Flowsheet

FLOWSHEETS
A plant design consists of words, numbers, and pictures. An engineer thinks in terms of sketches and drawings that are his or her ‘‘pictures.’’ To solve a material balance problem, the engineer will start with a block to represent equipment, or a process step and then will show the entering and leaving streams with their amounts and properties. When asked to describe a process, anengineer will begin to sketch equipment, show how it is interconnected, and show the process flows and operating conditions.

Such sketches develop into flow sheets, which are more elaborate diagrammatic representations of the equipment, the sequence of operations, and the expected performance of a proposed plant or the actual performance of an already operating one. For clarity and to meet theneeds of the various persons engaged in design, cost estimating, purchasing, fabrication, operation, maintenance, and management, several different kinds of flowsheets are necessary.

BLOCK FLOWSHEETS

At an early stage or to provide an overview of a complex process or plant, a drawing is made with rectangular blocks to represent individual processes or groups of operations, together withquantities and other pertinent properties of key streams between the blocks and into and from the process as a whole. Such block flowsheets are made at the beginning of a process design for orientation purposes, for discussions or later as a summary of the material balance of the process. For example, the coal carbonization process of Figure 2.1 starts with 100,000 lb/hr of coal and process air, involvessix main process units, and makes the indicated quantities of ten different products. When it is of particular interest, amounts of utilities also may be shown; in this example the use of steam is indicated at one point. The block diagram of Figure 2.2 was prepared in connection with a study of the modification of an existing petroleum refinery. The three feed stocks are separated into more than20 products.

PROCESS FLOWSHEETS

Process flowsheets embody the material and energy balances and include the sizes of major equipment of the plant. They include all vessels, such as reactors, separators, and drums; special processing equipment; heat exchangers; pumps; and so on. Numerical data include flow quantities, compositions, pressures, and temperatures. Major instrumentation essentialfor process control and the complete understanding of the flowsheet without reference to other information is required, particularly in the early stages of a design, since the process flowsheet is drawn first and is the only diagram available that represents the process. As the design develops and a mechanical flowsheet is prepared, instrumentation may be removed to minimize clutter. A checklist ofinformation usually included on a process flowsheet is found in Table 2.1.

Working flowsheets are necessarily elaborate and difficult to represent on the page of a book. Figure 2.3 originally was 30 in. wide. In this process, ammonia is made from available hydrogen supplemented by hydrogen from the air oxidation of natural gas in a two-stage reactor F-3 and V-5. A large part of the plant isdevoted to the purification of the feed gases—namely, the removal of carbon dioxide and unconverted methane before they enter the converter CV-1. Both commercial and refrigeration grade ammonia are made in this plant. Compositions of 13 key streams are summarized in the tabulation. Characteristics of streams, such as temperature, pressure, enthalpy, volumetric flow rates, and so on, sometimes areconveniently included in the tabulation, as in Figure 2.3. In the interest of clarity, it may be preferable to have a separate sheet if the material balance and related stream information is voluminous. | |

A process flowsheet of the dealkylation of toluene to benzene is in Figure 2.4; the material and enthalpy flows as well as temperature and pressures are tabulated conveniently, and basic...