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

3. 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

4. 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

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

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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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)

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

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

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

17. 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