A practical way of communicating information about a process is through the use of diagrams. There are four basic documents which belong to any separation process:
The BFD is a black-box approach. It takes a look from the outside only, looking what goes into the system and what leaves. Nobody cares how the system is designed, only what the result is. A BFD does not include any details of equipment.
There are several conventions regarding the drawing of BFDs:
What goes into the system must either come out of the system, or is used up or generated within the system, or remains in the system and accumulates. Best is to apply the mass conservation equations:
Accumulation = In - Out + Generation - Consumption
The conservation equation is very general.
In steady state there is no accumulation in the system so the overall mass and energy input matches its output. Variables are constant with respect to time. Dynamic models take into account the mass and energy rate of accumulation within the system, which allows one to determine the how long it would take to reach a stable condition starting from a specified initial state.
The conservation equation is easiest when set up for steady state conditions. The assumption of steady state avoids us from having to solve differential equations. The mass balance can be handled with algebra only. Steady state modeling is perfect for process conceptualization, design and evaluation.
However, for startup and over time, the parameters of steady state are changing with time and depend heavily on certain conditions within the system (temperature, fouling a.o.). It is therefoe not typical that a process runs linear, that material enters the system, goes through each process unit once, and the final product leaves smoothly. There are types of special streams that deserve special attention in a BFD:
A recycle stream is one where a portion of the outlet of a process unit is combined with fresh feed and sent into the same unit again.
A bypass stream is one where a portion of the inlet to a process unit is split from the feed and instead of entering the process is combined with the outlet from that process.
A purge stream is one where a portion of a recycle stream is removed from the system in order to avoid accumulation of undesired material in a recycled system.
A process flow diagram (PFD) is used to indicate the general flow of plant processes and equipment. The PFD displays the relationship between major equipment of a plant facility. The PFD does not show minor details such as piping details.
Process flow diagrams include the following:
Process flow diagrams generally do not include:
The PID is used to present “mechanical-type” details to piping and mechanical designers, electrical engineers, instrument engineers, and other engineers not directly in need of process details.
Piping and instrument diagrams are graphical summary of the actual hardware elements in a process plant and their interrelationships of connections.
The PIDs do not include piping lengths and bends.
The mass balance (material balance), is an application of conservation of material to the analysis of a process.
The general form quoted for a mass balance is "The mass that enters a system must either leave the system or accumulate within the system." The mass balance for a system without a chemical reaction is:
Input = Output + Accumulation
For a balance to be formed, the boundaries of the system must be clearly defined.
Mass balances are simplified with the assumption of steady state (in which the accumulation term is zero).