CHEMICAL REACTORS - PROBLEMS OF NON IDEAL REACTORS 61-78

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1 ourse HEMIL RETORS - PROBLEMS OF NON IDEL RETORS ccording to several experiments carried out in a continuous stirred tank reactor we suspect that the behavior of the reactor is not ideal. The response to a pulse tracer test is given by equation ( t) = 5t exp(.5t) (with t in min). It seems that the reactor can be modeled considering that the fluid elements of different ages do not mix with each other, appearing like if small batch reactors were operating inside the continuous reactor. a) Which fraction of the effluent remains in the reactor after the first minute after injection of the tracer? b) Which conversion could be achieved in this reactor if a second-order reaction were carried out in liquid phase with k o = 3.75 min -1? (k referred to-r ). 6.- The second-order reaction of dimerization B (r = -k ) takes place in liquid phase, with k = 0.01 dm 3 /(mol min) at the reaction temperature. The feeding is pure with a concentration of 8 mol/l. The theoretical volume of the reactor is 1000 L and the feeding rate for dimerization is 5 L/min. In order to better understand the behavior of the reactor, which does not behave ideally, a pulse tracer test has been performed with a volumetric flow rate of 5 L/min, and the results are shown in the following table: t (min) (mg/l) We want to know the limits within which the conversion can vary depending on the degree of micromixing pulse tracer test produces a signal with a parabolic shape that fits to the following equation (time in minutes, concentration in kmol/m 3 ): = ( t ), = 0, 0 t any other t alculate: a) The mean residence time. b) The equations for E(t), E(θ) and F(t). c) The conversion obtained under completely segregated flow conditions for a liquidphase reaction with a rate equation r = k, with k = 1 min -1. d) The parameters of the dispersion model that represent this curve, for closed-closed vessels.

2 64.- onsider the first-order reaction B carried out in a tubular reactor of 10 cm in diameter and 6.36 m in length in liquid phase. The kinetic constant is 0.5 min -1. pulse tracer test is performed in the reactor with the following results: t (min) (g/m 3 ) alculate the conversion in the system obtained using: a) the dispersion model for a closedclosed vessel, b) tanks-in-series model, c) ideal PFR model d) complete segregation model, e) ideal STR model In order to characterize the flow in a reactor a pulse tracer test was carried out, obtaining the following experimental values with a volumetric flow rate of 10 L/min: t (min) (mg/l) Based on these results it is suspected that the flow in this reactor could be modeled by a dispersion model or a tanks-in-series model. In the reactor, the reaction + B P is carried out in liquid phase. Since there is a large excess of B, the reaction can be considered pseudo first order. If the reactor behaved as an ideal PFR, it would yield a 99% conversion. Determine the conversion to be obtained in each of the following cases: a) ssuming a dispersion model for a closed-closed vessel. b) With the tanks-in-series model. c) From the direct use of the experimental tracer curve. d) With the model of an ideal STR We have a tank reactor of 6 m 3 that gives a conversion of 75% for the first-order reaction in liquid phase R. However, it is suspected that the mixture is incomplete and there is an undesirable flow pattern, since the reactor is stirred with a stir operating at low power. n experiment with pulse tracer input shows that this is so and provides the flow model shown schematically in the figure. alculate the conversion that can be obtained if the stirrer is replaced by a high-powered one to ensure the thorough mixing. What is the conversion in both cases if the fluid used is a macrofluid? 67.- tank of 860 L is used as a liquid-gas reactor. The gas bubbles rise through the reactor and exit the top part. The liquid flows in the opposite direction to the gas, with a volumetric

3 flow rate of 5 L/s. To get an idea of the pattern of fluid flow in this tank, a pulse tracer is injected (M = 150 g) at the entrance of the liquid and the concentration is measured at the exit as shown in the figure. a) Is this experiment well done? Is all the injected tracer collected? b) If so, calculate the fraction of reactor occupied by the liquid. c) Determine the E curve as a function of the concentration curve measured. d) Qualitatively, what do you think is happening in the reactor? e) If the liquid is a macrofluid ( 0 = mol/l) with rate constant k = 0.5 L/(mol min) (referring k to r ), what will be the final conversion? 68.- (exam feb 08) We want to perform in a tank reactor the liquid phase reaction B. It has been determined that the rate equation is: 0.5 r = (r in mol L -1 h -1 and in mol/l) In order to characterize the reactor, a pulse tracer test was performed, after which it was obtained that the tracer concentration measured versus time can be fitted to the following equation: ( t) = exp( t) ( in mg/l and t in h) a) alculate E(t) and the average time in the reactor t m. b) alculate F(t). Which fraction of the effluent will remain in the reactor after the first hour after injection of the tracer? c) alculate the conversion to be obtained in the reactor with the corresponding approximation of the reaction rate if the initial concentration of is low ( 0 = mol/l). d) alculate the expected conversion limits in the case of having a high initial concentration of ( 0 = 1 mol/l). e) ompare the results of parts c) and d) with the conversion to be obtained using the ideal STR model with the initial concentrations indicated in those parts. Suggestion: use time increments of 0.1 h (exam sep 07) onsider the following liquid-phase reactions taking place in parallel:

4 B r = k ( 1 ) r = k For a feeding stream containing only the reagent, determine the concentrations of this reagent according to the following models: a) Ideal STR. b) Ideal PFR. c) ompletely segregated flow with an E(t) identical to that obtained with a model of tanks in series for n =. d) Maximum mixedness model with the same E(t) used in the previous part. e) For the four previous cases, also determine the concentration of B, its yield (R) and selectivity (S). Data: 0 =.5 mol/l k 1 =3 L/(mol s) k =0.4 s -1 τ=1 s Suggestions: Present the model equations in terms of instead of ξ. In this case, for the complete segregation model ξ ( t), ξ can be replaced by ( t), in the equation governing the model. In the maximum mixedness model consider a value of λ λ = 10τ. B (exam feb 07) We have a reactor in which the elementary reaction B takes place in liquid phase with k = 0.15 min -1. The theoretical volume of the reactor is 430 L. In order to characterize the behavior of the reactor a pulse tracer test has been performed with a volumetric flow rate of 50 L/min. The concentrations measured at the exit of the reactor are the following ones: t (min) (mg/l) t (min) (mg/l) a) alculate and plot the corresponding curves E(t), F(t) and E(θ). b) alculate the degree of conversion for the reaction with the segregation model. c) Repeat the previous part using the maximum mixedness model. d) In view of the curves E and F, indicate a possible model which represents reasonably the behavior of the reactor. Determine the parameters of the model, compare the model with the actual behavior of the reactor and calculate the conversion using this model.

5 71.- (exam jul 08) onsider a tubular reactor with residence time τ = 10 s, where an irreversible first-order reaction products takes place in liquid phase, and whose rate constant (referred to the reaction rate r i ) is known, k = 0.1 s -1. a) The behavior of this reactor can be approximated to a PFR with laminar flow regime whose residence time distribution function is as follows: τ 0 t < E( t ) = 1 τ τ t 3 t alculate the average conversion using the segregation model. b) ompare the result in a) with the conversion that would result from the segregation model if the behavior of the reactor approaches that of an ideal STR and ideal PFR. c) Plot the cumulative distribution function F(t) for a laminar PFR, an ideal STR and an ideal PFR, and according to these curves comment on the appropriateness of approximating the residence time distribution of a laminar PFR with that corresponding to an ideal STR or an ideal PFR. 7.- The elementary reaction + B + D is carried out in a STR in which it is suspected there is bypass and dead volume. The table attached shows the step tracer concentrations at the output of the system. If the theoretical volume of the reactor is 1 m 3, the feeding volumetric flow rate is 0.1 m 3 /min, the rate constant is 0.8 m 3 /(kmol min) and the feeding consists of an equimolar solution of and B, with o = kmol/m 3, calculate the conversion that can be expected in the system. t (min) (mg/l) (exam jan 09) In a 10 L reactor the elementary reversible reaction B takes place in liquid phase at constant temperature. In order to determine whether the behavior of the reactor can be resembled to that of a continuous stirred tank reactor with dead volume and bypass a step tracer test has been performed using a volumetric input flow rate of L/min. The exit tracer concentrations are: 1 t (min) (mg/l) a) Obtain the model parameters and calculate the conversion at the outlet of the system for the reaction indicated using this model. b) Write the analytical expression of F(t) taking into account the previous model and obtain from it the analytical expression of E(t). Plot both functions. c) alculate the conversion to be obtained for the above reaction with the segregation model using the analytical expression of E(t) obtained in the previous part.

6 Data: k 1 (direct reaction) = 1 min -1 k (reverse reaction) = 0.3 min -1 in parts a) and c) consider B0 = 0 to calculate conversion 74.- The elementary second-order reaction + B R + S is carried out in aqueous phase inside a tank of 4 m 3. For an equimolar mixture of and B ( o = Bo ) the conversion of reactants is 60%. Unfortunately, the stirring in the reactor is not good and tracer tests conducted indicate that the reactor can be modeled according to the scheme shown in the figure. Which size of an ideal STR would be equivalent to the available tank? 0.5 Q v 9 m 3 15 m The liquid-phase elementary reaction B is carried out in a non-ideal STR, where k = 0.03 min -1. The model is similar to that of two STR with exchange. Reagent is introduced into the reactor with a volumetric flow of 5 L/min and a concentration of 0.0 mol/l, being the total reactor volume equal to 1000 L. When conducting a pulse tracer test the results shown in the table were obtained. Which conversion will be obtained in the system? t (min) (mg/l) (exam jul 10) In a 10 L reactor we want to carry out the elementary reversible reaction 1 B in liquid phase at constant temperature, with a feeding stream containing only reagent. Previously, the behavior of this reactor is going to be characterized, for which a tracer is introduced by a pulse signal, and the variation of tracer concentration with time at the reactor exit is measured. Data collected in the tracer experiment are shown in the following table: t (min) (t) (mg/l) t (min) (t) (mg/l)

7 a) Determine the plot of the normalized function of the residence time distribution E(θ) and that of the normalized function of the cumulative distribution F(θ). Which fraction of tracer remains inside the reactor at 6 minutes? alculate the conversion achieved in the reactor with the following models: b) Tanks-in-series model. c) Maximum mixedness model. Note: for part c) use the expression of E(t) corresponding to the tanks-in-series model used in part b), and use time increments no longer than 0.5 minutes. Data of kinetic constants: k 1 (direct reaction) = min -1 k (reverse reaction) = 0.6 min (exam jan 10) second-order reaction in liquid phase with a single reagent (α = 1) is 7 3θ carried out in a reactor whose RTD is E ( θ ) = θ e. We have studied the behavior of this reactor using various models for a range of values for the parameter R = k, obtaining the following plot: t 0 Obtain the values of the following models: for the largest possible value of the parameter = k 0 t = 50 0 R using

8 a) Ideal STR b) Ideal PFR c) Segregation model d) Maximum mixedness model e) 3 STR in series f) Each of the above models assumes a certain degree of both micromixing and macromixing. Place a point for each model in the shaded area in the following figure. Explain the answer. Suggestions: - value of normalized time (θ) over 4 can be assumed as infinite time. -Part c) This part can be solved numerically but it is faster if it is done analytically using the following recommendations: 1- For the general equation of the segregation model, ξ and ξ (t) can be replaced by ( t) and, respectively The approximation dθ dθ + Rθ is valid for R > Rθ 3- Integration by parts is useful for an integrand with the following expression: ax u = x and dv = e dx. 0 ax xe. hoose -Part d) 1- The following expression can be used: E( θ ) = 1 F( θ ) 7 θ. 9 θ + 3θ The following expressions can transform the differential equation of the maximum mixedness model in terms of the normalized time (θ): dξ dξ -for the conversion degree of the key component: = f ( λ, ξ ) = t f ( λ, ξ ), and dλ dθ E( λ) E( θ ) 1 also: =. 1 F( λ) 1 F( θ ) t

9 3- This part can be solved using Excel, but it is also solved faster using Matlab. 4- If we solve the ordinary differential equation by using Excel, you must take an increment of the independent variable small enough, θ (exam jan 11) In a reactor of volume V = 10 L you want to carry out the reaction B in liquid phase at constant temperature. The feeding is introduced with a volumetric flow rate Q v of.5 L/min and it contains only reactant, with 0 = 0. mol/l. In order to characterize previously the behavior of the reactor, and suspecting that it behaves like a tank reactor with dead volume, a pulse tracer test is performed using the above Q v and we measure the variation of tracer concentration with time at the output of the reactor. The results obtained are shown in the following table: t (min) (t) (mg/l) t (min) (t) (mg/l) a) alculate and plot E(θ) and F(θ). Which fraction of tracer will be out at 4 minutes? b) Which is the dead volume of the tank? alculate the conversion achieved in the reactor using the following three models: Notes: c) Ideal STR with dead volume d) Segregation model e) Maximum mixedness model f) ompare the conversion values obtained with the above three models For parts d) and e) use the analytical expression of E(t) from the model of an ideal STR with dead volume. Kinetic equation of the reaction: r = k, being k =.5 L mol -1 min -1, with r in mol L -1 min -1 and in mol L -1