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

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Schedule 1121-MarSequential Batch Processing HW-7 Assigned Sequential Batch Processing 23-MarSequential Batch Processing 25-MarProject Management 1228-MarAIChE Contest Problem Kick OffDesign Contest Problems Assigned 30-Mar AIChE Contest Problem for 30 DaysMemo 1 Due - Planning Memo Memo 1, HW-7

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 Specialty Chemicals, Pharmaceuticals, Drug Fermentations, Electronic Materials, Optical Materials, Steel/Aluminum parts Mfg. Chemicals that are toxic or hazardous Very High Purity Chemicals, zone refined materials Long residence times, Exothermic Reactions Sometimes 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 – start at t=0 Determine the temperature profile that gives min. batch time given T U. X

Batch Reactor – start at t=0 Determine the temperature profile that gives min. batch time given T U. As C A decreases C B increases changing T opt

Fed-Batch Reactors Reactor Volume increases with time V(t)=V o +v o *t Mass Balance, C A0 = inlet First Order Rxn Solution (t)

Batch Reactor A  B (desired)  C X A =exp(-k 1 t) X B =[exp(-k 1 t)-exp(-k 2 t)]*[k 1 /(k 2 -k 1 )] X C =1-X A - X B t B-opt = [1/(k 2 -k 1 )]*ln[k 1 /k 2 ] Reactor Separator Sequence C

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

Batch Distill Example Mixture 1/3 each Methanol Water Propylene glycol Steps Total Reflux R=3, distill 5 lbmole/hr until x water =0.001 R=5, distill until x water =0.001 Methanol cut Total Reflux R=3 distill until x proypylene glycol =0.001 Propylene glycol cut “slop cut” Dump contents of Still Also trays Water

Batch Reactor A  B  C X A =exp(-k 1 t) X B =[exp(-k 1 t)-exp(-k 2 t)]*[k 1 /(k 2 -k 1 )] X C =1-X A - X B t B-opt = [1/(k 2 -k 1 )]*ln[k 1 /k 2 ] t S =V r C Ao (1-X C )/F d t tot =t R +t S Reactor Separator Sequence A +B C F d =Distillate Flow Rate

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

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

Processing Sequences – see articles for reading materials 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, t j CT= max {t j } = max {2,6,4,3}=6 Max{2,6,4,3} =6 Parallel Tasks (n j in parallel) CT= max {t j /n j } = max{2,6/2=3,4,3}=4 Cycle Time (CT)

Multi-Product Processing Sequences Re Purposing the Plant Product A Task 1 –U1 Task 2 –U2, U3 Product B Task 1 –U1, U2 Task 2 – U2

Multi-Purpose Plant –general use equipment Product A Task 1–U1, U2, U3 Task 2–U1,U2, U3 Product B Task 1–U1, U2, U3 Task 2–U1, U2, U3

Costing a Sequential Batch Plant Equipment Cost EQ.

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. Determine the cost of total plant equipment and profitability measures

HW 7-1 Unit/Hour U1 (3 hr)xxxY` yyy U2 (5 hr)xxxxxyyyyy U3 (4 hr)xxxxYyyy U4-A (7 hr)xxxxxxxyyyyyyy U5 (2 hr)xxyy Bottleneck is U4*** y – Zero Wait Y` - intermediate storage in U1 until U2 is ready Intermediate storage for the product of U4-A is possible but only if U5 can not handle the total volume of the U4 batch or you can design the plant to have a much smaller and less costly U5 and have it operate 6 of 7 hrs.