6 replies to this topic

[raxza]

Relating to my post about the effect of the impurities of the natural gas.....
I have bad spec for sales gas (it has high HC dew point ranging from 150 to 85 deg F), this is because we have already signed the GSA and it has no dew point spec. Well now I want to build something to reduce the HCDP to 55 deg F. I've read the GPSA that the method is LT Separation and it can be achieved through JT Valve or Refrigeration System. Because the pressure is only 500 psig so we choose to build refrigeration system (through chilling in the chiller). My gas is 500 psig, 85 deg F and I want to cool the gas to 55 deg F. I have these problems:

1. I want to use glycol. Do you guys know the pressure and temperature in the chilling and after the chilling is the glycol vaporize? So we have to compress again using rotary/screw compressor?
(The Flow Process for glycol may be like this: Chiller-Compressor-Condenser-Accumulator-Expansion valve-Chiller)

2. I want to heat the gas back to 70-85 deg F. What heater should I use? Air heater is not enough to heat the gas because the air is only 85 deg F, gas-gas exchanger (use the gas inlet stream) is not possible (because inlet T is only 85 deg F). Do I need heating oil? Or can I burn my HC liquid from LT separator to heat the gas....


Thx.

[gvdlans]

Why don't you hire people who know what they are doing instead of trying to re-invent the wheel? See for example:http://www.abb.com/g...52568E10065860D]ABB Randall[/URL]
In my opinion, your questions go beyond what can be expected from participants of this forum.

[Art Montemayor]

Raxza:

You ask the following:
1. Do you know the pressure & temperature in the chilling and after the chilling is the glycol vaporize?
2. So we have to compress again using rotary/screw compressor?
3. What heater should I use?
4. Do I need heating oil?
5. Can I burn my HC liquid?

I don’t understand what the first question asks. This is either bad English or bad thinking; I hope it’s the former. In spite of furnishing a lot of basic data and details, as Guido infers you may not be aware of what you are confronting or have failed to understand your basic problem. It is either that, or you have not communicated it correctly. Allow me to explain:

You state you are presently receiving natural gas with a high “HC” (I assume you mean Hydrocarbon) dew point ranging from 150 to 85 deg F. You further state you want to consider something to reduce the HCDP (I presume you mean “Hydrocarbon Dew Point”) to 55 deg F. And you might follow GPSA guidelines on reducing the HCDP by employing refrigeration. You intend to cool the gas to 55 oF.

You follow this by mentioning the use of glycol for dehydrating natural gas. Now you start to confuse the issue by mentioning another process – which is used for removing water, not Hydrocarbons. Note that you stated you have a heavy hydrocarbon content that you want to reduce, and now you infer that you have a water-humid gas you want to “dry”. This is not only confusing, but it presents the possibility that you may not be differentiating between the hydrocarbon and the water content of the natural gas. Further, refrigerating the natural gas stream to reduce the amount of “heavies” (propane, butanes, etc.) present in the gas is a simple matter: you simply install a mechanical refrigeration unit and pass the natural gas through it, condensing the heavies and separating them out later with a mechanical separation drum. Then you employ a heat exchanger that heats up the product gas while cooling down the incoming feed gas. It’s that simple. However, that only reduces the heavy HCs.

If you have a large water content in your natural gas and wish to reduce it, you must consider the size of the flow stream. If the flow rate is relatively small, you can employ adsorption (activated alumina or silica gel) to drop the water dew point. If the stream is relatively large, then you may have to employ glycol to do the same thing – although the dew point won’t be as low as with adsorption.

So, in general, I would say this seems to be a case where you must decide on the scope of work you have before you (which you haven’t identified): what do you want to do with the natural gas you are receiving? Do you want to purify it (remove HCs)? Do you want to dry it (remove water)? And to what extent? Questions 2 through 5 do not apply until you identify the process you want to employ. You normally don’t have to recompress the gas. And the fuel you require to regenerate glycol is normally a portion of the product natural gas you process.

You really need to identify a clear Scope of Work, clearly and accurately identifying what it is that you want to do. If you can’t explain what you want to do, others can’t help you.

[raxza]

gvdlans :
I've contacted some vendors and they will come to our office in the next week. I want to hear other opinions in the forum so I got a 'vision' when the vendors come. And I want to try simulate the process in Hysys so I have the figure what is the size of the unit.

Art:
In the GPSA Section 16 about Refrigeration, in Figure 16-5 there is Glycol injection system to reduce higher HC and straight refrigeration system. So I think I didn't say about dehydration system/reducing water content UNLESS the Dehydration Unit can also reduce the higher HC. About the chilling process I think the one that I wrote in the first topic is about propane chilling process........So I guess I want to withdraw my question about the process condition to Do you know this glycol injection process and how about adsorption process (silica gel or molecular sieve) ?

Thank you for all your replies........

Regards,
Raxza

[Art Montemayor]

raxza:

You still persist in not giving us your Scope Of Work: What is it that you want to do with your natural gas? Do you want to remove condensable hydrocarbons? Do you also want to remove a water content? Please be specific and detailed about what your scope is. I am still confused because you haven't addressed the needs of those of us who are trying to identify what it is that you need. This is a lack of basic communications simply because you don't answer our questions.

You cannot employ a glycol and expect it to remove hydrocarbons (what you call "HC") . I am trying to understand your use of acronyms without your defining them. Please define your acronyms. Glycols are employed in removing moisture content to avoid hydrates' formation - not to remove hydrocarbons.

I have used glycol injection in pipelines and processes. This is a simple way to remove some of the water vapor in gas streams. However, you must modify your process to employ this type of operation. Also, you cannot expect to calculate the exact glycol amount nor the total trajectory. And you will inherit liquid glycol separation problems downstream. You also will have to regenerate the spent, recovered glycol.

What about adsorption? You still haven't revealed what it is you want to do. We can't guess what you have up your sleeve. We're not wizards, just engineers trying to communicate. The engineering design and calculations for what I believe you want to do are very simple and direct; it's trying to get basic data and scope of work out of you that is getting to be impossible to work out.

I'll wait for your details, if you would like to share them with us.

[raxza]

Art:
OK, My intention is reducing HC (HC=hydrocarbon) dew point from 85-150 deg F to 55 deg F by removing higher HC (C3,C4,C5,C6,C7,C8) not reducing water dew point because my gas already have tolerable water content (7 lb/MMSCF). And I don't care about the quality of the liquid because I only want lower HC dew point.
This is the gas comp. (used to be sales gas spec) in percentage:
C1 ---------------> 81.68
C2 ---------------> 4.97
C3 ---------------> 3.66
iC4 ---------------> 0.69
nC4 ---------------> 0.94
iC5 ---------------> 0.33
nC5 ---------------> 0.16
C8+ ---------------> 0.14
H2O ---------------> 0.015
CO2 ---------------> 4.32
O2 ---------------> 0.27
N2 ---------------> 2.82
(HC Dew point about 85 deg F)
The Pressure is 500 psig and temp. 95 deg F, the gas will be transported into 250 km offshore pipeline and 150 km onshore pipeline.

The method I preferred is mechanical refrigeration or adsorption not lean oil absorption or J-T Valve expansion (for LT separation).....

Once again, thank you very much......

[gvdlans]

Raxza,

In your second post in this thread you wrote:

In the GPSA Section 16 about Refrigeration, in Figure 16-5 there is Glycol injection system to reduce higher HC and straight refrigeration system.

To me, this confirms that you do not have a clue what you are talking about. Did you read the text in GPSA that describes this figure 16-5? It reads:

A further modification to this process (the LTS system with Joule-Thomson valve) is to add glycol to the high pressure gas to allow the achievement of lower water dew points when available pressure is limited. Fig 16-5 shows an LTS system with glycol injection. The use of the glycol eliminates the need to heat the LTS liquid phase and helps to ensure that no hydrate formation will block the process equipment upstream of the LTS separator.

So glycol is not used to cool the feed gas or to lower the hydrocarbon dewpoint. If you do not have the possibility of hydrate formation because of low water contents (did you check this? You should not have hydrate formation at the worst case conditions in the process (lowest temperatures)), you can forget about glycol injection. You can use a mechanical refrigeration process, that uses for example propane or propylene as a refrigerant. You can reheat your product gas by heat exchange with the feed gas, followed by a fired heater that uses part of the product gas or off-gases from the stabilizer as fuel gas. There must be many other possibilities to reheat your gas, but this one seems most logical to me. By the way, I do not really understand why you would want to reheat the gas to 70 - 85 °F. The gas will get same temperature as the seawater anyway when you send it through a 250 km pipeline... So probably heat exchanging with the feed gas will be sufficient!? The produced gas temperature would then basically be an economic optimum (a higher produced gas temperature results in less power consumption for your refrigeration, but the heat exchanger will be more expensive as well).

GPSA figures 16-6 and 16-10 show typical PFDs for a straight refrigeration process. The refrigeration cycle itself is described in GPSA chapter 14, with a typical PFD in figure 14-2.