Mechanical unit operation documents for Chemical engineering

Thanks

1. Preparing Flowsheets Lecture 5 Chapter Three

2. Introduction A flowsheet is the key document in process design & a diagrammatic model of a one process. It shows the arrangement of the equipment and unit operations selected for the process, the stream connections (i.e., inlet & outlet streams), stream flow rates and compositions, and the operating conditions. Flowsheet also shows every condition in each unit operation of a process in short and precise form. Flowsheet is used by professional design groups as the basis for their designs. These designs include piping, instrumentation, equipment design and plant layout. Flowsheet is also used by operating personnel for the preparation of operating manuals and operator training . Thus during plant startup and subsequent operation, flowsheets form a basis for comparison of operating performance with design.

3. The flowsheet is drawn up from material balances made over the complete process and each individual unit operation. Energy balances are also made to determine the energy flows and the utility requirements. Most flowsheet calculations are carried out using commercial process simulation programs. In which these process simulation programs contain models of most unit operations, thermodynamic models and physical property models.

4. Flowsheet Synthesis and Development Preparation of a flowsheet includes the steps of synthesis, development, evaluation, and selection of the most appropriate processing arrangement of a chemical process. The development of a flowsheet involves a branching nature resulting from both multiple possible types and arrangements of equipment and from the selection of process and equipment conditions. Selection of the flowsheet is one of the most important steps in the design of a chemical plant, because the most profitable, safe, and environmentally sound final design can be obtained only from the most optimal flowsheet. General Procedure There are six generalized procedures used for the flowsheet synthesis and development.

5. Process Information: Finding information about the product to be manufactured. Process information includes: Studying market conditions and the pricing of the product. Obtaining a significant property data from the literature (technical and patent literature) or through an experiment. For a new product, most probably there may be laboratory and perhaps pilot-plant data plus a market evaluation. Input/output Diagram: This step includes: Examining all potential chemical reaction paths/routes, and do a preliminary economic analysis of each path. Elimination of raw materials having higher value than that of the products, and those which appear infeasible for other issues.

6. Construction of an input/output (I/O) diagram showing all of the major material input and output streams with their stoichiometric balance. 3. Functions Diagram: Indicate all the major functions of the process, and the material flows in to and from these functions for each particular chemical reaction path. 4. Operations Diagram: In this step: Suitable reactors are chosen (batch, plug flow or continuous stirred tank reactor). Ranges of temperature, pressure, reactant conversion, and product selectivity are estimated. Purification and separation techniques are examined and selected. Finally, the technologies suited for accomplishing each of the operations/processes are selected. This is a difficult step, because it requires extensive information and understanding of the available technologies.

7. 5. Flowsheet: In this step several candidate qualitative flowsheets are constructed. The operations and steps of the process are shown with approximate mass and energy flows, where possible. Efficient utilization of energy is another important aspect of flowsheet development. 6. Base-Case Design and Optimization In this step, each of the constructed flowsheets are evaluated. To do so a detailed mass and energy balances are prepared for each flowsheet to provide a basis for the quantitative analysis of the flowsheet. A key design parameters of the process equipment such as reactor volumes and heat loads can also be calculated by using the mass and energy balances and the simulation software. Such design information provides a base-case design for each flowsheet.

8. Each base-case design is reviewed to determine whether it can meet the product specifications, environmental requirements, and health and safety needs. Here an economic evaluation is also conducted. With these reviews and evaluations, some flowsheets not met the process or economic requirements may be eliminated. However, before eliminated, the process should be determined whether it become within requirements by performing optimization studies i.e., by changing the process conditions and design parameters. All remaining candidate processes should be optimized, and the best alternative should be selected.

9. A single box represents the process; the chemical reactions are shown inside the box. Arrows into and out of the box show the raw material inputs and product outputs on the basis of 2 mols of vinyl chloride product. Figure 1. Input/Output diagram of vinyl chloride production E.g., Preparing a flowsheet for the production of vinyl chloride 1. I/O diagram

10. Figure 2. Functions diagram for vinyl chloride production 2 . Functions diagram

11. Figure 3. Operations diagram for vinyl chloride production 3. Operations diagram

12. Figure 4. Final flowsheet for vinyl chloride production. Stream quantities are in kg mol and heat quantities are in MJ, on a basis of 1 kg mol vinyl chloride produced. CW = cooling water 4. Final flowsheet Optimization& selection

13. Presentation of stream flow-rates Individual components flow-rate data, total stream flow-rate, and the percentage composition data can be shown on the flowsheet in different ways. There are two types of displaying modes: Setting the data (flow rate or percentage composition data) in blocks beside the process stream lines. It is the simplest and suitable presentation method for simple processes involving few equipment. The data is tabulated on the flow sheet. Figure 5 Symbols (i.e., number or letter) indicating the data are shown on the flow sheet. While the data are written alone e.g., in table. It is a better method for the presentation of data on flow-sheets because it reduce complexity and overlapping. Figure 6

14. Figure 5. Flowsheet of polymer production

15. Figure 6. Flowsheet of simplified nitric acid process

16. Information to be included on a flowsheet The amount of information shown on a flowsheet will depend on their purpose in the particular design. Essential Information Optional Information They are items must always be shown They are items added for the usefulness of the flowsheet but are not always included. Stream compositions: either flow rate of each individual component, kg/h or the stream composition as weight fraction. Physical property data such as: density, kg/ , and viscosity, mN.s / Total stream flow rate, kg/h. Stream name, and description of the nature of the stream, for example “ACETONE COLUMN BOTTOMS”. Nominal operating pressure (the required operating pressure). Molar percentages composition. Stream temperature, degrees Celsius preferred. Stream enthalpy, kJ/h. Essential Information Optional Information They are items must always be shown They are items added for the usefulness of the flowsheet but are not always included. Stream compositions: either flow rate of each individual component, kg/h or the stream composition as weight fraction. Total stream flow rate, kg/h. Stream name, and description of the nature of the stream, for example “ACETONE COLUMN BOTTOMS”. Nominal operating pressure (the required operating pressure). Molar percentages composition. Stream temperature, degrees Celsius preferred. Stream enthalpy, kJ/h. E.g.

17. Assignment Two Make a group having five members and prepare a flowsheet for the production of beer by using the six general flowsheet synthesis and development procedures. Submission date: on 23/12/22

18. Layout Layout is the sequence of the main equipment items shown symbolically on the flowsheet. Layout aims to show the flow of material from stage to stage, and to give a general impression of the layout of the actual process plant. The equipment should be drawn approximately to scale. Again, some license is allowed for the sake of clarity, but the main equipment items should be drawn roughly in the correct proportion/section while ancillary items can be drawn out of proportion. For a complex process, with many process units, several sheets may be needed, and the continuation of the process streams from one sheet to another must be clearly shown.

19. Overlapped lines are indicated by a double circular round the line number and the continuation sheet number written below it. All the process stream lines shown on the flowsheet should be numbered and the data for the stream is given. Precision of data The total stream and individual component flows do not normally be shown with high precision on the process flowsheet. One decimal place is usually justified as an accurate and sufficient for the flowsheet calculations. The flows should, however, balance to within the precision shown.

20. Imprecise small flows are shown as “TRACE”. If the composition of a trace component is specified as a process control agent i.e., for an effluent stream or product quality specification, it can be shown in parts per million, ppm. A trace quantity should not be shown as zero or the space in the arrangement left blank, unless the process designer is sure that it has no significance. For example, if the trace quantity opposites a particular component it is taken as insignificant. Trace quantities are important to determine the selection of the materials of construction. Further, a trace of an impurity is necessary to poison a catalyst.

21. Basis for the flow-sheet calculations a. Time basis No plant will operate continuously without shut-down. Planned shut-down periods will be necessary for maintenance, inspection/checkup, equipment cleaning, and for regeneration of catalysts and column packing. The frequency of shut-downs, and the consequent loss of production time, will depend on the nature of the process . Most chemical and petrochemical plant operates about 90-95% of the total hours in a year (8760hr). An average of 8000hr/yr. is used as basis production time. Unless the process is known to require longer shut-down periods.

22. b. Scaling factor Flow-sheet calculations is easy in one process step i.e., starting form the raw-material feeds, progressing stage by stage, where possible, to the final product flow. The required production rate will usually be specified in terms of the product rather than the raw-material feeds. So that, we take e.g., 100 kmol /h of the principal raw material as an arbitrary basis for the calculations. Then the required actual flows are calculated by multiplying each flow by a scaling factor determined from the actual production rate required .

23. Flowsheet calculations on individual units Reactors (i) Reactor yield and conversion specified. The reactor performance may be specified independently in the detailed design of the reactor. The optimum conditions or near optimum, and performance may be known from the operation of existing plant or from pilot plant studies. For processes that are well established, estimates of the reactor performance can often be obtained from the general and patent literature; for example, the production of nitric and sulfuric acids. If the yields and conversions are known, the stream flow rates and compositions can be calculated from a material balance.

24. (ii) Chemical equilibrium. For fast reactions, the reaction products can often be assumed to have reached equilibrium. The product compositions can then be calculated from the equilibrium data for the reaction at the chosen reactor temperature and pressure. 2 . Equilibrium stage In a separation or mixing unit, the estimated equipment performance is reasonable to consider the outlet streams as being in equilibrium; thus no significant inaccuracy is introduced by assuming that equilibrium is reached. The stream compositions can then be calculated from the phase equilibrium data of the components.

25. This calculation can often be made for single stages gas-liquid and liquid-liquid separators, such as quench towers, partial condensers and vessels. It is particularly useful if one component is essentially non-condensable and can be used as a tie substance. 3 . Fixed stream compositions If the composition (or flow-rate) of one stream is fixed by “internal” or “external” constraints, this may fix the composition and flows of other process streams. If sufficient design variables are fixed by external constraints, or by the designer, then the other stream flows round a unit will be uniquely determined.

26. For example, if the composition of one product stream from a distillation column is fixed by a product specification, or if an azeotrope is formed, then the other stream composition can be calculated directly from the feed compositions. The feed composition would be fixed by the outlet composition of the preceding unit. 4 . Combined heat and material balances Making material balance round a unit is possible and also the heat balance. The process temperatures may be set by other process considerations, and the energy balance can then be made separately to determine the energy requirements to maintain the specified temperatures. For other processes the energy input will determine the process stream flows and compositions, and the two balances must be made simultaneously; for instance, in flash distillation or partial condensation.

27. Example In the production of hydrogen by the steam reforming of hydrocarbons, the classic water-gas reaction is used to convert CO in the gases leaving the reforming furnace to hydrogen, in a shift converter. In this example the exit gas stream composition from a converter will be determined for a given inlet gas composition and steam ratio; by assuming that in the outlet stream the gases reach chemical equilibrium. In practice the reaction is carried out over a catalyst, and the assumption that the outlet composition approaches the equilibrium composition is valid. A typical gases composition obtained by steam reforming methane is: = 76,5 mol percent dry gas   If this is fed to a shift converter at 500K, with a steam ratio of 3 mol H 2 O to 1 mol CO, estimate the outlet composition and temperature. 4.1 144