10 replies to this topic

[flash]

Hi,

I would like to know the basic process operation when the road tankers are filled with ammonia. I understand these can be filled under pressure (for non-refrigerated tanker) or at atmoshperic pressure (under refrigerated tanker). So I guess my question is: if the system is to be filled under pressure how does the pressure build up? - is it due to some inert gas blanketing or because of its own flashing (say it reaches 1 bara initial pressure to final 10 bara at 25 ºC)?

I am assuming the road tankers are filled by a pump under certain pressure and flow regulating valve.

[Art Montemayor]

Flash:

Your query is said by you to be based on a “process operation when the road tankers are filled with ammonia” – but it is more complex than just “basic” when you condition it for both atmospheric temperature and refrigerated temperatures. These are two entirely different conditions. The nature of your question reveals that you are either unaware of phase equilibria or you don’t know what is involved.

I can give a general response, but if you are going to go into specifics, then you must furnish a detailed P&ID of the loading system under question. My response is as follows:

• I assume you have SATURATED liquid ammonia stored in a pressure vessel and will pump out of it to fill road tankers. I can then assume an atmospheric temperature of 30 ^oC that relates to the stored liquid ammonia. This temperature identifies the vapor pressure of the stored ammonia as 11.672 barA.
• You use a pump to pump the saturated liquid ammonia from the storage tank into the road tanker while having the vapor space of both vessels connected by an equalization line. The pump will pump the saturated liquid into the road tanker while the tanker is at the same 30 ^oC temperature and, consequently, at the same pressure.
• There is no flow regulating valve required if the transfer pump is designed appropriately for the required filling rate.
• There is NO “build up of pressure” taking place. This is obvious, since both vessels are interconnected by a vapor balance line.

[flash]

Thanks Art.

I do not have any P&ID or design with me at the moment. Since I have never come across or experience this process before, my question is more related to know the concept of ammonia filling and how this is done in the industry to gain knowledge. It makes it more interesting due to its phase change behaviour at different presssures and temperatures.

You mentioned about pressurised tank, however, could you please also mention what is the process if the ammonia is stored at atmospheric pressure (say 2 kPag) at (-)32ºC and then filled in the road tanker at 30ºC (approx. 12barA). How is the vapor equalisation line connected in this scenario?

Regards,

[breizh]

http://www.airgasspe...calLibrary.aspx

This resource should support your query.
Breizh

[flash]

breizh, it certainly appears to be a good source for ammonia but it does not discuss about ammonia loading into the road tankers.

[kkala]

Basic question was covered in the post No 2. Safety in handling liquid ammonia is a point of red alert, so http://www.osha.gov/...hib120505.html may be useful. Other data can be found by googling "liquid ammonia truck loading" or similar.

[breizh]

Hope this will help :
http://www.corken.co...ining/cp370.pdf

Breizh

[Art Montemayor]

Flash:

You continue to miss the whole concept of phase equilibria and the practical, everyday operation of transporting ammonia. Ammonia, like any condensable vapor or gas is always transported in the LIQUID phase when dealing with bulk quantities. You must focus on that reality in order to understand the operation of loading a bulk road tanker truck.

In the practical sense ammonia may be stored at ambient, compressed and saturated liquid conditions or it can also be stored at essentially ambient pressure conditions. The latter storage condition is selected when dealing with very large storage capacities because one can use an API 620 type of refrigerated storage tank that can tolerate the relatively low temperatures that this design can withstand. The large capacity and low pressure make this type of design more economically competitive than the ambient temperature, pressurized type of tank. HOWEVER, rarely – if ever – is ammonia transported at refrigerated, ambient pressure conditions. The reasons are based on common sense: the small capacity of the road tanker truck makes it more expensive and cumbersome because of the need for expensive alloy construction and portable refrigeration. Ambient temperature and pressurized road tanker trucks are used. In all of these cases – storage and transport – the ammonia is in the SATURATED LIQUID state.

Now, having established the basis of how ammonia is stored and transported, we can discuss your new query: “if the ammonia is stored at atmospheric pressure (say 2 kPag) at (-)32 ºC and then filled in the road tanker at 30 ºC (approx. 12 barA)”. In this case, you must again rely on common sense. Obviously, if you introduce saturated liquid ammonia at -32 ^oC into a vessel that is at +30 ^oC, you are going to generate a heck of a lot of vaporized ammonia that cannot be allowed to remain in the reception vessel. You are going to have to do one of three things to load refrigerated liquid ammonia into an ambient-temperature road tanker truck:

• Pre-cool the road tanker truck (and all associated piping) down to -32 ^oC;
• Recover the vaporized ammonia vapors using an ammonia compressor and condenser in order to return the recovered ammonia back to low-temperature storage; or,
• Pump refrigerated ammonia liquid into the road tanker truck at a definite, determined quantity and allow it to heat up while in transport and reach its saturated pressure corresponding to the temperature (maximum = ambient).

The first two options require a refrigerated, low pressure, alloy vessel. The third option may require an alloy pressure vessel, but no transport refrigeration. As I stated above, the normal method is the third option.

The Corken literature submitted by Beizh is very helpful and informative. It explains the two basic methods of displacing liquid ammonia (ambient temperature): vapor displacement and pumping. I have used both methods with success. Note that vapor displacement is not recommended for the refrigerated, low pressure storage type.

[flash]

Thank you heaps. That's really helpful. I understand the phase equilibria but did not know how exactly this is practised in the industry. Surprisingly I could not find a single relevant information on Internet on this.

Cheers,

[antrivedi]

What is recommended filling rate for  saturated liquid Ammonia at ambient condition( 80 Deg F) in Road tanker.. Is it possible to fill without opening vapor line to compressor 

[Molefe]

Thank you Flash for your concern,

 

I'm also gaining information from the responses to your concern, part of my task in my design project is to design the storage facility for the anhydrous ammonia produced in the plant (300 000 tonnes/yr).