This month’s puzzler we use a carbon-steel spiral-tube heat exchanger as a condenser in our spray tower
THIS MONTH’S PUZZLER
We use a carbon-steel spiral-tube heat exchanger as a condenser in our spray tower. Operating conditions normally are 100 psig at about 150°F. Treated chilled water at 45°F runs in the tube side. The shell side handles metal chlorides, e.g., FeCl2, VCl4 and TiCl4. These chlorides condense to form liquids and evaporate to form gases. Severe corrosion from chlorine and erosion was the reason why the spiral heat exchanger was chosen over a shell- and-tube one. The chlorine is usually dry but, occasionally, spikes of water upstream of the exchangers cause corrosion. One of our engineers is pushing the idea of cladding Type 304L stainless steel with a high nickel alloy. Coupon tests suggest this will improve service life. One concern is how bending the tube will affect cladding. How can we assure our nervous operations manager that this cladding will work? Do you have any other ideas? Keep in mind replacing the heat exchangers with another style will require a major change in piping around the units.
REPLACE THE EXCHANGER
Try looking at a hybrid heat exchanger that incorporates the strengths of both plate-and-frame and shell-and-tube exchangers without their weaknesses. It is a good compromise between cost and performance. My experience is only as a happy customer of an exchanger manufactured in France. Its design tends to provide the best lifecycle cost choice for our circumstances.
LOOK AT TANTALUM LINING
I think that you have not considered all of the options available. Cladding is a risky option. Maybe replacing the tubing with duplex steel would be a better option. Another idea would be to seek a consultant. There is a manufacturer in Germany that makes “tantalined” equipment for the fabrication of spiral exchangers with lined carbon steel pipe. This last item might be your best choice.
WATCH THERMAL EXPANSION
We have used nickel-plated copper tubing for similar heat-transfer applications. The plating can be applied after the tubing is bent, but you’ll need to be careful about the amount of expansion you get from temperature variation.
CONDUCT THOROUGH TESTING
Some years ago an engineer at our plant in Grimsby, U.K., looked at tantalum cladding for our chlorinators. These are large brick-lined reactors. Inside, chlorine comes in contact with carbon monoxide from burning coke; the chlorine reacts with ore to chlorinate it, producing titanium tetrachloride, TiCl4. Trace water and hydrogen chloride (HCl) in recycled chlorine is a continual problem. Unfortunately, the brick and carbon steel shell of the chlorinators is quickly attacked at the moisture penetration zone — where the water and HCl condense. These chlorinators are water-cooled by falling-film heat transfer, making it difficult to detect a breakthrough of the shell. Such a breakthrough posed a significant environmental risk.
The engineer thought tantalum, the only material not susceptible to attack by wet chlorides, would work. He was looking into coupons to test in the next chlorinator turnaround. These coupons included welds and other features.
With the heat exchangers, the combination of welds and bends offer a challenge for coupons. I suggest installing a coupon that includes welds and a distressed plate, not unlike the forming process for an elbow. Test in a large vessel, if possible, to avoid affecting the chlorination process. If the coupon survives, your cladding idea may succeed. The next test involves checking the manufacturing process. A very good test for identifying cracks in metal tubes is an acid dye-penetration test. It’s a destructive test that will reveal voids in the interstitial areas between crystals in nickel alloys and similar metals that ultrasonic means won’t identify. Use this method to validate the manufacturing process.
It would be really easy to grow shy about this application. When we considered cladding of our heating coils, which operated at about 1,000°C, we opted out. We felt that it was too risky and chose to go with high nickel alloy coils instead.
MARCH’S PUZZLER
Management wants to boost production from our batch polymer process by changing the operating procedure. Now, a portion of the acrylic monomer goes in with the bulk of the solvent and other ingredients. Then, monomer is slowly added until the reactor level rises to about 55%. Instead, management wants to increase the maximum level to about 65% and raise the final concentration of the polymer about 2%. The reaction is very exothermic; a chilled-water baffled jacket provides cooling. At the top of the reactor a horizontal shell-and-tube condenser that uses cooling tower water captures evaporated solvent. Agitation consists of a single axial impeller and the tank has reduced baffles to account for viscosity above 5,000 cP. To meet the production goal, it’s been proposed that we add all of the monomer at once; bench-scale tests have shown that this could work but our current approach seems safer. Management likes the plan and wants to put into practice immediately. Our production engineer is a little nervous — should he be concerned? How should we approach this problem? Are any process changes necessary? What do you think?
And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.
