Project Synopsis

Provide subcontracted process engineering services to the prime detail engineering company for the design of an outside battery limits (OSBL) relief system for a commercial scale polypropylene plastics recycling recycling facility.

Project Summary

Process Engineering Associates, LLC (PROCESS) was subcontracted by an engineering company to provide process engineering services for the design of an outside battery limits (OSBL) relief system for the ultimate client’s (a plastics recycling company) commercial scale polypropylene recycling facility, for which the inside battery limits (ISBL) design (including ISBL relief devices) was being prepared by a modular process systems design and fabrication company.  Scrap polypropylene will be processed and treated with supercritical butane to remove impurities, which will result in a recycled product with properties similar to virgin material.

The OSBL relief system consists mainly of multiple horizontal knockout pots that are sized to remove the majority of the solids and/or liquids from the discharge from ISBL relief devices before the remaining vapors are collected and sent on to an elevated flare.  The OSBL relief system also includes a thermal oxidizer (TOx) that will treat dilute VOC/air vent streams.

To perform this design, PROCESS was required to develop a method for representing a two and three-phase fluid system flashing down from supercritical conditions for which an adequate thermodynamic model was not available.

As part of this effort, PROCESS prepared the following deliverables for the OSBL relief system:

  • Design Basis – Summarized not only the information received from the engineering company, ultimate client, and ISBL designer upon which the design of the OSBL relief system was based, but also the key assumptions and design methods that Process applied to size the knockout pots, the associated piping, and the flare header networks.
  • Relief Device Specifications – Defined the sizes and sizing bases for the OSBL relief devices connected to the OSBL relief system.
  • Technical Evaluation Reports – Summarized the results of the sizing calculations for the knockout pots and for the piping leading to them from the ISBL relief devices. Also included the results of a Value Engineering Study that identified opportunities for combining several of the knockout pots according to function and location, and for reducing the overall flare load.
  • Flare Header and Thermal Oxidizer Header Sizing Reports – Included a summary of the required and rated flowrates from each of the ISBL and OSBL relief devices connected to the flare header networks and grouped them according to global relief scenarios. Summarized the results of the sizing calculations for the high-pressure and low-pressure flare header collection networks and for the thermal oxidizer header.
  • 30% P&IDs and Control Narrative – Depicted all the piping, instruments, and minor equipment items that interconnect the major equipment items and package equipment systems. Also included a description of the key process controls associated with these items and Instrument Performance Datasheets for some of the more specialized control loops included in the design.
  • Major Equipment List and Line List – Summarized all major equipment items and process lines included in the OSBL relief system design.
  • Major Equipment Datasheets – Defined the sizes and design requirements for the knockout pots, and the performance requirements for the other major equipment items and package equipment systems, including the elevated flare and thermal oxidizer.

Industry Type

  • Industrial Process EPC Projects
  • Plastics Recycling


Utilized Skills

  • Relief systems design
  • Process simulation

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