NGL Recovery Processes

Cryogenic low temp distillation processIn this process, the heavy hydrocarbons in the feed gas are removed using compression and cooling method. At first, the feed is flashed in a flash chamber. The vapour then undergoes a compression and then further cooled. The compression happens in high pressure while the cooling method is fulfilled using external refrigeration cycle or expansion of the products through a turbo-expander.

The next step involves a distillation column. Distillation column used in this process has many stages due to the low temperature in the process. Distillation column is used to separate methane from the natural gas liquids. The methane is processed further before it is being commercially sold.

Turbo expander or ISS

This process is regarded as the simplest and earliest forms of NGL recovery. The turbo expander represents a centrifugal flow axial turbine (expander shaft) where high pressure gas is expanded to produce work (cooling effect)(Park et al, 2014). This process is an isentropic process that’s work done highly depends on the pressure of the gas. This process has a maximum ethane recovery of 80%. The cold separator of this plant typically operates somewhere between -200C to -350C and these temperatures highly depend on the composition of the gas and the required recovery of ethane. The tops of the demethanizer operate at a range between -800C and -1100C while the bottom operates between 00C and -200C (Kidnay et al, 2011).

Some of the limitation towards the design and operation include; the stability of the process may be at harm due to the critical temperature and pressure of the mixture affecting the cold separator, occurrence of CO2 solidification and for a higher C2

+¿¿

recovery more compression is required.

The outcome of turbo expander is at a very low temperature hence a partial liquefaction of the gas is to be expected.

Figure: The figure on the left represents a turboexpander from a cutaway view, while the figure on the right represents an example of a turboexpander in a plant with its compressor at the side (Kidnay et al, 2011).

PFD Design

There are several designs in cryogenic low temperature distillation process. The original design is called the ISS Process. This process is the base case in cryogenic separation.

As described in the PFD above, the separation begins when stream 12 goes into a flash chamber (V-100). In the flash chamber, the mixture is flashed into stream 13 which consists of vapour and stream 14 that consists of liquid. Stream 13 goes into the turbo-expander (K-100) where refrigeration process happened. The outcome of this process is stream 16. The energy that is produced from refrigerating the vapour is collected in the turbo-expander. This energy is converted to mechanical energy to run the shaft to booster compressor (K-101).

As for stream 14, it undergoes expansion in expansion valve (VLV-100) to increase the pressure so that it matches the tower pressure. The outcome of this process is stream 15.

Stream 15 and 16 is then fed into the demethanizer column alongside with some recycled stream (PA1 and PA2). In the demethanizer column, methane is stripped from the stream as a gas and vent out as stream 17. The steam in stream 17 is used to cool stream 8 then partially compressed in K-101 before it is sent to the sale gas recompression unit (Getu, 2013).

Process Improvements

There are several alternatives available for this process, such as Gas sub-cooled process (GSP), cold residue gas-recycle (CRR), recycle split-vapor (RSV), Enhanced NGL recovery process (IPSI-1), and Internal refrigeration for enhanced NGL

recovery process (IPSI-2) that were developed to overcome these and other limitations (Getu, 2013) . Each of these processes has unique characteristics that will be explained below.

Gas sub-cooled process (GSP)

The main difference in this process compared to the ISS is that there is an additional flash chamber (V-101) after the first flash chamber (V-100) that separates the vapour and liquid of the feed. The liquid outcome stream of the additional flash chamber which is stream 18 enters from the top while the outcome from the turbo-expander (K-100) enters from the middle of the demethanizer column. The additional flash chamber (V-100) results in higher methane purity obtained. Hence, this process allows the demethanizer to reduce ethane loses (Kidnay et al, 2011).

The columns top temperature highly affects the amount of ethane recovery of the column. Hence a lower temperature is required in order to lower the ethanes volatility that results in more ethane being recovered at the bottom of the column. This can lead to the icing of the of the column due to CO2. However, this problem can be mitigated by using a flash drum at stream 16 that in turn warms the column and reduces the risk of CO2 freezing.

With the introduction of a separator V-101, the GSP is allowed to reduce the entrainment of ethane at the top of the column with the products. This allows the ethane to be stripped before being introduced into the demethanizer clolumn (Getu et al, 2013) . The composition of the reflux stream heavily influences the recovery levels of the process. Hence with the improvement of the split vapour concept resulted in more processes developed (Kidnay et al, 2011).

Figure: PFD for the GSP process (Getu et al, 2013).

Cold residue gas-recycle (CRR)

There are several differences in this process compared to the ISS Process. First, there is an additional compressor used in this process. The additional compressor is used to treat the vapour that comes out from the demethanizer column. Secondly, the vapour that comes out of from the flash chamber is being split into two streams. One stream goes straight to the turbo-expander while the other stream is mixed with the liquid stream and then being condensed before goes into the demethanizer column.

The result in these improvements is of higher ethane purity than the GSP process due to the addition of the split stream 18 that is used as a column top feed, which can now recover the power of expander K-100. However, compressor K-102’s horsepower will be increased due to this addition (Getu et al, 2013). Furthermore, the addition of the compressor relates to a higher CAPEX and OPEX (Park et al 2014). This process can also operate at an excess of 99% propane recovery for near complete rejection of ethane (Pitman et al, 1998).

Figure: PFD for the CRR process (Getu et al, 2013).

Recycle split-vapor (RSV)

This improvement is similar to the CRR process. The significant difference can be with the RSV process that provides the bulk ethane recovery in the tower by using split-vapour feed of steams 20 and 21 (Getu et al, 2013). A small portion of the methane is refluxed from the tower and recompressed, condensed and subcooled. The leaner methane is then flashed to the feed with the pressure of the top of the tower without using a compressor step. This design reduces the capital cost of the plant and overcomes the limits in recovery due to the vapour-liquid effect. As market prices change, the process can switch between both ethane recovery and ejection. Furthermore, this process allows the system to operate in GSP mode, at reduced ethane recoveries the gas processor is allowed to process higher inlet volumes. This design can accommodate higher demethanizer operating pressures in comparison to GSP design due to having a higher tolerance to CO2 at any recovery level.

Figure: PFD for the RSV process (Getu et al, 2013).

Enhanced NGL Recovery Process (IPSI-1)

This process was introduced for the purpose of high recovery and focused on the improvements at the bottom of the demethanizer column (T-100). This is done by introducing pumps PA, PA1 and PA2 as well as product stream 25. With PA2 being the most significant addition, this allows the splitting of steam 30 and 31 where stream 31 expanded in order to be used for cooling. Stream 25 and 35 are then mixed to form a NGL product stream. The flashed vapour stream from stream 34 is to compressed into streams 38 and 39, these streams mainly comprise of ethane and propane. The compressed stream 39 mixed with stream 41 after being cooled by cooler C-101. Stream 32 is then used to cool stream 5 and is then mixed with stream 43 before entering the dementhanizer (Getu et al, 2013).

The energy requirement of the reboiler can be significantly reduced due to the stripping gas stream PA2. This leads to the temperature of the demethanizer being reduced allowing heat integration to become more efficient. The reduction of external refrigeration and providing reboiler duties can be achieved due to the lowering of the temperature of the demethanizer by allowing more feed gas into the system. In addition, NGL recovery and separation efficiency will be increased due to the introduced stream from PA2 (Getu et al, 2013). For an increase in capacity, more refrigerant is to be used to compensate. In order to achieve a more flexible process, ISPI-2 was introduced to overcome the problem of needing more refrigerants by installing a close refrigeration loop (Park et al, 2014).

Figure: PFD for the IPS-I process (Getu et al, 2013).

Environmental Effects

This process has a bright future as no harmful material is produced. The turbine-expander process will also bring good feedback to the surroundings as no pollution and solid waste is produced. This will also further help with global warming matters as greenhouse gasses production can be avoided.

In addition, the turboexpander uses a non-oil based cooling lubricating fluid. This oil is made out of nitrogen fluid that is liquefied and is regarded as safe, non-combustible and non-corrosive. Furthermore, no residue for contamination (to surroundings and operators) as the liquid nitrogen evaporates and returns to the atmosphere. This can also save costs for disposal of the liquid nitrogen in comparison to the lean absorption process that uses oil (Pušavec* and Kopač, 2011).

Process Economics

In terms of process economics, this process requires a high amount of energy in comparison to the lean absorption process. For example, the demethanizer reboiler and salesgas compressor are the highest consuming units in the process. While there are no materials required by the process except for the feed gas, most of the operations cost comes from the energy required for the process to operate. This still gives us a rough idea on which technology provides a better cost on the long run.

Graph of energy consumed by demethanizer reboiler against NGL produced (Getu et al, 2013).

In order to compare the different technologies used to cryogenic low temperature distillation processes, according to () the graph above indicates that IPSI-1 uses the least energy in comparison to the other technologies. This in return saves a significant cost on electricity due to the efficiency of the self-refrigeration system.

However the IPSI-1 system requires higher energies when dealing with rich feeds but is still lower than the other technologies.

Graph of energy consumed by sales gas compressor against NGL produced (Getu et al, 2013).

Furthermore, the duties for compression tend to be lower for leaner feeds. The RSV technology requires a higher duty due to its large recovery stream that is needed for the ethane recovery while the other technologies use similar amounts of energy for the different quality of feeds. Hence, this factor plays a significant role while determining which technology is more economic.

The table above shows the common parameters for the turbo-expander technology (Getu et al, 2013).

Based on these parameters, a profitability analysis was conducted by (insert reference) that summarizes the cost in order to forecast the profitability of the project.

Total annualized capitol cost ($)

Lean feed = 737,537

Rich feed = 837,850

Total annual operating cost ($)

Lean feed = 1,433,227

Rich feed = 4,907,276

Sales gas product revenue ($/year)

Lean feed = 64,075,454

Rich feed = 51,428,826

NGL product revenue ($/year)

Lean feed = 8,291,994

Rich feed = 26,944,069

Gross profit ($/year)

Lean feed = 70,934,221

Rich feed = 73,465,618

Net profit ($/year)

Lean feed = 49,653,955

Rich feed = 51,425,932

Payback time (year)

Lean feed = 0.015

Rich feed = 0.016

Table 1:The table above shows a comparison between turboexpander plants and conventional plants (Bloch and Soares, 2011).

Comparison between absorption and cryogenic plants

Parameter Absorption Cryogenic

Temperauture 240-3000K 170K

Fuel Consumption 2-4% 1-2%

Ethane Recovery 0-35% 60-90%

Propane Recovery 50-90% 92-98%

Table 2 :EPA Multimedia Assesment of the Natural Gas Processing Industry

Table 3: Shows the amount of equipment required to operate a turboexpander plant and a refrigeration plant of 2.5 MMscmd capacities (Bloch and Soares, 2011)

According to tables (), () and (); it is evident that from an economics perspective that the turbo-expander process uses less energy, number of equipment and has a higher efficiency in comparison to the refrigeration/absorption plants. This will allow the company to save a substantial amount of costs. However the turbo-expander technology requires a higher CAPEX and this factor must be considered.

Safety Considerations

Due to using low temperatures for the gas cooler and demethanizer reboiler and the high pressures required for the turboexpander, safety considerations must not be overlooked for these units (Kidnay et al, 2011). To overcome the high pressures, pressure relief valves should be installed at the process to ensure that the system does not become over-pressurized especially when there is CO2 icing in the columns.. Furthermore, in case of icing of the column, safety precautions should be taken by the operators during handling. There are risks associated with the plant in terms of fires and explosions due to the NGL. However these are common throughout all of the processes suggested and can be overcome with

flares in times of an emergency when there is a leak or loss of containment from the pipelines. The nitrogen oil used does not pose any threats to the operators during leaks as the nitrogen does not carry any hazards.

Advantages and disadvantages of the turboexpander proces

Advantages

High ethane recovery Nitrogen oil used is non-hazardous and more efficient than the oil based lubricating

fluids The bottom of the demethanizer operates between 00C and -200C and this allows

for a higher recovery. Robust rotor design can handle deposit formed by CO2. Environmentally friends due to no harmfull emissions emitted.

Disadvantages

Low temperature demethanizer top Carbon dioxide icing may form A significant amount of compression is required in order to increase the C2

+¿¿

The use of a turboexpander has a high CAPEX and OPEX. Lose efficiency when feed rates are below 0.2x106 Nm3/d High pressure drop required for turbine-expander

References

https://books.google.com.au/books?id=Ro_LBQAAQBAJ&pg=PA297&lpg=PA297&dq=turbine+expander+NGL+recovery+safety+and+environment&source=bl&ots=h7WA2n4lLD&sig=n3cgm4KF5_lK0L5lJ-XeiENXHnk&hl=en&sa=X&ved=0CBwQ6AEwADgKahUKEwiFw-vO2bTHAhVlx6YKHXOmBeU#v=onepage&q=turbine%20expander%20NGL%20recovery%20safety%20and%20environment&f=false

http://www.researchgate.net/publication/270595616_Techno-economic_evaluation_of_a_novel_NGL_recovery_scheme_with_nine_patented_schemes_for_offshore_applications

https://books.google.com.au/books?id=Z-RY1Emx4JwC&pg=PA71&lpg=PA71&dq=safety+and+environment+consideration+on+turbine-expander+process&source=bl&ots=gWl0JAtqxa&sig=9dhw8_2_u8EVExppHCbeOvLeIBw&hl=en&sa=X&ved=0CCQQ6AEwAWoVChMI0auDsdC0xwIVZF2mCh37oQm9#v=onepage&q=safety%20and%20environment%20consideration%20on%20turbine-expander%20process&f=false (Bloch and Soares, 2011)

http://www.researchgate.net/publication/270595616_Techno-economic_evaluation_of_a_novel_NGL_recovery_scheme_with_nine_patented_schemes_for_offshore_applications (Park et al, 2014)

https://books.google.com.au/books?id=PiEtBAAAQBAJ&pg=PA187&lpg=PA187&dq=recycle+split+vapor+process&source=bl&ots=sBvMH7UdmP&sig=PU59Tq3j42WmUa7gBBfdKQu1Clc&hl=en&sa=X&ved=0CGMQ6AEwCWoVChMIj8j85quyxwIVgS6mCh1k4QBK#v=onepage&q=turboexpander&f=false

http://www.ou.edu/class/che-design/che5480-07/Next%20Generation%20NGL-LPG(Hudson%20et%20al)-98.pdf (Pitman et al, 1998)

, http://www.sciencedirect.com/science/article/pii/S026387621300035X# (Getu et al, 2013)

http://www.researchgate.net/publication/270595616_Techno-economic_evaluation_of_a_novel_NGL_recovery_scheme_with_nine_patented_schemes_for_offshore_applications (Park et al, 2014

http://www.sv-jme.eu/data/upload/2011/09/01_2010_249_Pusavec_04.pdf (Pušavec and Kopač, 2011)

https://books.google.com.au/books?id=Ro_LBQAAQBAJ&pg=PA297&lpg=PA297&dq=turbine+expander+NGL+recovery+safety+and+environment&source=bl&ots=h7WA2n4lLD&sig=n3cgm4KF5_lK0L5lJ-XeiENXHnk&hl=en&sa=X&ved=0CBwQ6AEwADgKahUKEwiFw-vO2bTHAhVlx6YKHXOmBeU#v=onepage&q=CRR&f=false (Kidnay et al, 2011)

https://books.google.com.au/books?id=PiEtBAAAQBAJ&pg=PA187&lpg=PA187&dq=recycle+split+vapor+process&source=bl&ots=sBvMH7UdmP&sig=PU59Tq3j42WmUa7gBBfdKQu1Clc&hl=en&sa=X&ved=0CGMQ6AEwCWoVChMIj8j85quyxwIVgS6mCh1k4QBK#v=onepage&q=recycle%20split%20vapor%20process&f=false