Page 44 - Chemical Process Equipment - Selection and Design

2

Flowsheets

plant design is made up of words, numbers, and elaborate diagrammatic representations of the equipment, the
pictures. An engineer thinks naturally in terms of the sequence of operations, and the expected performance of a
proposed plant or the actual performance of an already
sketches and drawings which are his "pictures. "
Thus, to solvi? a material balance problem, he will operating one. For clarityand to meet the needs of the
start with a block to represent the equipment and then will various persons engaged in design, cost estimating,
show entering and leaving streams with their amounts and purchasing, fabrication, operation, maintenance, and
properties. Or ask him to describe a process and he will begin management, several different kinds of flowsheets are
to sketch the equipment, show how it is interconnected, and necessary. Four of the main kinds will be described and
what the flows and operating conditions are. illustrated.
Such sketches develop into flow sheets, which are more

Characteristics of the streams such as temperature, pressure,
enthalpy, volumetric flow rates, etc., sometimes are conveniently
At an early stage or to provide an overview of a complex process or included in the tabulation. In the interest of clarity, however, in
plant, a drawing is made with rectangular blocks to represent some instances it may be preferable to have a separate sheet for a
individual processes or groups of operations, together with voluminous material balance and related stream information.
qoantities and other pertinent properties of key streams between A process flowsheet of the dealkylation of toluene to benzene
the blocks and into and from the process as a whole. Such block is in Figure 2.4; the material and enthalpy flows and temperature
flowsheets are made at the beginning of a process design for and pressures are tabulated conveniently, and basic instrumentation
orientation purposes or later as a summary of the material balance is represented.
of the process. For example, the coal carbonization process of
Figure 2.1 starts ,with 100,00OIb/hr of coal and some process air, 2.3. MECHANICAL (P&l) FLOWSHEETS
invailves six main process units, and makes the indicated quantities Mechanical flowsheets also are called piping and instrument (P&I)
of ten different products. When it is of particular interest, amounts diagrams to emphasize two of their major characteristics. They do
of utilities also may be shown; in this example the use of steam is not show operating conditions or compositions or flow quantities,
indicated at one point. The block diagram of Figure 2.2 was but they do show ail major as well as minor equipment more
prepared in connecticin with a study of the modification of an realistically than on the process flowsheet. Included are sizes and
existing petroleum refinery. The three feed stocks are separated specification classes of all pipe lines, all valves, and all instruments.
into more than 20 products. Another representative petroleum
refinery block diagram, in Figure 13.20, identifies the various In fact, every mechanical aspect of the plant regarding the process
equipment and their interconnections is represented except €or
streams but not their amounts or conditions.
supporting structures and foundations. The equipment is shown in
greater detail than on the PFS, notably with regard to external
piping connections, internal details, and resemblance to the actual
appearance.
Process flowsheets embody the material and energy balances
The mechanical flowsheet of the reaction section of a toluene
between and the sizing of the major equipment of the plant. They dealkylation unit in Figure 2.5 shows all instrumentation, including
include all vessels such as reactors, separators, and drums; special
processing equipment, heat exchangers, pumps, and so on. indicators and transmitters. The clutter on the diagram is minimized
Numerical data include flow quantities, compositions, pressures, by tabulating the design and opePating conditions of the major
temperatures, and so on. Inclusion of major instrumentation that is equipment below the diagram.
essential to process control and to complete understanding of the The P&I diagram of Figure 2.6 represents a gas treating plant
flowsheet without reference to other information is required that consists of an amine absorber and a regenerator and their
particularly during the early stages of a job, since the process immediate auxiliaries. Internals of the towers are shown with exact
flowsheet is drawn first and is for some time the only diagram locations of inlet and outlet connections. The amount of
representing the process. As the design develops and a mechanical instrumentation for such a comparatively simple process may be
Boarsheet gets underway, instrumentation may be taken off the surprising. On a completely finished diagram, every line will carry a
process diagram to reduce the clutter. A checklist of the code designation identifying the size, the kind of fluid handled, the
information that usually is included on a process flowsheet is given pressure rating, and material specification. Complete information
in Table 2.1. about each line-its length, size, elevation, pressure drop, fittings,
Working flowsheets are necessarily elaborate and difficult to etc.-is recorded in a separate line summary. On Figure 2.5, which
represent on the page of a book. Figure 2.3 originally was 30in. is of an early stage of construction, only the sizes of the lines are
wide. In this process, ammonia is made from available hydrogen shown. Although instrumentation symbols are fairly well standard-
supplemented by hydrogen from the air oxidation of natural gas in a ized, they are often tabulated on the P&P diagram as in this
two-stage reactor F-3 and V-5. A large part of the plant is devoted example.
to purification of the feed gases of carbon dioxide and unconverted 2.4. UTILITY FLOWSHEETS
methane before they enter the converter CV-1. Both commercial
and refrigeration grade ammonia are made in this plant. Com- These are P&I diagrams for individual utilities such as steam,
positions of 13 key streams are summarized in the tabulation. steam condensate, cooling water, heat transfer media in general,