L16 & 17- Statistical process control & Batch safety

Question 1

Batch advantage in process control & limitation

Answer 1

Check quality of individual batches and can be kept separate

However, sometimes physical form or functionality measurable only at end use

Question 2

Quality control system components

Answer 2

A standard

Means of measuring required attribute

Comparison of actual performance with standard fir basis of corrective action

Question 3

Variations come from

Answer 3

Variations inherent in raw materials
Variations in process conditions
Or both

Question 4

Assignable factors

Answer 4

Know what caused the variation

Question 5

Process capability & when established or valid

Answer 5

Standard deviation from mean that process is capable of achieving as it is currently operaing

Establish process capability when the process is under control and no attributable factors affecting samples

Check if values fit a normal distribution

Quality depends on many variables, subject to random variation

Question 6

Stewhart Control Charts Overview

Answer 6

Illustrate current process’ performance against standard - to identify when process not in control

Plots sample values against time

Values represent samples from same batch or same time

Question 7

Stewhart Control Charts: Limits

Answer 7

Warning limits: no more than 1/40 expected outside if under control - 2x standard deviation

Action limits: no more than 1/1000 expected if under control - 3x standard deviation

Question 8

Range charts

Answer 8

Record range of values in a set- difference of largest and smallest in set of samples

Action and warning limits and mean set - only high values of range are concern

Distinguish between normal, random variation and genuine operating problems - indicate when action needed

Question 9

When is a process in control?

Answer 9

No points outside action limits
No. of points above and below average equal
Points fall randomly above and below average
Most points near average line

Question 10

Indicators of problems & what to do after identified

Answer 10

One point outside of action limits
Series of points outside one warning limits
Points unevenly distributed around average line

Once out of control condition identified, look for assignable factors

Question 11

Correcting out of control problems

Answer 11

Monitor control chart- indicate with confidence when changes to control needed to compensate equipment changes

Question 12

Cumulative Sum (CuSum) Charts:

Answer 12

Info from previous values included in new information plotted for each point

Effective at detecting small process changes

S value plot, if process average increases -> general rise of CuSum

CuSum chart: gradient allow actual process average calculated- positive gradient = higher average (> k)

Question 13

Features of batch that increase scrutiny

Answer 13

Unsteady state ioeratuib
Increased operator interaction
Extreme conditions
Exotic materials
Complex chemistry

Question 14

Seveso incident

Answer 14

Vessel steam heated, with no automatic temp control/indicator
-> runaway reaction started so T&P increased leading to rupture and discharge of contents

Decisions led to disaster: operators not cooling vessel before leaving it
No weekend working so batches incomplete

Question 15

Batch HAZOP

Answer 15

• Look at process flowsheet one line at a time
• Process variables paired with guid words (none, more) to produce deviations
• If credible causes of deviation, consequences examined and actions recommened

Question 16

Example of deviations

Answer 16

Don’t charge A
Charge more A
Charge less A
Charge part of A
Reverse charge A
Charge A early

Question 17

What has to be considered more for batch system safety?

Answer 17

Consider water and air interaction more for batch

Question 18

Human error in batch processes & how to avoid

Answer 18

Batch likely to involve more operator intervention;

More potential for operators to come into contact with process materials by sampling, dosing, transfer, cleaning

Avoid by training and good design of equipment/instructions

Question 19

What is reaction runaway & what adds to this in batch systems?

Answer 19

Heat evolution rate of reaction greater than the rate of heat removed

(limited heat transfer capacities in batch vessels add to this hazard)

Question 20

Reaction runaway graph

Answer 20

Rate if heat produced/removed against temp of reaction mixture:

Cross at T1,T2: max rate of hear removal equal to rate of heat generation

Temp below T2: max rate of heat removal > heat generation so temp fall to T1

Temp controlled between T1 and T2: by reducing cooling water flwo until rate of removal = generation (T1 = max cooling flow)

Operation at T1: stable as any rise/fall in temp corrected naturally without change to cooling water flow

Exceed T2: heat generation rate exceed max heat removal rate and temp continue to rise and be out of control (runaway)

Question 21

Factors affecting Reaction runaway & heat removal/evolution

Answer 21

Fouling & loss of cooling water pressure will reduce heat removal capacity

Loss of agitation will increase reaction rate, and when agitation is restarted -> increase heat evolution rate

Question 22

Differential thermal colorimeter

Answer 22

Detect unanticipated reactions and sets safe temp range

One well and one control well of same thermal capacity heated- difference in temp between them as a function of time/temp indicates exothermic decomposition of sample