Water in a cryogenic vessel 4

[oddjobbob]

Hello, I'm a long time reader, first time poster. Thanks everyone for taking time to expand the knowledge of the human race.

I have a problem with a small scale LNG dispensing system. One part of the process allows small amounts of moisture (~1cc per event, 18 times per day) to be injected back into the system and we are now faced with moisture occasionally blocking a 10 micron filter screen in the pump discharge line. I see more blockages after we run the storage vessel level low and then fill the vessel close to capacity (20K Gal).

Does anybody know how water behaves while in a cryogen under pressure (90psi)? Will it sink, float or stay mixed into the LNG? I tested spraying water onto LN2 in an open vessel and it seemed to form chains but because the liquid was boiling it got very churned up. It was hard to see if it sank or floated.

I am considering circulating the whole volume of the vessel through the 10 micron strainer but this will take a long time. I can recirculate to either the top of bottom of the vessel. Any idea which would be better? Should I use a finer strainer?

Another option I considered was to empty the vessel and either flush it with more LNG or heat the inner vessel with warm N2 until the dew point is acceptable. The flush option is obviously preferable to save chilling the vessel again but I am worried that moisture may stick to the walls of the vessel and this won't solve the problem.

I appreciate any assistance you can give.

 

[iainuts]

Is the filter that's getting blocked always at low temperature? It isn't unusual to have a filter catching ice or frozen air, but they generally warm up and the contamination can be blown out.

Regarding how it behaves, I've seen solids block fairly large diameter pipes (maybe up to 1") but I would think the solids are generally going to take the same form as they get frozen in. That is, if water (or liquid air) lies in a small bowl, it'll freeze that way. If water gets in as a droplet, it'll freeze as a droplet. If it gets in as vapor or a fine mist, it'll freeze more like snow. In general, I wouldn't think the frozen particles would have any tendency to freeze together after they're already frozen. Kinda like very cold, dry snow doesn't make a good snow ball.

 

[oddjobbob]

Cheers iainuts. The filter goes from ambient to cryo roughly 18 times a day but not in even intervals. Do you think that the moisture will be blown through the 10 micron filter until it gets cold again? If that's the case we mightn't be capturing all the moisture before it leaves our system.

I agree that as the moisture starts as frost it probably stays as frost in the system. I just don't know how it will sit in the liquid when the vessel is idle. Will it sink or float? I'm assuming it must float because the blockages mainly occur after we've filled the vessel. If the water contaminant sunk the pump suction would pick it up constantly, the suction nozzle is at the very bottom of the vessel. The filling must churn up the moisture and keep it in suspension until the the liquid stops boiling in the vessel.

Do you have an opinion of the methods I've listed on how to remove the moisture for the vessel?

 

[iainuts]

The filter goes from ambient to cryo roughly 18 times a day but not in even intervals. Do you think that the moisture will be blown through the 10 micron filter until it gets cold again?

Generally, the filter and piping take a while to cool down after being warm, during which time there's a flow of gas through the filter blowing the water out. It's only after it gets cold again that it'll freeze and get trapped in the filter. That said, you could get a filter (might already have one?) that would allow the water to collect in a sump below the direct flow of gas. That sump could then be blown out on occasion (as they do for water separators on air compressors for example). That might be the best way to eliminate water. Certainly a whole lot easier and less expensive than emptying a tank and warming it up. I'm not familiar enough with LNG dispensing systems to know if this is commonly done or not, but I know it's not a common feature on typical cryogenic systems. I've seen something similar on aircraft fuel tanks, they can have a low point drain to remove water that is accessed by mechanics. Something similar might work well to remove water in your system.

Regarding whether water will sink or float, the density LNG is much lower than that of ice, so the ice will sink. The only way I see of it staying in suspension is if the ice particles are relatively small and they can be 'churned up' as you say.

If you absolutely had to remove the water from the tank, the only way I know of is to empty it, warm to atmosopheric temperature, then heat it by circulating hot, dry nitrogen through.

 

[oddjobbob]

the ice must be sinking to the bottom of the vessel and the majority of it would be away from the suction nozzle. I guess this is why it doesn't get pulled through the system all the time. I have a theory that the filter will hold the moisture like a sponge until it gets bad enough to affect the flow, it is a very fine stainless steel wool mesh, not unlike a paper oil filter. I don't think it would drop out when warm.

I think I will need to look at devising a water trap that will allow the moisture to drop away from the filters when warm.

Thanks again for your input this problem has stumped everyone I know.

 

[dcasto]

download the hydrate prediction program from CSM. At -200F, you have solids in your system, not liquids.

 

[oddjobbob]

thanks dcasto. I have limited knowledge of hydrates but I understand they are formed when natural gas and water is mixed under pressure at temperatures above freezing? With our situation the ice frost is injected into an LNG stream. Do you think the combination will form hydrates when mixed at -200?

and any idea how I can get it out of the solution?

 

[EdStainless]

perhaps a system that would keep the filter cold when in use, and then allow for a warm purge of it between cycles?
After all you do want to catch this stuff.
I would think about two stages of filtering. You could then trap more without restricting flow as much.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[oddjobbob]

cheers Ed that's a good idea. I would have to link it through the PLC somehow because it is an unmanned site but it is an option.
What do you know about filter media for cryogenic fluid? I'm thinking of recirculating all the fluid in the vessel through a high flow/capacity filter. I'll have to get a filter system fabricated and knowledge of cryo fluids is thin on the ground in my area. It would be nice to have some sort of coalescing filter to absorb the moisture but I'm not sure where to start.

 

[iainuts]

What do you know about filter media for cryogenic fluid?

For LNG the typical material is stainless steel wire mesh.

I'm thinking of recirculating all the fluid in the vessel through a high flow/capacity filter.

If all the liquid is circulated through a piping system that filters out the water/ice you can expect to get a lot of heat added to the tank as well. That will cause the pressure to rise and product will most likely end up being vented. So it might be an option for a tank that's heavily used (so tank pressure can be kept down), but if you're on the edge of venting product now, it'll be much worse when you add all that heat to it. You're essentially creating a PB (pressure build) loop.

It would be nice to have some sort of coalescing filter to absorb the moisture but I'm not sure where to start.

I don't know enough about coalescing filters to know if you could use one to separate solids but I'm thinking the filter is essentially going to be blocking solids, and those solids aren't likely to 'soak' the filter element. As soon as flow stops, I'd think the solids would simply drop to the bottom of the filter cartridge if they were big enough not to get stuck in the wire. Once they drop off the filter, they would be in the bottom of the filter bowl. At that point, once it warms up and turns to liquid, a port on the bottom of the filter bowl could be opened to a vent stack and the liquids blown out.

Imagine a T style filter with the bowl down as it normally would and the flow going from the OD of the filter element to the ID and then out. The solids are going to drop to the bottom of the bowl once flow stops. Now if there's a gas flow from the filter outlet which goes from the ID of the filter to the OD and then out the bottom of the bowl, any water trapped as liquid inside the filter element will be blown out as well.

You could test this out by soaking a filter element in water then blowing it out with compressed air, the same way you'd do with natural gas.

 

[oddjobbob]

Thanks mate some good advice there. We do have a bit of room for pressure rise so it should be OK.

We use Norman filters in our centrifugal pumps so I might inquire about a 2 micron filter element through them. The tee filter shouldn't be a problem and I think we could rig up a reverse N2 purge to run when the system is idle. The 10 micron filter in use currently does stay soaked when warm so I will need this to force the water out or we will have to constantly replace the filter with a dry unit.

Does anyone now if ambient dry N2 will actually carry moisture or it does need to be warmed to be effective?

 

[iainuts]

Interesting... so you're saying the filter has water in it when you pull it out? That surprises me, I've seen plenty of filters pulled out of cryogenic systems and they've never had liquid water in them.

The dry nitrogen will work, but how about hot nitrogen? Insulate the filter bowl and run the nitrogen through a heating unit to bring it up to about 250 F. Flow rate doesn't need to be very high, but warming it would help considerably and it isn't very expensive. You might even just wrap the nitrogen supply line with a heating tape, then insulate that.

 

[dcasto]

You need to to this.

Deriming an LNG plant(defrosting or deicing). Deriming (defrosting or deicing) means the removal by heat and evaporation, sublimation or solution of accumulated matter, such as water and CO2 from the low temperature process equipment.

 

[maddocks]

Given the density of water is about 0.9167, this solid mass of frozen water and hydrate conglomeration will likely sink reasonably quickly in an LNG mixture. It can be churned up with boiling, but bottom line is that it will act like dirt in your system and will sink to the bottom of vessels, exchangers, site glasses, and drains. Nothing good can come from water in a system like this.