Block Flow Diagram
Title: Block flow diagram
Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas
Date Presented: January 15, 2014 /Date Revised: January 18, 2014
A block flow diagram (BFD) is a drawing of a chemical processes used to simplify and understand the basic structure of a system. A BFD is 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.
Creating a BFD is often one of the first steps in developing a chemical process. Different alternatives can be easily and inexpensively compared at an early stage using simple BFDs. Once alternatives have been chosen, the BFD serves as a starting point for a complete process flow diagram (PFD).
A BFD is 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 an overall picture of what the plant does and how all the processes interact. These can be understood by people with little experience with reading or creating flow diagrams.
BFDs come in many forms and styles. They can be extremely simple or very detailed in their explanation of a process.
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.
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 documents 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 and format of BFDs that are commonly used in the engineering community. Some of the recommended conventions are:
- Operations/equipment are represented with blocks
- Material flows are represented with straight lines with arrows giving the direction of flow
- Lines are horizontal and/or vertical, with turns at 90 degree angles
- Flows go from left to right whenever possible
- If lines cross, the horizontal line is continuous and the vertical line is broken
- Light streams (gases) are typically closer to the top of the BFD than are heavy streams (liquids or solids)
- Critical information unique to the process (such as a chemical reaction) is supplied
- A 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 to a reactor, and 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 gas is collected as 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 they are not vital for an understanding of the main features of the process.
Example 2: Oxidation of Propene to Acrylic Acid
Propane is dehydrogenated to propene, which is oxidized to acrolein first and then further oxidized to acrylic acid. The products are separated in the end to give acrylic acid and various by-products. The by-products are further separated to yield a propane recycle stream. Each block in the BFD provided shows what each individual unit is doing along every step of the process. It also shows inlet and outlet streams, as well as byproducts and recycle streams. A BFD in this style is helpful so that all materials can be seen, every step of the process is outlined, and byproducts can be taken into consideration for waste removal/treatment.
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