cstr in series experiment

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EXPERIMENT 5: RESIDENCE TIME IN A MIXED REACTOR

Introduction:.

A continuous stirred tank reactor (CSTR) is a system where a liquid flows continuously into a tank which is agitated thoroughly. The following conditions prevail:

• The inlet flow rate is constant.
• The tank is always filled up to an overflow point near the top; any addition of liquid results in an outflow, so that the total volume of liquid in the tank remains constant.
• The volumetric flow rate out must therefore equal the volumetric flow rate in.
• The stirring is assumed to be good enough that the concentration of liquid throughout the tank is uniform. Thus, if a small amount of a dissolved component enters the tank, this is assumed to be distributed instantaneously and evenly through the whole volume of the tank.
• The composition of the liquid leaving the tank is, therefore, the same as the liquid in the bulk of the tank.   The concentration of a dissolved component in the tank at any one time may be found by sampling the liquid leaving.
• The concentration of any substance in the tank can change as a result of a reaction (hence, the name continuous stirred tank reactor) or dilution/strengthening through the concentration of the incoming solution.

A schematic of a single “continuous stirred tank reactor” (CSTR) is shown below:

Figure 1: Schematic of a single CSTR

When a stirred tank starts off filled with a solution, and a solution of a different composition is added continuously, the entering solution mixes with what is in the tank, so that the solution leaving is a different composition from the solution entering. The composition of a dissolved component in the tank gradually changes as the dissolved component either accumulates (if the entering solution is of higher concentration) or dwindles (if the entering solution is of lower concentration). Because of this change in the composition of liquid in the tank, this system is referred to as an “unsteady-state system”.

Considered a stirred tank with a dissolved component with initial concentration C 0 . If the inlet flow rate to the tank contains none of the dissolved component (C in = 0), the concentration of the dissolved component in the tank at any time “t” can be calculated through the formula:

This equation is an “exponential decay”.  The concentration of the dissolved component in the tank will slowly dwindle to eventually zero after, in theory, infinite time.

In the industry, it is very common to have multiple reactors in series.  In this case, the flow rate through all reactors is the same, to ensure steady state, but the concentration of components might change from one reactor to the other.  A schematic of a system with two reactors in series is shown in Figure 2 below:

Figure 1: Schematic of a two CSTRs in series

Considered two stirred tanks of similar volume in series with a dissolved component with initial concentrations C 1,0 and C 2,0 , respectively. If the inlet flow rate to the first tank contains none of the dissolved component (C in = 0), the concentration of the dissolved component in the second tank at any time “t” can be calculated through the formula:

where, similar to equation (1):

In the E030 lab, the stirred tanks system consists of two ChemGlass reactors, each equipped with an agitator. Either a single reactor can be used, or both reactors in series.  A pump is connected to the system to supply the feed liquid. Photos of the system with most important components highlighted are provided in Figures 3 and 4.

Figure 3: Front view of reactors set-up with major components highlighted.

Figure 4: Side view of reactors set-up with major components highlighted.

The purpose of this experiment is to compare the experimental observation of methylene blue concentrations exiting a single and a two-stirred tank(s) (reactors) system with the theoretical expectations.

In this lab, you will be working with two different set-ups.  First a single tank (reactor) and then two tanks in series.

Single Reactor

• The “stirred tank” in this experiment is a Chemglass glass reactor. Fill it with plain (tap) water manually and turn on the stirrer to a high speed (approximately 300 rpm) without excessive splashing at water’s surface . Turn on the pump and with water being added to the reactor, you will soon observe water being drained from the vessel. Stop the pump and close the bottom valve on the reactor. Prepare the graduated cylinder, reopen the bottom valve and drain the water into the cylinder. Measure the working volume of the reactor, which will be used in the theoretical calculations.
• Turn on the reactor’s agitator and set it to 300 rpm. Use the tachometer to measure the actual rpm of the motor and readjust the motor.  Make sure that the bottom drain valve on the CSTR is closed.
• Add 20ml of the supplied 500ppm methylene blue solution to the CSTR reactor.

The pump’s RPM can be adjusted using the touch-buttons while the rpm is displayed on the screen, as shown on Figure 6.

• Once the reactor is filled with water/Methylene blue solution, there will be a continuous flow of the product (Methylene blue solution being gradually diluted by fresh water fed into the reactor) from the bottom valve of the single CSTR. Make sure that there is a vigorous mixing in the reactor. Wait 10 seconds until residual water from the previous run, trapped in the discharge tube is being drained from the bottom valve. (This will prevent low concentration readings of methylene blue at time 0).
• Take a small sample of the outflow every 2 minutes for 20 min., noting the time at which each was taken. Find the absorbance of the sample using the HACH spectrophotometer at 610 nm. Absorbance measurements will be done once the run is completed.
• Meantime prepare a set of standards 5 and 10 ppm of methylene blue using the provided stock solution. The Absorbance measurement of the standards will be used to generate a calibration line. The calibration line will be generated by plotting Absorbance vs. Concentration.
• Analyze the samples using the HACH spectrophotometer set to measure Absorbance at 610 nm wavelength. Remember to also measure the Absorbance of each standard. The HACH spectrophotometer operating manual can be found near the instrument.

Two Reactors in Series

•  Measure the effective volumes of both tanks, if not done previously.
• Ensure that the feed line is connected to the top reactor.

• Plot absorbance versus ppm for the standards.
• Use the “calibration curve” to convert all the absorbance measurements to concentration.

• Discuss the plots. Why does the methylene blue concentration follow the observed curves?
• Comment on the results. How close ( be reasonably specific!) are the experimental results to the theoretical predictions?

PROCTECH 2CE3 Lab Manual Copyright © by Kostas Apostolou. All Rights Reserved.

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CHE506 Reaction Engineering Laboratory - CSTR in Series

Article Info The fundamental goal of this experiment is to figure out the moments that will be for the RTD function of the reaction inside a Continuous Stirred Tank Reactor (SOLTEQ BP107) in series. The effect of step input in the mixing process inside the CSTR is the main reasons of conducting this experiment. The RTD of a reactor is a measurement of how well a chemical reactor mixes its constituents. The RTD is determined experimentally by injecting an inert chemical, known as a tracer, at t=0 and determining the concentration of the tracer, C, at the exit as a function of time. The tracer is allowed to flow for five minutes before the reading at t = 0 min is taken as the initial concentration of tracer. Then, every 3 minutes the reading of the conductivity is taken for the three CSTRs in series. Three moments is determined to shows the behaviour of the RTD Function in the CSTR. The mean residence time for reactor 1, reactor 2 and reactor 3 is 3600142.01 min, 1424035.69 min and 866390.74 min respectively. The variance for reactor 1, reactor 2 and reactor 3 is 2.059325E+18 min2, 9.788092E+16 min2 and 2.029722E+16 min2, respectively. The skewness for reactor 1, reactor 2 and reactor 3 is-7.878923E+24 min3,-1.507332E+23 min3 and-2.049931E+22 min3, respectively. For all the reactors, it shows negative skewness. The C(t) curve, F(t) curve and E(t) curve has been plotted against time for all three reactors.

Related Papers

Nurlina Syahiirah

Residence Time Distribution (RTD) Function, E(t) for Continuous Stirred Tank Reactor is study in the experiment using SOLTEQ BP107 Continuous Stirred Tank Reactors in Series with step change input as the method of injection of tracer. The tracer is allowed to flow for five minutes before the reading at t = 0 min is taken as the initial concentration of tracer. Then, every 3 minutes the reading of the conductivity is taken for the three CSTRs in series. Three moments is determined to shows the behaviour of the RTD Function in the CSTR. The mean residence time for reactor 1, reactor 2 and reactor 3 is 7019.2579 min, 6790.2629 min and 2800.8597 min respectively. The variance for reactor 1, reactor 2 and reactor 3 is 20438359621.3811 min2, 22920136968.7755 min2 and 1794345612.9332 min2, respectively. The skewness for reactor 1, reactor 2 and reactor 3 is -2647659.9356 min3, -2633166.2558 min3 and -571150.8412 min3, respectively. All the reactors show negative skewness. The C(t) curve, F(t) curve and E(t) curve is successfully plotted against time for all three reactors. (Note: On the theory part, the formula for the 3/8 simpson rule is wrongly written, please refer books for the right formula. Or you can just change the h/8 to 3h/8)

The goals of the two experiment is to examine the effect of pulse input and step change input in tubular flow reactor and to construct a residence time distribution (RTD) in both experiment. In addition, a conductivity measurement of different conversion values between sodium hydroxide and ethyl acetate is also determined for the third experiment. The first and second experiment is conducted using the Tubular Flow Reactor (Model BP 101) where NaOH and Et(Ac) solution are fed into the reactor with de-ionized water flow at constant 700 mL/min for experiment 1 and salt solution at constant 700 mL/min for experiment 2. In experiment 3, the conductivity of NaOH concentration by mixing 100 ml of deionised water with different conversion is recorded for the calibration curve for the y – intercept value and the slope. The conductivity value for inlet and outlet in the TFR is observed for every 30 seconds and the data is recorded. The mean residence time, the variance, σ2 and skewness, s3 were three moment of the residence time distribution (RTD) function calculated for Pulse Input and Step Input experiment. Both experiment shows positive skewness, however, for Step Input Experiment the skewness cannot be verified since the curve shows a rather directly proportional relation over time and not a bell shape curve. (Note: On the theory part, the formula for the 3/8 simpson rule is wrongly written, please refer books for the right formula. Or you can just change the h/8 to 3h/8)

Nurdiana Syahira Azwan

Wishal Kurnia

Conductivity is observed to study the effect of the pulse input and step change input against residence time in a tubular flow reactor. The flowrate of de-ionized and salt solution is adjusted at constant flowrate of approximately at 700 mL/min in the reactor. The residence time distribution acquired is 1 for both experiments. Meanwhile, variance and skewness obtained are 0.631 and 25.328 for variance, 0.669 and 1109.5 for skewness for experiment 1 and 2, respectively.

Bryan Macarayo

Efrén Santillán

Minister Obonukut

Minhajur Rahman

Yalemzewd Yosef

The confines in which chemical reactions occur are called reactors. A reactor can be a chemical reactor in the traditional sense or other entities, for example, a chemical vapor deposition apparatus for making computer chips, an organ of the human body, and the atmosphere of a large city. In this chapter, the discussion of reactors is limited to topics germane to the determination of reaction rates. Later in this text, strategies for attacking the problems of mathematically describing and predicting behavior of reactors in general are presented. In practice, conditions in a reactor are usually quite different than the ideal requirements used in the definition of reaction rates. Normally, a reactor is not a closed system with uniform temperature, pressure, and composition. These ideal conditions can rarely if ever be met even in experimental reactors designed for the measurement of reaction rates. In fact, reaction rates cannot be measured directly in a closed system. In a closed system, the composition of the system varies with time and the rate is then inferred or calculated from these measurements. There are several questions that can be put forth about the operation of reactors and they can be used to form the basis of classifying and defining ideal conditions that are desirable for the proper measurements of reaction rates. The first question is whether the system exchanges mass with its surroundings. If it does not, then the system is called a batch reactor. If it does, then the system is classified as a flow reactor. The second question involves the exchange of heat between the reactor and its surroundings. If there is no heat exchange, the reactor is then adiabatic. At the other extreme, if the reactor makes very good thermal contact with the surroundings it can be held at a constant temperature (in both time and position within the reactor) and is thus isothermal. 64

NITIN KANSE

This paper proposes RTD studies in plug flow reactor and comparison of non-ideal reactors using residence time distribution function. The model also gives a prediction of the number of ideal continuous stirred tank reactors (CSTR) that could represent the non-ideal plug flow reactor (PFR) in question. Simulated results reveal that 10 numbers of ideal stirred tanks in series would represent the non-ideal plug flow reactor under study. The graphical result of all four reactors is generated directly by polymath. Most of the chemical reactors in the industries have non-ideal regime. The non-ideal plug flow reactor (PFR) is one whose attributes deviate from that of the ideal plug flow reactors. Therefore, an in-depth knowledge of the residence time distribution (RTD) of components in the reactor is necessary for its analysis. The residence time distribution indicates how much time each fraction of a charged material spends in the vessel. The residence time distribution of reactants or tracers in a flow vessel is a key datum for determining reactor performance.

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This simulation displays the residence time distribution (RTD), which is measured by injecting a tracer pulse into the first continuously-stirred tank reactor (CSTR) in a series and detecting the tracer concentration at the outlet of the last CSTR. The outlet of each CSTR is the inlet to the next CSTR. The RTD for the system of CSTRs is a probability that a molecule will spend a certain amount of time in the reactor system. Use the sliders to change the number of CSTRs in series and the space time of the system (i.e., the space time \( \tau \) is the total volume of CSTRs divided by the volumetric flow rate).

This simulation was created in the Department of Chemical and Biological Engineering , at University of Colorado Boulder for LearnChemE.com by Neil Hendren under the direction of Professor John L. Falconer. This simulation was prepared with financial support from the National Science Foundation. Address any questions or comments to [email protected]. Is your screen too small to fit this application? Try zooming-out on the web page (CTRL+"-" and CTRL+"=" on Windows, or ⌘+"-" and ⌘+"=" on Mac), then refreshing the page. This application is not compatible with Internet Explorer or web browsers that do not support WebGL and HTML5. Recommended browsers are the latest versions of: Chrome, Safari, Firefox, Edge, and Opera.