Shell and Tube HEX: Max slurry solids concentration to use for design

[plantprowler]

What is the max slurry concentration one should design using for a Shell and Tube HEX? The duty is as the Calendria of a forced circulation evaporator. Any guidelines based on what has worked in the field in the past experience? Above what concentration will I be facing operational / fouling / plugging problems?

The S&T is two pass on the tube side with 6000 mm long tubes with 1.25" dia. Mounting is vertical with a 3 m/sec liquid velocity.

What is the max conc. I should allow for in a conservative design? Any tips / heuristics? The solids are hard & crystalline NOT fibrous / slimy or pulpy.

 

[EdStainless]

Are the tubes U-bends?
At that velocity they should stay entrained, but if they are not then the velocity in the head at the far end will be a lot lower and you may have massive settling.
And what about erosion?
What level of loading are you thinking of?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[plantprowler]

Thanks @ed

No. No u-bends. These are straight tubes. HEX with a usual pass partition plate. I avoided U-tubes because I assumed they would be a nightmare to clean if a tube got plugged (not unusual for an evaporator duty). With straight tubes, & NaCl-plugging a water jet lance or rodding works well to clear plugging.

Yes, erosion is a risk. But not sure how to mitigate that.

The velocity selection is a tradeoff between plugging / settling vs erosion.

I'm thinking of very conservative loading. Say 5% solids conc. But I wanted an opinion from others. Is this too low a loading? The lower I want to keep the solids conc. the sizing of the solids removal loop increases.


Thoughts?

 

[georgeverghese]

At flow turndown on the tubeside, solids woud settle in the tubeside channel ?

Can you not do single pass on tubeside, with flow going down ?

 

[plantprowler]

Thanks @george

Not sure if downflow would work: This is a calendria for an evaporator. There is suppressed boiling in most of the tube length due to the liquid head above it.

But in the last parts of the tubes we have a vapor liq mix that then directly enters the vap liquid separator mounted next to it.

Something like this sketch below (but with 2 passes)

I could do a single pass though. Only reason I didn't was that single pass makes the pump size 2x to give the 3 m/sec velocity.

 

[EdStainless]

What Ni alloy are you using for tubes?
You need to look at single pass, up flow, with smaller diameter tubes.
I know people running in 5-10% solids range.
Worry about heat transfer and handle erosion with material selection.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[srfish]

There is a tip on

http://www.gulleyassociates.com

concerning tube velocity. The website shows the topic "minimum velocity inside tubing for slurries.

 

[plantprowler]

@ed

Thanks again.

Not Ni tubes. These are SS316Ti tubes. The backup option is pure Ti tubes or Ti-Gr12 / Ti-Gr16.

With Ti I expect erosion won't be as much of a problem even with high velocities?

 

[EdStainless]

Ti will have worse erosion than SS, but 316 is a poor choice.
If you need Gr12 or 16 I can't believe that 316 has enough corrosion resistance.
Why not a duplex like 2205 (as long as you are not over 305C)? It has better erosion and corrosion resistance than 316.
The high alloy stainless grades (like 6%Mo) have very good erosion and corrosion resistance.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[plantprowler]

@ed

You are absolutely right. Why SS316 (actually SS316Ti) has no good technical answer. Management stupidity mostly. [Rolls eyes]

They want to "prove" that SS316Ti won't work before they fund something "exotic" is what I surmise.

To their credit, the system has a cocktail of organic impurities and there's some (fuzzy) anecdotal evidence from a related site that the organics have some sort of inhibitory effect on pitting. For all I know this could be myth. We shall see.

Well, its a fairly innocuous system so a leaking bundle won't be a disaster.

A duplex would be cheaper than Gr12 or Gr16 but not cheaper than Gr2 Ti, right? Gr12 / Gr16 are our last resort.

 

[plantprowler]

Ti will have worse erosion than SS, but 316 is a poor choice.

Oh. That's interesting.

I always thought Ti had *lower* erosion than SS.

 

[EdStainless]

The low strength Ti grades used in chemical process (gr2 and other with similar properties) are very soft, and they don't work harden. I have seen cooling water erosion issues as well as steam impingement erosion of Ti in applications where SS stood up fine.

The tubes in 2205 should actually cost less than 316, buy the time you reduce the wall to adjust for the higher strength

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[plantprowler]

The tubes in 2205 should actually cost less than 316, buy the time you reduce the wall to adjust for the higher strength

Oh, thanks for that info. I did not know that either. I will explore 2205 tubes then.

OTOH, they don't make 2205 doped with Ti, do they? Then again I'm not sure how much of a difference that tiny bit of Ti doping makes to SS316 anyways. But there's this belief (myth?) that SS316Ti will hold up better than SS316.

 

[georgeverghese]

The sketch implies single pass or multiple odd passes only, and I dont see how you can set this up for 2pass tubeside flow.

Since you have a constant recirc pump, if the solids are small, you may be able to get away with high tubeside velocity and high piping velocities. Then again, there may be solids settling out in dead spots on the tubeside channels.

Else make provisions to accumulate these solids at some low points in this recirc loop for automated live or offline removal - can get messy.

Agree duplex 2205 is more erosion resistant that SS316, and you may want to need to do this if the solids are abrasive. Overall corrosion resistance is also much better than for SS316.

Esle see if you can install a hydrocyclone on the dilute liqour feed to remove most of these solids in a concentrated slurry form upstream of this forced recirc loop?

 

[EdStainless]

316Ti is used to prevent sensitization of the welds, the exact same thing happens when you use 3136L. Unless there is a lot of field welding there is no reason to use a stabilized alloy.
2205 is a lower C alloy and though it does have some special welding rules (2209 filler and restricted heat input) it welds very well and the welds have good corrosion resistance.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[itdepends]

I'm not clear what you're evaporating here- although you do have a reference to NaCl above. Is it a brine solution? Something else?

The operating temperature (under vacuum?) is also not provided which will impact on material selection for corrosion.

With a fixed speed recirculation pump turndown won't be an issue with respect to solids settling out. Your tube velocity should be based on the settling velocity of the solids in the process liquid.

Is the primary purpose of the evaporator to produce condensate/evaporate liquid (water?) or is the solid precipitate a product? If the solids are a product then that may drive the selection for the % solids.

5% solids is not particularly high for a shell and tube heater. In some cases operating at significantly higher slurry densities (e.g. >25w/w% solids) can be beneficial via hindered settling. We routinely operate slurry heaters at 40-50w/w% in 25-32mm tubes. The larger tube sizes are an advantage as they clear (water jet) more easily.

Horizontally mounted tubes are easier to water jet and retube (better access).

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"