Ethylene oxide tubular reactors

Ethylene oxide catalyst research is expensive and time-consuming because of the need to break in and stabilize the catalyst before rehable data can be collected. Computer controlled tubular microreactors containing as Httle as 5 g of catalyst can be used for assessment of a catalyst s initial performance and for long-term life studies, but moving basket reactors of the Berty (77) or Carberry (78) type are much better suited to kinetic studies. 

These equations hold if an Ignition Curve test consists of measuring conversion (X) as the unique function of temperature (T). This is done by a series of short, steady-state experiments at various temperature levels. Since this is done in a tubular, isothermal reactor at very low concentration of pollutant, the first order kinetic applies. In this case, results should be listed as pairs of corresponding X and T values. (The first order approximation was not needed in the previous ethylene oxide example, because reaction rates were measured directly as the total function of temperature, whereas all other concentrations changed with the temperature.) The example is from Appendix A, in Berty (1997). In the Ignition Curve measurement a graph is made to plot the temperature needed for the conversion achieved. 

Several patents exist on carrying out exothermic reactions for manufacture of reactive intermediates where high selectivity is essential. Even this author has a patent to make ethylene oxide in a transport line reactor (Berty 1959). Yet no fluidized bed technology is in use today. Mostly fixed bed, cooled tubular reactors are used for that purpose. 

So far, no reference has been made to the presence of more than one phase in the reactor. Many important chemicals are manufactured by processes in which gases react on the surface of solid catalysts. Examples include ammonia synthesis, the oxidation of sulphur dioxide to sulphur trioxide, the oxidation of naphthalene to phthalic anhydride and the manufacture of methanol from carbon monoxide and hydrogen. These reactions, and many others, are carried out in tubular reactors containing a fixed bed of catalyst which may be either a single deep bed or a number of parallel tubes packed with catalyst pellets. The latter arrangement is used, for exjimple, in the oxidation of ethene to oxiran (ethylene oxide)... 

Figure 17.14. Some unusual reactor configurations, (a) Flame reactor for making ethylene and acetylene from liquid hydrocarbons [Patton et al., Pet Refin 37(li) 180, (1958)]. (b) Shallow bed reactor for oxidation of ammonia, using Pt-Rh gauze [Gillespie and Kenson, Chemtech, 625 (Oct. 1971)]. (c) Sdioenherr furnace for fixation of atmospheric nitrogen, (d) Production of acetic acid anhydride from acetic acid and gaseous ketene in a mixing pump, (e) Phillips reactor for low pressure polymerization of ethylene (closed loop tubular reactor), (f) Polymerization of ethylene at high pressure.

Commercial production of ethanolamines (EOA) is by reaction of ethylene oxide with aqueous ammonia. The ethylene oxide reacts exothermically with 20% to 30% aqueous ammonia at 60 to 150°C and 30 to 150 bar in a tubular reactor to form the three possible ethanolamines (mono-ethanolamine - MEA, di-ethanolamine - DEA and tri-ethanolamine - TEA) with high selectivity. The product stream is then cooled before entering the first distillation column where any excess ammonia is removed overhead and recycled. In the second column, ammonia and water are removed and the EOA s are separated in a series of vacuum distillation columns. 

Ethylene oxide reacts exothermically with 20 to 30 percent aqueous ammonia at 60 to 150°C and 30 to 150 bar in a tubular reactor to form the three possible ethanolamines (mono-ethanolamine MEA, di-ethanolamine DEA and tri-ethanolamine TEA) with high selectivity. 

The industrial production of ethylene oxide is based on the direct oxidation of ethylene in the gas phase on a silver catalyst in cooled, tubular reactors. For a large excess of ethylene the reaction scheme can be simplified to ... 

For example, in one version of the Wacker process used for the oxidation of ethylene to acetaldehyde with soluble palladium-copper complexes, a tubular reactor containing ceramic rings is used to promote gas-liquid contacting and to impart a gas-liquid flow pattern that approaches ideal tubular reactor behavior. 

EO is mainly produced by the direct oxidation of ethylene with air or oxygen in a packed-bed, multi tubular reactor with recycle [2]. Catalysts for EO production... 

Glycol ethers are manufactured by adding ethylene oxide to alcohols. In these reactions, both reaction rate and product distribution are important. Reaction rate is important because the reaction is slow enough to require a large, costly reactor. Product distribution is important because the yields based on oxide and alcohols are lowered if an excess of the higher-molecular-weight by-products is formed. In this type of reaction, a tubular reactor would be desirable because it would give a better control of product distribution than a tank or tower reactor. 

GAS-PHASE PLUG-FLOW TUBULAR REACTORS THAT PRODUCE TRIETHANOLAMINE FROM ETHYLENE OXIDE AND AMMONIA... 

Triethanolamine is produced from ethylene oxide and ammonia at 5 atm total pressure via three consecutive elementary chemical reactions in a gas-phase plug-flow tubular reactor (PFR) that is not insulated from the surroundings. Ethylene oxide must react with the products from the first and second reactions before triethanolamine is formed in the third elementary step. The reaction scheme is described below via equations (1-1) to (1-3). All reactions are elementary, irreversible, and occur in the gas phase. In the first reaction, ethylene oxide, which is a cyclic ether, and ammonia combine to form monoethanolamine ... 

Experimental test of this mechanism was conducted by performing a competition study with ethylene/methane mixtures in the tubular reactor. The results, summarized in Table 5, demonstrate that ethylene oxidation competes readily with methane oxidation under the experimental conditions of the electrocatalytic cell. The ratios of 2/ 1 calculated for these experiments are 4.0 and 4.6. This is in reasonable agreement with the ratio derived from the methane coupling experiments. Thus, the consecutive reaction mechanism can be applied successfully to systems of this type. The inescapable conclusion is that methane dimerization is limited by the relative rates of methane and ethylene activation. 

Referring to the Encyclopaedia of Chemical Technology (1980), the process can be divided into three major sections reaction system, oxide recovery, and oxide purification. In the first section, as described in Chemical and process technology encyclopaedia (1974), a mixture of ethylene, air and recycle gas, in which the ethylene content is 3-5 vol% is conducted under a pressure of 10-20 atm gage to a tubular reactor with fixed-bed silver catalyst. The following reactions take place during the oxidation of ethylene. The per pass ethylene conversation in the primary reactors is maintained at 20-50% in order to ensure catalyst selectivity. 

The oxidation of ethylene in air on a Pt wire is a good example by which to demonstrate the ignition behavior of exothermic catalytic reactions. The experiment was conducted as follows (Table 4.5.4). A coil consisting of a thin Pt-wire is placed in a tubular reactor. Then an ethylene-air mixture of constant temperature and pressure (303 K, 1 bar) is fed into the tubular reactor. The wire is now electrically heated until ignition (jump in temperature) occurs. The current and the voltage is measured and, thus, also the temperature of the wire as the electrical resistance depends on temperature. 

In the process the preheated reactants, inerts (diluent methane and recycled CO2), and promoters are fed into the multi-tubular reactor The gas stream leaving the reactor is cooled by an external heat exchanger and sent to the ethylene oxide absorber column. In this column the relatively small amounts of ethylene oxide (concentration 1-2 mol.%) are absorbed in water. A minor part of the gas leaving the top of the absorber is purged to reduce the inerts concentration (mainly CO2, argon, and methane). The rest of the gas stream is sent to the CO2 absorber unit. 

The chlorohydrin process has largely been replaced by the direct oxidation of ethylene with silver as catalyst. By-products are CO2 and water, formed by total oxidation of ethylene or EO. The reactor design is dominated by the demand for an exact temperature and selectivity control. The conversion per pass is low (about 10%) to avoid the consecutive oxidation of EO, and the unconverted ethylene is recycled. A multi-tubular reactor guarantees efficient heat transfer. 

Tubular reactors are one of the basic types of chemical reactors. Such reactors can either be packed with catalyst or be empty, depending on the reaction system considered. Some examples for catalytic fixed bed tubular reactors are ethylene, sulphur or naphthalene oxidation reactors. On the other hand, hydrocarbon thermal Cracking, ethylene polymerization reactors are typical examples for empty tubular reactors. 

In spite of the numerous publications showing the potential of such periodic operations especially in the field of complex reactions, experimental studies are virtually non-existent. Of the few experimental works, Renken et al. ( ) compared the performance of a tubular reactor in which a single reaction, the hydrogenation of ethylene, took place, under periodic operation and at steady-state. He reported an improvement of 60% in conversion. In another publication, Renken et al. (3) showed experimentally that periodic operation can be used to eliminate the temperature problems associated with highly exothermic reactions, e.g. the oxidation of ethylene over a silver catalyst. In other experimental work Unni et al. W) showed that periodic variation of reactant composition improved the rate of oxidation of SO2 over a vanadium oxide by as much as 30%. Denis and Kabel ( 5) studied the cyclic operation of a heterogeneous reactor for the vapour phase dehydration of ethanol and observed that adsorption/desorption played a predominant role in the transients of the system. 

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