In a plant making Terephthalic Acid (a constituent for the production of Polyester fibres) the silos or hoppers which are filled by mechanical or pneumatic conveying of the material. Raw material (C.T.A.) feed silos in the preparation section empty into the mixing process, and finished product (P.T.A.) silos are discharged to bagging stations. The filling and emptying of these silos generated vent gas which has to be treated before going to atmosphere.
The vent gas treatment involves the removal of large amounts of C.T.A. or P.T.A. dusts, and can also require the odour abatement caused by the presence of acetic and benzolic acids. Typically up to 10 g/m3 of dust may be present and in some cases a first stage bag filter system is used. Alternatively, or in series after the bag filter, a scrubber is installed to polish the solids emissions below the required limit and also to perform the odour control duty. Where possible, the effluent water from the scrubber will be re-cycled back to the mixer stage in the preparation section.
A Begg Cousland BECOFLEX BF Rotary Brush Scrubber has proved to be ideal for this duty. Originally developed by I.C.I. and then licensed to Begg Cousland, it has been an integral part of the Invista P.T.A. technology package. The brush is made of PP or PVDF fibres and is rotated by a standard fan motor inside a volute casing. The brush is irrigated with the scrubbing solution / water, and creates a dynamic gas/liquid contact inside the volute. (For additional information look at Becolfex Rotary Brush Scrubber in the Products / Gas Cleaning section of this website).
Using 304L Stainless Steel for the casing and vessel, the BECOFLEX is compact and can normally be sited at the roof level of the silo. With the rotating brush acting as the air mover, there is no additional fan needed, and this has the benefit of enhancing the vacuum effect inside the silo. The whole unit is light enough to be mounted at the vent point, saving ducting costs.
When performing odour control functions, a 5% NaOH solution is used.
Gas volumes from 500 to 25,000 m3/hr can be handled in a single unit (see table below, showing flows & suctions, physical sizes, motor sizes and irrigation rates.
BECOFLEX BF Series Systems for PTA/CTA Silo Vent duty
A Becoflex BF 130 unit (left) and the installation on silo top (right)