Operation during transitions

Transcription

1 We will learn about operation Transitions involve planned transient behaviour between different initial and final conditions. Continuous processes Startup and shutdown Regeneration Blocked operation Load following Batch Processes No steady-state operation 1

2 Startup and shutdown Special and procedures are required for starting and stopping process operations. These are when most accidents and serious hazards occur - be very thorough in planning and training Need to load material startup and drain material for shutdown. Need to heat and/or cool to approach normal process conditions or return to ambient. 2

3 Startup and shutdown Equipment and process structure: Identify extra needed for startup of the exothermic reactor. FC 1 Cold product What must be added to start up? How will we control it? Cold feed Fast dynamics High gain Linear Symmetric Safe Hot effluent 3

4 Startup and shutdown Equipment: Identify extra needed for startup of the distillation column reboiler. Bottom tray Thermsiphon reboiler Bottoms product This is what the column operation looks like at full operation. But what about startup, when there is no vapour flow back up the column? 5

5 Startup and shutdown Equipment: Identify extra needed for startup of the distillation column reboiler. Bottom tray Before reboiler is functioning, no vapour flows occur, and the liquid weeps through trays Thermsiphon reboiler Bottoms product Valve is normally closed, opened only startup From: Lieberman, N.L., Process Design for Reliable Operations, Gulf Publishing, Houston,

6 Startup and shutdown Is the bypass valve required for these configurations? 7

7 Startup and shutdown Occasionally, must be shutdown for maintenance and modifications. What is required? Unit B Unit A 8

8 Startup and shutdown Occasionally, must be shutdown for maintenance and modifications. What is required? Unit B Unit A 9

9 Startup and shutdown Industrial practice The first step is to prepare a detailed startup (shutdown) procedure. Then, we check the availability of the appropriate to perform the procedure This is very detailed work and requires considerable experience in plant operation and plant (Talk with operators and shift supervisors.) 13

10 Regeneration Catalyst, adsorbents, fouled surfaces and some other require periodic regeneration. This can involve different materials flowing through the process and even in opposite directions. The switching period varies from hours to months. Example of regeneration include the following. Catalyst that loses activity Adsorbent that has active sites filled Equipment that has surface coated due to coke (reactor) or polymer (e.g. reboiler) Filter medium that must be backflushed (e.g. membrane) 14

11 Regeneration Regeneration involves different operating conditions that can challenge or be hazardous. Different feeds are required. Must have extra valves/ports. Hazards, such as introducing oxygen into an environment that normally contains hydrocarbons. Different operating conditions (e.g. high temperature steam is used) Special transition can involve corrosion, contamination, hygiene, toxicology, etc. 15

12 Regeneration We don t want to shut down the entire plant when regenerating an individual unit. How can we best maintain the continuous process operation? Parallel with isolation valves Recall the one-way valve discussion from the prior section Single with sufficient storage on both sides 16

13 Regeneration Example of olefins plant Ethane C 2 H 6 Propane C 2 H 4 C 3 H 6 Gas oil C 5 + C 4 H 10 Feed stocks Fired heaters crack the hydrocarbons Quench The operation of the heaters (reactor feed type, feed rate, temp, steam) influence the rate of coke and need for regeneration. Separation system Coke buildup: Requires periodic shutdown/decoke 17

14 Ethane Regeneration Example of olefins plant X Air and steam Exhaust safely C 2 H 6 Propane X C 2 H 4 C 3 H 6 Gas oil C 5 + C 4 H 10 Feed stocks Fired heaters crack the hydrocarbons Quench Separation system What happens when one furnace temporarily stops production for decoking (with air and steam)? Which of previous strategies is employed (parallel or storage)? Significant hazards can occur decoking 18

15 Regeneration Temperature (C) Tube wall temperature : reactor Time slots (3 days each) = decoke Example trend for one furnace The operators monitor the tube wall temperature. Roughly every 45 days the furnace is decoked 19

16 Regeneration Example heat exchanger is fouled and must be cleaned. Cold process fluid Hot process fluid (not a utility) TC Since both streams are process fluids, by-passes on each are required. If one were a utility (e.g., water or steam), no by-pass on the utility stream would be required. 21

17 Batch operations Batch operation Often the most economical method for manufacturing small quantities and very high purities. Is generally too expensive for producing very large quantities of material. A batch plant usually produces numerous products Each product is manufactured in a separate campaign involving unique feed materials and processing conditions and shared. The process followed is called a recipe Food, pharmaceuticals, fine chemicals, 33

18 Batch operations 34

19 Batch operations Batch operation: All materials provided at start of the process. P-3 P-1 feed time V-1 E-1 product P-2 P-4 Semi-batch operation: Some materials introduced after the start of the process feed V-3 time P-3 P-1 Reaction initiator time time V-1 E-1 P-2 V-3 P-4 product time 35

20 Batch operations Batch can have very different operating conditions. For example, it can be necessary to heat a reactor in the beginning of the batch and cool it thereafter. feed P-1 time V-1 P-3 product Flow of heat transfer medium E-1 P-2 V-3 P-4 = heating time time = cooling 36

21 Batch operations Some plants integrate batch and continuous units. The plant must contain storage capacity to allow the continuous parts to operate without frequent shutdowns. Batchwise operation Distillation feed Feed must be constant! V-1 P-3 P-1 time E-1 P-2 P-4 V-3 E-2 Reactor product flow time 37

22 All situations considered Industrial Practice The operating conditions change transients. For what operation do we size the? We must size for the most demanding condition of all operations experienced by the process. Never use the average operation, especially for these extreme transient operations. It may be required to have parallel with different capacities when the normal and maximum operations are very different. 38

23 2. Reliability DYNAMIC PERFORMANCE Let s recall that the process dynamics introduce the fundamental limit to feedback control performance. How does each parameter affect feedback performance? 3. Flexibility/ Time constant in disturbance, D Dead time in feedback, A 5. Efficiency 6. Startup & Shut down Steady-state Gain, K P Time constant in feedback, P

24 2. Reliability 3. Flexibility/ 5. Efficiency 6. Startup & Shut down DYNAMIC PERFORMANCE Let s recall that the process dynamics introduce the fundamental limit to feedback control performance. Controlled Variable Manipulated Variable S-LOOP plots deviation variables (IAE = ) Time CV limited set point overshoot, fast damping, and return to the set point CV does not change because of dead time Set point response Time

25 DYNAMIC PERFORMANCE 2. Reliability 3. Flexibility/ Let s recall that the process dynamics introduce the fundamental limit to feedback control performance. Can not prevent this deviation with feedback 5. Efficiency 6. Startup & Shut down Disturbance response

26 DYNAMIC PERFORMANCE 2. Reliability 3. Flexibility/ CLASS WORKSHOP: The feedback controller is performing well but the dynamic performance is poor. What improvements can we make? 5. Efficiency 6. Startup & Shut down AC 1

27 DYNAMIC PERFORMANCE 2. Reliability CLASS WORKSHOP CV selection: Which variables must be controlled? Why? 3. Flexibility/ 5. Efficiency 6. Startup & Shut down

28 DYNAMIC PERFORMANCE 2. Reliability 3. Flexibility/ 5. Efficiency 6. Startup & Shut down CLASS WORKSHOP CV selection: Which variables must be controlled? Why? Control unstable variables. For this process, the levels are non-self-regulating. Control variables that drift rapidly to unsafe or highly undesirable values. The pressure is stable but changes quickly for changes in the reboiler

29 DYNAMIC PERFORMANCE 2. Reliability CLASS WORKSHOP CV selection: Which variable(s) should be controlled? Why? 3. Flexibility/ 5. Efficiency 6. Startup & Shut down The bottoms product purity is important, but the cost of the onstream analyzer is very high. Is there an alternative CV?

30 DYNAMIC PERFORMANCE 2. Reliability 3. Flexibility/ 5. Efficiency 6. Startup & Shut down CLASS WORKSHOP CV selection: TC Inferential/Dominant Variables - Easily measured and strongly influence product quality and/or profit. In many (not all) distillation towers, tray temperatures are a good inference of product composition. (The profile moves

31 2. Reliability 3. Flexibility/ DYNAMIC PERFORMANCE Class Workshop on Control structure : These two controllers will interact strongly, What can we do? Fired heater Fluid to be heated by flowing in pipe through the fire box 5. Efficiency PC TC Fuel oil 6. Startup & Shut down Fuel gas distribution

32 2. Reliability 3. Flexibility/ 5. Efficiency 6. Startup & Shut down DYNAMIC PERFORMANCE Selecting Control Structure: For multivariable systems, options exist when determining the connection of CVs and MVs. These simple guidelines can be used in Pair selecting CV with loop an pairing. MV that gives Fast feedback response Strong effect (large gain) The most direct effect. This tends to reduce interaction among loops Large MV range of adjustment Caution: These guidelines are a good starting point. But, they can be violated in real designs.

33 2. Reliability DYNAMIC PERFORMANCE Workshop on Selecting Control Structure: Determine the loop pairings and sketch them on the drawing. 3. Flexibility/ v8 5. Efficiency 6. Startup & Shut down Feed tank F2 F1 T1 T3 v1 v2 T2 Hot Oil T4 v3 CSTR T5 L1 T9 F3 v5 T6 F4 v7 T8 Hot Oil v6 F6 T7 F5 P1 L2 Flash v4

34 DYNAMIC PERFORMANCE Workshop on Selecting Control Structure: See Marlin (2000) for explanation of this and other designs. 2. Reliability 3. Flexibility/ 5. Efficiency 6. Startup & Shut down

35 DYNAMIC PERFORMANCE 2. Reliability 3. Flexibility/ 5. Efficiency 6. Startup & Shut down INDUSTRIAL PRACTICE Good performance through process structure is more reliable and preferred if the cost is acceptable. We are guided by control objectives (safety,, smooth operation, product quality, and profit) We seek to provide good control with simple loop pairing control strategies, if possible. We control inferential/dominant