CE 10167 - CE Principles (Chemical Reaction Engineering) Flashcards
How can the conversion of a limiting reactant be calculated/written?
X = moles ‘A’ reacted/ moles ‘A’ fed
Where X is conversion (no units)
How can conversion of substance ‘A’ in a reaction be written?
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.
What does the yield of a reaction show?
Yield, Y, shows how much of a desired product was formed.
What is selectivity (of a reaction)?
How much desired product was formed in ratio to the undesired product.
What are reaction kinetics?
The measurement of how quickly reactions (in reactors) occur, and how long reactants should remain in the reactor.
What is space time (aka holding time or mean residence time)?
The time necessary to process one reactor’s volume of fluid based on a particular set of entrance conditions.
How is space time calculated?
τ = V / v 0 (= s) Where: τ is space time V is reactor volume v 0 is volumetric flow rate
What is space velocity?
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.
How can rate of consumption of species A be calculated?
-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.
What formulae are used to calculate the average and instantaneous rates of reaction?
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)
How is the instantaneous rate of reaction calculated?
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]
What’s rate of reaction?
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.
What is the rate law?
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.
What is the mathematical formula: ‘power law’ for describing the rate law?
-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
What factors affect the rate of reaction?
Temperature Order of Reaction Concentration Pressure Catalyst
How does temperature affect the rate of a reaction?
There is an exponential relationship between temperature and rate constant. k = Ae^(-E/RT) Therefore, as temp’ increases, rate increases.
How do reaction orders affect reaction rate?
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.
How can a zero order reaction be determined graphically?
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)
How can a first order reaction be determined graphically?
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)
How can a second order reaction be determined graphically?
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)
What are batch reactors?
Reaction vessels varying in size from <1L to 15,000L
What are Fed-batch reactors?
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.
What are the applications of batch reactors?
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.
What are the advantages and disadvantages of batch reactors?
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
