Infinite Recycle Impacts on Compression Systems

This TOTM will discuss the phenomenon of infinite recycles and their impacts on design, troubleshooting and optimization to increase oil production simply by adjusting setpoint. As well as flaring reduction, unconventionals, identification and solutions.

Question: If you have an infinite recycle of LPG in your gas processing plant, how much horsepower does your compression system need?

Answer: Infinite

Liquefied Petroleum Gas (LPG) is composed of propane and butane. The pressure at which LPG becomes liquid, called its vapor pressure, varies depending on composition and temperature; for example, it is approximately 220 kPa (32 psi) for pure butane at 20 °C (68 °F), and approximately 2,200 kPa (320 psi) for pure propane at 55 °C (131 °F).

In Figure 1 you have the standard propane refrigeration loop. Compressor (K-100) / Cooler (E100) / Separator (V-100) / JT Valve (VLV-100)/ Chiller Load (E101) and recycle back to the compressor.  Under normal operation, stream No 4 has zero flowrate and the system is a closed loop.  If we were to add an additional stream of pure propane to the system at the compressor suction it creates an infinite loop since the vapor out of V100 will still be zero due to the thermodynamic limits of the phase diagram. This system requires infinite horsepower and the rest of the equipment will need to increase in size as well. The molecules are trapped in a loop.

Figure 1 – Propane Refrigeration Loop

Steady State Process simulators generally handle this situation with an unconverged solution or in some instances negative flowrates.  Why is this important?  Although the simulator will not allow the condition to occur.  It can occur during actual operation.

Figure 2 shows a typical three stage compression system, it is like the propane refrigeration system process line up.  In a typical design where are the compressor discharge scrubber liquids sent?  Generally, it’s the previous stage suction scrubber.  Eventually it makes its way to the dry oil tank.  If the material in the scrubbers has propane and butane as a liquid and then sent to an essentially atmospheric tank the liquids will flash, and then be processed by the Oil Stock Tank Vapor Recovery compressor which will send the molecules back to the compression system.  This can set up an infinite recycle or very large recycle of LPG in the compressor system.

 

Figure 2 – Typical three stage compression system – notice a similar process in the boxes.

As shown in Figure 3 the system response to this recycle of LPG from compressor scrubbers will be to load the system to its capacity, and then flare.  This flaring breaks the recycle, but at a cost.

Indications that this is occurring at your offshore platform, gas plant, or unconventional tank battery is a hot flare with a smoky tail.  LPG’s have a significantly higher heating value than methane and the smoke is being caused by high temperatures in the flame cracking the LPG, and creating soot.  The flame will be a deep red rather than orange and we feel higher radiation from flare when walking in the area.

Figure 3 – System response to an LPG infinite recycle will be to completely load the system and then flare the excess gas.

Figure 4 shows how the flash gas composition becomes richer and richer in LPG.  This can lead to infinite recycles in the last two separators of the oil stabilization system. The last stage flash has 47% LPG in the vapor flash gas.

Figure 4 – Impact of stage separation on flash gas composition [1]

When we are in the design phase in field development planning gas compositions have a degree of uncertainty.  It is wise to have multiple dispositions for these LPG streams. They need a way out: sales gas, oil or fuel.

DESIGN SOLUTIONS:

1. Pump interstage liquids into the sales gas using a positive displacement pump.  The limitation here is the maximum heating value of the sales gas.  Liquids may need to be gas stripped of water to avoid hydrates.

2. Pump interstage liquids into the stock tanks oil.  The limitation is crude vapor pressure and additional recycling.

3. Operate your interstage compressor discharge coolers at a high enough temperature to avoid liquid accumulation in the discharge scrubbers.  This option increases the horsepower requirements of the next stage since you are sending higher temperature gas forward to the next stage of compression.  This is also a method to deal with recycles and reduce flaring in operation, but will reduce the inlet oil production since you will be using additional compression HP by operating at higher temperatures.

4. Fuel gas – spike the LPG into the low-pressure fuel gas system.  This is a preferred option, but the direct fire equipment must be designed to accommodate the higher heating value.

5. Change the gas composition by adding hydrocarbon dry stripping gas to the crude oil.

6. If unable to send the molecules to the gas (due to Btu limit) or oil (RVP limit) then a separate disposition like Pipeline. Pressurized trucks or a tanker is needed.

7. Add another process – in offshore consider power generator or low salinity waterflood- generation of distilled water using multi-effect evaporation. (e.g. Mechanical Flare)

Bottom line is that all the molecules need an evacuation route. 

DESIGN CASE No 1:  Offshore Platform Associated Gas Processing

The case in Figure 5 is taken from “Oilfield Processing of Petroleum-Volume 1- Natural Gas” [2].  No recycling of LPG occurs as all liquids from compressor discharge scrubbers are routed to the sales oil pipeline.

Figure 5 – All compressor discharge scrubbers directly sent or pumped into the sales crude pipeline [2]

CASE STUDY No 2:  NGL Recovery Plant with 50,000 bbl Floating Roof Tank Emissions Issues

The NGL recovery plant (Figure 6) processed a small amount of crude oil and was spiking liquids into the crude.  An infinite recycles developed and the stock tanks were degassing large volumes of LPG.  Notice the large temperature drop of the crude going from 145°F (63°C) to 102°F (39°C) as the fluid flashes into the last stage separator. The design intent was to maximize oil volume and eliminate discharge scrubbers, by utilizing the main train separators.

This recycle was broken by adding hydrocarbon dry residue gas from the NGL plant at the inlet to the Electrostatic Treater mixing with the crude oil, and stripping out the LPG breaking the LPG recycle. (Same location near DeC4 bottoms injection point) The blue and red lines show the paths for the LPG Traps.

Figure 6 – Operational problems caused by infinite recycles / butane trap in an oil processing train leading to floating roof storage tank degassing.

CASE STUDY No 3:

An offshore FPSO (Floating Production, Storage and Onboarding) in the North Sea was in operation for two years and its production was constrained by its compression system.  The flare had a smoky tail.

Question: What would you recommend looking for?

Answer: Infinite Recycles.

After some process simulations, the pressure in the last stage of oil separation was increased by 0.5 barg (7.25 psig).  This broke the internal infinite recycle of LPG in the oil dehydration train sending the LPG into the crude oil rather than the flash gas to the compressor.  The resulting oil was within contract vapor pressure limits once cooled.

The upside was that inlet oil production was increased 30% immediately with zero capex!

Lesson- you need to visit the site to identify opportunities.  And if the system is running it doesn’t mean there is no opportunity to improve.

UNCONVENTIONAL DESIGNS:

Many unconventional plays have standard tank batteries with Tank Vapor Recovery Systems and/or Thermal Oxidizers.  Numerous plays have very rich associated gas that form these infinite recycles in their flash gas compression and tank vapor recovery systems.  Simply sending the liquids back to the stock tanks or water tanks will not fix this problem.  The molecules must be sent forward to the gas gathering system.  Also, ensure your design will function in the winter and that LPG condensation in suction piping will not occur, and that stock tanks are insulated and heated to prevent reflux condensation in the stock tanks.

QUICK TIPS:

Do not design for a single point of operation. (i.e. 40,000 bopd and 100 MMSCFD)

To ensure that the desing will work make sure to design during the early start-up phase with high or low rates. Additionally, make sure to design for extreme ambient condition variations in Summer/Winter and Day/Night. The design should also work during shutdown and composition changes over the life of the development.

SUMMARY:

Recycles limiting production are frequently found in oil processing facilities worldwide. However, they are easy to identify and fix with setpoint adjustments to increase production. Now that you know how to identify, prevent and take advantage of it for your company…go out and make some process improvements to your systems and make your company more profitable!

To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Practical Computer Simulation Applications in Gas Processing), PF3(Concept Selection and Specification of Production Facilities in Field Development Projects), PF4 (Oil Production and Processing Facilities), and PF49 (Troubleshooting Oil & Gas Processing Facilities) courses.

PetroSkills offers consulting expertise on this subject and many others. For more information about these services, visit our website at http://petroskills.com/consulting or email us at consulting@PetroSkills.com.

By: James F. Langer, P.E.