Concentrated Liquid Mixture - Essay Example

This report aims to separate a concentrated liquid mixture of acetic acid and water. The feed of the liquid mixture is 3500 kg/hr (107.33  /hr) which has 35 mol per cent acetic acid. At the top of the column, the mixture of water compositions (90 mol per cent) collects at a rate of  /hr. Whereas, at the bottom compositions of acetic acid (98 mol per cent) collects at a rate of  /hr. The optimum reflux ratio is 1.90 and the boil-up ratio is 7.32.

The column efficiency is at 57% and the optimal number of trays required to separate the mixture is 30, with 9 trays required above and 20 trays required below the feed.  The feed plate is the 10th tray from the top of the column. The height of the column is 21m above ground level. The minimum number of trays required at total reflux is 18. The column diameter in the enriching section is 1.005 m and the column diameter in the stripping section is 1.39 m. The below feed is bigger than the above with an average column diameter. The biggest diameter selected 1.39 m was an estimation of the overall column diameter.

The type of tray used in this distillation design was a Sieve tray. It singly passes liquid flow across. The tray is 5 mm thick and spaced 0.45 m apart. Every tray contains 5888 active holes with a diameter of 5 mm in each at a hole pitch of 13.5 mm. The pressure drop across each tray is approximately 1.22 kPa, resulting in a total pressure drop down the column of 36.6 kPa. This is insignificant and the total column is considered to be operating at the atmospheric pressure. The saturated feed enters at 103˚C while temperatures at the top and bottom of the column are 100.86˚C and 117.48˚C, respectively.  The total condenser used is a double pipe heat exchanger with a duty of 2417.85 kW while a horizontal thermosyphon partial re-boiler with a duty of 1495.32 kW was chosen.   

The method used in designing the distillation column is the McCabe-Thiel Method and the design method outlined in Coulson and Richardson. An assumption of constant molar overflow (CMO) is made in this design. However; it was not an accurate assumption to make as it results in a significant error in the calculations. A summary of the key findings of the design is illustrated in the flow diagram on page 2.

6.0 Limitations

6.1. Using the constant K1 obtained from figure 11, the following restrictions apply:

6. Hole size less than 6.5 mm. Entrainment may be greater with larger hole sizes.
7. Weir's height is less than 15 per cent of the plate spacing.
8. Non-foaming systems.
9. Liquid surface tension is 0.02 N/m, for other surface tensions multiply the value of K1 by [/0.02]2.
10. Hole: active area ratio greater than 0.10; for other ratios apply the following corrections:

Hole: active area multiplied K1 by

           0.1                               1.0

0.08                            0.9

0.06                            0.8

6.2. The reflux calculations are limited by the equilibrium data and on the assumption of CMO. The optimum reflux ratio is selected in correspondence to the lowest operating cost of the column.  Hence if either of these assumptions is incorrect or the true optimum reflux ratio deviates significantly from the chosen R = 1.62, there is a possibility of the column failing to function.

6.3. When there is a drop below 27% in the minimum vapour flow rate, weeping will occur.

6.4. The reflux should be controlled and kept as close as possible to a saturated liquid.

6.5. Condenser and re-boiler duties were calculated assuming 100% efficiency. This cannot be assumed and therefore a more comprehensive analysis should be done as it will be much higher and more costly to run.

6.6. The diameter at the top of the column was found to be smaller than at the bottom. The bottom diameter was selected and the nearest standard pipe size (NPS STD 44) was found and used in further calculations as an estimate of the overall column diameter.

The construction of the plate design was based on a trial-and-error approach. Its design was modelled at the beginning by using certain values. The assumptions made helped in the improvement of the design plate.

3.5.1. Types of columns

In order to get the best efficiency of the separation of components, it is important to maintain a high level of contact between the vapour and the liquid phases. The use of a packed bed enhances the contact between two different phases.

3.5.2. Types of trays

In the distillation design, three types of trays are considered. These are the Sieve plates, Bubble-cap plates, and Valve plates. The following is a brief description of each plate.

Sieve plate:

Here, the vapour passes up through perforations in the plate and the liquid is recollected on the plate by the vapour flow. At low rates, the liquid will weep through the holes as the vapour liquid has no positive seal (figure12)

Figure 12: Sieve plate.[7]

Bubble-cap plates:

A Short pipe is used to allow the vapour to pass up which is called a riser that is concealed by a cap with serrated slots. Risers ensuring that the level of liquid is remaining the same on the tray despite any change in the vapour flow rates are used. (figure13).