The preparation and drawing of the first set of process flow-sheets (pro- duced at this stage as revision 0) represent the translation of the process block diagram and the chemical mechanisms concepts used by the R&D group into the usual chemical engineering methodology and practice of the chemical plant. This bridge between these two disciplines is an impor- tant creative task, requiring a fundamental scientific understanding of the process, as well as extensive experience in the design of chemical plants and their operating practice.
These process flow-sheets will represent a reference basis for every pro- fessional related to the project, and they should be clear also to engineers and technicians who do not have an extensive scientific background (or may have forgotten it since college). In these process flow-sheets, the different functional sections are presented in separate drawing sheets, each with its defined conditions. These drawings also include the systematic tag-number- ing of all pieces of equipment and all the main streams. These tag-numbers will then be used for reference in all future project documents. Some corporations have their own practice, but practically any one of the many numbering systems can be used, as long as the system chosen is clear and consistent.
These process flow-sheets include only such details on the piping, valves, and instruments that are essential for an understanding of the description of the process operation; there is no need to include at this stage all the valves, bypasses, drains, sampling points, instruments, and automatic loops that will be needed eventually for the convenience of the plant operations. Most of these items are not essential at this stage, and they may be introduced later into the P&ID (piping and instrumentation) drawings, at the detailed engineering stage. With progress of the development project, it can often be decided (or at least considered for alternative study) that certain pieces of equipment, streams, or control instruments should be added or removed, or that the routing of certain streams should be changed. Therefore, each of these pro- cess flow-sheet drawings will probably be revised several times at later stages. Three or four revisions are quite common in most projects.
The following five examples of small portions of (real) process flow- sheets illustrate different typical possibilities:
• Figure 7.1 shows a liquid–liquid extraction/back extraction scheme op- erated at two different temperatures. Each of the two stages consists of a liquid–liquid mixer, a phase separator (a settler or a centrifuge; it may not have been finalized at this stage), two collecting vessels for the separated phases, and two pumps. The solvent cycle (streams [10] and [11]) passes through three heat exchangers, one for heating with steam, one for cooling with cooling water, and an interchanger between them for energy economy. The feed (stream 1) is first heated then extracted at the higher temperature, and its residual stream exits as (2). Water (15) and possibly some reagents (16) are added to the hot extract (11), and result in the back-extraction aqueous product (3). • Figure 7.2 illustrates a single-stage, liquid–liquid contact pilot installa- tion, presented for actual use in an experimental program. Thus, it includes much more relevant details that need to be referred to in the operational procedure and measurements, such as sampling, tem- perature indication, liquid interface location, venting, etc.
• Figure 7.3 shows a process flow-sheet for a distillation section under vacuum, in which small quantities of a residual volatile organic sol- vent are eliminated from two aqueous streams (the product solution and the residual solution) in two stripping packed columns. The
Figure 7.1 Extraction/back extraction scheme at two different temperatures.
Trim heater H-02 Interchanger Trim cooler H-04 H-03
steam cooling water
T-01 T-02 T-03 T-04 steam H-01 TK-01 P-01 TK-03 P-03 TK-05 P-05 To DC-01 to DC-02 P-04 P-02 TK- 02 TK- 04 1 2 3 10 15 16 11
Figure 7.2 Pilot single-stage liq- uid–liquid continuous contact.
Figure 7.3 Recovery sec- tion for vacuum distilla- tion of residual solvent.
mixer VSD liquid- liquid settler overflow overflow closed vent solvent sump solvent pump aqueous sump
aqueous pump positive VSD aqueous out solvent out aqueous internal recycle aqueous in solvent in LI LI LI TI TI sample sample sample sample LI raffinate solution product solution live steam S S S LI AAA TI TI LI TI water to waste live steam to raffinate tank to product tank light vapors rectifier reflux light vapors condenser to solvent tank S vacuum TI TI TI LI CW
aqueous solutions are fed from the top of the columns and live steam is introduced at the bottom. The vapors from both columns, contain- ing solvent and water, are mixed and sent to the middle of a packed rectification column, which separates the solvent (top) from the water (bottom). The solvent vapors are condensed and part of the liquid is refluxed. Live steam is fed below the packing layer and water is removed from the bottom. The condenser is connected to a vacuum system through vacuum traps (not shown here). Four pumps are needed to remove the four exit streams and transfer them to their respective tanks. Since the operation of such pumps that are sucking liquids from a system under vacuum may be quite problematic, an experienced designer will do everything possible to replace them with “barometric legs” connected directly to the tanks. Therefore, such systems under vacuum are often found in the higher tower- structures in the plant.
• Figure 7.4 illustrates a process flow-sheet for the energy-efficient sep- aration, on a relatively large scale, of an extract stream containing a major proportion of a “light” water-soluble organic solvent, together with water and nonvolatile, water-soluble impurities. The extract (stream 1) is first clarified by passing through a pressure filter. In the append- ed textual description, it is explained that this operation is very important and that only one of two pressure filters in parallel is shown. One of these filters is in operation while the second is being washed and refurbished with filter-aid, but this standard feature does not have to appear on this flow-sheet at this stage. The stream is then fed first to a vapor-recompression evaporator, in which a great part of the volatile organic component can be evaporated in such condi- tions in which the vapor pressure of the water is still very low. The condensed solvent (10) flows to the recycled solvent tank and the remaining solution (2) passes into a double-effect co-current evapo- rator, heated with indirect steam in the first effect. Each effect is a forced-circulation evaporator, consisting of a vertical heat exchanger, a vapor–liquid separator, and a circulation pump. The organic vapors from the first effect (12) at the higher temperature are condensed in the heat exchanger of the second effect, and the organic vapors from the second effect (13), at the lower temperature, are condensed with cooling water. The remaining aqueous solution (4) still contains some dissolved organic and, thus, it is reheated and sent to the middle of a packed distillation column, with a steam-heated jacket at the bot- tom and a condenser with reflux at the top. All the solvent recovered from these four successive operations are mixed in the recycled sol- vent tank. Such a complicated flow-sheet is justified, or in fact dic- tated, by the allowable cost of the energy consumption in this process. • Figure 7.5 illustrates a process flow-sheet from a different field, the
preparation of dry granules of zircon and CaCl2, which constitute the starting section for a process described earlier (see Chapter 5,
Figure 5.2). It starts from the periodical reception of the merchant zircon sand concentrate in “big-bags” and its transfer into a silo. From there, it is fed at a controlled rate (1) to a wet ball-mill, where it is mixed with a carefully controlled stream (2) of a concentrated solu- tion of CaCl2. In the wet grinding mill, the zircon sand is reduced to very small particles in a concentrated slurry, which is passed through a wet magnetic separator into a holding and blending mixed tank. From there, this concentrated slurry is fed to a fluid-bed agglomer- ator/dryer, in which hot combustion gases are introduced from the bottom, below the “grid,” and part of the gases from the top are recirculated to maintain the FB and the partial pressure of the water vapor. In this case, complete evaporation of the water is not wanted; on the contrary, a certain concentration of water must be left in the granules, to prevent the beginning of thermal decomposition of the CaCl2 and the liberation of gaseous HCl in the granulator. A partial
Figure 7.4 Complex separation of a volatile solvent from an aqueous solution.
Aqueous residual solution solvent stripping column
steam
pressure filter extract
vapor recompression evaporator condenser CW reflux recycled solvent tank 1 2 3 4 5 10 12 13 12 10 15 17 4 3 steam CW steam 22 2 1
double-effect cocurrent evaporator
elimination of the water is sufficient to produce hard granules that can be transferred to the next stage of the process.