Fractional Distillation Column Temperature The fractional distillation column becomes cooler with height (because the heat source is at the bottom and) has different points at which substances are tapped but how do we know that the temperature at position Y is going to be X?
For example, fractional distillation of crude oil taps out several different substances, e.g. methane, gasoline, naphtha etc- how do we know the right points in the column at which the temperature is going to match their boiling points for them to condense there?
 A: That's where mathematical models of distillation columns come in.  These models solve the non-linear mass balance equations, energy balance equations, and phase equilibrium equations for both the liquid and vapor streams to and from each and every tray of the column, and for the reboiler and the condenser.  They include feed stream addition on indicated trays, and product removal from the condenser, reboiler, and indicated trays.  They also take into account reflux from the condenser to the top tray.  In the end, the models predict the temperature profile and species concentration profiles in both the liquid and vapor streams as a function of tray number.
Developing and applying such models is one way in which we Chemical Engineers earn our keep.  This falls within the realm of Chemical Process Modeling.
A: I'll add to Chester Miller's answer.  The equations that are commonly used to model crude distillation (usually, the MESH equations), assume equilibrium on each tray of the distillation column.  In actuality, crude distillation columns are run at their maximum throughput, so there isn't time to reach equilibrium in the column.  This means that the converged mathematical solution for such columns is an approximation.  Due to this, there are controllers at various points in the distillation column (e.g., on the product streams) that are continuously looking at things like temperature, boiling point, etc., and those controllers are adjusting flow rates in real time, such that issues like varying feed composition do not impact product specifications to a degree that would drive those products off-spec.
Now, regarding where products condense, reflux flow down the column is flowing counter-current to vapor that is going up the column.  These two streams continuously exchange heat with each other.  As a result, higher boiling components in the vapor stream condense as they cool down, and lower boiling components in the liquid stream boil as they heat up.  Assuming that the temperature that this occurs at is a "dependent" variable, the column pressure and the composition on each individual tray, determines the temperature at which these interactions occur.
