Natural Gas Liquefaction 2

[JimCasey]

Can anybody tell me where I can look up a reasonably representative schematic of a natural gas liquefaction process? The boss had some questions and the kind of stuff I am hitting on the web has more-than-vast oversimplification ( e.g.: two tanks and a pump) . Need a little detail; pressures and temps at various critical points in the process, etc.

Thanks in advance.

Jim

 

[BigInch]

Jim, Here's the best I ran into last year. A little short on temps and press, but pretty good PFDs.

http://www.ccivalve.com/pdf/429.pdf

http://virtualpipeline.spaces.msn.com

 

[JimCasey]

Thanks BigInch. A star for you.

 

[123sen]

Boil off gas CYCLE

The BOG cycle is an independent loop that consists of the pre-cooler, BOG-compressors, plate-fin cryogenic, separator, vent gas heater and LNG return pump. The plate fin cryogenic heat exchanger and the separator.

Boil-off gas is removed from the LNG tanks by means of a centrifugal BOG compressor. The BOG is compressed to 3.5 bar g and then cooled and condensed at this pressure.

To enable stable temperatures at inlet to the heat exchangers, a pre-cooler is installed upstream of the BOG compressor with a temperature controller. The pre-cooler is controlled based on both the BOG compressor suction temperature and discharge temperature.

The condensate (LBOG) leaves the cryogenic platefin heat exchanger at a temperature of about 159.5°C. Incondensable gases are separated in the separator and routed to the flare units where the entrained methane is burned. Liquid leaves the separator by level control and is returned to the storage tanks driven by the differential pressure between the separator pressure and storage tank pressure.

The LNG return pump is normally not used during normal operation. However, if reliquefaction is done by free flow, the differential pressure between the separator and storage tank is only the liquid column in the separator, which may not be enough to push the liquid to the tanks. During this operating mode the LBOG return pump are used to push the LBOG to the cargo tanks.


NITROGEN CYCLE
The refrigerant capacity is produced by a nitrogen compression/expansion cycle. Nitrogen gas at a pressure of 13 bar a is compressed to 56 bar a in a 3-stage centrifugal compressor. The nitrogen is cooled by fresh water after each stage of compression.

After the final compression and after cooling, the nitrogen is led to the “warm” section at the top of the cryogenic heat exchanger in which the nitrogen is pre-cooled to approximately –110°C. The nitrogen is then expanded from approximately 56 bar a to an outlet pressure of approximately 13 bar a and –163°C outlet temperature. The nitrogen is then routed to the “heat exchanging” section at the bottom of the cryogenic heat exchanger. The cold nitrogen then absorbs heat from the BOG and warm high-pressure nitrogen.

The nitrogen flows through from bottom of the cryogenic heat exchanger to top before it is returned to the suction side of the 3-stage compressor.


CAPACITY CONTROL
The system capacity is controlled as follows:

The BOG compressors are equipped with controle valves, which adjust the compressor flow rate to achieve the storage tank set pressure.

The N2 refrigeration system capacity control is done by changing the amount of N2 that is circulated through the N2 refrigeration loop. This is done by adding or removing N2 to the circuit, which will increase the pressure, density and therefore circulated mass flow.

Composition Mole %
Nitrogen 0.3242
Methane 93.1563
Ethane 6.2409
Propane 0.1758
i-butane 0.0001
n-butane 0.0065
i-pentane 0.0634
n-pentane 0.0321
Hexanes+ 0.0007

 

[dcasto]

T^he diagrams presented by BigInch leave a lot to you imagination. 123sen's is all verbal so need to see some more pictures.

The latest design I've come up with uses an 15% N2, 85% methane refrigeration loop. The refrigerant operates from 40 psig to 650 psig. The refrigerant goes through two expanders that provide 18% of the power for the entire loop. The whole system can produce LNG at around 1.15 KW-hr/gallon.

 

[BigInch]

dcastro, Feel free to "expand".

Fortunately no shortage of imagination around here.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

BigInch, my comment was that the link and PFD's were as you said short on temps and press'. Some of the diagrams also look like there is a distillation column and people not familiar as you might believe there is distillation taking place. Sorry if you took my note wrong.

Some of the process' use a mixed refrigerant that is a trade secret. Not the components, just the ratio of components. 123sen didn't have a diagram to relate to.

There are lots of ways to go here.

 

[BigInch]

No. No problem. Just my attempt at providing a bit of (probably too dry) humor in regard to your mention of expanders. I think the edition of expanders has become a rather standard item now and it is good that you mentioned them specifically.

http://virtualpipeline.spaces.msn.com