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Comparison of Different Reactor Types Used in the Manufacture of Ethoxylated, Propoxylated Products Martino Di Serio, Riccardo Tesser, and Elio Santacesaria* Dipartimento di Chimica, Universita` degli Studi di Napoli “Federico II”, via Cintia, 80126 Napoli, Italy
Ethoxylation and/or propoxylation are performed in industry to produce surfactants and polyglycols. The reaction is normally promoted by an alkaline catalyst (such as NaOH or KOH) and is performed in gas-liquid stirred reactors, Venturi loop reactors, and spray tower loop reactors. In the first two types of reactors, the gas phase is dispersed into the liquid phase; on the contrary, in the latter case, the liquid phase is dispersed into the gaseous one. In this paper, the key factors in ethoxylation/propoxylation technology will be examined: (i) the types of reactors employed and their performances; (ii) the role of kinetics and mass transfer in the process; and (iii) safety. 1. Introduction Polyethoxylation and polypropoxylation reactions are performed, in industry, to prepare nonionic surfactants and polymers. The starter (ROH) in the synthesis of a nonionic surfactant could be: a fatty alcohol, an alkyl phenol, or a fatty acid, i.e., a hydrophobic molecule containing a polar group with an active hydrogen.1 In this case, the starter will be reacted with ethylene oxide (EO) so as to insert a hydrophilic head in the molecules:
ROH + nEO f RO(EO)nH In some cases, propoxylation is performed before ethoxylation to increase the hydrophobicity of the starter (for example, in the case of hexanol) or after ethoxylation to change the foaming characteristics of the final product.2 The preferred processes are discontinuous semibatch processes (gaseous alkylene oxide (AO) continuously fed to the liquid substrate), mainly because of the necessity to perform post-treatment operations, such as catalyst removal or bleaching operations, to achieve the required quality features of the product.3 The reaction is, normally, promoted by alkaline catalysts, such as NaOH or KOH, and is frequently performed in stirred-tank reactors3 (see Figure 1a). However, the use of this type of reactor gives several problems related to productivity and safety.4 These problems are related to the difficulty in eliminating mass-transfer and heat-transfer limitations generally associated with conventional stirred-tank alkoxylators and with the presence of rotating mechanical parts in contact with the gaseous phase.4 These problems are particularly important in the case of ethoxylation, because this reaction is much faster than propoxylation2,5 and because the ethylene oxide can decompose in gaseous phases with a strongh exothermic reaction.4 The presence of a mass-transfer limitation can reduce the productivity of the reactor.4,6 Also, the heat-transfer limitation can limit the reactor’s productivity to below that set by mass transfer if the available heat transfer * To whom correspondence should be addressed. E-mail:
[email protected].
Figure 1. Schemes of semibatch reactors mainly employed industrially for ethoxylation/propoxylation processes: (a) stirredtank reactor; (b) Venturi loop reactor; (c) spray loop reactor.
surface per unit volume in the reactor is not high enough to ensure sufficient heat removal.4 These reactions are highly exothermic (∆H ) -92 000 J/(mol of ethylene oxide reacted)) and require efficient heat exchange to avoid the hazard of runaway reactions that are particularly dangerous because of the possible intervention, at high temperature, of explosive side reactions.7 The presence of the stirrer can cause gas mixture ignition (dipole formation between the stirrer and the reactor wall and overheating of the mechanical seal) or, if the mechanical seal is damaged or fails, oxide can leak out of the reactor or the seal flush can leak into the reactor.4 To avoid these problems, two alternative reactors can be employed in the alkoxylation reactions: Venturi loop reactors (VLR) (Buss Loop Reactor Technology, now licensed by Davy Process Technology AG (Switzerland)) and spray tower loop reactors (STLR) (Pressindustria Technology, now licensed by Scientific Design Company, Inc. (U.S.)). The schemes of the two mentioned reactors are respectively reported in parts b and c of Figure 1. In VLR, the gas phase, as for stirred reactors, is dispersed into the liquid phase; on the contrary, in STLR, the liquid phase is dispersed into the gaseous one. In both reactor types mentioned, there are no rotating metallic devices present and the efficiency of heat transfer is ensured by an external thermal exchanger. Both reactors give high productivity. The VLR have excellent mass-transfer performance with less power requirement in comparison to the case of con-
10.1021/ie0502234 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/01/2005
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ventional stirred-tank reactors,8 and STLR have shown, for example, in the production of nonylphenol + 9EO, a diminution from 210 to 60 min of required reaction time with respect to that seen in stirred-tank reactors.4 Kinetics and mass-transfer models for simulating the performances of well-stirred gas-liquid reactors used in the ethoxylation/propoxylation reactions are reported in the literature.1,2,5,6,9 On the contrary, very few paper have been published on the use of spray loop reactors10-12 and no detailed papers can be found in the scientific literature about the use of the Venturi loop reactor in the ethoxylation/propoxylation reactions. In this paper, a detailed comparison between the performances of VLR and STLR in the ethoxylation reaction will be done, studying the maximum productivities of the two reactors in safe conditions. 2. Reaction Mechanism and Kinetic Model In the case of ethoxylation performed by using fatty alcohols as a starter in the presence of an alkaline catalyst, the following reaction scheme has been suggested by Santacesaria et al.:9
ROH + M+OH- a RO-M+ + H2Ov Catalyst formation k0
RO-M+ + EO 98 RO(EO)1-M+
Initiation
kpi
RO(EO)i-M+ + EO 98 RO(EO)i+1-M+ Propagation i ) 1, 2, ... K0i
RO-M+ + RO(EO)iH {\} ROH + RO(EO)i+1-M+ Proton transfer On the basis of this mechanism, the following rates of EO consumption in each step can be written:
Initiation rate: -
So the rate of EO consumption depends on the concentration of catalyst and EO, respectively. The unique proton-transfer equilibrium constant (K01 ) K02 ) ... ) K0i ) ... ) Ke ) 4.1) influences the oligomer distribution that can be calculated using the Weibull and Nicander equations, which link oligomer distribution and overall ethylene oxide consumption.13,14 It is important to point out that during the reaction the liquid mass in the reactor increases. Moreover, the density of the reacting mixture changes during the reaction time and, accordingly, the alkylene oxide solubility changes, too.9 The density of the liquid phase as a function of temperature and the ethoxylation degree of dodecanol (the number of EO moles adducted per initial mole of hydrophobic substrate, nEO/nROH°) can be calculated using the following empirical polynomial correlation:15
FS ) 0.86 + 2.50‚10-2 2.69‚10-5
+
r0 ) k0[RO M ][EO]
(1)
i ) 1, 2, ...
( ) nEO nROH°
3
( )
( )
nEO nEO - 4.76‚10-4 nROH° nROH°
2
-
- 7.7‚10-4(T - 273) (g/cm3) (6)
(2)
Experimental data on EO solubility can be interpreted by the Wilson method, giving excellent performances for dodecanol and the ethoxylated derivatives.15 In Figure 2, the calculated concentrations of EO in dodecanol and dodecanol + 10EO as a function of temperature and pressure are reported.
(3)
3. VLR Model
Propagation rates: ri ) kpi[RO(EO)i-M+][EO]
Figure 2. Calculated ethylene oxide concentrations in dodecanol and dodecanol + 10EO.26
So overall EO consumption is: n
rEO )
ri ∑ i)0
In the case of the ethoxylation of dodecanol, catalyzed by KOH, the chain length does not influence either the rate of the reaction steps or the value of the protontransfer equilibrium constants.1,9 Therefore, because only one kinetic constant (k0 ) kp1 ) ... ) kpi ) ... ) k) is to be considered, eq 3 can be rewritten:
rEO ) k[cat][EO]
(4)
where1,9
k ) (6 ( 2)‚108 exp[-(6640 ( 150)/T] cm3 mol-1 s-1 (5)
In the VLR, the pumped liquid passes through a nozzle that provides a high velocity jet of fluid to create suction of the gas. In a mixing tube, the high velocity jet attaches itself to the mixing tube wall, resulting in a rapid dissipation of kinetic energy, which creates an intensive mixing with the production of a fine dispersion of gas bubbles. The two-phase mixture that “jets” into the reaction autoclave here also causes intensive mixing.8 Extensive studies have been reported in the literature that can be useful for a correct design of VLR.16-22 As shown by Stefoglo et al., the ejector of VLR in many cases can be considered as a device for the complete saturation of the liquid with gas.16 This occurs because of a high kLa value17 (10 s-1), and the concentration
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variation for the reactions is insignificant due to a short contact time.16 The holding vessel can be considered as a well-stirred reactor with a very high kLa value. Values of kLa in the range 0.2-1.5 s-1 for the total reactor system are reported.18 Taking into account that the Hatta number of ethoxylation reactions6 is 1000 rpm),6,23 which is not easy to achieve in industrial reactors.4 In the case of VLR, the required kLa values can, on the contrary, be achieved with a power input per unit volume 10 times less than that for stirred reactors.8 The influence of mass transfer is stronger in the case of other substrates, such as the nonylphenol, which is characterized by a higher kinetic constant.1,6 The productivity of an industrial reactor depends on the following operating conditions: temperature, catalyst concentration, EO feed, and the fixed maximum total pressure of the reactor. The correct choice of the last two operating parameters is also important for safety. The total pressure also depends on the presence of an inert gas (nitrogen). Nitrogen is introduced into the reactor before ethylene oxide to remove oxygen (the combustible range of EO-air mixtures is between 2.6 and 100%3) and to prevent EO decomposition that can also happen suddenly in the absence of air.3 To prevent EO decomposition, the gas phase in the reactor must be rendered inert with a sufficient amount of nitrogen.3 Moreover, at the normally used reaction temperature, to limit the formation of other ethylene oxide byproducts which adversely affect the use properties, the inert gas pressure has to be at least 80% of the alkylene oxide partial pressure,7 i.e., a molar gaseous fraction of EO 1.5-3 m), since ejectors can be viewed as space concentrated distributors,18 more than one jet has to be used to obtain satisfactory performances. Moreover, in alkoxylation reactions, as the liquid strongly increases as a consequence of the reaction (up to a 50-fold increase in the production of some products, such as the polyglycols, is possible) and for a reactor with a small diameter (
