1 Example SPC-2: Effect of Increasing Column P on a C3 splitter Consider the separation of a mixture of 50 mol/hr of propane C 3 H 8 (1) and 50 mol/hr propene, C 3 H 6 (2) at a pressure of 1.1 bar and a bubble point feed temperature of 230 K (-43 o C).Under these conditions, P 0 1=930.5 mm and P 0 2=724.1 mm and a 1/2 = Setting the recoveries (percentage) of the two components at 2=0.99 and 1=0.01, we find out that minimum number of trays, N m is: Nm ln. / ln / Now, if the pressure is increased tenfold to P=10.94 bar, we have a bubble point feed temperature of 300K (27 o C) and P 0 1= mm, and P 0 2= mm And separation factor, 1/2 = As a result, for the same recoveries, the separation becomes more difficult and the minimum number of trays increases to N m = Separation factor 1/2 has to larger than 1.0 for good separation!!
2 Time Out Session SPC-1 - Setting Column Pressure for a C3 splitter A liquid mixture containing 50 mole % propane (C3-), 50 mole % propene (C3=) is fed at a rate of 1000 mole/h to a distillation unit. Estimate the operating pressure for a column separating C3- from C3=, assuming cooling water at 30 o C is available for use. What would be the top temperature of the column? TABLE 3: BOILING POINT ( o C) OF COMPOUNDS AT PRESSURES BETWEEN 1 atm AND 40 atm
3 Flowsheeting: Process Conditions (T and P levels) non-condensibles 1.1 bar (feed from ambient conditions) R1 P reactor : 40 bar T reactor : 250 C F1 P flash : 3 bar (from flash calculations) C1 P top,c1 =1bar P bot,c1 = 1.2bar Set column pressures based on product bubble points Negative pressure gradient downstream? Rearrange/Readjust pressure levels to achieve positive gradient and facilitate flows? C2 P top,c2 =1.5bar P bot,c2 = 1.7bar P top,c2 =1.8bar C3 P top,c2 = 2.0bar heavies
4 Flowsheeting: Process Conditions (T and P levels) non-condensibles 1.1 bar (feed from ambient conditions) R1 P operation : 40 bar T operation : 250 C F1 Rearrange/Readjust pressure levels to achieve positive gradient and facilitate flows. Check that specs and flowrates are met. Recalculate Ps and Ts as necessary. C1 P top,c1 = 2.2bar P bot,c1 = 2.4 bar C2 P top,c2 =2.0 bar P bot,c2 = 2.2 bar P top,c2 =1.8bar C3 P top,c2 = 2.0bar
5 Flowsheeting: Process Conditions (T and P levels) non-condensibles 1.1 bar (feed from ambient conditions) R1 P operation : 40 bar T operation : 250 C F1 1.5 bar AVOID ROUND TRIPS! (unnecessary ups and downs in Pressure and Temperature levels C1 P top,c1 = 2.2bar P bot,c1 = 2.4 bar C2 P top,c2 =3.0 bar P bot,c2 = 3.5 bar C3 P top,c2 = 2.0bar
6 1 st column to separate HCl (overhead). For our VC production For column P equal 1 atm, the condenser has to operate at low temperature (-84.8 o C), expensive refrigeration. Patent suggest P=12 atm, so outlet from condenser will be o C. The bottom product will be at bubble temp of 93 o C. Higher pressure is avoided too prevent operation near critical regions. This bottom temperature will allow the used of lower pressure steam (lps) for reboiler. Feed will enter at bubble point, so the T is 6 o C at 12 atm. Higher feed T (e.g. 30 o C), will introduce vapor to column that will increase condenser duty. Not a good idea.
7 For our VC production 2 nd column to separate VC (overhead). For P=4.8 atm, the condenser outlet is at 33.1 o C. So could use CW at 25 o C The bottom product will be at bubble temp of 146 o C and could use medium pressure steam (mps) for reboiler
8 With separation system Fig 4.6 There may be other, possibly better alternative configurations, as discussed in (Chapter 5) later.
9 Synthesis Tree
10 Preliminary Process Synthesis Process Synthesis involves: Selection of processing mode: continuous or batch Fixing the chemical state of raw materials, products, and by-products, noting the differences between them. Process operations (unit operations) - flowsheet building blocks Synthesis steps: Eliminate differences in molecular types (chemical reaction) Distribute chemicals by matching sources and sinks (mixing & recycle) Eliminate differences in composition (separation) Eliminate differences in temperature, pressure and phase Integrate tasks (combine tasks into unit operations)
11 Eliminate differences in temperature, pressure and phase In other words, identify all T, P and phase change to determine the required utility systems
12 How do we set the T and P?
13 General Guidelines for Range of Process Operations Sensible operating range to avoid severe processing difficulties Parameter Range Rationale Pressure Between 1 to 10 bara Temperature 40 to 250ºC Most equipment can go up to 10 bar without increase in capital cost Limited by utilities available: cooling (30ºC) and steam between 40 to 60 bar, to 260ºC
14 Sensible Pressure Range The decision to operate outside the range of 1 to 10 bar usually is a compromise between performance and the capital and operating costs of process equipment. Conditions Justification Penalty e.g. in gas operations, increased density, lower volume, smaller more costly, P higher than 10 bar equipment (for the thicker-walled same residence time), equipment needed higher quality heating media (steam) P lower than 1 bar Prevent degradation of heat-sensitive materials Larger equipment. Need special equipment construction for vacuum operations
15 Sensible Temperature Range Temperature Several critical temperature limits apply to chemical processes. At elevated temperatures, common construction materials (primarily carbon steel), suffer a significant drop in physical strength and must be replaced by more costly materials (see next example SPC-1).
16 Effect of Temperature Excursions Example SPC-1 The maximum allowable tensile strength for a typical carbon steel and stainless steel, at ambient temperature, 400 o C, and 550 o C is provided below (from Walas ) Tensile Strength (bar) of Material At Temperature Indicated Temperature Ambient 400 o C 550 o C Carbon Steel (grade 70) Stainless Steel (Type 302) Determine the fractional decrease in the maximum allowable tensile strength (relative to the strength at ambient conditions) for the temperature intervals: (a) ambient to 400 o C and (b) 400 o C to 550 o C.
17 Effect of Temperature Excursions Answer to Example SPC-1 a. Interval ambient to 400 o C: Carbon Steel: ( )/1190 = 0.18 Stainless Steel: ( )/1290 = 0.0 b. Interval 400 o C to 550 o C: Carbon Steel: ( )/1190 = 0.67 Stainless Steel: ( )/1290 = 0.67 Example SPC-1 shows that carbon steel suffers a loss of 18% and the stainless steel suffers no loss in tensile strength when heated to 400 o C. With an additional temperature increase of 150 o C to 550 o C, the stainless steel suffers a 67% loss while the carbon steel suffers an additional 67% loss in strength. At operating temperatures of 550 o C, the carbon steel has a maximum allowable tensile strength of about 15% of its value at ambient conditions. For the stainless steel, the maximum allowable strength at 550 o C is about 33% of its ambient value. Conclusion: Carbon steel is unacceptable for service temperatures above about 400 o C, and that the use of stainless steel is severely limited. For higher service temperatures, more exotic (and expensive) alloys are required and/or equipment may have to be refractory lined. A decision to operate above 400ºC must be justified
18 Relative Cost/Btu Temperature Excursions Effects on Operating Cost Cooling All other equipment being equal (T & P). However, avoid excursions in T and P, but aim high T, rather than low T (due to higher cost to operate at lower T. Heating -150 Room Temperature 400 F Cost of temperature excursions from [Rudd 72]
19 Hot Oil Flue Gas T( C) 300 Typical Utility Levels Medium Pressure (MP) Steam Air Preheat HP Steam Lower 150 Pressure (LP) Steam 100 Boiler feed water C.W T ambient Refrigeration (high level) Refrigeration (low level)
20 Utility Systems General Rule for Cost Effective Operations In general: for hot utilities, USE at the LOWEST level and GENERATE at the HIGHEST level possible for cold utilities, USE at the HIGHEST level and GENERATE at the LOWEST level possible
21 Equipment General Guidelines for Range of Process Operations Guidelines for Setting Process Conditions Reactor Pressure and Temperature depends on favourable conversion & yield Separator: Distillation Azeotropic conditions Absorption Flash LL-Extraction S-L Separation Adsorption Membrane Set top column pressure assuming (if possible) a water-cooled condenser As in distillation, but may operates for special pressure to avoid azeotrope Operation between ambient pressure and 10 bar to prevent solvent loss Operation at much lower pressure (and temperature) than upstream equipment depending on the required split fraction of key component Operation around ambient pressure and temperature Operation around ambient pressure and temperature Find the optimum (high) pressure to enable separation Find the optimum (high) pressure to enable separation General Rule : Pressure : Between 1 to 10 bara Temperature : 40 to 250 o C Azetrope: a mixture of two liquids that has a same/constant boiling point and composition in distillation proceses.
22 Eliminate differences in T, P and phase Fig 4.7 UTM Computer-Aided Process Simulation Project
23 Synthesis Tree UTM Computer-Aided Process Simulation Project Intro to PD - 23
24 Preliminary Process Synthesis Process Synthesis involves: Selection of processing mode: continuous or batch Fixing the chemical state of raw materials, products, and by-products, noting the differences between them. Process operations (unit operations) - flowsheet building blocks Synthesis steps: Eliminate differences in molecular types (chemical reaction) Distribute chemicals by matching sources and sinks (mixing & recycle) Eliminate differences in composition (separation) Eliminate differences in temperature, pressure and phase Integrate tasks (combine tasks into unit operations)
25 Integrate tasks (combine tasks into unit operations) In other words, specify all the tasks in term of unit operations within the flowsheet
26 Chlorination reactor Cl2 and C2H4 in vapor phase bubble through the liquid C2H4Cl2 (with dissolve ferrite catalyst). Heat is released as C2H4Cl2 formed causing some of the product to vaporise and rise up through rectifying section (trays) Condenser is added the top of reactor to condense the vapor with CW. Some condensate is return to rectifying section.
27 Pump To increase the pressure of liquid C2H4Cl2 from 1.5 to 26 atm. Evaporator To increase liquid temperature and to boil it to saturated vapor at 26 atm, 242 o C Pyrolysis Furnace with Economizer The economizer (function similar to HEX) at inlet is to preheat the sat vapor to 500 o C and followed by pyrolysis reaction in the present of catalyst (Nickel).
28 Spray quench tank and cooler To rapidly quench the pyrolysis effluent to avoid carbon deposition? in HEX tubes (if send directly to HEX). The outlet stream is at its dew pressure of 170 o C, 26 atm Hot gases are showered with cold liquid. The heat transferred to liquid is removed when the liquid is circulated in the adjacent cooler (using CW). Condenser and valve To reduce temperature and then condense the stream to its bubble point of 6 o C and 12 atm
29 Recycle cooler To cool the recycle dichloroethane from o C to to 90 o C prior mixing with dichloroethane from chlorination reactor.
30 Fig 4.8 Integrate tasks (tasks unit operations)
31 Assembly of synthesis tree The bold branches trace the development of only one flowsheet of VC process.
32 What next? Identify promising alternative flowsheet(s) for further development We call this base-case design (s) First we need to create detailed PFD Gather additional database Do pilot plant test Simulation Continuous improvement
33 Block Flow Diagram (for VC) Fig 4.18 (Almost similar to Figure 4.5)
34 Process Flow Diagram (PFD) Much more detailed view of the process. Usually constructed using process simulators. Updated throughout much of process design. Almost similar to flowsheet develop after the completion of task integration (with stream and unit operation numberings). Unit Operation is identified with U-XYY,(e.g. P-102) U Unit type (P for pump), X single digit identifying process area, YY two-digit equipment number. Each stream in label as a numbered diamond.
35 Process Flow Diagram (PFD) Information (T,P, vapor fraction, component molar flow rates, total mole and mass flow rates, also enthalpy, density, heat capacity, viscosity) for each stream is tabulated and place at the bottom of PFD (e.g. 3.6) Equipment summary, see Table 3.8 Note: Mass and Energy Balance are almost complete for the flowsheet. More details information to begin the construction engineering work are in the Piping and Instrumentation Diagram (see Ulrich 1984, Sandler & Luckiewicz 1993). Contain additional info on location and type of control instruments, valves (isolation and control), MOC of piping (size, schedule) etc
36 Fig 4.20 Icons for process unit
37 Utilities Cooling Cooling water (cw), Refrigerated Brine (rb) propane refrigeration (pf) Heating fuel gas (fg), high-pressure steam (hps) medium-pressure steam (mps), low-pressure steam (lps) See Table 4.7 for typical operating range.
38 Development of Base-case Design Develop one or two of the more promising flowsheets from the synthesis tree for more detailed consideration.
39 Check assumptions? What next? Refining and adding preliminary database (e.g. kinetics data) More simulation (using process simulator and/or simulation model) for any changes (different unit operation or process parameters) Pilot-plant testing by development team (need planning as time consuming/expensive) Process Integration Heat and Power Integration (for Energy saving), see Ch 10 Mass Integration (e.g. minimize solvent used), see Ch 11 Detailed design calculations
40 Further Reading for Hierarchical Approach to Process Synthesis: J. M. Douglas, Conceptual Design of Chemical Processes, Chapter 1, McGraw-Hill, Biegler et al. Systematic Methods of Chemical Process Design, Chapter 2, Prentice hall, 1997.