Mass Integration CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 9 – Synthesis of Mass Exchange Networks II February 7, 2013

The Pinch Diagram 1:6 Amount of Mass Transferred by Rich Streams

The Pinch Diagram 2:6 Constructing Rich Composite using Superposition

The Pinch Diagram 3:6 Amount of Mass Accepted by Process MSA’s

The Pinch Diagram 4:6 Constructing Lean Composite using Superposition

The Pinch Diagram 5:6 Constructing the Pinch Diagram Plot the two composite curves on the same diagram Pinch Point Move the lean composite vertically until the entire stream exists above the rich composite. The point closest to the rich composite is the Pinch.

DON’T TRANSFER MASS ACROSS THE PINCH! The Pinch Diagram 6:6 Decomposing the Synthesis Problem Creates two subregions, i.e. a rich end and a lean end Above the Pinch Mass exchange between rich and lean process streams No external MSA’s required Below the Pinch Both process and external MSA’s are used If mass is transferred across the pinch, the lean composite moves upward, thus: DON’T TRANSFER MASS ACROSS THE PINCH!

Example No. 2 1:17 Dephenolization of Aqueous Wastes

Example No. 2 2:17 Rich Stream Data Candidate MSA’s Two process MSA’s Three external MSA’s

Example No. 2 3:17 Process MSA Data External MSA’s Absorption using activated carbon (S3) Ion exchange using a polymeric resin (S4) Stripping using air (S5)

Example No. 2 4:17 Equilibrium Data General equation for transferring phenol to the j’th lean stream Minimum allowable composition difference

Excess Capacity of Process MSA’s Example No. 2 5:17 The Pinch Diagram Excess Capacity of Process MSA’s 0.1224 – 0.1040 = 0.0184 kg phenol/s Pinch Point y = 0.0168 x1 = 0.0074 x2 = 0.0100 External MSA Load 0.0124 kg phenol/s

Example No. 2 6:17 Removing Excess Capacity of Process MSA’s Can be eliminated by lowering the flowrate and/or outlet compositions of the process MSA’s. If elected to lower the flowrate of S2 then:

Example No. 2 7:17 Activated Carbon Adsorption isotherm is linear up to mass fraction 0.11 Above 0.11 activated carbon becomes saturated Thus x3t is taken at 0.11 Corresponding composition on y-scale: Less than supply compositions of R1 and R2 Thus feasible to transfer phenol from both streams to S3 Less than value of tail end of lean composite Hence S3 will not eliminate any phenol that can be removed by the process MSA’s.

Example No. 2 8:17 Activated Carbon (Continued) Cost of using activated carbon (A) Amount of activated carbon required to remove 1 kg of phenol can be calculated from a mass balance (B1) (B2)

Example No. 2 9:17 Activated Carbon (Continued) Multiplying equations (A) and (B2) provides the cost of removing 1 kg of phenol from the waste streams using activated carbon Substituting x3t = 0.11 kg phenol/kg activated carbon into (A) and (B2)

Example No. 2 10:17 Ion Exchange Regeneration Cost of using ion exchange (C) Amount of activated carbon required to remove 1 kg of phenol can be calculated from a mass balance (D1) (D2)

Example No. 2 11:17 Ion Exchange (Continued) Multiplying equations (C) and (D2) provides the cost of removing 1 kg of phenol from the waste streams using ion exchange (E) No mass should be transferred across the pinch Optimum target composition of S4 is the pinch composition y = 0.0168. Corresponding to: The higher the value of x4t, the lower the removal cost. So what is the highest possible value of x4t that can be used?

Example No. 2 12:17 Ion Exchange (Continued) Substituting x4t = 0.186 kg phenol/kg ion exchange resin into (C) and (E)

Thus thermodynamically feasible! Example No. 2 13:17 Air Stripping Based on the cooling duty of the phenol condensation unit, the cost of using air stripping is given as: (F) The outlet composition should be 50% of the Lower Flammability Limit (LFL) of 5.8 weight%: (G) Corresponding y-scale composition Less than supply composition of rich streams and pinch composition. Thus thermodynamically feasible!

And the winner is Activated Carbon!!! Example No. 2 14:17 Air Stripping (Continued) Since it is feasible to use air stripping for the phenol removal, the removal cost can be calculated Summary And the winner is Activated Carbon!!!

Example No. 2 15:17 Summary (Continued) Flowrate of activated carbon Minimum Operating Cost Ahead of design!!!

Example No. 2 16:17 Trading Off Fixed Vs. Operating Cost Minimum allowable composition differences can be used to trade off fixed vs. operating cost. When waste streams are mixed, the number of mass exchangers and, consequently, fixed cost decrease. On the other hand, mixing various waste streams normally increases the MOC of the system Waste stream data for mixing of waste streams

Example No. 2 17:17 Trading Off Fixed Vs. Operating Cost (Continued) New Pinch Diagram for mixed waste streams Values Obtained Pinch location as well as external MSA load is unchanged, i.e. so is the MOC. MOC = $288,000/yr

Screening External MSA’s 1:3 Questions How do we screen candidate external MSA’s? Is the cost of each MSA ($/kg recirculating MSA) a proper screening criterion? Example - Refinery Hydrogen Removal Need to remove 10 kg H2/hr from refinery gasses Two candidate MSA’s Absorption on sand (Cost $10-4 /kg sand) Absorption on activated carbon (Cost $1.0 /kg carbon)

Screening External MSA’s 2:3 Example - Refinery Hydrogen Removal (Cont’d) 1 kg of sand can remove 10-9 kg of H2 1 kg of activated carbon can remove 0.1 kg of H2

Screening External MSA’s 3:3 Proper Screening Criterion Removal cost: $/kg removed of targeted species Conversion to Removal Cost How to convert from $/kg MSA to $/kg removed? (xt – xs) kg of targeted species is removed per kg of the MSA IMPORTANT!!! (3.30)

No Process MSA’s 1:2 Screening the External MSA’s If C2r < C1r then eliminate S1 from the problem, as it is thermodynamically and economically inferior to S2 If C2r < C3r retain both MSA’s THERMODYNAMIC FEASIBILITY

No Process MSA’s 2:2 Constructing the Pinch Diagram

Example No. 3 1:4 Toluene Removal from Wastewater Flowrate of wastestream: G = 10 kg/s Supply composition of toluene: ys = 500 ppmw Target composition of toluene: yt = 20 ppmw Three external MSA’s Air (S1) for stripping Activated carbon (S2) for adsorption Solvent (S3) for extraction Removal Cost C1r = $0.38/kg C2r = $5.53/kg C3r = $43.90/kg

Example No. 3 2:4 Screening the MSA’s S2 and S3 are capable of removing the entire toluene load. S1 has limited removal capability Air is least expensive, thus S1 should be used to remove all the load to its right, i.e. 0.0045 kg/s Activated carbon is less expensive than the solvent, thus S2 should be used to remove the remaining load, i.e. 0.0003 kg/s

Example No. 3 3:4 Constructing the Pinch Diagram

Example No. 3 4:4 Minimum Operating Cost (MOC) Solution Flowrate of air: Flowrate of activated carbon: Assuming 8000 hr/yr:

Other Business Next Lecture – February 14 Graphical mass integration techniques SSLW pp. 297-308