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==Introduction==
==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.
The implementations of separation processes are of utmost importance in designing chemical plants. The product stream from a reactor is rarely close to the purity desired by the operator. There are either impurities from undesirable side reactions or unreacted species from the inlet stream. As a result, most chemical processes use an array of separation processes designed to purify the desired product and recycle remaining reactants back to the reactor feed. Aspen HYSYS V8.0 has an array of units that are able to simulate separation processes and estimate the composition of the product streams. By choosing parameters in the system in an consistent way, the operator can fully specify the system in a way that HYSYS can solve for the remaining parameters.  


==Types of Separation==
There are many different types of separations utilized by engineers to purify their desired product. Though there are numerous uncommon types of separations that use niche techniques, most separations used in industry involve multi-phase separation. Multi-phase separation refers to the heterogeneous composition of the product and its tendency to split into multiple discrete mixtures.


==Specification of Process==
==Multi-Phase Separation==
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.
Multi-Phase separation plays a very important role in the purification of a desired product. It involves manipulating the conditions of a stream to cause the mixture to separate into distinct phases.  Two common modes of this separation that can be modeled in HYSYS are flash separation and 3-phase separation. The common theme in these separations is that the mixture in the column is heterogeneous and one outlet stream will contain the desired product in a higher purity than it was at the inlet.  
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.  


===Flash Separation===
Flash separation is a technique used to separate the gas and liquid species in a stream that is at vapor-liquid equilibrium (VLE). Either the species can be at VLE before entering the separator, or the separator may add or remove heat to bring the system to VLE (Towler and Sinnott, 2013). If there is no duty in the system, a fully specified feed attached to a separator will result in a fully specified system. If duty exists, either the duty must be specified or anther aspect of the product needs to be chosen for HYSYS to converge.


==HYSYS Setup==
===3-Phase Separation===
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.  
Sometimes, especially in the presence of hydrocarbons and water, the liquid phase of the separation will not be homogeneous. This results from the fact that species within the liquid phase are not miscible with each other. The resulting separation will result in 3 unique streams, one in the vapor phase and two in the liquid phase. The vapor will still be in VLE with the liquid. As with the flash separation, a fully specified feed will result in a fully specified system if not duty is added. If there is duty in the separator, then another variable will need to be chosen.
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 Simulation==
To simulate a separation in HYSYS, first the simulation environment must be initialized. This includes first choosing all of the individual compounds that will exist in the overall system, and then choosing a fluid package that will accurately simulate all compounds in the range of expected temperatures and pressures. Once this is complete the simulation environment may be entered.


==HYSYS Operation==
To begin building the process, first it is important to specify the feed stream that you are attempting to separate. These specifications include the composition, flow rate, temperature, and pressure of the stream. Once these parameters are entered, it is then necessary to connect the feed to the desired separator type and then run the simulation.
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.  
 
The following two case studies demonstrate how to accomplish a simple separation with a flash separator and a 3-phase separate. The components of the feed will be equal fractions water and ethanol for the flash separation and will be equal fractions water, ethanol, and benzene for the 3-phase separation. The flow rate will be 100 mol/hr in both systems and the NRTL fluid package will be utilized. The feed will be at 1atm pressure and have a .5 vapor fraction.


===Flash Separator===
===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.
To simulate the flash separator, first the feed must be fully specified as shown below.
 
[[File:mwh1.png|thumb|center|'''Figure 1''' Feed Specification|450x500px]]
 
Then the feed can be connected to the separator, and outlet vapor and liquid streams can also be added. If the separator is left without a duty, then HYSYS will be able to solve for the outlet steams based on the VLE composition of the feed.
 
[[File:mwh3.png|thumb|center|'''Figure 2'' Flash separator|450x500px]]
 
If a duty is added, the amount of heat added or taken away will determine the final separation. Either you can specify the duty or another parameter. If there are desired outlet compositions or flow rate from the system, it can be useful to specify one of those characteristics. After being specified, HYSYS will converge to a solution and the compositions of the outlet streams can be seen in the workbook.
 
[[File:mwh4.png|thumb|center|'''Figure 3'' Flash separator with duty|450x500px]]
 
In figure 3, the composition of the liquid stream was specified and HYSYS was able to calculate the vapor composition as well as other flow and energy parameters.
 
===3-Phase Separator===
To simulate a useful 3-phase separator, a feed is needed that will separate into two liquid phases. The addition of benzene to the water ethanol system adds the desired element of immiscible liquid phases. The feed can be then specified as before and connected to the 3-Phase Separator. Vapor, light liquid, and heavy liquid product streams are added. As with the flash separator, the 3-phase separator is fully specified once the feed is specified.
 
[[File:mwh5.png|thumb|center|'''Figure 4'' Converged 3-Phase Separator|450x500px]]
 
However the addition of a duty allows the user to specify one aspect of the product streams. Usually it is useful to specify concentrations of one component in a product stream or possibly the flow rate of one of those streams.
 
[[File:mwh6.png|thumb|center|'''Figure 5'' Converged 3-Phase Separator with duty|450x500px]]
 
As seen above, the user specified the vapor flow rate and HYSYS was able to converge and solve for the remaining unknowns in the system.
 
==Conclusion==
The process of simulating separation in HYSYS is fairly straightforward once the user has a fully specified feed. The feed specifications come from the description of the problem that the user is attempting to solve. Without added duty, the full specification of the feed is all the information that HYSYS needs to simulate the separation. Within the feed stream, two phases exist, and the composition and flow rate of each phase will determine the composition and flow rate of the two phases leaving the separator. The addition of duty complicates the problem slightly more, though it also gives the user more flexibility as they are able to specify one aspect of the product streams and customize their separation.
 
The choice between using a normal flash separator and a 3-phase separator depends on the species in the feed stream. If there are only two phases, the flash separator is sufficient, thought the exact same separation can be simulated with a 3-phase separator. In that case, the light liquid flow rate would be 0. However, if there are 3 phases in the feed stream it is necessary to simulate with a 3-phase separator. The flash separator would not work as the liquid stream would still contain two distinct phases.


===Distillation Column===
==References==
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.
# G.P. Towler, R. Sinnott, Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design, Elsevier, 2013.
# R.H. Perry, D. W. Green, Eds., Perry’s Chemical Engineers’ Handbook, 6th Ed., McGrawHill: New York, 1984.
# Ullmann’s Encyclopedia of Industrial Chemistry, 5th Ed., VCH: Deerfield Beach, 1988.

Latest revision as of 02:56, 2 March 2015


Author: Matthew Hantzmon [2015]

Stewards: Jian Gong and Fengqi You


Introduction

The implementations of separation processes are of utmost importance in designing chemical plants. The product stream from a reactor is rarely close to the purity desired by the operator. There are either impurities from undesirable side reactions or unreacted species from the inlet stream. As a result, most chemical processes use an array of separation processes designed to purify the desired product and recycle remaining reactants back to the reactor feed. Aspen HYSYS V8.0 has an array of units that are able to simulate separation processes and estimate the composition of the product streams. By choosing parameters in the system in an consistent way, the operator can fully specify the system in a way that HYSYS can solve for the remaining parameters.

Types of Separation

There are many different types of separations utilized by engineers to purify their desired product. Though there are numerous uncommon types of separations that use niche techniques, most separations used in industry involve multi-phase separation. Multi-phase separation refers to the heterogeneous composition of the product and its tendency to split into multiple discrete mixtures.

Multi-Phase Separation

Multi-Phase separation plays a very important role in the purification of a desired product. It involves manipulating the conditions of a stream to cause the mixture to separate into distinct phases. Two common modes of this separation that can be modeled in HYSYS are flash separation and 3-phase separation. The common theme in these separations is that the mixture in the column is heterogeneous and one outlet stream will contain the desired product in a higher purity than it was at the inlet.

Flash Separation

Flash separation is a technique used to separate the gas and liquid species in a stream that is at vapor-liquid equilibrium (VLE). Either the species can be at VLE before entering the separator, or the separator may add or remove heat to bring the system to VLE (Towler and Sinnott, 2013). If there is no duty in the system, a fully specified feed attached to a separator will result in a fully specified system. If duty exists, either the duty must be specified or anther aspect of the product needs to be chosen for HYSYS to converge.

3-Phase Separation

Sometimes, especially in the presence of hydrocarbons and water, the liquid phase of the separation will not be homogeneous. This results from the fact that species within the liquid phase are not miscible with each other. The resulting separation will result in 3 unique streams, one in the vapor phase and two in the liquid phase. The vapor will still be in VLE with the liquid. As with the flash separation, a fully specified feed will result in a fully specified system if not duty is added. If there is duty in the separator, then another variable will need to be chosen.

HYSYS Simulation

To simulate a separation in HYSYS, first the simulation environment must be initialized. This includes first choosing all of the individual compounds that will exist in the overall system, and then choosing a fluid package that will accurately simulate all compounds in the range of expected temperatures and pressures. Once this is complete the simulation environment may be entered.

To begin building the process, first it is important to specify the feed stream that you are attempting to separate. These specifications include the composition, flow rate, temperature, and pressure of the stream. Once these parameters are entered, it is then necessary to connect the feed to the desired separator type and then run the simulation.

The following two case studies demonstrate how to accomplish a simple separation with a flash separator and a 3-phase separate. The components of the feed will be equal fractions water and ethanol for the flash separation and will be equal fractions water, ethanol, and benzene for the 3-phase separation. The flow rate will be 100 mol/hr in both systems and the NRTL fluid package will be utilized. The feed will be at 1atm pressure and have a .5 vapor fraction.

Flash Separator

To simulate the flash separator, first the feed must be fully specified as shown below.

Figure 1 Feed Specification

Then the feed can be connected to the separator, and outlet vapor and liquid streams can also be added. If the separator is left without a duty, then HYSYS will be able to solve for the outlet steams based on the VLE composition of the feed.

'Figure 2 Flash separator

If a duty is added, the amount of heat added or taken away will determine the final separation. Either you can specify the duty or another parameter. If there are desired outlet compositions or flow rate from the system, it can be useful to specify one of those characteristics. After being specified, HYSYS will converge to a solution and the compositions of the outlet streams can be seen in the workbook.

'Figure 3 Flash separator with duty

In figure 3, the composition of the liquid stream was specified and HYSYS was able to calculate the vapor composition as well as other flow and energy parameters.

3-Phase Separator

To simulate a useful 3-phase separator, a feed is needed that will separate into two liquid phases. The addition of benzene to the water ethanol system adds the desired element of immiscible liquid phases. The feed can be then specified as before and connected to the 3-Phase Separator. Vapor, light liquid, and heavy liquid product streams are added. As with the flash separator, the 3-phase separator is fully specified once the feed is specified.

'Figure 4 Converged 3-Phase Separator

However the addition of a duty allows the user to specify one aspect of the product streams. Usually it is useful to specify concentrations of one component in a product stream or possibly the flow rate of one of those streams.

'Figure 5 Converged 3-Phase Separator with duty

As seen above, the user specified the vapor flow rate and HYSYS was able to converge and solve for the remaining unknowns in the system.

Conclusion

The process of simulating separation in HYSYS is fairly straightforward once the user has a fully specified feed. The feed specifications come from the description of the problem that the user is attempting to solve. Without added duty, the full specification of the feed is all the information that HYSYS needs to simulate the separation. Within the feed stream, two phases exist, and the composition and flow rate of each phase will determine the composition and flow rate of the two phases leaving the separator. The addition of duty complicates the problem slightly more, though it also gives the user more flexibility as they are able to specify one aspect of the product streams and customize their separation.

The choice between using a normal flash separator and a 3-phase separator depends on the species in the feed stream. If there are only two phases, the flash separator is sufficient, thought the exact same separation can be simulated with a 3-phase separator. In that case, the light liquid flow rate would be 0. However, if there are 3 phases in the feed stream it is necessary to simulate with a 3-phase separator. The flash separator would not work as the liquid stream would still contain two distinct phases.

References

  1. G.P. Towler, R. Sinnott, Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design, Elsevier, 2013.
  2. R.H. Perry, D. W. Green, Eds., Perry’s Chemical Engineers’ Handbook, 6th Ed., McGrawHill: New York, 1984.
  3. Ullmann’s Encyclopedia of Industrial Chemistry, 5th Ed., VCH: Deerfield Beach, 1988.
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