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.

Flowsheet Presentation

Different types of flowsheets exist for illustrating a process. These different types of flowsheets contain different degrees of detail and are usually drafted at different stages in the development of the process. This information also varies slightly from company and department to department as to what information is contained on which type of flowsheet.

Block Diagrams

Block diagrams are usually generated very early in the design process, and utilize labeled boxes to illustrate process equipment. These diagrams communicate the general idea behind a process in terms of what types of equipment will be present, and what order the process stream passes through the equipment, but it does not contain detailed information regarding equipment design or stream conditions. Usually, material balance information and flow rates of many streams are available, but some streams may be unspecified. For this reason, they are not useful as final engineering schematics, but are more useful as a tool for communicating during early stages in process development.

Process Flow Diagrams

Process Flow Diagrams (PFDs) contain more detail than block diagrams. They contain details of all process equipment present; while in a Block Diagram, separations may be represented by a box labeled "separator," in a PFD all equipment, including flash separators, absorption columns, and distillation columns, are represented and connected by the appropriate piping. Additionally, PFDs contain all pumps, compressors, and heat exchangers as well, while these less important pieces of equipment may be absent from a Block Diagram.

Furthermore, industry standard symbols are used to represent different types of equipment within a PFD. Examples of some symbols used in PFDs to denote certain types of equipment can be found at the following website: http://www.edrawsoft.com/pfdsymbols.php. These symbols also come standard with the flowsheeting software tool Microsoft Visio.

While mass flow rate data may or may not be specified on Block Diagrams, PFDs contain detailed information about every stream including flow rate, composition, pressure, temperature, enthalpy, and any other relevant information. If this information is difficult to display on a PFD in an organized way, a stream table may be attached as a reference, containing this information. Typically, the operating basis, such as the operating hours per year, is also indicated on the PFD. It is normal practice not to display utilities on the PFD in order to avoid clutter.

A PFD is typically organized to reflect the proposed layout of the chemical plant, with different layouts already being experimented with at this early phase of design.

Piping and Instrumentation Diagrams

The most detailed level of flowsheeting is a Piping and Instrumentation Diagram (P&ID). A P&ID will contain all of the detail on a PFD, but will another level of detail such as pipe diameters and construction, valves, actuators, measuring equipment, and all equipment related to process controls. P&IDs will also contain relevant utility information.

The Anatomy of a Chemical Manufacturing Process

Chemical manufacturing process Components of a chemical process can be divided by these categories: raw material storage, feed preparation, reaction, product separation, purification, and product storage. Unless made on site or provided by neighboring companies, raw material or supply of reactant has to be bought and stored. The feeds have to be prepared and let the reaction run. The product has to be separated and maybe sometimes recycled. The product is purified as a product and is stored for its final usage or transportation. Continuous and batch processes Processes are usually continuous or batch. Batch processes are commonly used for food products, pharmaceutical products, personal care product, and specialty chemicals. Continuous processes are economical for large scale production. Batch allows production of multiple different products based on the reactor time; quality control is much easier. It is also easier to clean and maintain a sterility. Downside of the batch is that the scale of the production is limited and scale up is hard. Recycle and heat recovery is much harder, producing more waste byproduct and energy loss. Conversion and yield Conversion measures a fraction of the reagent that has reacted. Yield is a measure of a plant performance. Yield is usually defined by moles of product formed over moles of reactant times multiplied by a stoichiometric factor. One thing to note is that the conversion is related to reactants and yield is related to the products. Selectivity measures efficiency of the reactor in converting reagent to the desired product. It is calculated by moles of product produced over moles of product that could have been formed if all the reactant had converted completely. Recycles and Purges Recycle refers to processes in which a flow stream is returned to an earlier stage. This is common process when valuable reagent is not fully reacted; unreacted material is separated and added back. Recycle stream complicates the mass balance of the processes and necessitates a purge stream, which prevents a buildup of unwanted material in 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.