Process Design and the Strategy of Process Design 8.1 INTRODUCTION TO PROCESS DESIGN Process design is the distinguishing feature which differentiates the chemical engineer from all other engineers. As the field of air-pollution control engineering has evolved over the past several decades, it is obvious that the air-pollution control engineer must adopt the ideas and principles of process design. In defining process design, we first define design as the development of a plan to accomplish a goal. The engineer must modify that definition to consider engineering design . Borrowing from a 19th century definition for railway engineering and extending the definition to our time, engineer- ing design is “The art of engineering a safe, environmentally sound system for $1.00 which any fool can do for $2.00.” The engineer must now accept the responsibility to see that any engineering system is not only economically feasible, but it must be safe and meet environmental regulations. Two books that would be of assistance in developing the ideas of process analysis, synthesis, and design principles are the texts by Seider, Seader and Lewin 1 and by Turton et al. 2 Both of these books emphasize the use of computer-aided process design and are up to date in their viewpoint. 8.2 THE STRATEGY OF PROCESS DESIGN This book focuses on environmental process design and specifically air-pollution control design. This design proceeds in a sequence of steps from the planning stage to the equipment stage of an “air-pollution control” project. As each decision point is reached, the engineer must evaluate alternatives. The most technically and eco- nomically feasible alternatives must be chosen; these alternatives must also be safe and environmentally sound . Consider the following process as an example: It has been determined that NO 2 emissions from the GREASKO LTD. plant have exceeded emission limitations. The orange cloud continues to be visible every day. The company management is deeply concerned for the community surrounding the plant and has determined that action must be taken to eliminate these emissions. Figure 8.1 illustrates the following likely sequence of events. • Calculated emission rates are verified by measurement, and sampling and labo- ratory analysis determine if emission limitations are being exceeded. • Equipment for the reduction of the NO 2 must be installed. 8 9588ch08 frame Page 91 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC FIGURE 8.1 The process of process design. 9588ch08 frame Page 92 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC © 2002 by CRC Press LLC • Final problem definition will identify the following factors as part of the design basis: • Composition of the stream • Flow rate • Temperature • Pressure • Variation of the operating parameters A series of decisions must now be made. 1. Decision Point 1: Alternative Control Techniques • Condensation • Absorption — Chosen • Adsorption 2. Decision Point 2: Absorption — Type of Absorber • Continuous — Packed Column — Chosen • Stage-wise 3. Decision Point 3: Flow Pattern • Counterflow — Chosen • Crossflow • Concurrent flow 4. Decision Point 4: A Major Impact — Disposing of the Collected Pollutant • Recycle • Recover • Incinerate • Landfill • React • Neutralize Emphasizing the decision process, Phillips 3 presents a detailed decision tree for the selection of the right device for removing particulates from a gas stream. He states that his process will help the designer to rapidly identify processes that are suitable for a particular application from 40 or more classes of processes available. In a letter to the editor, Nair 4 takes issue with this detailed decision tree. He points out that a knowledgeable air-pollution control process designer will contact a vendor with the minimum information necessary for the design, and the vendor will do the design. Nair’s method leaves the air pollution control process designer with the question: What is an acceptable and feasible design? The authors present the material in this handbook to assist the air pollution control engineer in determining and recognizing a safe, environmentally sound, and feasible process design. 8.2.1 P ROCESS F LOWSHEETS Beginning the process design, a simple basic flowsheet should be made. Even a pencil sketch can help explain better what will be required. Adding the process variables and the flowsheet can be a useful tool at every step. One way to begin is to draw a boundary around the whole process. This diagram can then serve to initiate 9588ch08 frame Page 93 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC the basic mass and energy balances which will be discussed in the next section. As the process design develops, the recycle streams can be added to this input–output sketch. The flowsheet should then be modified and moved toward the final document to be used for the detailed design and cost estimation. Start with the air pollution control equipment, then add the needed appurtances such as ducting, piping, pumps, fans, and tanks. Finally, the instrument and automatic controls can be added. 8.3 MASS AND ENERGY BALANCES The basis for all process design is the mass and energy balances. Thermodynamics serves as the scientific background for these calculations. For a treatment featuring chemical processes see Smith, Van Ness, and Abbott. 5 The basic mass and energy balance concept is presented by Felder and Rousseau. 6 Figure 8.2 presents the general mass and energy balance. Here the “system” is receiving two input streams from the “surroundings” and is producing one outlet stream to the surroundings. Each stream enters the system at a height z i above a reference plane. Heat, Q, and shaft work, W S , are being supplied and considered positive transferred to the system from the surroundings. The overall mass balance in terms of flow rate is (8.1) A component mass balance can be made where the concentration x ij is given in mass or mole fractions depending on the units of the flow rate. Here “i” designates the component and “j” designates the stream. (8.2) FIGURE 8.2 The general mass and energy balance. MMM ABC += xM xM xM AA A AB B AC c += 9588ch08 frame Page 94 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC The energy balance can then be written using the convention of exit quantities minus inlet quantities. (8.3) where U j = internal energy P j = pressure ρ j = density of fluid — U j = average velocity of stream α = a correction factor of 1.0 for turbulent conditions and 1/2 for laminar conditions g = acceleration due to gravity g c = gravitational constant Enthalpy is defined as (8.4) where v = specific volume = 1/ ρ The enthalpy can be calculated from specific heat at constant pressure. (8.4) (8.5) Equation 8.3 can be simplified for a single stream in, A, and a single stream out, C. Then, Using ∆ to indicate the output–input difference, MU P U g zgg M U P U g zgg MU P U g zgg Q W CCCC C c CC AAAA A c AC BBBB B c BC S +++       −+++       −+++       =+ ρ α ρ α ρ α 2 2 2 22 2 HUPv=+ C H T P P ≡ ∂ ∂       ∆H C dT C T T P T T P avg == − () ∫ 1 2 21 MM AC = 9588ch08 frame Page 95 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC Equation 8.3 may now be written, (8.6) 8.3.1 A M ASS -B ALANCE E XAMPLE Two streams enter a separations apparatus as shown in Figure 8.3. The streams mix and are separated. Two streams also leave the apparatus. The following lists the stream compositions. It is also known that the ratio of the flow rate of Stream 2 to Stream 1 is 5/1. The composition of Stream 4 is unknown and must be determined. All of the component A goes into Stream 3. Set the basis for calculation. FIGURE 8.3 The mass balance. Stream 1 Stream 2 Stream 3 60% A 100% N 95% A 40% B 5% B ∆ ∆ ∆ HH H UUU zz Z CA CA CA =− =− =− 222 MH U g zgg Q W C CS ∆ ∆ ∆++       =+ 2 2α 9588ch08 frame Page 96 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC Basis : 100 units of Stream 1. The units could be lb mass, kg mass, lb moles, or kg moles. Use kg moles. Stream 1 = 60 kg moles of A, 40 kg moles of B Stream 2 = 5 × 100 = 500 kg moles N A is a “tie substance” which can be used to determine how much of the total flow goes into Stream 3. Stream 3: 60 kg moles A = 95% of the stream 60/0.95 = 63.16 kg moles total flow 63.16 – 60 = 3.16 kg moles B Check: (3.16/63.16) × 100 = 5.00% B — Ok! Stream 4: N = 500 kg moles B = 40 – 3.16 = 36.84 kg moles Total moles = 500 + 36.84 = 536.84 kg moles % N = (500/536.84) × 100 = 93.14% % B = (36/536.84) × 100 = 6.86% Total 100.00% 8.3.2 A N E NERGY -B ALANCE E XAMPLE Hot dirty air at 10,000 acfm, 1.0 atm, and 500°F is blown by a fan into a heat exchanger and then into a waste treatment process. Figure 8.4 illustrates the process. The air enters the blower at 125 ft/s. The blower has a 15 HP motor operating at 85% efficiency. The dirty air is cooled to 130°F at Point 2 and leaves the heat exchanger at 275 ft/s. Water at 80°F is available as a coolant. A 10°F temperature rise is allowed. Determine the amount of coolant required. FIGURE 8.4 The energy balance. 9588ch08 frame Page 97 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC D ata: Solve Equation 8.6 for Q. (8.6) Assume z = z 2 – z 1 = 0.0 and α = 1.0 or Air density at 500 F: Air specific heat: C - Water specific heat C - Air enthalpy at 1: H Air enthalpy at 2: H air P avg P avg 1 2 °= =° =° = = ρ 0 0412 0 241 10 231 1 141 1 3 . . . . . lbm ft BTU lbm F BTU lbm F BTU lbm BTU lbm QM H U g zg g W C CS =++       −∆ ∆ ∆ 2 2α M ft lbm ft lbm H C T T BTU lbm =××= =− ( ) =− ( ) =− 10 000 0 0412 1 0 60 6 87 0 241 130 500 89 17 33 21 , min . . min sec . sec ∆ P avg ∆ ∆ ∆ ∆ H H H BTU lbm H BTU lbm ft lbf BTU lbm s ft lbf U g ft s ft lbm lbf s ft lbf lbm U g ft lbf lbm c c =−= − =− =× ×=− = ( ) − ( ) ×         = = 21 2 22 22 2 2 141 1 231 1 90 0 90 778 6 87 476 370 2 275 125 2 32 174 932 4 2 932 4 . . , sec . . . - - α α ××= =× × = =− + − =− ( ) =− × =− 6 86 6402 7 0 80 15 550 6600 0 476 370 6403 6600 476 567 476 576 778 612 6 ,, ,. lbm s ft lbf s W HP ft lbf s HP ft lbf s Q ft lbf s Q ft lbf s BTU ft lbf BTU s S - - - Shows H to be the most significant factor. ∆ 9588ch08 frame Page 98 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC For the coolant Q C = –Q, REFERENCES 1. Seider, W. D., Seader, J. D., and Lewin, D. L., Process Design Principles — Synthesis, Analysis, and Evaluation , John Wiley & Sons, Inc., New York, 1999. 2. Turton, R., Bailie, R. C., Whiting, W. B., and Shaeiwitz, J. A., Analysis, Synthesis, and Design of Chemical Processes , Prentice-Hall, Englewood Cliffs, NJ, 1998. 3. Phillips, H. W., Select the proper gas cleaning equipment, Chem. Eng. Prog., 96(9), 19, 2000. 4. Nair, C., Letters — Gas cleaning equipment, Chem. Eng. Prog., 97(1), 10, 2001. 5. Smith, J. M., Van Ness, H. C., and Abbott M. M., Introduction to Chemical Engi- neering Thermodynamics, 6th ed., McGraw-Hill, New York, 2001. 6. Felder, M. R. and Rousseau, R. W., Elementary Principles of Chemical Processes, 3rd ed., John Wiley & Sons Inc., New York, 2000. QmC T CT lbm s m lbm s s lbm q lbm gal lbm gpm C w =° = × = =×= =× = p avg C P avg T limited to 10 F m= Q ∆∆ ∆ , . . . min . min . min . . 612 6 110 61 26 61 26 60 3675 3 3675 3 1 0 8 34 440 9588ch08 frame Page 99 Wednesday, September 5, 2001 9:49 PM © 2002 by CRC Press LLC . engineer from all other engineers. As the field of air- pollution control engineering has evolved over the past several decades, it is obvious that the air- pollution control engineer must adopt the ideas and. – z 1 = 0.0 and α = 1.0 or Air density at 500 F: Air specific heat: C - Water specific heat C - Air enthalpy at 1: H Air enthalpy at 2: H air P avg P avg 1 2 °= =° =° = = ρ 0 0412 0. Nair’s method leaves the air pollution control process designer with the question: What is an acceptable and feasible design? The authors present the material in this handbook to assist the air