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CHE 415: Chemical Engineering Laboratory Winter 2018

GAS ABSORPTION

Equipment Description

A packed tower for gas absorption experiments is located on the east wall of Johnson 214. The tower is packed to a height of 4 feet with ½” Raschig rings and has an inner diameter of 4 inches. Pure carbon dioxide gas is supplied from a gas cylinder and regulator. The carbon dioxide is mixed with utility air and fed to the bottom of the column. NaOH solution is pumped to the top of the tower from a Nalgene feed tank. Rotometers, mounted on a control panel to the right of the tower measure carbon dioxide, air, and liquid flow rates entering the column. The valves for controlling each flow rate are located directly below their respective rotometers. Calibration data for each flowmeter are provided in Table 1. A manometer with taps at the top and bottom of the column measures the pressure drop across the column.

Two types of continuous-flow gas absorption experiments can be performed with the packed tower. First, the gas flow rate is kept constant while the liquid flow rate is varied. Second, the liquid flow rate is kept constant while the gas flow rate is varied. For both experiments, the gas mixture entering the column is fixed at 0.5 mole % carbon dioxide.

Figure 1. Gas absorption pilot plant in JOHN 214. The 180 L reservoir at left feed a pump that both recycles fluid back to the tank and delivers it through a rotameter and to the 4 ft packed section. Air (from house air) and CO2 (from the cylinder at left) flow through rotameters, then combine and feed the column bottom. CO2 sensor are positioned at both column feed and outlet. A gas sample port is located on the inlet.

Equipment Operation

Note: Personal protective equipment is of paramount importance in this lab. In addition to the lab minimum of protective eyewear and a lab coat, you will wear an apron and face shield during concentrated solution preparation (see below) and nitrile gloves if operating or working around the batch reactor or PFR. Be sure to identify roles so that the dry working lab space and computer are not exposed to gloves of chemicals.

Your lab group will develop detailed procedures for operating the equipment and acquiring the data. Therefore, you should study the equipment carefully before performing any experiments.

The sodium hydroxide scrubbing solution should be prepared at the start of the lab session across from the fume hood in the chem prep room (JOHN 210D). The feed reservoir volume is 180 liters. Determine in advance the mass of sodium hydroxide pellets required, mass the pellets on the top-loading balance, and then dissolve using the provide stir plate and 2 L beaker. Make sure the pellets have completely dissolved, then transfer the solution carefully to the provided Nalgene bottle(s). This concentrate will be diluted in the feed reservoir.

CAUTION! Significant heat evolves when NaOH dissolves in water! Start with cold water and continuously stir while slowly adding pellets to the water to dissipate the heat of solution. The resulting concentrated NaOH solution is also extremely caustic – wear appropriate PPE (double gloves, goggles, face shield, and lab coat) at all times when mixing and handling the solution. Transport the solution in capped 1 L bottles.

Fill the feed reservoir approximately half-full with process water and turn on the recirculation pump. Carefully pour the concentrated sodium hydroxide solution into the tank while continuing to fill the tank with water from hose (wear your PPE and pour along the side of the tank to prevent splashing). NOTE: Do not walk away from a reservoir as you fill it with a hose. Fill the tank to 180 liters. Continue mixing the solution using the pump and the column bypass line. Determine when you think it’s sufficiently well-mixed. If desired, liquid samples leaving the packed tower can be removed through the sampling line at the bottom of the tower.

Figure 2. Chemical preparation area for concentrated NaOH solutions. Be extra careful and communicative while working in close quarters with others. Leave the workspace as you found it, with stir plate, stir bar, graduated cylinders, etc.

IMPORTANT: Gas composition is assessed using Vernier LabQuest® units with CO2 sensors. The power requirements of the CO2 sensors allow only one CO2 probe per LabQuest®, so two are provided. During column operation, water can collect near the sensors, potentially damaging them or leading to measurement error. Make sure that any time water is flowing through the column, the gas is flowing as well. If you have trouble with the CO2 probe readings:

1. Verify that all inlet gas plumbing is connected and leak free

2. Verify that there is not liquid near the inlet and outlet CO2 probes

3. Double-check your calcs for total flow and %CO2

4. Pull probe and check for 400 ppm

5. Contact instructor if problems persist

Shut Down

Turn off the carbon dioxide gas cylinder and bleed the pressure in the carbon dioxide delivery line. Pump out the NaOH solution to the drain using the bypass valve behind the panel. Add fresh water to the feed tank until the tank is one-third full. Pump water though the system to flush sodium hydroxide solution from the pump, column packing, and tubing.

Appendix I. Rotameter Calibration

Do not attempt to calibrate the rotameters and assume the data below is sound.

Table 1. Rotameter calibration data at 21 oC for the packed gas absorption tower.

Flow (mL/s)

MeterAir**CO

2

*MeterWater

5976.21511.0

1019316.02017.2

1528626.42520.3

2037636.73026.3

2546249.13532.3

407034037.4

508484540.0

7011015044.0

901303

*CO

2

measured at middle of the ball

**Air measured from bottom disc

Flow (mL/s)

Note:

The calibration data does not cover the full range of the rotometers. Plot out this data before the first laboratory session, fit flow rate vs. scale reading data for each rotometer to determine the linear range of each. Decide on the range of gas and liquid molar flowrates you expect to use in the pilot-scale absorption tower before you begin the experiments.

� Performed by Dusty Berggren, March 2009