Distillation is one of the most common types of separation techniques employed in the oil and gas, chemical and petrochemical industries. A good column design means that the desired products are separated effectively and with the minimum consumption of energy. This guide will give an overview of the major considerations that one has to make when designing a distillation column.
1. Understanding the Basics of Distillation
Distillation is a process of separating the components of a liquid mixture by utilizing the differences in boiling points of the various components. The liquid mixture is vaporized to the top and the remaining liquid or bottom product is being flowed downward in the column. This process goes on in stages where the components with higher volatility will evaporate on the surface while those with lower volatility will condense at the bottom.
2. Types of Distillation Columns
There are two primary types of distillation columns: tray columns and packed columns.
- Tray Columns: Are composed of several trays in which vapor and liquid phases are at equilibrium. These are suitable applications involving high liquid and vapor flow rates and are applicable in large scale manufacturing plants.
- Packed Columns: Add packing material so as to enhance the area of contact between vapor and liquid. They are more effective in the industries that involve relatively less complex operations such as the pharmaceutical or the specialty chemical industries.
The decision between a tray or packed column is based on factors such as throughput, efficiency or operation conditions.
3. Column Diameter and Height
The two most important geometrical characteristics that determine the column’s performance are its diameter and height. The diameter is dependent on the vapor flow rate and the height is dependent on the number of stages that the separation will require.
- Column Diameter: If it is too small, then pressure drops and flooding become a problem. On the other hand, a large diameter is inefficient and costly in terms of capital expenditure.
- Column Height: The height depends on the number of theoretical trays which is a number of vapor-liquid equilibrium stages. Thus, more stages offer better separation but at the same time produce a taller and more costly column.
4. Operating Pressure
The operating pressure inside the distillation column has a direct impact on the capital cost as well as the operating cost. The pressure is reduced to allow the boiling point of the components to be lowered for separation, but it implies that large equipment is required. It is likely that high pressure operations will necessitate the use of more costly materials that can handle it.
5. Feed Composition and Location
The location where the feed is introduced into the column determines the success of the process efficiency. Placing the feed at the right location means that it enters at the point of balance between vapor and liquid phases. The feed introduced at a wrong level disrupts the balance of the equipment and decreases the separation efficiency.
6. Energy Considerations
Energy consumption is high in distillation hence energy conservation is a major consideration in distillation design. Some of the heat integration include using the heat from the bottom product to heat the incoming feed thus reducing energy consumption.
7. Material Selection
The materials used in construction of the column must be able to withstand the operating temperatures, pressure and chemical makeup. Carbon steel is suitable for the low-pressure and non-corrosive processes while stainless steel is suitable for high pressure and corrosive processes.
Conclusion
Designing a distillation column involves a number of factors such as type and size of the column, pressure, feed composition and materials of construction and all these have to be achieved while considering energy efficiency and cost. Optimization of the design not only improves the efficiency of separation processes, but also minimizes operating expenses.
