The BR&E Blog

Troubleshooting Amine Unit Simulations (by Lili Lyddon)

b.cochran@bre.com — Mon, 02 Jun 2008 19:24:00 GMT

If problems occur during execution of an amine sweetening unit simulation, you should always first look at the Warnings list and Message Log for clues as to what went wrong. If there is too little or too much acid gas absorption, check the following:

If too much acid gas is absorbed, be sure the Column Type (Process Data tab) is set to TSWEET Kinetics and not Equilibrium. Also be sure a Residence Time is being calculated (Stage Data tab, Hardware Grouping) and that it is reasonable (approximately 1 - 4 seconds). If Residence Time is not set or calculated, the results will be the same as if the Column Type were set to Equilibrium.
If there is not enough acid gas absorption, be sure the Property Package is set to Amine Sweetening or Electrolytic ELR. If the Property Package is set to an EOS (SRK or Peng-Robinson), very little acid gas pickup will occur.
If there is not enough acid gas pickup and the rich loading is too high, check the circulation rate as it may be too low.
Also check the lean loading. If the lean loading seems too high, you should check the reboiler duty or steam rate, which should correspond to about 0.8 to 1.2 lb steam per gallon of amine solution circulated (0.096 to 0.144 kg steam per liter of amine solution circulated).
If a trayed absorber is modeled using ideal stages, be sure the Real/Ideal Stage Ratio (Stage Data tab, Hardware Grouping) is set correctly. For a packed absorber, be sure the total column liquid residence time is maintained if the number of stages is changed.

If the absorber fails to converge, the Convergence tab contains a number of parameters which can be manipulated to help the absorber column converge. Try changing the following parameters:

Try the "Boston-Sullivan Nonideal" Inner Loop model instead of the default "Boston-Sullivan" (Parameters Grouping).
If the outer loop error appears to be steadily decreasing but the Column does not converge before reaching the maximum number of loops, increase the Maximum Iterations parameter in the Convergence tab, Solver Grouping.
The absorber normally does not require the "Composition-Dependent" Enthalpy Model (Parameters Grouping) even for absorbers with high acid gas loadings, however, this model might be required in some cases. Try this model only after first trying the other parameters mentioned above.
Try enabling the Boston-Sullivan Kb method by selecting the checkbox (Parameters Grouping).
If the absorber fails to converge after making a very large change to the sour gas composition/flow rate or to the lean amine flow rate, try making the change in increments. You can also try the Delete Last Solution button on the Column Convergence tab, Parameters Grouping.

If the stripper fails to converge, the Column Convergence tab contains a number of parameters which can be manipulated to help the stripper column converge. Try changing the following parameters. Change only one parameter at a time initially and in the order indicated:

If reboiler temperature is set, DELETE THIS SPECIFICATION. The column will have difficulty converging since reboiler temperature is determined by the stripper operating pressure. Set reboiler duty or steam rate instead.
The "Composition Dependent" Enthalpy Model (Parameters Grouping) is usually required for amine stripper columns.
If specifications such as composition (e.g. lean loading) or reflux ratio are set, change to specifications which allow easier column convergence, such as reboiler steam rate and condenser temperature. After the column converges and a profile is established, you may change to the more difficult specifications.
If reboiler duty is specified, be sure the value is not too high or too low. The duty should usually fall within the range of 800 to 1200 Btu per gallon of amine solution circulated (206 to 309 kJ/liter or 49 to 74 kcal/liter). This duty corresponds to a steam rate of 0.8 to 1.2 lb steam per gallon of amine solution circulated or 0.096 to 0.144 kg steam per liter of amine solution circulated.
Check the rich amine feed temperature to the stripper. If it is too low, the column will have trouble converging. The minimum rich feed temperature should be about 180 F (82 C). Temperatures of 200-220 F (93 to 104 C) are typical.
If the Column Type (Process Data tab) is set to TSWEET Kinetics, try using TSWEET Alternate Stripper instead.
Strippers operating near minimum reflux can often be solved using the "Boston-Sullivan Nonideal" Inner Loop model. instead of the default "Boston-Sullivan" (Parameters Grouping).
If the outer loop error appears to be steadily decreasing but the Column does not converge before reaching the maximum number of loops, increase the Maximum Iterations parameter in the Convergence tab, Solver Grouping.
Try enabling the Boston-Sullivan Kb method by selecting the checkbox (Parameters Grouping).
If the stripper fails to converge after making a very large change to the sour gas composition/flow rate, reboiler duty/steam rate, or lean amine flow rate, try making the change in increments. You can also try the Delete Last Solution button on the Column Convergence tab, Parameters Grouping.

If the Recycle has trouble converging in the specified number of iterations, you can increase the Maximum Iterations parameter. Open the Recycle block dialog, click the Process Data tab, click the "Advanced..." button, then click the "Solver Options..." button. Increase Maximum Iterations to 30 or 40. Scroll up through the Message Log and observe the Recycle error for each iteration. If the error is not changing or is oscillating, it may be necessary to decrease the default Weight for some components. For example, if the Recycle is placed in the rich leg or the semi-lean leg of an amine sweetening process, you may have to decrease the Weight for the reactive components (acid gas, amine, water) to obtain flowsheet convergence. If Weights are decreased, be sure to check the sensitivity of the results to the weighting.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Simulating Change in a Steady-State Simulator (by Craig Spears)

b.cochran@bre.com — Wed, 30 Apr 2008 16:53:00 GMT

With feed streams changing, and with the ever watchful eye on the bottom line, you need to have simulation software that can help steer you in the right direction.

In 2006 Bryan Research & Engineering, Inc. and Crosstex Energy Services, L.P. published an article for Hydrocarbon Engineering titled “Steady-State Simulators are Developing a Dynamic Personality.” This article helps step through one case of designing a ProMax simulation utilizing our Scenario Tool to model changing conditions, and finding an optimum operating condition.

The first step is to model the current plant accurately. This step cannot be stressed enough, because without verifying that your model matches your plant data, how can any changes be successfully modeled?

Sizing and ratings for all equipment should also be set and verified. These calculations will give you information about how close you are to capacity, and whether your exchangers can handle the load being given to them under new cases. Exchangers should have calculators set to drive the percent overdesign to zero. Even if the exchanger was designed with a ten percent overdesign, remember that once it’s placed in the field, all area available to it is being used for heat exchange. Adjusting the outlet temperature or duty of the exchanger to give an overdesign of zero will best match the exchanger operation.

The scenarios should then be decided, and set within our Scenario Tool. The process requirements should be set: overhead specifications, product quality, etc… Also, choosing the variables that might be limiting for any changes made to a process is paramount. Some to think of: actual pressure drops in an exchanger, calculated nozzle sizes in an exchanger, tray flooding in towers, etc...

Once these are set, run the Scenario Tool, and then check the results. Which cases improve plant production or profitability? Which decrease the energy consumption? How much more inlet flow can the plant handle? The possibilities are endless and you can almost always make your company money.

This is not a process that can completely do away with dynamic simulators, as the time element is completely missing. ProMax can tell you what the result will be, but cannot tell you how long it will take to get there. Reminds me a little of thermodynamics.

Authored by Craig Spears - BR&E Sales Department\

Amine Thermal Degradation (by Lili Lyddon)

b.cochran@bre.com — Tue, 08 Apr 2008 19:04:00 GMT

Process engineers often express concern about amine reboiler temperatures being high enough to cause thermal degradation of the amine. However, thermal degradation is generally not a concern in amine reboilers heated with steam or heat transfer fluids.

In “DEA degradation mechanism,” Hydrocarbon Processing, October 1982, A. Meisen and M. L. Kennard discuss the fact that DEA and MDEA thermal degradation is minimal up to 400°F. Although the degradation of DEA is caused by reaction with CO₂ and not temperature alone, temperature does affect the rate of degradation caused by reaction with CO₂. This reference states: “Degradation increases strongly with temperature. This is not due to thermal breakdown of DEA, but it requires the presence of carbon dioxide. Design and operation of DEA units must avoid creation of elevated temperature throughout the plants. Heat transfer surfaces of DEA stripper-reboilers (especially when gas fired) are particularly prone to formation of localized hot spots. To prevent such hot spots in operating plants, DEA circulation through the stripper-reboiler should be kept high and steam (or gas) temperature kept low. In many DEA units only the bulk solution temperatures are measured. It must be remembered that the skin temperatures of heat transfer surfaces can be very much higher, particularly during process upsets. Reliance upon bulk temperatures is therefore inadequate.”

In the paper “Reduce amine plant solvent losses, Part 2” from Hydrocarbon Processing, June 1994, E. J. Stewart and R. A. Lanning mention that the thermal degradation of amines accelerates above 350°F, so the skin temperature of direct fired reboilers should be kept below 350°F. They recommend a reboiler operation with an amine bulk temperature below 260°F. This reference goes on to say: “With hot oil and steam heating systems, risk of thermal degradation is low since the heat media is usually not operated at high temperature. However, in fired-reboiler operation, the temperature of amine on the tube’s surface can easily exceed 350°F. In fired reboilers, forced circulation is often used to maintain low skin temperatures. The rule of thumb is to maintain amine skin temperatures between 300°F and 325°F, and not exceed 350°F. for these temperatures a conservative design heat flux of less than 8000 Btu/ft² of tube area is recommended.”

Other references indicate 400°F as the thermal degradation temperature of MEA. Keep in mind that the reboiler temperature is set by the stripper operating pressure. To reduce the reboiler temperature, the stripper pressure can be reduced.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Problems with Crude Column Cut Point Temperature Specifications (by Lili Lyddon)

b.cochran@bre.com — Wed, 19 Mar 2008 13:20:00 GMT

When modeling crude distillation columns, boiling point curve temperature specifications are often used to characterize products (e.g. ASTM D86 90 Volume % Cut Point Temperature). In the early stages of the model development it may be easier to monitor the product boiling point curve temperatures rather than converge on a particular specification. Once the column has been converged with relatively "easy" specifications such as draw rate or flow ratio for the product draw streams, those can be used as estimates and the specifications changed to the more difficult cut point temperature specifications to fine tune the simulation.

If the column will not converge to a desired temperature specification at what appears to be a reasonable operating condition, a converged solution is sometimes obtained by re-characterizing the crude to add additional components. The additional components add additional temperature resolution which makes it easier to obtain a desired temperature specification. To add additional components, you can increase the number of cuts for each temperature range on the Cut Points tab of the ProMax oil specification dialog. One option is to increase the components in the large interval. As an example, the number of cuts in the 100-825°F interval could be increased from 29 to 58. Or you could just increase the number of cuts near the problem temperature. For instance, make the interval 100-400°F 12 cuts, 400-500°F 8 cuts, 500-825°F 13 cuts. The 400-500°F originally was about 4 cuts and you could double or triple it while not changing the cuts per temperature interval for the remainder of the oil.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Do You Know Where Your Water Is? (By Craig Spears)

b.cochran@bre.com — Tue, 11 Mar 2008 18:20:00 GMT

Typically March in Texas is not a time to expect snow to fall; but it did. While we don’t know when we are going to have freezing water fall from the sky, hopefully we have more control over our operating units – freezing water in our sky is much better than freezing water in our gas plants.

Water content is important for predicting these freeze conditions, including hydrate formation, as well as corrosion in the plants. This is especially true in units containing H2S and CO2 as these tend to exacerbate these issues.

Phase envelopes with ice, dry ice and hydrate lines are very helpful in determining how safe your unit is operating. However, these diagrams are only as good as the simulator that generated them. ProMax has the unique ability to predict multiple hydrate, ice, or solid CO2 formation points. Most simulators only calculate the highest formation temperature; however ProMax will predict up to three solids formation temperatures and plot them directly on the phase envelope. Depending on the operating conditions of the plant, the highest formation temperature may not be the one of most relevance.

Critical of all these calculations is the proper prediction of water content during processing, especially in the presence of H2S and CO2. What may seem to be a trivial difference, can actually have a very significant impact. For additional information on this topic, please refer to the John M. Campbell website and read the December 2007 “Tip of the Month”.

So keep the freezes outside by truly knowing where your water is.

Authored by Craig Spears - BR&E Sales Department

Presentation at GPA (by Lili Lyddon)

b.cochran@bre.com — Mon, 03 Mar 2008 17:27:00 GMT

BR&E will be presenting a paper entitled “A Comparison of Physical Solvents for Acid Gas Removal” at the 87th Annual GPA Convention in Grapevine on March 3, 2008. This paper compares the acid gas removal ability, required equipment, and power requirements for the four physical solvents DEPG, Methanol, NMP, and Propylene Carbonate. If you would like a copy of the paper, please submit your request to sales@bre.com to receive an electronic copy. Shortly after the GPA convention the paper will be available for download.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Actual Volumetric Flow Rate vs Standard Liquid Volumetric Flow Rate (by Michael Hlavinka)

b.cochran@bre.com — Thu, 28 Feb 2008 18:11:00 GMT

Why does the actual volumetric flow rate in ProMax not equal the standard liquid volumetric flow rate if the stream temperature and pressure are at standard conditions?

The only time the actual and standard liquid volumetric flow rates will be equal at standard conditions is if the system forms an ideal solution. Solutions that are not ideal can have significantly different actual and standard liquid densities.

The actual density of the liquid is computed using the user selected liquid density model in the ProMax environment. This is normally either the COSTALD or the Rackett model. The predictions from these models are mixture densities which include a volume change on mixing effect.

The standard liquid densities are computed using GPA Standard 8173. This computation is simply a molar average of the liquid densities of the individual components that comprise the mixture. No volume change on mixing is calculated. GPA Standard 8173 states that the component liquid densities needed for this averaging are to be obtained from the following sources:

The current edition of GPA Standard 2145. In the current release of ProMax, GPA Standard 2145-03 is used. A revised GPA Standard 2145-08 was to be promulgated on 01 January 2008. However, due to changes made, this revision has not been formally approved. When the new standard is approved, it will be included in ProMax in its next release.
If the component is not listed in GPA Standard 2145 (there are only 19 components in GPA Standard 2145-03), the next source is the current version of the GPSA Engineering Data Book. ProMax includes the density data from the 12th edition of the GPSA Engineering Data Book.
If the component is not listed in either of the above two sources, the API Technical Data Book is used.

Most of the components present in the gas processing industry are available in one of the three above sources. Also note that some components are supercritical and cannot exist in the liquid phase at standard conditions (e.g., H2, CH4, N2, etc.). For these components, estimated values are provided in the tables for use in these calculations.

GPA Standard 2145 contains tables of properties in both FPS (English) and metric units. The GPSA Engineering Data Book is also available in FPS and in SI units. The properties in the respective tables of these two sources are not merely unit conversions between the two sources. The standard temperature for the FPS sources is 60°F (15.56°C) while the standard temperature for the SI sources is 15.0°C. In 1980, GPA adopted a standard temperature for SI units of 15.0°C (see 12th Edition GPSA Engineering Data Book, page 1-13). ProMax includes both the SI and the FPS data internally. However, the standard temperature must be changed to 15.0°C to use the SI data. This is done by selecting the ProMax->Project Options… menu item.

In addition to standard liquid density computation from the above sources, the gross (higher) and net (lower) heating values are computed using data from the same sources. As with standard liquid densities, the heating values are also impacted by the choice of standard temperature and pressure.

Finally, another common standard for gases is known as normal conditions (as opposed to standard conditions). These conditions are widely used in Europe and other locations. Normal conditions refer to gas volumes measured at 1 atm pressure and 0°C. ProMax provides a normal volumetric flow property in the stream that can be used for specification of normal volumetric flow rates. Note that if the ideal gas reference temperature in ProMax->Options is set to 0°C, the standard gas flow rates will equal the normal gas flow rates.

Authored by Michael Hlavinka (BR&E Technical Director)

Customizing ProMax by Editing Options.xml (by Lili Lyddon)

b.cochran@bre.com — Thu, 21 Feb 2008 17:31:00 GMT

The Options.xml file specifies ProMax program defaults such as stream properties, units sets, items appearing in Tooltips, etc. Some default settings can be changed using Project Options, however, extensive changes may be inconvenient if they are to be performed for each new Project. Other default settings cannot be accessed through the ProMax program. By editing the Options.xml file, default settings can be specified such that each new Project will use these modified default settings. Examples

You can change the default stream properties and order of properties appearing in the stream Properties tab. All available stream properties are displayed by default and these properties appear in a certain order. To change these properties and the order of properties, the user must open the Project Options dialog, click on the Stream Properties tab, remove all unwanted properties, and drag properties to the desired order in the list. If the same properties and order of properties are to be used in future Projects, this procedure must be repeated for each new Project. To avoid modifying the default properties for each new Project, the Options.xml file may be edited such that the user selects certain properties and places them in the desired order. Each new project will use these new default settings.
The Options.xml file defines the content of the unit sets available in ProMax. You may add more unit sets if desired or you may modify the existing unit sets.
Default Units for properties appearing in the ProMax Report can be specified. For example, the default units for Standard Liquid Volumetric Flow are sgpm in the Report, however, you would like to see liquid flow listed in standard barrels per day (bbld). The Options.xml file can be modified such that the default liquid flow units in the Report are bbld instead of sgpm.
You can add properties to the “Tables” in a heat exchanger. You can also remove properties.
Properties can be added or removed from the tooltips. For example, if you want to see molecular weight displayed for a stream via the tooltip, you can modify Options.xml to display molecular weight, or any other desired properties.

How to Edit Options.xml

If Options.xml is altered, the new default settings will be in effect for ALL future projects. Be sure to save a back-up copy of Options.xml prior to editing so that original default settings can be easily restored if necessary. Options.xml may be found by copying and pasting or typing in the following in a browser Address field:
%allusersprofile%\Application Data\Bryan Research & Engineering Inc\ProMax2\Data
Alternatively, you can use Start—>Search—>For files or Folders... to locate Options.xml on your computer. To edit Options.xml, do the following:
1. Go to the above location and open the file in Notepad or some other XML editor.
2. To open in Notepad, right click "Options.xml", select "Open With", and "Notepad".
3. Note that Options.xml must be edited in the English language and all information in Options.xml is CASE SENSITIVE.
4. Be sure ProMax is closed before editing and saving the file. Any changes made while ProMax is currently running will not be saved.
Alternatively, custom Options.xml files can be created by BR&E for the user at no charge. Viewing Permissible Values for Options.xml

To view the correct nomenclature and available permissible values, do the following:

From the ProMax Flowsheet View, click on "Tools" in the main menu and choose "Macros".
From the Macros list, select "Visual Basic Editor ".
In the Visual Basic Editor, click on the "View" menu item and select "Object Browser".
The drop-down list in the top left corner lists Type Libraries with a default of . To narrow the number of Classes, select "ProMax" from the drop-down list.
For each of the Classes listed on the left, all permissible values are displayed on the right.
For example, click on pmxPhysPropEnum under Classes. On the right side is a list of every physical property name as used in ProMax. This list gives the correct spelling and all available items to be used when modifying the Options.xml file.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

User Value Sets and Short Monikers (Authored by Jeff Melland)

b.cochran@bre.com — Wed, 06 Feb 2008 18:00:00 GMT

Recovery values are very useful to see on a process flowsheet. However, sometimes you may want to display these values with a custom name. Let us say you want to see the elemental sulfur recovery for a Claus unit. There is a simple way to display this value as described in another blog entry. A custom name can be assigned by defining a user defined value. After you create your recovery object on an atomic basis, right-click on the “User Value Sets” from the Project Viewer tree control, select “Add” to create a new group, and then select “Add…” again at the bottom left to define a specific value. For this case choose “Fraction” for the standard units, select the “Associate with a New Specifier” box, and type a name for the value. Next, choose the variable that represents the desired recovery value through ProMax’s specifier technology. This will make your user value have the same value as that calculated in the recovery table. Lastly, go to the moniker builder, the button with the “a!b” symbol. Select the user value you just specified, and come up with a descriptive moniker (i.e. Sulfur_Recovery); this is what will be displayed on the flowsheet.

When open your property table now, you can select “moniker” and then “short moniker” in the view selection at the bottom, you can now select your recovery value with a short descriptive label for your flowsheet.

Authored by Jeff Melland (BR&E Technical Support)

BR&E to Present 2008 GPA Convention in Grapevine (Authored by Gavin McIntyre)

b.cochran@bre.com — Thu, 31 Jan 2008 20:59:00 GMT

BR&E will be participating in three papers and presenting two next month at the 87th Annual GPA Convention in Grapevine Texas. One is titled “Industrial Design and Optimization of CO2 Capture, Dehydration, and Compression Facilities” and written in partnership with HTC Purenergy of Regina, SK, Canada. The main design and engineering factors affecting the CO2 capture, dehydration, and compression processes have been highlighted in this paper. The study provides a feasible engineering design and acceptable production cost taking into consideration all the technical, economic, and plant location factors. In you are unable to attend GPA and would like a copy of the paper, please submit your request to sales@bre.com to receive an electronic version. A download version will be available shortly after the convention.

Authored by Gavin McIntyre (BR&E Sales)

UA Wizard (Authored by Jovita Duran)

b.cochran@bre.com — Tue, 29 Jan 2008 22:43:00 GMT

Have you ever needed to add a solver to your simulation that could calculate exchanger temperature change to achieve an Approach Temperature or End Point UA? The ProMax Property Stencil Add-in offers what is known as a UA Wizard that will achieve the same results as a solver. Unlike a solver, UA Wizard does not require you to build a calculator. Once you attach the UA Wizard to a valid integrated exchanger and double-click on it to open its dialog, it allows you to select the target property, target value/units, and control property. The UA Wizard can have Log Mean Temperature Difference, Minimum End Approach Temperature, Effective UA, End Point UA, Effective Mean Temperature Difference, or Minimum Effective Approach Temperature as the target property. A control property could be Exchanger Duty, Temperature Change (of highest mass flow stream), or Outlet Temperature (of highest mass flow stream). The Property Stencil containing the UA Wizard Add-in installs with ProMax 2.0 under the Program File directory and can be added to any ProMax project.

Authored by Jovita Duran (BR&E Technical Support)

Why do I see a temperature increase predicted across a JT (throttling) valve? (Authored by Michael Hlavinka)

b.cochran@bre.com — Fri, 26 Oct 2007 11:15:00 GMT

There have been many questions concerning temperature rises predicted by ProMax across JT valves and other adiabatic flashes. Depending on conditions, ProMax may predict the temperature to rise in the flash. I have created a document that explains the cause of this temperature rise and mathematically proves its existence for simple systems.

Explanation of Temperature Rise across JT Valves and Adiabatic Flashes.pdf

Authored by Michael Hlavinka (BR&E Technical Director)

Improving Amine Regenerator Convergence (Authored by Luke Addington)

b.cochran@bre.com — Tue, 23 Oct 2007 09:00:00 GMT

Amine systems can be tough to calculate. However, there are a few things that you can do to help out. In this post we're going to discuss a few options on the convergence tab, especially the Enthalpy Model and the Inner Loop Model.

The amine regenerator can, at times, be especially challenging. Sometimes the column will refuse to converge or will converge quite slowly. A few select components with non-ideal volatilities such as ammonia can make things especially tough. Below is some general information about the various options and some best practices to use while working with your model. These parameters can be found on the "Column Convergence" Tab in ProMax.

Enthalpy Model

Boston-Britt - is selected by default and calculates the total molar enthalpy for each of the phases on each stage as a function of temperature only. As there is only one variable, iterations for this method are very fast.

The Composition-Dependent - method is more rigorous than Boston-Britt and calculates the partial molar enthalpy in the liquid phase for each of the reactive components in an electrolytic system, or all components in a non-electrolytic system. This method can help convergence issues, especially in cases where the composition changes significantly with each outer loop calculation.

Inner-Loop Model

The Boston-Sullivan - Loop Model is selected by default and linearizes the enthalpy and volatility during column calculation. The number of variables in the inner loop is thus reduced to the number of stages plus the number of specifications in the column.

Boston-Sullivan Non-Ideal - is a more rigorous inner loop method. Instead of linearizing enthalpy and volatility, this method calculates partial molar liquid fugacity for each component in non-electrolytic systems or for each reactive component in electrolytic systems.

As an example, let's take the case of an amine regenerator containing ammonia. If you run this column with the default settings you will likely have difficulty with convergence. In real column operation, these units have a tendency to build up ammonia in the overhead loop due to the volatility profile of ammonia and the column operating temperatures. If you watch the flow rates on each stage, or the overhead loop, during the column convergence you will notice a similar build up in ProMax. Perhaps the first outer-loop showed 0.1% NH4 in the overhead. The next loop showed 0.2%, then 0.5%, then 1% and so on. This is a pretty sizable change in the composition for each loop, so Boston-Britt may not be the best Enthalpy Model to use. Composition-Dependent would give you a more accurate enthalpy for each iteration as it takes the changing composition into account and would thus help with convergence.

Now that we set the enthalpy model to Composition-Dependent we see that the column gets a little closer to convergence, but still solves to an approximate solution at the end. Perhaps we should take a look at the Inner-Loop model.

Below is a graph of the volatility of ammonia as a function of temperature. We should note standard temperatures for amine regenerators are around 50˚C for the condenser and around 120˚C at the reboiler.

Looking at the graph, making the volatility linear may not be a good idea. While the column may at times converge without the Inner-Loop model set to Boston-Sullivan, it will almost certainly converge with fewer steps with Boston-Sullivan non-ideal.

There is a trade-off, of course, using these settings. While we have fewer iterations, we have greatly increased the number of variables and thus the speed of calculating each iteration. This is why Boston-Britt and Boston-Sullivan are selected by default; they are the fastest settings available. But in cases where the column does not converge well with these two, it may be faster--or necessary--to use a different combination to get our solution.

It should be noted than none of these settings affect the actual solution. They are merely the approach taken to calculate the column. The end result is still a function of the thermodynamic package chosen.

Amine_Regen_Convergence.pdf

Authored by Luke Addington (BR&E Sales)

Ammonia in your amine unit? (by Lili Lyddon)

b.cochran@bre.com — Mon, 09 Jul 2007 21:18:00 GMT

What seems like an insignificant amount of ammonia in the sour feed gas to an amine unit can be detrimental to the sweetening process. Ammonia in an amine sweetening system can cause reduced absorption of acid gas, as well as greatly increased stripper condenser and reboiler duties due to build up of NH3 in the system. Corrosion in the stripper condenser loop is also a significant problem. We recently looked at a refinery DEA unit which had only a small amount of NH3 in the feed to one absorber (0.7%), and with no water wash ProMax was calculating a duty 3 times the actual. Finally we discovered that a water wash was being used on the feed containing ammonia to remove as much NH3 as possible from the feed. When the water wash was simulated, the reboiler duty predicted by ProMax matched the plant operating data. A good source of information concerning ammonia may be found in a BR&E Technical Paper at the following location on the BR&E website:

http://www.bre.com/portals/0/technicalarticles/Influence%20of%20Ammonia%20on%20Gas%20Sweetening%20Units%20Using%20Amine%20Soluti.pdf

This paper describes operating problems due to ammonia in amine unit feed. Although an ammonia stripper is the solution used in this paper to solve the problems caused by ammonia, sometimes just a purge stream from the stripper reflux is sufficient to stop the buildup of ammonia in the system and to allow the unit to operate more efficiently. The paper mentions 0.5% ammonia as being the maximum amount of ammonia that can be present in the amine unit feed. If the ammonia content of the feed is above this amount, the gas would have to be pre-treated to remove the ammonia before being sent to the amine sweetening unit.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Relief Valve Sizing Analysis (by Lili Lyddon)

b.cochran@bre.com — Thu, 28 Jun 2007 20:18:00 GMT

The Relief Valve Sizing Analysis in ProMax calculates the Effective Discharge Area, Relief Pressure, and other parameters. This analysis reports estimation of latent heat of vaporization for mixtures (Inlet Latent Heat parameter) which may be used for other applications. This sizing analysis applies only to relief devices intended to protect unfired pressure vessels against overpressure only. The pressure relief devices do not protect against structural failure when the vessel is exposed to extremely high temperatures such as during a fire. The Analysis is based on recommended practices for sizing of relief valves as laid out in several published international standards. These methods are a guide for selection of pressure relief devices for equipment with maximum working pressures exceeding two atmospheres (absolute). Selection of valves from manufacturer data sheets is made via the effective discharge area calculated by this analysis. A valve with an effective area close to but exceeding this value is recommended. The implemented standards suggest final selection be made in consultation with your relief valve manufacturer.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Where’s the Water? (by Craig Spears)

b.cochran@bre.com — Fri, 11 May 2007 17:50:00 GMT

If you are trying to find the water content or water dew point of your gas, add a “Freeze Out, Hydrate, H₂O Dew Point” analysis to the stream. The temperature of the water dew point is given, as is the water content of that stream (reported as lbm/MMSCF, pounds per million standard cubic feet).

What if there is not a number here, though – what if the spot is just a red blank? This occurs in ProMax version 1.2 (and previous versions) for the water dew point value and simply indicates that there is not enough water in the stream to form a liquid phase; instead it will form another phase such as ice or hydrate.

Many customers requested us to report the water dew point, and with our focus on meeting the customers' requests, we have changed this in our latest release. In version 2.0, the water dew point is reported along with a message "Warning: Water dew point temperature is thermodynamically unstable and will not form a free aqueous phase." This message is indicating that the amount of water is not sufficient to form its own liquid phase and will instead form another phase such as ice or hydrate.

Authored by Craig Spears - BR&E Sales Department

Oil/Water Viscosity (by Lili Lyddon)

b.cochran@bre.com — Fri, 04 May 2007 22:00:00 GMT

ProMax uses the arithmetic average method for calculating mixed liquid viscosity because that method is used by all of the pipeline pressure drop models included in the program (see Brill and Beggs, 1991) except the OLGAS correlations. However, the viscosity of oil/water mixtures is extremely difficult to predict. In Brill and Mukherjee, SPE, Multiphase Flow in Wells, 1999, they say: “Studies have shown that Eq. 3.18 [mL = mofo + mwfw, where m is viscosity and f is the fraction of oil or water] often is not valid for the viscosity of two immiscible liquids, such as oil and water..… The viscosity of a dispersion or emulsion has been found to depend mainly on the determination of which phase is continuous. The apparent liquid viscosity then will be governed primarily by the viscosity of the continuous phase, because this is the phase that predominates at the pipe wall where most of the friction losses occur. Other factors, such as the dispersed-phase viscosity and the droplet-size distribution of the dispersed phase, also are important. For some oil/water systems, the viscosity of the liquid mixture can be several times greater than the oil viscosity when the continuous phase is oil but the water fraction is approaching the point where an inversion of the dispersion or emulsion will occur…. The inversion point of an oil/water mixture occurs at water fractions ranging from 0.2 to 0.5 with inversion taking place at lower water fractions when oil viscosities are high..….Although Eq. 3.18 is the most common way to treat the apparent viscosity of an oil/water mixture, a more accurate method is to use the oil viscosity when oil is the continuous phase and the water viscosity when water is the continuous phase…. An even better alternative is to conduct flow tests on actual crudes and water to determine the rheological characteristics and the probable inversion point.” So this tells us that there is no totally accurate way to determine the viscosity of oil/water mixtures short of taking physical samples and testing.

Property Stencil (by Lili Lyddon)

b.cochran@bre.com — Thu, 19 Apr 2007 15:14:00 GMT

The Property Stencil distributed with ProMax 2.0 includes tools such as Copy Stream Conditions, Flow Duplicator Example, Property Calculator, Cn+ GPM Calculator, Cn+ GPM Solver Example, UA Wizard, and many others. The Property Stencil may be found in the C:\Program Files\Bryan Research & Engineering Inc\ProMax2\AddOns\Visio Property Stencil folder. To open the Property Stencil in ProMax, click the File menu item, select “Shapes”, then click on “Open Stencil…” and browse to the above mentioned location. The upcoming version of the Property Stencil (available now) includes a flow multiplier which copies a stream and allows the user to conveniently multiply the total flow rate by some number such as 2 or 3.5. This feature can be useful when modeling multi-train plant simulations or performing case studies to determine the effects of increased (or decreased) flow without overwriting the original results. Also included is a Membrane Tool. Contact BR&E for the latest version of the Property Stencil.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Energy Budgets and Recoveries (by Lili Lyddon)

b.cochran@bre.com — Tue, 03 Apr 2007 22:33:00 GMT

The 2.0 version of ProMax includes energy budget and recovery calculation objects that are fully customizable by the user. The recovery objects provide a summary of the project inlets, outlets, losses (due to convergence tolerances), and component relative outlet recoveries. For example, it is very easy to compute the recovery of components in any number of selected outlets relative to any number of selected inlets by creating a user defined recovery object. The recovery object is the method to calculate the overall elemental sulfur recovery of a Claus sulfur plant. The duty and power budgets summarize the heat and power consuming or producing units in the project.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Plug Flow in ProMax (by Lili Lyddon)

b.cochran@bre.com — Fri, 30 Mar 2007 15:55:00 GMT

Plug flow reactors in ProMax can be used to model tubular flow reactors in which there is no mixing in the horizontal direction and perfect mixing in the radial direction. Kinetic information must be known and the reaction does not have to come to equilibrium. Reaction set data must be completed which includes stoichiometric equations, reaction order information for some combination of forward, reverse, and equilibrium reactions, rate constant information including pre-exponential and activation energy in a Arrhenius type expression, adsorption term information, and concentration type and units. In addition, heterogeneous catalysis reactions may have the rate specified per mass rather than per volume and require the catalyst particle density. Catalytic reformers are often modeled as a plug flow reactor. A lumped model may be used where a naphtha feed is represented by model paraffinic, naphthenic, and aromatic compounds and reaction equations for naphthene dehydrogenation, paraffin cyclization, and hydrocracking are included.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Equilibrium in ProMax (by Lili Lyddon)

b.cochran@bre.com — Fri, 30 Mar 2007 15:54:00 GMT

Like the Gibbs Minimization reactor, the Equilibrium reactor in ProMax also calculates chemical equilibrium. However for this reactor stoichiometric equation information must be entered by completing the data for a Reaction Set. The required equilibrium constant may be determined by one of two options. ProMax can calculate the equilibrium constant from the Gibbs free energy or it can be entered by the user as a function of temperature. A temperature approach to equilibrium can be specified based on pilot plant data or plant experience. Isomerization reactors for paraffin or xylene isomerization are often be modeled as equilibrium reactors.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Gibbs Minimization (by Lili Lyddon)

b.cochran@bre.com — Fri, 23 Mar 2007 20:03:00 GMT

The Gibbs Minimization reactor in ProMax has the advantage that stoichiometric equations are not required. Equilibrium is determined from the free energy and the heat of reaction is calculated automatically. The method is completely general and predictive. Processes which come to equilibrium or close to equilibrium may be modeled with this technique. ProMax allows the user to choose which components are reactive and which are inert. Setting components reactive and inert is an important aspect in modeling sulfur plants. In the refinery, a hydrogen plant may be modeled using two Gibbs Minimization reactors for the reformer and shift converter. In the higher temperature reformer most or all components are allowed to react. The temperature in the shift converter is lower and only the components in the water-gas shift reaction (CO + H2O = CO2 + H2) are allowed to react.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Hill Notation (by Lili Lyddon)

b.cochran@bre.com — Wed, 21 Mar 2007 13:57:00 GMT

When entering components in ProMax, the components may be filtered based on chemical formula. If the exact formula is known, you may enter the formula in the field and all compounds matching that formula will be listed, subject to other filters. This is a case sensitive match and elements should be entered using their standard upper and lower case combinations. Additionally, a wildcard search technique is available. The percent sign (%) represents one or more of any character. The underscore ( _ ) is used to match any single character. You may place any number of percent signs or underscores in a filter string. If wildcards are used, the filter must be created using CAS Hill notation for the compound. Hill notation requires carbon to be specified first, followed by Hydrogen. After Hydrogen, the remaining elements are entered in alphabetical order. Some examples are: CH4 (Methane), H3N (Ammonia), CH4O (Methanol), ClNa (Sodium Chloride), and CHBr2F (Dibromofluoromethane).

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

Options for Color Blind Users (by Lili Lyddon)

b.cochran@bre.com — Thu, 15 Mar 2007 14:43:00 GMT

In ProMax, color indicates the status of blocks and streams. For the user with normal vision, a red block is unconnected, a blue block is unsolved, an orange block has an approximate solution, and a green block is solved. The color blind user cannot distinguish between some of these colors and thus cannot visually determine block/stream status. ProMax includes some color scheme options which can aid the color blind user in distinguishing between the status of blocks and streams. This is accomplished by modifying the Settings.xml file to change the color scheme. Color schemes are available for Protanope (red deficiency), Deuteranope (green deficiency), and Tritanope (blue deficiency). Note that the Settings.xml file is not generated until ProMax is executed for the first time.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)

User Value Objects in ProMax 2.0 (by Lili Lyddon)

b.cochran@bre.com — Tue, 13 Mar 2007 18:35:00 GMT

New in ProMax 2.0 is the ability to create User Value objects which are values or properties defined by the user. Related User Value objects are grouped into User Value Sets as defined by the user. Any of the predefined unit combinations present in ProMax (e.g., mass flow rate, temperature, density) may be selected for a user value. Alternatively, a custom unit may be defined which combines standard units in ProMax or utilizes unknown units such as currency. The results from a user value can be displayed on the flowsheet in Visio or utilized in dependent calculations in specifiers and solvers. A User Value is typically a property defined by the user which is not available in ProMax. Examples of User Values might be steam rate to the stripper reboiler in an amine unit (lb steam per gallon solution circulation), circulation rate in a glycol dehydration unit (gallon glycol per lb water in the wet feed), a component ratio, acid gas composition (grains per gallon or grains per standard cubic foot,) etc. A User Value can also be a reference value, composition, or property.

Authored by Lili Lyddon (BR&E Technical Support and Help Author)