L16 & 17- Statistical process control & Batch safety Flashcards
Batch advantage in process control & limitation
Check quality of individual batches and can be kept separate
However, sometimes physical form or functionality measurable only at end use
Quality control system components
A standard
Means of measuring required attribute
Comparison of actual performance with standard fir basis of corrective action
Variations come from
Variations inherent in raw materials
Variations in process conditions
Or both
Assignable factors
Know what caused the variation
Process capability & when established or valid
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
Stewhart Control Charts Overview
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
Stewhart Control Charts: Limits
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
Range charts
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
When is a process in control?
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
Indicators of problems & what to do after identified
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
Correcting out of control problems
Monitor control chart- indicate with confidence when changes to control needed to compensate equipment changes
Cumulative Sum (CuSum) Charts:
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)
Features of batch that increase scrutiny
Unsteady state ioeratuib
Increased operator interaction
Extreme conditions
Exotic materials
Complex chemistry
Seveso incident
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
Batch HAZOP
- 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
Example of deviations
Don’t charge A
Charge more A
Charge less A
Charge part of A
Reverse charge A
Charge A early
What has to be considered more for batch system safety?
Consider water and air interaction more for batch
Human error in batch processes & how to avoid
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
What is reaction runaway & what adds to this in batch systems?
Heat evolution rate of reaction greater than the rate of heat removed
(limited heat transfer capacities in batch vessels add to this hazard)
Reaction runaway graph
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)
Factors affecting Reaction runaway & heat removal/evolution
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
Differential thermal colorimeter
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
