Framework for Selecting Thin-Cake Candle Filter Technology for Removing Solid Contaminant Fines from Recirculating Acid Gas Scrubbing Fluid Streams

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Source: BHS-Sonthofen Inc.

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ABSTRACT
The need for filtration systems in scrubbing units has been well documented. Filtration provides the benefits of reduced hot spots in the regenerator reboiler. reduced heal exchanger fouling and reduction of foaming in the absorbers. This paper discusses thin-cake candle filter technology that is used to remove trace amounts of solid contaminant fines from the recirculating fluid stream. These contaminants originate from various sources and are generally less than 3-5 microns in size, which makes their removal very difficult. Candle filler technology and the process of thin-cake building, is a new approach that is employed for high-efficiency and cost-effective fines removal.

KEYWORDS
Automatic Filter, Cake Filtration, Candle Filter, Catalyst, Catalytic Cracking, Diatomaceous Earth, Filter Cake, Filter Cake Discharge, Filter Cake Wash, Fine Particle, Precoal Filtration

INTRODUCTION
Sulfur dioxide (SO2) removal systems are in many types of applications such as fluid catalytic cracking (FCC) and Coker units at refineries, acid plant tail gas, spent acid recovery plants, smellers, pulp mills, natural gas, power generation, cogeneration, chemical process plants as well as process vent streams in sulfur plants. While the plant location and type of removal process may vary, there is a need to remove fine particulate matter from the recirculating liquid streams for efficient scrubbing operation and to protect absorbers, condensers and reboilers, heat exchangers, pumps, downstream purification systems, etc. The particulate matter can be catalyst fines and other contaminants in the feedstock. It is difficult or impossible to remove particles of this size in settling tanks, hydrocyclones or centrifuges, so the particles must be removed by filtration. The use of thin-cake candle filter technology has been proven to be a cost-effective and reliable approach to removing the contaminants, recovering the scrubbing liquids and drying the cake for easy landfill disposal.

PROBLEM DEFINITION
Various catalyst and carbonized particles are carried into the gas and are captured by the scrubbing fluid. The flow rale of the scrubbing can be as high as 150 m'Yhr so the fluid must be regenerated and reused to make the process economical. The fine particles are less than 3 to 5 microns and they will accumulate in the scrubbing system unless they are removed from the scrubbing fluid. These particles cause fouling in heal exchangers and foaming in the trays of the scrubbers, so the particles must be removed from the scrubbing fluid before it can be reused. The scrubbing fluid can fluid can be water, an amine salt fluid (e.g. CanSolv®) or some other proprietary fluid (e.g. the LabSorb® phosphate salt system). This paper discusses specifically the removal of fines from an amine fluid, but the theoretical basis and the technology can be used for other scrubbing fluids.

THEORETICAL BASIS
The following equation is the basic equation that relates the flow rate through a filler for constant pressure filtration. Various forms of this equation are used to evaluate the lab data gathered and then to scale up the lab data to the required production system.

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