Process alternatives and flowsheeting

Authors: Alex Chandel, Eric Jiang, Minwook Kim, Todor Kukushliev, William Lassman (ChE 352 in Winter 2014)

Steward: David Chen, Fengqi You

Date Presented: 2/9/2014

Introduction

Any given chemical process is composed of a series of chemical operations, performed by one or several related pieces of equipment working to accomplish a given task. However, as more of these elements within a process accumulate, it becomes difficult to track the progress and conditions of a process or utility stream throughout the process. For this reason, process flowsheets are developed to better visualize and summarize information about a process. Having a process flowsheet also allows design engineers to visualize the many alternatives, and how those alternatives affect the rest of the process.

Synthesis of Novel Flowsheets

Design of completely new flowsheets are usually avoided due to the financial and safety risk they carry. Process synthesis attempt to minimize risk and maximize potential process such that the financial reward is greater than the risk. Process synthesis has been aided by process simulation programs as well as experience in similiar processes.

Overall Procedure for Flowsheet Synthesis

1. Generate Process.
2. Initial Economics.
3. Set Yield Targets.
4. Preliminary Economic Assessment.
5. Refine Process Structure.
6. PFD Review.
7. Preliminary Process Hazard Analysis (PHA).
8. Revise Economic Assessment.
9. Optimization. While it might not be possible to have sufficient data to properly optimize the system at the time the design team is performing processes, the design team should optimize the design based on the data available. While it might be necessary to perform optimization on different sections of the process due to complexity, the overall optimization with advantages and disadvantages should be considered.

Set Targets in Process Synthesis

Applicable heuristics can be used to check answers or generate preliminary values if insufficient data are available.

PFD Review

Review of the PFD is an important part of the design process whether the flow sheet is newly generated or altered from existing designs. The process of PFD is usually done in committee consisting of the design team and relevant unbiased consultants.

PFD Review Procedures

1. PFD Printout. Display the PFD on a wall such that it is visible to all members of the review committee. Allow enough space between equipment for addition and notes.
2. Walkthrough. Introduce the PFD, describing all streams and process operations.
3. Questions. The review group should challenge the design team, paying special attention to potential missing equipment or redundant equipment. Safety and adequate control systems should also be questioned.
4. Follow-up. If there are unanswered questions which need to be addressed, the design team should perform the necessary analysis. Corrections made during the review should be noted and added to the PFD. Notes describing issues, concerns, and future steps should be distributed after the meeting adjourns.
5. More PFD Reviews. Depending on the number of changes performed during the review process, further review process may be necessary.

References

1. Gavin Towler and Ray Sinnott, Chapter 2 – Process Flowsheet Development, Chemical Engineering Design, 2nd Ed., edited by Gavin Towler and Ray Sinnott, Butterworth-Heinemann, Boston, 2013, Pages 33–102, ISBN 9780080966595, http://www.sciencedirect.com/science/article/pii/B9780080966595000079.