Absorption column design costing

The absorption column and its associated structures (platforms, ladders and railings) are expected to cost in the region of AS870 000. The final design comparesfavourably with similar units already found in industry. 

The absorption column is sized according to two key parameters, these are to design for optimum mass transfer and optimum unit cost. A column internal diameter can be estimated according to the liquid and gas flowrates by utilizing graphs and nomographs such as those contained in Ref. A3. These recommendations have been refined using a computer-based mathematical model. The model predicts the required number of trays for a specified column internal diameter. These results enable a compromise to be achieved between tower cost and tower performance. 

Random-packed absorption columns have seldom been used for large commercial-size dehydrators because of the very low liquid flow rates usually employed. As pointed out in Chapter I, a low liquid flow rate normally leads to the selection of a bubble-cap column. The recent trend toward extremely low dew-point requirements (-40°F and below) has resulted in a reexamination of contactor designs to attain more theoretical stages at a reasonable cost. Structured packing, which is offered by several vendors, is claimed to offer greatly increased throughput, lower pressure drop, and lower column height than bubble-cap trays for the same service. 

Determining the number of theoretical and actual trays in a distillation column is only part of the design necessary to ensure system performance. The interpretation of distillation, absorption, or stripping requirements into a mechanical vessel with internal components (trays or packing, see Chapter 9) to carry out the function requires use of theoretical and empirical data. The costs of this equipment are markedly influenced by the column diameter and the intricacies of the trays, such as caps, risers, weirs, downcomers, perforations, etc. Calcvdated tray efficiencies for determination of actual trays can be lost by any unbalanced and improperly designed tray. 

When absorption is used for VOC control, packed towers are usually more cost effective than impingement-plate towers. However, in certain cases, the impingement-plate design is preferred over packed-tower columns either when internal cooling is desired or where low liquid flow rates would inadequately wet the packing. 

In an overall process of absorption and stripping, the cost of steam is often a major expense, and the process is designed to use as little steam as possible. The stripping column is operated at close to the minimum vapor rate, and some solute is left in the stripped solution, rather than trying for complete recovery. When the equilibrium line is curved upward, as in Fig. 22.17, the minimum steam rate becomes much higher as Xj, approaches zero. 

It is important to know the total pressure drop Ap of the irrigated packed bed when designing packed columns for gas/liquid systems in counter-current flow of the phases. In absorption as weU as desorption processes, the total pressure drop Ap of the packing determines the blower capacity and thus the major part of the operating costs of the process. In rectihcation, the sum of the top pressure pT and the total pressure drop Ap gives the bottom pressure pw, which determines the bottom temperature tw- The bottom temperature, in turn, determines the effective temperature difference, at a given heating medium temperature, at which the reboiler in the distillation column must be operated. 

Dr. J. Madkowiak has committed to paper his 20 years experience as practitioner while operating company ENVIMAC GmbH and as researcher, publishing his results in journals and conference papers. Calculation methods of packed columns, presented in the book will increase the design accuracy of distillation, absorption and extraction which cause up to 60% of total processing costs in chemical industry. 

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