8 replies to this topic

[herrani]

Hi everyone

We are exporting propylene from our refinery, almost in a continuous manner. The produced propylene is temporarily stored in a tank with a constant inlet flowrate (approx. +5 deg C, 2 bar g storage conditions and 150 ppm water dissolved) and exported once a week.

In order to meet the product spec. we need to reduce the water content down to 60 ppm or less. At the moment we are pumping the propylene throuhg a salt bed that dissolves the water. The water level in the salt dryer is kept to the lowest feasible point (less than 5%). However, we are experiencing a number of operational problems, and have observed that there is channeling in the salt bed, with small chunks of salt ending up at the export point further downstream. The salt beds are not insulated, and our plant is in scandinavia.

We have a coalescer installed upstream the salt beds to separate any free water, but no water is accumulated in the boot.

We are using up 3 to 4 times more salt than the original design documentation states and this is quite costly. We believe that the channeling effect and the higher salt consumption problems are related.

Has any of you experienced the same problem or have any good piece of advice?

Any comments will be appreciated!

Edited by herrani, 01 June 2010 - 02:00 AM.

[Profe]

Hi Herrani
About your operational problem, its necessary some design data of your salt dryer: propylene inlet flow rate, dimensions of equipment like heigth and diameter, maximum heigth of salt bed, type of salt used, type of flow: upward or downward?, minimum salt heigth before reload salt. If it is possible to attach the equipment diagram with internals?
How was determined the channeling?

Good luck.

Edited by Profe, 01 June 2010 - 08:14 AM.

[herrani]

Hi profe

I apologize for the missing design data. Here is the information I have available:

Normal inlet flowrate: 285 m3/hr
Design flowrate: 300 m3/hr
Inner diameter: 1676 mm
Height: 6096 mm seam/seam
Salt type: calcium chloride
Flow type: bottom to top
Salt bed height: 995 to 5400 mm(approx.); salt bed height approx. 4.4 m
Water drain height: bottom of dryer
Normal operating T: +5 deg C
Normal operating P: 10 bar g

I do not have any drawings yet of the internals of the tank available, but there is an inlet distributor plate, and a bed support plate close to the dryer bottom with (995 mm height). I can provide a sketch if you think it will help the discussion.

As for the channeling, the performance of this unit is closely monitored. When the outlet water content becomes higher than the design values, we open the dryers for inspection and change the salt eventually. In these situations we have observed that there are big void spaces in the salt layer, and parts of the salt layer have "solidified" together instead of remaining in the original pellet form.

As said in the previous mail, small lumps of salt have been collected in equipment downstream the dryers, which adds to these observations.

The unit is only operational 1 day/week more or less, and we have observed that the outlet water content increases over time. We have been informed that this is something to be expected, even if we drain water from the sal bed continuously (which is our normal operating procedure).

I hope this is sufficient to give an idea of our unit. I am looking forward to your comments.

[Profe]

Hi Herrani again.

Do you use Anhydrous Calcium Chloride as dessicant?

If yes, do you regenerate the dessicant?, or there is an operational procedures for regenerate? Or have instalations for regenerate the bed?

I review your data and for dessicant are ok. But if you has to drain the brine frecuently you are operating the equipment improperly. And you have to review the operational procedures for use the dryer and refill the calcium chloride.

Good luck.

[herrani]

Hi Herrani again.

Do you use Anhydrous Calcium Chloride as dessicant?

If yes, do you regenerate the dessicant?, or there is an operational procedures for regenerate? Or have instalations for regenerate the bed?

I review your data and for dessicant are ok. But if you has to drain the brine frecuently you are operating the equipment improperly. And you have to review the operational procedures for use the dryer and refill the calcium chloride.

Good luck.

Hi Profe

Thank you for your reply! As for your questions:

Yes, we use anhydrous calcium chloride.

We do not regenerate the dessicant. We do not have any equipment installed for this purpose, and I have not found any operational procedures in this respect. I am confident that the equipment was desgined correctly, but I have problems with much higher than design salt consumption and channeling in the salt bed.

The operating procedures for refilling seem sensible. The short summary is as follows:

1. Completely drain and vent the dryer
2. Inspect existing salt bed, and determine if what is left can be re-used or removed
3. Steam out is required if all salt is removed
4. Ensure that it is dry before refilling salt
5. Refill salt. Air is blown from the bottom to ensure that all dust is collected at the top of the tank and/or vented
6. Leak testing and re-start

Could you give some advice on the draining the equipment during operation? We have been discussing whether we should maintain the brine level at almost 0% or if we should try to keep it at 25% closer to the bottom of the salt bed. Do you think this is relevant?

Why do you say that I have a problem if I have to drain frequently? I would expect that this is just a matter of the amount of water that enters the equipment.

Thanks again for the discussion

[Profe]

Hi Herrani again.

I apologize for the delay of answer you.
By the way, the CaCl2 it is not regenerable.

For solve your operational problems I think the following hints:

1. To identify the source of additional water, make an upstream dryer follow up.
2. What is the total volume of propylene processed in each batch, the number of batch processed before problems arise?
3. Is there a sigth glass under the bed of CaCl2 to know the level of brine?
4. It is advisable to maintain the minimum level of brine, as you say almost 0%. Make at least two drains each shift when the dryer is operating.
5. The holes of charge distributor point up or down? I think it is better downward. Check his mechanical integrity, corroded holes etc...
6. The anhydrous CaCl2 reaches up to 55% by weight of hydration water before it begins to form brine.
7. Is there quality control product by cobalt bromide test NF F 41-004, or some other method?
8. Review the process of vessel opening, ..., refill with CaCl2, blowing dust with air, sweep air with inert gas, and slow feed admission to the dryer.
9. CaCl2 can be dissolved with warm water, and heat up slowly with steam for increase disolution.
10. For blowing dusts, do you use dry air or hot air or an inert gas?
11. I think that the flow rate is too high, try at 20% – 25% of design to avoid carryover of salt and channaling, (if you prefer maintain for 2 hours and increase 5% and maintain for 2 hours and so on… until the problem arise, this is to get the safe operating flow rate.
12. The connections used for steam out are temporary or permanent? If it is permanent, use blind flanges for isolate it from process when the dryer is operating.

Good luck.

Edited by Profe, 04 June 2010 - 04:48 PM.

[herrani]

Hi Profe

This was really useful. I have already started a sampling program to review water content in and out of the unit, and will try and implement your advice.

Since we are using the dryers for export to ships, we do not have the option of using low flowrates for a long time, but i will keep this in mind.

We are using our own lab to certify the product quality, but I believe we are using another water measurement method. I will get back to you on that.

I am also thinking of inviting the salt manufacturer on site for further advice.

I will let you know if we manage to improve operations!

Thanks again for the good tips.

Edited by herrani, 05 June 2010 - 02:37 AM.

[Shauno]

I had similar experience in a refinery when we switched a salt dryer service from light atmospheric gas oil (LAGO) to a heavy atmospheric gas oil (HAGO). Basically, the higher viscosity of the HAGO resulted in salt carryover (the drag force on the calcium chloride flake was too large and resulted in the HAGO carrying the salt particles and suspended brine downstream of the salt dryer). End result was that product was offspec and the downstream filter plugged too frequently (several times/week during high flowrates).

We mitigated by doing the following:
1) Switched to Peladow DG briquettes to have less suspended flakes/smaller particles
2) Added lighter process material to reduce viscosity. You could do the same by heating, if heating is possible/acceptable.
3) Limited flowrates to allow sufficient time for brine and particles to drop out in dryer. Flowrates were estimated using Stokes' Law and then tweeked by using operating data.

To avoid chanelling, ensure you check loading of the bed prior to startup to ensure it is evenly distributed. Confirm your distributor is working properly and not physically degraded.

Hi everyone

We are exporting propylene from our refinery, almost in a continuous manner. The produced propylene is temporarily stored in a tank with a constant inlet flowrate (approx. +5 deg C, 2 bar g storage conditions and 150 ppm water dissolved) and exported once a week.

In order to meet the product spec. we need to reduce the water content down to 60 ppm or less. At the moment we are pumping the propylene throuhg a salt bed that dissolves the water. The water level in the salt dryer is kept to the lowest feasible point (less than 5%). However, we are experiencing a number of operational problems, and have observed that there is channeling in the salt bed, with small chunks of salt ending up at the export point further downstream. The salt beds are not insulated, and our plant is in scandinavia.

We have a coalescer installed upstream the salt beds to separate any free water, but no water is accumulated in the boot.

We are using up 3 to 4 times more salt than the original design documentation states and this is quite costly. We believe that the channeling effect and the higher salt consumption problems are related.

Has any of you experienced the same problem or have any good piece of advice?

Any comments will be appreciated!

[Profe]

Hi Herrani.

For additional help about this subject, next I transcribe the general coments about CaCl2 as solid dryer from: Gas Processors Suppliers Association, Section 20: Dehydration that says on page 20-42:

"Calcium Chloride
Calcium chloride (CaCl2) can be used as a consumable desiccant to dehydrate natural gas. Solid anhydrous CaCl2 combines with water to form various CaCl2 hydrates (CaCl2 · XH2O). As water absorption continues, CaCl2 is converted to successively higher states of hydration – eventually
forming a CaCl2 brine solution.
10 to 20 mm CaCl2 pellets are installed in a fixed bed much like a dry desiccant tower. Gas flow is upflow. The more efficient designs utilize 3-4 trays below the solid bed to pre-contact the gas with the brine solution. This removes a portion of the water from the gas before contact with the solid CaCl2 and increases unit capacity. One such unit is shown in Fig. 20-74.
The solid CaCl2 near the bottom of the fixed bed will typically be CaCl2 · 4H2O or CaCl2 · 6H2O and the CaCl2 at the top of the fixed bed will be anhydrous CaCl2 or CaCl2 · H20. In this way the gas contacts successively drier CaCl2 as it flows upwards and in theory leaves the fixed bed in equilibrium with the CaCl2 at the top of the bed.
Outlet water contents of 16 mg/standard m3 have been achieved with CaCl2 dehydrators. Typical CaCl2 capacity is 0.3 kg CaCl2 per kg H20. Superficial bed velocities are 6-9 m/min and length to diameter ratio for the bed should be at least 3 to 4:1.
CaCl2 dehydrators may offer a viable alternative to glycol units on low rate, remote dry gas wells. The CaCl2 must be changed out periodically. In low capacity – high rate units this may be as often as every 2-3 weeks. Brine disposal raises environmental issues. In addition, under certain conditions the CaCl2 pellets can bond together to form a solid bridge in the fixed bed portion of the tower. This results in gas channeling and poor unit performance."

I think that will be useful for your operation.

Good luck