Mist eliminators in sulfuric acid plants are selected to give an agreed efficiency (defined either by % for one or more micron mist size ranges, or by a total residual mg/Nm3 content), while respecting a pressure loss value in clean condition. This article focuses on some examples of external factors that interfere with the performance of mist eliminators.
In sulfur-burning sulfuric acid (H2SO4) plants the drying towers are generally equipped with either a knitted wire meshpad type demister or a set of short, high velocity, cylindrical fibrebed candle filters. The meshpad is designed typically with a pressure loss in the range of 40 – 80 mm H2O and removes all particles greater than 5 microns – or all that are greater than 2 microns if co-knit layers with fibre yarn are included. The high velocity candle filter may run in the range of 100 – 180 mm H2O and removes all particles greater than 3 microns and an estimated 80% of particles between 1 – 3 microns. The drying tower exit content is typically specified as 40 – 50 mg/Nm3.
The air that feeds into the drying tower should be clean, so it is filtered to remove solid dust particles from the ambient conditions. The degree of filtration varies, with some systems using quite coarse media, while others are more efficient. It is common for these air filters to be neglected or poorly maintained, and as a result dust – often phosphate rock dust from an adjacent fertilizer plant – enters the drying tower and slowly but surely collects in the meshpad or fibrebed, restricting the free volume void of the filter. This blockage increases the pressure loss and the localised air velocity, with negative results on efficiency. The collected sulfuric acid cannot drain freely as it should and, as it floods the filter media, it is entrained downstream by the higher velocity air flow.
Sulfur dioxide (SO2) off-gas from non-ferrous metal smelter operations is the feed into the drying tower that has first passed through a series of gas cleaning stages, e.g. electrostatic precipitator (ESP)/quench/wet electrostatic precipitator (WESP). There are often operational problems with parts of the gas cleaning train, resulting in solids and sulfuric acid mist entering the drying tower. Filter media blockage is also an issue here, but in addition the filter may be receiving smaller sized mist particles than their design can handle. Particles below 2 microns will not be efficiently collected and will impact the downstream catalyst pressure loss and corrode any downstream blower.
Troubleshooting mist eliminator issues
Regular monitoring of the stability of the pressure loss of the filter is a valuable tool in preventing serious consequences, as is performing stick tests. Stick tests are a cheap way of generally observing the amount and size of liquid particles exiting the drying tower. Figure 1 shows heavy droplet marks (large black burns on the wood), while Figure 2 shows a mix of mist and droplets observed (pin-prick size mist and larger spots).
Figure 1. Stick test with droplets.
By using regular pressure loss and stick test information the plant operators can establish a degree and rate of change in mist eliminator performance, and plan to wash or replace the mist eliminator(s) accordingly.
Figure 2. Stick test with mix of mist and droplets.
The main role of mist eliminators in an intermediate absorbing tower is to protect the expensive gas-gas heat exchanger immediately downstream, minimising corrosion and blockage. The tower generates the highest amount of acid entrained to the mist eliminators. In addition the majority of the acid particles are below 3 microns, with a very large proportion smaller than 1 micron. This is due to the changes in vapour pressures in the tower when hot gas (±200°C) contacts the circulating acid at 70 – 80°C. There are also larger droplets entrained from the acid distribution system, which is the case in all towers. This all means that the mist eliminators used here must be of a very high efficiency, able to collect sub-micron mist even below 0.5 microns in size. Such Brownian diffusion type candle filters operate with a gas velocity in the fibrebed below 0.25 m/sec.
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