Batch Processing Optimal Design and Scheduling Chapter 11 Terry A. Ring Chemical Engineering

Chemical Processes Continuous Processes – AspenPlus Large production rates Commodity chemicals Bulk chemicals, plastics, petroleum, paper Batch Processes – Aspen BatchPlus, BatchSep Multiple products from a single plant Contract Manufacturing Small and or intermittent production rates Speciality Chemicals, Pharmaceuticals, Electronic Materials, Optical Materials Chemicals are toxic or hazardous Very High Purity Chemicals Long residence times, Exothermic Reactions Multiple process steps in the same tank

Batch Processes Batch Semi-batch Semi-continuous Fed-Batch Feed continuously, then remove, clean Batch-product removal Mixing and removal are combined Optimal operating profiles Recipe or sequence of tasks Optimal control problems

Batch Reactor Determine the temperature profile that give min. batch time given TU. As CA decreases CB increases changing Topt

Fed-Batch Reactors Reactor Volume increases with time V(t)=Vo+vo*t Mass Balance, CA0 = inlet First Order Rxn Solution

Batch Product Removal Batch Distillation (Aspen BATCHFRAC) Still Charged Heat on Lights distill out first Control needed to Maximize Reflux ratio increases with time Condenser collector changes Pressure changes Heavies left in reboiler Reflux control Timed Reboiler dumps

Reactor Separator Sequence Batch Reactor ABC XA=exp(-k1t) XB=[exp(-k1t)-exp(-k2t)]*[k1/(k2-k1)] XC=1-XA- XB tB-opt= [1/(k2-k1)]*ln[k1/k2] tS=VrCAo(1-XC)/Fd ttot=tR+tS A +B Fd=Distillate Flow Rate C

Batch Times Reactor Separator Controlled by kinetics Temperature Concentration Mixing Separator Controlled by minimum of Heating rate or Cooling rate

Cases Vr>>Vc  multiple distillation batches Storage Tank for Reactor Product = Vr Storage Tank for Products Vc>>Vr  multiple reactor batches Storage Tank for Reactor Product = Vc tr>tc  Reactor is idle Add more reactors to balance the unit times tc>tr  Distillation column is idle Add more distillation columns or a larger column to balance unit times Available equipment sizes determine the batch volumes and times

Processing Sequences Optimal Batch Times Recipe of Tasks Numerous Batch Steps Each with a program and tasks to follow Each with a given batch size Task Integration Sequence of steps that take place in one piece of equipment Gives batch time determined by batch size and processing time Optimum Cycle times for Recipe (sequence of batch steps) Using rates of production & yields, vessel sizes are determined to minimize the cost of the plant and determine the cycle times for a given recipe

Production Line Set list of equipment items assigned to tasks Tanks, mixers, reactors, separators Equipment can be used for two or more tasks, if free Prevents contamination Cycle time = time between the completion of batches Schedule of Production = Gantt Chart of the flow of material from start to finish showing times for each task in recipe Bottleneck = unit having longest batch time Zero-wait strategy = no intermediate storage tanks

Example Distillation Task – Many Steps Charge Still Heat and Condense (may include heat up) Empty the Reflux Accumulator Empty Reboiler Clean (may include cool down)

No parallel Tasks with batch times, tj CT= max tj Parallel Tasks (nj in parallel) CT= max tj/nj

Multi-Product Processing Sequences Product A Task 1 –U1 Task 2 –U2, U3 Product B Task 1 –U1, U2 Task 2 – U2

Multi-Purpose Plant Product A Product B Task 1–U1, U2, U3

What you need to know. Know about process units executed in batch mode and approaches for optimization of their design and operation Know how to determine the optimial reaction for a batch reactor- separator process. Be able to schedule recipes for the production of a single chemical product. Understand how to scheudle batch plants for the production of multiple products