Catalytic reactive distillation process development for 1,1 diethoxy butane production from renewable sources

Bioresource Technology 102 (2011) 1289–1297

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Catalytic reactive distillation process development for 1,1 diethoxy butane production from renewable sources I. Agirre ⇑, V.L. Barrio, B. Güemez, J.F. Cambra, P.L. Arias Chemical and Environmental Engineering Department, Engineering School of Bilbao Alameda Urquijo s/n, 48013 Bilbao, Spain

a r t i c l e

i n f o

Article history: Received 31 May 2010 Received in revised form 19 August 2010 Accepted 22 August 2010 Available online 26 August 2010 Keywords: 1,1 Diethoxy butane Reactive distillation Steady-state model Amberlyst Katapak

a b s t r a c t Some acetals can be produced from renewable resources (bioalcohols) and seem to be good candidates for different applications such as oxygenated diesel additives. In the present case the production of 1,1 diethoxy butane from bioethanol and butanal is presented. Butanal can be obtained from biobutanol following a partial oxidation or a dehydrogenation process. In this paper innovative process development about the synthesis of the mentioned acetal including catalytic reactive distillation experimental and simulation results will be presented and discussed. Katapak SP modules containing Amberlyst 47 resin were used as structured catalytic packings. This reactive system allowed reaching higher conversions than the equilibrium ones at the same temperatures. All the experimental data gathered allowed to tune a simulation model for the reactive distillation operation which showed a fairly good behavior in order to perform initial 1,1 diethoxy butane production process design studies. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction In the last years, production of oxygenated compounds like ETBE as petrol additive from isobutylene and bioethanol has increased significantly. Nowadays, the use of different biofuels in conventional car engines has become one of the technological goals towards a sustainable development. Biodiesel is an alternative fuel obtained from vegetable oils or animal fats and it has several technical advantages over petro-diesel such as the reduction of exhaust emissions, improved lubricity and biodegradability, higher flash point and reduced toxicity. There are some other properties like cetane number, gross heat of combustion and viscosity that are very similar in biodiesels and in conventional diesels. But biodiesels are worse than conventional ones in terms of oxidation stability, nitrogen oxides emissions, energy content and cold weather operability (Moser and Erhan, 2008). A possible solution to these limitations is the use of additives. Metal based additives (manganese, iron, copper, barium. . .) were the most important ones so far (Burtscher et al., 1999). However, due to the harmful emissions that this kind of additives imply, the use of renewable ashless diesel combustion enhancer additives like acetals seem to be the most suitable ones (Capeletti et al., 2000). The acetals can be obtained from reactions between alcohols (bioalcohols) and aldehydes. The aldehydes can be obtained from their corresponding alcohols ⇑ Corresponding author. Tel.: +34 946017297; fax: +34 946014179. E-mail addresses: [email protected] (I. Agirre), [email protected] (V.L. Barrio), [email protected] (B. Güemez), [email protected] (J.F. Cambra), [email protected] (P.L. Arias). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.08.064

following a partial oxidation or a dehydrogenation process (Agirre et al., 2010b). Thus, the acetals can be produced from just renewable raw materials. Acetals are produced via homogeneous catalytic processes using strong mineral acids as catalysts such as H2SO4, HF, HCl or p-toluensuphonic acid (Frusteri et al., 2007; Green, 1981; Kaufhold and El-Chahawi 1996). Kaufhold et al. proposed in a patent (1996) an industrial process for acetal production. In this process, apart from a homogeneous strong acid catalyst an entrainer (hexane, pentane) is used with a normal boiling point between 298.15 K and 348.15 K. This entrainer must be water insoluble (<3% soluble in water), thus the water is continuously removed from the reacting phase shifting the acetalization reversible reaction to the desired direction. However, these processes entail corrosion problems, are uneconomical and they are not environmentally friendly. The use of a heterogeneous catalytic process would overcome most of the previously indicated problems. As a consequence, several solid acid catalysts are being tested currently. Capeletti et al. (2000) reported the performance of several solid acid catalysts, from commercial, natural and laboratory sources. They concluded that exchange resins show better performance than other catalysts allowing reaching equilibrium values much faster than with other alternatives. Acetalization reactions show high thermodynamic limitations achieving really low equilibrium conversions (around 50% depending on the operating conditions) if they are carried out in a conventional batch reactor (Agirre et al., 2010; Capeletti et al., 2000; Chopade and Sharma, 1997a,b; Mahajani et al., 1995; Sharma, 1995). The reactive distillation (RD) technology seems to be one

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Nomenclature ac CiL C si h H kc L Q r

catalyst surface area, (m2/kg) concentration of component ‘‘i” in the liquid bulk, (mol/L) concentration of component ‘‘i” in the catalyst surface, (mol/L) enthalpy of the liquid phase, (J/h) enthalpy of the vapor phase, (J/h) mass transfer coefficient, (m/s) liquid flow, (mol/h) heat, (J/h) reaction rate, (mol/h)

of the most promising alternatives (Benedict et al., 2006; Calvar et al., 2007; Chopade and Sharma, 1997a,b; Domingues et al., 1999; Lim et al., 2002; Sanz and Gmehling, 2006a,b; Sharma, 1995; Xianshe and Huang, 1996; Zhu et al., 1996) to overcome these limitations. RD combines chemical reaction and thermal separation in the same unit. Thus, the reaction products are being removed from the reaction mixture and thermodynamic limitations can be overcome achieving higher conversions. For RD Katapak catalytic structured column packings offer important catalyst contents, moderate surface areas as compared to nonreactive structured packings, low pressure drops (Klöker et al., 2004) and a good axial liquid distribution (Taylor and Krishna, 2000). Sharma and Chopade (1997a,b) and Dhale et al. (2004) used RD columns for acetalization reactions achieving high conversions. Calvar et al. (2007) and Klöker et al. (2004) also showed the benefits of using RD systems in similar reactions like the esterification of acetic acid with ethanol and in the ethyl acetate synthesis. RD presents several advantages like capital savings, achievement of high conversions, improved selectivity and reduction of the catalyst requirement. However, it also presents some difficulties and constraints like the volatility or the short residence time of the reactants in the reactive section. The formation of 1,1 diethoxy butane was chosen since it fulfills diesel specifications. Other smaller acetals like 1,1 diethoxy ethane do not fulfill some specifications like the flash point. The flash point for 1,1 diethoxy ethane is 21 °C which is much lower than 55 °C, the flash point that a diesel must have.

2. Methods 2.1. Materials Ethanol (99.5% w/w for synthesis) from Panreac and butanal (99% w/w) from Merck were used as reagents. 1,1 diethoxy butane (97% w/w) for GC calibration was obtained from Chemos. Amberlyst 47 was the selected catalyst in order to place it in the Katapak SP-11 structured packings.

V x y

vapor flow, (mol/h) liquid molar fraction vapor molar fraction

Greek letters DHr enthalpy of reaction, (J/mol) t reaction volume, (L) Subscripts i step in the distillation tower j compound

2.3. Reactive distillation column The reactive distillation experiments were carried out in a packed catalytic glass distillation column. The inner diameter of the column was 0.050 m while the total height was 0.750 m. The column was divided in three different sections of 0.30 m, 0.30 m and 0.15 m. In principle, the intermediate section was used as the reactive one (see Fig. 1a). Amberlyst 47 was supported on KATAPAK SP-11 modules (Sulzer). Each KATAPAK module had 0.010 m height and a diameter of 0.050 m and contains 20 g of catalyst. In the rectification and stripping sections Multiknit structured packing was used (supplied by Pignat). The reactants could be fed to the column at two different heights of the column, at the upper and the lower parts of the catalytic section. Moreover, the feed could be pre-heated before being fed to the column. The set up was also provided with a reboiler, a total condenser and a reflux controlling valve. 3. Results and discussion As the objective of this study was to explore the possibility of an innovative production processes for the 1,1 diethoxy butane acetal using catalytic reactive distillation all the experimental work to be presented in the following sections was required to develop and to tune a process simulation model for further engineering studies. 3.1. Previous studies Prior to all reactive distillation experiments a kinetic study was performed in a batch stirred tank reactor testing different operating temperatures, feed compositions, Amberlyst resins and different catalyst loadings. Low conversions were achieved at acceptable temperatures due to the high thermodynamic limitations that this exothermic reaction shows. Moreover, it was checked that the reaction follows an elemental reaction rate and the kinetic of the reaction was determined obtaining the corresponding Arrhenius’ parameters (Table 1) (Agirre et al., 2010a). The stoichiometry of the reaction between ethanol and butanal is the following one:

2Ethanol þ Butanal () 1; 1Diethoxy butane þ water

2.2. Analysis

3.2. Reactive distillation

Both reactants (ethanol and butanal) and reaction products (1,1 diethoxy butane and water) were analyzed by gas chromatography (Agilent 6890 N) using a flame ionization detector (FID) and a thermal conductivity detector (TCD). Agilent DB-1 60 m  0.53 mm  5 lm capillary column was used with Helium as the carrier gas.

The use of non conventional reaction systems like reactive distillation where the reaction and the simultaneous separation of the products (or one of the products) take place in the same unit can be extremely useful in catalytic reversible reactions like the acetalization ones. Several configurations were tested (different feed points,

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Fig. 1. Scheme of the reactive distillation semi-pilot plant.

Table 1 Arrhenius’ correlation’s parameters for the acetalization reaction.

Ea (J/mol) A

Forward reaction

Reverse reaction

35,505  1.08

1.06E + 5



ðm3 Þ3 2 mol s kgcat

59,752



ðm3 Þ2 mol s kgcat



feed mole ratios, catalyst amount and location of the catalytic section) and all of them were carried out at the maximum reboiler power since it was checked that a better liquid–catalyst contact was achieved. In all the cases the reactants, ethanol and butanal, were fed at 343 K and 338 K, respectively. In each experimental test, only the desired parameter was changed in order to see its effect in the performance of the column, keeping the rest of the variables as same as in the standard conditions. Most of the experiments were carried out in the following standard configuration: – Rectification section: 0.15 m. Multiknit non-catalytic structured packing – Reactive section: 0.30 m. 3 KATAPAK SP-11 modules with Amberlyst 47 as catalyst. – Stripping section: 0.30 m. Multiknit non-catalytic structured packing – Feed (stoichiometric ratio): o Ethanol: 1.14106 m3/s from the upper part of the catalytic section at 343.15 K. o Butanal: 8.61107 m3/s from the lower part of the catalytic section at 338.15 K. – Total condenser. – Reboiler duty and the reboiler liquid level were kept constant in all the cases (2 kW and 2.2 103 m3). – Reflux ratio (R): for each configuration several experiments were performed varying R from 0.5 to total reflux.

Under these conditions, the column operates with adequate fluid dynamics, with good liquid–vapor contact, good liquid–catalyst contact and near flooding conditions but without reaching them. 3.2.1. Catalyst loading effect The length of the reactive section is one of the critical parameters for process optimization. Three different column configurations were tested:  Only with 1 KATAPAK module in the middle of the central section.  With 3 KATAPAK modules in the central section.  With 5 KATAPAK modules, three in the central section, adding two more packings one just above the central section and another one just below it. One experiment was performed with each configuration at total reflux ratio in order to perform an initial screening and to check the importance of the catalyst loading. The achieved conversion with 1 KATAPAK module was 18.35%, 45.34% with 3 modules and 44.84% with 5 modules. After these results, more experiments were carried out with 3 and 5 KATAPAK modules varying the reflux ratio (Figure 2). As it was expected at higher reflux ratios, greater conversions were achieved in both cases. This can be explained since at high reflux ratios, the reactant molecules have more opportunities to react because they pass through the catalyst bed more times. It seems that at very low and very high reflux ratios, the amount of catalyst is not so critical since the achieved conversions are quite similar. However, the equilibrium conversion was overcome working only with total reflux. This can be explained by the limited achievable separation among water and non-reacted ethanol and butanal in the rectification section. As a result, significant amounts

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I. Agirre et al. / Bioresource Technology 102 (2011) 1289–1297 Table 3 Temperature and concentrations in the reboiler at different reflux ratios. Experiments performed with 5 Katapak modules. R

T (°C)reb

(Acetal molar fraction)reb

0.5 2 5 8 Rtot

136.0 79.4 77.0 76.4 75.1

0.789 0.291 0.211 0.193 0.163

ature in the reactive section was quite similar to the temperature measured in the experiments performed at higher reflux ratios (around 347 K) but the observed temperature in the reboiler increased considerably (Table 3). Moreover, according to a brief thermodynamic study performed by ASPEN PLUS using RGIBBS module, acetal decomposition to 1-ethoxy-1-butene, which is an endothermic reaction, is thermodynamically favored only at reboiler conditions when R 6 0.5 (temperature around 413.15– 416.15 K and high acetal concentrations). Fig. 2. Catalyst loading effect for different reflux ratios.

Table 2 Distillate composition at R = 0.5 and the corresponding equilibrium composition in a conventional reactor.

3.2.2. The height of the stripping section Two column configurations were used as it is depicted in Figure 1a and b. In both cases 3 KATAPAK modules were used and the variation stems from the modification of stripping height,

Molar%

R = 0.5 Equilibrium

Acetal

Water

Ethanol

Butanal

2.5 15.4

14.6 15.4

57.2 46.0

25.7 23.0

of ethanol and butanal left the column before they can react except in the case of total reflux ratio. Furthermore, at low reflux ratios the concentrations in the downflow reaching the reactive section are not far away from the equilibrium ones except for the acetal (Table 2). Reactive distillation systems would overcome more efficiently thermodynamic limitations operating with reactants that can be separated from water more easily by distillation. Another important aspect is the acetal concentration in the output. At low reflux ratios high acetal concentrations were obtained in the bottoms. In case of 3 KATAPAK modules, 65 M% of acetal can be achieved in the reboiler, facilitating its later purification. With 5 KATAPAK modules one experiment was performed at R = 0.5 reaching acetal molar concentrations of 80% (Fig. 3) but one side reaction was observed: 1,1 diethoxy butane (acetal) was converted into 1ethoxy-1-butene and ethanol. It seems that this reaction took place in the reboiler due to the achieved high temperatures. The temper-

Fig. 4. Conversion vs reflux ratio with the two column configurations.

Fig. 3. Volumetric flow rate and acetal concentration in the outputs. Catalyst loading: 5 Katapak SP-11 modules.

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and therefore, the rectification height. Fig. 4 shows the variation of the conversion with the reflux ratio. It can be observed that at low reflux ratios the conversion with additional stripping height is higher, but at higher reflux ratios this trend changes, achieving lower conversions with this column configuration. In principle, more stripping height implies higher concentration of the volatile compounds (the reactants) and lower acetal concentrations through the reactive section. According to this explanation, the achieved conversion should have been higher with the second column configuration for all the reflux ratios but this is not true for the highest ones. This fact can be explained in the following way: as a consequence of having a higher stripping height, the top part of the catalytic section works better and therefore, more acetal is formed there. Due to its low relative volatility, the acetal goes down towards the reboiler as soon as it is formed. Thus, acetal concentrations in the lowest part of the catalytic section could be important enough in order to favor the reverse reaction and as a result reach lower conversions. This effect will be described in a more detailed way in Section 3.3.3.1 since the process mathematical model also predicts this effect. 3.2.3. Different feeding configurations All these experiments were performed with 5 KATAPAK modules since it seems that it offers the best column performance. 3.2.3.1. Feed flow rate. Total feed flow rate was lowered from 2  106 m3/s (1.14  106 m3/s of ethanol and 8.61  107 m3/s of butanal) to 1  106 m3/s and 5  107 m3/s keeping constant the ethanol:butanal mole stoichiometric ratio (2:1). The fluid dynamics of the column changes and the vapor–liquid and liquid–catalyst contact could be a bit worse. However, decreasing the total feed flow rate and keeping constant the amount of catalyst, slightly higher conversions were achieved: 44.84%, 49.61% and 50.75%, respectively. 3.2.3.2. Feeding position. Two different feeding configurations were tested: the first one introducing the ethanol to the upper part of the reactive section and the butanal to the lower part of the reactive section and the second one, introducing a stoichiometric feed mixture to the upper part of the catalytic section. Butanal is a bit more volatile than ethanol and that is the reason why ethanol is fed to the top side and butanal to the bottom side of the catalytic section. However, the volatility difference between them is quiet small (Tb,EtOH = 351.15 K and Tb,but = 347.15 K) and that is the reason why the second configuration was also tested. In order to test the second configuration, the feed was prepared in the corresponding storage drum and, before introducing it to the column, it was pre-heated to 341.15 K. The used total feed flow was 2  106 m3/s, in order to maintain the same fluid dynamics in the distillation column. At all the tested reflux ratios the conversion was higher when all the feed was introduced to the upper part of the catalytic section. For R = 0.5, 15.5% vs. 19.2%, for R = 5, 38.2% vs. 42.1% and for total reflux ratio, 44.8% vs. 47.6%, respectively. So, it seems that feeding a reactant mixture is a better strategy than trying to get some advantage from the small volatility difference showed by the reactants. 3.2.3.3. Feed composition. Two different compositions were tested, 2:1 (the stoichiometric one) and 4:1 (excess of the cheapest reactant) EtOH:Butanal molar ratios. In this case, the reactants were fed only to the upper part of the catalytic section since, as it is showed in Section 3.2.3.2., the use of this configuration leads to higher conversions. 74.5% of conversion can be achieved working at total reflux ratio and with a 4:1 ethanol to butanal feed ratio, being 61.8% the equilibrium conversion at these conditions. How-

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ever, the acetal concentration in the reboiler was still quite low (17 mol%). As in all the previous cases, working with R = 0.5 high acetal concentrations were achieved in the reboiler (81 mol%) but obtaining only 36.6% of conversion. 3.3. Reactive distillation model A mathematical model was developed in order to obtain further insights and to predict results without carrying out any additional experiments. In the open literature two model types are the most common ones: steady-state equilibrium models, where the vapor and liquid phases in contact are assumed to reach phase equilibrium, and non-equilibrium stage models, where mass transfer rates across the vapor–liquid interface are taken into account. Some authors concluded that a non-equilibrium model describes better the performance of a reactive distillation process (Baur et al., 1999) and some others (Peng et al., 2002) indicate that both approximations show very few differences. A non-equilibrium model requires accurate estimations of mass transfer coefficients, binary diffusion coefficients, surface tensions values, etc., and as these estimations are a big challenge, as a first approximation, an equilibrium model was developed. It was checked that introducing only one empirical tuning factor the model predicted results were comparable to the experimental results. Moreover, Taylor and Krishna (2000) concluded in a extensive review that equilibrium models are valid for preliminary designs. 3.3.1. The model and its simplifications An equilibrium stage model was used to model the packed column. Every stage was considered adiabatic and non-ideal liquid mixture behavior was considered (NRTL model was chosen as the most suitable one (Letcher et al., 1996)). A total condenser was supposed and the possibility of two different feed points was taken into account. The equilibrium model consists on the conventional MESH equations.  Material balance

Li1 xj;i1 þ V iþ1 yj;iþ1  Li xji  V i yji þ r j t ¼ 0

ð1Þ

 Enthalpy balance

Li1 hj;i1 þ V iþ1 Hj;iþ1  Li hji  V i Hji þ DHr rj t þ Q i ¼ 0

ð2Þ

 Equilibrium equation

yji ¼ K ji xji

ð3Þ

 Summation equations c X

xji ¼ 1

ð4Þ

yji ¼ 1

ð5Þ

j¼1 c X j¼1

After the degrees of freedom study it was concluded that two variables had to be fixed apart from the input data,(number of stages, feed stages, reflux ratio, feed flows and compositions, total pressure) one in the reboiler (bottoms flow rate) and another one in the condenser; it was supposed that the input and output temperatures in the condenser were the same, i.e. the condenser duty value is the latent heat value. 3.3.2. Model validation In order to generate comparable predicted results with the experimental ones, the number of stages was adjusted as well as an empiric tuning factor had to be introduced. The tuning factor in-

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cludes several effects like the equivalent plate efficiencies and the wetting factor of the catalyst. However, the possibility of external mass transfer limitations was checked. For this purpose a first approximation was done using the Frösling correlation.

Sh ¼ 2 þ 0:6 Re1=2 Sc1=3

ð6Þ

From Sherwood’s number the mass transport coefficient was obtained. In order to know if the reaction is controlled by the kinetics or by the external mass transfer, the kinetic low and the average molar flux from the liquid bulk to the catalyst surface were matched.

 r ¼ k’ðC SA Þ2 C SB ¼ ðkc ac ÞB ðC BL  C SB Þ ¼ 0:5 ðkc ac ÞA ðC AL  C SA Þ  r ¼ k’ðC SA Þ2

ðkc ac Þ C BL k’ðC SA Þ2 þ ðkc ac Þ

¼

1 kc ac

C BL þ k’ðC1S Þ2

ð7Þ ð8Þ

A

Comparing kc1ac and 1S 2 it was concluded that the effect of the k’ðC Þ external mass transfer Awas completely negligible. Several column configurations with different stage numbers were tested and the optimum tuning factor in each case to match experimental conversions was calculated. It was observed that this factor behaved approximately constant in all the cases as it can be expected for similar gas–liquid–solid contacts. After several tests is was concluded that the column configuration that better describes the experimental results reported in the previous sections (3 KATAPAK modules were considered) must include six stages: condenser, 1 rectification stage (where one feed stream is placed), 1 reaction stage, 2 stripping stages (another feed stream is placed in the 4th stage) and the reboiler. The optimum tuning factor found was 0.13. The order of magnitude of this number was the expected one. According to the literature 0.3 m of Katapak SP-11 modules are equivalent to 0.66 stages as the NTSM number (Number of Theoretical Stages per Meter) for this type of structured packing is 2 (Götze et al., 2001). On the other hand, all the catalyst may not have taken part in the reaction due to some wetting limitations and moreover, the model assumes that the column is completely adiabatic when the experimental facility could not be fully isolated. The comparison among experimental and predicted data is shown in Table 4. 3.3.3. Model application for initial process design Once the model was tuned several column configurations were tested in order to study the effect of each parameter and establish an appropriate column configuration. For this purpose the number of different stages (reaction, rectification and stripping stages) were varied as well as different feed parameters like flow rate, temperature, composition and feeding stage. For each reflux ratio, experimentally obtained feed-bottom flow rate ratios were used for the simulation calculations. 3.3.3.1. Variation of equivalent stage numbers. From the base column configuration described in Section 3.3.2, stripping, rectification and reaction stages were varied in order to find the optimum column configuration. First of all the number of rectification stages was varied and increasing it, as it was expected, the conversion de-

creased. With more rectification stages, there is less acetal in the distillate and therefore, there is more acetal in the reactive section lowering the forward reaction and as a result the conversion decreases. In terms of separation, with 1 rectification stage the acetal molar fraction in the distillate was 5  103, with 2 rectification stages 2.8  103 and with three stages 1.7  103. It is observed that each added rectification stage decreases around 50% the acetal molar fraction. However, in all the cases this fraction is really low. All these predictions were performed using a stoichiometric feed composition and the experimental feed flows and bottoms flow rate. In this case the used reflux ratio was 1. In order to study the relation of reaction and stripping stages further simulations were performed varying simultaneously these two kinds of stages. The reaction stages were varied from 1 to 5 and in each case three different stripping stages were tested (3– 5) for each number of reaction stages. Thus, the obtained predictions are depicted in Fig. 5. It is clear that with more than three reaction stages no significant improvement can be achieved. Moreover, additional stripping stages imply that using more than three reaction stages the conversion improvement is almost zero. Table 5 shows that increasing the number of stripping stages, the reaction heat effect of the bottom side reaction stages decreases probably due to the higher acetal concentrations coming from the upper stages. Besides, it can be observed that operating at low reflux ratios, this heat effect can change its sign. This means that the reverse reaction is more important than the forward reaction. Therefore, adding more catalyst (or reaction stages) does not imply higher conversions in this kind of systems. With these series of simulations it is confirmed that, regarding the stages and reflux ratios, the optimum column configuration for the used superficial feed flows (1.578 m3/(h m2) of butanal and 2.124 m3/(h m2) of ethanol) has the following characteristics:    

3 reaction stages 3 stripping stages 1 rectification stages R=5

In terms of concentration, as well as in the experimental part, the model predicts high acetal concentrations in the reboiler operating at low reflux ratios. It predicts acetal molar fractions up to 90% in the reboiler operating with 2 reaction stages, 2 stripping stage and 1 rectification stages. However, the model does not take into account the effect of side reactions, i.e. the 1,1 diethoxy butane conversion to 1-etoxy-1-butene and ethanol that happens when high acetal concentration and thus, high temperatures are reached in the reboiler.

3.3.3.2. Bottoms flow rate. For each simulation experimentally obtained feed to bottoms rate ratio was used so far. Varying the bottoms rate it will be checked if any enhancement can be achieved. Fig. 6a shows that an optimum bottoms flow rate can be achieved. Mention that 0.0159 mol/s is the experimental bottom rate value. Extracting 0.011 mol/s from the bottoms the maximum conversion

Table 4 Comparison of some other experimental and predicted parameters. R

1 3.5 5 6.5 Total

Acetal molar fraction in bottoms

Acetal molar fraction in the distillate

Distillate rate (m3/s)

Bottoms rate (m3/s)

Exp.

Pred.

Exp.

Pred.

Exp.

Pred.

Exp.

Pred.

Exp

Pred.

0.65 0.22 0.19 0.19 0.17

0.76 0.23 0.22 0.2 0.18

0.014 0.01 0.0084 0.0076 –

0.0053 0.004 0.0039 0.0039 –

1.5E08 8.0E07 6.0E07 4.7E07 –

1.5E08 8.0E07 6.3E07 4.9E07 –

4.5E07 1.1E08 1.3E08 1.4E08 2.0E08

4.8E07 1.2E08 1.3E08 1.5E08 2.0E08

26.5 33.4 36.5 38.6 45.3

25.8 34.4 36.5 36.8 47.4

Conversion (%)

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Fig. 5. Conversion vs. reflux ratio for several reaction stages. (a) 3 stripping stages, (b) 4 stripping stages and (c) 5 stripping stages.

Table 5 DHr r j t predicted values for 2, 3 and 4 stripping stages and 5 reaction stages. Reflux ratio

Reaction stage

EDHr r j tðJ=sÞ 2 stripping stages

3 stripping stages

4 stripping stages

R=5

1 2 3 4 5

106.38 31.777 12.731 77.513 16.914

108.25 31.467 11.894 67.922 1.023

109.95 31.657 11.65 62.131 0.17468

R=2

1 2 3 4 5

– – – – –

– – – – –

96.691 24.226 7.309 36.247 10.402

was predicted. In terms of concentrations, promising acetal molar fractions around 0.5 can be achieved in the reboiler decreasing the bottoms rate.

3.3.3.3. Feeding configuration. 3.3.3.3.1. Feed temperature. Studying the optimal feed temperature, it was observed that it depends on the number of reaction stages. Fig. 6b shows the effect of the feed temperature for 1 reaction stage and for ‘‘0.3 reaction stages”. In principle, it is not possi-

ble to model 0.3 reaction stages but this simulation was done in order to model the system with only 1 Katapak module, which was the catalyst loading used in the experimental part when feed temperature effect was measured. Remember that 1 reaction stage was defined as an equivalent of 3 Katapak SP-11 modules. Instead of introducing 0.3 reaction stages, which is not possible, the tuning factor was divided by three. In the case of ‘‘0.3 reaction stages”, the followed trend agrees with the effect observed in the experimental part. Probably, when the catalyst amount is really low, increasing the feed temperature increases the conversion because the residence time is low enough and equilibrium limitations have not an important influence. But, when the catalyst amount is high enough, the residence time is higher and the effect of the reverse reaction becomes significant and therefore, the conversion decreases. 3.3.3.3.2. Feed composition. Three different feed composition were tested: 2:1, 3:1 and 4:1 of EtOH:Butanal molar ratio. Ethanol was the reactant used in excess since, from an industrial point of view, it is cheaper and safer than butanal. As it was expected, when working with an excess of one of the reactants higher conversions were achieved. In each case, the difference between the predicted and the equilibrium conversion was approximately constant and very similar to the differences obtained experimentally. The predicted conversions by the model for 2:1, 3:1 and 4:1 ethanol/but-

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Fig. 6. Effect of the bottoms rate (a) and the feed temperature with 1 reaction stage and ‘‘0.3 reaction stages” on the conversion (b).

anal molar ratios were 47.0%, 61.0% and 70.4%, respectively, while the corresponding equilibrium conversions for each initial composition are 38.6%, 53.2% and 61.2%. 3.3.3.3.3. Feed position. Three different feeding configurations were predicted: the first one introducing ethanol from the top side of the catalytic section and butanal from the bottom side of the catalytic section; the second one, introducing a stoichiometric feed composition through the top side of the catalytic section; and the third one, introducing a stoichiometric feed composition through the bottom side of the catalytic section. The 2nd configuration offers better conversions and the 3rd configuration offers the worst ones. The achieved conversions were 47.0%, 51.6% and 33.7%, respectively for each configuration. This effect was also checked experimentally so it is confirmed that the volatility difference between ethanol and butanal is not high enough in order to justify feeding them at different column heights. In those configurations with a unique feed point, a feed rate of 2  106 m3/s was used in order to simulate similar fluid dynamics within the column. 4. Conclusions The thermodynamic limitations that the 1,1 diethoxy butane production presents can be overcome using reactive distillation. Higher conversions than the equilibrium ones were only observed for high reflux ratios due to the limited achievable separation of water from non-reacted reactants. The best results were achieved using at least 3 KATAPAK modules for the used equipment and conditions. Feeding a reactant mixture to the top of the reactive section is better than feeding them separately due to their low volatility difference. An equilibrium steady-state model simulates adequately the experimental results and can be used for initial process design of acetal production. Acknowledgements The authors gratefully acknowledge the financial support of this work by the Spanish Ministry of Science and Technology (ENE2006-15116-C04-03/CON), the Basque Government (IE06171) and the University of the Basque Country (UPV/EHU). Besides, the authors want to mention the collaboration of Rohm and Haas for kindly supplying different Amberlyst resins.

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