1. Batch Processing Optimal Design and Scheduling
Chapter 22
Terry A. Ring
Chemical Engineering

2. 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., Surfactants and Additives
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

3. Batch Processes Batch Semi-batch or Semi-continuous
Fed-Batch
Feed continuously until you fill the tank, then complete the reaction, then remove, clean
Batch with product removal
Quickly fill the reactor, react, then remove product and clean
Optimal operating profiles
Recipe or sequence of tasks
Optimal control problems

4. Batch Reactor–start at t=0, => tfinal = minimum
Exothermic Batch Reactor
Determine the temperature profile that gives min. batch time given TU.
Plug Flow Reactor X
CAodXA/dt=-rA
FAodXA /dV=-rA
CAodXA/dτ=-rA

5. Batch Reactor – start at t=0
Determine the temperature profile that gives min. batch time given TU.
dr/dT=0
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
Vo=Initial Fill
vo= flow rate
V(t) dCA/dt+Cavo = voCAo+rAV(t)
(t)

7. Reactor Separator Sequence
Batch Reactor
AB (desired) C
XA=exp(-k1t)
XB=[exp(-k1t)-exp(-k2t)]*[k1/(k2-k1)]
XC=1-XA- XB
tB-opt= [1/(k2-k1)]*ln[k1/k2]

8. Batch Product Removal 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

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

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

11. Batch Times Reactor Separator Controlled by kinetics
Temperature
Concentration
Mixing
Separator
Controlled by minimum of
Heating rate or
Cooling rate
Which determines the distillate flow rate

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

13. Processing Sequences – book
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

14. 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

15. Example Distillation Task – Many Steps Reactor Task – Many Steps
Charge Still
Heat and Condense (may include heat up)
Run at R=∞
Run at R=C1
Switch Valve
Run at R=C2
Empty the Reflux Accumulator
Empty Reboiler
Cool down
Clean
Reactor Task – Many Steps
Charge reactor
Heatup
May include temperature following program
React for a given time
Cool down or quench
Empty Reactor
Clean

16. Cycle Time (CT) No parallel Tasks with batch times, tj
CT= max {tj} = max {2,6,4,3}=6
Max{2,6,4,3} =6
Parallel Tasks (nj in parallel)
CT= max {tj/nj} = max{2/1,6/2=3,4/1,3/1}=4

17. Multi-Product Processing Sequences
RePurposing the Plant
Product A
Task 1 –U1
Task 2 –U2, U3
Product B
Task 1 –U1, U2
Task 2 – U2

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

19. Costing a Sequential Batch Plant
Equipment
Cost EQ.
min (𝐸𝑞𝑢𝑖𝑝𝑚𝑒𝑛𝑡 𝐶𝑜𝑠𝑡)= 𝑗=1 𝑀 𝑛 𝑗 𝑎 𝑗 𝑉 𝑗 α 𝑗
𝐶𝑇 𝑖 ≥ τ 𝑖𝑗 𝑛 𝑗 product i cycle time
𝑖=1 𝑁 𝑄 𝑖 𝐵 𝑖 𝐶𝑇 𝑖 ≤𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝐻𝑜𝑢𝑟𝑠/𝑦𝑟
Qi= annual demand for product i.
nj= number of identical units.
Bi = product batch size (plant can produce several products, i’s)

20. 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 schedule batch plants for the production of multiple products.
Determine the cost of total plant equipment and profitability measures

21. HW
A batch process requires the following operations to be completed in sequence: 3 hr mixing, 5 hr heating, 4 hr reaction, 7 hr purification and 2 hr transfer/cleaning. When the five operations are carried out in vessels: U1, U2, U3, U4A and U5, respectively, determine the cycle times and construct Gantt charts corresponding to the zero-wait, intermediate storage and unlimited intermediate storage inventory strategies.
Unit/Hour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
U1 (3 hr) x Y` y
U2 (5 hr)
U3 (4 hr) Y
U4-A (7 hr)
U5 (2 hr)
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.