Liquid Phase Reactor: Sucrose Inversion

Overview

Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

Both batch and continuous flow reactors are used in catalytic reactions. Packed beds, which use solid catalysts and a continuous flow, are the most common configuration. In the absence of an extensive recycle stream, such packed bed reactors are typically modeled as "plug flow". The other most common continuous reactor is a stirred tank, which is assumed to be perfectly mixed.¹ One reason for the prevalence of packed bed reactors is that, unlike most stirred tank designs, a large wall area to reactor volume ratio promotes more rapid heat transfer. For almost all reactors, heat must either be added or withdrawn to control the temperature for the desired reaction to take place.

The kinetics of catalytic reactions are often more complex than the simple 1^st order, 2^nd order, etc. kinetics found in textbooks. The reaction rates can also be affected by rates of mass transfer - reaction cannot take place faster than the rate at which reactants are supplied to the surface or the rate at which products are removed - and heat transfer. For these reasons, experimentation is almost always necessary to determine the reaction kinetics prior to designing large-scale equipment. In this experiment, we explore how to conduct such experiments and how to interpret them by finding a reaction rate expression and an apparent rate constant.

This experiment explores the use of a packed bed reactor to determine the kinetics of sucrose inversion. This reaction is typical of those characterized by a solid catalyst with liquid phase reactants and products.

sucrose → glucose (dextrose) + fructose(1)

A packed bed reactor will be operated at different flow rates to control the space time, which is related to residence time and is analogous to elapsed time in a batch reactor. The catalyst, a solid acid, will first be prepped by exchanging protons for any other cations present. Then, the reactor will be heated to the desired temperature (isothermal operation) with the flow of reactants. When the temperature has equilibrated, product sampling will begin. The samples will be analyzed by a polarimeter, which measures optical rotation. The mixture's optical rotation can be related to the conversion of sucrose, which can then be used in standard kinetics analyses to determine the order of the reaction, with respect to the reactant sucrose, and the apparent rate constant. The effects of fluid mechanics - no axial mixing (plug flow) vs. some axial mixing (stirred tanks in series) - on the kinetics will be analyzed as well.

Procedure

Catalyst properties are: size = 20 - 40 mesh; weight = 223 g; water content = 30 wt. %; apparent (bulk) density = 1.01 g/mL; acid site concentration = 4.6 mmol acid sites/g dry weight; surface area = 50 m²/g; macroporosity (macropore volume/total volume of cat.) = 0.34; average macropore size = 80 nm. A P&ID diagram of the unit is shown in Figure 2. For this experiment, only bed #1, the organics tank, pump and rotameter are used. Samples are collected at the upper

Results

The polarimeter determines the fractional conversions of sucrose after reaction in a packed bed reactor. A previous polarimeter calibration for a three different sucrose feeds is shown in Figure 3.

Figure 3. Relationship between degree of rotation and fractional conversion o

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Application and Summary

The reaction does not behave exactly as expected because the apparent order n is > 1. Of all the phenomena that can cause such deviations in real reactors, deviations from ideal PFR behavior caused by axial mixing are suggested by the fact that fitting to the tanks-in-series model gives only a small number of tanks - for a perfect PFR, N should be at least 6. Such deviations are often found in relatively short beds, especially if the flow is multiphase (some water is vaporized in the reactor). However, another cause

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References

J. Sauer, N. Dahmen and E. Henrich. "Chemical Reactor Types." Ullman's Encycylopedia of Industrial Chemistry (2015). Web. 15 Oct. 2016.

H.S. Fogler, "Elements of Chemical Reaction Engineering," 4^th Ed., Prentice-Hall, Upper Saddle River, NJ, 2006, Ch. 2-4; O. Levenspiel, "Chemical Reaction Engineering," 3^rd Ed., John Wiley, New York, 1999, Ch. 4-6; C.G. Hill, Jr. and T.W. Root, "Introduction to Chemical Engineering Kinetics and Reactor Design," 2^nd Ed., John Wiley, New York, 2014, Ch. 8.

N. Lifshutz and J. S. Dranoff, Ind. Eng. Chem. Proc. Des. Dev., 7, 266-269 (1968).

E.R. Gilliland, H. J. Bixler, and J. E. O'Connell, Ind. Eng. Chem. Fundam., 10, 185-191 (1971).

"Sulfuric Acid." The Essential Chemical Industry. Univ. of York, 2016. http://www.essentialchemicalindustry.org/chemicals/sulfuric-acid.html. Accessed 10/20/16.

E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce and J.G. Goodwin, Jr., Ind. Eng. Chem. Res.,44, 5353-5363 (2005); A. Buasri, N. Chaiyut, V. Loryuenyong, C. Rodklum, T. Chaikwan, and N. Kumphan, Appl. Sci.2, 641-653 (2012); doi:10.3390/app2030641.

Tags

Liquid Phase Reactor Sucrose Inversion Chemical Reaction Reactor Types Catalysts Reaction Rate Heat Transfer Mass Transfer Packed Bed Reactor Batch Reactor Kinetics Reaction Order Rate Constant Tanks in series Model Feed Concentration Fractional Conversion Residence Time Forward Rate Catalytic Reaction

Figure 2. P&ID representation of the apparatus (control system interface).

1. Starting up the Reactor

Access the distributed control system interface. From the Unit item on the menu, select Perm. The Perm P&ID schematic (Figure 2) will appear. All data can be collected into an Excel spreadsheet. Click "Trend 50" on the interface to obtain the key process variables with respect to time.

Open the inlet and exit valves to the catalytic reactor bed (Bed #1). Be sure that the inlet and exit valves to the other beds are closed, and that the control valve (F531) and on-off valve (D531) on the city water supply are also closed.

Add dilute acid (0.25 M H₂SO₄) to the feed tank (2 L).

Turn on the feed pump to a constant speed (at least 3 on the dial), and set the rotameter to give the desired flow rate (40 - 70 mL/min). If the rotameter cannot control flow in this range, increase the pump speed to the next higher setting.

Once acid has been fed, add 200 mL or more of DI water to the feed tank. The acid protons will exchange for whatever other ions (Na⁺, Ca²⁺) are attached to the sulfonic acid anions. This is because even DI water contains impurities and municipal water contains even more. This is called "regenerating" the catalyst.

Prepare at least 10 L of feed solution with 15 wt% sucrose in DI water. When preparing the sucrose solution, add the sucrose slowly to the water while stirring at room temperature. Use both a magnetic stirrer and a paddle.

Add the feed into the organics tank.

2. Packed Bed Reactor Operation

To begin sucrose feed flow, turn on the pump, and adjust flow as desired using the pump's speed controller and the rotameter. The speed controller is for gross adjustment, whereas the rotameter is for fine adjustment.

Program the setpoint for T505 to 50°C, and set it to AUTO. The system is programmed to cut off the heaters if the bed temperature reaches 90°C, or if the pressure drop, as measured by the d/p transmitter, falls below 1 in. on the H₂O column pressure, which indicates that there is no feed.

A typical temperature for the reaction is 60°C. When the reactor temperature first exceeds 50°C, move the set point to this desired final temperature. A temperature profile may be observed across the catalyst section, which can be visualized by examining thermocouples T502 and T503.

Collect 25 mL samples of the reaction products at the upper drain in test tubes or sample bottles 10 min apart. Do not collect samples until at least two bed residence times (the bed open volume is ~1.2 L) have passed.

3. Shutting Down the System

Set the T-505, Bed #1 temperature control to zero output on MANUAL.

Shut off flow to the reactor once the temperature begins to decrease.

Close the block valves on Bed #1.

4. Using the Polarimeter

Analyze reaction product samples and the initial feed with the polarimeter. For a 15 wt. % sucrose feed, conversions should be in the range of 80 to 120 mL/min. Pure sucrose, glucose and fructose should be used as the calibration standards. See the appendix for more details on polarimetry.

Turn on the sodium lamp and allow it to warm up for approximately 5 to 10 min. A yellow light will be visible.

Check the zero position of the dial. At zero, a uniform dark field with no dark/light fringes, should be observed.

Add 25 mL of the initial feed solution to a clean tube. Make sure the tube is completely filled. Use the same tube for both calibrations and measurement because the reading is path length dependent.

Place the tube in the polarimeter. The tube should be facing up to remove any trapped air from the line of sight and the bulb should be near the eyepiece.

Close the cover. If the solution rotates polarized light, dark/light fringes should be observed through the lens.

Rotate the dial until the fringes disappear and a uniform dark field is observed.

Read the rotation angle through the magnifying glass using the vernier scale. The dial divisions are 1 degree each and each vernier division is 0.05°. To adjust the focus, rotate the black dial located below the eyepiece.

Repeat for each sample. Use DI water to clean the tube before each measurement.

건너뛰기...

0:07

Overview

1:04

Principles of Reaction Kinetics in Packed Bed Reactors

3:26

Packed Bed Reactor Start-up

4:21

Catalyst Regeneration and Sucrose Feed

5:27

Sample Collection and Polarimeter Analysis

7:07

Results

9:29

Applications

10:37

Summary

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