Project Synopsis

Prepare a process design package for a small, continuous hydrofluoric acid (HF) recovery system to reclaim valuable spent acid from the manufacturing process.

Project Summary


Process Engineering Associates, LLC (PROCESS) was contracted by the client, a specialty metals manufacturer, to provide process design and engineering services in preparation of a design package for a pilot-plant size hydrofluoric acid (HF) recovery system. Specifically, PROCESS was contracted to prepare the following engineering documents:

  • Mass and energy balance
  • Process flow diagrams (PFDs)
  • Piping and instrumentation diagrams (P&IDs)
  • Process control description
  • Equipment duty specifications

The feed to the pilot system is a waste raffinate stream from the client’s extraction process.  The waste stream contains a mixture of free HF, fluosilicic acid (H2SiF6 also referred to as HFS), complex metal fluorides, various elemental metals, and water.  The recovery process involved two stages.  In the first stage, the waste is stripped of excess water to concentrate the HF.  In the second stage, the waste stream is fed to a high-temperature distillation column to separate the HF from the remaining water and other constituents.

The client contracted with an outside laboratory to collect process-specific vapor liquid equilibrium (VLE) data.  PROCESS was involved in defining the scope of work for the outside lab as well as reviewing the VLE data results to define additional data point collection requirements to ensure that the requisite data was available for simulation model VLE regression.

During the separation process, the HFS dissociates (decomposes) to HF and silica tetrafluoride (SiF4).  The extent of the dissociation is a function of temperature and pressure, and a literature search by PROCESS uncovered this relationship expressed as a curve showing the partial pressure of SiF4 versus HFS concentration in the liquid.  PROCESS generated a similar curve with the client’s process-specific VLE data.  The curve was regressed into PROCESS’ licensed computer process simulation software, CHEMCAD, to model the separation process.  PROCESS had to determine if the client’s performance specification for HF purity in the distillate product could be met with a range of waste feed constituent concentrations.  The design team also had to determine if one pilot-plant column, operated batch-wise, could be used for both the stripper and distillation unit operations.

PROCESS used CHEMCAD to develop a complete mass and energy balance for the pilot stripper/distillation column.  Sensitivity studies were performed to arrive at the optimal number of stages, feed location, reflux ratio, reflux temperature, and product recovery.  These studies were used to define the operating conditions shown in the PFD.  From this design information, the P&IDs and process control description were developed.  Equipment duty specifications were prepared for all new equipment items associated with the process: the stripper/distillation column, condenser, reboiler, product cooler, bottoms cooler, pumps, and blower.

The design of the acid recovery stripper/distillation process presented challenges to the design team.  The nature of the separation process and the fact that the feed stream constituent concentrations are variable will make the system challenging to operate while maintaining consistent quality.  The design team took additional steps to validate the design by reviewing literature for additional component-specific data and completed sensitivity studies comparing product purity and required number of stages to gauge the robustness of the design with respect to changing feed streams.

Industry Type

  • Specialty Metals Manufacturing

Utilized Skills

  • Pilot-plant size continuous process design
  • Vapor/liquid equilibrium (VLE) data regression
  • Sensitivity analysis

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