‘A’, r(A) = -(1/V)(dN (A)/dt) [1/V * rate of moles used]

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12
Q

What’s rate of reaction?

A

The speed at which a chemical reaction proceeds in terms of disappearance of a reactant (how quickly they are used up) or as the rate of appearance of a product (how quickly they are formed) per unit time.

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13
Q

What is the rate law?

A

The Rate Law describes the relationship between reactant rates and reactant concentrations (or pressures) in terms of a mathematical equation which then describes the progress of the reaction.

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14
Q

What is the mathematical formula: ‘power law’ for describing the rate law?

A

-rA = k CᵐA CⁿB (r = k[A][B]) Where: - C is concentration of species - k is reaction rate constant - m and n are reaction orders

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15
Q

What factors affect the rate of reaction?

A

Temperature Order of Reaction 
Concentration 
 Pressure 
Catalyst

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16
Q

How does temperature affect the rate of a reaction?

A

There is an exponential relationship between temperature and rate constant. k = Ae^(-E/RT) Therefore, as temp’ increases, rate increases.

17
Q

How do reaction orders affect reaction rate?

A

Zero order - constant rate. Rate is independent of conc’. First order: rate is proportional to conc’. Second order: rate is proportional to square of the conc’ of a reactant.

18
Q

How can a zero order reaction be determined graphically?

A

A concentration-time graph will show a straight line with a negative gradient. Conc’ falls at a steady rate with time. (This wouldn’t work with 1st and 2nd order reactions, which have curved conc’-time graphs)

19
Q

How can a first order reaction be determined graphically?

A

Plot the graph of ln(C₀/C) against time. [C is conc’] If it produces a straight line with positive gradient, it is first order. (Gradient = k)

20
Q

How can a second order reaction be determined graphically?

A

Plot the graph of 1/C against time. [C is conc’] If it produces a straight line with positive gradient (and non-zero origin), it is second order. (Gradient = k)

21
Q

What are batch reactors?

A

Reaction vessels varying in size from <1L to 15,000L

22
Q

What are Fed-batch reactors?

A

Reaction vessels where reactants enter and remain in the system until the reaction is complete. However, gases produced during the reaction can be removed, or additional reactants can be added throughout the reaction. It doesn’t reach steady state.

23
Q

What are the applications of batch reactors?

A

Wide variety of uses. Mainly used for liquid phase reactions requiring long period of time. Used when a small amount of product is wanted; thus it’s preferred during product testing phases.

24
Q

What are the advantages and disadvantages of batch reactors?

A

Adv: - High conversions can be obtained by leaving reactants in reactor for longer times - Batch reactor jackets allow the system to change heating or cooling power at constant jacket heat flux. - Can make many products consecutively - Easy to clean - Good for producing small quantities Disadv: - High cost of labour per unit production - Hard to maintain - Long downtime for cleaning = less production

25
Q

What are continuous flow reactors?

A

Reaction vessels where the reactant is continuously flowing in and products are flowing out. (Examples include tubular flow reactor and CSTR). They continuously flow at steady state.

26
Q

What are tubular flow reactors:

A

A type of continuous flow reactor where reactants continuously flow at steady state e.g. plug flow reactors. As the reactant travels along the tube, conc’ of reactant decreases.

27
Q

What are the advantages and disadvantages of tubular reactors?

A

Advs: - Used large scale - High conversion per unit volume - Good heat transfer - Low operating cost Disadv: - May have thermal gradients - Poor temp’ control - Channelling may occur - Shutdown and cleaning is expensive

28
Q

What’s a CSTR, Continuous Stirred Tank Reactor?

A

A type of continuous flow reactor. reactants flow in and products flow out, and the system is continuously stirred throughout. They’re open systems and steady state. A steady state must be reached where the flow rate into the reactor equals the flow rate out and the conditions in the reactor do not change with time, otherwise the tank would empty or overflow. CSTRs are very well mixed, so the contents have relatively uniform properties such as temperature, density, etc. throughout the operation.

29
Q

What are the advantages and disadvantages of CSTR?

A

Adv: - Easy to maintain temp’ - Cheap to build - Has large heat capacity Interior of reactor is easily accessed Disadv: - Conversion of reactant to product per volume of reactor is small compared to other flow reactors

30
Q

How can reactors be classified?

A

Mode of operation e.g. batch, semi-batch, continuous End use e.g. chemical, biological, electrochemical or nuclear Catalyst (or phase) based e.g. homogenous (single phase) or heterogenous (multi-phase)

31
Q

What is the overall material balance on a system?

A

Mass flow in - mass flow out = accumulation

32
Q

What is the material balance for a certain species, A, on a system?

A

(Molar flow of species A in)-(Molar flow of species A out) + or - (rate of species A produced/consumed) = (Rate of accumulation of species A in time t)

33
Q

What’s the symbol for molar flowrate?

A

FA

34
Q

What are the assumptions in deriving a design equation for a batch reactor?

A

Closed system Perfectly mixed / uniform composition Conditions can vary over time

35
Q

What are the rate constant equations for batch reactors for zero, first and second order reactions? (Should be derived from rate/-dC/dt and not memorised)

A

Zero: k = 1/t (C₀ - C) First: k = 1/t ln(C₀/C) Second: k = 1/t (1/C - 1/C₀)

36
Q

How is the design equation for a CSTR reactor formulated?

A

Molar inflow - Molar outflow + rate of species produced - rate of species consumed = rate of accumulation. However, steady state therefore no accumulation. F in -F out - (-rV) = 0 V = (F in - F out) / (-rA) In terms of conversion, X: V = XF in / (-rA) F in = v₀CA₀ -rA = kCA₀ (1 - X) V = (Xv₀CA₀) / (kCA₀ (1 - X))

37
Q

What’s the Levenspiel plot?

A

A plot to determine the required volume of a chemical reactor using the experimental data on the reaction taking place in the reactor. It is a plot of the inverse of the reaction rate (y axis) as a function of conversion (x axis). The area is therefore equal to V/F A₀. Knowing F A₀ means that reactor volume can be estimated.

38
Q

What is the Levenspiel plot used for?

A

It is a plot of the inverse of the reaction rate (y axis) as a function of conversion (x axis), used to determine the required volume of a chemical reactor using the experimental data on the reaction taking place in the reactor. The area is therefore equal to V/F A₀. Knowing F A₀ means that reactor volume can be estimated.

39
Q

How is a design equation for a PFR (plug flow reactor) determined?

A

(The reactor is divided into sub-volumes and a mass balance is carried out on the sub-volume rather than the whole volume, ΔV) Therefore: [molar flow of A into ΔV] - [molar flow of A out of ΔV] +/- [rate of formation/consumption in ΔV] = [Rate of accumulation in ΔV during Δt] Fₐ₍ᵥ₎ - Fₐ₍ᵥ₊Δᵥ₎ - (-rₐΔV) = 0 Rearrange to find -rₐ and take the limit as ΔV approaches 0. The design equation becomes: -dFₐ/dV = -rₐ Integrate both sides: V = ∫ dFₐ/rₐ from Fₐ to Fₐ,₀ Writing in terms of conversion, (Fₐ = Fₐ,₀ - Fₐ,₀X) so Fₐ,₀*dX/dV = rₐ Now, integrating from X = 0 to X = X: V = Fₐ,₀*∫dX/-rₐ