Separator

Author: Matthew Hantzmon ^[2015]

Stewards: Jian Gong and Fengqi You

Introduction

In chemical processes, separation steps are usually among the most important and costly steps to manage. At nearly every step in a process there is an impurity that needs to be removed or a product that needs to be isolated. As a result, the study of different separation methods is an area of great importance in Chemical Engineering. As it is costly and time consuming to build actual models of each potential separation process, engineers often use software like Aspen: HYSYS to simulate their idea. Knowing the process for specifying all constraints of a system through HYSYS allows for an easy study of process options without the need for lab scale testing. HYSYS can model many different types of separators so it is important to be familiar with the differences in their software.

Specification of Process

For a system to be solved with software, there must be a variable or parameter specified for each governing equation on the separator. When initially presented with a system of equations, usually there will be less variables than equations, leaving the problem initially unsolvable. The designer of a process will continue to specify parameters until the equations are solved and all aspects of the process are determined. For the system to be solved, the number of unspecified variables must equal to the difference between the number of mathematical equations that govern the system and number of constants that have already been specified. Each separator unit in HYSYS needs a different number of variables specified because they have a different number of equations that govern their physical and chemical processes.

HYSYS Setup

Prior to setting up a process with HYSYS, the user needs to know a few of the specifics of the separation they are trying to achieve. These specifications include basics of the separation such as the species of interest, and all known temperatures and pressures. Once all information is collected, the user may start organizing it to input in to HYSYS. Though there are dozens of specific types of separators that HYSYS can model, there specific ones that Chemical Engineers come across on a daily basis. These common units include distillation columns, flash separators, absorbers and strippers. HYSYS has a specific user interface for each of these types of separators that assists the user in correctly specifying all of the known parameters within their process.

HYSYS Operation

When setting up a separation process in HYSYS, the user must first specify the components of the mixture that they are trying separate. The user will usually have a feed stream of known compositions, temperature and pressure. They will have to decide what aspects of the products of the separation are important to them. Usually there is a desired product purity that is specified by the process. The remainder of the variables can be specified to minimize duty on the separator while maximizing efficiency. Then they must choose a fluid package that will best represent their system. Once the simulation environment has been entered the user can then choose the type of separator that they would like to use.

Flash Separator

A flash separator is the simplest type of column for separation. It has one inlet stream and two outlet streams. By adding to or removing heat from the feed stream the stream “flashes” into one vapor and one liquid. With a fully specified feed stream including compositions and enthalpy, the operator only needs to fix one aspect of the product streams to fully specify the entire system. This can be product purities, product flow rates, or even column duty. As the system only has one degree of freedom after the feed has been introduced, only one more parameter can be chosen before everything is determined.

Distillation Column

A distillation column is slightly more complicated than a flash separator though it still only consists of one feed and two product streams. It has multiple trays of unique vapor-liquid-equilibrium compositions. Products will become more pure as they move further above and below the feed stage. The column is more complicated than the flash separation and the degree of freedom analysis finds that two more parameters must be specified besides the feed to fully specify the system. Besides the product purities, flow rates and duty from before, within the distillation column there is also a reflux ratio to specify that will count as an additional variable. There are also individual duties on the reboiler and condenser that can also be specified to result in a fully determined column.

References

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3. R.H. Perry, D. W. Green, Eds., Perry’s Chemical Engineers’ Handbook, 6th Ed., McGrawHill: New York, 1984.

4. Ullmann’s Encyclopedia of Industrial Chemistry, 5th Ed., VCH: Deerfield Beach, 1988.