Free Report On And Materials Engineering

PEME2010

Summary
The experiment is meant to study the carbon dioxide absorption process in a packed tower mechanism column. The pressure performance of a packed column was determined experimentally. The calculations shoed that the pressure difference between the top and bottom part of the column is directly proportional to air flow rate; as the water flow rate increases, the pressure difference increases as well. However, at high water and air flow rates, the column flooding appears. The rate of carbon dioxide absorption was calculated. It was determined, that absorption increased carbon dioxide concentration by 80%.

Introduction

Gas absorption is an industrial process used for separation or recovery of gas mixture components. During absorption, gas stream is placed into contact with water or other liquid. The gas components are transferred to the liquid phase via physical or chemical interaction. In other words, the gas components either dissolve in liquid (physical absorption) or react with it (chemical absorption) (Yildirim et al. 2012). The absorption processes are widely applied in chemical, cellulose, petrochemical and gas industry, metal and packaging industry, for environmental protection purposes. Both organic and inorganic substances can be separated by absorption. Hydrocarbons, ethylene oxide, acrylonitrile, hexane, triethylamine, etc. are often concentrated or removed from gas by absorption. The large-scale processes involving inorganic components are carbon dioxide, hydrogen sulphide, hydrogen chloride, ammonia, and nitrous gases recovery (Chattopadhyay 2007).
Since absorption is limited by mass transfer kinetics, engineering science aims to improve kinetics rate. In most cases, this is realized with advance in absorption units. Depending on phase flow types and interaction, the absorption apparatus form the following groups: continuous interaction of phases, gas dispersion in a continuous liquid phase, and liquid phase dispersion in a continuous gas phase. Various reactive units have been developed for each group, namely packed columns, laminar jet absorbers, plate, bubble, spray columns, Venturi scrubbers (Sundmacher et al. 2005).
In packed towers, the liquid phase flows through the packing material surface, increasing the surface of gas – liquid contact. The column is referred as 'packed' because it is filled with Raschig rings packing material. Raschig rings are pieces of tube with length equal to their diameter.
The laboratory work aims to illustrate the theory of absorption in a packed tower. It consists of two experiments and involves theoretical calculations. The experiments are hydrodynamics study and determination of the carbon dioxide absorption rate.

Methodology

Equipment
The gas absorption tower used for the experiment is a 75-mm column filled with Rascheg rings. Figure 1 presents the experimental setup: 1 – packed column with Raschig rings, 2 – cylinder with carbon dioxide, 3 – water sump tank, 4 – pressure manometer, 5 – water flowmeter (F1) and water control valve (C1), 6 – carbon dioxide flowmeter (F2) and carbon dioxide control valve (C2), 7 – air flowmeter (F3) and air control valve (C3), 8 – volume indication tube, 9 – sodium hydroxide globe.
Figure 1: Absorption unit used for the experiment.
The column is equipped with pressure taps to determine pressure at the top, centre and base of the column. The same sections are used for gas sampling. Rate of water absorbent is measured by a flowmeter (F1). Flowmeters F2 and F3 are used to measure air and carbon dioxide rate, respectively. The valves C1-C3 control rate of water, carbon dioxide and air. Carbon dioxide is mixed with air at a pre-determined variable ratio. The gas mixture is fed to the base of the tower, and the effluent gas is exhausted from the top of the column to atmosphere.

Experiment 1. Hydrodynamic test

The water flow rate was set to reach 3 l/minute; in 30 seconds, the pump was switched off and C1 valve was closed. The column was left for about 5 minutes to drain. Then, the packed tower was applied water and air flow rates of 1.5-4 l/min (interval 0.5 l/min) and 20-160 l/min (interval 20 l/min), respectively. The pressure difference readings were measured at the top and bottom of the column at different ait and water flow rates. It should be noted that the column performance has to be monitored as the pressure readings cannot be obtained at column fluctuation (bubbling).

Experiment 2. Carbon dioxide absorption rate determination

The absorption analysis globe 8 and 9 were filled with 1 M solution of sodium hydroxide. The level of the globes was adjusted to zero mark on the indication tube (drain valve of the globe 9 was used).
The sump tank 3 was filled with water at closed control valves C1 and C3. The water pump was started to reach 6 l/min flow through the column (F1) by adjusting valve C1. Then, the compressor was run to get air flow of 30 l/minute (F2), control valve C2.

The pressure valve of the carbon dioxide cylinder 2 was opened (C3), and 50% of the F2 flow was set at F3.

The column was left for 15 minutes to reach steady operation. The gas samples were taken simultaneously from points S1 and S2 and analysed for carbon dioxide content.
The sucking from the sample line was repeated to flush the sample line with the gas piston. The contents of the cylinder was expelled to atmosphere. The atmosphere vent and absorption globe were closed, the cylinder was filled and left for two minutes to allow gas to come to the cylinder temperature.
The cylinder was isolated from the column and absorption globe and vented to atmospheric pressure for 10 seconds. Then, it was attached to absorption globe. The liquid level was monitored to stay the same. After the indicator tube had set to zero level (pressure in the cylinder became atmospheric) the piston was closed to empty the cylinder into the absorption globe, and then it was drawn out again until no significant changes occurred. The indicator tube readings were written down as volume of the sampled gas.

Safety notes

Gloves and safety goggles have to be word when dealing with sodium hydroxide.
The carbon dioxide cylinder valve has to be opened cautiously.
If carbon dioxide concentration exceeds 8%, the liquid might be suck into the cylinder. This will destroy the experimental set up.

Results

Experiment 1. Hydrodynamic test
The pressure readings were recorded for various air and water flow rates. The raw data were recorded in mm and are presented in Table 1. Table 2 presents the same data converted to standard CI measurement units (m). The conversion was performed as the following: Pressure difference (m) = Pressure difference (mm) / 1000.
For example, the pressure readings at air flow 20 l/min and water flow of 1.5 l/min was 14 mm, and after conversion it is 0.014 m.

The data obtained in Table 2 were used for calculation of pressure differences in Pascal, using the formulae.

*Note: F stands for column flooding of the column due to high water flow rate.

Subsequently, the pressure difference equation was used to calculate the pressure in Pascal, and this was done as following (Coulson et al. 2009):
∆p=ρg∆h,
where Δp is pressure difference, Pa; ρ is the column density, kg/m3; g is the constant gravitational force, m/s2, Δh is the experimental pressure difference, m.

Since Δp and g are constant for the experiment, we their product can be calculated to obtain a conversion factor:

Conversion factor = Δp ∙ g = 784 ∙ 9.81 = 7691.04 (the units are omitted since they have no physical sense in this case).

Hence, the pressure readings in Pa can be calculated as:

Pressure difference (Pa) = Pressure difference (m) ∙ Conversion factor.
For example, the pressure readings at air flow 20 l/min and water flow of 1.5 l/min was 0.014 m, and after conversion it to Pascals gives:
Pressure difference (Pa) = 0.014 ∙ 7691.04 = 107.67 Pa.
Thus, multiplying the data in Table 2 by 7691.04, the pressure difference data in Pa are obtained. The data are presented in Table 3.

The Table 3 data were plotted as a graph of air flow rate as a function of pressure difference (Figure 2).

Figure 2: The relationship between air flow rates and pressure differential values for different water flow rates.
The logarithmic functions were taken of pressure readings data and air flow rate readings (Table 4). For example, the pressure readings at air flow 20 l/min and water flow of 1.5 l/min was 107.67 Pa, and the respective logarithm value is log (107.67) = 2.03. It should be noted, that only pressure difference and air flow were converted to logarithmic scale, but not the water flow (it is not necessary).

The logarithmic graph is presented on Figure 3.

Figure 3: The relationship between the logarithms of air flow rates and pressure differential values for different water flow rate.
Experiment 2. Carbon dioxide absorption rate determination
The air and carbon dioxide flow rates were set at 30 l/s and 6 l/s, respectively. At the top of the column, carbon dioxide sample was measured as 5 ml (V2), while the volume of the gas was 35 ml (V1). Table 5 presents the raw data obtained in the experiment.

Discussion

Experiment 1. Hydrodynamic test
The processed pressure readings are difference between pressure values at the top and bottom of the column. Pressure difference is the efficiency factor of absorption: the higher difference in pressure, the higher the absorption capacity of an absorbent, and thus the more efficient is the overall process. This happens because the gas solubility and reactivity in water increases at high pressure.
The data obtained from the experiment were in mm, and they were converted first to m, and then to Pascals with application of the physical law (Coulson et al. 2009).
The processed data of hydrodynamic test (Table 3) show that the pressure difference increases with increase of air flow rate. Comparably, the pressure difference increases with increase of water flow rate. This can be graphically observed in Figure 2 (flow rate – pressure dependence) and Figure 3 ( logarithm flow rate – logarithm pressure dependence). The dependences are well fitted with straight lines. Thus, there is direct proportional dependence between the flow rate and pressure dependence.
There are some fluctuations for flow rates of 1.5 and 2 l/min which are caused by errors in manometer reading. At high rates of water and air flow (namely, 4 l/min and 100 l/min, 3.5 l/min and 120 l/min,  3 l/min and 160 l/min) the column flooding is observed. Thus, these parameters cannot be applied for absorption process.
Experiment 2. Carbon dioxide absorption rate determination

The mole fraction of carbon dioxide at the inlet was determined using the following equation (Coulson et al. 2009):

y1=F3F2+F3.
Thus, y1=630+6=0.1667.

At the outlet, the mole fraction of carbon dioxide was calculated:

y0=V2V1.
During the experiment, the values of V1= 35 ml and V2=5 ml were obtained. Therefore,
y0=535=0.1428.

The carbon dioxide absorption rate was calculated (Coulson et al. 2009):

Fa=(y1-y0)(F2+F3)(1-y0).

The experimentally obtained absorption rate:

Fa=(0.1667-0.1428)(30+6)(1-0.1428)=1.004 l/min.

The results of calculations are presented in Table 5.

Carbon dioxide absorption rate was 1.004 l/min (Table 6), and this shows that the gas was absorbed significantly, mainly by 80%. The experimental results and calculations prove the efficiency of packed column application. The Raschig rings play a significant role in absorption process. They increase the surface of contact between water absorbent and gas flow. Therefore, the mass transfer kinetics advances and the process efficiency improves. This fact causes the extend application of the packed columns and constant interest to their improvement (Yildirim et al. 2012).

Conclusions

The experimental study of carbon dioxide absorption in a packed column was carried out. There were two set of experiments: hydrodynamic test and absorption rate determination.
At first experiment, the pressure difference between top and bottom of the column were measured. Then, with theoretical calculations, they were transformed to standard CI pressure units (Pascals) and plotted as a function of air flow rate. Thus, the curves were obtained for each water flow rate.
The hydrodynamic test showed that the values of pressure difference were directly proportional to air flow rate, or the increase of air flow rate caused the increase in pressure difference. The same way, the pressure difference increased with increase of water flow rate at fixed air flow rate. Although in general the logarithmic pressure difference as a function of logarithm of air flow could be approximated with a straight line, some pressure fluctuations appeared. In addition, high water and air rate caused column flooding, therefore the process cannot be run at these values.
The determination of carbon dioxide absorption rate is mainly the matter of calculations. The feed data are water, air and carbon dioxide rates, and gas volumes at the inlet and outlet. The absorption of carbon dioxide was estimated as 80%.
Therefore, the absorption in a packed column is and effective method for separation of gas components, and particularly carbon dioxide, as was studied in the present laboratory work.

Reference List

Chattopadhyay, P, 2007, Absorption & stripping, Asian Books: New Delhi.
Coulson, JM, Richardson, JF, & Sinnott, RK 2009, Chemical engineering, Pergamon: Oxford.
Sundmacher, K, Kienle, A, & Seidel-Morgenstern, A 2005, Integrated chemical processes: synthesis, operation, analysis, and control, Wiley-VCH: Weinheim.
Yildirim, O, Kiss, AA, Hüser, N, Leßmann, K & Kenig, ET 2012, ‘Reactive absorption in chemical process industry: A review on current activities’, Chemical Engineering Journal, vol. 213, p. 371-391.