Block Flow Diagram

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Title: Block flow diagram

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: January 15, 2013 /Date Revised: January 18, 2013



Block Flow Diagrams, BFDs, are process flow models used to simplify and understand the structure of a system. BFDs are the simplest form of the flow diagrams used in industry. Blocks in a BFD can represent anything from a single piece of equipment to an entire plant. For a complex process, block flow diagrams can be used to break up a complicated system into more reasonable principle stages/sectors.



BFD's are a useful tool for reports, textbooks and presentations when a detailed process flow diagram is too cumbersome. These models allow for the reader to get a complete picture of what the plant does and how all the processes interact. These can be understood by people with little experience reading or creating flow diagrams. To remain uncluttered, the drawback of these diagrams is that they provide limited information about the the process units or the system in general.


BFDs come in many forms and styles. They can be extremely simple or very detailed in their explanation of a process.

I/O Diagrams

The simplest form of BFD, the I/O (input/output) Diagram provides the material streams entering and exiting the process. The diagram is modeled below using arrows entering and exiting a process box to represent the inputs and outputs respectively.

Figure1. I/O Diagram

Block Flow Plant Diagram

This model of flow diagram is used to explain the general material flows throughout an entire plant. They will be generalized to certain plant sectors or stages. These document would help orient workers to the products and important operation zones of a chemical facility.

Block Flow Process Diagram

This model will concentrate on a particular sector/area of a chemical plant. This would be a separate flow diagram that details what would have been present inside of one of the blocks in the plant diagram. These diagrams may be more or less complicated than the plant diagram but will focus on only a small sub-section of the overall process.


There are several conventions regarding the construction of BFDs that are used in the engineering community. These conventions are recommendations for laying out a block flow process diagram:

1. Operations shown by blocks

2. Major flow lines shown with arrows giving direction of flow

3. Flow goes from left to right whenever possible

4. Light stream (gases) towards the top of the block with heavy streams (liquids or solids) towards the bottom

5. Critical information unique to process supplied

6. Lines are straight turning at 90 degree angles.

7. If lines cross, the horizontal line is continuous and the vertical line is broken

8. Simplified material balance should be provided

Example 1: Production of Benzene

Toluene and hydrogen are used as feed stocks for the production of benzene. The toluene and hydrogen are sent into a reactor and then the effluent is sent to a gas separator where the noncondensable gases are discharged from the system. The bottoms of the separator provides a liquid feed to a still where the lighter benzene gases are collected through the distillate and the bottom toluene draw is recycled back into the reactor. The BFD provided shows the reaction, the stream names, and the mass flow of the inlets and outlets. There are many components of this system (heat exchangers and pumps etc.) that are not represented because there are not vital for understanding the process.

Block flow process diagram for the production of benzene

Example 3: Caprolactam From Toluene

Block Flow Diagram Example 3: Caprolactam From Toluene


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