Visbreaking unit

VISBS

Visbreaker reactors comprise two sections: a heater, which heats up the feed to thermal cracking temperatures, and a soaker drum where most of the cracking occurs. Economically the visbreaking severity should be maximized, up to asphaltene solubility constraint; however it is difficult for the operator to determine the correct COT (heater coil outlet temperature) because:

• The visbreaker feed properties, which affect fuel stability are not fully known, Refinery wide constraints dictate visbreaker feed flow (and reaction space velocity) variations daily or sometimes hourly.

• Secondary parameters: pressure and steam injection into the transfer line, affect the severity in a way that cannot be readily estimated.

• Laboratory fuel stability tests are typically carried out only once per day.
  

VISBS is a first principles calculation package designed to determine the best setting for the reactor and fractionator. These calculations become control variables for advanced control applications using multi-variable predictive control technology.

VISBS CALCULATIONS

EVT (equivalent visbreaking time)

• EVT reduces the time temperature history of the reaction into an equivalent residence time at 468.33ºC (875ºF). This model responds to throughput, coil steam or pressure changes. The APC controller would then correct the COT, avoiding over or undercracking.
  

Visbreaking conversion.

• Conversion is defined as the 211ºC- yield, and it is calculated via a GCC fractionator package. VISBS uses the slower conversion model to dynamically reset the equivalent visbreaking time.
  

Maximum conversion.

• The maximum conversion model relies on feed Watson (Kw) coefficient, and the use of density and viscosity measurements to predict Kw. Alternatively, a GCC inferential model on the vacuum unit, can estimate Kw from vacuum cutpoint and resid density.
  

• Visbroken product properties calculated by a GCC package on the fractionator:

• Naphtha ASTM 90% point

• LGO flashpoint, ASTM 90% point

• Internal reflux on key trays and flooding constraints.

VISBS CALCULATIONS:

Able to keep the severity constant while unit conditions vary..

• Permits maximizing applications to work with no need to keep unit conditions steady.

Responsive to feed property changes.

• Automatically adjusts the maximum permitted severity upon as needed. There is no need to input any data whatsoever before feedstock switches. Detection of feed characteristics is completely automatic.

Simple to calibrate.

• VISBS (and GCC) require only a simple calibration procedure, which makes use of steady state data. The technique itself is robust enough to work without calibration, but given inaccuracies of flow, pressure and temperature measurements, precision improves after steady state calibration.

Self tuned.

• Commissioning does not require an elaborate tuning procedure. The model will work instantly upon commissioning.

Easily understood.

• The model follows standard chemical engineering and API procedures.

Multi variable dynamic capability.

• The inferential calculations integrate with multi variable control packages available on the market.

Fractionator GCC is reliable enough to replace distillation analyzers.

• Inferential precision is for example 2ºC for Naphtha 90% distillation point, or 4ºC for 90 % sidestream distillation point. The model can work with analyzer resetting a bias in the calculation via a dead time compensator.