Thermal shocks in vertical falling thin film evaporators

[aahmed]

Hi
am a chemical engineer and new to this forum.Below is my problem statement:

We have a vertical thin film evaporator used to remove solvent from a slurry ( containing 10 - 12 % solids ).HP steam ( 100 barg ) is let down through a PCV upto 50barg ( obviously a SH steam) and goes to four jackets of evaporator through 4 parallel individual lines. condensate at approx 265 C is collected from all 4 jackets and sent back. Process Feed at a temp of 125 C enters from and then leaves from bottom of evaporator in the form of this pasty slurry. We have experienced internal cracks on process feed nozzle welding re-inforcement pads.Some of the guys at our plant say that its due to thermal shocks due to less temp of feed ( 125 C ) whereas evaporated is heated upto 50 barg steam pressure. Is this true ?Secondly we think that steam flow should be lowered in first top jacket where coldest feed is introduced.Will it help to reduce any thermal shocks?
My second question is that...We operate evaporator for 4 hrs / day @ 4te/h feed rate. We want a continuous operation of evaporator @ 500 kg/h..Is this possible theoratically and practically?

will appreciate your response

Best Regards

 

[SREisme]

My guess is that the batch operation of the evaporator is more to blame for thermal cycling cracks (fatigue?) than the actual operating conditions. Assuming you run this equipment 4 hrs per day and shut it down for 20 hours per day, you are cycling the evaporator from approximately room temperature to somewhere less than 100bar saturated temp (311C). Letting down 100bar steam to 50bar will superheat the 50bar steam but not change its temperature much. I expect you are heating the slurry with 311C/50bar steam that then condenses to 48.3bar saturated condensate at 265 C. Your evaporator shell is cycling from cold to hot to cold 365 times per year.

Have you been able to cut out a sample of the repad and look at the cracks internally? You should be able to confirm the cracking mechanism with the help of a good metallurgist. I would start by suspecting fatique cracking of the nozzle repad.

I don't understand your units of measurement on feed rate of 4te/h, but it seems logical that if you are processing more than 500kg/h in a batch process you should be able to maintain that rate on a continuous basis, unless tis is a fill and dump operation that depends on residence time in the evap to remove the solvent.

 

[aahmed]

Thanks SREisme

Units of measurement is 4 tonnes/ hr ( equal to 4000 kg/h ).. I understand what you said.Unfortunately,here we don't have facility of internal inspection of crack mechanism.
Say we start operating evaporator on continuous basis ( process wise,we don't have ant problem with continuous operation), will it have any adverse effect on residue slurry film formation? i foresee that residue might overcook / over dry if we operate @ 500 kg/h due to less residence time and this can damage rotor . What you say regarding this?

Secondly , can we operate first shell at low temperature compared to other shells to avoid thermal shock with coldest feed in contact with 1st shell jacket ? will it cause damage to internal rotor , due to clearance change or any other such phenomenon?

Reagards