Project Synopsis

Provide an independent process engineering design review of a high-pressure letdown system for a pilot-scale foods related process.

Project Summary


The client, the R&D group of an international producer of food products, was piloting a high-pressure slurry letdown process at one of its U.S. facilities.  The slurry consists of pressurized hot water and a granulated organic food material.  The process configuration consists of an orifice, control valve, flash tank, and small heat exchanger.  The heat exchanger is cooled with cooling water supplied from a cooling tower.  Pressurized slurry feed source is from a plug flow reactor (PFR).  The reactor is operated weekdays during day-shift and then shut down. The high-pressure letdown system is used for system shutdown.

High-pressure slurry from the PFR enters the letdown system and flows through an orifice that drops stream pressure significantly.  The slurry then flows through a control valve used to maintain back-pressure on the PFR.  Flow from the control valve enters a flash tank that separates the vapor and liquid phases.  Slurry solids go out the bottom of the flash tank with the liquid stream.  Flash tank overheads are condensed in a shell-and-tube exchanger.  Vapor condensate and the liquid slurry bottom stream is sent to waste.

Process Engineering Associates, LLC (PROCESS) was contracted by the client to provide an independent third-party review of the process design developed by the client.  PROCESS performed computer simulations and calculations to verify the client’s design.  Results were summarized and tabulated, and then submitted to the client for review.

For the first step of this evaluation effort, PROCESS prepared a Design Basis that summarized the input parameters used in the technical evaluation and submitted the document to the client.  After the client’s acceptance of the Design Basis, PROCESS generated a CHEMCAD simulation model of the process and prepared a Process Flow Diagram (PFD).  Results of the simulation model were used to develop a Heat and Material Balance (H&MB) of the system.  H&MB results, which were used in the subsequent analysis of the process equipment, were presented on the PFD for easy reference.

Pipe sizes were also analyzed using CHEMCAD.  Velocity and other parameters such as pressure drop were evaluated and found to be acceptable.  The commercially available Computational Fluid Dynamics (CFD) software, MStar, was used to model the orifice.  Use of CFD model provided a visual result of the length of the vena-contracta of the orifice.  This variable was of intense interest to the client, who wanted to ensure that the velocity profile downstream of the flow orifice did not create unwanted pressure drop, turbulence, or erosion.

Computations using Excel’s spreadsheet software were used to examine the tangential entry of the flash tank.  API-14 and API-521 were used to aide in evaluating the inlet nozzle velocity and to determine if vessel erosion would be an issue.  It was found that the nozzle entry to the flash tank was adequate for the process design conditions.  The spreadsheet also examined the diameter, sump depth, and disengagement height to help ensure that two-phase separation would be complete with minimal carry-over of liquid.  In all cases it was found that the client design was adequate, if not more than adequate, for the process design assumptions outlined in the Design Basis.

Results were tabulated, and a report generated and submitted to the client.  The client used the analysis to continue their pilot activities.  As of the writing of this document, the client is considering utilizing PROCESS for additional follow-up activities with a focus on orifice sizing calculations (for alternative pressure drop specifications).

Industry Type

  • Food and Beverage Manufacturing

Utilized Skills

  • Independent process design review
  • Process simulation
  • Computational Fluid Dynamics (CFD) modeling.

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