Control and Optimal Operation of Oil Refinery and Pulp Mill Cooling Towers

摘要

For cooling large volumes of water, cooling towers (CT) in their various forms are universally used. In oil refinery practice, large volumes of water are used for cooling distillation tower side streams or cuts of certain boiling ranges. The overhead condenser for producing the reflux flow and product stream also requires large volumes of cooling water. Other refinery processes also require copious amounts of cooling water. Cooling towers reject low grade heat, tower water temperature differences tend to be in the range 8-12 C. In the case of high temperature water streams, in oil refineries, higher temperature water streams may be used to preheat boiler feedwater, heat exchanged with input process streams or may be cooled by using fan coil heat exchangers. Paper mills also require cooling water for various processes. In pulp mills these are to cool various liquid and vapour flows. The requirements for cooling water in paper mills are smaller but include vacuum pump sealing water. If a condensing turbine is installed, this will also require cooling water Cooling tower water must be properly dosed to stop build up of algae, bacteria etc. Legionaires disease is a well known problem with poorly treated CT water. Blowdown of a few percent is also required to control the build up of dissolved salts. In order to optimise cooling tower use, energy use particularly for fan operation or water supply needs to be minimised. Makeup water use should be optimised by minimising blowdown and spray and entrainment losses. Maintenance and cleaning of tower internals should be minimised by careful treatment of the recirculating water by dosing with a combination of pesticides. Fouling, Scaling and biological growths are all potential problems in cooling tower operation. These areas will be covered in this paper.