Freezing in the gulf oil pipeline

[Mark24]

I was wondering if someone could explain how ice crystals are being formed in the tank/pipeline BP is trying to use to stem the flood of oil pumping into the ocean.

A friend and me were discussing it and a few questions came to mind about it, not just related to this issue. He suggested that the higher pressures deep in the ocean lower the freezing point of water enough so that it does not freeze, which seemed plausible to me. But after looking it up, it sounds as though pressure has a very small effect on the freezing temperature - something like 135atms of pressure being necessary to lower the freezing temp by 1C.

But ice always seems to form at the surface of a body of water and never in the deep. Is this because the ground at the bottom of the sea floor is always warmer than freezing temperature and the temperatures at the surface?

 

[skeptic2]

I was a little surprised to hear that the temperature of the water at the bottom was about 10 F above freezing. I had expected it to be the temperature at which water is the most dense, which for fresh water is about 39 F. I don't know what it is for salt water.

Suppose there are bubbles of methane rising with the petroleum. As they rise they should expand about 150 times due to the reduction in pressure. Would that expansion cause the methane to cool and perhaps freeze the water next to it?

 

[eachus]

No, the problem/issue is that methane clathrates are stable at room temperature under high pressure. These are a combination of methane (CH4, a gas at standard temperature and pressure), and water. It is not a compound, but the presence of the methane (or other molecules in non-methane clathrates) stabilizes the crystalline structure and increases the freezing point of the water.

In other words, the clathrate is energetically favorable when the methane gas is under pressure. Water normally expands as it freezes, but methane clathrate takes up less space than the water and methane it is formed from.

Incidentally, like ice, methane clathrate will sit around melting at room temperature and pressure, and you may be able to find photos of burning methane clathrate on the web. But it is pretty tricky to try to extract large amounts of methane from clathrates. The methane contains about as much energy when burned as the clathrate takes to melt. So if you mine chunks of methane clathrate from the sea, you need to let them melt, probably in a sea water bath, over a period of weeks--depending on the size of the chunks you mine. It would be nice to be able to use that cooling somehow, but transporting the clathrate probably costs too much.

Oh, and part of the problem with extracting the methane from the clathrate is that it is a much better insulator than regular ice. That and the fact that it is heavier than water means that all the scare stories about global warming from clathrates are bunk. Clathrates fall once they are formed, then effectively tunnel into the seabed over time. The ocean level would have to drop about 100 meters for a significant amount of methane clathrate to be where it could melt--and then it would take a couple thousand years. Most climate models predict ocean levels rising due to global warming, anyway.

Oh, and there is a really neat trick which may be the solution to the CO2 problem. Liquid CO2 is heavier than water, and forms clathrates just like methane. So collect CO2 pressurize it to make it a liquid, and dump it in the deep ocean. It will eventually form clathrates that will stay in place for millions of years.

Why is no one advocating this? Where do you get the energy to compress the CO2? Nuclear power is the only real answer. (Well, knock the price of solar cells down by a factor of ten and they may work.) Now in either case, there is no real reason to burn coal, then spend most of the energy generated dealing with the CO2 formed.

 

[PRDan4th]

No, the problem/issue is that methane clathrates are stable at room temperature under high pressure. These are a combination of methane (CH4, a gas at standard temperature and pressure), and water. It is not a compound, but the presence of the methane (or other molecules in non-methane clathrates) stabilizes the crystalline structure and increases the freezing point of the water.

In other words, the clathrate is energetically favorable when the methane gas is under pressure. Water normally expands as it freezes, but methane clathrate takes up less space than the water and methane it is formed from.

Incidentally, like ice, methane clathrate will sit around melting at room temperature and pressure, and you may be able to find photos of burning methane clathrate on the web. But it is pretty tricky to try to extract large amounts of methane from clathrates. The methane contains about as much energy when burned as the clathrate takes to melt. So if you mine chunks of methane clathrate from the sea, you need to let them melt, probably in a sea water bath, over a period of weeks--depending on the size of the chunks you mine. It would be nice to be able to use that cooling somehow, but transporting the clathrate probably costs too much.

Oh, and part of the problem with extracting the methane from the clathrate is that it is a much better insulator than regular ice. That and the fact that it is heavier than water means that all the scare stories about global warming from clathrates are bunk. Clathrates fall once they are formed, then effectively tunnel into the seabed over time. The ocean level would have to drop about 100 meters for a significant amount of methane clathrate to be where it could melt--and then it would take a couple thousand years. Most climate models predict ocean levels rising due to global warming, anyway.

Oh, and there is a really neat trick which may be the solution to the CO2 problem. Liquid CO2 is heavier than water, and forms clathrates just like methane. So collect CO2 pressurize it to make it a liquid, and dump it in the deep ocean. It will eventually form clathrates that will stay in place for millions of years.

Why is no one advocating this? Where do you get the energy to compress the CO2? Nuclear power is the only real answer. (Well, knock the price of solar cells down by a factor of ten and they may work.) Now in either case, there is no real reason to burn coal, then spend most of the energy generated dealing with the CO2 formed.

You are right about methane clathrates forming at deep water pressure and low, but above freezing, temperatures. You are wrong about clathrates being heaver than water, they float. The large sub-sea methane clathrate beds are under the sea floor and form a matrix with sand or mud and stay below the sea-bed. The reason the funnel box in the Gulf of Mexico is fouling with methane/ice is that they form above the sea-bed and float to the top and stick to the box and accumulate like snow or ice blocks stopping the flow of oil and water/gas through the funnel.

 

[Xnn]

Methane clathrate floats in water as its density is around 0.9 gm/cc.

http://en.allexperts.com/e/m/me/methane_clathrate.htm

It can quickly form when methane gas leaks out of a well and comes in contact with high pressure water. At great depths and high pressures, it can be stable at temperatures as high as 18C. Closer to the surface, it is unstable above OC.

As permafrost melts, methane is released.

 

[croghan27]

As methane is released from deep in the earth, where it is under terrific pressures, it expands rapidly ... this is the same phenenoma that operates refridgerators and air conditioning systems ... the refridgerant expanding. Would not this, when it happens to the methane, tend to lower the temperature of the water about the emerging gas and cause it to form ice - giving a misture of methane and ice?

 

[eachus]

You are wrong about clathrates being heaver than water, they float. The large sub-sea methane clathrate beds are under the sea floor and form a matrix with sand or mud and stay below the sea-bed.

Sigh! It is not what we don't know that is a problem, it is what we know that ain't so. At STP (standard temperature and pressure, well not quite) methane clathrate has a density slightly higher than regular water ice, and melts. So take any listing of STP numbers for methane clathrate with a large grain of salt. ;-) There are many books and references that will tell you that methane clathrates are also unstable beyond 1.2 GPa pressure, and turn into ice and gaseous methane. This is totally wrong, and anyone looking into the issue should smell a large rat. You might want to read: http://www.informaworld.com/smpp/content~content=a714008099&db=all to find some rat free information, although the authors were interested in conditions on Titan.

As Loveday and Nelmes found out there are three solid forms of methane clathrate. (Ice has more. ;-) The low pressure form has a density around 0.9. The other two forms are heavier than water. Expecting otherwise, is to venture into rat infested territory. Huge amounts of methane clathrate all around the continental shelves, and chunks not commonly found floating to the surface? How could that be? In fact it was only a few years ago that scientists thought that methane clathrates could actually be found on Earth outside the laboratory. So methane clathrates do exist at depth in the oceans, and don't float--until you bring them close to the surface.

Oh, and once the knowledge that methane clathrates are stable at ocean floor depths spreads around, look for another factor of ten increase in the estimates of the amount methane clathrate in the oceans that can be mined for energy. I don't know that large amounts of methane are to be found on the ocean floors, but it would be interesting to look under the sediments there. (Hmm. To some extent that is what BP just did. Four thousand feet is above the normal ocean floor depth, but it is also below the normal continental shelf depths--and about the depth at which methane clathrates were believed to become unstable--and decompose into methane gas and water. The blow out was probably caused by methane clathrates several hundred feet deeper than the actual wellhead.)

Finally, if you look deeper into the formation of methane clathrates, you can kiss anthropocentric global warming goodbye. Why? Studies of C13 isotope concentrations in methane clathrates shows that they are formed from dissolved CO2! Scientists have been trying to close the global carbon budget, but a lot of CO2 was disappearing somewhere. It now seems that microorganisms are turning CO2 into methane (CH4) in the deep ocean, and the CH4 gets trapped in methane clathrates. I personally think that high CO2 levels in the atmosphere are not good for people and other animals. But the connection between CO2 levels and global temperatures seems to work the other way around. (Warmer oceans dissolve less atmospheric CO2, to carry it down to where it gets turned into methane clathrates.)

 

[croghan27]

there are three solid forms of methane clathrate. (Ice has more. ;-) The low pressure form has a density around 0.9. The other two forms are heavier than water.

http://en.wikipedia.org/wiki/Ice-nine"