Project Synopsis

Computer process simulation and process design of a multi-tower HCl absorption system in India, designed to produce concentrated hydrochloric acid from two production offgas streams. Simulation Diagram

Project Summary


The client, an engineering consulting firm, was contracted by a foreign consortium to perform a detail design for upgrades to a specialty chemical production facility, and subsequently subcontracted Process Engineering Associates, LLC (PROCESS) to perform a detailed computer process simulation and process design for a multi-tower HCl absorption system designed to produce concentrated hydrochloric acid from two production offgas streams.  The primary purpose of the project was to execute rigorous computer process simulations for a basic three-tower absorption system design to demonstrate that the system can achieve the difficult goal of simultaneously producing 25% hydrochloric acid product and a maximum 15 ppm HCl concentration in the absorption system offgas. 

The following variables could be adjusted to meet the system performance goals:

  • Tower heights
  • Tower recycle flow rates
  • Quench/absorption water flow rates
  • Feed gas and intermediate product and offgas stream routings
  • Recycle stream temperatures (i.e., degree of system heat rejection)

The simulation was challenging due to the non-ideality of the HCl/water system, the complicating presence of ferrous chloride (FeCl2), and the presence of an azeotrope.  Vapor pressure data was regressed in the simulator thermodynamics package to help ensure accurate vapor-liquid equilibrium data for this application.  By methodically testing the above variables versus overall system performance, PROCESS was able to determine the optimum system configuration and operating conditions to achieve the required absorption system performance.  Use of the commercial process simulation software for the modeling allowed for completing the multiple runs required for system optimization in a relatively short time frame.

After the optimum system configuration and operating conditions were determined, the simulation software was used to perform sensitivity analyses on the key variables for the purpose of determining the “size of the operating envelope”, i.e., how much inlet conditions (e.g., inlet gas stream HCl concentration, quench/dilution water temperature and flow rate, cooling water flow rate, etc.) could vary without causing excursions in hydrochloric acid and offgas HCl concentrations.  It was determined that, with the incorporation of a well-designed feedback control system, that the operation should be relatively stable under expected operating condition variations.

The simulation software was also used to determine: the total number of required theoretical equilibrium stages, the overall packed height based on the calculated height equivalent to one theoretical plate (HETP), and the optimum column diameter for all three towers; and recycle cooler heat duties.  PROCESS also selected the packing type and materials of construction, and calculated packing HETPs, based on knowledge of, and experience with similar applications.

Based on all the information determined during the course of this project, PROCESS prepared duty specifications for the absorption towers and associated equipment for use by the client during detail design of the absorption system.

Industry Type

  • Chemical Manufacturing

Utilized Skills

  • Computer process simulation
  • Absorption tower design
  • Required tower HETP determination
  • Azeotrope and non-ideal system VLE
  • Design optimization
  • Basic engineering design (BED)

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