Replacement of gas phase with liquid, for hexamine production

Transcription

1 Proeedings of the International Conferene on Ciruits, Systems, Signals Replaement of gas phase with liquid, for hexamine prodution Anita Kovač Kralj¹ Davorin Kralj² Faulty of Chemistry and Chemial Engineering, University of Maribor,Smetanova 17, Maribor, Slovenia ¹Art-K BusinessConsulting, Na griu 47, 2000 Maribor ² Abstrat: Chemial kinetis, also known as reation kinetis, is the study of hemial proesses rates. Chemial kinetis inludes investigations into how different experimental onditions an influene the speed of a hemial reation, and yield information about the reation's mehanism and transitional states, as well as the onstrution of mathematial models that an desribe the harateristis of a hemial reation. The physial state (solid, liquid, or gas) of a reatant is also an important fator regarding rate of hange. Many syntheses arried out during the gas-phase, providing a synthesis of the same produt, an also be performed during the liquid phase, for simpliity, for major opportunities regarding analysis, and lower risk. This paper studies the kineti determination of hexamine prodution in an aqueous solution of ammonia hydroxide and formaldehyde. In the literature, a lot of information an be found about the kineti determination of hexamine prodution during gaseous phases. Hexamine (C 6 H 12 N 4 ) was produed from aqueous ammonia hydroxide (NH 4 OH) and aqueous formaldehyde (CH 2 O) in a bath reator at different temperatures (T 1 = 23 ºC, T 2 = 24 ºC, T 3 = 26 ºC and T 3 = 28 ºC): 6 CH 2 O + 4 NH 4 OH = C 6 H 12 N H 2 O. The reation was observed by measuring a ertain onentration by titration, in our ase, non-reative ammonia hydroxide. The onentration and kineti parameters of the ammonia hydroxide s reation (reation rate onstant, rate order, ativation energy) were determined by using experimental parameters. Keywords prodution of hexamine, onentration, kinetis, titration, reation rate onstant, rate order I. INTRODUCTION Chemial kinetis deals with the experimental determination of reation rates from whih rate laws and rate onstants are derived [4]. Relatively simple rate laws exist for zero order reations (for whih reation rates are independent of onentration), first order reations, and seond order reations, and an be derived at for others. In onseutive reations the rate-determining step often determines the kinetis. In onseutive first order reations, a steady state approximation an simplify the rate law. The ativation energy for a reation is experimentally determined through the Arrhenius and Eyring equations. The main fators that influene the reation rate inlude: the physial states of the reatants, the onentrations of the reatants, the temperature at whih the reation ours, and whether or not any atalysts are present during the reation. The physial state (solid, liquid, or gas) of a reatant is also an important fator regarding the rate of hange. When reatants are in the same phase, as in aqueous solution, thermal motion brings them into ontat. However, when they are in different phases, the reation is limited to the interfae between the reatants. Reation an only our at their area of ontat and, in the ase of a liquid and a gas, at the surfae of the liquid. Vigorous shaking and stirring may be needed to bring the reation to ompletion. This means that the more finely divided a solid or liquid reatant, the greater its surfae area per unit volume, and the more ontat it makes with the other reatant s, the faster the reation. An aqueous solution is a solution in whih the solvent is water. It is usually shown in hemial equations by appending (aq) to the relevant formula. The word aqueous means pertaining to, related to, similar to, or dissolved in water. As water is an exellent solvent, as well as naturally abundant, it has logially beome a ubiquitous solvent in hemistry. In 1864, Peter Waage pioneered the development of hemial kinetis by formulating the law of mass ation, whih states that the rate of a hemial reation is proportional to the amounts of reating substanes. Hexamine or hexamethylenetetramine is used in pharmaeutial industries as a primary feed material, and in hemial industries as an intermediate material. It is highly soluble in water and, like some other tertiary amines, has an inverse solubility at low temperatures [1]. An optimum method for produing hexamine from formaldehyde and gaseous ammonia is the Meissner proess, in whih the reation and rystallization stages take plae simultaneously to produe rystalline hexamine. Due to a high dissolution rate of ammonia and formaldehyde in the aqueous solution and a high reation rate during 158

2 Proeedings of the International Conferene on Ciruits, Systems, Signals hexamine prodution, the rystallization stage ontrols the Meissner proess [2]. The values for kineti parameters regarding hexamine rystallization from aqueous solution, neessary for ontrolling industrial rystallizers, were alulated by Alamdari and Tabkhi [6]. Many syntheses arried out during the gas-phase, provided a synthesis of the same produt an also be performed during the liquid phase, for simpliity, for major opportunities regarding analysis, and lower risk. This paper studies the kineti determination of hexamine prodution, by titration of an aqueous solution of ammonia hydroxide and formaldehyde with reation: 6 CH 2 O + 4 NH 4 OH = C 6 H 12 N H 2 O In the literature, a lot of information an be found about the kineti determination of hexamine prodution during gaseous phases: 6 CH 2 O + 4 NH 3 = C 6 H 12 N H 2 O I REACTION RATE For a onstant-volume bath reator [3], the rate at whih reatant A s appears (ammonia hydroxide), r A, is given by: da r = A dt (1) where r A an be expressed as: n m r A = k (2) A B where k is the reation rate onstant, n and m are the reation orders with respet to speies A (ammonia hydroxide) and B (formaldehyde), respetively. Sine formaldehyde is in exess, B remains essentially unhanged during the ourse of the reation: r = k' n A A (3) where k' is a pseudo rate onstant k' = k m B k B0 The reation order and rate onstant an be determined by the integral analysis method. In this method, the rate expression is guessed and the differential equation used to model the bath system is integrated. If the assumed order is orret, the appropriate plot (determined from the integration) of onentration-time data should be linear. For the first-order ase where r A = k A, the integration of equation 3.1 yields: k t=ln A0 (5) A (4) where A is ammonia hydroxide onentration and A0 is the initial ammonia hydroxide onentration. For the seond-order ase where r A = k A 2, the integration of equation 3.1 yields: 1 A 1 = A0 + k t The speifi reation rate, k, is a funtion of reation temperature and is given by the Arrhenius equation: k= k e Ea R T (6) 0 (7) where k 0 is a pre-exponential fator, E a is the ativation energy for the reation, and T is the absolute temperature. II CASE STUDY Hexamine, also alled hexamethylenetetramine or methenamine (INN), is a heteroyli organi ompound that an be prepared by the reation of formaldehyde and ammonia [5]. It is rystalline and white in appearane. Partile sizes range from mirometres. It is moderately soluble in water and very soluble in most organi solvents. It has a age-like struture similar to adamantane. It has two very different types of usage: as an antibioti (ommonly hippurate salt, methenamine hippurate), and as a solid fuel tablet used for ooking while amping or hiking. The name methenamine is more ommon regarding its mediinal uses and hexamine is more ommon in ommerial uses. Methenamine hippurate is mainly used for the prophylaxis (prevention) of urinary trat infetions, where it is hydrolysed in the aidi ph of the urine to form formaldehyde. Formaldehyde is an effetive antisepti against bateria and fungi in vitro, but may be ineffetive in vivo - it has been shown to be ineffetive for the treatment of urinary trat infetions. Furthermore, ertain bateria (inluding Proteus and Pseudomonas) inrease urinary ph, rendering methenamine ineffetive. In China, a 10% solution is used as a deodorant. As a food additive, hexamine is used as a preservative in heeses. It has E number E239 but in some ountries is banned for this use. Hexamine an be absorbed through the skin, and some people are allergi to it. This is normally manifested as a rash. Hexamine is used as a omponent with 1,3,5- trioxane in hexamine fuel tablets (often alled Esbit). This solid fuel in tablet form is used by ampers, the military and relief organizations for heating amping food or military rations. It is smoke-free, has a high energy density, does not liquefy while burning, and leaves no ashes. Another important area for the use of hexamine is in the prodution of powdery or liquid phenoli resin 159

3 HCNaOHProeedings of the International Conferene on Ciruits, Systems, Signals preparations and phenoli resin moulding ompounds, where hexamine is added as a hardening omponent. These produts are used as binders, e.g. in brake and luth linings, abrasive produts, non-woven textiles, formed parts produed by moulding proesses, and fireproof materials. It is used in rubber and textile adhesives, in paints and laquers, in the photographi industry, in the prodution of explosives suh as RDX, as a orrosion inhibitor, as a protein modifier, as a reagent in hemial analysis, and in the prodution of deodorants and hair fixers, among many other uses. The degree of onversion, X A, is: X A0 A A= (11) A0 The initial ammonia hydroxide onentration, A0 was analysed by the first measurement (Number 1; Table 1). Figure 2 illustrates the plot of A as a funtion of time at different temperatures. III EXPERIMENT A shemati diagram of the laboratory apparatus is shown in Figure 1. A bath reator with a stirrer was used in the experiment. Ammonia hydroxide (2.15 ml 25 %) was dissolved in water (0.35 L) and heated into a reator at different temperatures (T 1 = 23 ºC, T 2 = 24 ºC, T 3 = 26 ºC and T 3 = 28 ºC). Surplus formaldehyde (4 ml 36.5 %) was added: 6 CH 2 O + 4 NH 4 OH = C 6 H 12 N H 2 O B A stirrer Figure 2: Conentration of ammonia hydroxide, A, as a funtion of time at different temperatures. reator Figure 3 presents the plot of onversion degree, X A, as a funtion of time at different temperatures. solution Figure 1: The apparatus stirred bath reator. Both substanes were mixed. The onentration of nonreative ammonia hydroxide, A was analysed by using a titration analytial method depends on time (Table 1). 10 ml 0.1 mol/l HCl was added to the 10 ml sample, whih stopped the reation (reation 1), and retitration took plae with 0.1 mol/l NaOH (reation 2): = mol ( V ) = ( V ) + ( V (8)A(9) mol HCl mol NaOH + sample = 0.1 V NaOH + 10 A, (10) l) NH 4 OH + HCl NH Cl + H 2 O (reation 1) NaOH + HCl Na + + Cl + H 2 O (reation 2) Figure 3: The plot of onversion degree X A, as a funtion of time at different temperatures The plot of ln( A0 / A ) as a funtion of time is non-linear. As Figure 5 illustrates (Table 2), the plot of (1/ A ) as a funtion of time is linear, whih suggests that the rate law is seond order with respet to ammonia hydroxide onentration under given reation onditions for exess formaldehyde. The slope represents the rate onstant, k. The rate onstant was found to be L/(mol min 1 ) at 23 o C. The data show the reation is seond order at all temperatures studied 160

4 Proeedings of the International Conferene on Ciruits, Systems, Signals (Table 3). The initial parameters deviated from the linear plot beause of the non-homogeneous solution. Table 1: The experimental data and alulation at T= 23 C. Nu. t/min V NaOH/ml A/(mol/L) ln( A0/ A) Figure 5: The plot of (1/ A) as a funtion of time at 23 ºC. Table3: The reation rate onstant, k: T/ºC k/ L/(mol min 1 ) Figure 4: The plot of ln( A0 / A ) as a funtion of time at 23 ºC. The experiment at temperatures 24 ºC, 26 ºC and 28 ºC was repeated using the same proedure. Table 2: The experimental data and alulation at T= 23 C. Zap. št t/min V NaOH/ml C A/(mol/L) (1/C A)/(L/mol) X A Taking the logarithm of the Arrhenius equation (7): Ea ln k = ln k0 (12) R T 161

5 Proeedings of the International Conferene on Ciruits, Systems, Signals the plot of ln(k) and 1/T should be a straight line and the slope is proportional to the ativation energy (Table 4). The ativation energy is kj/mol (Fig. 6). 72, HEKSAMIN - standard Table 4: The data for determining ativation energy T/ C T/K (1/T)/K -1 k/(l/mol min) lnk , , , , , %T , , , , ,04 833, , , , ,61 11,6 1238, ,22 811,40 673,14 512, , ,0 m-1 Figure 7: The peaks from the standard s hexamine. 74,3 70 HEKSAMIN - vzore , , , , ,01 833, ,99 40 %T ,25 Figure 6: The plot of ln k and 1/T. The slope of the graph is: E a /R = 3877 K (13) E = 3877 K 8.314J/(mol K) kj/mol (14) a = This reation took plae at room temperature, thus onfirming the ativation energy value, whih was low. The peaks from the standard s and sample s hexamine (Figures 7 and 8) were analysed using infrared spetrosopy. The important peaks were the same, thus onfirming that hexamine was produed , , , , , ,06 811,50671, , ,90 3,7 4000, ,0 m-1 Figure 8: The peaks from the sample s hexamine. IV. CONCLUSION Many syntheses arried out during the gas-phase, providing a synthesis of the same produt an also be performed during the liquid phase, for simpliity, for major opportunities for analysis, and lower risk. This paper studies the kineti determination of hexamine prodution in an aqueous solution of ammonia hydroxide and formaldehyde. In the literature, a lot of information an be found about the kineti determination of hexamine prodution during gaseous phases. We determined the onentration and 512,46 162

6 Proeedings of the International Conferene on Ciruits, Systems, Signals kinetis of ammonia hydroxide to hexamine by measuring the onentration using titration - ammonia hydroxide in a bath reator. The ammonia hydroxide reats as a seondorder reation. The rate onstant was found to be L/(mol min 1 ) at 23 ºC. The hexamine an be produed from an aqueous solution of ammonia hydroxide and formaldehyde with a high-degree of onversion under low temperatures. The ativation energy was kj/mol. The same peaks for the standard s and sample s hexamine onfirms that hexamine was produed. REFERENCES [1] Smolin E.M. and Rapoport L., The hemistry of heteroyli ompounds, S-Triazines and derivatives, Intersiene publiation, New York, (1959). [2] Meissner F., Shwiedessen E and Othmer D.F. Shatyski, Continuous prodution of hexamethylenetetramine, Ind. Eng. Chem. 46 (1954) 724. [3] Haji S. and Erkey C., Kinetis of hydrolisis of aeti anhydride by in situ FTIR spetrosopy, Chemial Engineering Eduation, (2005) [4] hemial kinetis [5] hexamine [6] Alamdari A. and Tabkhi F, Kinetis of hexamine rystallization in industrial sale, Chem. Eng. And Proessing 43 (2004)