Assignment Task

Introduction Design assumptions and Given values

Distillation is a practice widely used in industry to fully or partially separate liquids with different boiling points. The working principle is that the liquid with the lower boiling point will vaporise first, and rise to the top, where is it collected and condensed back into liquid. This is processes that does not require any moving parts, only a source of heat input. Systems can be designed to operate continuously, or just for smaller batches. Batch designs would be more versatile but less efficient, while continuous would be designed to run for days or weeks at a time, with very little variation. Components in a distillation system include a reboiler, used to boil the mixture, and create vapour to rise through the distillation column.

The column is where the separation occurs, using sieve plates or packed cages to expose the vapour to as much un-vaporised liquid as possible. The condenser then removes enough heat from the enriched vapour so it liquifies. From the condenser the product then travels through the reflux valve. A portion goes back into the distillation column to be re-processed, while the rest is the final product.

The ratio at which this separate is called the reflux ratio, which plays a role in determining the purity of the final product. The more product that is re processed, the higher the purity will become, but this takes more time and energy. When designing the Distillation plant, the most important factors are running efficiency and build cost. Measures will be taken to ensure a cost-effective solution. This is done through an iterative approach in finding the sufficient device geometry. The Plant will be designed to process an aqueous solution of 12% ethyl alcohol concentration and achieving a final product of 78.5% purity. In this situation, ethyl alcohol has a lower boiling point then water at 78.3℃. The ethyl alcohol will therefore boil first and rise to the top of the column, becoming the top product. This purity has been selected as it is relatively easy and economical to achieve. Higher purity requires much more cycling through the system and therefore use a lot more energy and time, requiring a larger system for the same volumetric output.

The system will be designed to operate continuously which increases efficiency and decreases costs and maintenance requirements. Additionally, the distillation column will be insulated, to reduce heat losses by 97%. This further reduces running costs, and in turn is more environmentally friendly. Other energy saving measures include mounting the condenser above the reflux inlet on the distillation column. This eliminates the need to run a pump in the reflux line.

Condenser description

The distillation plant uses a shell and tube design condenser, with a 1 shell pass and 4 tube pass configuration. The shell end caps have been designed to accommodate this and are seen in appendix 7.1. The tubes are arranged in a triangular configuration with a pitch of  At each end of the shell are floating split ring caps. The tubes are made from copper alloy for its high conductivity and are 20 mm diameter with 2 mm wall thickness. The tubes are horizontally orientated. There are 5 baffles inside the shell, with varying spacing. The cool end of the shell having the smallest spacing, and towards the hot side larger spacings. The condenser mounts are tilted at a gradient of 1/100 towards the shell outlet, to assist in drainage of condensate. The tube side pressure drop is within acceptable limits at 18.15 kPa. The overall heat transfer coefficient of the condenser.

Reboiler design

The distillation column will be heated by a pool boiling style of reboiler. A tube bundle immersed in the feed liquid at the base of the column will be used as the hot surface. A Split ring floating head style at the tube ends has been adapted, the same way it has been in the condenser. Saturated steam at 128 ℃ will run through the tubes, providing the heat input though dispelling its latent heat of condensation. The steam will make 4 tube passes. The tubes are arranged in a triangular configuration with a pitch of 1.25 x Tubeod. The tubes are made from copper alloy for its high conductivity and are 20 mm diameter with 2 mm wall thickness. As the in-tube steam is driven by a pressure of 2.54 bar, horizontal tube orientation can be utilised, making the integrated column/reboiler geometry possible. The Actual flux of the boiling pool is within the critical flux limit, by roughly 30%. The overall heat transfer coefficient of the reboiler is 905.86 W/m2c.

Distillation column description

The distillation column’s purpose is to perform the key process of feeding the required concentration of ethyl alcohol concentration towards the top product outlet, while directing the undesired products towards the bottom effluent outlet. In this design this is done using sieve plates and downcomers. Hot vapour bubbles through the sieve plates, taking with it more of the liquid with a lower boiling point. The downcomers allow excess liquid to move to lower stages to be reprocessed.

The column has 6 rectifying plates and 7 stripping plates, all spaced 500 mm apart. The equilibrium chart for determining the number of plates can be seen in appendix 3. The plates are 1742 mm in diameter, with vapour traveling through the column at a velocity of 2.97 m/s. The downcomers are designed so that their outlet is submerged in a pool on the lower plate, preventing the rising vapour from bypassing the sieve plate bubble holes. The total height of the column is 9.5 meters. This includes 1.5m of service space above the top plate, and 2 meters below the bottom plate.