Evaporating liquid C4 in sales gas stream 1

[Iradah]

Hello all,

I am thinking of mixing a 1,600 bbls of liquid C4 stream with 40 mmscfd natural gas stream.
They will meet in 16" header and flow for ~ 50 meters before entering an amine plant.

the stream should be totally in the gas phase before it enters the amine plant as it will go to the contactor. there is an inlet scrubber, but we do not want to knock out C4 there.

Can someone advise on how I can know if 50 meters travelling in 16" ID line is enough to evaporate the whole C4 before it reaches Amine plant?

 

[zdas04]

You really can't. All of the evaporation calculations are based of statistics and the tail end of the curve will always bite you. If you find an equation that predicts 99.999% ±1% of the butane will evaporate, then the equation is still "correct" is 1% of the butane still liquid.

As I recall, MDEA has a very high propensity to extract C3 and C4 from a gas stream, which is why VOC is such a big deal on Amine reboiler exhaust. The way I understand it the chance of your C4 surviving the Amine contactor is not very high.

You didn't say why you were doing this. Most pipeline tariffs that I've seen around the world call for a maximum of something like 0.05% C3 and zero C4. I hope you've verified your sales contract before you get very far.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[LittleInch]

Iradah,

As with many issues, it depends on your particular conditions.

You don't say what flowrate you want to use for your 1600 barrels. , an hour, a day, continuous or batch mode?

What is the pressure and temperature if the gas as this would affect the boil of rate of your C4 and how much will remain in the gaseous phase.

What do you mean by "they will meet"? Are you just talking about a tee connection?. I would suggest either a static mixer or inject the liquid into the stream using an injection tube with multiple small holes to increase mixing rate.

If the flowrate is anything serious or gas pressure fairly low, 50m looks very short to me to get complete mixing.

I think you need some better analysis than we can offer here, HYSYS maybe?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Iradah]

Hi zdas04, LittleInch,

The reason I want to do that is the old story of our stationary pig in C4 line that I discussed with you under:

http://www.eng-tips.com/viewthread.cfm?qid=354614

The pig is still stuck, and we are preparing to divert C4 temporarily somewhere until the line is fixed.
I was looking at the plant piping and trying to find a way to send C4 to the sales gas.
I used the simulation for that as follows:
mixing 1,600 bbl/day of C4 at 340 psig and 100 oF with 40 mmscfd sales gas of 0.64 S.G at 340 psig and 90 oF, will result in a stream that is totally vapor and ~30 oF above the dew point. This will be a continuous process. C4 liquid will be pumped in 6" line that joins the 16" header at 340 psig. The 16" header has the 40 mmscfd sales gas that is going to the amine plant. After C4 meets the sales gas, they will travel for 50 meters that should be enough to evaporate the C4.

I cannot make modifications like using an injection tube. In the simulation, the resulting stream will be totally vapor, but I am not sure if it will evaporate before it reaches the amine plant, but I am not sure how is it going to be in reality??

 

[LittleInch]

One thing you seem to have neglected here is the latent heat of vaporisation of C4 (380 KJ/kg)

I've just done a quick calc and reckon for your flow which equates to 1.76kg/sec, you need 680 kW of heat to maintain your temperature. I might be wrong in my quick calc, but anyone who has used a butane supply for cooking knows the canister gets pretty cold pretty fast once you turn the burners on, hence the pre heaters on those large hot air balloon burners.

If you try this I honestly think the temperature will fall dramatically as your gas has no real heat capacity and 50m of pipe will not be able to absorb heat from the air fast enough. Unless you can adds loss of heat to your pipe it will just freeze everything and you might even hydrate your gas supply. It sure won't all evaporate that's for certain. At 40mmscfd in your 16" line at 22bar, you're doing about 5m/sec. So you have 10 seconds to evaporate your c4. Can't see it happening, sorry.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way