Spiral Heat Exchanger
Figure 1. Respondent recommends unit that has channels without spacers.
Switch To Shell-And-Tube
I see that you have a problem but you only gave us half the story. We have no details of the sludge side of your process other than temperature. Your flowsheet shows exchanger outlet pressures of about 10 psig on the glycol side.
Spiral heat exchangers are great for providing large exchange surface at minimal cost. They work well so long as the differential pressure across the heat transfer surface is fairly low and neither side is prone to fouling.
[callToAction ]
My preference is for shell-and-tube heat exchangers when one of the fluids is a slurry. However, without a compelling reason not to, my designs put the slurry inside the tubes. It is much easier to clean the inside of a tube than the outside.
All this being said, there is a simple trick that I used in the 1970s to prevent leakage between a glycol heat transfer fluid and a process fluid. Our issue was keeping the glycol out of the process side, so we kept the process pressure higher than the glycol side. In your case, you should keep the glycol pressure higher than the process pressure, assuming of course that the presence of glycol doesn’t adversely affect your digester operation.
Vic Justes, consultant
St. Augustine, Fla.
Stay With Spirals
I believe that a spiral exchanger still should be your solution. The design needs to have welded end construction (for glycol solution side) and channel spacing without spacers. This design should provide excellent heat transfer and anti-fouling characteristics. It also eliminates cross contamination while providing excellent cleanability should fouling occur (opening of both ends and easily high-pressure blasting through the unit). If your unit is not of this design, you should get a replacement unit that is; I do not believe modifications would be viable or cost effective.
Jim Bliss, maintenance/utility engineer
Occidental Chemical Corp., Niagara Falls, N.Y.
Monitor For Leaks
Detecting leaks is relatively easy in closed system because the offending chemical doesn’t dissipate. However, detection can be tough if the chemical reacts with the circulating fluid.
Some H2S monitors can work well with an aqueous stream. I suggest having operators use lead acetate test strips weekly. These strips can detect H2S concentrations down to 5 ppm-m. Take precautions by using an enclosed plexiglass sampler.
It’s important to catch H2S as quickly as possible, especially in a closed system. H2S is a reducing agent (electrophile) and is flammable. Note in particular that H2S, if present, in the PG/aq. solution will react with aldehydes (violently) and dioxalanes.
Corrosion occurring in the spiral heat exchangers most likely will appear first at the welds. I suggest cutting up one of your leaky exchangers and dye-testing the welds. Changing heat exchanger type won’t make the problem go away. Switching to shell-and-tube exchangers might even make the problem worse because they are more complex that spiral units.
One common solution to leaks is to de-pressurize the most offensive chemical. Why not lower the pressure on the manure side or raise the pressure on the PG/aq. side?
You can handle the odor complaints in a number of ways, including by planting vegetation that will mask the odor with a pleasant scent or absorb the odor. Another option is using a fogger to spray a fine mist over the area where the odor is produced; a mild carbonate solution or even water should suffice to eliminate much of the foul smell.
Dirk Willard, consultant
Wooster, Ohio
