Two-column Process Research Articles

Experimental study of removing trace H2S using solvent coated adsorbent for PSA

Application of compressed natural gas (CNG) as the fuel of motor vehicles will continue to grow. A key requirement for this application is the removal of trace content of H2S from the fuel. A pressure swing operation was proposed to take the task. The key technique therein is to modify the ordinal adsorbents with a film of solvent. The dual modes of absorption by solvent and adsorption by the interior adsorbent provide a higher H2S capacity than each mode alone and render the sorbent regeneration possible at ambient temperature. Experiments are performed on a two-column process to study the effect of operation condition on process performance. Consecutive cycling of the operation proves that the pressure swing technique can be used to remove the trace content of H2S in practice, which represents a major breakthrough in natural gas purification technology. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Application of a fully thermally coupled distillation column for fractionation process in naphtha reforming plant

Naphtha reformate is extracted for aromatic compounds and the aromatics are separated into benzene, toluene and xylene in sequence. This separation is conducted using a series of binary-like columns. In this study, the first two columns of the separation process are replaced with a fully thermally coupled distillation column (FTCDC) also known as the Petlyuk column. Though feed of the process contains 18 components, the FTCDC developed for the separation of a ternary system is effectively implemented in the industrial-scale application. A structural design to determine the optimum structure of the column from the minimum tray configuration is utilized, and the minimum tray configuration is found from the stage-to-stage computation with ideal tray efficiency in equilibrium condition. The operating condition to yield a given set of product specifications is found from the HYSYS simulation. The performance of the FTCDC is compared with a conventional two-column process to indicate an energy saving of 13% and the investment cost reduction of 4%.

Economic and Dynamic Impact of the Use of Excess Reactant in Reactive Distillation Systems

Reactive distillation columns are sometimes operated with an excess of one of the reactants, but this mode of operation may have the disadvantage of requiring the recovery and recycle of the reactant that is in excess. The flowsheet typically consists of a two-column system: a reactive column and a recovery column. An alternative is to use a single reactive column that is operated “neat” (exact stoichiometric amounts of the reactants fed and consumed). The one-column process has lower capital investment and energy costs than the two-column system. However, Al-Arfaj and Luyben1 recently explored the control of a one-column “neat” system and demonstrated that a composition analyzer may be required to detect the inventory of one of the reactants so that the fresh feed can be adjusted. Because composition analyzers are expensive and sometimes unreliable, the economic advantages of the single-column system need to be assessed. The purpose of this paper is to quantitatively compare the single-column (neat operation) and the two-column system (excess of one of the reactants fed to the reactive column). The comparison includes both steady-state economics and dynamic controllability. The reversible reaction is A + B ⇌ C + D. The relative volatilities are favorable for reactive distillation; i.e., the reactants are intermediate boilers between the light product C and the heavy product D. In the two-column system, an excess of B is fed to the reactive column. The distillate from the reactive column is product C. The bottoms is a mixture of components D and B, which are separated in the second column. The distillate of the second column is mostly B and is recycled back to the first column. Steady-state economics show a very significant incentive for the single-column system. Total annual cost of the two-column system is 50% more than that of the single-column process if a 10% reactant excess is needed to compensate for flow-measurement uncertainties and fresh-feed composition changes. If a 20% excess is needed, the total annual cost is 100% more. Both systems are dynamically controllable, but the dynamic response of the two-column process is slower because of the presence of the recycle and is more complex. Only tray temperature control is required in the two-column process.