Process Equipment Design & Unit Operation Economics [PDF]

Zitiervorschau

Plant and Equipment Design

DR. LEONARDO C. MEDINA, JR.

Plant Design & Economics for Chemical Engineers 5th edition Peters & Timmerhaus

Perry’s Chemical Engineers Handbook 7th Edition

8th Edition

Section 9 Process Economics

Section 9 Process Economics

Section 28 Materials of Construction

Section 25 Materials of Construction

Plant Design  Plant design  includes all engineering aspects involved in the development of a new, modified, or expanded commercial process in a chemical or biochemical plant.

 Process engineering  used in connection with economic evaluation and general economic analyses of commercial processes

 Process design  actual design of the equipment and facilities necessary for providing the desired products and services

Plant Design  General Overall Design Considerations

1. PROCESS DESIGN DEVELOPMENT  Inception of basic idea (salesman, customer report  New or modified product  Pilot plant can be constructed

The engineer should have the ability to eliminate unprofitable ventures before the design project approaches a final proposal stage.

Plant Design  General Overall Design Considerations

1. PROCESS DESIGN DEVELOPMENT PILOT PLANT

COMMERCIAL DEVELOPMENT PLANT

 small-scale replica of the  usually constructed from odd pieces of equipment that full-scale final plant are already available and is not meant to duplicate the exact setup to be used in the full-scale plant

Plant Design  General Overall Design Considerations

2. FLOWSHEET DEVELOPMENT  Chemical engineer creates one or more solutions  Different feeds and intermediates  Performs mass and energy balances

Plant Design  General Overall Design Considerations

3. COMPUTER AIDED DESIGN  Allows rapid calculations, large storage  Allow examination of effect that various design variables will have on the process or plant design more rapidly than manual calculation  Use of simulation programs  Can regress experimental data obtained in the laboratory or pilot plant for empirical or theoretical curve fitting

 Use of spreadsheet programs

Plant Design  General Overall Design Considerations

4. COST ESTIMATION  As final process design is completed it becomes possible to make accurate cost estimations  Pre-design cost estimation  Provide basis for company management to decide to infuse further capital

Plant Design  General Overall Design Considerations

5. PROFITABILITY ANALYSIS OF INVESTMENTS  When a company invests money it expects to receive a return  Rate of return (minimum acceptable)  Time value of money

Plant Design  General Overall Design Considerations

6. OPTIMUM DESIGN

 Cost Minimization  Profit Maximization  Capacity Maximization

Plant Design  General Overall Design Considerations

6. OPTIMUM DESIGN  Several alternative methods can be used for any given process or operation  Formaldehyde  Catalytic dehydrogenation of methanol  Controlled oxidation of natural gas  Direct reaction between CO and H2 under special conditions of catalyst, T and P

Plant Design  General Overall Design Considerations

6. OPTIMUM DESIGN If there are two methods for obtaining exactly equivalent final results, the preferred method is the one involving the LEAST TOTAL COST. Optimum Economic Design

Plant Design  General Overall Design Considerations

7. OPTIMUM OPERATION DESIGN  Optimum conditions for specific conditions of temperature, pressure, contact time, or other variable Assumptions are made only when they are necessary and reasonably correct and will not adversely affect the overall design and its economic conclusions.

Optimization Applications: Unit Operation, Process/Equipment

Variable

Symbol

Flow of Fluids

Economic Pipe Diameter

Di

Flow of Heat

Optimum Insulation Thickness

XI

Evaporation

Optimum Number of Effects

N

Drying

Temperature Difference

ΔT

Distillation

Optimum Reflux Ratio

R = L/D

Condenser

Cooling Water Flow Rate

w

Filtration

Filter Capacity= volume filtrate delivered per V/Өf filtering time

Leaching

Optimum Solvent to Feed Ratio

S/F

Unit Operation, Process/Equipment

Variable

Symbol

Solvent Extraction

Optimum Solvent to Feed Ratio

S/F

Adsorption

Optimum Adsorbent to Solution Ratio

L/V

Scale Formation in Evaporation

Optimum Cycle Time = Operating or Boiling Time + Emptying, Cleaning & Recharging Time

Өt = Ө b + Ө c

Reactor

Reactant Conversion

XA

Pressure Vessel

Optimum Diameter

D

Humidification

Mass velocity of vapor

Gy

Gas Absorption

Mass Velocity of Liquid

Gx

Ion Exchange

Mass Velocity of Liquid or mass velocity of vapor

Gx or Gy

Scope of a Plant Design Project Market Study Technical Analysis Economic Analysis Site Selection Plant Layout Environmental Impact Assessment Safety Process Control

Cost Estimation

Engineering Economics A practicing engineer’s decision are heavily influenced by the economics of the project. -beneficial to man: safe and environmentally benign. -Though environmentally friendly, a project will not be pursued if it is not economically attractive.

Cost Estimation A Chemical Engineer must be able to estimate the following in designing a plant: Capital costs (Total Capital Investment) Equipment costs Manufacturing costs General expenses Taxes

Total Capital Investment Fixed Capital Investment (FCI)  manufacturing capital investment

80-85%

 equipment, installation, instrumentation, piping, insulation, site preparation etc. directly related to process operation  non-manufacturing capital investment • land, offices, warehouses, utility generation, waste disposal etc.  all except land are depreciable

•Working Capital Investment (WC) 15-20%  raw materials and supplies, finished and semi-finished products, accounts receivable, cash for expenses/accounts payable, taxes payable, nondepreciable

Estimating Purchased Equipment Cost cost indexing of past purchase orders scaling on basis of capacity cost/capacity ratio 6-10th’s rule firm vendor quotes

Cost Indexing of Past Purchases  Cost Index –a value for a given point of time showing the cost at that time relative to a certain base time (usually in the past)

Cn

index value at present time C0 index value at time of original purchase

 Cn=cost of equipment to be estimated (new cost)  Co=known cost of existing equipment (old cost)

Cost Indexes  Chemical Engineering Plant Cost Index (CEPCI)  published monthly in Chemical Engineering  base value= 100 in 1957-59  Value=395.4 in May 2001.

 Marshall and Swift Equipment Cost Index (MS)  published monthly in Chemical Engineering  base value= 100 in 1926  Value=1092.0 in 2Q 2001.

 CEPCI and MS are most widely accepted in chemical process industries..  Other cost indices  Nelson-Farrar Refinery Construction Cost Index  Engineering News –Record Construction Index

Index Values (CEPCI)

Marshall and Swift Equipment Cost Index

Capacity Scaling (6-10th’s rule)  Good approximations are often obtained using an exponent of 0.6

Cn

C0

Pn P0

0. 6 See Table 19/187 PT for x values See Table 9-48/9-67/Perry

 Cn=cost of equipment to be estimated (new cost)  Co=known cost of existing equipment (old cost)  Pn=capacity of new piece of equipment  Po=capacity of existing piece of equipment  Actual exponents vary from 0.2 to >1,  Do not use beyond a 10-fold range of capacity  Use only for similar types of equipment

Estimation of Capital Investment

Estimating Equipment Installation Costs: percentage of purchased equipment cost varies from 20-90% •Firm contractor quotes

Costs of Instrumentation, Controls and Insulation •Instrumentation and Control – major component of chemical processing plant – estimated as fraction of purchased equipment cost (preliminary) or from P&ID’s and instrument index (detailed and definitive) •Insulation – major component for very high or very low temperature service – estimate from fraction of purchased equipment cost (preliminary) or material take-offs (detailed and definitive)

Buildings and Yard Improvements • control rooms, maintenance shops, warehouses, etc. – includes plumbing, heating, lighting, ventilations • fencing, grading, roads, sidewalks, railroad sidings, and landscaping • estimate as a percentage of purchased equipment cost

Service Facilities and Land • utilities –steam, water, power, compressed air and fuel • waste disposal, fire protection, shop, first aid, cafeteria • estimate as a percentage of purchased equipment cost • necessary at a new plant site • at existing plant site, new facilities may not be needed, but project may have to pay for future expansions. • Land –purchase may be needed for grass-roots plant –not depreciable

Engineering and Services percentage of fixed capital investment

Construction Expense and Contractor’s Fee  percentage of fixed capital investment

Total Capital Investment

Total Capital Investment

Total Product Cost Cost necessary to produce and sell the product •Total Product Cost consists of:

•Manufacturing Costs –Directly related to the manufacturing process •General Expenses –not directly related to manufacturing, but necessary for running the business

Direct Product Cost • raw materials • operating labor • direct supervisory and clerical labor • utilities • maintenance and repairs • operating supplies • lab charges • patents and royalties • catalysts and solvents

Fixed Charges • depreciation • local taxes • insurance

• rent

Plant Overhead Cost •hospital and medical •general engineering •safety services •cafeteria and recreation facilities •payroll and overhead •janitorial services •warehouses •shipping and receiving

Materials of Fabrication Selection  Corrosion – chemical or electrochemical attack  choose combination of metals that are close as possible in the galvanic series  corrosion rate → affected by pH  aluminum and zinc dissolves rapidly in either acidic and basic solutions  oxidizing agents  powerful accelerator of corrosion  cathodic protection  widely used in the protection of underground pipes and tanks from external soil corrosion and in water systems.

Materials of Fabrication Selection  Ferrous Metals and Alloys  steel  carbon steel is commonly used  low cost  ease of fabrication  limited corrosion resistance

 stainless steel  martensitic  ferritic  austenitic

Process Equipment Design Factors considered in the selection of materials of construction for process vessels

1. CHEMICAL FACTOR  Resistance to corrosion  Table in Peters et al., corrosion resistance  Table of Materials of Construction in Chemical Engineers Handbook  Reagent → Vessel material

Process Equipment Design Factors considered in the selection of materials of construction for process vessels

2. PHYSICAL FACTOR  Ability to resist expansion  Material properties: elasticity, machinability, porosity, hardness, softness, conductivity of heat and elasticity, etc.

Process Equipment Design Factors considered in the selection of materials of construction for process vessels

3. ECONOMIC FACTOR  Cost of Material of Construction  Fabrication cost

Process Equipment Design Pressure Vessels

1. RIVETED (seldom given in BE, not used in industries) Longitudinal joint (tangential stress)

circumferential joint (longitudinal stress)

Process Equipment Design Pressure Vessels

2. WELDED (commonly used in industries)

Design Equation

Material Selection

Material Selection

Material Selection

Pressure Vessels  Type  Riveted or bolted  Welded  Standards  ASME Boiler and Pressure Vessel Code  British Code or British Standards (BS)  (West) German Code (A. D. Merkblätter)  Variable  Working or Operating  Design

Design Considerations  Pressure  Internal  External  Temperature  Material of Construction  Design Stress  Welded Joint Efficiency  Corrosion Allowance  Design Loads  Major  Subsidiary  Minimum Practical Wall Thickness

Welded Joint Efficiency  Single-welded butt joint with bonding strips  0.90 for fully radiographed  0.80 for spot examined (radiographed)  0.65 if not radiographed  Double-welded butt joints  1.00 for fully radiographed  0.85 for spot examined (radiographed)  0.70 if not radiographed

Welded Joint Efficiency  In general, for spot examined (in the absence of available precise data)  0.85 for electric resistance weld  0.80 for lap welded  0.60 for single-butt welded  1.0 for seamless shells and heads

Design Equations • Shells – Cylindrical – Spherical

• Heads/Closures/Ends – Flat • Plates • Formed ends

– Domed • Types – Pierced – Unpierced

Design Equations – Domed • Hemispherical • Ellipsoidal • Torispherical (or dished ends) • Conical

Design Equations (Internal Pressure) • Cylindrical Shells t

t

P ri Cc SEJ 0.6 P

SEJ ri SEJ

P P

1 2

where t

or P 0.385SEJ

where t

ri Cc

0.5ri

0.5ri

or P 0.385SEJ

Design Equations (Internal Pressure) • Spherical Shell t

t

P ri Cc 2SEJ 0.2 P

2SEJ 2 P ri 2SEJ P

where t

0.356ri

or P 0.665SEJ

1 3

where t

ri

Cc

0.356ri

or P 0.665SEJ

Design Equation (Internal Pressure) • Hemispherical Head (formula of spherical shell can be used) • Ellipsoidal Head (for 2:1 ratio)

t

PDa Cc 2SEJ 0.2 P

Design Equation (Internal Pressure) • Torispherical Head

t where

0.885 PLa Cc SEJ 0.1P Knuckle radius 6% Crown radius 3t

Design Equations (Internal Pressure) • Conical Head (for any point on a cone)

t

PDc 1 2 SEJ P cos

Cc

Design Equation (Internal Pressure) • Conical Head (at cone-cylinder junction)

t

Cs

C s PDc 2 SEJ P 20o 1.00

30o 45o 1.35 2.05

Cc 60o 3.20

Rules of Thumb (Heuristics) • Design Pressure

• Design Temperature • Corrosion Allowance

Design Pressure • Allowance is either 10% of max operating P or 70–175 kPa which ever is greater. Design P = max operating P + Allowance • When no data is available for max operating P, Design P = normal operating P + 175 kPa • For vessels operating at 0.32–1 atm and 316– 538oC, Design P = 377 kPa • For vacuum operation, design P is 200 kPa outside and full vacuum inside

Design Temperature • For operating temperature between –30 to 350oC, Design T = Operating T + 30oC • Below –30oC, special steel required • Above 350oC, allowable design stress falls sharply

Piping System • Pipe – Nominal Diameter – Schedule Number Schedule Number 1000

 Tubing – Outside diameter – Wall Thickness/Gauge

P S

Pipe and Tubing • Pipe – Heavy walled – In moderate length of 20-40 feet – Slightly rough – Can be screwed, welded or flanged – Made by welding, casting or piercing – Relatively large

• Tubing – Thin walled – In coils of several hundred feet – Very smooth wall – Connected by compression, flaring or soldering – Made by extrusion or cold-drawned

Friction Factor Calculation • Conduit Configuration • Friction Factor • Temperature Correction

Conduit Configuration • Diameter for circular conduits • Equivalent Diameter for non-circular conduits (Turbulent flow only)

Deq

cross - sectional area 4 wetted perimeter

Isothermal Friction Factor • Skin – Equations – Moody Diagram or Friction Factor Chart

• Form – Resistance Coefficient (K) – Equivalent Length

Pipe Friction Equation • Fanning Equation

• Darcy-Weisbach Equation

2

F

2 f Fanning v Le

F

f Darcy v 2 Le

gc D

 Friction Factor Relationship

f Darcy

4 f Fanning

2 gc D

Moody Diagram (Newtonian)

Affinity Laws [Constant Impeller Speed] • Flow Q1 Q2

D1 D2

• Head H1 H2

D1 D2

2

• Power BHP1 BHP2

D1 D2

3

PUMP CURVES

Gas Motive Devices • Fans – Low pressure service of up to 0.5 psi – Volume service of up to 130,000 ft3/min – Gas compressibility usually assumed negligible • Blowers – Pressure service of up to 1.5 psi – Volume service of up to 200,000 ft3/min • Compressors – For large volume and higher pressure service

Process Equipment Design

Problem Solving  Problem No. 17 Four inch (4.0 inch) schedule 40 steel pipes are to be used to transport high pressure steam. The pipe joints are to be butt-welded. The safe working fiber stress for butt welded pipes is 457.1 kg/cm2. The maximum steam pressure, in kg/cm2, the pipes can handle is a. b. c. d.

50.8 kg/cm2 18.28 kg/cm2 25 kg/cm2 55 kg/cm2

Problem Solving  Problem No. 18 A spherical carbon storage tank for ammonia has an inside diameter of 30 ft. All joints are butt welded with backing strip. If the tank is to be used at a working pressure of 50 psig and a temperature of 80 ̊ F, estimate the necessary wall thickness. Assume no corrosion allowance is necessary. Efficiency is 80% and allowable tensile strength is 13,700 psi. a. b. c. d.

1/2 in 1/4 in 7/16 in 5/16 in

Problem Solving  Problem No. 19 A reactor will operate at 300 psi and 600 ̊ F. Height = 12 ft; crown radius = 66 in; diameter = 6 ft; double welded butt joint, efficiency = 80%. Allowable tensile strength of material is 12,000 psi. The thickness of the shell is a. b. c. d.

1 and 3/16 in 1 and 1/16 in 2 in 1 and ¼ in

Problem Solving  Problem No. 20 A reactor will operate at 300 psi and 600 ̊ F. Height = 12 ft; crown radius = 66 in; diameter = 6 ft; double welded butt joint, efficiency = 80%. Allowable tensile strength of material is 12,000 psi. The thickness of the head is a. b. c. a.

5/2 in 1/2 in 3/4 in 3/2 in

Problem Solving  Problem No. 22 A water tank 30 ft. in diameter has a thick steel plate available at 3/8 in thick. Assume the allowable stress of steel is 15,000 psi and a joint efficiency of 80%. Provide a corrosion allowance of 1/16 inch. The maximum height of the water tank is a. b. c. a.

84.3 ft 43.6 ft 68.05 ft 48.03 ft

Problem Solving  Problem No. 15 A chemical engineer was commissioned to design a vertical cylindrical tank with a flat bottom and a conical roof. The tank must be able to hold a maximum of 4,500 m3 of water for firefighting purposes. Ease of climbing the tank and bearing capacity allows a maximum height of 16.5 m from the bottom of the tank up to the rim of the tank cylinder. Normal working practice dictates that the maximum working capacity of the tank is 90% of the total tank volume. The tank roof has a 10% incline. Suitable steel plates available for constructing the tank come in size 4’x8’ sheets. The number of steel plates needed is…

Problem Solving  Problem No. 15 The number of steel plates needed is… a. b. c. a.

540 575 610 525

Problem Solving  Problem No. 23 A cylindrical water tank with a hemispherical dome has the dimensions shown below: The tank is full. 10 ft

20 ft

20 ft

The total force, in lbf, exerted by the water on the base of the tank is most nearly a. 500,000 b. 520,000 c. 550,000 d. 590,000

Problem Solving  Problem No. 21 A horizontal cylindrical tank is used for the storage of motor gasoline in the bulk plant of an oil company in Pandacan. The tank has an inside diameter of 3 m and an inside length 10 m. The suction line of the tank is located 30 cm from the tank bottom to avoid sucking out the sludge. To prevent overfilling, the maximum height of liquid in the tank is not made to exceed 90% of the vertical height of the tank. The working capacity of the tank in kiloliters is a. b. c. a.

36.23 63.32 45.23 68.32

Unit Operation Economics

Problem Solving  Problem No. 13 A smelting furnace operating at 2,400˚F is to be insulated on the outside to reduce heat losses and save on energy. The furnace wall consists of a ½ inch steel plate and 4-inch thick refractory inner lining. During operation without outer insulation, the outer surface of the steel plate exposed to air has a temperature of 300 ˚F. Ambient air temperature is at 90˚F. Operation is 300 days per year. Thermal conductivities in BTU/hr-ft-˚F are: steel plate = 26; refractory = 1; insulation to be installed=0.025. The combined radiation and convection loss to air irrespective of material exposed is 3 BTU/hr-ft2.˚F, annual fixed charge is 20% of the initial installation cost. If heat energy is P5.00 per 10,000 BTU and installed cost of insulation is P100/in-ft2 of area, the optimum thickness of the outer insulation that should be is…

Problem Solving  Problem No. 11 A multiple effect evaporator produces 10,000 kg of salt from a 20% brine solution per day. One kg of steam evaporates 0.7 N kg water in N effects at a cost of P25/1000 kg of steam. The cost of the first effect is P450,000 and the additional effects at P300,000 each. The life of the evaporator is 10 years with no salvage value. The annual average cost of repair and maintenance is 10% and taxes and insurance is 5%. Assume 300 operating days per year. The optimum number of effects for minimum annual cost is a. b. c. d.

3 effects 5 effects 4 effects 2 effects

Problem Solving  Problem No. 12 A process requires 20,000 lb/hr of saturated steam at 115 psig. This is purchased from a neighboring plant at P18.00 per short ton and the total energy content rate (mechanical) in the steam may be valued at P7.5x10-6 per BTU. Hours of operation per year are 7200. The friction loss in the line is given by the following equation:

F Cf

187.5 Lq1.8 mc0.20 0.20 4.8 d Di 1.44 Di

1. 5

in ft-lbf/lbm

in P/yr

L

where L = length of straight pipe, ft. q = steam flow rate, cu.ft. per sec. mc= steam viscosity, cp d = steam density, lb per ft3 Di= inside diameter of pipe, in. The optimum pipe diameter that should be used for transporting the above steam is a. 6 in b. 4 in c. 3 in d. 5 in

Problem Solving  Problem No. 15 One hundred gram moles of R are to be produced hourly from a feed consisting of a saturated solution of A (CAO = 0.1 gmol/L). The reaction A → R with rate ra = (0.2/hr)CA. Cost of reactant at CAO = 0.1 gmol/L is P3.75/gmol A; cost of backmix reactor, installed complete with auxiliary equipment., instrumentation, overhead, labor depreciation, etc is P0.075/hr-L. The % conversion of A that should be used for optimum operation is

a. b. c. d.

45% 60% 50% 40%