Distillation Control Flashcards

Distillation Control Notes

Objective One: Distillation Process

• Distillation: A process separating liquid mixture components through vaporization and condensation, leveraging different vapor pressures.

  • Vapor pressure: Pressure exerted by a gas in equilibrium with a solid or liquid in a closed container at a given temperature.

  • Volatile components: Evaporate more easily and are found in higher concentrations in the vapor.

• Binary Liquid: A liquid composed of two components.

  • Example: 40% methanol and 60% water at 80°C.

    • Methanol is more volatile than water.

    • Vapor pressure of water: $47 \text{ kPaa}$ (6.8 psia).

    • Vapor pressure of methanol: $178 \text{ kPaa}$ (25.8 psia).

• Partial Pressure:

  • The partial pressure of each component equals the vapor pressure of the pure component multiplied by its concentration in the liquid mixture.

  • Partial pressure of methanol: $0.4 \times 178 \text{ kPaa} = 71 \text{ kPaa}$.

  • Partial pressure of water: $0.6 \times 47 \text{ kPaa} = 28 \text{ kPaa}$.

• Concentration in Gaseous Mixture:

  • For ideal gases, the concentration equals its partial pressure divided by the total pressure.

  • Total pressure: $71 \text{ kPaa} + 28 \text{ kPaa} = 99 \text{ kPaa}$.

  • Concentration of methanol in vapor: $71 \text{ kPaa} / 99 \text{ kPaa} = 72\%$.

  • Concentration of water in vapor: $28 \text{ kPaa} / 99 \text{ kPaa} = 28\%$.

• Repeated Vaporization and Condensation:

  • Repeated vaporizing and condensing at progressively lower temperatures separates components to a higher degree.

  • Example: 72% methanol and 28% water at 75°C.

    • Partial pressure of methanol: $0.72 \times 149 \text{ kPaa} = 107 \text{ kPaa}$.

    • Partial pressure of water: $0.28 \times 39 \text{ kPaa} = 11 \text{ kPaa}$.

    • Concentration of methanol in vapor: $107 \text{ kPaa} / 118 \text{ kPaa} = 91\%$.

    • Concentration of water in vapor: $11 \text{ kPaa} / 118 \text{ kPaa} = 9\%$.
      Note: Vapor pressure of both methanol and water decreases due to lower temperature.

• Practical Continuous Distillation Process:

  • Feed enters at stage two (e.g., 31% methanol, 69% water at 80°C).

  • Each stage holds a fixed amount of liquid; excess flows down.

  • Heat is supplied to the bottom stage to boil liquid.

  • Hot vapor rises, containing higher percentage of volatile components.

  • Vapors from below boil cooler liquid above, causing condensation of less volatile components and vaporization of more volatile components.

  • As vapor rises, volatile component percentage increases; as liquid descends, it decreases.

  • Vapor exiting the top stage (e.g., stage 4) contains a high percentage of methanol (e.g., 86%).

  • Reflux condenser condenses vapor, producing distillate.

  • Some distillate is used as reflux, flowing back into the top stage.

  • Liquid in bottom stage (stage 1) contains lower methanol percentage (e.g., 20%).

  • Bottoms product is drawn off to maintain constant level.
    Note: For a continuous process to work, a decreasing temperature gradient and continuous vapor/reflux flow are essential.

• Distillation Column:

  • Provides necessary separation; design determines the number of stages.

  • Trays or packing maximize vapor-liquid interaction.

  • Vertical vessels containing stages.

  • Tray weir maintains liquid level.

  • Vapors enter bubble caps through chimneys, condense, and cause liquid to boil.

  • Vapors leaving the tray are enriched with volatile components.

  • Liquid enters and leaves through downcomers (internal reflux).

  • Packed columns use packing material for liquid/vapor movement.

  • All columns have theoretical or physical stages.

• Associated Equipment:

  • Feed tray: Feed composition and temperature match liquid on the tray at design conditions.

  • Rectifying section: Above the feed tray, enriches vapor with volatile components.

  • Reflux condenser: Cools overhead vapor to liquid.

  • Distillate (tops): Liquid removed from the top, highest concentration of volatiles.

  • Accumulator (reflux drum): Stores distillate, returns it to column (external reflux).

  • Stripping section: Below feed tray, strips volatiles from descending liquid.

  • Bottoms: Liquid removed from the bottom, lowest concentration of volatiles.

  • Reboiler: Provides heat input, typically using steam.

Distillation Steady State Model

• Defines the distillation process with equations from material and energy balances.

• Aids in defining control variables.

• Key component recovery is often crucial.

• Light key: More volatile component in binary distillation. Definitions of Variables in the Distillation Process

  • F = Molar flow rate of the feed

  • D = Molar flow rate of the distillate

  • B = Molar flow rate of the bottoms

  • L = Molar flow rate of the external reflux

  • V = Molar flow rate of the overhead vapour

  • $$ =Molar percent composition of the light key in the feed

  • $$ =Molar percent composition of the light key in the distillate

  • $$ =Molar percent composition of the light key in the bottoms

  • $Q_B$ = Heat energy supplied by the reboiler

  • $Q_T$ = Heat energy removed by the condenser

• Two Fundamental Variables:

  • Split (Cut):

    • Amount of feed exiting as distillate or bottoms.

  • Fractionation:

    • Amount of separation that occurs or composition of distillate and bottoms.

• Material Balance Equations (Binary Distillation):

  • Overall Material Balance Equation

    • $F = D + B$

  • Light Key Material Balance Equation

    • $(F)(\%LK)=(D) \times (\%LK<em>D)+(B) \times (\%LK</em>B)$

• Solutions for Bottoms Split and Distillate Split:

  • Bottoms Split

    • $B/F = (\%LK<em>D - \%LK</em>F) / (\%LK<em>D - \%LK</em>B)$

  • Distillate Split

    • $D/F = (\%LK<em>F - \%LK</em>B) / (\%LK<em>D - \%LK</em>B)$

• Distillate and Bottoms Material Balances Equation

  • The Distillate and Bottoms material balances equation illustrates how the vapour flow (V) and reflux (L) influence the distillation column's split (D and B values). Since the split affects fractionation, vapour and reflux flows indirectly affect the distillate and bottoms composition.

    • $D= V-L$

  • Bottoms Material Balance Equation

    • $B=(F+L)-V$

• Vapour Flow (V)

  • Depends on reboiler heat energy ($Q_B$) and vapour released from feed.

  • Boil up: Manipulating heat energy controls vapor flow.

    • Boil up most significantly affects the split.
      Note: Increasing heat input increases the distillate product removal rate.

• External Reflux:

  • Colder than feed mixture, removes heat from the column.

  • Total reflux: All overhead vapor condenses and returns, removing maximum heat (equal to condenser heat removal, $Q_r$).

    • Results in maximum light key concentration but no distillate production.

  • Reflux Ratio (R):

    • $R = L/D$

    • Ratio between reflux flow and distillate flow.

    • Infinite reflux ratio: Total reflux, maximum heat removal, maximum light key concentration.

    • Zero reflux ratio: Zero reflux, minimum heat removal, minimum light key concentration.

    • Relative measure of heat removed, essential for setting fractionation or light key concentration.
      Note: Increasing the reflux ratio increases the concentration of the light key in the distillate.

Objective Two: Column Control

• Distillation columns designed for specific feed rate, composition, separation, and reflux ratio.

• Design process determines trays, feed tray location, column diameter, operating pressure, and energy input.

• Distillation variables set at design conditions:

  • Feed flow rate, temperature, and composition

  • Feed tray location

  • Column pressure

  • Reflux temperature

  • Distillate's composition and flow rate

  • Bottom's composition and flow rate

• Example Specifications:

  • Feed rate: 150 mol/hr

  • Feed composition: 20% light key

  • Distillate composition: 98% light key

  • Bottoms composition: 3% light key

  • Reflux ratio: 2
    Note: Equations convert between molar, mass, or volume flow based on mixture composition.

• Calculations for Distillate and Bottoms Flow Rate at Design Conditions:

  • $F = D + B$ therefore $B = F - D$

  • $(F) \times (\%LK) = (D) \times (\%LK) + (B) \times (\%LK_B)$

  • $(F) \times (\%LK) = (D) \times (\%LK) + (F - D) \times (\%LK_B)$

  • $(150 \text{ m}) \times (20\%) = (D) \times (98\%) + (150 \text{ m} - D) \times (3\%)$

  • $3000\% \text{ mo} = 98\%D + 450\% \text{ mor} - 3\%D$

  • $2550\% \text{ mo} = 95\%D$

  • $D = 26.8 \text{ mor}$

  • $F = D + B$

  • $150 \text{ mo} = 26.8 \text{ mor} + B$

  • $B = 123.2 \text{ mor}$

• Reflux Flow Calculation:

  • $R = L/D$

  • $2 = L / 26.8 \text{ mor}$

  • $L = 53.6 \text{ mo}$

• Heat Input:

  • Energy to vaporize enough liquid to ensure a flow from the condenser of 80.4 mol/hr (D+L).

• Manipulated Flows (at design conditions):

  • Manipulating cooling flow controls condenser cooling.

  • Manipulating steam flow controls heat input (boil up).

  • Manipulating reflux flow controls tower cooling.

  • Manipulating distillate flow controls distillate production.

  • Manipulating bottom's flow controls bottoms production.
    Note: The above control variables interact, influencing each other, usually negatively.
    for example:

  • Increasing reflux increases distillate quality but decreases distillate flow.

  • Increasing heat input increases distillate flow but decreases quality.
    Note: A column’s design establishes its split and fractionation capabilities, with only small adjustments possible via the control system.

• Pressure Control:

  • Essential for column operation, often considered separately from product composition control.

• Lower Pressure Advantages:

  • Facilitates easier evaporation at lower temperatures.

  • Reduces required distillation heat.

• Pressure Control Strategies (using a total condenser):

  • Water throttling

  • Overhead throttling

  • Flooded condenser

Water Throttling

• Coolant flow through condenser is manipulated. The pressure controller's output (PC-2) is the RSP for the coolant flow control loop (FC-2).

• Slow-acting; appropriate for processes near design conditions and with treated cooling fluid.

• Untreated cooling fluid can foul condenser tubes.

Overhead Throttling

• Overhead vapour flow to condenser is manipulated. The pressure controller's output (PC-2) goes directly to the overhead control valve.

• Separate flow control loop (FC-2) independently controls condenser coolant flow.

• Fast-acting, provides excellent pressure control but is not common due to cost and accumulator pressure fluctuations.

Flooded Condenser

• Condensed (liquid) overhead product flow out of the condenser is manipulated. The pressure controller's output (PC-2) goes directly to the condenser outlet control valve which manipulates the liquid level inside the condenser.

• Separate flow control loop (FC-2) independently controls condenser coolant flow.

• Varies surface area of tubes in condenser to control pressure.

• Used when coolant flow rate is constant and cannot be manipulated.

Product Composition Control

• Requires maintenance of reflux ratio and heat input.

• Can be grouped into two categories:

  • Conventional energy balance

  • Conventional material balance

• Columns separating multiple components can use either strategy.

Conventional Energy Balance

• Either sets column heat input or column cooling directly.

Heating Control

• Operator directly controls heat input by setting the SP of the steam flow (FC-3).

• Column pressure controlled via cooling fluid flow controller (FC-2) set by pressure controller (PC-2).

• Accumulator level controller (LC-5) controls distillate flow by setting the RSP of the distillate flow controller (FC-5).

• Bottoms level controller (LC-6) controls bottoms flow by setting the RSP of the bottoms flow controller (FC-6).

• The operator adjusts the setpoints of the steam flow controller (FC-3) and the column temperature controller (TC-4) to operate the column at its design conditions.

• Tray Temperature control (TC-4) near the top to control distillate composition.

Reflux Control

• Operator directly controls the column's cooling by setting the SP of the reflux flow (FC-4).

• Column pressure controlled via cooling fluid flow controller (FC-2) set by pressure controller (PC-2).

• Accumulator level controller (LC-5) controls distillate flow by setting the RSP of the distillate flow controller (FC-5).

• Bottoms level controller (LC-6) controls bottoms flow by setting the RSP of the bottoms flow controller (FC-6).
  *The operator adjusts the setpoints of the column temperature controller (TC-3) and the reflux flow controller (FC-4) to operate the column at its design conditions.

• Tray temperature (TC-3) near the bottom to control boil up.

• Reflux flow controller (FC-4) sets reflux ratio.

Conventional Material Balance

• Either sets the distillate flow or bottoms flow directly.

Distillate Control

• Operator directly controls the distillate flow rate by setting the SP of the distillate flow (FC-5).

• Column pressure controlled via cooling fluid flow controller (FC-2) set by pressure controller (PC-2).

• Accumulator level controller (LC-4) sets reflux flow controller (FC-4) to control the reflux flow.

• Bottoms level controller (LC-6) sets bottoms flow controller (FC-6) to control the bottoms flow.

• The operator adjusts the setpoints of the column temperature controller (TC-3) and the distillate flow controller (FC-5) to operate the column at its design conditions.

• Bottoms flow is determined by the column’s material balance.

• Tray temperature (TC-3) near the bottom to control boil up.

• Distillate flow sets reflux ratio.

Bottoms Control

• Operator directly controls the bottoms flow rate by setting the SP of the bottoms flow (FC-6).

• Column pressure controlled via cooling fluid flow controller (FC-2) set by pressure controller (PC-2).

• Accumulator level controller (LC-5) sets the distillate flow controller (FC-5) to control the distillate flow.

• The bottoms temperature controller (TC-3) sets the RSP of the steam flow controller (FC-3) to control the column's heat input (boil up).

• The operator adjusts the setpoints of the column temperature controller (TC-4) and the bottoms flow controller (FC-6) to operate the column at its design conditions.

• Distillate flow is set steady state via the column's material balance.

• Steam flow (FC-3) is controlled by the bottoms temperature (TC-3).

Advanced Control Strategies

• Maximize productivity; use online analyzers for composition control.

Analyzer Control

• Temperature measurement (TT-5) to indirectly control distillate composition.

• Periodic lab analysis validates distillate composition; adjustments made to TC-5 SP if needed.

  • The temperature controller (TC-5) slows the flow of distillate, which causes the level in the accumulator to rise.

  • The level controller (LC-4) reacts in order to keep the level constant in the accumulator by increasing the flow of reflux.

  • The increase in reflux flow lowers the temperature of the tray to bring it back to the required SP of the temperature controller (TC-5).

• Online composition measurement (AT-5) directly controls distillate composition.

• Productivity is enhanced because the distillate composition is continuously monitored and controlled.

Feed Rate Changes

• Require adjustment of control points; proportional changes in distillate, bottoms, reflux, and heat input.

• Static feedforward control strategies developed from material and energy balance equations.

• Dynamic compensation (dead-time and lead-lag function blocks) is also required.

• Feedforward system changes reflux and steam flow in response to feed rate changes.

  • Relay FY-1A provides the required dynamic compensation to time the change correctly for the reflux flow change.

  • Relay FY-1B provides the static change based on the distillation columns reflux to feed split (L/F). This split is calculated from steady-state equations.

  • Relay FY-1C provides the required dynamic compensation to time the change correctly for the steam flow change.

  • Relay FY-1D provides the static change based on the distillation columns heat input to feed split (Q/F). This split is calculated from steady-state equations.

  • Hand station (HS-3) provides a bias input to the steam flow so the operator can fine tune the split as required.

Feed Rate and Composition Changes

• Feedforward compensation to minimize off-specification product; requires measurement of feed and product composition.

• For constant feed composition, distillation splits are fixed.

• Feed composition changes require recalculation of splits.

• Feedforward system adjusts distillate flow in response to feed rate or composition change.

Model Predictive Control (MPC)

• Uses a process model to determine timed changes to key control points; minimizes the effect of measured disturbances.

• Disturbances include feed composition, feed rate, distillate composition, bottoms composition, column pressure, and cooling fluid flow.

• Conventional control strategy (reflux and steam flow) still operates the process.

Multiple Component Separation

• Products removed from the side of the distillation column at different temperatures (fractional distillation).

• Crude oil distillation unit is a fractionation tower at atmospheric pressure.

• Heated crude oil enters and flashes to vapor; unvaporized portion separated using vacuum distillation.

• Reflux cools crude oil; pump-around removes additional heat.

• Products are groupings of molecules with similar vapor pressures; fractionation can occur in multiple towers (stepwise fractionation).

• For example, a natural gas liquid (NGL) feed can be fractionated into:

  • ethane,

  • propane,

  • butane and

  • gasoline.

• Components separate in order of decreasing vapour pressure; more volatile components distill under higher pressures.

Objective Three: Distillation Column Problems

• Operational problems: from operator adjustments that the column design can't handle; often in response to system disturbances.

• Feed-related problems: operational or feed component related.

  • Feed flow rate and composition changes require advanced control or experienced operation.

  • Feed component problems (e.g., incondensable gases) require column design solutions.

Operational Problems

• Unstable columns can take up to 12 hours to stabilize.

• Include:

  • Flooding

  • Dry trays

  • Weeping

Flooding

• Excessive accumulation of liquid; blocks vapour movement.

• Excessive vapor flow (boil up) causes liquid trapping and frothy buildup on trays.

• Excessive reflux flow overwhelms downcomer.

• Runaway flooding: liquid level rises until the column is filled with frothy liquid.

• Differential pressure increase with differential temperature decrease indicates flooding.

• Detection of incipient flooding is ideal.

• Recovery involves reducing boil up and reflux flow; if flooding persists, reduce feed rate.

Dry Trays

• Insufficient liquid; upward vapor velocity drives liquid off the tray.

• Downcomer loses liquid seal.

• Decrease in differential pressure and differential temperature indicates dry trays.

• Recovery involves reducing boil up and increasing reflux flow.

Weeping

• Liquid leaks through vapor passages; caused by low vapor flow.

• Excessive weeping can lead to dumping: liquid cascades through the column.

• Sharp differential pressure drop and reduced separation efficiency indicates weeping.

Feed Component Problems

• Pressure control is essential; special problems arise from vapor distillate or incondensable gases.

Vapour Distillate

• Distillate removed as a vapour; next process requires vapour feed, or very light components present.

• Use partial condenser: only partially liquefies overhead vapour to maintain reflux flow.

Feed with Inert Gases

• Gases do not condense at condenser conditions; can block the condenser.

• Use flooded condenser.