Calculating Plant-Wide Emissions in VMGSim
Oil and gas companies worldwide are required to report emissions to obtain permits at various stages of design and operation. VMGSim is continuously evolving to meet all your emissions prediction and reporting needs within your plant model. In VMGSim 9.5 users can calculate flash, tank, and loading emissions from the different sources in a plant model, and analyze results in great detail as well as in an overall summary view. In this article, we show you how you can take advantage of this full set of features to complete your environmental work.
The article includes:
- Background on emission definitions and methodology,
- Description of emissions-specific features in VMGSim,
- An example of a simple model, where
- part 1 goes through calculating the flash and tank emissions of a crude oil storage unit starting with fluid characterization, and
- part 2 is focused on plant-wide emissions,
- An example of a simple model, where
- A brief look at calculating emissions in a rigorous plant model.
Environmental agencies classify air pollutants in a variety of ways. Hazardous air pollutants (HAPs) are pollutants that have been linked to serious health, environmental or ecological effects. These air toxics include benzene and carbon disulfide. Pollutants like sulfur dioxide and nitrogen oxides are known to cause acid rain. Greenhouse gases like methane and carbon dioxide contribute to climate change. The oil and gas industry is the largest industrial source of emissions of volatile organic compounds (VOCs), which contribute to the formation of smog and are thereby linked to a wide variety of health effects.
Sources of emissions in oil and gas plants include flares, venting from separators, breathing and working losses from storage tanks, and losses from fluid transfer. Flash emissions occur when vapor is released from a fluid as a result of a pressure drop, and this vapor is vented to the atmosphere; for example, during the transfer of a fluid from a separator to a vented storage tank. Breathing losses occur as a result of vapor space breathing in the storage tank, as changes in atmospheric conditions cause the vapor in the tank to expand and contract. Working losses occur during tank filling or emptying operations, as liquid clings to the exposed surface area in the tank and eventually evaporates. Loading losses occur during the transfer of liquid from storage tanks into cargo tanks, as stored and generated vapors are released to the atmosphere.
Total emissions from a site have to be estimated and reported to an environmental agency, often before a building or operation permit can be obtained. Emissions are reported in the form of emissions factors, developed by the United States Environmental Protection Agency (EPA) to quantify the pollutants released to the atmosphere as a result of an activity. These values are meant to convey representative long-term averages of emissions for each type of source. The methodology for emissions factor calculation was published by EPA in AP-42, Compilation of Air Pollutant Emission Factors.
VMGSim 9.5 provides all the tools necessary to calculate emissions from your model, so these calculations can be updated and analyzed automatically as the model develops.
The built-in Tank Emissions feature takes care of estimating emissions from any vessel. It provides the ability to compare emissions from alternative tank designs and changing ambient conditions. Loading emissions can be activated in any tank, with estimates taking into account the loading method and control mechanisms in place. Flash emissions can be reported from any vent stream, with rigorous fluid and oil characterization that maximizes the accuracy of these predictions.
Once sources are selected and specified, users can analyze total emissions by component group, emission type, and by source. They can run case studies, add variables to Visio PFD, extend calculations in spreadsheets, generate emissions reports, and more; all inside the VMGSim case.
This example will demonstrate the basic steps needed to estimate and report flash and tank emissions using VMGSim. It will also touch on how other features of the simulator can be used to analyze, optimize, and report emissions in the model.
We will start by calculating the flash and tank emissions of a crude oil storage unit, followed by the emissions from storage of downstream products.
Emission factors calculations are highly sensitive to stored fluid properties, particularly vapor pressure. These properties are rigorously calculated in VMGSim using equation of state or activity coefficient models. The program offers a variety of thermodynamic models tuned for different industries and applications, and the right selection for the system allows for more accurate estimates of flash and tank emissions.
Proper fluid characterization is essential when modeling oils and heavy hydrocarbons. VMGSim has the capability to characterize oil using either the traditional pseudo-component approach or the new PIONA (n-Paraffin, Iso-paraffin, Olefin, Naphthene, and Aromatic) approach. The latter is the state of the art in its ability to match experimental properties of a fluid even with limited laboratory data.
In this example we use the Advanced Peng-Robinson thermodynamic model for fluid property calculations, and the PIONA approach to characterize the crude oil. A recent newsletter delved into the details of this particular PIONA characterization, and following is a brief preview of the data and results.
We set up an Oil Source unit operation to enter laboratory data for the reservoir: a C7+ analysis, MW, API gravity, and the reservoir saturation pressure. After setting operating conditions for the reservoir stream (temperature, pressure, and flow) we click “Regress Parameters” to match the oil properties.
Once the regression is complete, the PIONA oil composition is available, and the Oil Source can be connected to a material stream that we name “Reservoir_Oil”.
We may now use a PVT Analysis unit operation to validate the regression. This unit operation analyzes the behavior of the oil by means of the simulation of some PVT experiments, like Differential Liberation or Separators, and we are then able to compare these results to available experimental data. Details on this functionality will be demonstrated in a future newsletter article. The following graph shows the result of a separator gas analysis on the oil in VMGSim compared with experimental data. The match of results was similar at 100, 200, and 300 psig.
Characterization that predicts fluid behavior so accurately is fundamental to obtaining better emission estimates.
Flash emissions that occur during the transfer of the crude oil to a storage tank can be calculated using a separator. We add a heater in between the reservoir oil stream and the separator to simulate the change in conditions during the fluid transfer.
The separator is set to be at atmospheric pressure and the average flash gas temperature, 90F. The vapor port of the separator represents the flow of gases that is vented to the atmosphere, while the liquid port represents the composition of the liquid that is stored in the tank.
Flash emissions are the mass flows of the vapor port. We can see these in emissions reporting format by opening the connected stream, OilVent, and in the Settings tab opting to show Emissions details.
This enables a “Flash Emissions” tab to appear in the stream, displaying the mass flows in ton/y or ton/d. Emissions are presented by component as well as by pollutant type.
The level of flash emissions in this stream is very high and after analysis it would be clear that the vapor outlet of the separator likely cannot be vented directly and some mechanism for recovery must be added.
We can customize how flash emissions are presented from here, for example adding them to a PFD Datasheet, exporting them to a process calculator, or exporting them to excel. Below is an example of how these values can be displayed on a data sheet.
Emissions that occur during tank breathing or liquid filling/emptying operations can be estimated using the Tank Emissions utility. This tool is accessible through the “Tank Emissions…” button in the separator.
The first objective in this example is to estimate the emissions that would be vented from a vertical fixed roof tank holding the crude oil.
An emissions case is created by default when we first open the Tank Emissions tool. We rename this “Emissions_Case” to “Emissions_VFRT”.
Tank design parameters are input in the Main Data tab, including tank type and dimensions, paint details, and vent settings. The stored liquid is defined as a Crude Oil and its composition is read from the liquid port of the separator, Liq0. The tank is heated, so the liquid bulk temperature is directly specified as 85 F. Ambient data is loaded from the database as we specify that the nearest city to the tank is Houston, Texas. The tank has 80 turnovers per year.
Overall emissions results are shown in the Emissions Summary frame on top of the tabs. These are broken down into standing storage losses and working losses and calculated on an annual basis. The Time Basis parameter can be changed to calculate emissions over a monthly time period instead.
Detailed results are presented in the Storage Tank Loss tab. Values like vapor space volume, vapor pressure of the stored liquid, and losses per component are available here. Component results can be filtered by pollutant type (HAPs, VOCs, GHGs, etc).
All specs and calculations for the emissions case can be exported to an excel template directly from the Report tab.
The results estimate 0.14 tpy (320lb/y) of propane emissions from this tank. We want to analyze the effect that using a floating roof tank instead of a fixed roof tank would have on this value.
A new emissions case is created with the name “Emissions_EFRT” and the new design parameters are input in the Main Data tab.
Initially there is no fitting loss because fitting information has not been added. This can be done in the Fittings tab, where for this example we elect to use the AP-42 recommended set of fittings based on the tank type and dimensions. This list could be customized after deselecting the option to “Use Recommended Set”.
There is now a fitting loss. The design specification is complete and results can be compared. The floating roof is predicted to decrease propane emissions over 50% from 0.14 tpy (320lb/y) to 0.06 tpy (145lb/y).
Values in the Tank Emissions tool can be exposed in the separator form. Enable the Tank Emissions tab in the separator by checking “Is Source of Emissions” in the separator’s Settings tab. The Tank Emissions tab displays the breakdown of storage loss by loss type, pollutant type, and component.
Values in the Tank Emissions tool (and tab) can be used like values in any unit operation. For example, they can be added to a PFD Datasheet as shown below.
These variables can also be used in case studies to analyze the effect of process or operating conditions on emissions. In this example we do a simple case study to view the effect of manipulating the number of annual turnovers on tank emissions.
VMGSim allows users to calculate, analyze, and report emissions from all sources in a site using a single simulation case. Part 2 of this case study will show what this might look like in a basic simulation model, and then a more complex model.
Emissions from crude oil storage were estimated in part 1; in part 2 we will be estimating emissions from storage of the downstream liquid products. Since products heavier than naphtha are unlikely to have significant emissions of the lighter pollutants in the case, we will focus only on the storage of naphtha and straight-run naphtha.
We can use component splitters to model ideal distillation of the crude oil by boiling points. A tower unit operation could be used to obtain rigorous results, but at this point the goal is to get a rough approximation of the stored liquid composition in each tank.
The flowsheet is expanded as shown below, with the stored liquid stream from the crude oil tank connected to a component splitter.
We use the “Assign Splits by Group” option in the component splitter to separate products by boiling point. The bottom splitter sets all components with a normal boiling point higher than 380 F to be heavier products.
All other components are sent to the second component splitter to be separated at 200F, into naphtha and lighter products.
Finally, a separator is used to model the partial condenser at the top of the tower that separates straight-run naphtha from light ends. The condenser is assumed to operate at 100 F and 20 psia.
Product storage tanks are modeled by connecting separators to the naphtha streams. Each separator is specified to operate at atmospheric pressure and flash gas temperature. A cooler is used where required to model changes in fluid conditions during transfer to the tanks.
Once this rough model is set up, we can open the Emissions Summary form to manage multiple sources of emissions in the case. This form is accessible through the Earth icon in the toolbar, and from a link in the Flash Emissions and Tank Emissions tabs.
The Emissions Summary form reports total emissions from all sources in the case: vent (flash emissions), storage (tank emissions), and transportation (loading emissions). Emissions are summarized by pollutant group, by source type, and by source/unit operation.
In the Venting tab, the only stream that is selected as a source (“Is Vented”) is the OilVent stream because earlier we selected the option to show Emission details for the stream. We can also check “Is Vented” for the two vent streams that we have just added, SRNaphthaVent and NaphthaVent.
Next, we look at the Liquid Storage tab. Once again only OilTank is selected as a source (“Is Tank”) because earlier to view the Tank Emissions tab we indicated in its settings that it is a source of emissions. We can now select “Is Tank” for SRNaphthaTank and NaphthaTank to activate them as sources of emissions.
The cell under “Edit” with an ellipsis is a shortcut to open the Tank Emissions form and view or edit details of the tank emissions case in the previous cell.
Finally, we can switch to the Transportation tab and indicate which separators are sources of loading emissions. Assuming liquid product in the straight-run naphtha tank and the naphtha tank will be transported by ground, we select these two separators.
Once we have selected all sources, the form can be simplified by checking the “Sources Only” option in the top frame.
Loading emissions are calculated inside the Tank Emissions tool, alongside storage tank emissions. These calculations can be activated from the summary form as above, from the Tank Emission tab, or within the Tank Emissions form.
When loading calculations are active, users can specify the loading method, carrier history, and control mechanism details. These are all used to determine the most appropriate emission factors to use in calculations. Detailed results are reported in the Loading Loss tab.
In this case specifying the loading method as submerged loading and indicating a control mechanism with 70% collection efficiency and 95% control efficiency (typical of VRU units) is enough to reduce the loading loss from 750lb/y to 100lb/y.
The summary for the case indicates that by far the biggest issue is flash emissions in the OilVent stream and control mechanisms must be added.
Simple models like this can provide initial estimates for plant-wide emissions at early stages of a project. After this initial setup the model is free to continuously evolve as more field data and/or design specs become available, and emission estimates will be updated automatically with the model.
It is just as easy to add emission estimates to a complete and rigorous plant model. For example, open the PIONA fluid catalytic cracking plant example in VMGSim. This is accessible through the main menu’s File > Plant Examples.
Add a new subflowsheet in the case and set up a storage tank in it for the plant’s gasoline product. Spec the separator at atmospheric pressure and flash gas temperature, and connect the “Gasoline(145)” stream from the main flowsheet to the cooler.
Flash emissions from the tank transfer are now calculated and available in the Flash Emissions tab of the stream, and the tank emissions can be estimated using the Tank Emissions tool in the separator.
This simple process can be repeated for any liquid feeds or products in the case that go to storage. The rigorous model is expected to give precise emissions estimates.
Angela Solano, EIT, Process Simulation Software Developer & Herbert Loria, Ph.D., P.Eng., Sr Reasearch and Development Engineer
Please contact your local VMG office for more information.
- “National Air Pollutant Emissions”, Environment Canada, 9 May 2014. Web. 24 Jan. 2015.
- “Oil and Natural Gas Air Pollution Standards”, US EPA. Web. 22 Jan. 2015.
- US EPA, Document AP-42 Chapter 7, Organic Liquid Storage Tanks, Fifth Edition, 2006
- McCain, W.D. The Properties of Petroleum Fluids, PennWell Books 1990
- US EPA, Document AP-42 Chapter 5.2, Transport and Marketing of Petroleum Liquids, Fifth Edition, 2006