Reflux is a distillation technique in which a portion of the condensate is returned to the distillation vessel from which it originated. This process is used to maximize the purity of the products taken off of the still.
Types of reflux
Reflux in a system can either be passive reflux or forced reflux. Passive reflux is reflux that arises because of the heat loss of the system to the environment. Energy from the distillate is transferred to the column and out to the environment, causing the distillate to condense. The amount of passive reflux in a still can change based on environmental factors such as the temperature of the room or wind.
Reflux still designs almost always use forced reflux. Forced reflux is the process of using a condenser in the still to send distillate back down the column. The exact location of the condenser is dependent on the type of reflux still design. See Liquid management, Vapor management, or Cooling management for more information on reflux still designs.
Effect of reflux on product purity
Each time the product is condensed and sent back down the column, in effect another distillation has taken place on the distillate. With a pot still, each batch is only a single distillation, so the product is still relatively impure. The number of distillations that occur in a column can be approximated by the number of theoretical plates in that column.
A theoretical plate is a zone where the liquid and vapor phases of the distillate establish an equilibrium. The more theoretical plates in a still, the more pure of a product it can produce. Plates can either be physical plates or trays, such as bubble caps, or they can be a packing material, such as steel or copper mesh. In the case of mesh, the Height Equivalent to a Theoretical Plate (HETP) is based on the the physical characteristics of the packing material and can be calculated using the Van Deemter equation. To calculate the amount of theoretical plates in a column, divide the height of the column packing by the height equivalent to a theoretical plate of the packing.
The amount of theoretical plates and column performance can be calculated here.
The efficiency of your column is related to both the height and diameter of the column. Since the height controls the number of theoretical plates, it determines the purity of the product. The column diameter determines the amount of vapor and reflux that can be traveling up and down the column at any time, and therefore determines the maximum speed that product can be produced.
Column diamneters should range between 2" and 4". Column height is best determined as a ratio with column diameter. The absolute minimum height:diameter ration would be 12:1. Around 20-25:1 would be ideal and above 30:1 the environmental effects of cooling on the column will impact power efficiency.
The reflux ratio is defined as the ratio of the boil-up rate to the take off rate. If 500mL of product was condensed back down into the pot and 100mL of product was taken off, this would be a reflux ratio of 5:1. If all product was taken off the still and no product was condensed back down into the pot, this would give a reflux ratio of 0:1. If no product is taken off, this is called total reflux.
The higher the reflux ratio, the more vapor/liquid contact can occur in the distillation column, so higher reflux ratios usually mean higher purity of the distillate. It also means that the collection rate for the distillate will be slower.