Finding the Root Cause of Acid Carryover

When foggy gas, acid draining, and burnt sticks are observed, the immediate concern of all operators is mist eliminator efficiency. Properly sized and built, previ­ously tested, and carefully installed mist eliminators are extremely reliable pieces of equipment. In most cases, mist eliminators should be the last and not the first concern, especially when evaluating their costs.

This article shows a successful case in which filters were not responsible for ob­served acid carryover.

Problem identification 

Stick tests, as shown in Figs. 1 and 2, are the simplest qualitative way to reveal fine mist carryover and/or acid droplet re-entrainment (dark wet spots), an inefficient SO3 absorption, or submicron mists (uni­form wood burn) inside an absorption or drying tower. An ideal stick test retrieves a clean stick. Fig. 1 shows, from left to right, a 1-minute stick test realized at the drying (DT), interpass (IPAT), and final (FAT) towers of a 600 MTPD sulfur burning sul­furic acid plant. It is easy to see that the IPAT was operating with significant car­ryover—the stick was so dark and wet that it is almost impossible to pinpoint possible causes. A major mist eliminator vendor im­mediately suggested that candles were not working properly and that replacement was necessary.

Fig 1. Original Stick Test

Fig 2.Closer stick test view shows dark spots (acid droplets).

Operating with these conditions may result in downstream damage. Clark Solu­tions and the customer chose a programmatic problem solving ap­proach to identify possible issues and correct flaws, prior to condemning the candles.Fig. 1 shows, from left to right, a 1-minute stick test realized at the drying (DT), interpass (IPAT), and final (FAT) towers of a 600 MTPD sulfur burning sul­furic acid plant. It is easy to see that the IPAT was operating with significant car­ryover—the stick was so dark and wet that it is almost impossible to pinpoint possible causes. A major mist eliminator vendor im­mediately suggested that candles were not working properly and that replacement was necessary.

September 2015 inspection 

In September 2015, the plant was shut down for a brief inspection. The IPAT in­spection identified several cuts along the support beams, most probably because of a modification to fit candle filters that were acquired with different flange bolting hole patterns. Originally, the tube sheet was de­signed to support seventeen candles with 16 flange bolts on each candle. However, a different situation was found, with a mix of 12 and 16 flange bolts as well as two block­ages at tube sheet center. When observing these conditions there was suspicion of possible tube sheet warping.

Besides the cuts made in the support beam, there were changes in the original tube sheet project.

Downcomers 

Fig 6. Downcomers pins and trough fixation.

Most downcomers were heavily fouled, directly affecting acid flow on dis­tribution troughs. This caused heavy acid overflow and resulted in intense acid-mechanical carryover toward candles that overloaded due to draining capacity. All downcomers were cleaned and/or repaired. Downcomer extensions are attached by pins that must be properly designed to avoid fouling. Teflon pins are shown in Fig. 6. Properly designed pins keep debris that may be circulating with the acid from block­ing the downcomers. All pins were replaced with Clark Solutions designed pins. Improper trough fixation can result in unbalanced troughs, which causes vibra­tion and damage, and results in acid spills and carryover.

Tube sheet drain seal legs.

All IPAT tube sheet drain seal legs were completely fouled and blocked by sulfate, preventing continuous drainage of collected acid by the tube sheet mist eliminators. At the time, the only way to drain collected acid was through a side drain, which operators opened once every shift. All draining seals were changed with newly designed ones, properly sized for the operating pressure drops in a more open design to reduce fouling effects.

Since there were candles of different types and suppliers inside the IPAT, there was not a pattern on the mist eliminators. Fig. 11 shows two hole patterns on the same flange. In addition, some candles included an internal reinforcement structure and others did not. In September, 18 Fiberbeds® were bought from Clark Solutions. Because a new tube sheet with one single candle bolting hole pattern was scheduled to be installed in 2016, the candles were designed and built with a stainless steel pedestal so that they could be adapted and reused in the future.

Fig 11.New Fiberbed® with pedestals to fit different hole patterns.

Due to the short working timeframe in September, it was not possible to implement further actions. After starting the plant, a new stick test was conducted. The improvements were noticeable, but not complete, so another inspection was scheduled for January.

Stick Test Result

January 2016 inspection 

In January 2016, the plant was shut down for a brief inspection prior to the new tube sheet installation. Due to both the mist eliminators’ weight in operation and the support beams’ “cut windows” (Fig. 3), the tube sheet showed warping, creatingpreferential gas paths between the filter’s flange and the tube sheet.

The January inspection revealed that the Teflon gaskets on some filter candles were clearly not tight, indicating warping and gas passage. Fig. 15 shows that even anti-acid bricks were removed to fit bolt­ing. Even 6 mm full face Teflon gaskets were not able to correct the spacing caused by the tube sheet warping, confirming that a new tube sheet was necessary.

In order to improve acid distribution in the IPAT acid troughs, new flow-control orifice plates were designed. Differences in the plate designs are shown in Fig. 17. The plates were changed during the Janu­ary inspection. After these orifice plates’ modification, smoother flow was observed over all acid troughs, eliminating overflow and minimizing acid entrainment.

Fig 3.Tube sheet support beams cut to fit bolts.

Fig 15. Warping detail.

Fig 17. Old (below) and new (above) orifice plates.

Tube sheet replacement and final results 

Because of the warped and damaged structure of the IPAT tube sheet, a full re­placement was made in May 2016. The new tube sheet was designed to evenly support 19 filter candles as shown in Fig. 19. After the tube sheet replacement, a proper stick test was finally achieved. The black marks in the final stick test are typi­cally seen as a result of acid residue in the line. All candle filters that were installed in September 2015 had their pedestals adapt­ed to the new candle bolt hole standard and were installed in the new tube sheet as planned. The plant has now prevented fur­ther damage of downstream equipment and is properly operational.

Fig 19. New tower head with new tube sheet.

Conclusion 

Fine mist carryover, acid droplets re-entrainment, and inefficient SO3 absorption can result from many sources, including un­even acid distribution (distributor or packing flaws), fouled drain seal legs, bad mist elim­inator installation, gasket leakage, tube sheet preferential gas paths, and others. If only the mist eliminator candles were exchanged in these situations, the stick test would show little to no improvement.

When observing acid content down­stream from towers it may be necessary to inspect and evaluate the plant, considering dry tower inefficiency, water leaks, bad acid or gas distribution inside towers, in­tense acid fine mist condensation, as well as other non-conformities discussed in this article. Clark Solutions is always available to conduct such inspections.