Category: Gas Processing

In a previous “Tip of the Month” we briefly discussed the need for understanding a phase diagram in a gas processing system. We also defined the areas of a light mixture phase envelope and the terms necessary to “talk intelligently” about the shape of a mixture phase diagram. This allowed us to look at the methods of calculation and their limitations in another tip and eventually defined our areas of risk in the operation or design of a facility based on the phase diagram.

In this Tip we will explain how a phase envelope is generated and what factors affect the shape and accuracy of a phase envelope. There are two methods of generating a phase envelope: a) by conducting a series of bubble point and dew point measurements in a PVT laboratory b) Using a cubic EoS such as SRK or PR and performing a series of bubble point and dew point calculations. The triangle symbols in Figure 1 present a phase envelope measured in a laboratory for a synthetic natural gas.

For the same mixture, we used GCAP for Volume 2 of Gas Conditioning and Processing Software to generate the phase envelope using SRK and PR EoSs. The dashed line represents the SRK EoS and the solid line represents the PR EoS. As can be seen, for this case the SRK EoS gives a perfect match with the experimentally measured dew point curve but PR predicts a lower cricondentherm point. The built in and in–house binary interaction parameters for SRK and PR were used to generate these two diagrams; however, an experienced engineer is able to produce a close match for either of these two EoSs by tuning the binary interaction parameters and/or heavy end properties.

The best practical method for generating an accurate phase envelope by any commercial software is to utilize a limited number of VLE measurements and tune one or more properties of the heavy ends (C₇₊). In other words, we suggest using combination of methods “a” and “b”.

For real natural gas mixtures, the components and composition of the heavy ends are not well defined and laboratory measurements are not accurate enough. Therefore, different techniques as described in the literature and Volume 3 of Gas Conditioning and Processing are used to properly characterize the heavy ends resulting in an overall acceptable match.

Since the laboratory reported values of molecular weight (MW) and specific gravity (SG) of C₇₊ are questionable, in the proceeding section we will demonstrate the impact of these two properties on the shape of overall phase envelope.

Figure 2 presents the impact of molecular weight of C₇₊ on the phase envelope of a rich natural gas mixture. As the C₇₊ molecular weight increases, the cricondentherm temperature and cricondenbar pressure will increase and the two phase region expands.

Figure 3 shows that as the C₇₊ specific gravity increases, the cricondentherm temperature and cricondenbar pressure will also increase and the two phase region expands.

From these two diagrams, one can see that improper characterization of heavy ends may result in a bad design or troublesome operation (e.g. designing a dry gas pipeline instead of two phase gas-liquid flow).

In the next tip of the month we will demonstrate how to tune computer software to generate proper and relatively accurate phase envelopes.

By: Dr. Mahmood Moshfeghian

Reference:

1. C. Jarne, S. Avila, S. T. Blanco, E. Rauzy, S. Otin, I. Velaso, Ind. Eng. Chem. Res. 43 (2004) 209-217.

In a previous “Tip of the Month” we briefly discussed the problems of how to “name” the various regions of the phase diagram. To decide if this “naming game” can be a problem, look at Figure 1.

This is a typical two-stage compressor system with intercoolers. This particular system however is being used to compress gas to a pressure above the cricondenbar of the gas for “dense phase” transportation. Note that the streams are only labeled with letters because I used numbers on the phase diagram. As a “process engineer” I find it difficult to discuss streams with out seven digit numbers identifying them.

Point A is easily “defined” as a saturated vapor. Point B will be superheated vapor or possibly dense phase. Point C might be 2 phase, super heated vapor, dense phase, or liquid phase. From an equipment point of few in all cases but the two-phase case, separator V-2 is not needed, in theory. In practice, we would always install V-2 as very minor changes in temperature or pressure can sometimes dramatically change the quantity of liquid in stream E.

For computer simulation of this process, the designation of the “phases” of stream C becomes quite important. Assuming the simulator has the ability to calculate Vapor-Liquid equilibrium properly, the two-phase case is the only case where the simulator can be trusted to give the proper duty requirements for compressor C-2.

Figure 2 is the “generic” phase diagram for a light mixture that we previously discussed. The general areas are: to the left of the phase diagram – liquid; area to the right of the phase diagram – vapor; the area inside the phase diagram – two phase; and the area above the phase diagram – dense phase.

My personal preferences for the sub areas are listed below:
Area 1 – Vapor
Area 2 – Vapor
Area 3 – Vapor
Area 4 – Vapor
Area 5 – Dense phase
Area 6 – Liquid
Area 7 – Liquid
Area 8 – Liquid
Area 9 – Vapor

The stream labels from figure 1 are plotted on the diagram. Note this assumes that the phase diagram represents the composition of the gas leaving separator V-1.

If stream C is “named” vapor by the simulator, then most computer simulations will properly calculate the power requirements of C-2. If stream C is “named” liquid by the simulation program then C-2 will have no flow through it. The program will either “fail” or worse yet give meaningless answers. Worse yet some simulators identify point C as two-phase with an arbitrary split between the flow rates of streams D and E.

The current trend in commercial simulators is to always “give an answer” no mater how “stupid the question is.” Most simulators will generally give warnings when calculating a “meaningless” answer, but with the current shortage of engineers, the chances of these being identified are fairly small.

This is a simple example of how “naming” can get you in trouble. There are many more that occur subtly inside simulations. Many of these errors go undetected because the program is quite “happy” with whatever phase it calculates.

For examples of how the choice of the name might be important in your process simulation calculations, contact JMC@jmcampbell.com for test simulations that you can use with your process simulator.

At a future date we will discuss the meaning of “condensate” and why this is so politically important even today.

In the meantime Chapter 4 and 5 of Volume I and chapter 15 of Volume II of Gas Conditioning and Processing and Chapter discusses these topics in more detail.

The stream labels from figure 1 are plotted on the diagram. Note this assumes that the phase diagram represents the composition of the gas leaving separator V-1.

WHAT If stream C is “named” vapor by the simulator, then most computer simulations will properly calculate the power requirements of C-2. If stream C is “named” liquid by the simulation program then C-2 will have no flow through it. The program will either “fail” or worse yet give meaningless answers. Worse yet some simulators identify point C as two-phase with an arbitrary split between the flow rates of streams D and E.

The current trend in commercial simulators is to always “give an answer” no mater how “stupid the question is.” Most simulators will generally give warnings when calculating a “meaningless” answer, but with the current shortage of engineers, the chances of these being identified are fairly small.

This is a simple example of how “naming” can get you in trouble. There are many more that occur subtly inside simulations. Many of these errors go undetected because the program is quite “happy” with whatever phase it calculates.

For examples of how the choice of the name might be important in your process simulation calculations, contact JMC@jmcampbell.com for test simulations that you can use with your process simulator.

At a future date we will discuss the meaning of “condensate” and why this is so politically important even today.

In the mean time Chapter 4 and 5 of Volume I and chapter 15 of Volume II of Gas Conditioning and Processing and Chapter discusses these topics in more detail.

By: Dr. Larry L. Lilly

In a previous “Tip of the Month” we briefly discussed the need for understanding a phase diagram in a gas processing system.  In this Tip we will try to clearly define the areas of a light mixture phase envelope and the terms necessary to “talk intelligently” about the shape of a mixture phase diagram. This will allow us to look at the methods of calculation and their limitations in another tip and eventually define our areas of risk in the operation or design of a facility based on the phase diagram.

The figure is a “generic” phase diagram. The general areas are 1) to the left of the phase diagram – liquid; 2) to the right of the phase diagram – vapor; 3) inside the phase diagram – two phase; and 4) above the phase diagram – dense phase.

However, note that there are several sub-areas that might be questioned as to their exact phase name.  For instance the area marked “3” is “above the phase diagram but below the highest pressure that two phases can exist.  Should this region be called “vapor” or “dense phase?”  Or perhaps the more important question is:  “Do I care?”  The answer to the second question is “yes, sometimes.”  The answer to the first question is “maybe, sometimes.” With these two very clear answers in mind, maybe the general criteria needs to be further defined.

For a quick review let’s start with pure component and mixture phase diagram differences.

For a pure component an acceptable definition of the CRITICAL POINT is “the highest temperature and pressure that two phases can exist for a given component.”  The critical point for our generic mixture is point C. This is an experimentally determinable point and is the point where the mixture properties in the vapor phase and the liquid phase are the same.  As shown on the diagram there is a considerable region at temperatures higher than the critical point that is two phase and a smaller region of pressures higher than the critical pressure that is also two phase.

Two new terms are introduced to describe these regions and their limits.  The first term is the name given to the highest temperature at which two phases can coexist.  This is point T in the figure.  This point is called the cricondentherm. The second term is the highest pressure at which two phases can coexist – point B. This point is called the cricondenbar. Any facility that operates at a temperature always higher than the cricondentherm will never condense liquids.  Any process that is operated at a pressure higher than the cricondenbar will not be two-phase. A process in this region can be liquid, vapor or “dense phase”, but never two at the same time. Dense phase pipelines are designed to always be at pressures above the cricondenbar. Dewpoint control is often a matter of controlling the temperature of the cricondentherm for sales gas so that the gas pipeline is not two-phase at the coldest temperature in the system.

Now back to defining each of the areas in the figure.  The easy areas first:  Areas 1 and 2 are generally called vapor. Areas 7 and 8 are generally called liquid.  Areas 5a and 5b are generally called dense phase.

Area 3 (vapor or dense phase) is generally called a vapor because it is below the cricondenbar (B) and condenses liquid when the pressure is decreased at constant temperature.

Area 4 (vapor or dense phase) could be called vapor or dense phase depending on the exact definition used. For instance, if all areas above the cricondenbar (B) are defined as dense phase then area 4 is dense phase. If all areas to the right of the cricondentherm (T) are defined as vapor then area 4 is vapor.

Area 6 (liquid or dense phase) could be called liquid or dense phase again depending on our exact definition.  If all areas above the cricondenbar (B) are defined as dense phase then it is dense phase. If all areas to the left of the critical point (C) are defined as liquid then it is liquid.

Area 9 (liquid, vapor, or dense phase) could be interpreted as any of the phases depending on your definitions.  Some people would define any area to the left of the cricondenbar (B) as a liquid.  This would imply that areas 5b, 6 and 9 are liquid.  Most people would define any single-phase fluid outside of the dewpoint line and below the cricondenbar (B) as a vapor.  This would imply that area 9 is a vapor.  Some people would define any single-phase fluid above the critical point (C) as dense phase.

My personal preferences are:

Area 1 – Vapor
Area 2 – Vapor
Area 3 – Vapor
Area 4 – Vapor
Area 5 – Dense phase
Area 6 – Liquid
Area 7 – Liquid
Area 8 – Liquid
Area 9 – Vapor

These choices are made based on the answer to the “Do I care?” question.  In a future tip we will discuss how these names (doesn’t a rose by any other name smell just as sweet?) do make a difference from a simulation point of view and from an interpretation of real world problems.

In the meantime Chapter 4 of Volume I Gas Conditioning and Processing discusses these topics in more detail.

By: Dr. Larry L. Lilly

 

In facilities operations the understanding of where the process is on a phase diagram can often help the engineer and operator avoid extremely embarrassing design and operating mistakes. The oil and gas industry is full of many “war stories” about “phase diagram disasters.” Most instances are never related back to the phase diagram misunderstanding.  In one well-documented but poorly published case a “dry gas” pipeline that was pigged flooded miles of sandy beach.  In another case thousands of kilowatts of compression power were installed to maintain the pressure of a reservoir above the dew point when in fact the reservoir was at a temperature above the cricondentherm.  In many cases equipment manufacturers and purchasers of gas have specifications of “superheat” or dew point that have not been met and led to upset customers and/or millions of dollars of lawsuits.

One of the first issues to be resolved by a facilities engineer working in a gas plant or gas production facility is where is the process operating with respect to the phase diagram.  A general knowledge, if not a detailed knowledge, will allow the design engineer and the facilities operator to make intelligent decisions that have significant impact on the profitability of a gas production facility.

The following figure is a “generic hydrocarbon mixture” phase diagram for a lean gas.  The area to the left of the Bubble Point line is the sub-cooled liquid region.

The area to the right of the Dew Point line is the super-heated gas region.  Between these two lines the mixture is two-phase. Other areas of interest are the retrograde region and the supercritical region. Each of these regions provides advantages and disadvantages for operations.

This month we will start to define the points of interest so that we may choose proper operating points for various types of processes.  The first point to define is the cricondentherm.  The definition of this point is the highest temperature at which two-phases (liquid and vapor for most processes) can coexist.  In the drawing above this is point M.  Point M has considerable theoretical and practical importance.  For example, if the cricondentherm for a sales gas (point M) is 0 ºC (32 ºF) cooling the gas to 4 ºC (40 ºF) at any pressure will not result in condensation of liquids.  This type of operation is typically the type used for cross-country transportation of gas in pipelines.  Operation with this type of system will not require “slug catchers” at the end of the pipeline and will significantly decrease pressure drop in the pipeline.

If the gas were processed in a cold separator such that point B (a dew point) was 0 ºC (32 ºF) problems could occur in the same conditions as the pipeline mentioned above.  If the pressure of the pipeline was between the pressure of point B and E and the pipeline cooled to 4 ºC (40 ºF) there could be significant quantities of liquid in the pipeline.  If the operations people were not familiar with the phase diagram they might increase the operating pressure of the cold separator and still keep the temperature at 0 ºC (32 ºF).  This action would result in increased liquids in the pipeline, not decreased.  However, if the cold separator was operated at the pressure of point M, at a temperature of 0 ºC (32 ºF), in theory there would be no liquids in the pipeline again. (More about the difference between theory and practice in future tips).

If you want more information about how to use phase diagrams to improve profitability of operations or how to generate a phase diagram similar to the calculated drawing below, from a process simulator using equations of state, use our search engine above.  You may look for a course to attend, books or software to buy or other articles on this web site.

Suggested search words: phase diagram, dew point control, equation of state, process simulation, gas conditioning and processing.

By: Dr. Larry L. Lilly