ENCH 445: Lecture 6 --  Staging of Separation Processes

There are two main reasons why staging is used in a separation process: (i) to amplify the separation achieved in a single stage (an example being a distillation column) and (ii) to increase the energy efficiency of a single stage process (an example being a multieffect evaporator).

In a multistage process, there are two choices concerning the flow of contacting streams: (i) cocurrent and (ii) countercurrent.  Generally, countercurrent is advantageous since it premits a product stream to approach equilibrium with a feed stream.   In a cocurrent process, the best performance possible corresponds to equilibrium between product streams.

Heat is generally supplied to chemical processes using steam.  One major reason for this is that steam heating is a constant temperature source while, e.g., electrical heating is a constant energy source.  If fouling of the heat exchanger surface occurs, the heating coils of an electrical heater will increase in temperature in order to supply a constant amount of heat while the temperature of steam heated coils remain constant.  The later situation is safer.

In order to calculate the heat transfer rate in a steam heating system, consider that the steam supply pressure and the temperature of the fluid to be heated is known.  Then the temperature in heating coil, the pressure in the heating coil, the steam flow rate (f), and the heat transfer rate (Q) can be determined using the following four equations:

               f = K_valve (P_supply - P_coil)                    (Relation describing the flow of steam across the  control valve)

               Q = h A (T_coil - T_fluid)

                T_coil  = g (P_coil)                                      (Equilibrium relation between T and P for saturated steam)

                Q = (Heat of vaporation of water) * f