Synthesis of Acrolein through Packed Bed Reactor Design

Department of Chemical Engineering and Materials Science University of California Davis

Reactions of Acrolein by Catalytic Air Oxidation of Propylene

By

Travis Silva, Max Oppedahl, and Brett Koehn

Report Submitted: March 14, 2016

Table of Contents

Introduction……………………………………………………………………………………………….

Results and Discussion…………………………………………………………………………………

    Part I…………………………………………………………………………………………...

        Figure 1………………………………………………………………………………..

        Figure 2………………………………………………………………………………..

    Part II………………………………………………………………………………………

        Figure 3…………………………………………………………………………………

        Figure 4…………………………………………………………………………………

        Figure 5………………………………………………………………………………

    Part III………………………………………………………………………………………

        Figure 6…………………………………………………………………………………

        Table 1…………………………………………………………………………………

    Part IV………………………………………………………………………………………

        Figure 7…………………………………………………………………………………

        Figure 8…………………………………………………………………………………

        Figure 9…………………………………………………………………………………

Conclusion………………………………………………………………………………………...

References………………………………………………………………………………………..

Introduction

An acrolein production reaction is being studied for a scale up design. The reaction that is being used is propylene and oxygen to acrolein with a catalyst to help the reaction forward. Acrolein, the main product of the chemical plant that is being designed, is used by itself as a herbicide or as a reactant for many other products, mainly methionine which is an amino acid. Another use of acrolein is as an intermediate to acrylic acid. The feed to this reaction is propylene which is also known as propene. This chemical is one of the most widely used starting products for many processes and is more commonly used in the production of plastic polypropylene.

This process of converting propylene to acrolein is usually done in a packed bed reactor. Different values are used for large scale processes when compared to values used for lab processes. Propylene and oxygen have a problem where they combusted if propylene is around 2.5 to 11 mole percent. This means that at no point during the conversion in the packed bed reactor can the compositions of propylene and oxygen be in this range. For large scale processes a common composition of the inlet stream to the reactor would be around 10 to 12 mole percent of propylene and oxygen and then the rest inerts. Propylene gas is usually stored at around 10 atm of pressure but a more common reaction pressure is around 2 atm. The reaction temperature can range anywhere from 350 to 450 degrees celsius. A typical conversion of propylene to acrolein is around 50 percent. For lab scale experiments the conversion can be anywhere from 70 to 90 percent and the composition does not need to have inert gas.

Results and Discussion

Part I

For Part I we had to design an isothermal reactor that would produce 135,000 metric tons/yr of acrolein from the partial air oxidation of propylene. The reaction is as follows

where the reactants are propylene and oxygen, and the products are acrolein and water. The type of reactor used was an isothermal packed bed reactor. In order to model this type of reactor the following equation is used.

This is used where the catalyst density is used to convert weight to volume. So there are multiple unknown variables in this design equation. First the conversion is unknown and the volume is also unknown. So one of the values needs to be set in order to calculate all the variables that describe the reactor. The conversion was set. The kinetics for the reaction rate part of the design equation were found from the paper Catalytic Air Oxidation of Propylene to Acrolein: Modeling Based on Data from an Industrial Fixed-Bed Reactor. The reaction followed power law kinetics. The reactants coefficients were both -1, and the products coefficients were both 1. The reactant exponential coefficients were .44 and .93 for propylene and oxygen respectively. The product exponential coefficients were both 0.  The k value was taken to be 1.67E-6 kmol/m^3/s/Pa. The activation energy was 4.74E7 J/kmol. We choose a temperature of 350oC. For the composition of the inlet stream, 98% oxygen and 2% propylene were chosen. This is because the composition limit  will always be below the lower flammability limit, but also because the exponents of the reaction show that the partial pressure of oxygen is more important than the partial pressure of propylene for getting higher conversion. We then plotted conversion of propylene against reactor length for a corresponding reactor diameter of 1.5 meters. This allowed us to see how long of a reactor we would need to achieve 80% conversion. We back calculated that when we have 80% conversion of propylene to acrolein, we would need 4.95E6 metric tons per year of total inlet stream in order to make the desired amount of acrolein. We found that we would need a reactor with a length of 25.5 meters. This will give a volume of 45.06 meters cubed. Using the bed voidage and catalyst density from the literature we calculated 30956 kg of catalyst will be needed. Next we varied the temperature betweenr 350oC and 450oC. We plotted this versus reactor volume and conversion of propylene. The reactor converted more effectively at higher temperatures. At a temperature of 450oC the reactor reached 80% conversion with a length of only about 7 meters. At a temperature of 400oC the reactor needed a length of about 23.5 meters to reach 80% conversion.

Figure 1: The figure displayed above show the effects of temperature and reactor volume on conversions. Each contour line represents a different temperature varying between 350oC and 450oC. Conversion increased as temperature and reactor volume increased.

We then plotted conversion versus the heat duty of our reactant. The heat duty decreases as conversion increased. This means the reaction is exothermic and therefore more heat is created as the reactants react more and there is more energy in the reactor to remove in order to keep it isothermal.

Figure 2: The figure displayed above shows the effect of conversion of our reactant on the heat duty. The conversion of our reactant increased as the magnitude of the heat duty increased.

Part II

For part II we had to design an multitubular fixed bed reactor. The reasons for using a multitubular reactor are that it is much easier to pack catalyst into the tubes and secondly the heat transfer coefficient is higher with more tubes because there is more surface area to cool the reactor. The problem is there is a pressure drop. First we accounted for the momentum balance with the Ergun equation.

We chose a pressure drop scaling factor of 1 and roughness of  4.572E-5. We calculated a number of 27918 1 inch tubes to reach our previous reactor volume. We found a pressure drop of .00003 Bars.

FIgure 3: The figure displayed above shows the pressure drop versus reactor length for two different catalyst volumes.

Next we calculated drop versus reactor length making sure to change the diameter in order to keep the reactor at a constant volume. We found that the pressure drop increased as the length of the reactor increased.

Figure 4: This figure shows the pressure drop versus length for a particle diameter of .0053.

Then we increased our particle size by a factor of two. The pressure drops were half as big when we doubled the particle size. Both of these can be clearly seen in the ergun equation to calculate pressure drop. First there is the diameter on the bottom of the fraction so when it is increased the pressure drop become lower and the length on the top of the fraction where if length is increase so does pressure drop.

Figure 5: This figure shows the pressure drop versus length for a particle diameter of .0075. This particle has volume which is twice as large as the particle shown in figure 3. This plot shows that the pressure drops are half as large for a particle twice the size.

Part III

For part III, we accounted for multiple reactions to occur. We accounted for three more reactions as follows

The reaction pathway is shown below.

Next we developed a reactor simulation that included mole balances of the multiple reactions. The primary product was acrolein and side products were carbon monoxide, carbon dioxide and acrylic acid. We also used a momentum balance with the Ergun equation. Then we plotted the mole flow rate of each species as a function of the catalyst weight by using the volume of the reactor and the bed voidage.

Figure 6: The figure displayed above shows the molar flow rate of each species as a function of Volume.

This graph shows that acrolein and all the side reactions have similar trends even though they are not on the same order of magnitude where they are increasing linearly until propylene starts to run out. Acrylic acid is an outlier because it is made from both propylene and acrolein in separate reactions. The trend of acrylic acid is linear production but as propylene is depleted the line starts to flatten out. Unlike the other side products the line does not become completely constant because acrolein was made and is being used to create the acrylic acid at a slow but constant rate.

5 reaction temperatures were chosen that would fit the literature values and the reactor was simulated and the molar flow rates of the products were recorded. These values were then manipulated to find the selectivity of the desired product and any valuable by-products. Acrolein was the product but acrylic acid is a valuable by product. Selectivity is defined to be the amount of desired product divided by the amount of undesired products. For the creation of acrolein, carbon dioxide, acrylic and carbon monoxide are undesired. Water is not undesired because it is created equally along with acrolein according to the reaction. The conversion is kept constant by changing the volume for these values.

Table 1: This table shows the selectivity of acrolein and acrylic acid as a function of temperature

Figure 7: This figure is a graph of the table above showing the selectivity of acrolein as a function of temperature. It is linear down because the reactions from acrolein to side products requires a high temperature.  So as temperature increases more acrolein converts to side products.

Figure 8:  This figure is a graph showing the selectivity of a desired product acrylic as a function of temperature.  It increases as temperature increases, because more acrolein is converted to acrylic acid as temperature increases.  

This shows that as the temperature increases so does the amount of the acrylic acid compared to acrolein. The highest percentage of acrolein is created at 350 degrees celsius while the highest percent of acrylic acid is made at 450 degrees celsius. This is because the acrolein reaction does not increase in efficiency as fast as the other reaction when temperature is increased.

Part IV

For part IV we designed a non-isothermal reactor by factoring in an energy balance to our reactor.

The reaction is exothermic so energy is added through the reaction. The enthalpies of the inlet stream and outlet stream are also taken into account when determining the temperature change in the reaction. So the heat taken away from the reactor is achieved in the coolant stream. This energy balance allows one to find the temperature of the reactor at any length and then also figure out how the coolant temperature.

Figure 8: The figure displayed above shows the reactor temperature versus length for varying inlet stream temperatures.  

This graph shows us where the hot spot of our reactor is located, and confirms that we can vary the hotspot location by varying the inlet temperature.  If you look at the inlet feed of 360 degrees celsius you see that the hotspot is located around .72 meters and 371 degrees celsius, however if you look at the inlet feed of 330 degrees celsius the hotspot is around 1.1 meters and 368 degrees celsius.     

Figure 9: This figure shows the selectivity of acrolein to the undesired products when the temperature of the reactor is being varied because of changes to the inlet stream shown in figure 8.  

The trend is the same as shown in Figure 7, which is the same graph but with an isothermal reactor.  The graphs differ, however, in the magnitude of the change of selectivity.  The isothermal reactor selectivity for acrolein drops significantly more than the reactor with a coolant stream.  This is logical since the coolant stream keeps the reactor at a lower temperature.  The higher the temperature of the reactor the more acrolein is converted to side products.  Thus the coolant stream increases the selectivity towards our acrolein, by keeping the reactor temperature lower.   

Figure 10: The figure displayed above shows the reactor temperature versus the length of the reactor for varying coolant temperatures.

From this figure we see that the coolant temperature does not appear to affect the location of the hotspot, but it does affect the magnitude of the hotspot.  The line that corresponds to a 350 degrees celsius coolant stream has a hotspot at .97 meters and 410 degrees celsius, while the line that corresponds to a coolant temperature of 380 degrees celsius has a hotspot at .97 meters and 371 degrees celsius.  

Figure 11: This figure shows the selectivity of acrolein against the undesired products like carbon monoxide. The figure is based on the temperature in the reactor when the coolant temperature is changed.

As the temperature of the coolant increases, we see a large decrease in the selectivity of acrolein.  This is logical, since the increase in coolant temperature increases the reactor temperature. As reactor temperature increases the side products production increases decreasing the acrolein selectivity.  

Figure 12: This graph shows how the energy changes with respect to both reactor coolant temperature and

The gain within the reactor was about 1. This gain is stable dynamically because it is less than 2.

Conclusion

The reactor that was made through this project is clearly possible. The start of the project ignored almost everything and used a design equation to find a volume of a packed bed reactor at a desired conversion for a given outlet flow rate. This volume was converted to catalyst weight by hand to given an idea of how much catalyst would be necessary for a reactor this size. This reactor was isothermal but the design was not easy to make isothermal so first the reactor was changed to a tubular reactor. This gives a new term, the pressure drop, but makes it much more feasible to implement because tubes are easier to cool and pack with catalyst. The one reaction was changed to 4 reactions that can occur at the same conditions as the original reaction. This led to undesired byproducts. The last part added to the original design was the coolant stream to help make the reactor isothermal. The hotspots were analyzed and the reactor was made sure to be dynamically safe meaning it would not heat up too much because the reaction is exothermic. THis design with all the added aspects gave a very similar result to the original design. The original design has an extremely high flow rate and a very exact composition while in theory works might not work in practice.

This reactor is not economically feasible yet. There are many parameters to lower the reactor volume and amount of catalyst used that can still be tried. There are other feed compositions that need to be explored which might be more realistic. The main problem with this design is the price of propylene because this gas is used to make plastic. Companies like Dow chemical have worked around this problem by starting with a gas like propane which is far cheaper and making propylene as an intermediate and then converting it to acrolein.

References

1. https://en.wikipedia.org/wiki/Acrolein
2. “Acrolein Production from Propane”, August 2003. IHS Incorporated https://www.ihs.com/products/chemical-technology-pep-reviews-acrolein-production-from-propane-2003.html
3. https://en.wikipedia.org/wiki/Propene
4. “Production of Acrolein” 2002. Univeristy of West Virignia http://www.che.cemr.wvu.edu/publications/projects/large_proj/Acrolein.PDF
5. Arntz, D., Knapp, K., Prescher, G., Emig, G., Hofmann, H., “Catalytic Air Oxidation of Propylene to Acrolein: Modeling Based on Data from an Industrial Fixed-Bed Reactor” Degussa AG, Hanau, Federal Republic of Germany, Inst. f. Techn. Chemi I, Universitat Erlangen-Numberg, Federal Republic of Germany, 1982 http://pubs.acs.org/doi/pdf/10.1021/bk-1982-0196.ch001