To increase the concentration of product formed in a PFR, what should we do?

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Nickolas Cannon
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1 To produce more moles of product per time in a flow reactor system, what can we do? a) Use less catalyst b) Make the reactor bigger c) Make the flow rate through the reactor smaller To increase the concentration of product formed in a PR, what should we do? a) ecrease the reactor diameter b) ecrease the reactor length c) ecrease the feed flow rate When designing a PR, why would we ever want to use a bunch of skinny tubes instead of one large-diameter tube? a) Easier catalyst replacement b) etter heat transfer c) Lower gas compression costs n decrease in which of the following variables will increase the concentration of product formed? a) Volumetric flow rate b) Initial oxygen concentration c) Initial butadiene concentration d) Rate constant k e) atalyst mass or the reaction below, which is the limiting reactant? a) b) c) + + products = mol/sec, = = mol/sec What is the volume of a STR needed to achieve 6% conversion for the liquid-phase reaction + if the inlet volumetric flow rate of liquid is 1 L/min, and the inlet concentration is mol/l. The reaction rate -r has been measured to be mol/l-min at = mol/l and 1 mol/l-min at = 1. mol/l. a) L b) 6 L c) 9 L d) 18 L 1

2 In the previous STR sizing example, what happens to the conversion if you double the volume? a) It doubles b) It goes up by less than a factor of c) It goes up by more than a factor of d) It goes down 1/-r Which reactor type will require the lowest volume to achieve a conversion of 4%? STR PR Need more data.4 X What batch time is required to achieve 8% conversion for the following kinetics, if the beginning concentration of the limiting reactant is 1 mol/l?.9 min 5.6 min 9 min (d) 56 min (e) an t tell. ata only work for flow reactors. ssuming that the reaction rate decreases continually as conversion increases, what will be the effect on a process of taking two equal-size STRs in series and placing them in parallel? (In a parallel set-up, the feed is divided in half and each stream is routed through one STR; the product streams are then combined.) a) onversion will go down b) onversion will go up c) onversion will remain the same d) onversion will change to become more PR-like The figure below describes the kinetics with different reactors in series indicated. y swapping the order of the reactors, it may be possible to increase the conversion. What would you do? a) Swap 1 and b) Swap 1 and c) Swap and d) Leave them the way they are Which of the following reactions does not follow an elementary rate law? a) H l + r - H r + l - Pd b),5-h,-h c) 4 H 6 g + ½ O 4 H 6 O -r = k Hl r- -r = k,5-h -r = k O

3 Under what condition will the rate appear to be 1 st -order in EO and zero-order in water? a) When there is a large excess of water b) When there is a large excess of EO c) When the feed rates of EO and water are equal Given the following liquid-phase reaction and feed conditions, what is the correct expression for as a function of X? + + = = 1 mol/min = mol/min = (d) (e) = = = = = (1 + X ) (1 X / ) 1 X + 1 X or the gas-phase reaction below, what is the relation between the total and initial number of moles? ssume that X is based on the limiting reactant = + + (d) T T T T T T T T X = 1 X = 1 X = 1+ X = 1 or the gas-phase reaction below, what is the relation concentration of exiting the reactor? ssume that X is based on the limiting reactant = + + X 1 = X 1 X 1 = X 1 X = X 1 Solve this first. n encyclopedia that consists of 5 volumes is resting on a library bookshelf. In an individual volume, each cover is.5 thick and the pages are cumulatively thick. What is the linear distance between the first page of the encyclopedia and the last? a) 8 b) 1 c) 1 d) 1.5 e) 1 The irreversible liquid-phase reaction + + occurs in a batch reactor. When the initial concentration of is twice that of, the conversion of after 6 minutes of batch time is measured to be 6%. If the initial concentration of is increased so that it is three times that of (leaving all other independent variables constant), what batch time (in minutes) will be required to achieve 6% conversion of?

4 What is the material balance (appetizer) for this problem? V X = r dv dt = r = r What is the rate expression for this problem? r r r = k = k = k What are the appropriate expressions for and as a function of conversion of in this problem (first part)? = (1 X ), = (1 X ) = (1 X ), = (1 X ) =, = ( X ) What material balance expression do you obtain when you combine the first three steps? t =.6 k k t = t =.6.6 What is the concentration of as a function of conversion of for part (ii) of the problem? = = = ( X ) What material balance expression do you obtain when you combine the first three steps? k k k t = t = t = (/ X ) (/ X ) 4

5 When is the equation listed below valid? If K τk m <<, how would you express a? X = 1 + τk vmax a) or any reaction in any kind of reactor b) or any liquid-phase reaction in any kind of reactor c) or any liquid-phase reaction in a STR d) or a first-order, irreversible reaction of a liquid in a STR (d) a = v vmax a = V K v vmax a = v v a = v V m max V V How will the magnitude of the effect of replacing a single STR with two STRs change as the reaction order increases? a) Increase b) ecrease c) Remain the same regardless of order or the isothermal, elementary gas-phase reaction -->, how does the PR volume required for a given conversion change with increasing mole fraction of if the inlet concentration of and total volumetric flow rate remain constant? a) V increases b) V decreases c) No change in V ogler Problem 4-1: ompound undergoes a reversible reaction,, over a supported metal catalyst. The equilibrium constant for the reaction is 5.8, and the reaction is elementary. In a PR, a feed of pure undergoes a net conversion to of 55%. If a second, identical flow reactor at the same temperature is placed downstream from the first, what overall conversion of would you expect? Which of the following expressions results from the first two steps of ogler s methodology? dw dw dv = k' = k ( + K ) = k' K 5

6 Which result is obtained after the combine step? dw k' 1 = 1 X v 1 + K dw dv 1 = k' v 1 X 1 K k' = ( 1 X ) K Which of the following steps will decrease the P drop in a PR for the gas phase reaction? a) Increase flow rate b) ecrease reactor diameter c) Increase particle size d) ecrease entering mole fraction of (at constant total flow). e) oth and (d) To increase the conversion from a membrane reactor for ethane dehydrogenation, what can be done? a) Increase the thickness of the membrane b) Increase the temperature c) ecrease the flow rate of the permeateside stream d) Increase the diameter of the reactor (keeping total volume constant) Which of the following is an advantage for membrane reactors? a) Low capital cost b) Low complexity compared to other reactors c) Reduced reactor volume How many moles of hydrogen are consumed during the reaction when a final conversion of 8% is achieved? The initial number of moles of polyunsaturated fatty acids in the reactor is 4 moles. What will be the trend in the flow rate of hydrogen into the reactor versus time? a) 5 b) 4 c) d) H t 6

7 The reaction (l) + (l) (g) + (l) is being conducted in a batch process. ecause is produced as a gas, it tends to accumulate in the headspace of the reactor. Which of the following strategies would be useful for increasing the conversion? a) Increase agitation to mix the vapor and liquid streams better b) ttach a pipe to the top of the reactor and allow vapor streams to exit the reactor through it c) Make the reactor smaller to minimize the head space available for vapor to accumulate rom the following plot, what is the reaction order with respect to? ln (-d /dt) a) b).5 c).78 d) 1 e) y=1.1 x +.78 ln ( ) ifferential reactor data are shown below for a reaction of the type + +. What are the reaction orders in each reactant? P P 4 8 mmoles in outlet sample a) 1 st order in, 1 st order in b) nd order in, nd order in c) nd order in, 4 th order in. d) 1 st order in, nd order in. ifferential reactor data are shown below for three different catalysts. Which catalyst has the highest activation energy? Inlet P T(K) at at a) atalyst. b) atalyst. c) atalyst. d) atalyst. mmoles in outlet stream at at In the batch phase reaction, +, the concentration of is measured after 1 minute of reaction time for several sets of initial conditions. What are the approx. reaction orders of and? a) and b) and c) 1 and 1.1 d) and. e) and onsider the reaction network below: if the overall selectivity of W to P is 4., what is the overall yield of P? W P a). b).75 c).8 d) 4. 7

8 If the following reactions are elementary as written, which reactor configuration would you use to improve selectivity? + + U a) eed to a PR, inject side streams of low concentration b) Load into a semibatch reactor, add as a side-stream c) eed through a series of STRs, add a little to each one Which of the reactor conditions would you choose to optimize selectivity to in a PR for the following reaction network? 1 U Reaction order α = 1 for rxn 1, α = for rxn E 1 = kcal/mol, E = 16 kcal/mol Note that is a slightly endothermic, reversible process. a) eed pure at high temperature; keep space time long. b) eed diluted at high temperature; keep space time short. c) eed pure at low temperature; keep space time short. d) eed diluted at low temperature; keep space time long. The endothermic reaction + + is conducted adiabatically. Initially, the feed is stoichiometric. In a process change, the concentration of in the feed is doubled, but the conversion of is not changed. How will the outlet temperature change from the stoichiometric case? a) T will go up b) T will go down c) T will not change The reaction is conducted adiabatically. The heat capacities of and are approximately equal at cal/mol-k. When a conversion of 5% is achieved in a PR, the outlet temperature is 5 K higher than the inlet. What is the heat of reaction? a).5 kcal/mol b) -.5 kcal/mol c) kcal/mol d) - kcal/mol What is the enthalpy change of reaction for the following system at 5 K? + + How will the slope of the X vs. T plot for an adiabatic STR change if the flow rate of water is increased above the stoichiometric level? H f (kj/mol) ata collected at 98 K. p (J/mol-K) kj/mol kj/mol 5 kj/mol (d) 1 kj/mol a) Slope will increase b) Slope will decrease c) Slope will remain the same 8

9 What would we do to make the STR operate isothermally? a) Isothermal operation is not possible b) Use a heat exchanger with a coolant flow c) Use a lower conversion Which of the following will increase the needed heat exchange area to maintain isothermal operation in a STR? a) n increase in feed temperature. b) decrease in the overall heat transfer coefficient. c) decrease in the molar flow rate of inert relative to (leaving the production rate of constant). d) ll of the above. What heat exchange area is needed for isothermal operation? H Rx = -4 kcal/mol U = 5 kcal/hr-m -K T T a = 1 K 1 moles/hr formed a).8 m b) 1. m c) 4. m d) Impossible to compute from given info. The temperature curve for the adiabatic reaction is shown in black. What will the curve the reaction with a constant cooling water temperature (high coolant flow) look like? T d a c b W If we increase the feed temperature, which of the following is a possible temperature profile? T a c b Which of the following will make the temperature easier to control? a) llow coolant temperature to increase down the length of the reactor. b) Make the feed stoichiometric. c) Increase the reaction temperature so that the initial rate is very high. d) None of the above. W 9

10 Which of the following approaches will increase equilibrium conversion? a) ecrease the feed temperature b) Increase the mole fraction of inert c) Increase the stoichiometric ratio of steam to O d) ll of the above or an endothermic reaction, which of the following approaches will experience the most severe equilibrium limitations? a) diabatic b) Isothermal c) Use of a heat exchanger so that T out >T. Suppose that the reaction can still continue to occur in the heat exchanger (but that the outlet T from the exchanger is to remain the same). Which of the following is true? a) The equilibrium conversion after the nd reactor will increase b) The equilibrium conversion after the nd reactor will decrease c) Impossible to predict the direction of the effect You are in charge of managing a STR for a 1 st - order, adiabatic and exothermic reaction. ecause of a process upset, the inlet temperature is nearing that where one would expect thermal runaway. n operator suggests using valves on the inlet and outlet to cut the flow rate through the reactor. Is that a good idea? a) Yes b) No c) Maybe What would be a good idea? a) Open a valve that allows inert of the same inlet temperature to flow into the reactor b) If you have a way of cooling the reactor, turn on the coolant c) lose the valves into/out of the reactor d) and e) ll of the above What is the form of the energy balance when the combustion of ethylene oxide is included? dt dw dt dw Ua ( T Ta ) ρ = Ua ( T Ta ) ρ = ( r1 H Rx1 r H Rx r H Rx) piθi ( r H r H r H ) 1 Rx1 θ pi i Rx Rx Equation doesn t change, because reaction is a series reaction. 1

11 onsider the following system of reactions in a STR. How much heat must be removed to maintain isothermal operation? + + H Rx = kcal/mol + E H Rx = 1 kcal/mol = 1 mol/l = mol/l r = 1 mol/sec r E =.5 mol/sec a) 1 kcal/sec b) 15 kcal/sec c) kcal/sec d) 5 kcal/sec e) None of the above or the following reaction, batch experiments are conducted to evaluate the reaction kinetics. The slope of a ln (r ) versus ln ( ) plot is measured to be 1. and the intercept is.4 when is in great excess. What is the overall reaction order? + products a).4 b) 1. c) 1.44 d) an t tell on the basis of this experiment In a particular differential reactor experiment for, the entering volumetric flow rate is cut in half, leaving all intrinsic variables constant. How would you expect the measured outlet flow rate of to change under conditions of differential conversion? a) low rate of product will go up b) low rate of product will go down c) No change If the following reactions are elementary, which would be an appropriate scheme for increasing S U? + + U a) Semibatch reactor in which is slowly added to a vat of b) Semibatch reactor in which is continuously removed c) Series of STRs with a :1 feed of : or the following system of reactions, what is the mole balance on for a PR? E + r 1 = k 1 r E = k r = k or the following plot, which shows energy balance lines for a series of reactors with intermediate heat exchange, at what point is the reaction rate highest? d dv d dv d dv = k 1 = k 1 k1 = + k k + k k + k k X equil. line E T 11

12 The following shape is observed for an rrhenius plot in an enzymatic reaction at [S]>>K M. What is likely happening? Which of the following is true about the energy balance for an isothermal reaction in a STR at fixed conversion? a) The amount of inert has a strong effect on the heat exchange requirements b) hanges in the feed ratio have no effect c) The term Ua is set to zero ln (rate) 1/T a) s T is increased, transition state becomes lower in energy than reactants b) s T is increased, enzyme begins to unfold and lose activity c) s T is increased, all enzyme becomes bound to substrate, choking off reaction How will the Lineweaver-urk plot change as [I] is increased for competitive inhibition? a) Slope and intercept will go up b) Slope and intercept will go down c) Slope will go up, intercept will stay the same d) Intercept will go down, slope will stay same In a particular fermentation reaction, the Y /S =.. If the inlet concentration of substrate is 4. g/l and the substrate conversion is 75%, what concentration of cells has been produced? a).8 g/l b). g/l c).6 g/l d).8 g/l or what space time will wash-out be achieved for an inlet substrate concentration of 5. g/l, K S = 1. g/l, µ max = 1. hr -1? a). hr b) 1. hr c). hr d) 1 hr The following data were measured in a differential reactor for the O oxidation reaction. Which model is consistent with the data? P O.5.5. P O rate a) Reaction-limited b) O adsorption-limited c) Neither d) oth 1

13 or butadiene () oxidation to epoxybutene (E), the following differential kinetic data are obtained. Which of the following statements is true? P P O P E rate a) The product binds strongly to the catalyst surface b) Surface reaction of and O is likely the rate determining step c) The surface coverage of O is high d) None of the above particular catalyst is used in a reactor where the rate determining step is surface reaction. That same catalyst is loaded into a different reactor operating at the same temperature, but where external mass transfer is slow and limits the reaction. oes the rate go up or down? a) Up b) own c) Stays the same It is known that a catalyst particle with a diameter of.5 cm exhibits no internal MT limitations. When the catalyst size is increased to 1. cm, the rate of reaction goes down by a factor of 4; when it s increased again, the rate is a factor of 1 lower than for the.5 cm particles. What is the particle size in the final measurement? a). cm b).5 cm c) 7.5 cm d) 5 cm step tracer experiment is conducted, and the tracer concentration is reported as a function of time below. Where is the value of E highest? time The black curve shows E(t) for a STR with perfect mixing. Which curve corresponds to a STR with a significant dead volume? The black curve shows (t) for a tubular reactor with perfect plug flow. Which curve corresponds to a tubular reactor in which the fluid velocity at the centerline is higher than that near the wall? (no change) E(t) (t) (no change) time time 1

CBE 142: Chemical Kinetics & Reaction Engineering

CBE 142: Chemical Kinetics & Reaction Engineering Midterm #2 November 6 th 2014 This exam is worth 100 points and 20% of your course grade. Please read through the questions carefully before giving your