With expanded engineering involvement, the design team sets out to create a : detailed process flow diagram and to improve the task integration begun in preliminary 2 process synthesis..
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Phase 3 begins (see Section 1.2) Consequently, emphasis is normally placed on the rapid de- velopment of a promising design, and less on design optimization Stated differently, for high-priced pharmaceuticals, it is far more important to be first-to-market rather than achieve relatively small savings in the capital investment or operating expenses for the plant through design optimization
Finally, before leaving this section on preliminary process synthesis, the limitations of the heuristic approaches should not be overlooked Many algorithmic methods are very effective for the synthesis of alternative flowsheets, their analysis, and optimization These methods are usually used by design teams in parallel with their work on the development of the base- case design, which is the subject of the next section The algorithmic methods are easily im- plemented and are illustrated with many examples in Part Two of this text (Chapters 6-12)
Flow Diagrams
At some point in the synthesis of alternative flowsheets, it becomes important to select one or two of the most promising alternatives for further development into the so-called base-case design(s) To accomplish this, the design team is usually expanded, mostly with chemical en- gineers, or assisted by more specialized engineers, as the engineering workload is increased © significantly With expanded engineering involvement, the design team sets out to create a : detailed process flow diagram and to improve the task integration begun in preliminary 2 process synthesis Then, in preparation for the detailed design work to follow, a detailed © database is created, a pilot plant is often constructed to test the reaction steps and the more | important, less-understood separation operations, and a simulation model is commonly pre- : pared As the design team learns more about the process, improvements are made, especially changes in the flow diagram to eliminate processing problems that had not been envisioned
In so doing, several of the alternative flowsheets generated in preliminary process synthesis gain more careful consideration, as well as the alternatives generated by the algorithmic methods, in detailed process synthesis [which often continues as the base-case design(s) is being developed]
As the engineering work on the base-case design proceeds, a sequence of flow diagrams is used to provide a crucial vehicle for sharing information The three main types are intro- : duced in this subsection, beginning with the simplest block flow diagram (BFD), proceeding
to the process flow diagram (PFD), and concluding with the piping and instrumentation dia- gram (P&ID) These are illustrated for the vinyl-chloride process synthesized in the previous section (see Figure 3.8)—the so-called base-case design
Block Flow Diagram (BFD) The block flow diagram represents the main processing sections in terms of functional : blocks As an example, Figure 3.18 shows a block diagram for the vinyl-chloride process, :
in which the three main sections in the process, namely, ethylene chlorination, pyrolysis, ; and separation are shown, together with the principal flow topology Note that the dia- gram also indicates the overall material balances and the conditions at each stage, where appropriate This level of detail is helpful to summarize the principal processing sections and is appropriate in the early design stages, where alternative processes are usually ' under consideration
Trang 23.5 Development of the Base-Case Design 97
C,H, Cl, Recycle
105,500 lb/hr
Figure 3.18 Block flow diagram for the vinyl-chloride process
Process Flow Diagram (PFD)
Process flow diagrams provide a more detailed view of the process These diagrams display all of the major processing units in the process (including heat exchangers, pumps, and com-
pressors), provide stream information, and include the main control loops that enable the
process to be regulated under normal operating conditions Often, preliminary PFDs are con- structed using the process simulators Subsequently, more detailed PFDs are prepared using software such as AUTOCAD and VISIO, the latter having been used to prepare Figure 3.19 for the vinyl-chloride process The conventions typically used when preparing PDFs are 1- lustrated using this figure and are described next
Processing Units Icons that represent the units are linked by arcs (lines) that represent the process streams The drawing conventions for the unit icons are taken from accepted standards, for example, the ASME (American Society for Mechanical Engineers) stan- dards (ASME, 1961) A partial list of typical icons is presented in Figure 3.20 Note that each unit is labeled according to the convention: U-X YY, where U is a single letter identi- fying the unit type (V for vessel, E for exchanger, R for reactor, T for tower, P for pump, C for compressor, etc.), X is a single digit, identifying the process area where the unit is in- stalled, and YY is a two-digit number identifying the unit itself Thus, for example, E-100
is the identification code for the heat exchanger that condenses the overhead vapors from the chlorination reactor Its identification code indicates that it is the 00 item installed in plant area 1
Stream Information Directed arcs that represent the streams, with flow direction from left to right wherever possible, are numbered for reference By convention, when stream-
lines cross, the horizontal line is shown as a continuous arc, with the vertical line broken
Each stream is labeled on the PFD by a numbered diamond Furthermore, the feed and product streams are identified by name Thus, streams 1 and 2 in Figure 3.19 are labeled as the ethylene and chlorine feed streams, while streams 11 and 14 are labeled as the hydro- gen chloride and vinyl-chloride product streams Mass flow rates, pressures, and tempera- tures may appear on the PFD directly, but more often are placed in the stream table instead, for clarity The latter has a column for each stream and can appear at the bottom of
the PFD or as a separate table Here, because of formatting limitations in this text, the
stream table for the vinyl-chloride process is presented separately in Table 3.6 At least the following entries are presented for each stream: label, temperature, pressure, vapor frac-
tion, total and component molar flow rates, and total mass flow rate In addition, stream
properties such as the enthalpy, density, heat capacity, viscosity, and entropy, may be dis- played Stream tables are often completed using a process simulator In Table 3.6, the con- version in the direct chlorination reactor is assumed to be 100%, while that in the pyrolysis reactor is only 60% Furthermore, both towers are assumed to carry out perfect separa- tions, with the overhead and bottoms temperatures computed based on dew- and bubble- point temperatures, respectively
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98
Trang 43.5 Development of the Base-Case Design 99
Compressors
Heat Exchangers
[> Process Input
or Output
I>k{ Manual Valve
Separation Columns 3 Control Valve Figure 3.20 Icons in process flow
diagrams
Utilities As shown in Figure 3.19, various utility streams are utilized for heating or cool- ing the process streams For example, E-100, the overhead condenser for the direct chlorina- tion reactor, which operates at 90°C, is cooled using cooling water (cw) The other cooling utilities are refrigerated brine (rb) and propane refrigerant (pr), each selected according to the temperature level of the required utility Heating utilities are fuel gas (fg), high-pressure steam (hps), and medium-pressure steam (mps) A list of heating and cooling utilities, with temperature ranges, and the abbreviations commonly used on PFDs is presented in Table 3.7 (see also Table 17.1 and the subsection on utilities in Section 17.2)
Equipment Summary Table This provides information for each equipment item in the PFD, with the kind of information typically provided for each type of unit shown in Table 3.8 Note that the materials of construction (MOC), and operating temperature and pressure, are required for all units Also note that suggestions for the materials of construction are pro- vided in Appendix III
In summary, the PFD is the most definitive process design document, encapsulating much
of the commonly referred to design information As such, it is used and updated throughout much of process design However, it lacks many details required to begin the construction engineering work for the plant Many of these details are transmitted in the Piping and In- strumentation Diagram
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Table 3.6 Stream Summary Table for the Vinyl-Chloride Process in Figure 3.19
8
Component molar flow (lbmol/hr):
4.8
Component molar flow (Ibmol/hr):
Table 3.7 Heating and Cooling Utilities—Identifiers and Temperature Ranges
Hot Utilities—In increasing cost per BTU:
Ips Low-pressure steam, 15 to 30 psig 250 to 275°F
Cold Utilities—In increasing cost per BTU:
approach to process 40°F
tower), return at 110°F
tower), return at 115 to 125°F
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3.5 Development of the Base-Case Design 101
Table 3.8 Equipment Summary Specifications
Vessel Height, diameter, orientation, pressure, temperature, materials of construction
(MOC)
height and type of packing, MOC
MOC
MOC
operating temperature, pressure, pressure drop, and MOC
area, MOC
Piping and Instrumentation Diagram (P&ID)
This is the design document transmitted by the process design engineers to the engineers re- sponsible for plant construction It is also used to support the startup, operation of the process, and operator training Consequently, it contains items that do not appear in the PFD, such as the location and type of all measurement and control instruments, the positioning of all valves,
including those used for isolation and control, and the size, schedule, and materials of con-
struction of piping As a result, a number of interconnected P&IDs are prepared for a process that is represented on a single PFD For more details on the preparation of P&IDs, the reader
is referred to the books by Sandler and Luckiewicz (1993) and Ulrich (1984)
Calculations Supporting Flow Diagrams
As indicated when discussing the stream table (Table 3.6), and emphasized when synthesiz- ing the vinyl-chloride process in the previous section, the material balances for the process streams are nearly complete after preliminary process synthesis These are conducted by means of spreadsheets and by process simulators, as discussed in Chapter 4 At this stage, the design team checks the assumptions It also completes the material and energy balances as- sociated with heat addition and removal, without attempting to carry out heat and power inte- gration As indicated in the section on process integration that follows, the design team carries out heat and power integration just prior to the detailed design stage
It should also be noted that, during the synthesis of the vinyl-chloride process, no attempt was made to complete calculations to determine the number of stages and reflux ratios for the distillation towers, and furthermore, perfect splits may be assumed Hence, the condenser
and reboiler heat duties are not yet known The vapor stream, S1, is assumed to be saturated,
pure dichloroethane, which releases its heat of vaporization, 143.1 Btu/lb, to cooling water, which is heated from 30° to 50°C Both the direct chlorination reactor and the pyrolysis fur- nace are assumed to operate adiabatically, and natural gas is assumed to have a lower heating value of 23,860 Btu/lb (heat of combustion at 25°C) The liquid effluent from the quench is assumed to have a composition in vapor-liquid equilibrium at 150°C and 26 atm The stream
is cooled to 50°C with cooling water to release the heat necessary to cool the pyrolysis prod- ucts from 500 to 170°C (4.66 x 107 Btu/hr) No attempt is made to calculate the amount of propane refrigerant necessary to remove the heat to cool the pyrolysis effuent to its bubble point at 6°C (5.20 X 10’ Btu/hr); this calculation is completed during process integration, when the heat and power integration is completed