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Mining and metals refining
Apr 1, 2020

WESP technology and advances in digital support for our customers

By precisely adjusting the operation of the Outotec EDITUBE® Wet Electrostatic Precipitator, our newly developed Outotec EDITUBE® Process Advisor maximizes removal efficiency during upset conditions, improves availability through reduction of unplanned stoppages and – for plastic units – increases material protection and integrity during low load conditions.
Wet electrostatic precipitation at the Dundee Precious Metals sulfuric acid plant

Gas cleaning solution with superior removal efficiency

The wet electrostatic precipitator (WESP) is a wet gas cleaning solution designed for removal of the finest fractions of dust particles and mist aerosols. The equipment comprises a bundle of tubes through which the gas flows vertically.  A discharge electrode is placed in the center of each tube and with power supplied to the unit, an electric field is created around the discharge electrodes and a flow of electrons is achieved. As the gas passes through the tubes, particles and aerosols become electrically charged through collisions with the fast-moving electrons. Once charged, the particles and aerosols migrate through the electrical field towards the tube wall where they are collected in a liquid film on the tube surface. The gas direction can be either top to bottom or bottom to top.

Outotec EDITUBE WESP
Outotec EDITUBE WESP

WESPs are a necessity prior to sulfuric acid plants processing metallurgical off-gas to ensure a high-quality acid but have also become increasingly common before stacks in non-sulfur applications with wet gas cleaning because of higher demands on low dust particle emissions. Outotec EDITUBE WESP has a very high removal efficiency potential where dust levels in the outlet are commonly measured at below 0.5 mg/Nm3.

The tube bundle in each EDITUBE is enclosed in a casing. A gas distribution system inside the casing, before the tubes, ensures an efficient distribution of the gas in each tube. A uniformly spread gas is vital to ensure a high removal efficiency, which is heavily dependent upon retention time. The emission system, carrying the discharge electrodes, is free-hanging, with the upper frame connected through suspension rods to the insulators located on the top of the unit. The insulators are enclosed in hoods and kept dry by heating of the encapsulated air. The heated air acts as a cushion, protecting the hoods from the process gas and eliminating the need for pressurized air. The free-hanging design of the EDITUBE electrode system effectively reduces the inherent forces that the electrodes are exposed to, thus greatly increasing their lifetime.

Our EDITUBE WESP is self-supported, with the Transformer and Rectifier (T/R) unit and insulator hoods conveniently gathered on the top. Additionally, with the correct design considerations, it can even carry the gas inlet and outlet duct, as well as the relevant access platforms. With access ladders attached to the upper parts of the unit, EDITUBE requires no surrounding steel structure. 

Outotec EDITUBE is equipped with a flushing system to clean the tubes from any build-ups. The flushing is performed once per shift and normally circulates the acid mist condensate having been removed from the process gas, thus consuming very small amounts of water. In some cases, when the process gas itself does not contain any or enough sulfuric acid aerosols, a second auxiliary water system is installed: the fogging system. Captured aerosols form the liquid film flowing on the tube surfaces, which not only prevents re-entrainment of particles to the gas but also protects the material in plastic units. The fogging system is a spraying system, forming an artificial mist of aerosols that achieve the same effect.

Outotec EDITUBE WESP
Compact 2x2 design

With over 30 years of WESP experience, our experts have built a vast technical knowledge that allows them to continuously improve the design and performance of Outotec EDITUBE. With increasingly automated and digitalized plants, new ways of transferring this knowledge to the products have become available. Now, more than ever before, we can provide a flexible WESP process section, where our units in real-time adapt to changes in the process conditions, ably assisted by the Outotec expertise through our new digital process advisor for the EDITUBE WESP.

Digital tool for optimized operation

The control system of modern plants offers the operators an abundance of data. The WESP section alone provides information based on which conclusions can be drawn not only on WESP performance but the performance of the upstream plant in general. Knowing how to interpret that information and understanding what is relevant and how it fits together with readings in other parts of the plant is a difficult task. But Outotec EDITUBE Process Advisor can handle a large amount of data, interpret it and present the essential information to the operators.  

Maximizing removal efficiency during offset conditions, when the concentration of impurities in the gas is very high, poses a challenge to the unit meant for cleaning of the very finest fractions of impurities. Foreseeing a maintenance need of the equipment which requires very little attention is also difficult. And for any plastic filter in operation, there is always a risk of running the tubes dry during low load conditions, which may result in material damage. Managing these challenges requires attention to the conditions under which the equipment operates. Our process advisor continuously monitors the process conditions and provides a combination of automatic responses, information and suggested actions for operators to contemplate.

The process advisor performs real-time classification of load conditions to the WESP section based on the plant measurements. For applications with SO2, acid mist estimations are made based on automatic condensate measurements. Over time, plant-specific rates for SO2 to SO3 conversion are calculated and based on just the on-line measurements of gas flow and SO2 concentration, our digital tool not only adjusts the operation of the WESP section but also provides the operator with useful information on the overall performance of the upstream plant which may have led to those conditions.

Based on the load conditions, our process advisor will determine the optimal parameter setting on the T/R-unit. For plastic filters during low load conditions, to protect the material from damage, it strategically places a voltage limitation on one or more WESPs to ensure material integrity while still achieving the performance targets of the plant section. During extremely high loads caused by upset plant conditions, the advisor adjusts the parameters again to keep the filters operating and maximize removal efficiency until the plant is back to normal operation. 

Besides, the EDITUBE process advisor assists plant staff with information on maintenance requirements. It monitors the flushing and – if applicable – fogging nozzles and water filters to provide information on when cleaning or replacement is necessary. It also quickly identifies and provides information on conditions that require operator attention.

Different control options

Outotec EDITUBE Process Advisor is installed in an industrial PC, which, either directly or through a separate PLC, is communicating with the plant DCS. As the function is merely advisory, the tool can be disconnected at any time, leaving the WESPs operating as normal, without any impact on their availability.  

The optional separate PLC can also control the auxiliary water systems, flushing and fogging, of the WESP section in addition to supporting the advisor-DCS communication. The complete control option is performed by the Outotec EDITUBE Control Cabinet (ECC), in which the PLC is situated. Using a well-proven and rigorously tested control system reduces the amount of automation design and plant down-time. The ECC has a touch panel with individual control of each EDITUBE and corresponding auxiliary equipment.

Plug-and-play solution

With the ECC, Outotec has opened the door to a plug-and-play solution that maximizes the fabrication, installation, and control logic implementation that can be done before arrival on site.

We can provide the client with a WESP plant in its entirety, ready for CE-marking: including pumps, valves, tanks, piping, and all instrumentation and cabling, all efficiently designed to minimize the overall footprint of the section. Combined with the EDITUBE self-supporting properties, the pre-programmed plant that doesn’t require supporting steel structure is simply plugged into the gas duct.

The number of steps taken towards plug-and-play is decided in each case: the process advisor, PLC control of auxiliary equipment and prefabrication and programming can all be tailored to the specific needs and requests of the client.

For further information on Outotec EDITUBE WESP or the process advisor please contact Leif Skilling leif.skilling@outotec.com

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