CHARACTERIZING THE KINETICS OF HETEROGENEOUS EXO- THERMIC REACTIONS

Transcription

1 CHARACTERIZING THE KINETICS OF HETEROGENEOUS EXO- THERMIC REACTIONS Kathrin Bisup 1,HeioBothe 1, Günther Hessel, Günther Hulzer 1, Holger Kry, Wilfried Schitt, Nurelegne Tefera 1. Introduction In the pharaceutical and fine cheical industry the nowledge of thero-inetic reaction paraeters is of practical interest to the process optiization for econoic and environental reasons as well as to the ris anageent of exotheric reactions with a high hazardous potential. Since the thero-inetic data are ainly deterined at the laboratory scale, the scaleup proble plays a crucial role when the results have to be adapted to the production plant. For this purpose, reaction calorieters are a well-suited tool because they allow to wor under conditions of stirred industrial tan reactors. In recent years the perforance of calorietric ethods has been proven for hoogeneous reactions. The ai of this paper is to present a set of experients which allow to characterize exotheric reactions in heterogeneous systes. Results are described for the catalytic hydrogenation of a nitro aroatic copound.. Fundaentals of the heterogeneous process The inetics of hoogeneous exotheric reactions can be characterized by the reaction enthalpy and the thero-inetic paraeters such as the rate of reaction, the activation energy and the order of reaction. Due to ultiphase reactions in heterogeneous systes, the reaction inetics is additionally affected by the phase equilibriu and the ass transfer. Therefore, the gas solubility and the ass transfer coefficients have to be deterined if they significantly affect on the reaction rate. As an exaple for coplex heterogeneous reactions, the process inetics of the catalytic hydrogenation of a substituted nitrobenzoate (SNBE) to a substituted ainobenzoate (SABE) is described in the following. The three-phase process (gas-liquid-solid) of the catalytic hydrogenation is not only influenced by the reaction inetics, but also by the hydrodynaics and the ass transfer. During the hydrogenation reactions which are catalysed by solids, the reactants ust be transferred to the reaction site. The cheisorption onto the catalyst surface could be a rate deterining step. Depending on the process conditions and the effects of the ass transfer, two pathways for the reduction of nitro copounds to aines are possible (Fig. 1). The first path a) proceeds fro the sequential reduction of nitro copounds to nitroso copounds, next to hydroxylaine, and finally, to aine. This path a) is favoured when the reduction of nitro copounds is conducted with etals and ineral acids. The second path b) is possible under process conditions that lead to an increase of concentration of the interediates nitroso and hydroxylaine. Both interediates can cobine to the azoxy copound followed by reduction to the azo copound, hydrazo copound, and finally, to the aine (SABE). The inetics of this pathway of the SABE foration is uch slower. Due to the accuulation of interediates, the 1 Arzneiittelwer Dresden GbH (AWD) 71

2 hazardous potential of the catalytic hydrogenation is increased extreely since it can lead to side-reactions which are strongly exotheric. Furtherore, the azoxy foration, in contrast to the hydrogenation, cannot be controlled by the hydrogen supply because this side-reaction taes place without hydrogen uptae. Therefore, it is necessary to now whether accuulations of interediates occurs. Path a): R NO H R NO+H O H R NHOH H R NH +H O Nitro Nitroso Hydroxylaine Aine Pathb): R N=N R+H O H Azoxy O H R N=N R+H O Azo H R NH NH R Hydrazo Fig. 1: Reaction pathways for the reduction of SNBE (nitro copound) to SABE (aine). Measuring ethods.1 Measureent of the gas solubility and ass transfer coefficient To investigate the gas solubility and the voluetric ass transfer coefficient fro the gas to liquid phase, the batch absorption of hydrogen in a stirred tan reactor was easured [1]. The advantage of this ethod is that both paraeters can be deterined without H -concentration easureents in real reaction ixtures. The pressure profile during the batch absorption of H in the reaction ixture is depicted in Fig.. Before the reactor is pressurized with hydrogen, first the liquid ust be degassed under agitation to reach an equilibriu at nown pressure P and teperature T. Then the agitation is stopped and the reactor is pressurized with H to the desired pressure P. When the theral equilibriu is reached, the stirrer is started. During starting the agitation, the gas absorption will be accelerated and the pressure continues to drop until the saturation point P " is reached. The gas solubility α can be deterined by onitoring the initial and final pressures: α = P P P P V V G L 1 RT = 1 He (1) where V G gas volue [l] V L liquid volue [l] R general gas constant R = 8.1 J/(ol ( K) gas absorption coefficient [ol/(³ ( Pa)] He Henry constant [(³ ( Pa)/ol] 7

3 The integration of the ass balance between gas and liquid phase gives the following equation: P ln P where P is the pressure at the tie t. P P = L P a P P P Equation () shows that the value of the voluetric ass transfer coefficient L a as the product of the ass transfer coefficient and the specific boundary surface can be deterined fro the slope of the pressure drop (Fig. ). t () 4.5 P P" 4 P[bar] P[bar] P t[s] t[s] Fig. : Pressure profile during the batch absorption of H. Reaction enthalpy and thero-inetic paraeters Since the heat release rate Q r of exotheric reactions is proportional to the rate of reaction r, to the volue of the reaction ixture V, and to the enthalpy (- H r ), calorietric ethods can be used to deterine the reaction enthalpy and the overall rate of reaction: ( ) Q r = rv H r () According to Arrhenius law, the rate of reaction r is an exponential function of teperature: r = ( E / RT ) C ( 1 ) n exp a A n x where pre-exponential factor (unit depends on reaction order) E a activation energy [J/ol] C A initial concentration of reactant [ol/³] n reaction order. x conversion of reactant during the reaction r rate of reaction [ol/(³ ( s)] (4) 7

4 To deterine the activation energy and the pre-exponential factor, a set of isotheral experients at different teperatures have to be carried out in the reaction calorieter. 4. Experiental equipent The experients were carried out in the METTLER high pressure reaction calorieter RC1e/HP6 that was equipped with a high speed gassing-stirrer. To insure a good gas dispersion, the stirrer operated at 1 rp. This reaction calorieter was ainly used to deterine the reaction enthalpy with in situ high accuracy and to investigate the reaction inetics. An FTIR spectroeter was applied to onitor the concentration profiles of reactants, products, and, if possible, of interediates. The hydrogen uptae was onitored by gas flow easureents. The studies of ass transfer processes were carried out in the autoatic laboratory reactor (ALR) that was additionally equipped with a fast-response pressure sensor. 5. Results 5.1 Gas solubility and ass transfer According to equation (1), the H solubility αh was deterined fro the pressure values P, P " and P. The solubility easureents in the reaction ixture, consisting of isopropanol and SNBE without the catalyst, provided a ean solubility coefficient α H at C 5 ol αh,ix = 4 1 Pa In isopropanol (tech), a little lower H solubility at C was obtained 5 ol αh,isop =,9 1 Pa As presented by the Arrhenius plot (Fig. ) of the H solubility in isopropanol (tech), there is an exponential dependence on the teperature of the liquid /T [K -1 ] Fig. : Arrhenius plot of the H solubility in isopropanol (tech.) using a gassing stirrer. 74

5 Fro the Arrhenius plot (Fig. ), the following dependence of the H solubility on the teperature was deterined α ( T ) 17 RT 5 æ ö H,isop = exp ç è ø ol Pa (5) Obviously, the low content of water in technical isopropanol sees to be responsible for the opposite dependence on the teperature []. The results of the L a-deterination, represented in Tab., show that the H transfer to the liquid phase of the isopropanol depends on the stirrer speed and on the teperature of the liquid. For coparison, the ass transfer coefficients for a three-blade stirrer are also given. The gassing stirrer provides higher L a-values because the H gas is well-dispersed into the liquid volue in sall bubbles as observed in a glass reactor. Table : Experiental L a-values for the H transfer into isopropanol (tech.) at different teperatures and two stirrer speeds of the gassing stirrer. T r n L a [ C] [1/in] [1/s] 1,6 4 1,1 6 1,57 6,1 6,*,* 6 6 1,6,1,175 * -blades ipeller stirrer 5. Reaction enthalpy The overall heat Q r of the reaction was deterined fro isotheral experients in the RC1 reaction calorieter. The olar reaction enthalpy H r is defined by Qr H r = n (6) SNBE where n SNBE is the aount of oles of SNBE. The experiental results for different reaction conditions and SNBE charges are listed in Tab.. Assuing a coplete conversion of SNBE, an average value of the olar reaction enthalpy H can be calculated: H r = 558 J/ol SNBE This ean value is in good agreeent with the nown olar reaction enthalpy for nitrobenzene []: H r = 56 J/ol r A significant dependence of the reaction enthalpy on the teperature was not found. 75

6 Table : Heat of reaction and olar reaction enthalpy for the hydrogenation of SNBE to SABE Experient SNBE** b SNBE18** a SNBE *a SNBE4** SNBE5** T r [ C] 6 n SNBE [ol] 1,5,5,5,5,5 cat c [g],75,75,75,75,5 Q r [J] -879,6-19,7-16, -149,4-15,6 H r [J/ol] Average -557,8 * SNBE(new charge), ** SNBE(old charge) a with after ore than 15h hydrogenation, b concentrated SNBE, c catalyst 5% Pt in Carbon 5. Reaction rate Experiental results show that the overall reaction rate r and therefore the heat release rate Q r is directly proportional to the aount of the added catalyst. This is illustrated in Fig. 4 for,54 and 1,8 wt% Pt wt% Pt 1,8 wt% Pt t[in] Fig. 4: Influence of the catalyst concentration on the heat release rate. The influence of ph on the reaction rate was also studied. In the acidic solution (with HCl), the reaction rate is very slow, whereas in the basic solution (ph = 8) the reaction rate is very high. This result is in agreeent with the investigation of Ture and Co-worers [4] for the hydrogenation of glucose. Experiental investigation of another charge of SNBE (new charge) shows a higher reaction rate copared to the old charge at the sae reaction conditions (see Fig. 5). FTIR spectroscopy of both SNBE charges gives the sae result. There is only a slight difference in colour of the SNBE. 76

7 old charge new charge t[in] Fig. 5: Influence of the SNBE quality on the heat release rate. It is advisable to investigate whether path a) or path b) is favoured under the above experiental conditions. In a basic solution of an alcohol, the rate of the reaction between nitroso and hydroxylaine, foring the azoxy copound, is uch higher than the reduction step. If this is true, the rate of foration of aine through path b) sees to be the probable echanis. According to the inforation of the industrial partner (AWD), the reason for the azoxy foration lies in the type of SNBE that was used. 5.4 Reaction inetics To investigate the inetics of the hydrogenation of SNBE to SABE, the following process odel is assued. The solubility of H in the reaction ixture is odelled as follows H 1 ( g) H ( l) (7) [ H () l ] r1 = 1 PH and r = (8) The equilibriu condition is given by 1 [ H () l ] = PH = α H P H (9) Assuing that only Hydrogen is adsorbed onto the catalyst surface and that the adsorption/desorption of H (l) onto the catalyst is the rate deterining step, the following echanis can be forulated: Adsorption/desorption of hydrogen onto the catalyst (E) H () l E H E + (1) 4 [ H () l ][ E] and r = [ H E] r = 4 4 (11) 77

8 Interediate foration (hydroxylaine) and desorption of the hydroxylaine 5 R NO + (H E) R - NHOH + H O + E (1) [ R NO ][ H E], r,5 r5 = 5 H (1) Product foration (aine): reaction between the adsorbed hydrogen and hydroxylaine fro the liquid phase to aine and desorption 6 R NHOH + H E R NH + H O + E (14) [ R NHOH ][ H E], r,6 r6 = 6 H (15) The above equations are used to fit the experiental data of the heat release rate Q r by eans of the progra RATE (BATCHCAD). To deterine the odel paraeters, the olar reaction enthalpy should be nown for each reaction step. In [] the following olar reaction enthalpies were experientally deterined for the hydrogenation of nitrobenzene to aniline for path a): H r,5 = J/ol and H r,6 = 5 J/ol Using these olar reaction enthalpies, the odel paraeters, 4, 5 and 6 were adjusted with isotheral experients. Fig. 6 shows experiental data Q exp and odel predictions Q odel1 for the best fit using these enthalpies. There is a big discrepancy between the predicted and experiental values of Q r. Next, the olar reaction enthalpies H r,5 and H r,6 were assued to be proportional to the hydrogen uptae for each step and were set to be / of the overall reaction enthalpy for the foration of hydroxylaine and 1/ for the foration of aine, respectively. The overall olar reaction enthalpy is 56 J/ol SNBE. Using these olar reaction enthalpies H r,5 = -7 J/ol and H r,6 = -187 J/ol, the isotheral odel paraeters were estiated. The results are also plotted in Fig. 6 ( Q odel ). This odel provides a better approxiation of the experiental data. Therefore, in this study H r,5 = -7 J/ol and H r,6 = -187 J/ol were adopted t[in] Fig. 6: Coparison between experiental data ( Q exp ) and odel ( Q odel1/ ) heat release rate. ( Qexp (Tr = 6 C) ( Qodel1 ( Qodel 78

9 Using the above olar enthalpies, the odel paraeters, 4, 5 and 6 were estiated for the isotheral hydrogenation of SNBE to SABE in the teperature range of C to6 C. The nuerical values of the adjusted paraeters are given in Table 4. Table 4: Estiated paraeters for the isotheral hydrogenation of SNBE to SABE Paraeter C 4 C 5 C 6 C [³ /(g) s)] 4 [1/s] 5 [³/(ol ) s)] 6 [³/(ol ) s)],514 4,47 ) 1-5,114 5,488 ) 1-4, ,47 ) 1-5,9 1,79 ) 1-1,6119 4,47 ) 1-5,1171,848 ) 1-1,794 4,47 ) 1-5,7696 9,715 ) 1 - The paraeters and 4 are the adsorption and desorption rate constants for hydrogen onto the catalyst. 4 appears to be independent of the teperature and constant, where as, 5 and 6 increase with the reaction teperature. The Arrhenius plot is illustrated in Fig /T [1/K] Fig. 7: Arrhenius plot of the reaction rate constants. The following dependencies on the teperature were found: =.58 1 exp RT g s = s (16) = exp RT ol s = exp RT ol s Figure 8 copares siulation and experiental results of the heat release rate at different teperatures. Now there is a good agreeent between the odel and experiental results. Qualitative analyses of the reaction product show that an interediate product which sees to be the azoxy copound was produced. Therefore, the reaction echanis given in path b) 79

10 (Fig. 1) should also be considered to odel the hydrogenation of SNBE to SABE, especially if the overall reaction rate is low. Heat release rate [W] Q odel (Tr = C) Q odel (Tr =4 C) Q odel (Tr =5 C) Q odel (Tr =6 C) Q exp (Tr = C) Q exp (Tr = 4 C) Q exp (Tr = 5 C) Q exp (Tr = 6 C) t[in] Fig. 8: Coparison of the heat release rate between siulation and experiental results. 6. Conclusions The catalytic hydrogenation of SNBE to SABE is a coplex exotheric process influenced by the copeting effects of ass transfer and inetics. In reaction calorieters, several ethods were applied to deterine the paraeters of inetics and ass transfer under different process conditions. The reaction pathway is deterined by the reactant quality and the process conditions. It was found that a bad quality of SNBE and disadvantageous process conditions can cause an accuulation of interediates which probably desactivate the catalyst and lead to low process rates. Further wor is necessary to investigate the gas solubility and ass transfer in the production plant as well as to siulate and to study the industrial conditions in the laboratory reactor. References [1] E. Dietrich, C. Mathieu, H. Delas, J. Jenc, Raney-Nicel Catalyzed Hydrogenations: Gas-Liquid Mass Transfer in Gas-Induced Stirred Slurry Reactors, Cheical Engineering Science, 199, Vol. 47, No. 1/14, [] Landolt-Börnstein, 6. Aufl., Bd. II/ b, pp [] F. Stoessel, Experiental study of theral hazards during the hydrogenation of aroatic nitro copounds, J. Loss Prev. Process Inc., 199, Vol. 6, No., [4] Ture, Dissertation, Technische Hochschule Merseburg, 198 8