Plate-and-frame (P&F) heat exchangers were first manufactured 100 yr ago for use in the dairy industry. They have since been used in the process industries, as they offer considerable advantages in true counter-current flow, high heat transfer rates, low manufacturing costs and a small footprint. These benefits lead to lower capital and operating costs than can be achieved with shell-and-tube (S&T) heat exchangers. The limitation on their use has been the elastomer gasket between each plate, which is susceptible to leakage from chemical attack or to penetration by aromatics. In addition, operating pressure and temperature are typically limited to 20 bar at 180[degrees]C (356[degrees]F) for water--a condition that precludes its use in many downstream processes, especially refining, where aromatics can soften gasket materials at considerably lower temperatures.
Welded plate exchangers arose from these shortcomings in the P&F type. Two types exist: plate-and-shell and plate-and-block. Both were initially tungsten inert gas (TIG) welded, but are now fabricated with laser-welded plates. Production of the laser-welded plate-and-shell type started in 2006, and some 1,200 units have since been delivered, mainly for use in the chemical industry or for utility systems in large buildings. The shell type differs from the block type in two vital respects: it has true counter-current flow, whereas the block type is a cross-flow design; and the service pressure for the plate-and-shell heat exchanger is potentially as high as 400 barg, comparable to the S&T exchanger, whereas the block type is limited to around 30 barg by its cuboid shape.
Part 1, featured in the February issue of Hydrocarbon Processing, detailed the construction of a plate-and-shell heat exchanger, plate packs assembly, flow directors, arrangements, reliability and maintenance, and process applications. The following will present study results of a welded plate-and-shell heat exchanger in a crude distillation unit (CDU). The possibility of using a parallel scheme rather than a series flow heat exchange scheme for the crude preheat train is also investigated for both heat exchanger types.
CDU example. The main column pump-arounds, distillate products and atmospheric residue (the largest heat source) are available to heat incoming crude upstream of the furnace. Three banks of heat exchangers are present. The first one heats the crude feed to desalting temperature; the second one heats the desalted crude to 180[degrees]C (356[degrees]F)--at which point it enters a flash separator, the gas from which is fed directly to the main crude column. The flashed crude oil is then pumped through a third bank of heat exchangers, the fired heaters and into the flash zone of the main column at a temperature at or above 370[degrees]C (698[degrees]F). The hot vapor rising in the column is cooled by the pumparound streams, and the resulting condensate is drawn off into side strippers. Three distillate products are normal, i.e., kerosine, diesel or light gasoil (LGO) and heavy gasoil (HGO).
Main column overheads are condensed, and offgas is compressed and re-contacted with the distillate naphtha from the reflux drum. The combined naphtha stream with the absorbed gas is sent to...