ENCH 445: Separation Processes

     Professor Frey's Separation Processes WebBook

 


 

Chapter 6: Staging and Steam Heating in Separation Processes.

 

Reasons for Staging a Separation Process

 

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).

 

Cocurrent Versus Countercurrent Flow Between Stages

 

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.

 

Steam Heating in Separation Processes

 

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 a steam heated coil remains constant.  This means that steam heating is an inherently safer method to supply heat as compared to electrical heating.

 

 

Consider the calculation of the heat transfer rate in a steam heating system as shown in the above figure that uses a steam trap, and where steam is condensing on the inside (the tube side) of a tubular heating coil. It is assumed that the steam supply pressure and the temperature of the fluid to be heated on the outside (the shell side) of the heating coil are both specified.  It is also assumed that there is no pressure drop in the heating coil, but there is a pressure drop from the steam supply pressure to the pressure inside the heating coil across the steam flow control valve, and also a pressure drop from the pressure inside the heating coil to the pressure at the steam trap outlet (assume to be atmospheric pressure). Finally, it is assumed that the steam at the upstream side of the flow control valve is saturated vapor and the liquid that exits the steam trap is saturated liquid water.

 

Under the conditions just described, the temperature and pressure of the condensing steam inside the heating coil (T_coil and P_coil), the steam flow rate (f), and the heat transfer rate (Q) can be determined by solving simultaneously the following four equations:

 

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

                                                                   in terms of the valve constant K_valve)

 

Q = h A (T_coil - T_fluid)                         (Heat tranfer relation using a heat transfer coefficient)

 

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

 

Q = (Heat of vaporation of water) * f        (Determination of heat transfer from steam energy balance)

 

 

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