Study of a sieved tray column for methanol water distillation

 

Study of a sieved tray column for methanol water distillation

 

 

1. Introduction

Multistage distillation is one of the most important techniques used in industrial processes for the separation of components in a liquid mixture. Although the design and operation procedures in commercial distillations are well established, in situations when vapour-liquid equilibrium, azeotrope formation and other process data are uncertain, a laboratory or pilot plant study is necessary before the full scale design. In addition, laboratory scale distillation columns are needed to provide adequate practical training for student engineers and plant operators in a safe environment.

In this experiment, you will study the behaviour of a column operating with a water-methanol binary mixture. You will operate a batch sieved tray column at total reflux. Given that no product is withdrawn from the column during operation, there is no need for differential calculations and the same principles as for continuous distillation can be applied.

Objectives

1. To study the variation of column pressure drop with vapour velocity;

1. To determine the variation of overall column efficiency with vapour velocity;
2. To identify the occurrence of extreme, far from equilibrium conditions (weeping and flooding).

2. Health, safety and environment (HSE)

The distillation column was designed to be safe when operated according to the instructions. The column should only be operated by Yr2 students in the presence of a demonstrator or a supervisor.

Before starting the experiment, you should read the process assessment and:

– identify the hazards associated with this experiment;

– identify the risks associated with these hazards;

– identify the precautionary measures that are in place in order to minimise the risks

Summarise these in a table your report (qualitative HSE assessment).

3. Theory

Performance data of distillation columns are generally measured at conditions of total reflux, to eliminate fluctuations from steady state (due to feed, or to withdrawal of the top product).

For a high efficiency tray: (1) vapour flows only upwards, through the open regions of the tray between downcomers; (2) liquid flows flows downwards from tray to tray only through the downcomers; (3) liquid never weeps through the tray perforations, nor is carried by the vapour as entrainment to the tray above; (4) vapour is never carried down by the liquid in the downcomer, nor allowed to bubble through the liquid in the downcomer. The tray therefore operates in a stable region of vapour and liquid flowrate (Kister HZ, Distillation design, McGraw-Hill, New York 1992). The efficiency will vary with both vapour and liquid flowrate.

The total pressure drop across each tray is the sum of that caused by the restriction of the holes in the sieve tray, and that caused by passing through the liquid (foam) on top of the tray. As the velocity of the vapours passing up the column increases then so does the overall pressure drop. Under conditions with no liquid present, the sieve trays will behave like an orifice in that the pressure drop will be proportional to the square of the velocity. Due to the fact that there is a liquid head however, this square relationship does not become apparent until the head of liquid has been overcome and foaming is taking place. In a graph of pressure drop vs. vapour flowrate rate (log/log), at low flowrates the pressure drop will remain fairly constant until foaming occurs when the pressure drop would be expected to rise sharply for unit increases in vapour flowrate.

Knowing the compositions of the top and of the bottom products and the equilibrium data for the two components, the theoretical number of stages at total reflux can be determined by using the McCabe-Thiele diagram (see H82SP1 notes).

4. Experimental procedures

1. Equipment

The UOP3BM is a batch distillation equipment. Identify the following constituents:

– the 50mm diameter column (with sieve trays);

– the insulated reboiler, containing a flameproof immersion type heating element

– the coil-in-shell condenser, using water as coolant

– the decanter that collects the condensed vapour. With valve (V10) open, condensate from the condenser outlet passes directly through the decanter to the inlet of the reflux ratio control valve.

– thermocouple sensors, used to monitor temperatures at strategic positions in the system:

T5 = 5th tray = top tray of the bottom glass section

T6 = 6th tray

T7 = 7th tray

T8 = 8th tray = bottom tray of the bottom glass section

T9 = temperature of liquid in the reboiler

– the U-tube manometer indicates the total pressure drop across the column. Water is used as a liquid.

1. Methodology
2. The column should be operating at atmospheric pressure under total reflux. Start-up was performed by the demonstrator prior to your arrival and the reboiler should be set at 0.8 kW. The reboiler contains a methanol-water mixture.
3. Identify the sieve plates and their different areas: weir, downcomer and bubbling area. A non-operating column is available for inspection (handle with care!)
4. Toggle through the temperature readings on the control console; identify and record the temperature at each stage (T5-T9).
5. Describe the appearance of the liquid-vapour mixture on the top tray (T5) by the degree of foaming on the trays (i.e: liquid weeping, no foaming, gentle localised, violent localised, gentle over the whole tray, violent over the whole tray, liquid flooding).
6. Measure the liquid boil-up rate (mL/s) by performing a timed volume collection: Operate valve V3 so that all the condensate is diverted into a measuring cylinder. First take a small discarding sample into an alternative vessel (to drain the condensate feeding pipe), then divert the condensate into the measuring cylinder and use the stopwatch to measure the time required to collect a set measurable volume. ATTENTION: SAMPLE MAY BE HOT!
7. Use a refractometer to determine the refractive index of the collected methanol-water mixture at the top of the column. Measurements should be taken at room temperature.
8. Take readings of the pressure drop over the trayed column section by opening the valves V6 and V7 on the manometer. When opening the valves, make sure always to open valve V6 then V7 to prevent vapour from the column entering the manometer. Close the valves in the same order after taking the pressure drop reading. Repeat these readings three times, waiting 2-3 minutes between each measurement.
9. Adjust the reboiler power to 0.5 kW (on the control panel). Wait for minimum 20 min to reach equilibrium (no variation in temperatures is seen at equilibrium). Repeat steps 3-7.
10. Repeat steps 3-7 for powers 0.65 kW, 0.95 kW and 1.10 kW (wait for 10 min to reach equilibrium after adjusting the reboiler power).
11. Results
12. Tabulate your results under the following headings:

Power (kW)	T9	T8	T7	T6	T5	Boil-up rate (L/h)	Pressure drop				Degree of foaming on T5
							1	2	3	Average
0.50
0.65
0.80
0.95
1.10

 

2. From the temperatures, estimate and tabulate liquid and vapour compositions at T5, T8, T9, assuming equilibrium conditions. At which reboiler duties is this assumption valid?
3. From the refractive index and using the calibration curve provided, determine the molar fractions of methanol and of water in the 5 mixtures collected at the top of the column.
4. For each of the 5 reboiler duties, use top and bottom compositions to draw the McCabe-Thiele constructions, then identify the number of theoretical stages and the overall column efficiencies.

Hints:

– Identify which compositions you will use as column top (x_D) and bottom (x_W);

– Consider whether the reboiler should be included in the calculation of column efficiency.

5. Using the volumetric liquid boil-up rate (V in L/h) and the molar compositions at the top of the column, determine the vapour velocities (V’ in m/s) inside the column, for each of the 5 reboiler duties.
6. On separate graphs, plot the overall column efficiency, the average pressure drop and the top product composition in function of the vapour velocity (plot log DP vs log V’). On the graphs, identify the regions where extreme conditions occur (flooding, weeping). Identify the optimum vapour velocity and reboiler duty for this column.
7. Discussion

Compare compositions estimated by using the temperature at the top of the column, and by using refractive index measurement of the collected sample. Discuss any differences observed, and justify the values of x_D and x_W chosen to draw the McCabe-Thiele diagram and to perform calculation of V’.

Describe the relationship between vapour velocity and pressure drop and compare with expected variations for sieved trays.

Describe the relationship between vapour rate and overall column efficiency and explain why the column efficiency decreases above and below the optimum boil-up rate value. Hint: relate to the weeping / foaming / entrainment / flooding situation on the trays.

7. Conclusions: Summarise your findings.
8. References

Attached: T-x-y and x-y diagrams for methanol/water at 1 atm; calibration curve refractive index vs mol percent methanol in the methanol / water mixture (at room temperature).

Produce a maximum 10 page-report