Introduction to Chemical Batch Processing Introduction to Chemical Batch Processing Introduction to Chemical Batch Processing

Introduction to Chemical Batch Processing


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Outline Characteristics of Chemical Batch Processes Analysis and Optimization of Chemical Batch Processes Examples for Sizing and Inventories


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Characteristics of Chemical Batch Processes Chemical Batch Processes: Definition of Terms Batch vs Continuous


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What is chemical batch processing? • The discontinuous, “charge wise” production of chemicals • Several units are designed to be started and stopped frequently (i.e. in a cycle-mode) for: Charging (fill with material) Task performing for a specified period of time Shutting down and draining (discharging) Cleaning • Combinations of batch and continuous mode (using holding tanks as an interface) are possible.


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Connecting Continuous and Batch Processes: Holding Tanks Feed holding tank Continuous stream for batch charge Batch Charge transferred once each cycle Optional stream for continuous feed

Reactor Product holding tank Reactor product transferred once each cycle


Continuous stream for reactor product

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Semi-Batch There are two classes of semi-batch processes: • Fed-batch processes with some or all chemicals being fed continuously during the processing (or some time of the processing). When the processing is finished the products are removed batchwise. • In batch-product removal the chemicals are fed to the process before processing begins, and then the product (or some of the products) is removed continuously as the processing occurs.


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(Some) History of Chemical Batch Processing

“In the early days of chemical reaction engineering (1950s) students might well have gained the impression that the ultimate mission of the chemical engineer was to transform old-fashioned batch processes into modern continuous ones…”

D.W.T. Rippin, 1983. Computers & Chemical Engineering 7: 137-156


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(Some) History of Chemical Batch Processing “…With such a perspective it would be surprising to find that, today, thirty years later, a significant proportion of the world’s chemical production by volume and a much larger proportion by values is still made in batch plants and it does not seem likely that this proportion will decline.” D.W.T. Rippin, 1983. Computers & Chemical Engineering 7: 137-156

“…With the recent trend of building small flexible plants that are close to the markets of consumption, there has been renewed interest in batch processes.” L.T. Biegler, I.E. Grossmann, A.W. Westerberg 1997. Systematic methods of chemical process design. Prentice Hall, Upper Saddle River. Introduction to Chemical Batch Processing

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(Some) History of Chemical Batch Processing


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Hierarchy of Decisions in Chemical Process Design 1. 2. 3. 4.

Batch versus continuous Input-output structure of the flowsheet Recycle structure of the flowsheet General structure of the separation system a) Vapor recovery system b) Liquid recovery system 5. Heat-exchanger network J.M. Douglas, 1988. Conceptual Design of Chemical Processes. McGrawHill, New York.


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Batch vs Continuous (Capacity aspects) 1. Production rate a. Sometimes batch if less than 10 ktonnes/year b. Usually batch if less than 1 ktonne/year 2. Market forces a. Seasonal production or uncertain demand pattern b. Short product lifetime c. The process setup/design has to be fast (market competition)


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Batch vs Continuous (Technical Aspects) 3. Scale-up problems a. Very long reaction times b. Handling slurries at low flow rates c. Rapidly fouling materials 4. Flexibility a. Operational problems b. Feedstock variations J.M. Douglas, 1988. Conceptual Design of Chemical Processes. McGraw Hill, New York Introduction to Chemical Batch Processing

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Batch vs Continuous (Conclusively) •

Because of their greater flexibility, batch plants are most common, when a large number of products can/should be produced in essentially the same processing equipment.

•

For seasonal products high storage cost arise when they are produced over the complete year.

•

Batch production is typically used for high-value added chemicals, e.g. pharmaceuticals, fine chemicals, pesticides, bio-products, foods, polymers etc. Introduction to Chemical Batch Processing

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Batch vs Continuous (Conclusively) “There are indeed some products, for which it is not possible or at least would be unreasonably demanding in time and resources, to develop reliable continuous processes. However, many more products which could be manufactured continuously are in fact made in batch plants on economic grounds.”

D.W.T. Rippin, 1983. Computers & Chemical Engineering 7: 137-156


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Outline Characteristics of Chemical Batch Processes Analysis and Optimization of Chemical Batch Processes Examples for Sizing and Inventories


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Analysis and Optimization of Chemical Batch Processes Design and Operation of Batch Process Units Design of Reactor-Separator Processes Dedicated, Multiproduct, Multipurpose Batch Plants Key parameters in the analysis of batch processes Debottlenecking strategies Introduction to Chemical Batch Processing

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The batch reactor exoth.

 → n2 B Reaction: n1 A ←  0 A

0 B

r=−

C , (C ) [mol / l ] Batch Charge transferred once each cycle

V [l ]

= k1o e

dC A = k1C An1 − k2CBn2 dt −

E1 RT

C An1 − k2o e

−

E2 RT

CBn2

Reactor

(A formulation of) Optimal Control Problem: Reactor product transferred once each cycle

Determine the profile of operating temperature T(t) that reaches a certain conversion (or yield) in the minimum batch time (τmin). nA0 − nA (t ) C A0 − C A (t ) XA = = nA0 C A0 ( density = const.)


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Optimal Control Problem (A general formulation of the) Optimal Control Problem: Determine the profile of (all or some) operating parameters (e.g. temperature, feed rate, removal rate, reflux ratio,…) to achieve a certain performance (e.g. minimum batch time for a given conversion, minimum batch size etc.)


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Coding in Matlab


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Results for the Isothermal case

450 K

CA(t)

470 K

530 K 510 K 490 K

t Introduction to Chemical Batch Processing

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Deriving the Optimum Profile The minimum batch time is achieved by applying a temperature profile that maximizes the reaction rate at each point in time: r = k1o e

−

E1 RT

C An1 − k2o e

dr = 0 ⇒ Topt = dT

−

E2 RT

CBn2

E2 − E1 CBn2 k2o E2 R ln n1 o C A k1 E1


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Deriving the Optimum Profile 900

1

850

0.9

800

0.8 750 Temperature [K]

solution Ca

0.7

0.6

700

650

0.5 600

0.4

550

0.3

0.2

500

450

0

500

1000

1500

2000

2500 time t

3000

3500

4000

4500

5000


0

500

1000

1500

2000

2500 time

3000

3500

4000

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4500

5000

Batch Distillation

A mixture of methanol, water and propylene glycol has to be separated using a batch distillation operation at normal pressure.


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Batch Distillation Operation: Methanol Recovery: 1) Bring the column to total reflux operation, with the distillate valve closed. 2) Using a constant reflux ratio distill with a constant rate to the methanol receiver. Continue until the mole fraction of water exceeds a specification. 3) Bring the column to total reflux. 4) Using a higher constant reflux ratio distill with a lower rate to the methanol receiver. Continue until the same water specification is reached. Introduction to Chemical Batch Processing

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Batch Distillation Operation: Propylene Glycol Recovery: 1) Bring the column to total reflux operation, with the distillate valve closed. 2) Using a constant reflux ratio distill with a constant rate to the water receiver. Continue until the mole fraction of propylene glycol exceeds the given specification. 3) Pump the contents of the still pot into the propylene glycol receiver.


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Optimizing a Reactor-Separator Process1 in Batch Mode The process consists of two units (with given capacities Vr, Vc) with unlimited intermediate storage between them:

1Barrera

et al., 1989. Chem. Eng. Comm., 82, 45-66.


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Optimizing a Reactor-Separator Process in Batch Mode Reactions are first order irreversible reactions following the kinetics:

The molar density C (mol/L) in the reactor is assumed to be constant.


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Optimizing a Reactor-Separator Process in Batch Mode The column is assumed to operate in a way that it produces perfect splits (A, then B, then C). The operation time to recover product B is simply given by:

where Fd (mol/hr) is the constant distillate rate.


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Optimizing a Reactor-Separator Process in Batch Mode The objective is to minimize the total cost of the campaign to produce a required amount of product Btot (mol) in a given horizon time Thor (hr). The following operational and cost factors are given: tci: cleaning time between batches for i-equipment (i=r, c) (hr) Pj: cost (or credit) of j-material (j=A, rA, C) ($/mol) rk: equipment rental rates (k=r, s, c) ($/hr) Ccli: equipment cleaning cost ($/batch) Pu: distillation utility cost ($/mol) Introduction to Chemical Batch Processing

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Optimizing a Reactor-Separator Process in Batch Mode Objective function


Equality and inequality constraints

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Optimizing a Reactor-Separator Process in Batch Mode Feasible region


Unfeasible region

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Optimizing a Reactor-Separator Process in Batch Mode


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What is needed to create a batch process ? •

A market, a demand pattern, and product requirements

•

A recipe (or: process step procedure), i.e. a list of physicochemical operations (tasks) and their duration

•

Available equipment units


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The Batch Process Recipe – an Example List of tasks to be completed: • • • • •

Mix raw materials A and B. Heat to 80 oC and react during 2 hours to form product C Add raw material D and react during 1 hour at 80 oC to obtain product E Mix with solvent F for 1.5 hours at ambient conditions; cool and age for 3.5 hours; E will crystallize Centrifuge for 2 hours to separate solid product E Dry in a tray for 1 hour at 60 oC.


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Available Equipment Units – an Example

3 reactors, 1 centrifuge, 1 dryer


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Challenges of Chemical Batch Processing The general flexibility of batch processes offers a wide range of possibilities. To take full advantage of this flexibility advanced planning and modeling tools are required for: • • •

Dynamic nature of operations/processes Dynamic nature of plant operation (demand patterns, new products, scheduling changes) Good manufacturing practice (GMP rules)

Not taking full advantage might have severe economic implications ! Introduction to Chemical Batch Processing

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Design Tasks for Batch Processes •

Which units in the flowsheet should be batch and which continuous?

•

Which processing steps should be carried out in which equipment with or without other steps?

•

When is it advantageous to use parallel batch units to speed up production?

•

How much intermediate storage is required, and where should it be located? Introduction to Chemical Batch Processing

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The Batch Process Layout – one Example


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Types of Chemical Batch Plants •

Single-product / dedicated plants: only one product

•

Multiproduct / flowshop plants: every product follows (approximately) the same sequence through all the process steps

•

Multipurpose / jobshop plants: each product follows its own distinct processing sequence by using the available equipment in a product specific layout; either only one production runs in the plant at a given time or many run concurrently


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Single Product / Dedicated Batch Plant

Similarities with continuous processes: • Only one product is produced • Highly automated • Several continuous operations Introduction to Chemical Batch Processing

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Multiproduct / Flowshop Batch Plant

In this type of plant every product follows the same sequence of operations and uses the same equipment units for (almost) all the process steps. Introduction to Chemical Batch Processing

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Multipurpose / Jobshop Batch Plant

Here each product follows its own distinct processing sequence by using the available equipment in a product-specific layout. Introduction to Chemical Batch Processing

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Design Levels for Multipurpose Batch Plant •

Plant design choice of equipment types, volumes and specifications choice of transfer and intermediate storage policy

•

Process Design selection of available equipment units allocation of operations to equipment units

•

Plant Operation campaign type and duration sequencing of products and scheduling Introduction to Chemical Batch Processing

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Definition of “Times” Occupancy time (OTj) of an equipment is the time that a stage taking place in this equipment needs to be completed (ti). If more than one stages take place in an equipment then OTj=∑ ti. Cycle time, CT= tf - ts

final time– initial time of a cycle

Batch time, BT

the time required to produce 1 batch

Makespan, MT (or Campaign time)

the time required to produce N batches


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Gantt Chart 1.5 hours

3.5 hours

Occupancy time (OTj) of an equipment is the time that a task taking place in this equipment needs to be completed (ti). If more than one tasks take place in an equipment then OTj=∑ ti. Each bar indicates the occupancy time of the corresponding equipment unit. Introduction to Chemical Batch Processing

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Occupancy times – one Example OT1=2 h

OT2=1 h

OT3=5 h


OT4=2 h

OT5=1 h

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Gantt Chart During transfer two equipment units are occupied at the same time for the same operation (i.e., material transfer).

1.5 hours

3.5 hours

OTj= ttrj-1/j+∑ tji +ttrj/j+1 Introduction to Chemical Batch Processing

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Modes of Processing Subsequent Batches •

Non-overlapping mode A subsequent batch is only started when the previous one is completed.

•

Overlapping mode Several batches are processed simultaneously; this reduces the idle (or dead) time of an equipment significantly.


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Modes of Processing Subsequent Batches


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Cycle Time

In overlapping mode the maximum occupancy time of all equipment units in a process defines the process cycle time. CT=max(OTj) Introduction to Chemical Batch Processing

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Modes of Processing Subsequent Batches

CT=11 h BT=11 h

CT=5 h BT=11 h


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Batch Time (BT) The time to produce one batch, from the start of the first task to the end of the last task.   BT = ∑  t trj −1/ j + ∑ ti j  j  i  ?

BT = ∑ ( OT j ) j

Overlapping: BT > CT Non-overlapping: BT=CT Introduction to Chemical Batch Processing

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Makespan or Campaign Time (MT) The time to produce N batches of a campaign. Cycle time

Overlapping: MT =(N-1)·CT+BT Non-overlapping: MT=N·CT=N·BT

Cycle time Cycle time

CT=5 hours, BT=2+1+5+2+1=11 hours MT=(3-1)·5+11=21 hours (for 3 batches) Introduction to Chemical Batch Processing

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Campaign Types in Flowshop / Jobshop batch plants •

Single Product Campaign (SPC) All batches of one product are manufactured before switching to another product (A......AABB.........B).

•

Mixed Product Campaign (MPC) The various batches are produced according to some selected sequence (......AABAABAAB.........).


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Makespan or Campaing Time (MT) CT=3*6+2*6+3+2=35

CTA=6h, CTB=6h CTAAABBB=35h BTA=13h, BTB=11h, BTAAABBB=43h MT=43h

MT=13+2*6+11+2*6-3-2=43 CTAB=6+3+2=11h CTAB=6+3+2=11h

CTAB=11h, (CTABABAB=33h) BTAB=13+11-5=19h MT=41h

MT=19+2*11=41 Introduction to Chemical Batch Processing

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Transfer and Storage Policies for Batch Plants Zero-wait (ZW) At any stage the material is transferred immediately to the next stage. No Intermediate Storage (NIS) It is possible to hold the material inside the production vessel. Unlimited Intermediate Storage (UIS) The batch can be stored without any capacity limit in vessels.


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Makespan or Campaing Time (MT)

CTUIS

 P  = max  ∑ Ni ⋅ OTij  j =1...T  i =1 

T: # of Tasks P: # of Products N: # of batches Introduction to Chemical Batch Processing

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Bottlenecks in Batch Processes


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Time limiting Equipment Unit / Task

The equipment (or stage) defining the cycle time is the time-limiting one. It represents a bottleneck. Introduction to Chemical Batch Processing

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Effect of Waiting Times

Szijjarto et al., 2008, Ind. Eng. Chem. Res. 47, 7323 Introduction to Chemical Batch Processing

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Volume limiting Equipment Unit / Task

The equipment (or stage) showing the highest capacity utilization (maximum task volume / equipment volume) is the volume limiting one. It also represents a bottleneck. Introduction to Chemical Batch Processing

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Volume limiting Equipment Unit / Task

If one equipment unit shows 100% capacity utilization, then the batch size Mb can not be increased with a linear scale-up of the production recipe. Introduction to Chemical Batch Processing

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Resource Bottlenecks Other resources can also limit the production capacity: • availability of raw materials • availability of utilities • size of storage tanks • workforce In most cases resource bottlenecks will have an impact on scheduling, hence they create time bottlenecks.


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Resource Bottleneck Example

Here resource bottlenecks will not be further investigated. Introduction to Chemical Batch Processing

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Productivity The productivity of a batch process per time (Pt) or per total equipment volume and time (Ptv) can be defined as follows:

Mb Pt = CT

Mb Ptv = CT ⋅Vt

The productivity can be increased by increasing the batch size (Mb) or by decreasing the cycle time and/or the total equipment volume used. Introduction to Chemical Batch Processing

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Increasing Productivity – Recipe Changes The productivity of a batch process can be increased by removing bottlenecks (debottlenecking) with regard to time and volume. Bottlenecks might be removed by changing the operation time of single tasks, in particular the time limiting one, or by changing the required operating volume of single tasks, in particular the volume-limiting one. These two possibilities can be achieved by changes in the production recipe, which needs additional research effort. Introduction to Chemical Batch Processing

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Increasing Productivity – Allocation Changes Both types of bottlenecks might also be removed by changing the allocation of tasks to the given set of equipment units or by providing additional equipment and identifying the new optimized allocation. These actions might either require an additional investment or reduce the design options for processes to be designed later in the same multipurpose plant.


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Debottlenecking with Regard to Time When debottlenecking a single process the following aspects should be considered: • equipment allocation time limiting task or unit multiple tasks in the same unit adding transfers (in-series design) • out-of-phase parallel design


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Recipe Example • • • • •

Mix raw materials A and B. Heat to 80 oC and react during 2 hours to form product C Add raw material D and react during 1 hour at 80 oC to obtain product E Mix with solvent F for 1.5 hours at ambient conditions; cool and age for 3.5 hours; E will crystallize Centrifuge for 2 hours to separate solid product E Dry in a tray for 1 hour at 60 oC


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Recipe Example

The tasks have been grouped (Tasks 1-5) and assigned to equipment units Introduction to Chemical Batch Processing

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Recipe Example The Gannt-chart section to the left has a duration of one cycle-time and shows parts of three batches. Because the sum of the durations of the tasks in Units 1 and 2 is shorter than the cycle time (2+1<5), these tasks may be conducted in the same unit without any increase in the cycle time. However, physicho-chemical reasons or required equipment specifications may not allow it. Introduction to Chemical Batch Processing

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Example Process - Layout #1


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Example Process - Gantt Chart of Layout #1

Now less equipment units (NoE=4) are required: • reduced investment cost • increased plant flexibility (more available equipment for the design of new processes) Introduction to Chemical Batch Processing

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Adding Transfers & Equipment

The time-limiting task in the example process is actually a combination of multiple tasks: 3a. Mix with solvent F for 1.5 hours at ambient conditions 3b. Cool and age for 3.5 hours; E crystallizes If physicho-chemical constraints allow it, a transfer might be added between the two tasks. Introduction to Chemical Batch Processing

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Now again three reactors are used (NoE=5), but CT=3.5 hours and thus productivity per time (Pt) has been increased.


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Analysis of Layout #2

In Layout #2 the time limiting task is the crystallization (Task 3b). This task contains only one single unit operation, hence no transfer can be added during this task. As the cycle time is now determined by a single task, adding transfers elsewhere would just increase the number of units without improving the productivity.


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In-series Design Conducting a transfer during a unit operation leads to an in-series design, i.e. the same operation is conducted in several equipment units in-series. This reduces the cycle time, increases the productivity per time, but again requires more equipment units. Many unit operations can not be conducted in an in-series mode because of physico-chemical constraints, e.g. crystallization.


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Out-of-phase Parallel Design When unit operations such as crystallization are time-limiting, the cycle time can be further reduced by using equipment units in parallel in an out-of-phase design, e.g. batch #1 uses equipment A, while batch #2 uses equipment B, and so on. For nj parallel equipment units per unit operation j the cycle time becomes:  OT j CT = max   nj 


  

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Now four reactors are used (NoE=6), but: CT= 3 hours if tasks (stages) 1 and 2 are grouped CT= 2.5 hours if tasks 2 and 3a are grouped CT= 2 hours if all tasks are ungrouped (NoE=7) Introduction to Chemical Batch Processing

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Debottlenecking with Regard to Time: Summary of Results


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Debottlenecking with Regard to Volume When debottlenecking a single process the following aspects should be considered: • equipment allocation improving the capacity utilization assigning a larger unit • cycle multiplication (repeat task multiple times) • in-phase parallel design


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Recipe Example 1. Mix raw materials A and B. Heat to 80 oC and react during 2 hours to form product C (3 m3) 2. Add raw material D and react during 1 hour at 80 oC to obtain product E (4.2 m3) 3. 3.a Mix with solvent F for 1.5 hours at ambient conditions (9.7 m3) 3.b Cool and age for 3.5 hours; E will crystallize (9.4 m3) 4. Centrifuge for 1.5 hours to separate solid product E (0.42 m3) 5. Dry in a tray for 1 hour at 60 oC (0.42 m3) Product obtained 0.3 m3. Introduction to Chemical Batch Processing

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Available Equipment Units Reactors: • R1: 10 m3 • R2: 10 m3 • R3: 16 m3 • R4: 16 m3 Centrifuge C1: 0.44 m3 (solid) Dryer D1: 2 m3 (solid)


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Volume Analysis of Layout #2

A scale-up of the recipe should be conducted so that for at least one equipment unit the capacity utilization is equal to 100%.


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Volume Analysis of Layout #2 – Scale-up

Some operations may require more time after scale-up!


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Volume Analysis of Layout #2


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Assignment of Larger Equipment Units

In general, the tasks requiring large volumes should be conducted in the largest available units. Considering the example process after scale-up, Task 3a utilizes 100% of the R2 capacity. However, a larger reactor (R4) is available. Replacing R2 by R4 (Layout #2a) leads only to a small increase in productivity because then centrifuge C1 is volume-limiting.


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Volume Analysis of Layout #2a


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Equipment Time Utilization


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Equipment Time Utilization – Cycle Multiplication


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Cycle Multiplication


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In-phase Parallel Design


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Optimizing Cycle Time and Batch Time


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Optimizing Cycle Time and Batch Time


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Debottlenecking with Regard to Time and Volume: Summary of Results (Pareto front) 4.5 4.0

Ptv-eq

3.5 3.0 2.5 2.0 1.5 1.0 30.0

50.0

70.0

90.0

110.0

130.0

150.0

170.0

Pt


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Special Characteristics of Batch Processes


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Special Characteristics of Batch Processes


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Storage / Inventory •

Storage capacities have to be planned and managed well.

•

Cost factor.

•

Planning required, i.e. matching raw material availability and customer requests.

•

Supply chain management is important.


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Structure of a Multipurpose Batch Plant


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Structure of a Multipurpose Batch Plant


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Typical Equipment Units in Batch Plants


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Literature


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Introduction to Chemical Batch Processing Introduction to Chemical Batch Processing Introduction to Chemical Batch Processing

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