Examples and Exercises Process simulation

Ammonia production: Haber Bosh process

Example: Ammonia production Haber Process: N2 + 3 H2  2 NH3 Reaction conditions: T= 930 °F, P= 7350 psi Feed composition: 74% H2, 24.5% N2, 1.2% CH4, 0.3% Ar; T= 300°F; P = 500 psi Feed flow rate: 100 lb-mole/hr Conversion per pass: 65% of N2 Thermodynamic model: Equation of state (Peng Robinson) Reaction products are refrigerated to separate 75% mole of NH3 product per pass (no pressure drop). The product is pure NH3 at -20 °F (i.e. all other compositions in bottom product is zero). The remaining product is recycled back after purge (10%) i.e. no gases in the product stream. T=300°F.

Example: Ammonia production Goal 1: adjust the purge flow rate so that the stream to the reactor contains 0.11 CH4(mole fraction) Goal 2: calculate all streams for two values of initial molar flow rate of methane (2 and 3 lbmole/hr). Prepare a graph with % CH4 in carica  spurgo con vincolo 1 Goal 3: maximize the value of the product: Equal to the difference between the flow rate of NH3 in S-5 and S-7. Subject to constraint: total flow rate to the reactor less than 240 lbmol/hr

Example: Ammonia production Run base case with the stream manipulator Run the base case with the flash Flash is substituted to SM (T= -20°F; P=500 psi) Perform sensitivity analysis on the methane concentration vs recycle ratio (purge from .92 to .98) Set a design specification to keep methane mole frac. = 0.11 at the inlet of the reaction (take a suggestion from sensitivity) Save a final flowsheet with the desired value of purge and without any design spec or sensitivity Optimize the problem with the following Objective function FOB = Flow rate of NH3 in product – Flow rate NH3 in purge Constraint: the total flow rate to the reactor be less than 240 lb mol/hr Suggestions: Reinitialize the simulation, particularly after sensitivity analysis If needed modify tolerances for convergence If convergence is slow, increase the number of iterations for Wegstein method Force the tear stream to be S-3

Ammonia: the real process

Optimizing steam consumption for solvent recovery Typical example of organic aqueous stream separation The final goal is to comply the emission specification ( in this case 150 ppm) by reducing the amount of steam injections to a minimum reducing the operation cost addressing the environmental restrictions. Goal recovery of methylene chloride from waste Use of a liq-liq decanter for separating organic rich phase Lower organic content to less than 150 ppm in water stream

The process Simulation details Process Input calculation of the steam consumption identification of the optimum conditions (sensitivity analysis) Process Input Feed stream containing an aqueous stream of Methylene chloride (S1): total flow rate of 100,000 lb/hr, Temperature at 100 0F, pressure of 24.7 psia and components in the feed stream are 0.0140 of CH2Cl2 and 0.986 of H2O in mass fraction Steam Streams injected to the two towers ( S2 & S4) Steam saturate vapor at 200 psi flow rate of 10.000 lb/hr is fed to the primary tower of. Steam (S2) at a flow rate of 5000 lb/hr at the secondary tower. The top product of both towers (primary & secondary) or vapor overheads are collected in the mixer and condensed at a Temperature of 75 0F and pressure of 14.7 psi. The remaining stream of the process is carried off to a decanter to separate the condensate into a methylene–chloride-rich stream and a water-rich stream.

Methylene chloride recovery

Process Simulation goal Recovery of methylene chloride in waste water; Sensitivity analysis 1 to obtain methylene chloride concentration of 150 ppm in the bottom of the secondary tower (Stream 7 in Figure 2) by regulating the flowrate of the steam inlet to the primary tower; Simulate sensitivity analysis 2 to obtain a methylene chloride concentration of 150 ppm in the bottom of the second tower in (Stream 7) by regulating the flow rate of the steam inlet to the secondary tower; Optimize the total stream injections in the process.

Input specifications

Thermodynamic validation Pure component properties

Thermodynamic validation Binary LLE

Sensitivity analysis 1 Independent Variable: Steam flow rate injected to the primary tower Dependent Variable : Methylene Chloride concentration in Bottom 2 Fixed Parameters: Steam flow rate injected to the secondary tower fixed at 5000 lb/hr and Feed conditions

Sensitivity analysis 1

Sensitivity analysis 2 Independent Variable: Steam feed rate inlet to the secondary tower Dependent Variable: Methylene chloride concentration in Bottom 2 Fixed Parameters: Steam feed rate inlet to the primary tower fixed at 10,000 lb/hr, feed conditions

Sensitivity analysis 2

Design specification Controller Optimizer set design specification on the concentration of methylene chloride in stream 7 is equal to 150 ppm the variable is the flow rate (lb/hr) of steam injected to the primary and secondary tower (S2 & S4). Optimizer The sensitivity studies show that several solutions exist to give a methylene chloride concentration of 150 ppm at the bottom of the secondary tower. The optimizer is introduced to determine among them the solution with the minimum consumption of energy. The optimization procedure followed is to couple the controller with an optimizer to minimize the total steam consumption and the variable is the steam flow rate injected to the first and second tower.

Optimization results Results savings in steam usage realized without any major process or equipment changes Steam saving of 30000 $ per year (assuming $ 2.5/ 1000 lb steam)

Cyclohexane production Reaction section Benzene + 3 Hydrogen = Cyclo hexane Separation section Recovery of cyclohexane

Cyclo hexane production process The production process The objectives of the case study are the following: Verification of the thermodynamic data and models Simulate the base case of the reaction section Simulate the base case of the entire process with a simple model for the distillation column Simulate the base case of the separation section such that the bottom liquid product from the distillation column is equal to 135 kgmol/hr Modify the process condition to obtain high recovery of cyclohexane (99.5 %) at the bottom product stream (S5) Maintain a flow rate of 5.3 kmol/hr in the distillate stream (S4) Simulate the complete process with a distillation column

Thermodynamic and data bank

Vapor pressure of cyclohexane

Binary VLE cyclo hexane - benzene Thermodynamic model: RKS (or GE )

Reaction section Results Reaction is a catalytic hydrogenation with total conversion = 0.998 Feed 1 Gas: T= 48°C - P= 22 atm – Rate 523 kgmol/hr H2= 465 – N2= 15 – CH4= 43 Feed 2 Benzene: T=40°C - P= 20.4 atm – Rate 144.4 Results

Cyclohexane production

Separation section The initial feed flow rate is 232 kg-mol/hr at a temperature of 200 °C and pressure of 15 atm Inlet to a flash separator, operated at a Pressure of 15 atm and Temperature of 50 °C. The vapor out of the flash is an output of the process, The liquid product is fed in the middle tray of a distillation column with a reboiler and partial condenser. The internal design specification of the process is the reflux ratio is equal to 1.2. Thermodynamic: RKS or GE

Separation section N stages= 15 (feed at 8) P= 13.33 atm Ref R= 1.2 Vapor dist (1: vap; 15: liq) T= 200 C P= 15 atm H2 = 30.0 kmol/hr N2 = 15.0 CH4 = 43.0 CYC6 = 144.2 Bz = 0.2 T= 50 C P= 15 atm Rate initial = 135 kmol/hr

Base case: results Stream Name S1 S3 S2 S4 S5 Stream Description Phase Vapor Liquid Temperature, C 200 50 169.2375183 199.5368652 Pressure,ATM 15 13.32999992 Flowrate,kg-mol/hr 232.3999939 86.66875458 145.7312469 10.73137283 134.9998779 FLUID RATES, Kg-mol/hr HYDROGEN 30 0.3855 29.6145 1.27E-11 NITROGEN 0.3108 14.6892 2.37E-11 METHANE 43 3.0901 39.9099 2.53E-08 CYCLOHEX 144.2 141.7496 2.4504 6.9231 134.8266 COMPOSITION 0.129087776 0.341697156 0.002645517 0.035925929 9.40E-14 0.064543888 0.169486165 0.002133012 0.028966138 1.76E-13 0.185025826 0.460488141 0.021203889 0.287947208 1.87E-10 0.620481908 0.028272673 0.972678483 0.645144701 0.998714685 BENZENE 0.000860585 5.59E-05 0.001339122 0.002016034 0.001285313

Base Case results Only 90 % of cyclohexane is recovered. Concentrations of cyclohexane and benzene emitted in the vapor stream of the flash (S3) are within the TLV standard of NIOSH. The third goal to maintain a minimum flow rate of 5.3 k-mol/hr in Stream 4 is not reached.

Internal design specification To obtain a high recovery of cyclohexane at the bottom product stream (S5) an additional design specification is established Adjustment of the flow rate of cyclohexane at the bottom product stream (S5) with reference to the bottom of the flash (S2) be 0.995 The variable is the duty of heater in the condenser. The other design specification is the reflux ratio be equal to 1:2 The variable is the duty of the heater in the reboiler. The total flowrate in Stream 5 is equal to 141.04 kg-mol/hr which indicates that 99.5 % of cyclohexane is recovered. The second goal to attain high recovery of cyclohexane is reached.

Internal design specification STREAM ID S1 S2 S3 S4 S5 NAME PHASE VAPOR LIQUID FLUID RATES, KG-MOL/HR 1 HYDROGEN 30 0.3855 29.6145 3.35E-11 2 NITROGEN 15 0.3108 14.6892 6.40E-11 3 METHANE 43 3.0901 39.9099 7.24E-08 4 CYCLOHEX 144.2 141.7496 2.4504 0.7088 141.0409 5 BENZENE 0.2 0.1952 4.85E-03 4.20E-03 0.1909 TOTAL RATE, KG-MOL/HR 232.4 145.7312 86.6688 4.4994 141.2318 TEMPERATURE, C 200 50 102.2286 199.5356 PRESSURE, ATM 13.33 ENTHALPY, M*KCAL/HR 2.2402 0.2559 0.0917 0.0131 1.2584 MOLECULAR WEIGHT 57.3255 82.3734 15.2082 26.4565 84.1548 MOLE FRAC VAPOR 1 MOLE FRAC LIQUID

Sensitivity Analysis 1 The independent variable for this parametric study is the variation of the temperature in the flash. Independent Variable: Flash T 10.°C to 50 °C Fixed Parameters: Feed Conditions & Column Conditions Dependent Variable: Flow rate of all components in the Vapor Stream (S4) In kg-mol/hr

Sensitivity analysis 1

Sensitivity Analysis 2 The independent variable for this parametric study is the variation of the pressure in the flash. Independent Variable: Flash P 10 to 35 atm Fixed Parameters: Feed Conditions & Column Conditions Dependent Variable: Flow rate of all components in the Vapor Stream (S4) In kg-mol/hr

Sensitivity analysis 2

External design specification Sensitivity analysis gave indications and feasibility of process modification (T is the best variable) The target of of 5.3 kg-mol/hr in the distillate or Stream 4 of the process is reachable An external design specification is established to this aim. The flow rate of 5.3 kg-mol/hr in the distillate is reached with out affecting the product quality ( purity of cyclohexane recovered at the bottom product stream).

Design specification Stream Name S1 S3 S2 S4 S5 Stream Description Phase Vapor Liquid Temperature, C 200 2.957733154 25.61639404 199.5356445 Pressure, ATM 15 0.99999994 13.32999992 Flowrate,kg-mol/hr 232.3999939 83.74118805 148.6588135 5.300059795 143.3587494 Composition HYDROGEN 0.129087776 0.354866177 0.001904261 0.053411685 6.42E-14 NITROGEN 0.064543888 0.175634116 0.001965511 0.055129658 1.56E-13 METHANE 0.185025826 0.465723932 0.02690541 0.754656792 2.39E-10 CYCLOHEX 0.620481908 0.003767717 0.967884004 0.135731429 0.99864918 BENZENE 0.000860585 8.07E-06 0.001340818 0.001070414 0.001350815

The complete process Stream manipulator  distillation column

Cyclohexane production process

Cyclohexane production

Process data Components: Hydrogen, Nitrogen, methane, Cyclohexane, benzene Thermodynamics: Equation of State (Peng Robinson) Reaction: Hydrogenation of benzene to cyclohexane with conversion 0.998 at T= 400°F

Reaction section Reaction hydrogenation with total conversion = 0.998; T= 400°F Feed 1 Gas: T=120°C - P= 335 psi – Rate 823.43 lbmol/hr H2= 802.73 – N2= 4.15 – CH4= 16.55 lbmol/hr Feed 2 Benzene: T=104.1°F - P= 300 psi – Rate 256 lbmol/hr pure benzene Mixer: no specifications Heater: process stream exit temperature = 300 °F

Separation and recycle Flash: temperature 120°F, pressure drop 5 psi Vapor recycle: Splitter: purge ratio (purged stream/feed to the splitter) = 0.08 Compressor: outlet pressure 382 psi Liquid recycle: Splitter: recycle ratio (recycle stream/feed to the splitter) = 0.30 pump: pressure rise 5 psi Separation section Pump: outlet pressure 200 psi Separator (stream calculator): Overhead temperature = 120°F - Bottom temperature = 120 °F Specifications recovery in overhead: H2=1, N2=1, CH4=0.8 Specifications recovery in bottom: Benzene=1, CyC6=1

Excercise Setup the Units, Description of the flowsheet, autosave off Name streams and units Components Thermodynamic specification SRK and UNIFAC Data retrieval Vapor pressure – Enthalpy of vaporization Verification of VLE for benzene – cyclo hexane Reaction specification Base case calculation Include a stream property table Verify: Total flow rate in overhead product S15 = 5.1 lb mol /hr Composition of Bottom product S16 of CyC6 = 0.996 Recovery of cyclo hexane in the separation section =