Etherification Introduction

Transcription

1 4.4 Etherification Introduction In the petroleum industry the term etherification is used to mean the addition reaction of an alcohol to a tertiary olefin leading to the formation of oxygenates (ethers) with excellent octane characteristics. Oxygenates were first added to gasoline in the 1970s following the first energy crisis that led to the search for alternative fuels and components for oil and the progressive elimination of lead-based additives. Oxygenates thus initially had the double target of increasing the octane number (octane boosters) and adding to gasoline compounds obtained from alternatives sources (volume extender). Initially, alcohols (methanol and ethanol) were used directly as oxygenate compounds. Later, they were substituted with ethers such as methyl tertbutyl ether (), ethyl tert-butyl ether (ETBE) and tert-amyl methyl ether (TAME), whose structural formulas are as follows: C C CH 2 C O OCH 2 O ETBE TAME Thanks to its properties,, immediately came to the forefront as the oxygenate capable of dominating the market. joins high-octane properties with a lower volatility with respect to alcohols, it mixes completely with gasoline, it entails no problems related to phase separation in the storage and distribution systems of the gasoline and has the great advantage that it can be easily synthesised starting from compounds generally not used in the production of gasoline, such as methanol and isobutene. The use of oxygenate compounds in gasoline rose sharply in the 1980s when it was realized, in the United States, that has an oxygenating effect, i.e. the capacity to improve the quality of combustion. thus became the chemical compound with the highest growth rate in the 1980s and 1990s. Its production, which began in 1973 with the start up of the Snamprogetti/ANIC plant in Ravenna (100,000 t/y), exceeded twenty million tonnes annually (in approximately 150 plants) in just twenty years. As a rule, and ETBE are produced industrially from the following feedstocks: a) refinery streams derived from FCC (Fluid Catalytic Cracking) units; b) C 4 cuts from naphtha fed steam crackers after elimination of butadiene by extraction or hydrogenation; c) isobutene from dehydrogenation of isobutane obtained by isomerization of the field butanes; d) isobutene from the dehydration of tert-butyl alcohol (TBA) co-produced in the synthesis of propylene oxide. At the beginning of the Twenty-first century, most plants were located inside the refineries (60%), used FCC cuts and were quite small (30,000-60,000 t/y). In practice, over half the world production was realized in dehydrogenation and dehydration plants. While few in number (approximately 15%), these plants are quite large (500,000 t/y). The etherification reaction can also be applied to light FCC gasoline, also known as LCN (Light Cracked Naphtha), which represents approximately 50% all FCC gasoline and is characterized by a distillation temperature range of C. This stream is already a gasoline component, and in the United States represents 15-20% of all gasoline produced. However, it can be fractionated to recover the C 5 cut (which constitutes 30-50% of the LCN) and then etherified to obtain TAME or it can be VOLUME II / REFINING AND PETROCHEMICALS 193

2 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES directly etherified to produce a mixture of TAME and higher ethers obtained by conversion of the C 6 and C 7 reactive olefins. In both cases, the etherification reaction makes it possible to improve some fraction characteristics (reduction of volatility and olefin content). It has, however, a limited octane effect since the formation of ethers takes place utilizing olefins already possessing good motor properties. In contrast to and ETBE, TAME and higher ethers are not merchant chemicals, but are produced in refineries for captive use. Etherification of olefins with a number of carbon atoms above 7 (heavy gasoline from FCC) does not have practical interest since equilibrium conversions are very low and the ethers octane properties are very similar to the original olefins Aspects related to use of oxygenates in fuel formulation As mentioned above, the growth of oxygenates as gasoline components was motivated, during the 1970s, by an attempt to reduce dependence on oil-producing countries. The idea was to use new fuels as an alternative to gasoline (methanol from coal or natural gas and ethanol produced by biomass fermentation). It was also a result of the growing sensitivity towards environmental programs that led to increasingly severe emission limitations and therefore progressive reduction/elimination of lead compounds. These compounds (tetra-ethyl lead or TEL and tetra-methyl lead or TML) were normally added in small quantities ( g/l) to gasoline and allowed a considerable increase to the octane number of the hydrocarbon base material. Their removal was found to be necessary however, both because of the formation of harmful inorganic compounds (lead bromides and chlorides) during combustion and, above all, because lead rapidly poisoned the catalysts used in catalytic mufflers. Nonetheless, elimination of these lead additives made it necessary to identify alternative methods to maintain the octane quality that gasoline needed. The octane loss was thus compensated for by the use of processes to upgrade gasoline such as alkylation, isomerization and, above all, reforming (more severe operation conditions and therefore an increase in the concentration of aromatics) and, at the same time, introduction of a new class of compounds consisting of oxygenates. Oxygenates clearly do not have the same efficiency as lead compounds in increasing the octane number and so cannot be considered simply antiknock additives. Indeed, they are more correctly referred to as high octane compounds since, based on the larger quantities used (1-15% in volume), they also have a marked impact on many other characteristics of the hydrocarbon base (density, distillation curve, volatility, heat capacity, etc.). Starting in the 1980s, it was also noted that these compounds were also oxygenating, i.e. capable of improving combustion quality while reducing harmful emissions. In 1981, the Environmental Protection Agency (EPA) regulated the addition of oxygenates in the United States. According to its Substantially Similar Rule, gasolines with a content of less than 2% of their weight in oxygen were to all intents and purpose similar to completely hydrocarbon gasolines. The greatest impetus to the use of oxygenates and came in 1990 with the approval of the Clean Air Act Amendments (CAAAs) in the United States, which required a minimum oxygen content both in oxygenated gasoline (2.7% of weight in oxygen, corresponding to an volume of 15%) in the regions that did not respect CO emissions and in RFG (ReFormulated Gasoline) (2% of its weight in oxygen, corresponding to a volume of 11%) in the regions that did not respect specifications on the presence of ozone. This latter alone represents approximately 30% of all US gasoline. As can be seen in Fig. 1, one result of the CAAA is that the United States became the major market for (with a consumption of over 60%). California alone consumes about 40%, for a total consumption of more than 4 million t/y (Methanol [ ], 2003). The requirement for a minimum oxygenate content and therefore of compounds produced primarily outside refineries inevitably aroused the opposition of the US petroleum industry, which subsequently undertook 12% global capacity 15% 15% 4% 1% others Europe Asia 53% Fig. 1. market, % global demand 17% 5% 2% 2% Middle East South America North America 63% 194 ENCYCLOPAEDIA OF HYDROCARBONS

3 ETHERIFICATION unsuccessful campaigns in the attempt to demonstrate the alleged toxicity of. Nevertheless, in-depth studies carried out by many groups have excluded any carcinogenic effect in this ether. Finally, in 1996, the discovery of traces of in some wells used for the production of drinking water in California led to a nation-wide debate in the United States on the safety of the use of this ether. Contrary to other gasoline components, given its greater solubility in water and lower biogradability, tends to build up in water: roughly 15 ppb are enough to alter the organoleptic properties of water. The cause of the contamination of drinking water wells is to be found primarily in the leakage of gasoline from underground service station storage tanks (which are not double-wall as they are in Europe) or from pipelines. The first consequence of the dispute over this contamination was the publication of the EPA s Blue Ribbon Panel which reconsidered the role of in gasoline and called for a significant reduction. Later came California s drastic decision to ban as of the 1 st of January 2004 (an example followed by the states of New York and Connecticut). While the problem of underground storage tanks was solved, the situation in the rest of the United States is still uncertain, even though reduction in consumption is envisaged. In Europe, the second most important market, the use of as a gasoline component is still looked upon with favour thanks to its many positive aspects. Its use could diminish however with the drop in gasoline demand and the competition of compounds from renewable energy sources (ethanol and ETBE). Conversely, under the impetus of growing respect for the environment, the demand for has grown in emerging markets such as China. The effect of ethers on gasoline properties Among the oxygenates, immediately established its place as compound par excellence thanks to its physico-chemical and motor properties and the particular characteristics of its synthesis. Aside from being simple and selective, the synthesis reaction of has the great advantage of allowing the addition to gasoline of large amounts of both methanol and, above all, of a C 4 compound, such as isobutene, that cannot be added directly because of its high volatility. This reaction is also marked by a notable flexibility in raw materials since can be produced both inside the refinery with raw materials derived from petroleum and outside, using compounds derived from natural gas. Table 1 shows the main absolute and blending properties of oxygenates used as gasoline components. It should be pointed out that in this case of oxygenates, blending properties are much more indicative since they highlight the real behaviour of the compound according to its interaction with the hydrocarbon gasoline used as a base. The main advantage of using oxygenates for refining is their high octane blending capability. Ethers are preferable to alcohols due to their lower sensitivity value the difference between the Research Octane Number (RON) and Motor Octane Number (MON) which is approximately 15 points, compared to the over 30 points of alcohols. They are therefore more in line with the sensitivity value of the hydrocarbon base (about 10 points). The octane blending power is strongly affected by the characteristics of the base, as shown in Fig. 2. For this reason it would be more correct to supply a range of values rather than a single value (Pecci and Floris, 1977). Still, the advantage of ethers over alcohol is most clearly seen in the vapour pressure (Reid Vapour Pressure or RVP) and the boiling temperature, which are directly linked to gasoline volatility and therefore to engine performance. Good gasoline Table 1. Properties of oxygenates used in gasolines ETBE TAME Methanol Ethanol Chemical class Ethers Ethers Ethers Alcohol Alcohol Oxygen (% in weight) Boiling temperature ( C) RVP abs. (psi) Density (g/cm 3 ) RON, blending MON, blending RVP, blending VOLUME II / REFINING AND PETROCHEMICALS 195

4 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES RON blending (volume %) base A; RON 85 base B; RON 91 base C; RON 94 Fig. 2. Blending Research Octane Number (RON) of with different bases. should have a volatility curve (between approximately 25 and 190 C) that is well balanced in its light, medium and heavy components (obtained by appropriate dosing of the constituent cuts) and an RVP lower than 7 psi which allows the engine to run better in various conditions. Thanks to their compatibility with hydrocarbons, ethers not only do not modify the shape of the distillation curve, but they also contribute to lowering the T50 (the temperature at which 50% by volume of the gasoline boils), therefore improving the cold engine performance. On the contrary, alcohols cause extreme changes in the distillation curve since they tend to form azeotropes with the various components. This difference in compatibility is also reflected in the value of the gasoline s RVP which, as shown in Fig. 3, is diminished by the addition of ethers and increases in the case of addition of alcohols that also exhibit absolute vapour pressure values lower than the ethers (Bott and Piel, 1991). Another important advantage of ethers is their complete miscibility with hydrocarbons, thus avoiding the adoption of particular procedures to store and distribute the gasoline. In contrast, alcohols, and methanol in particular, have a significant tendency to set off phase separation in the presence of small quantities of water, with obvious problems for network distribution. Among the ethers, is the oxygenate with the best combination of properties, while ETBE albeit having very similar and in some cases better characteristics (lower solubility in water) is penalized by the fact that its production cost depends on subsidies granted to ethanol. TAME and the higher ethers deserve separate attention. Despite their high octane characteristics, they cannot be considered authentic octane suppliers since their production takes place by the consumptions of olefins which are characterized by good octane properties. Instead, etherification of these cuts allows improvement of general fraction characteristics since: a) it introduces oxygen into the blend, therefore improving combustion quality; b) the contents of the more volatile and photoreactive olefins are reduced (see below); c) the RVP of the blend is reduced; d) the stream s sulphur and aromatic contents are reduced thanks to the dilution effect due to the introduction of methanol. Thanks to these series of advantages, the synthesis of TAME and higher ethers seemed to have a very high growth potential in the 1990s. However, the ban of in the United States has practically blocked its market penetration. Effect of ethers on air quality The effects of ethers on air quality can be classified as direct and indirect. Indirect effects are those due to the reduction of sulphur, olefin and aromatic contents in gasoline as a result of the dilution by the addition of ethers. Direct effects, on the other hand, are those due to a reduction of emissions of polluting compounds whose formation is tied to the air/fuel ratio used in engine fuel supply. These polluting compounds include CO, gasoline RVP (psi) (weight %) oxygen in gasoline methanol ethanol ETBE Fig. 3. Effect of oxygenate concentration on RVP. 196 ENCYCLOPAEDIA OF HYDROCARBONS

5 ETHERIFICATION days year Fig. 4. Days that cities in USA exceeded CO emission limits but was only studied in depth following the use of ethers as gasoline components. The addition of methanol to isobutene is an exothermic equilibrium reaction that is industrially performed in the liquid phase at pressures of 8-20 bar and relatively low temperatures (40-70 C) in order to favour thermodynamic conversion: C CH 2 OH C nitrogen oxides (NO x ) and unburned hydrocarbons (HC). The addition of to gasoline allowed a significant reduction of CO emissions as can be seen in Fig. 4 (Sanfilippo et al., 1994) which shows the results obtained in the United States in the winter of (first year of actual application of the CAAA). It also made it possible to obtain a slight decrease of unburned hydrocarbons while nitrogen oxides emission remained largely unchanged. The use of ethers has also made it possible to reduce the formation of ozone in the lower layers of the atmosphere where, in contrast to what happens in the stratosphere (where it acts as a filter for ultraviolet radiation), it has a harmful effect on human beings and flora. The synthesis of ozone is linked to complex reactions of Volatile Organic Compounds (VOCs), present in the atmosphere with hydroxyl radicals and nitrogen oxides in the presence of sunlight. VOC reactivity (expressed as the rate of reaction of hydroxyl radicals in the gas phase) is closely related to their chemical nature and volatility. Table 2 (Pescarollo et al., 1993) highlights how photochemical reactivity generally tends to increase with the molecular weight for every class of compound. It also shows that olefins are the most harmful compounds, aromatics have an intermediary behaviour, while saturated hydrocarbons and oxygenates are instead the compounds with the lowest reactivity. Therefore MBTE can be considered to have an indirect effect on ozone reduction, while TAME and higher ethers have also a direct effect due to the transformation of highly photoreactive olefins to more stable compounds Chemistry and thermodynamics Etherification is an exothermic equilibrium reaction between a primary alcohol and an isoolefin (with a double bond on a tertiary carbon atom) that takes place in the presence of an acid catalyst in a gaseous or liquid phase as a function of the operating pressure. This etherification reaction has been known since O One important characteristic of this reaction is its high selectivity: industrially produced has a purity that exceeds 99% and the only by-products formed are the dimers of isobutene (DIB, diisobutene), tert-butyl alcohol (TBA), dimethyl ether (DME) and methyl sec-butyl ether (MSBE), as shown in Fig. 5. The formation of DME and DIB is thermodynamically favoured in conditions of synthesis. However, if the reactor is run correctly (controlled temperature and suitable contact times), it is possible to Table 2. Atmospheric reactivity of hydrocarbons and oxygenates (Pescarollo et al., 1993) Compounds Atmospheric reactivity* RVP blending (psi) Alkanes Propane n-butane n-pentane n-hexane Olefins Propylene butene pentene hexene methyl-1-butene Aromatics Toluene p-xylene m-xylene Alcohols Methanol Ethanol Oxygenates ETBE TAME *Coefficient of reaction rate of hydroxylic groups: kcm 3 /molecole per second VOLUME II / REFINING AND PETROCHEMICALS 197

6 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES dimethyleter (DME) formation 2 OH O 2 diisobutenes (DIB) formation 2 C CH 2 CH 2 C CH 2 C C CH C ter-butyl alcohol (TBA) formation C CH 2 2 C OH methyl sec-butylether (MSBE) formation CH CH OH CH 2 CH O Fig. 5. side reactions. limit their formation to a few hundred ppm. The formation of TBA due to the competitive addition of water to the isobutene is much faster and approaches thermodynamic equilibrium. Nonetheless, since the amount of water normally present in the reagents is fairly limited (1,000 ppm in methanol and 300 ppm in the C 4 compounds), the concentration of TBA in the remains lower than 1% in weight. Finally, linear olefins, present in the FCC and steam cracking streams, can react with methanol to form a more linear ether (MSBE) with lower octane properties. The reactivity of these olefins is however much lower than that of the isobutene so that the concentration of MSBE in the product is also limited to a few hundred ppm. Despite the high selectivity of the reaction, formation of by-products must be carefully monitored, not so much for the specifications of ether quality as for the oxygenate (mainly DME) content in the outgoing C 4 stream because these compounds are potential poisons for the downstream olefin conversion plants (metathesis and polymerization). Since etherification reactors must be designed to run in conditions as close to thermodynamic equilibrium as possible, knowledge of the equilibrium conversion is essential in determining correct plant size. The equilibrium constant for a liquid phase reaction of a non-ideal system, as in the case of, is given by the following expression: x g g IB g MeOH K eq 1111 x 1111 K x K IB x MeOH g where x is the liquid molar fraction, g is the activity coefficient, K x the ratio of the molar fractions and K γ is the ratio of the activity coefficients. K x can be evaluated experimentally, so that the solution of this equation at various temperatures only depends on the method used to describe the nonideality of the liquid phase. Since and isobutene have practically ideal behaviour (g is practically to 1), the equilibrium condition can thus be simplified as follows (Di Girolamo and Tagliabue, 1999): K x 1 K eq if K g 111 g MeOH g MeOH It is clear that, the more accurate the method (Unifac, Unifac Dortmund or Lyngby, Uniquac, Wilson, NRTL) used to describe the non-ideality of the system, and therefore to calculate the methanol 1,000 K eq temperature ( C) Rehfinger Izquierdo Zhang Fig. 6. equilibrium constants. 198 ENCYCLOPAEDIA OF HYDROCARBONS

7 ETHERIFICATION activity coefficient, the greater the accuracy of the description of thermodynamic equilibrium. There have been numerous studies (Rehfinger and Hoffmann, 1990; Izquierdo et al., 1994; Zhang and Datta, 1995) on the thermodynamic equilibrium of and the equations used to evaluate the equilibrium constants as a function of the temperature are in sufficient agreement, as shown in Fig. 6. ETBE ETBE, on the other hand, is synthesized using isobutene and ethanol: The main differences with respect to the synthesis of are: The thermodynamic equilibrium is less favoured at the same operating conditions, as shown in Fig. 7 (Izquierdo et al., 1994). The reaction is less selective. Side reactions are practically the same with the formation, along with DIB and TBA, of diethyl ether (DEE) through selfcondensation of ethanol, and ethyl sec-butyl ether (ESBE) following etherification of linear butenes. In the synthesis of ETBE however, an increase in the formation of TBA is observed due to the greater amount of water in the reactor inlet because of its greater concentration in both fresh ethanol (0,5-1 weight %) and in the recycle stream (azeotrope ethanol /water 94/6 in weight) obtained from the alcohol recovery section. TAME and higher ethers Analogous to the synthesis of and ETBE, the formation of TAME is also an equilibrium, acid catalyzed, methanol addition reaction. In this case however, there are two reactive isoolefins in the C 5 fraction: 2-methyl-1-butene (2M1B) and 2-methyl- 2-butene (2M2B), which simultaneously react with alcohol and undergo isomerization (kinetically quicker reaction). The reaction scheme is thus composed of three equilibrium reactions: C CH 2 CH 2 OH C OCH 2 CH 2 OH C CH 2 CH 2 C CH C O OH Thermodynamically, conversion of 2M1B is very favoured since it has an equilibrium constant near to that of (Fig. 8). However, global formation of TAME remains low (Serda et al., 1995) because the equilibrium between the two olefins is not greatly affected by temperature and is clearly shifted towards the more internal olefin (2M2B), which instead has an equilibrium constant of an order of magnitude lower (Rihko et al., 1994). The reaction scheme becomes much more complex when etherification of light gasoline is considered, since the number olefins that can react with the alcohol increases considerably. Thus, in the C 6 fraction there are 8 reactive olefins which can lead to 4 ethers, while in the case of C 7 hydrocarbons there are 22 reactive olefins that give rise to 12 ethers, as shown in Table 3. Rigorous thermodynamic treatment of the etherification of LCN is very complicated. It is however possible to discern general information from the examination of the thermodynamic equilibria of some C 6 olefins (2-methyl-1-pentene; 2-methyl-2- pentene; 2,3-dimethyl-1-butene; and 1,000 K eq temperature ( C) ETBE Fig. 7. Etherification equilibrium constants. 1,000 K eq temperature ( C) TAME from 2M1B 2M2B from 2M1B TAME from 2M2B Fig. 8. TAME equilibrium constants. VOLUME II / REFINING AND PETROCHEMICALS 199

8 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES Table 3. Reactive olefins and corresponding ethers Reactive olefins Boiling temperature ( C) Ethers C 4 isobutene 6.3 C 5 2-methyl-1-butene 31.1 TAME 2- methyl-2-butene 38.6 C 6 2- methyl-1-pentene methyl-2-methoxypentane 2-methyl-2-pentene 67.3 cis-3-methyl-2-pentene methyl-3-methoxypentane trans-3-methyl-2-pentene ethyl-1-butene ,3-dimethyl-1-butene ,3-dimethyl-2-methoxybutane 2,3-dimethyl-2-butene methyl-cyclopentene methyl-1-methoxycyclopentane C 7 2-methyl-1-hexene methyl-2-methoxyhexane 2-methyl-2-hexene 95.4 cis-3-methyl-2-hexene methyl-3-methoxyhexane trans-3-methyl-2-hexene 95.2 cis-3-methyl-3-hexene 95.4 trans-3-methyl-3-hexene ethyl-1-pentene ,3-dimethyl-1-pentene ,3-dimethyl-2-methoxypentane 2,3-dimethyl-2-pentene 97.4 cis-3,4-dimethyl-2-pentene ,3-dimethyl-3-methoxypentane trans-3,4-dimethyl-2-pentene methyl-2-ethyl-1-butene ,4-dimethyl-1-pentene ,4-dimethyl-2-methoxypentane 2,4-dimethyl-2-pentene ethyl-2-pentene ethyl-3-metoxypentane 2,3,3-trimethyl-1-butene ,3,3-trimethyl-2-metoxybutane 1-ethylcyclopentene ethyl-1-methoxycyclopentane 1,2-dimethyl-1-cyclopentene ,2-dimethyl-1-methoxycyclopentane 1,5-dimethyl-1-cyclopentene ,3-dimethyl-1-cyclopentene ,3-dimethyl-1-methoxycyclopentane 1,4-dimethyl-1-cyclopentene ,4-dimethyl-1-methoxycyclopentane 1-methylcyclohexene methyl-1-methoxycyclohexane 1-methylcyclopentene) used as reference point compounds (Rihko and Krause, 1996). Fig. 9 shows that the thermodynamic equilibrium is not favoured by the increase of the number of carbon atoms in the olefin (comparison of 2-methyl-1-butene and 2- methyl-1-pentene) and is greatly dependent on the olefin structure. It is thus penalized in the case of: more branched structures (comparison of 2-methyl-1- pentene and 2,3 dimethyl-1-butene); internal double bonds (comparison of 2-methyl-1-pentene and 2- methyl-2-pentene); and cyclic structures (comparison of 2-methyl-1-pentene and 1-methylcyclopentene) Catalysts The acidic catalysts used in industrial etherification processes consist of cation exchange 200 ENCYCLOPAEDIA OF HYDROCARBONS

9 ETHERIFICATION K eq temperature ( C) 2-methyl-1-butene 2-methyl-1-pentene 2,3-dimethyl-1-butene 2-methyl-2-pentene 1-methylcyclopentene Fig. 9. TAME and C 7 ethers equilibrium constants. macroporous resins, commercialized by Bayer, Dow Chemical, Purolite and Rohm & Haas. These resins are prepared by polymerization in a suspension of styrene in the presence of an appropriate cross-linking agent (divinylbenzene or DVB). The spherical polymer matrix obtained in this way, is insoluble in water and organic solvents and is functionalized by means of sulphonation with sulphuric or chlorosulphonic acid. The diameter of the obtained resin spheres normally varies between 0.3 and 1.2 mm. This parameter can be easily modified during the synthesis phase by acting on the type of compound used for suspension, the monomer concentration and stirrer rate. The use of an adequate amount of cross-linking agents is fundamental to confer the desired qualities on the polymer matrix. Depending on the amount of DVB used, it is possible to obtain gel or macroporous resins, characterized by different porosity values. With an amount of DVB lower than 4%, gel resins characterized by a continuous polymer phase (microporosity) and a negligible surface area are obtained. These resins can be used as catalysts only in the presence of a suitable swelling agent capable of allowing the reagents to enter inside the polymer matrix. In the case of macroporous resins, the conditions of polymerization (greater concentration of DVB and use of inert compounds as diluents) allow the creation of a structure composed of agglomerates of cellular microspheres joined to each other so as to form canals (macropores), therefore giving the resin a high artificial porosity and a structure similar to that of inorganic supports. These are the most commonly used resins in industry since aside from the fact that they have greater mechanical resistance they exhibit both permanent porosity (macroporosity) and microporosity and can therefore be used as catalysts in the presence of any type of solvent (polar or non-polar). Even macroporous resins tend to swell and shrink in volume although to a smaller extent than the gel resins. The intensity of this swelling and shrinking depends on the degree of cross-linking (swelling inversely proportional to the DVB content), the type of ion bound to the functional group and the solvent polarity. Polar molecules such as water tend to solvate the sulphonic groups and therefore swell the matrix, while on the contrary solvents with little polarity cause a contraction of the resin structure. This is the case of industrial etherification catalysts for which a volume reduction of approximately 20-30% is observed as a result of the shift of the resin from the hydrated form (the catalyst is generally charged in water) to a more contracted form due to a much less polar reacting medium (a hydrocarbon/methanol mixture). Anyhow, the most important parameter to discern resin activity is the total exchange capacity (expressed in hydrogen ion equivalents per unit of weight or volume) which represents the number of active sites available. At an industrial level, two classes of catalysts are used: along with the traditional type, a new generation of catalysts have been introduced with a greater exchange capacity (5.2 compared to 4.8 meq H /g) which assure greater activity and resistance to poisons. Table 4 summarizes the main characteristics of the two types of industrial catalysts, comparing the two most widespread resins of the market, which differ primarily by the value of surface area (and by this value s relation to their pore diameter). The life of industrial catalysts, on the other hand, is based on process characteristics and the level of poisons and/or impurities present in the feeds. From the point of view of the process, the life of the catalyst is affected by two main parameters: The type of reactor: given an equal concentration of poisons present in the different feed, and therefore degree of deactivation, resin life is greater in a reactor receiving thermal support from outside than in an adiabatic reactor (see Section 4.4.6). The operating conditions: the main limitation of acidic resins is the easy thermal degradation with breakage of the carbon-sulphur bond and subsequent VOLUME II / REFINING AND PETROCHEMICALS 201

10 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES Table 4. Properties of industrial resins Traditional catalysts Rohm & Haas Amberlyst 15 Purolite CT 175 New generation catalysts Rohm & Haas Amberlyst 35 Purolite CT 275 Functional group RSO 3 H RSO 3 H RSO 3 H RSO 3 H Acidic sites (eq H /kg) Surface area (m 2 /g) Pore diameter (Å) release of sulphuric acid. In the case of commercial resins used for etherification, degradation becomes significant only for temperatures above 130 C. Operating conditions must therefore be optimized to avoid the temperature within the catalytic bed reaching these values. The main cause of resin deactivation is however the neutralization of acidic sites due to the interaction with the contaminants present in the feed. Neutralization may be due to: Cations: in this case, deactivation takes place by ion exchange with the protons of the functional groups. The main cations in the feed are sodium and calcium (present in the wash water or as contaminants of methanol due to contact with sea water), iron, aluminum and chromium (due to solubilized rust) and aluminum and silicon (from the zeolite catalyst of the FCC unit). Strong nitrogen bases: included in this category are ammonia and amines with low molecular weight found in FCC C 4 fractions that have a deactivating action similar to that of cations. Weak nitrogen bases: this type of deactivation takes place due to action of nitriles (acetonitrile and propionitrile), whose presence in the FCC stream is due to the formation of azeotropes with C 4 and C 5 hydrocarbons, of N,N-dimethylformamide and of N-methylpyrrolidone which are used to extract butadiene from the steam cracking cuts. Dienes: these compounds (butadiene, isoprene, etc.) in the presence of acidic groups can polymerise inside the hydrocarbon matrix and plug the pores, in fact making a number of acidic sites inaccessible. This type of poisoning is a function of operating conditions (temperature and methanol content) and the concentration of the diene in the feed, which increases, passing from a C 4 cut to light gasolines. For the latter, selective hydrogenation is needed before the etherification stage. According to the type of contaminant, two extreme forms of deactivation can be identified: plug-flow and diffuse deactivation. Plug-flow neutralization takes place by interaction with cations or strong bases which tend to neutralize the acidic sites as soon as they enter in contact, thus provoking a complete neutralization of the first part of the catalytic bed, which practically acts a guard bed. Diffuse neutralization occurs in the presence of weak bases (amides, nitriles) which do not react immediately with the acidic centre but are adsorbed uniformly along the entire catalytic bed. Once adsorbed, they react with water (hydrolysis) or methanol (alcoholysis or Pinner reaction) to form ammonia, which rapidly neutralizes the acidic sites. It is clear that this latter type of deactivation has a much heavier impact on the life of the resin, since the catalytic bed is neutralized homogeneously and in fact, the nitriles are the main cause of industrial catalyst deactivation (Trotta et al., 1994). The introduction of a washing tower is the system generally adopted to reduce the content of contaminants agents in the case of FCC or steam cracking fractions. This system works very well with cations and strong bases while it is less effective with nitriles and amides due to their low distribution coefficient. The life of the resin is approximately two years in steam cracking plants and despite the washing section, 6-12 months in refineries. On the other hand, feeds from dehydrogenation have a much lower level of contaminants and allow the catalysts to live up to 4-5 years Kinetics In the case of reactions catalysed by resins, the rate of reaction depends substantially on two parameters: acidity, which in turn depends on the type and number of acidic sites, of the DVB content and reacting medium; and accessibility, which depends on the DVB content, porosity, particle diameter and treacting medium. While the characteristics of the resin chiefly affect its accessibility, the reacting medium is the basic parameter which influences the kinetics, since it heavily influences both accessibility (more or less accentuated swelling) and acidity, or more precisely, the catalytic activity. 202 ENCYCLOPAEDIA OF HYDROCARBONS

11 ETHERIFICATION According to the characteristics of the medium, two extreme situations can be identified: in the absence of polar molecules, the resins have a very active functional group structure ( RSO 3 H ) and are closely interlinked by hydrogen bonds (general catalysis). On the contrary, a high concentration of polar molecules, such as water and methanol, provokes dissociation of the functional groups with the solvated proton which becomes the true authentic catalytic species (specific catalysis). This transition towards specific catalysis is always accompanied by net reduction of resin activity (Gates and Rodriguez, 1973). The rate of reaction for this type of catalysis can be calculated by means of a model that uses a pseudo-homogeneous approach (Helfferich, 1962). According to this model, in the absence of diffusion phenomena, the reaction rate can be described in terms of homogeneous catalysis confined to the catalyst pores where the reactant concentration is in partition equilibrium with the corresponding concentrations in the external solution. Reaction order and mechanism are identical to those of the homogeneously catalyzed reaction. This approach, however, does not allow the interpretation of results when anhydrous solvents or non-polar reactants are used. Under these conditions, it is necessary to use a heterogeneous approach described by the Langmuir-Hinshelwood or Rideal-Eley models, which are more suitable for extrapolating experimental data in a wide range of concentrations (Ancillotti et al., 1977). In the specific case of etherification reactions, the real situation is more complex since the reduction of the concentration of the alcohol along the entire catalytic bed leads to a transition in the catalysis mechanism. At the inlet of the bed, a high concentration of methanol is encountered, which tends to dissociate the functional groups, reducing their activity. Conversely, as alcohol is converted, the polarity of the medium gradually diminishes and the undissociated sulphonic group becomes the catalytic species. Therefore, in the case of etherification, it is the isobutene/alcohol ratio which determines the catalytically active species and thus the reaction rate, as shown by Fig. 10 (Ancillotti et al., 1978). For molar ratios near the stoichiometric ratio (<1.7), a rate of reaction in the order of one for isobutene and zero for methanol has been observed. The latter is preferably adsorbed and tends to saturate the catalytic sites. Under near-stoichiometric conditions, the dominant stage becomes the interaction of the isobutene with the solvated proton. For higher values of the isobutene/alcohol ratio, a remarkable increase of the rate of reaction has been demonstrated. It is therefore possible to make the hypothesis that the mechanism initial rate (moles of ether/s.acid eq.) a 10.0 a a 2.7 a methanol initial concentration (mol/l) Fig. 10. Effects of reactants concentration on etherification initial rate. includes competition between the olefin and alcohol for assumption of the sulphonic group proton. Finally, for ratios greater than 10, a decrease of the reaction rate was observed, a result of the passage to a concerted mechanism (a single state), where the isobutene is coordinated with associated sulphonic groups and interaction with the alcohol becomes the reaction rate determining step. The dependence of catalytic activity on the reduction of the methanol content in the feed is also seen in the behaviour of the reaction rates of the secondary reactions, that is isobutene dimerization and bond isomerization (1-butene to 2-butene) as shown in Fig. 11 (Ancillotti et al., 1978). To describe this transition, a number of mechanisms and fairly complex kinetic equations have been proposed. One such example is that given below, which shows a non-linear dependence of the methanol concentration (Miracca et al., 1996): a a a 1 IB aa IB a MeOH K eq r k (1 K MeOH a MeOH ) b isobutene initial concentration (mol/l) 6 8 VOLUME II / REFINING AND PETROCHEMICALS 203

12 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES isobutene dimerization initial rate isobutene/methanol molar ratio Fig. 11. The effect of the methanol concentration on the initial rate of isobutene dimerization (moles 10 3 / acid eq.) and 1-butene double-bond isomerization. 1-butene double-bond isomerization initial rate where r is the rate of reaction, k the Arrhenius constant, a the activity, K MeOH the methanol absorption constant, K eq the equilibrium constant and a and b are exponents > 1. In industrial operations, the addition reaction methanol to isobutene is favoured to a much greater extent than other olefins found in the C 4 feeds, as can be seen in the following relative reactivity orders: isobutene 50,000 butadiene 10 2-butene 1 which are in agreement with the carbocation stability order (Ancillotti et al., 1987): In comparison to what happens from a C CH CH CH 2 CH 2 CH thermodynamic point of view, kinetics are not greatly affected by the change from to ETBE since the rates of reaction are comparable when anhydrous ethanol is used. Water has, in fact, a high dissociating capacity on the sulphonic functional group and therefore exerts a strong depressant effect on the catalyst activity; water content of 1% in weight is enough to reduce etherification rate by 25%. In the case of etherification of the C 5 cut and light gasolines, a rigorous study kinetic becomes difficult due to the high number of isomers and reactions involved. However, from the literature data (Rihko and Krause, 1996), it is possible to deduce some general indications on the rate of reaction: In the case of the C 5 cut, isomerization is the kinetically favoured reaction, and the rate of etherification is not affected by the initial ratio between the two reactant olefins (2M2B/2M1B). The rate of reaction declines with the increase of the number of carbon atoms in the olefin. The ratio between the initial rate of etherification of the C 6 and C 7 olefins with respect to that of C 5 is constant and practically independent of temperature. The greater the molecule s steric hindrance, the lower its rate of etherification; in the case of the C 6 olefins, the reactivity order is as follows: methylcyclopentene > 2-methylpentene > 3-methylpentene > 2,3-dimethylbutene. The rate of reaction decreases in the case of internal double bonds, for example in the case of a passage from 2-methyl-1-pentene to 2-methyl-2-pentene there is roughly a 30% reduction of the reaction rate Processes All commercial etherification technologies use similar operating conditions but different plant layouts (number and type of reactors). These latter on the kind of feed treated and therefore on the required conversion of the isoolefin. The feeds used to synthesize and ETBE (steam cracking, FCC, dehydrogenation of isobutane) differ in isobutene and linear butenes content as shown in Table 5. In the case of refineries (FCC cuts), a high conversion of isobutene (about 95%) is not required since an alkylation plant capable of completely converting all olefins in the exit stream is normally applied downstream of etherification. The standard refinery layout is thus the simplest possible and calls for two reactors in series with interstage cooling. Instead, in the case of feeds from steam cracking or isobutane dehydrogenation plants, more than 99% of the isobutene must be converted to minimize its content in the output flow. In fact, in steam cracking plants, isobutene is an impurity of the downstream treatment of residual C 4 (metathesis and polymerization), while in the case of dehydrogenation feeds, the output flow from the etherification plant (consisting primarily of isobutane) is recycled to the dehydrogenation reactor Table 5. Characteristics of C 4 feeds Feed Isobutene (% in weight) n-butenes (% in weight) Steam cracking FCC Dehydrogenation ENCYCLOPAEDIA OF HYDROCARBONS

13 ETHERIFICATION where any isobutene present forms coke. This results in loss of raw material and reduction of the catalyst life cycle. With these feeds, therefore, the plant layout is more complex and based on a double reaction stage as shown in Fig. 12 A, with two reactors separated by a fractionation column to remove the ethers produced in the first stage. By contrast, the separation section of the reactants is similar for all technologies and includes the following stages: a) an initial separation tower after the first reaction stage which allows recovery of the C 4 /methanol azeotrope from the top and recovery of the produced from the bottom; b) a second separation tower after the second reaction stage which allows recovery of the C 4 /methanol azeotrope from the top and recovery of a stream containing primarily from the bottom, which is sent to the first tower for recovery of the ethers; c) a washing tower with water to remove the methanol from the C 4 ; and d) a distillation tower to separate water and methanol which is recycled to the reaction. In the case in which high C 4 purity is required, it is possible to perform a more efficient removal of the oxygenate by-products present (mainly DME) by adding an additional distillation tower (stripper) or a removal unit with molecular sieve units know as an Oxygen Removal Unit (ORU). In some technologies, the two methanol recovery towers (washing-distillation) can be substituted with a methanol recovery system known as a Methanol Recovery Unit (MRU), based on the use of molecular sieves. In any event, independent of plant layout, the reaction is executed with a pressure greater than 8 bar (to keep the C 4 in the liquid phase) and a Liquid Hourly Space Velocity (LHSV) of 2-7 h 1. The methanol/isobutene molar ratio used industrially is slightly greater than the stoichiometric ratio in order to increase thermodynamic conversion. In fact, this ratio cannot be raised too much since the nonconverted alcohol can only be recovered from the bottom of the first fractionation column together with the product, or from the top of the second column together with the C 4 components (see again Fig. 12 A). For both solutions, the quantity of methanol in the stream is limited; in the first case by the specifications of the and in the other by the C 4 /methanol azeotrope composition (97/3 in weight). In the case of ETBE synthesis, the situation is more critical since: Thermodynamic equilibrium is less favoured and cannot be compensated by an increase of ethanol since the C 4 /ethanol azeotrope is also much poorer in alcohol (98/2 in weight). methanol C 4 cut 1 st reaction stage A methanol a) C 5 cut b) LCN 1 st reaction stage B fract. tower alcohol recycle 2 nd reaction stage 2 nd fract. tower alcohol recycle a) hydroc. C 5 b) hydroc. C 5 -C 6 fract. tower a) TAME b) hydroc. C 6+, TAME and heavy ethers 2 nd reaction stage washing tower washing tower C 4 out water methanol recovery column a) hydroc. C 5, TAME b) hydroc. C 5 -C 6, TAME and ethers water methanol recovery column Fig. 12. Etherification simplified flow diagrams: A, synthesis; B, TAME and etherified LCN synthesis. product The presence of a greater amount of water in the feed penalizes the reaction kinetics. In order to obtain isobutene conversion similar to synthesis, it is therefore necessary to increase reactor number or size. In the case of synthesis of TAME and higher ethers, the plant configuration is slightly different since it is not restricted by the type of feed, which comes exclusively from the FCC unit. The feeds (C 5 cut or LCN) have a reactive olefins content that depends on the severity of conditions used in catalytic cracking and that can vary between 20 and 40% (Trotta, 1996). These streams are already components of the gasoline pool so that there is no need to separate the ethers produced and maximize the conversion. Obviously, the greater the conversion VOLUME II / REFINING AND PETROCHEMICALS 205

14 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES the higher the methanol content that can be introduced in the gasoline. A traditional gasoline etherification plant, as shown in Fig. 12 B, is thus very similar to a double stage without the second fractionation column, since the output of the second reaction stage can be sent directly to the washing tower to remove the methanol. Elimination of the fractionation column allows removal of restrictions on the quantity of methanol and the use, in the second reaction stage, of a considerable excess of methanol to compensate the less favourable thermodynamic equilibrium. It is clear that, depending on the type of feed, composition at the head of the product recovery column may vary. In the case of TAME, all the C 5 hydrocarbons are recovered from the head of the first column and then sent to the second reaction stage. Instead, in the case of LCN etherification, it is impossible to completely separate the product from the feed due to the overlap of the ether (TAME and C 7 ethers) and C 6 -C 7 hydrocarbon boiling temperatures. Thus, only C 5 and part of the C 6 hydrocarbons can be recovered from the head of the first column and sent to the second etherification stage. The temperatures overlapping penalizes the conversion (20-40%) of C 7 reactive olefins that can only be converted in the first reaction stage. In the case of etherification of light gasoline, it is necessary to selectively hydrogenate the dienes which may be present in the feed in quantities up to several percent. Hydrogenation may be carried out with traditional reactors and catalysts (respectively adiabatic and noble-metal supported) or with innovative systems such as the column reactor where a trifunctional catalyst (acidic resin with added palladium) simultaneously allows hydrogenation of the dienes, isomerization of the olefin bond (from 3-methyl-1- butene to 2M1B and 2M2B) and etherification of the reactive olefins. Industrial reactors Three types of reactors are used in industry: the adiabatic reactor, the tubular reactor and the reactor column. Adiabatic reactor. The reactor can be used either as a front-end reactor or for finishing. It is certainly the simplest and most economical configuration since it consists of a catalyst-filled vertical recipient. In this reactor, reaction heat is not removed so that the temperature rises (as a function of the concentration of the isobutene in the feed and the conversion) in the direction of the flow along the catalytic bed. These conditions clearly tend to penalize the thermodynamic equilibrium of etherification and, on the contrary, favour the formation of by-products. The temperature increase in this type of reactor is controlled only by using low concentrations of isobutene in the feed and limiting the conversion. For this reason, adiabatic reactors are suitable for treating feed with low concentration of isobutene (from FCC). Instead, in the case of concentrated feedstock, the feed must be diluted with C 4 compounds output from the plant or an externally cooled reactor must be used, where part of the outflowing stream is cooled and recycled in the reactor. It is clear that recycles make it possible to achieve high global conversion (low conversion by passage), even if that means an increase in reactor size. Some improvements have been made to the traditional adiabatic reactor to obtain greater control of reaction temperature. These are: The boiling point reactor, which works at a lower operating pressure than a traditional adiabatic reactor, with vaporization of part of the reactants to absorb the reaction heat when the desired temperature (which is, in fact, the maximum possible temperature of the reactor) is reached. The expanded bed reactor where the catalyst is kept in motion by the reactants, thus avoiding local overheating or bad distribution of the liquid. Tubular reactor. The tubular reactor or Water Cooled Tubular Reactor (WCTR) is used exclusively as a front-end reactor and is practically a vertical tubular-shaped heat exchanger (Fig. 13) where the catalyst is lodged in the tubes and in the bottom (as well as in small amounts above the tubes). Furthermore, the cooling water, which flows in the reactor shell, can be sent in either co- or countercurrent flow according to the amount of heat to be removed. Since there are no support devices for the catalyst in the tubes, this reactor is equivalent to an adiabatic reactor in terms of the ease of loading and unloading. Moreover, even though the WCTR is clearly more complex and costly, it entails the following advantages in comparison to an adiabatic reactor: Greater operating flexibility: the rate of reaction may be controlled by acting on the temperature and the tempered water flow as well as on the entry temperature of the reactants. Excellent thermal profile: as shown in Fig. 14, the temperature is high in the first part of the catalytic bed to ensure kinetic support for etherification, and as low as possible at the reactor outlet to take full advantage of thermodynamic conversion and curtail production of by-products. Greater resistance to poisons: thanks to the particular thermal profile, only the initial part of 206 ENCYCLOPAEDIA OF HYDROCARBONS