Flooding and Gas Absorption in a Packed Column

Process Engineering Laboratories:

Flooding and Gas Absorption in a Packed Column

 

  1. Aims and Objectives

Packed columns, through which two fluids flowing in opposite directions enabling component(s) to be transferred from one fluid phase to the other, are key units is almost all chemical plants. Examples of their applications include distillation, gas absorption (for example Carbon Capture and Storage units), solvent extraction and chemical reactions. Given their importance in developed and emerging chemical plant, it is important to have a knowledge and understanding of both fluid flow and mass transfer in these plant operations

 

This laboratory has two key aims:

  1. To study the hydrodynamic performance of a packed column.
  2. Investigate the performance of the mass transfer process involved in gas absorption.

 

  1. Learning Outcomes

Students should demonstrate an understanding of:

 

S4 Awareness of the framework of relevant legal requirements governing chemical/environmental engineering activities, including personnel, health, safety, and risk (including environmental risk) issues.

 

CCEPS3 Methods of identifying process hazards (e.g. HAZOP), and of assessing environmental impact, with quantification appropriate to the programme level.

 

E1 Fundamental core chemical/environmental engineering principles and the ability to apply them to analyse key chemical/environmental engineering processes, in particular:

 

CCEF2 Transport properties of fluids

CCEF5 Principles of momentum, heat and mass transfer (including the applications of these principles to the solution of problems involving flowing fluids and multiple phases)

 

E2 Ability to identify, classify and analyse the characteristic performance of chemical/environmental engineering systems and components through the use of analytical methods and modelling techniques.

 

CCEPPT1 and be able to apply methods to analyse the characteristics and performance of mixing, separation and similar processing steps (in particular the design and operation of a mixing reactor vessel).

 

P1 Understanding of and ability to use relevant materials, equipment, tools, processes, or products.

 

P4 Ability to use and apply information from the technical literature.

 

P8 Ability to work with technical uncertainty

 

 

  1. Description of Experimental Rig

The schematic of the packed column rig (UO7) is provided in Figure 1. The rig is composed of two acrylic sections joined end to end to give a total column height of approximately 1.4 m and is installed vertically on a mild steel floor standing framework. The column is packed with glass Raschig rings as detailed below:

 

Top/Bottom Column

Total height of packed section: 1.4 m

Diameter of packed section: 80.0 mm

Surface area               of wall:                         0.352 m2

 

Packing

Raschig rings (10 mm x 10 mm) 8917 pieces, 7 litre (total vol. of the packing material in the column)

Total surface area       of         packing:                     2.79 m2

 

  1. Safety

Before starting any experiments you will need to familiarise yourself with the experimental rig and also complete a Process Assessment. You will need to discuss this with the rig supervisor and your demonstrator before starting any work in the laboratory.

 

It is important that you familiarise yourself with the process assessment, layout and control on the rig before starting any experiments.

 

  1. Methodology: Experimental Design and Planning

You are required to investigate the hydrodynamics and mass transfer operation in the column for CO2 capture from air using water. The experiment will involve 2 principal stages:

 

Part 1. Determination of the loading and flooding point of both columns.

Ensure you are familiar with the concept of loading a flooding applied to packed columns. The academic and demonstrator in charge of the experiment will discuss these with you before you start. You should be aware of the visual as well as experimental techniques that can be used to determine both phases.

 

Experimental – Characterisation of the dry column:

  1. The first part of the experiment is to characterise the columns in a dry state. Make sure the column is dry before starting.
  2. Characterise dry column varying the flow rate of air through in turn. Use the manometer to determine pressure drop for the various flow rates. Ensure the manometer is set for the column you are working on (check valve positions). Plot the results on a log / log scale axes of the pressure drop versus flow rate as you do the experiments. To do this you will need to be able to understand the flow devices and the manometer to determine:
    1. The absolute pressure drop across the bed.
    2. The actual flow rate of air from the rotameter
  3. Take measurements across the range of possible air flow rates. It is important that you do not exceed the maximum flow rate.

 

Questions:

  1. What trend do you get from plotting this data and can you explain it?
  2. Have you accurately calculated the flow and pressure drop?
  3. Can you calculate the dry void fraction of the column? Does the data from the packings supplied earlier agree with your calculation?

 

Experimental – Determination of the loading and flooding points.

  1. Before conducting experiments in this part characterise the columns in a wet state. Make sure the water is completely drained after you wet the column before running experiments with wet column (no water flow).
  2. Prior to carrying out any experiments with CO2 it is important to determine the loading and flooding conditions for the column. Can you explain why?
  3. Using the range of flow rates you set/available for the air and water in the column determine a matrix of experimental conditions you think will be required to achieve this.
  4. Working with the different flow rates, measure and plot the pressure drop across the system. Adjust the flow rates gradually, and again ensure you reset the manometer between experiments. Try to plot these as you make the measurements, this will help you take more measurements close to the loading and flooding points.       Whilst doing so one member of the team should check for visual signs of changes in the flow regime.       Do this for both top/bottom column sections.
  5. Determine the loading and flooding point for a number of liquid and gas flows.

 

Questions:

  1. What is the best experimental strategy to get accurate data from the experiment?
  2. How do the pressure drop curves with the counter current liquid flow compare to those of the dry column?
  3. Have you determined the loading and flooding point of the column? Can you correlate the changes in flow regimes determined by pressure drop with the visual observations?
  4. Under which flow regime is it ideal to operate a packed column? Why?
  5. A lot of studies have tried to predict the flooding point of packed columns, for example Sherwood’s generalised flooding correlation. Why is this important?
  6. Plot the same data according to Sherwood’s generalised flooding correlation and insert Sherwood’s flooding line on your graph.       How does your data compare?

 

Part 2 – CO2 absorption in a packed column.

Once you have characterised the flow in the column and determined the loading and flooding conditions, design a set of experiments to measure the efficiency of the CO2 absorption. The key aim of this part of the work is to determine the number of transfer units (NTU) and the height of transfer units (HTU) for the column packing.

 

Experimental – Absorption in a packed column.

  1. Once you have a proposed experimental matrix, discuss this with your academic and demonstrator. This is to ensure suitable flow rates and CO2 concentrations (check max allowable CO2 amount in air for the rig) are being used.
  2. As this part of the experiment involves the use of CO2, safety is paramount at this point.
  3. Start-up: Set water flow to desired column. Turn up air flow gradually to required value. For absorption experiments, ask the demonstrator to turn on the CO2 cylinder valve when you are ready. Adjust air and CO2 flow rates only.       BE CAREFUL WHILE ADJUSTING THE CO2 FLOW RATE.
  4. To Shut down, it is important that you turn off the CO2 cylinder valve, water rotameter followed by the air. This is important to avoid any possible leaks into the laboratory.
  5. EMERGENCY SHUT DOWN.       Should you observe or suspect a leak from the rig, follow the following instructions:
    1. IF CO2 LEAK: Turn off CO2 cylinder if safe to do so. Evacuate area, warn other students on neighbouring rigs and inform demonstrator or technician immediately.
  6. Using your experiment matrix conduct a range of experiments to determine the CO2 absorption performance of the column. Ensure that you conduct each set of experiments, for example fixed liquid and air flow rates, by setting CO2 flow rate in air, from the same starting tank of water to ensure results are comparable:
    1. The initial concentration and mass flow of air can be determined from the rotameter readings.
    2. Analyse the exit gas stream by means of a gas chromatograph (or CO2 analyzer). Your demonstrator / technician will show you how to use this.
    3. It is also possible to determine the concentration of the CO2 in the liquid phase.

 

Questions:

  1. What is taking place in the column?
  2. Can you calculate a CO2 balance for each of the runs and conditions?
  3. From the data and measurements you have, what assumptions do you need to make to calculate the NTU?
  4. Calculate the NTU and HTU units for the column. How do your results compare with those from the literature?

References

  • Perry and Green Chemical Engineer’s Handbook P18-22 (6 th Ed 1984).
  • Coulson and Richardson Vol 2 P222 (5th Ed 2002)p(6s)

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