Enriching section operating line intercept at origin in McCabe-Thiele method

The McCabe-Thiele method is a graphical technique used in chemical engineering to analyze and design distillation processes. It involves constructing a graph known as the McCabe-Thiele diagram, which helps determine the number of theoretical stages required for separation.

In the McCabe-Thiele diagram, the enriching section operating line represents the relationship between the liquid and vapor compositions in the enriching section of the distillation column. The intercept of this operating line at the origin (0,0) on the graph is of particular interest.

Let's analyze the intercept at the origin for different reflux ratios:

a) Reflux ratio = Zero
When the reflux ratio is zero, there is no reflux of liquid from the condenser back to the column. This means that the liquid leaving the condenser is entirely removed as the distillate. In this case, the intercept of the enriching section operating line at the origin would be at the point (0,0). The entire feed mixture is vaporized and removed as the distillate without any liquid leaving the column.

b) Reflux ratio = Infinity
When the reflux ratio is infinity, it means that an infinite amount of reflux liquid is returned to the column. This results in an operating line with a slope of 1, passing through the point (0,0). In this case, the intercept of the enriching section operating line at the origin is at the point (0,0). This indicates that all the liquid leaving the condenser is returned to the column as reflux, and no distillate is obtained.

c) Reflux ratio = One
When the reflux ratio is one, it means that the amount of reflux liquid returned to the column is equal to the amount of distillate removed. The operating line in this case will have a slope equal to the relative volatility of the key components being separated. However, the intercept of the enriching section operating line at the origin will not be at (0,0). It will be at some point above the origin, indicating that a certain amount of the feed mixture is vaporized and removed as the distillate.

d) Reflux ratio = 1.5 to 2 times Reflux minimum
When the reflux ratio is between 1.5 to 2 times the reflux minimum, the intercept of the enriching section operating line at the origin will be slightly above the point (0,0). This indicates that a small amount of the feed mixture is vaporized and removed as the distillate, while the majority of the liquid leaving the condenser is returned as reflux to the column.

Conclusion
Among the given options, the correct answer is option 'B' (Reflux ratio = Infinity). When the reflux ratio is infinity, all the liquid leaving the condenser is returned to the column as reflux, resulting in an intercept of the enriching section operating line at the origin (0,0). This means that no distillate is obtained in this scenario.