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Operating and Simulation Modes

Modes of Operating Distillation Columns

The HYSYS system can be used to model and simulate almost any operating configuration that can actually be attained with the distillation columns in the UF Unit Operations Laboratory. 

• Continuous (steady state and transient) 
• Batch (a special form of transient) 
• Semi-batch (feed to the reboiler for a while) 
• Total reflux (steady state and transient) 
• Distillate-to-Pot (steady state and transient) 

A few comments on the modeling and simulation of the operating modes are given below.

Continuous Steady State Operation

 In Continuous operation you may specify up to one feed and drawoff per stage.  A drawoff may be liquid or vapor. A drawoff CAN NOT be returned directly to the column as a feed stream. 

Note that the random choice of a feed plate is unlikely to pick the optimum feed plate location. The user is advised to find out what his feed is going to be, pick his operating conditions, and then find the optimum feed plate location by experimenting with the computer model before going to the laboratory. 

Transient Operations - Holdup

The characteristic property of a transient operation is that something is changing with time. In particular, in transient distillation, compositions throughout a column are changing. This means that the material balances around the stages are actually differential equations and each equation includes holdup as a variable. 

Since the average molecular weight is changing on each plate it is obvious that MOLE HOLDUP on each plate is also CHANGING. However with the alcohol system that we use, density changes very little with composition and it is reasonable to assume that WEIGHT HOLDUP in each stage stays CONSTANT. 

Therefore, all transient operations of any kind require plate and condenser holdup, in pounds. For transient CONTINUOUS operation it is probably most convenient to also set the reboiler holdup. For all other types of transient operations it is more convenient to designate the total original charge. 

For mathematical reasons plate holdup can NOT be specified to be ZERO. 

Transient Operations - Initial Conditions

 In order to simulate any type of transient distillation, it is necessary to solve the material and energy balance differential equations which describe the system. To get solutions to such a set of differential equations, it is necessary to have an INITIAL CONDITION for each differential equation. Use as initial conditions the liquid plate compositions in each stage when the calculation is started. 

The USER MUST supply the INITIAL CONDITIONS for each transient run If they have been generated by a previous computer run, they may be conveniently transferred to input data.  It should be emphasized here that to get good agreement between the computer and experimental transient results, the initial conditions used should ALWAYS included the actual observed compositions in the condenser and reboiler at the beginning of the Batch or other type transient operation. 

Total Reflux And Distillate-to-Pot

Note that both total reflux and distillate-to-pot are characterized by having NO PRODUCTS (distillate or bottoms) taken out of the system. Therefore Distillate and Bottoms must both be 0. 

Since there are no external streams, any overall material balance must satisfy the original charge. Therefore, for these modes of operation, it is necessary to specify the original charge and its composition, plus the weight holdup on each plate , the condenser, and in the bottom of the column.  

Startup

At this time we do not have a simulation of a cold start up. The above remark is very important in considering any type of transient operation because of the initial condition problem. The program always assumes that YOU can provide the required initial conditions - that is, that you have already started up and know the plate compositions at the time you start your experimental run.

 WARNING !!!! For good agreement between computed and experimental results, the initial conditions must include the actual observed compositions x_distillate and x_bottoms at the beginning of the transient run.