Validation of the kinetics of the Hydrogen Peroxide Propene Oxide

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Kinetics, Catalysis, and Reaction Engineering

Validation of the kinetics of the Hydrogen Peroxide Propene Oxide (HPPO) process in a dynamic Continuous Stirred Tank Reactor (CSTR) Vincenzo Russo, Elio Santacesaria, Riccardo Tesser, Rosa Turco, Rosa Vitiello, and Martino Di Serio Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03233 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 16, 2018

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Industrial & Engineering Chemistry Research

Validation of the kinetics of the Hydrogen Peroxide Propene Oxide (HPPO) process in a dynamic Continuous Stirred Tank Reactor (CSTR) V. Russo1, E. Santacesaria2, R. Tesser1, R Turco1, R. Vitiello1, M. Di Serio1,3* 1Università

di Napoli Federico II, Chemical Sciences Department, and CIRCC. IT-80126 Napoli, Italy.

2Eurochem

Engineering, IT-20139 Milano, Italy.

3International

Research Organization for Advanced Science and Technology (IROAST), University of

Kumamoto, 860-8555 Kumamoto, Japan

* [email protected]

Abstract A continuous HPPO lab-scale pilot plant was designed and tested for the production of propene oxide via HPPO (Hydrogen Peroxide Propene Oxide) process. The plant was equipped with a continuous stirred tank reactor, able to work under nitrogen pressure with liquid propene. Attention was paid on the feed system and the plant control, being the labscale pilot plant completely automated. Hydraulic tests were performed to check the performance of the plant; a fluid-dynamic characterization was conducted to evaluate the residence time distribution. Propene oxide synthesis experiments were carried out to evaluate both hydrogen peroxide conversion and propene oxide selectivity. The collected data were interpreted with a recently published kinetics, validating the developed model, obtaining satisfactory results, also in simulating the start-up transient state of the reactor. The model can be considered of high utility in designing and optimizing HPPO process, to achieve high reactants conversion and propene oxide yields. Keywords: HPPO, propene oxide, lab-scale pilot plant, kinetics, CSTR.

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1.

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Introduction

Propene oxide is a high reactive chemical used as raw matherial for the production of several commercial materials (i,e. polyether polyols, propene glycols and propene glycol ethers)1. Propene oxide synthesis has a relative long history. Many processes were already developed and some of them even applied at the industrial scale1-5. The direct oxidation of propene with oxygen in gas phase would be in theory the best synthesis route, as it would lead to total selectivity without the need of using a solvent. At the present state, this process is still a holy grail, as the results reported in the literature are very far from the industrial targets6. Among the current technologies, one of the most promising is the HPPO process (Hydrogen Peroxide Propene Oxide), because hydrogen peroxide is used as direct oxidant in propene oxidation and only water is obtained as theoretical byproduct. The reaction is catalyzed by TS-1, a titanium silicalite MFI zeolite, and it is carried out under mild conditions of temperature and pressure (40°C, 20 bars)6,7. Methanol is used as solvent. Despite its toxicity, it is still used in the current technology: it allows to reach high conversion as it stabilizes the key intermediate of the propene oxidation reaction6. The process temperature ranges between 30-50°C, while the pressure is, in majority of cases, between 20-25 bar to keep propene in the liquid phase: liquid-solid (catalyst) system. In the last 25 years, this process is under development and recently some industrial plants, based on this technology, are already running4,5. The HPPO process reaction network scheme is reported in Scheme 1. As evident, beside the main reaction that occurs between propene and hydrogen peroxide, some other side reactions can occur, lowering the selectivity of the process7.


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HCOOCH3 + H2O TS-1 CH3OH HCOOH + 1.5 H2O TS-1 0.5 CH3OH H2O2 +

O

TS-1

+ H2O

TS-1

H2O OH

H2O + 0.5 O2

CH3OH OH

OH

n

OCH3 OCH3

O

OH

OCH3

OH O n

Scheme 1 - Overall reaction network for the HPPO process.

The reactions reported in the reaction scheme 1 are related to either the ring opening reactions, or the hydrogen peroxide decomposition or to methanol oxidation. Propene oxide can react with either water or methanol, giving place to propene glycol and methoxypropanol, respectively. These two products are the major by-products and, among them, methoxypropanol is the most abundant, being usually methanol concentration greater than the water one. Of course, propene oxide can react with methoxypropanol or propene glycol giving place to dimers that can react further forming heavier oligomeric adducts. It was shown in the scientific literature that an increase of the reaction temperature corresponds to an increase of the hydrogen peroxide conversion, but to a corresponding decrease of the propene oxide selectivity7. This fact shows that temperature needs to be kept low to avoid the ring opening reactions. Therefore, being the epoxidation reaction extremely exothermic6-10, it is not very easy to keep the system at the desired temperature values, so attention must be paid to heat exchange aspect. Hydrogen peroxide decomposition must be carefully considered mainly at high temperatures (T>50°C with 2wt.%TS-1 and 9wt.% H2O2 in methanol, in a batch reactor7), as a temperature increase leads to a faster hydrogen peroxide 3 ACS Paragon Plus Environment


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decomposition, lowering the yield of the process and leading to the formation of molecular oxygen, giving place to explosive mixture with propene7. Concerning the reaction mechanism, Clerici et al.11 have published that the most accredited mechanism, based on spectroscopic evidences, is the Eley-Rideal one. Recently, our group published a kinetic study based on gas-liquid-solid propene oxide synthesis runs, performed in a fed-batch reactor at low pressure (propene was fed in gas phase), using TS-1 in microspheres, that brought to the development of a complete reaction mechanism that considers all the main and side reactions of the HPPO process7 (see Scheme 2).

H

OSi CH3OH + SiO SiO

O

Ti

OHOCH3 H

OSi H2O

Ti

kD1*

H2O

CH3OH

Kads,3 OSi

OSi

CH3OH + Ti SiO OSi SiO

Kads,2

TiC3H6

OSi SiO SiO

Ti

O

OSi

Ti

O

Kads,4 CH3OH +

SiO SiO

H3C SiO O H Ti O H O SiO OSi HOSi

CH3 H

H2O2

Kads,1 H3C SiO O Ti SiO OSi

k1

O

H3C O H Ti O H O SiO OSi O HOSi

SiO

Kads,5

Ti*

Ti*PO H2O

kD3*

HOSi

OHOH

O

Ti

O

OSi

+ H2O

TiPO

Scheme 2 - Overall reaction scheme of the HPPO process7.

Satisfactory results were obtained, as the kinetic model was able to correctly interpret a large number of experiments conducted in a wide range of operation conditions, studying all the possible reactions separately. To extend the validity of the model, a lab-scale continuous pilot plant was installed in our laboratory. The core of the pilot plant was a continuous stirred tank reactor (CSTR), as the TS-1 catalyst was in form of a fine powder, that would eventually lead


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to high pressure drops if used in packing a tubular reactor. The choice of CSTR allows, at labscale, to obtain a very good heat exchange compared to packed bed reactors where temperature profiles (i.e. hot spots) could arise both in the principal axis of the pipe and within the catalyst; as it was recently demonstrated by Lin et al.12, in packed bed reactors temperature profiles were obtained within the axis of the pipe, leading to a lowering of the propene oxide selectivity. Selectivity decrease was described in terms of propene oxide ring opening reactions, catalyzed by two different sites (e.g. sinalols and titanium hydroperoxide), as demonstrated in our previous studies7 and recently by Signorile et al.13. The developed system was first characterized in terms of residence time distribution, then used to conduct propene oxide synthesis experiments. The aim of the present work is to extend the investigation from semi-batch to continuous reactor to validate the developed kinetic model, demonstrating that the kinetics can be used for optimizing the yield of the HPPO process. Thus, the developed model can be of high interest for both process design and optimization.

2.

Materials and methods

2.1 Reagents Propene was supplied by SIAD with a purity of 99.5 wt.% (0.5 wt.% propane), methanol by Clean Consult at 99.8 wt.% purity, hydrogen peroxide (60 wt.%) by Mythen S.r.l. All the other reagents employed were supplied by Aldrich at the highest level of purity available and have been used as received without further purification. TS-1 catalyst has been supplied by Conser S.p.A. in microspheres form obtained by spray-drying technique. The microspheres, characterized by an average diameter of 35μm, are composed by crystalline TS-1 (Ti content



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of 3 wt.% and average size of crystallites of 30 nm) and silica as binder in a ratio 1:1 g/g (information given by the catalyst supplier). 2.2 Analytical methods The products distribution and the PO conversion were analytically determined by gaschromatographic analysis, using a gas chromatograph (HP 6890), equipped with a flame ionization detector (FID), a split-splitless column injector, and employing a Restek RT-QBond Plot column (30m x 0.32mm ID, 0.1μm film). Before the analysis, 100μL of ethyl acetate was added to 5 cm3 of sample, as internal standard. The residual hydrogen peroxide concentration was determined by iodometric titration14. Propene conversion was calculated by stoichiometry from the hydrogen peroxide conversion values. The UV-Vis measurements for the CSTR fluid-dynamic tests were conducted by using a Jasco UV-975 spectrophotometer in continuous, setting the wavelength at λ=286nm, using toluene as tracer molecule.

2.3 Experimental apparatus The propene oxide synthesis experimental runs were carried out in a lab-scale pilot plant, where the reactor consists of a continuous stirred tank (CSTR). A sketch of the realized plant is reported in Figure 1. A piston pump was used to feed the oxidizing solution composed by 93.2wt.% methanol, 3.5wt.% hydrogen peroxide and 3.3wt.% water. Propene was fed to the reactor through a liquid mass-flow-meter controller based on the Coriolis principle. Propene is stored in a tank under the pressure of 35bar, thus pressure is the driving force responsible of the feed, while the flow-meter is the regulator. Both the pumping systems were fixed at the desired flow-rate that is kept constant during the test. A static mixer was installed and tested to verify if the oxidizing solution and propene were properly mixed before feeding them in the CSTR. The mentioned mixer is an AISI 316 stainless steel tube of 10cm length and 3/4" OD


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(1.54cm ID) filled with glassy spheres of 2.4mm diameter. The mixer is characterized by a void degree of ε=0.4. The reactor was a continuous stirred tank reactor (CSTR). The design of this unit was not trivial, as there was the necessity to build up a complex reaction system, deriving from different properties of the reaction itself. Being the reaction extremely exothermic (ΔH = -57 kcal/mol15), a proper heat exchange system was needed, able to remove the heat released by all the reactions and keep the reaction temperature at the desired level. For this reason, a stainless steel AISI 316 cooling coil of 2m length and 1/8” OD was installed, where water flows at al flow-rate of 6L/min. A thermocouple was installed in the reactor to measure the reaction temperature.

Secondly, the system must be pressurized at pressure greater than 16bar (propene partial pressure value estimated using ChemCAD suite16), to keep propene in the liquid phase. For this reason, an auxiliary tank connected to the top of the reactor was connected to a nitrogen bottle through a pressure regulator. All the tests were performed at 20bar. A filter was installed on line, to avoid any catalyst drainage from the CSTR to the mentioned tank. There was the necessity to keep all the reactor filled with liquid, avoiding high catalyst concentration in the CSTR and to warrantee a good heat transfer. For this reason, a liquid level sensor was installed in the auxiliary tank. As oxygen can be released due to the eventual hydrogen peroxide decomposition, it would be dangerous an eventual accumulation in the auxiliary tank. Thus, the top of the mentioned tank was continuously flushed by regulating the gas outer flow with a mass flow-meter controller. This equipment is driven by the pressure of the gas phase with a PID controller. For the sake of safety, a mechanical relief valve was installed on the outlet stream, calibrated at 50bars, thus if pressure gets higher than the mentioned value, the valve opens, and the mixture is sent to the final collecting tank. By



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considering that the pumps were always in action during the tests, it was necessary to warrantee a continuous and regular outlet liquid flux from the reactor, to avoid either a liquid accumulation or a reactor emptying. A liquid mass flow-meter controlled based on the Coriolis principle was installed on the outlet line to solve this problem. The mentioned flowmeter is automatically driven by the liquid level sensor previously described, so if the liquid level increases, the flow-meter proportionally opens. This operation was made by a PID controller. An adequate filtering system was installed on the outlet line being the catalyst in an extremely fine powdered form. The outlet liquid phase consists of a pressurized and warm stream of the reaction products and the unreacted reagents. This mixture must be de-pressurized at atmospheric pressure. In this way, propene, methanol and propene oxide would directly flash in two streams: a liquid phase containing of the heavy products; a gas phase containing mainly propene and a very small quantity of methanol and traces of propene oxide. In order to keep propene oxide and methanol in the liquid phase and to properly flash the product stream, two operation units were put in series: (i) a heat exchange system, that allows to cool the product stream at 5°C, composed by a stainless steel AISI 316 cooling coil of 2m length 1/8” OD immersed in a cylindrical vessel; (ii) a flash unit composed by a stainless steel AISI 316 pipe of 30cm length and 1” OD, fed at the center. The flash unit was connected to a second auxiliary tank where a liquid level sensor is installed to keep the flash unit filled with liquid for 1/3 of its length. The liquid stream coming out from the flash unit is measured through a liquid mass flowmeter controller, based on Coriolis principle, and further collected and analyzed to determine the reaction conversion and the propene oxide selectivity. The liquid solution is stored in a 5 L tank continuously flushed with nitrogen to avoid eventual ignitions.


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The gas stream is measured through a mass flow-meter controller and analyzed to quantify propene oxide, that is present in the gas phase (less than 0.1%). Then, the gas phase is sent to a burner working at 900°C for safety reasons. A thermocouple is installed on the flame, while a pilot flame was provided by feeding external butane to the burner to avoid its turning off. All the gas purges of the system are connected to the same burner. All the described operations and all the valves connecting the described units were automated through to a data acquisition and control system (NI cDAQ-9178) provided by National Instruments and a dedicated software written in LabVIEW 2011 by the authors. The mentioned software was written to include alarms on the main operating variables, to keep the execution of the experiment always in safe conditions. Before each test, the lab scale continuous plant was firstly loaded with the liquid methanolic solution under atmospheric and inert pressure. In the meantime, both the reactor and the cooler were brought at the set temperature value, while the stirring rate set at 300 rpm. As the system was full and at the desired temperature level, a pressure of 20bar of nitrogen was applied. Dependently on the test, two different procedures were applied. (i)

Fluid-dynamic tests: the flow-rate is adjusted and the solution tracer (toluene 9wt.% in methanol) was pumped to the reactor. The tracer concentration was monitored by UV-Vis detector.

(ii)

Kinetic tests: both the oxidizing solution and propene are pumped to the reactor at a fixed flow-rate.

At the end of each test, the reactor was washed with methanol, then the liquid phase discharged.



3.

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Results and discussion

3.1. Fluid-dynamic tests A fluid dynamic characterization was conducted to determine the residence time distribution function. In Figure 2, an example of the experimental profiles obtained for the fluid-dynamic tests is reported, where F represents the dimensionless tracer concentration for a fixed feed composition, obtained dividing toluene concentration by its feed value.

The obtained concentration profile is typical for an ideal CSTR and the average residence time, t , can be obtained by fitting on the collected experimental data, applying Eq. 1, valid for a CSTR ideal behavior17. F  1  e t / t

(1)

The fit can be appreciated in Figure 2. The corresponding measured residence time is 6.7±0.1min. The results confirm that the unit can be modelled as an ideal continuous stirred tank reactor (CSTR).


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3.2. Kinetic tests Different experimental runs were conducted in the designed CSTR investigating the influence of several operation variables: i.e. residence time, propene/hydrogen peroxide ratio and temperature. The mentioned strategy was to validate the kinetic rate expressions, with related parameters, reported in our previous work in a gas-liquid-solid fed batch reactor7. A list of the performed experimental experiments with related operation conditions is reported in Table 1. All the experiments were performed by feeding methanol/H2O2 solution to the reactor loaded with TS-1. The temperature values reported in Table 1 are referred to both feed and jacket temperatures. This means that at start-up the system is isothermal. Then, propene was fed to the reactor and the reaction started. Concerning the use of the optimal stirring rate, thus avoiding eventual fluid-solid mass transfer limitations, preliminary experiments were performed finding that 300rpm can be considered an optimal stirring rate to work under kinetic regime. The absence of intraparticle mass transfer profiles was already demonstrated in our previous work7. In particular, the Weisz-Prater correlation was applied, finding in all cases an effectiveness factor always near to the unity. Catalyst loading was fixed at ρTS-1=19.2g/L.

Table 1 – Summary of the performed tests with related experimental conditions. Stirring rate was fixed at 300rpm, ρTS-1=19.2g/L. Run T [K] τ [min] C3H6/H2O2 [mol/mol] 1 303 6.7 0.24 2 313 6.7 0.23 3 313 6.6 0.47 4 313 13.0 0.96 On the basis of the experiences collected during the kinetic study performed in a fed-batch reactor, to keep the experiments in safe conditions, it was decided to work at low temperature and low TS-1 loading. In this way, it was possible to work under isothermal conditions, 11 ACS Paragon Plus Environment


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avoiding hydrogen peroxide decomposition. The first run was carried out for 5h to verify that the system reached the stationary state after an initial transient behavior. Moreover, the experiment was prolonged also to check the eventual catalyst deactivation, demonstrating that the catalyst is stable in the adopted operation conditions. As all the experiments were conducted with the same TS-1 sample, we can conclude that the tested catalyst shows no deactivation for at least 20h of operation.

The profiles of propene conversion/propene oxide selectivity and reactor's temperature are reported (see Figure 3). The semi-log plot starts from 3.6s of reaction for graphical reasons. The readers must be aware that at time on stream equal to zero temperature is the fed value reported in Table 1. As revealed by Figure 3, the system can be considered stationary already after 0.5h of timeon-stream. Temperature profile shows a first peak due to the exothermicity of the reaction, balanced by the heat exchanged by the cooling system. The heat balance allows to reach a steady state value, with an overall maximum ΔT=2K, due to the good heat exchange of the system. Moreover, it is possible to observe that the catalyst activity is constant for 5h time-onstream. Further experiments were conducted and the comparison of the data in terms of propene conversion (XC3H6) and propene oxide selectivity (ΦPO) are reported in Figure 4. The collected results showed that by increasing the oxidizing solution flow-rate (Runs 3-4), so decreasing the residence time, a decrease in the propene conversion was observed but the selectivity to propene oxide significantly increases (Figure 4). This result is reasonable because by lowering the residence time the conversion of the main reaction decreases as expected but as the side reactions are consecutive to the main one the occurrence of by products is lowered.


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The results of the two runs performed at different temperatures (Runs 1-2) demonstrate that by increasing the temperature, the propene conversion increases while the propene oxide selectivity decreases (Figure 4). Two experiments were carried out at different propene/hydrogen peroxide molar ratio, keeping constant the temperature and, roughly, the residence time (Runs 2-3). The results of the two runs showed that by doubling the propene concentration in the reactor, the conversion increases only by a factor of 1.7 (Figure 4), fact in agreement with the not linear dependence of the propene concentration on the propene oxide formation reaction rate7.



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3.3. Kinetic modelling and simulations The collected experimental data were interpreted with the kinetic model previously derived from fed-batch experiments. The stochiometric matrix was written as in Table 2. The reaction rate expressions comprised in the overall reaction network are reported in Table 3, together with the related parameters calculated at a reference temperature fixed at 313K.

Table 2 – Stochiometric matrix for the HPPO kinetic model. i/m H2O2 C3H6 PO H2O O2 CH3OH MetOx Gly Oligom

1 -1 -1 +1 +1 0 0 0 0 0

2 -1 0 0 +1 0.5 0 0 0 0

3 0 0 -1 0 0 -1 +1 0 0

4 0 0 -1 0 0 0 -1 0 0

5 0 0 -1 -1 0 0 0 +1 0

6 0 0 -1 0 0 0 0 0 +1

7 0 0 -1 0 0 -1 +1 0 0

8 0 0 -1 0 0 0 -1 0 0

9 0 0 -1 -1 0 0 0 +1 0

10 0 0 -1 0 0 0 0 0 +1


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Table 3 – Reaction rate laws and parameters for the HPPO kinetic model. Kinetic constants are calculated at T=313K. m 1

2 3

Reaction PO synthesis

H2O2 decomposition Ring opening catalyzed by Ti-OOH

4

1  K1cH 2O2  K 2cC3 H6  K3cH 2O  K 4cPO

r2  k2 TS 1cH 2O2 r3 


1  K1cH 2O2  K 2 cC3 H 6  K 3cH 2O  K 4 cPO

r6  Ring opening catalyzed by silanols

k3 TS 1 K1cH 2O2 cPO cCH3OH k4 TS 1 K1cH 2O2 cPO cCH 3 O ( PO ) H

r5 

6

8

k1 TS 1 K1cH 2O2 cC3 H6

r1 

r4 

5

7

rm

k5 TS 1 K1cH 2O2 cPO cH 2 O 1  K1cH 2O2  K 2cC3 H6  K3cH 2O  K 4cPO k4 TS 1 K1cH 2O2 cPO cOligom

Parameters Ea1=10.9 kcal/mol k1,ref=2.98·101 L/g/min K1=3.47·10-3 L/mol K2=8.28 L/mol K3=4.54·10-1 L/mol K4=7.58 L/mol Ea2=19.5 kcal/mol k2,ref =2.45·10-5 L/g/min Ea3=25.4 kcal/mol k3,ref =1.18·10-1 L2/g/mol/min Ea4=30.2 kcal/mol k4,ref =3.11·10-4 L2/g/mol/min Ea5=35.1 kcal/mol k5,ref =4.40·10-2 L2/g/mol/min As reaction 4


r7  k7 TS 1cPO cCH3OH r8  k8 TS 1cPO cCH 3 O ( PO ) H

9

r9  k9 TS 1cPO cH 2O

10

r10  k8 TS 1cPO cOligom

Ea7=10.7 kcal/mol k7,ref =9.48·10-6 L2/g/mol/min Ea8=34.0 kcal/mol k8,ref =6.07·10-6 L2/g/mol/min Ea9=16.0 kcal/mol k9,ref =1.14·10-5 L2/g/mol/min As reaction 8

The kinetic rate laws were fixed, together with the kinetic and equilibrium parameters. The experimental data collected in the HPPO lab-scale plant were simulated by solving a dynamic CSTR heat exchanged model. The mass and heat balance equations that have been used to interpret the collected data are reported in Eqs. 2-3, solved numerically by using ode23s ODE solver algorithm implemented in MATLAB.

dci ci , feed  ci ,out    i ,m  rm dt  m

 mix c p ,mixV

(2)

dT  Q feed  mix c p ,mix (T feed  T )  (HrV 1 )  UA(T  TJ ) dt

(3) 15

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Jacket temperature was set to be constant, controlled through the cryostat unit and measured through a dedicated thermocouple. The value was fixed to the feed temperature value lo allow the system to reach the steady-state condition as fast as possible; reaction heat due mainly to the epoxidation reaction (ΔH=-57 kcal/mol), neglecting the effect of the side-reaction heat. This assumption is valid as in the experiments conducted, the propene oxide selectivity was always very high, thus the side-reactions rates were very low. Overall heat exchange coefficient was calculated aside with heat exchange characterization tests (UA=0.42·102 cal/K/min). Step-wise experiments were conducted by loading the reactor with methanol and heating/cooling it between two temperature levels. The collected data were interpreted using Eq. 3, putting the reaction term equal to zero. Density and specific heat of the liquid mixture were calculated by weighting the corresponding values of pure components by their mass fraction. The density and specific heat values of the pure components as a function of temperature were taken from ChemCAD libraries16, with general equations reported below, Eqs. 4-5.

i  1000Mwi

A1,i  T 1 1  A 3,i  2,i

A

c p ,i 

  

(4)

A4,i

P1,i  P2,iT  P3,iT 2  P4,iT 3

(5)

4187 Mwi

The parameters needed for the calculations are reported in Table 4.


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Table 4 – Density and specific heat parameters for the components involved in the HPPO process. I CH3OH PO MetOx H2O Gly H2O2 C3H6

A1 2.288 1.486 0.928 5.459 1.092 3.215 1.525

A2 0.2685 0.2763 0.2728 0.3054 0.2611 0.2498 0.2752

A3 512.64 482.25 566.00 647.13 626.00 730.15 364.76

A4 0.2453 0.2935 0.2052 0.0810 0.2046 0.2877 0.3025

P1 105800 165050 67929 276370 58080 63850 105800

P2 -362.2 -629.5 448.1 -2090.1 445.2 72.7 -234.4

P3 0.938 2.011 0 8.125 0 0 0.755

P4 0 -0.0013 0 -0.0141 0 0 0

The dynamic model was used to simulate the transient and steady-state profiles collected in the lab-scale pilot plant. In Figure 3, it is evident that the model (related to Run 1) prediction is very satisfactory, interpreting the experimental data, both the dynamic and steady-state data. The model is able to follow the temperature profile accurately, both in transient and steady-state sections. This achievement is of high importance, as the model can be used even in predicting the start-up procedure of a HPPO plant.

The developed model was applied to describe also the other experimental data available. To appreciate the goodness of the fitting, in Figure 5 the parity plot, between calculated and experimental data, is reported, concerning both the propene conversion and selectivity to propene oxide, and reactor temperature. As it can be seen, conversion and selectivity data are in a window of 15%, while temperature data in a window of 0.5%, thus the model can be considered validated as no further adjustable parameters were introduced with respect to previous findings7.



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4. Conclusions A continuous lab-scale pilot plant has been designed, installed and tested for producing propene oxide via HPPO process, to validate the recently published kinetics. The plant is complex but fits the main requests that the HPPO process call. The heart of the plant is a continuous stirred tank reactor (CSTR), that was tested both in terms of fluid-dynamics and kinetics. The fluid-dynamic characterization experiments were conducted to determine the flow-regime operating in the reactor vessel. It was demonstrated that the F experimental function can be correctly described by an ideal continuous stirred tank model. This result allowed us to write the mass balance equations in correct form. Several experiments were conducted to investigate the influence of the main operation conditions on propene conversion and propene oxide selectivity. The results were coherent with our previous experiences made by conducting propene oxide synthesis experiments in a fed-batch reactor. The collected data were interpreted using the kinetic rate laws and related parameters derived from fed-batch experiments. A dynamic continuous stirred tank reactor model was implemented in MATLAB and solved with built-in ODE solver functions. The results of the predictive simulations were encouraging, demonstrating that a 15% window of error can be derived from the overall parity plot. The model predictions were in good agreement both for transient and steady-state measurements, fact that will allow to forecast the start-up procedure of the HPPO pilot plant. In conclusion, the present work confirms the high potential of the developed kinetic model and high reliability of the kinetic parameters determined previously by our research group. The developed tool allows to forecast the best operation conditions to maximize the reactants conversion and propene oxide yield, allowing to design and optimize larger scale HPPO


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plants. In perspective, the model could be used in simulating propene oxide experiments conducted in other devices, as in packed-bed reactors, making further efforts in simulating non-isothermal cases that could be dominated by mass and heat transfer effects. In this way, the robustness of the model would be further improved, and the model considered of general application.

Acknowledgments CONSER S.p.A. is acknowledged for funding this study.



Page 20 of 29

Cited literature (1) Nijhuis, T.A.; Makkee, M.; Moulijn, J.A.; Weckhuysen, B.M. The Production of Propene Oxide: Catalytic Processes and Recent Developments. Ind. Eng. Chem. Res. 2006, 45, 3447– 3459. (2) Bassler, P.; Weidenbach, M.; Goebbel, H. The new HPPO Process for Propylene Oxide: From Joint Development to worldscale Production. Chem. Engineer. Trans. 2010, 21, 571576. (3) Short, P.L. BASF, Dow Open Novel Propylene Oxide Plant. Chem. Eng. News 2009, 87, 21. (4) http://www.knak.jp/big/evonik-hppo.htm (accessed 15 June 2018). (5)

http://corporate.evonik.com/en/media/search/Pages/news-details.aspx?newsid=26719

(accessed 15 June 2018). (6) Russo, V.; Tesser, R.; Santacesaria, E.; Di Serio, M. Chemical and Technical Aspects of Propene Oxide Production via Hydrogen Peroxide (HPPO Process). Ind. Eng. Chem. Res. 2013, 52, 1168−1178. (7) Russo, V.; Tesser, R.; Santacesaria, E.; Di Serio, M. Kinetics of Propene Oxide Production via Hydrogen Peroxide with TS-1. Ind. Eng. Chem. Res. 2014, 53, 6274–6287. (8) Wang, T.-S.; Liu, S.-H.; Lin, Y.-C.; Chen, Y.-C.; Shu, C.-M. Green Process of Propylene Oxide Reaction for Thermal Hazard Assessment by Differential Scanning Calorimetry and Simulation. Chem. Eng. Technol. 2015, 38(3), 455–462. (9) Shilcrat, S. Process Safety Evaluation of a Tungsten-Catalyzed Hydrogen Peroxide Epoxidation Resulting In a Runaway Laboratory Reaction. Org. Process Res. Dev. 2011, 15, 1464–1469


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(10) Leveneur, S. Thermal Safety Assessment through the Concept of Structure−Reactivity: Application to Vegetable Oil Valorization Sebatien Leveneur. Org. Process Res. Dev. 2017, 21, 543−550. (11) Clerici, M.G. TS-1 and Propylene Oxide, 20 Years Later. Erdöl, Erdgas, Kohle 2006, 122, OG77-OG82. (12) Lin, M.; Xia, C.; Zhu, B.; Li, H.; Shu, X. Green and efficient epoxidation of propylene with hydrogen peroxide (HPPO process) catalyzed by hollow TS-1 zeolite: A 1.0kt/a pilotscale study. Chem. Eng. J. 2016, 295, 370–375. (13) Signorile, M.; Crocella, V.; Damin, A.; Rossi, B.; Lamberti, C.; Bonino, F.; Bordiga, S. Effect of Ti Speciation on Catalytic Performance of TS-1 in the Hydrogen Peroxide to Propylene Oxide Reaction. J. Phys. Chem. C 2018, 122, 9021–9034. (14) Kolthoff, I.M.; Sandell, E.B.; Meehan, E.J. Treatise Analytical Chemistry, Vol. 2. Wiley: New York, 1978. (15) Santacesaria, E.; Tesser, R.; Di Serio, M.; Turco, R.; Russo, V.; Verde, D. A biphasic model describing soybean oil epoxidation with H2O2 in a fed-batch reactor. Chem. Eng. J. 2011, 173, 198-209. (16) ChemCAD, Chemstations. https://www.chemstations.com/ (accessed 15 June 2018) (17) Levenspiel, O. Chemical Reaction Engineering, John Wiley & Sons: New York, 1999.



Page 22 of 29

List of symbols Ak,i

Density parameter of component i (k=1-4)

ci

Concentration of the i component [mol/L]

ci,feed

Feed concentration [mol/L]

ci,out

Outlet concentration [mol/L]

cp,i

Specific heat of component i [cal/g/K]

cp,mix

Liquid mixture specific heat [cal/g/K]

Eam

Activation energy of reaction m [kcal/mol]

F

Dimensionless tracer concentration [-]

i

Component [-]

km

Kinetic constant of reaction m

km,ref

Reference kinetic constant, at T=313K, of reaction m

Kj

Adsorption parameter (j=1-4) [L/mol]

m

Reaction [-]

Mwi

Molecular weight of component i [g/mol]

Pk,i

Specific heat of component i parameter (k=1-4)

Qfeed

Feed volumetric flow-rate [L/min]

rm

Reaction rate of the reaction m, [mol/(L∙min)]

t

Time [min]

t

Average residence time [min]

T

Temperature [K]

Tfeed

Feed temperature [K]

TJ

Jacket temperature [K]

UA

Overall heat transfer coefficient [cal/K/min]

V

Volume [L]


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wcat

Catalyst mass [g]

XC3H6

Propene conversion [-]

Greek letters ΔH

Reaction enthalpy [kcal/mol]

ΔT

Temperature difference [K]

ε

Void fraction [-]

νi,m

Stoichiometric coefficient for component i in reaction m [-]

ρi

Liquid density of component i [g/L]

ρmix

Density of the liquid mixture [g/L]

ρTS-1

Catalyst load density, wcat/V [g/L]

τ

Residence time [min]

ΦPO

Propene oxide selectivity [-]

Abbreviations Gly

Propene glycol

HPPO

Hydrogen Peroxide Propene Oxide

ID

Internal diameter

MetOx

Methoxypropanol

OD

Outer diameter

Oligom

Oligomer

PO

Propene oxide

TS-1

Titanium silicalite



Figures

T

P

NITROGEN BOTTLE

RID-1 FLAME

P

RID-2 P

L P

GAS SAMPLING MASS FLOW-METER REGULATOR

MASS FLOW-METER

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

P P

L T

PROPENE TANK

VENT

COOLER

STATIC MIXER

CSTR

OXIDIZING SOLUTION TANK

T

MASS FLOW-METER REGULATOR

MASS FLOW-METER REGULATOR

PISTON PUMP

CRYO UNIT

CRIO UNIT

MASS FLOW-METER

LIQUID SAMPLING

Legend Check valve COLLECTING TANK

Two ways valve Three ways valve Relief valve Sensor P

Pressure reducer DISPOSAL

Figure 1 – Flow-sheet of the HPPO lab-scale continuous plant.


Page 25 of 29

1.0 0.9 0.8 0.7

F [-]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

2

4

6

8

10

12

14

16

18

20

Time on stream [minutes] Figure 2 – Fluid-dynamic test: F function versus time on stream.



308

1.0 0.9

0.7

XC3H6 [-]

307

PO [-]

T [K]

306

0.6 305

0.5 0.4

T [K]

0.8

XC3H6 [-], PO [-]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

304

0.3 0.2

303

0.1 0.0 0.001

302 0.01

0.1

1

Time on stream [hours] Figure 3 – Propene conversion, propene oxide selectivity and CSTR temperature, against time-on-stream, of Run 1 of Table 1. Symbols are the experimental data, lines the simulated profiles.


Page 27 of 29

Run 1 - T=303K, =6.7min, C3H6/H2O2=0.24 mol/mol Run 2 - T=313K, =6.7min, C3H6/H2O2=0.23 mol/mol Run 3 - T=313K, =6.6min, C3H6/H2O2=0.47 mol/mol

1.0

Run 4 - T=313K, =13.0min, C3H6/H2O2=0.96 mol/mol

XC3H6 [-], PO [-]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4 0.2 0.0 Run 1 XC3H6

Run 2

EXP

XC3H6

Run 3 SIM

PO

EXP

Run 4 POSIM

Figure 4 – Propene conversion and propene oxide selectivity experimental and simulated results for all the experiments conducted in the CSTR.


1.0

Experimental data [-]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Experimental temperature [K]


0.9 0.8 0.7 0.6

 15%

0.5 0.4 0.3 0.2

XC3H6 [-]

0.1

PO [-]

0.0 0.0

A.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Calculated data [-]

0.9

1.0

Page 28 of 29

306

305

0.5%

304 T [K]

303 303

B.

304

305

306

Calculated temperature [K]

Figure 5 – CSTR parity plot for: A. propene conversion and propene oxide selectivity; B. reactor temperature.


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For Table of Contents Only


Validation of the kinetics of the Hydrogen Peroxide Propene Oxide

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