L'Auberge Seminar 2012 -...

Reboiler TrainingReboiler Training

Property of ChemTreat – do not reproduce without permission

Topics to be Covered• Importance of venting – removal of non‐condensable

gassesCarbon Dioxide– Carbon Dioxide

– Oxygen

• Reducing non‐condensable gasses by pretreatment• Reboiler Temperature Control

– Reduce the overall solubility of gasses in condensate carbon dioxide / oxygendioxide / oxygen

– Prevents steam collapse– Improves reboiler / column operation

C d id b il f li– Can reduce process side reboiler fouling


Reboiler VentingWh V t?• Why Vent?1. Prevent the concentration of non‐condensable gasses in

reboiler free space.pa. Reduces corrosion from carbonic acid b. Reduces corrosion from oxygenc Reduces neutralizing amine feedc. Reduces neutralizing amine feedd. Improves heat transfer by removing the build‐up of non‐

condensable gasses. It h b h littl 500 f d bl– It has been shown as little as 500 ppm of non‐condensable gasses can reduce heat transfer by 50%

2. Increases asset life3 R d T t d I B k t th B il3. Reduces Transported Iron Back to the Boiler

Sources of Carbon DioxideSources of Carbon Dioxide

• Surface WaterSurface Water– Typically about 5 ppm free CO2

Alkalinity varies from 5 ppm to 150 ppm– Alkalinity varies from 5 ppm to 150 ppm

• Well Water– No typical Carbon Dioxide level. Varies from 0 ppm to 100s of ppm depending on the well.

Alk li it l l t i ll hi h i ll t– Alkalinity levels are typically higher in well water than surface water.


AlkalinityAlkalinity

• Defined as substances that neutralize acidsDefined as substances that neutralize acids.– Bicarbonate HCO3

‐ Most common form of alkalinity in surface and well wateralkalinity in surface and well water.

– Carbonate CO3‐2 Found occasionally in some

surface and well water.surface and well water.

– Hydrate OH‐ Very rarely found in well or surface water.


Boiler ReactionsBoiler ReactionsAlkalinity Breakdown


ReactionsReactions CO2 + H2O → H2CO3

• Solubility is directly proportional to partial pressure.

P [CO ] [H O] ([H O] 1)• PCO2 = [CO2] x [H2O] ([H2O] = 1)– PCO2 = [CO2]

PCO2 is the solubility constant of carbon dioxide at a given temperature and pressure. It is proportional to

d l d lpressure and partial pressure, and inversely proportional to temperature.


HCO3 to CO2 Conversion 3 2

Pressure, psig Soft Water 15 10%15 10%50 33%

100 63% 150 79%150 79%200 90%


ReactionsReactions H2CO3 → H+ + HCO3

‐

• Lowers Condensate pH

• Low pH causes corrosionoReduces reboiler life

oCauses reboiler tube leaks

C i lt i t t d I t i th• Corrosion results in transported Iron entering the boilers and causing boiler deposits.

oBoiler deposits reduce boiler efficiencyoBoiler deposits reduce boiler efficiency

oCan eventually result in tube failures.


Ways to Reduce Carbon DioxidePretreatment – reduces carbon dioxide entering the

steam system.• Reverse Osmosis

– Unless carbon dioxide is converted to bicarbonate or carbonate alkalinity, carbon dioxide will largely pass through RO membranes.

– Very effective for removing 98% of alkalinity

• Demineralization

– Effective for removal of both carbon dioxide and alkalinity

• Other

Dealkalizers Decarbonators / Degasifiers Hot lime / cold lime– Dealkalizers, Decarbonators / Degasifiers, Hot lime / cold lime softening


OxygenOxygen

• Primary source of oxygen is the atmospherePrimary source of oxygen is the atmosphere.

• Methods of entranceAlth h t t li d f d t li– Although most steam lines and feedwater lines have a positive pressure, oxygen can be educted across valves and pumpsacross valves and pumps.

– Condensing steam frequently creates a vacuum.


Ways to remove Carbon Dioxide / OxygenWays to remove Carbon Dioxide / Oxygen

• DeaerationDeaeration– Effective deaeration will remove carbon dioxide from ppm levels to < 10 ppBfrom ppm levels to < 10 ppB.

– A properly operating deaerator will remove the majority of free carbon dioxide remaining aftermajority of free carbon dioxide remaining after demineralization and reverse osmosis.

– Deaeration will not remove alkalinity.y

– A properly operating deaerator will remove oxygen to down to 7 ppB.


Chemical Treatment• Volatile alkaline amines

– Neutralize carbonic acid and raise pHp

– Will not protect against oxygen attack

• Volatile oxygen scavengers / metal passivatorsVolatile oxygen scavengers / metal passivators– Protect against small amounts of oxygen ingress

Passivate metal surfaces and decrease iron and– Passivate metal surfaces and decrease iron and copper transport


Chemical TreatmentChemical Treatment

• Volatile filming aminesVolatile filming amines– Applied correctly they will form a wax like barrier on metal surfaceson metal surfaces

– Protects against oxygen

– Over feed will plug steam traps and increase– Over feed will plug steam traps and increase condensate iron levels

– Can cause super heater tube deposits and failuresCan cause super heater tube deposits and failures


Reboiler VentingReboiler Venting

• Reduces carbon dioxide concentration inReduces carbon dioxide concentration in reboilers

• Raises reboiler pH• Raises reboiler pH

• Reduces reboiler corrosion

• Reduces iron transport back to the boilers

• Reduces treatment costs


Reboiler VentingReboiler Venting


Vent LocationsVent Locations• Horizontal Reboiler


Vent LocationsVent Locations

• Horizontal ReboilerHorizontal Reboiler


Vent LocationsVent Locations

• Vertical ReboilerVertical Reboiler

Rebo

iler


Venting GeneralVenting General

• Best vent location is just above the reboilerBest vent location is just above the reboiler condensate outlet.– Carbon dioxide tends to concentrate in this– Carbon dioxide tends to concentrate in this location

– Vent will blow condensate if reboiler becomesVent will blow condensate if reboiler becomes flooded

– Virtually all reboilers have a vent locationVirtually all reboilers have a vent location installed by the manufacturer


When Not to VentWhen Not to Vent

• Reboiler is under vacuumReboiler is under vacuum– Venting will only pull in non‐condensable gasses

• OxygenOxygen– accelerates reboiler, condensate tank, and condensate line corrosion

I t f ith h t t f– Interferes with heat transfer

• Nitrogen – generally not a problem, but it can interfere with heat transfer


How much to vent?How much to vent?Method 1

Open vent – check condensate pH – repeat untilOpen vent check condensate pH repeat until condensate pH no longer increases.

Method 2With steam load and pressure, calculate the vent

orifice size need to vent 0.1 to 0.15% of the steam. This is the same amount whether for a reboiler or a deaerator. Install orifice and open valve completely. Orifice will restrict vent to thevalve completely. Orifice will restrict vent to the correct amount.


Vent SizingVent Sizing

Spreadsheet


Condensate Temperature ControlCondensate Temperature Control

Benefits of Condensate Temperature Benefits of Condensate Temperature Control Reduce the overall solubility of gasses in condensate: y gcarbon dioxide & oxygen

Prevents steam collapse

Temperature control improves reboiler / column operation

C d id b il f li Can reduce process side reboiler fouling

Improves asset life


Reduce the overall solubility of gasses in d t b di id & condensate: carbon dioxide & oxygen

Henry’s Law:The solubility of a gas in water directly decreases as the temperature of water increases. When the saturation temperature, (boiling point), of water is reached, all p , ( g p ), ,uncombined gasses are insoluble and can be removed.

Therefore, maintaining hot condensate exit temperatures minimizes the amount of carbon dioxide and oxygen that can be dissolved.


Preventing Steam CollapsePreventing Steam Collapse• Steam collapse is a very aggressive form of erosion corrosionerosion corrosion.

• Steam collapse on metal surface can have a shearing force of 100,000 psi and will rip chunksshearing force of 100,000 psi and will rip chunks of metal from the surface.

• It cannot be treated for chemicallyy• Causes rapid metal loss followed by reboiler failures.

• Characterized by under cut pits.


Typical Undercut Pit Due to Steam Collapse

Property of ChemTreat – do not reproduce without permissionProperty of ChemTreat – do not reproduce without permission

Poor Temperature Control Can Result InPoor Temperature Control Can Result In• Hot spots

May cause surface chemical reactions that promote– May cause surface chemical reactions that promote surface process side fouling

• Vapor Fraction Increases• Vapor Fraction Increases– An increase in the vapor fraction results in an elevated level of non‐volatile materials in contact with the heatlevel of non volatile materials in contact with the heat exchange surface. This increases the fouling potential, and at a 0.5 vapor fraction, dry‐out and hot spots can form When this continues on an intermittent basis theform. When this continues on an intermittent basis, the fouling potential is increased.


Basic Temperature ControlBasic Temperature Control

• Condensate Level Control

• Steam Flow Control with Level control on Condensate Pot


Temperature Control• Condensate Level Control

– Reboiler temperature is controlled by maintaining condensateReboiler temperature is controlled by maintaining condensate level. In effect, this controls steam flow by throttling condensate flow.

Reboiler dynamics prevent quick response The temperature– Reboiler dynamics prevent quick response. The temperature transition period is longer and overshoot is more likely.

– Best if used on slow columns, steady state loads, or base load columns.


Temperature ControlTemperature Control

• Condensate Level Control• Condensate Level Control– Condensate hold up increases subcooling resulting in increase

carbon dioxide absorption, decreased pH and increased corrosion potential.

– Flooded reboiler surfaces increase the potential for corrosion and iron transport.p

– Flooded reboiler conditions result in increased bundle depreciation from thermal stress especially on welds and from steam collapse (erosion corrosion )steam collapse (erosion corrosion )

– Flooded reboilers can result in increased process side fouling.


Signs of reboiler floodingSigns of reboiler flooding


Temperature ControlTemperature Control• Steam Flow Control with Level control on C d PCondensate Pot– Good for high and medium pressure steam. Apply with care to

low pressure steam or compensate through controlslow pressure steam or compensate through controls.

– Steam pressure required in the reboiler needs to be low enough to allow for a reasonable pressure drop across the

t l lcontrol valve.

– Clean exchanger conditions and turndown should be carefully checked to assure the steam pressure in the reboiler does not drop below the pressure required to force condensate out of the condensate pot.


Temperature Controlp

• Steam Flow Control with Level control onSteam Flow Control with Level control on Condensate Pot– Steam flow and hence heat input is controlled directly withSteam flow and hence heat input is controlled directly with

quick response.

– Use where non stringent process or product is sufficient.

F ll d b d i l f– For small systems, a condensate trap may be used in place of the condensate pot and level controller.


Temperature Control• Steam Flow Control with Level control on Condensate Pot– A change in required duty requires a large change in steam pressure.

For example: A reboiler with nearly constant process side temperature of say 360 oF that requires 420 psig steam (452 oF) at p y q p g ( )design would require 344 psig (434 oF) at 80% of design. The steam control valve pressure drop would go from 30 to 106 psi. As delta P increases, temperature / losses go up. High delta P / and low % valve openings leads to hunting and poor temperature control. For high delta P use SVCs as shown below.



• Feed Forward / Feedback Trim ControlFeed Forward / Feedback Trim Control– Use where stringent product control or process control is requiredcontrol is required

– Dual steam flow control valves allow for varying loads or startup.loads or startup.

– Uses a direct synthesis control, lead / lag feedback, dynamic compensation., y p

– Decoupling consideration for manual operation during startup.


General CommentGeneral Comment

The reboiler outlet condensate temperatureThe reboiler outlet condensate temperature should be no greater than 10 oF below that of the inlet steam saturation temperatureof the inlet steam saturation temperature.

For example at 50psig, the reboiler inlet saturated steam temperature is 298 oF The outlet condensatetemperature is 298 oF. The outlet condensate temperature should be no less than 288 oF. Essentially all heat input is latent heat of vaporization.


Q = F ΔH = U A (T ‐T )Q = FsΔH = U1A1(TV‐TB)Where

A1 = the heat transfer area in the steamcondensate region

Q = the heat input to distillationFs = the flow rate of the vapor heatings p g

mediumTB = the temperature of the process fluidT = the temperature of the steamTV = the temperature of the steam

providedU1 = the temperature coefficient in the

steam condensation regionΔH = the latent heat of vaporization


Steam psig Latent Heat BTU/lb

Saturation Temperature oF

Desired Condensate Temp oF

450 763 450 440160 852 371 361140 861 361 35125 934 267 2575 961 227 2175 961 227 217


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