Rate-Based Concepts Best Practices for When and How Mass Transfer is - - PowerPoint PPT Presentation

Rate-Based Concepts Best Practices for When and How Mass Transfer is

Applied in a Column Simulation

Peter Poellmann, AQSim Europe

OLI Simulation Conference 2014 22 October 2014

http://www.sulzer.com/en/ Products-and-Services/ Separation-Technology/ Distillation-and-Absorption/Distillation

Vapor to condenser Reflux from condenser Liquid feed Liquid to reboiler Vapor from reboiler

Distillation Column

Manway Manway Liquid distributor Structured packing Random packing Trays Liquid collector with packing support Support device Liquid distributor

Q1 Q2 Qj QN-1 QN Stage 1 2 Stage j N-1 N F1 F2 Fj FN-1 FN W2 W3 Wj+1 WN-1 WN Wj U1 U2 Uj Uj-1 UN-2 UN-1 L1 Lj-1 LN-2 LN-1 V2 V3 Vj+1 VN

Modeling Distillation

F feed V vapor L liquid Q heat U liquid side product W vapor side product Countercurrent cascade of stages – generic model LN V1

Q1 QN 2 Stage j N-1 F U1 L1 Lj-1 LN-2 LN-1 V2 V3 Vj+1 VN

Modeling Distillation

F feed V vapor L liquid Q heat Countercurrent cascade of stages –

• rdinary distillation

column LN V1 V1 vapor distillate product U1 liquid distillate product L1 reflux LN bottom product Q1 condenser duty (-) QN reboiler duty (+)

1 2 Stage j N-1 N F1 FN L1 Lj-1 LN-2 LN-1 V2 V3 Vj+1 VN

Modeling Distillation

F feed V vapor L liquid Countercurrent cascade of stages – absorption column LN V1 FN rich/dirty feed gas F1 fresh liquid absorption medium V1 lean/clean off gas LN product/spent absorption medium

Qn Stage n yi,n HV,n Tn pn yi,n+1 HV,n+1 Tn+1 pn+1 xi,n HL,n Tn pn xi,n-1 HL,n-1 Tn-1 Pn-1 Wn Un Ln Vn Fn zi,n HF,n TF,n pF,n

Equilibrium Stage

Tn, pn, xi,n and yi,n are related by Vapor- Liquid Equilibrium VLE F feed V vapor L liquid Q heat U liquid side product W vapor side product H enthalpy T temperature P pressure x,y,z liquid, vapor or feed compositions i index for component n index for stage number Ln-1 Vn+1 => MESH equations

Problems Usually Encoutered in Dealing with Distillation Models

• Column solving algorithm usually fails to converge on

infeasible specifications of separation or numerics

• Solver may diverge even on feasible specifications of

the separation job, caused e.g. by numerical trouble, missing estimates, many stages, large flow feeded

• Feasibility of separation non-trivial to ensure, e.g. in

cases of pinch conditions or distillation boundaries

• Divergence behaviour rarely helpful for correction
• Among different specifications, some may lead to

trouble, while others may run fine

• Still today, automated methods for doing distillation

design, e.g. finding numbers of stages, locations of feeds or side-stream draw-offs, are not at hand

QV,n Segment n yi,n HV,n TV,n pV,n yi,n+1 HV,n+1 TV,n+1 pV,n+1 xi,n HL,n TL,n pL,n xi,n-1 HL,n-1 TL,n-1 PL,n-1 Ln Vn FL,n xF,i,n HL,F,n TL,F,n pL,F,n

Mass-Transfer Segment

• Bulk and film regions

are distinguished in each phase

• Films have contact

along a V-L interface

• Each bulk has its own

feed of material and heat

• Material and heat fluxes

across interface are defined

• Exiting streams are not

related by VLE – rather are conditions at V-L interface N material flux q heat flux IF vapor-liquid interface Ln-1 Vn+1 FV,n yF,i,n HV,,F,n TV,F,n pV,,F,n QL,,n qV,n qIF,n qL,n NV,n NIF,n NL,n Bulk vapor Bulk liquid Vapor film Liquid film xIF,i,n yIF,i,n TIF,n V-L interface => MESHNQ equations

When is Mass-Transfer Applied in a Column simulation ?

• When a good VLE model of the physicochemical

system is at hand, then the rate-based model is a useful fine-tuning of the column simulation.

• Primary motivation for applying mass-transfer in a

column simulation is design of the height of the equipment.

• Several mass-transfer devices, e.g. random packing

elements, structured packing, or trays, can be rated against each other, provided m-t coefficients exist.

• Diameter and height lead to volume of packing, thus

cost comparisons can be done.

• Generally, mass-transfer is applied whenever product

purities or emission limits need to be respected.

• Hydrochloric acid, i.e. Water / HCl

– xy Diagram – Separation Factor – Acid concentration column (vacuum distillation) – Desorption / Specific energy consumption

• Water / HCl / Chlorine

– Solubility of Chlorine in Water – … in hydrochloric acid

Examples for Significance

• f VLE for Distillation

There is no point trying to apply a rate-based model for distillation, until the VLE is done properly

Hydrochloric Acid / boiling point

1.013 bar

• 100
• 50

50 100 150 200 0,05 0,1 0,15 0,2 0,25 0,3 0,35 x_HCl (kmol/kmol) t_bub (°C) Aspen ElecWiz Aspen EHCLFF AspenOLI MSE AspenOLI aq. OLI reference

HCl Acid / xy Diagram / Pressure

6 bar 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5 x_HCl (kg/kg) y_HCl (kg/kg) Aspen ElecWiz Aspen EHCLFF AspenOLI MSE diag

HCl Acid / Separation Factor / Pressure

6 bar 0,01 0,1 1 10 100 1000 10000 100000 1000000 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5 x_HCl (kg/kg) alpha Aspen ElecWiz Aspen EHCLFF AspenOLI MSE

HCl Acid / Desorption Column

Acid Flowback ~ 18% HCl Feed Acid 32% HCl by wt. HCl Gas Product

Motivated by the separation factor, the OLI MSE model should be applied for desorption of HCl gas from hydrochloric acid at elevated pressure.

HCl Acid / Desorption / Reboiler Duty

6 bar 0,7 0,75 0,8 0,85 0,9 0,95 1 1,05 1,1 1,15 1,2 0,9 0,91 0,92 0,93 0,94 0,95 0,96 0,97 0,98 0,99 1 D / D_max Qr / D (kWh/kg) Aspen ElecWiz OLI MSE

Desorption of HCl gas from acid 32% wt. using 10 equilibrium stages at 6 bar(a). Qr reboiler duty (kW) D product gas flow (kg/h) D_max assumes desoption down to azeotrope

Specification-to-Column-Design Workflow

1. Assume desired separation job as feasible 2. Set up and experiment with equilibrium-staged model, finally put solver loops (design-specs) in, ensuring product purities met all the time (by separate investigation, make sure such loops work robustly) 3. Select numbers of stages and feed locations (integer variables), then specify continuous variables (like, e.g. flow rates, temperatures of feed streams, pressures) 4. Run equilibrium-stage based, analyse, (if necessary, go back to 3.) – until staged column configuration and L, G (i.e. operating point) are fixed 5. Select internals, do hydraulic design, i.e. find diameters of every section 6. Set up for rate-based, e.g. numbers of mass-transfer segments, internals, bed heights, numbers of trays, not to forget numerics of model 7. With solvers (2.) still in, run mass-transfer based model, vary bed heights

• r tray numbers for minimal energy consumption

8. Establish a cost function for investment of column, decide about energy consumption cost, establish a total cost objective function 9. Vary type of internals, calculate volumes of packed sections, evaluate the total cost objective function (if too large, go back to 7) 10. Design column periphericals, e.g. reboiler, condenser, pumps, valves

Aspects of Applying Mass-Transfer in Column Simulations

• Have good VLE model at hand, since m-t model is like

fine tuning on eq.-stage based solution

• See that eq.-stage based model converges robustly,

before attemptimg to go for m-t

• Get and accept information on empirical correlations

involved for :

– Mass-transfer coefficients – Heat-transfer coefficients – Diffusion coefficients – Interfacial area – Pressure drop

Applying Mass-Transfer in Column Simulations – Tricks of the Trade

• While a variation of number of stages is hardly ever found in simulators

(some can vary down), the m-t model allows for some kind of design by variation of height for fixed number of segments (but see also next point)

• Do not over-do with number of segments – rule of thumb is height of

segment should not exceed 1/10 of nominal size of packing element – however insert more segments in sections of significant change, look at profiles and go for smoothness

• Complex columns with pumparounds may converge better overall, after

being cut into pieces made of purely countercurrent or pumped-around sections, even though a flowsheet tear stream is created

• If a specific operating point will not converge at all, then try to specify

another, easier-to-conerge, maybe even trivial, operating point, and establish a homotopy from the latter to the former (simulators keep results obtained in a run as starting values for the next run)

• „No estimates are better than bad estimates“ – they should be inside the

range of variation the model will probably take – it can be severly disturbed, if estimates are infeasible, e.g. forgotten from earlier runs

Typical Applications with Mass-Transfer from the Experience of the Author

• Adiabatic absorption of HCl from gases in

countercurrent or recirculated packed beds, for keeping emission limits, e.g. TA Luft

• Non-adiabatic absorption of greater amounts of HCl

from synthesis or other flue gases, for production of concentrated acid, and keeping emission limits at the same time

• Simultaneous removal of HCl and Chlorine from flue

gases of chlorinated chemical waste incinerations

• Judgement of random or structured packed bed

efficiencies during desorption of HCl gas from acid

• Checking preconditions for aerosol formation in HCl

absorbers after incineration units

Discussion

• Questions
• Other experiences

Backup

HCl Acid / Azeotrope / Temperature

HCl-water azeotropic T-p 50 100 150 200 250 2 4 6 8 10 12 14 16 18 20 pressure (bar) temperature (°C) Aspen ElecWiz Aspen EHCLFF OLI MSE Ullmann

HCl Acid / Azeotrope / Composition

HCl-water azeotropic x-p 0,075 0,1 0,125 0,15 0,175 0,2 0,225 0,25 2 4 6 8 10 12 14 16 18 20 pressure (bar) HCl mass fraction (kg/kg) Aspen ElecWiz Aspen EHCLFF OLI MSE Ullmann Römpp Gmelin's

Separation Factor / Definition

αij = Ki / Kj = yi/xi / yj/xj The separation factor is a key figure for the change of composition of a chemical mixture by a technical process, by separation in

• particular. The ratio is usually chosen for a

value of greater than unity. A large value denotes good separability of components i and j. In terms of distillation, the separation factor is defined via the compositions in liquid and vapor phases. A value of unity indicates the separation technique is infeasible. This is the case at the azeotropic point, where distillation fails.

HCl Acid / xy Diagram / Vacuum

0.1 bar 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5 x_HCl (kg/kg) y_HCl (kg/kg) Aspen ElecWiz Aspen EHCLFF AspenOLI MSE diag

HCl Acid / Separation Factor / Vacuum

0.1 bar 0,0001 0,001 0,01 0,1 1 10 100 1000 10000 100000 1000000 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5 x_HCl (kg/kg) alpha Aspen ElecWiz Aspen EHCLFF AspenOLI MSE

HCl Acid / Concentration Column

Waste Water ~ 1% HCl (vacuum) Feed Acid 18% HCl by wt. Product Acid 22% HCl

Motivated by the separation factor, the OLI MSE model should be applied for concentrating hydrochloric acid under vacuum.

Water / Chlorine

Solubility of Chlorine in Water 2000 4000 6000 8000 10000 12000 14000 16000 10 20 30 40 50 60 70 80 90 t / °C ppm Cl2 Aspen ElecWizard Oli aqueous Oli MSE Schönfeld (1855) Perry (50th edn.) Winkler (1907)

Water / HCl / Chlorine / 25°C

Solubility of Chlorine in Hydrochloric Acid (25 °C) *) Oliveri-Mandala data hold for 20 °C 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 5 10 15 20 25 30 35 HCl (mass-%) Cl2 (ppm) Aspen ElecWizard Oli aqueous Oli MSE Oliveri-Mandala (1920) Sherrill-Izard (1928) Curda-Holas (1964)

Water / HCl / Chlorine

Solubility of Chlorine in Hydrochloric Acid solid lines calculated using AspenPlus standard electrolyte model (2006.5) 5000 10000 15000 20000 25000 30000 10 20 30 40 50 60 70 80 90 t (°C) ppm Cl2 0 % HCl 10 % HCl 20 % HCl 32 % HCl Perry (50th edn.) - 0% Curda-Holas (1964) - 10% ... - 20% ... - 32%

Water / HCl / Chlorine

Solubility of Chlorine in Hydrochloric Acid solid lines calculated by Oli MSE model 5000 10000 15000 20000 25000 30000 10 20 30 40 50 60 70 80 90 t (°C) ppm Cl2 0 % HCl 10 % HCl 20 % HCl 32 % HCl Perry (50th edn.) - 0% Curda-Holas (1964) - 10% ... - 20% ... - 32%