Chapter Ten: Blending process

Blending process

Components of the Gasoline Pool Butane Isomerate LN Alkylate Reformate FCC HN All have different properties Lots of different Grade

Gasoline Blending with n-Butane Gasoline is blended with n-butane to promote high antiknock quality, ease of starting, and quick warm-up. The vapor pressure [expressed as the Reid vapor pressure (RVP)] of gasoline is a compromise between a high RVP to improve economics and engine starting characteristics and a low RVP to prevent vapor lock and reduce evaporation losses. RVP of gasoline changes with the season of the year and varies between 7.2 psi (49.6 kPa) in the summer and 13.5 psi (93.1 kPa) in the winter.

The desired RVP of a gasoline is obtained by blending gasoline with n-butane. The amount of n-butane required to give the needed RVP is calculated by:

RVP of water ~6.60 kPa

is mole fraction Reid vapor pressure Partial vapor pressure=RVP*xi PVP=Partial vapor pressure= mole fraction* RVP. LSR: is the light straight-run (LSR) portion (i.e., the material boiling below 180°F) of the full range naphtha is separated for blending into the gasoline product. is mole fraction

In-class exercise Assume that we need to blend a gasoline pool consisting of Component A : flow rate= 440 mol/h, RVP= 13.0 psi Component B : flow rate= 660 mol/h, RVP= 5.5 psi Component C : flow rate= 330 mol/h, RVP= 6.5 psi Component D : flow rate= 770 mol/h, RVP= 4.5 psi Calculate the amount of n-butane (in mol/h) required to blend the above mixture to produce a gasoline pool having 11.5 psi RVP. N-butane MW= 58 g/mol and its RVP=52 psi

Using Vapor Pressure Blending Indices (VPBI) Vapor pressure blending indices (VPBI) have been compiled as a function of the RVP of the blending streams Reid vapor pressure of the blend is closely approximated by the sum of all the products of the volume fraction (v) times the VPBI for each component. In the case where the volume of the butane to be blended for a given RVP is desired:

BPD Barrels per calendar day (BPCD) Average flow rates based on operating 365 days per year. BPD Although this differs slightly from the result obtained in Example 1, they agree well within the limits required for normal refinery operation.

Blending Based on Octane Number Octane number : Is a measure of a gasoline’s resistance to knock in a cylinder of a pertol engine. The higher octane number, the higher the resistance to knock, the higher is the efficiency of the fuel to produce work. RON (Research Octane Number) is determined in a single cylinder variable compression ratio engine that operates at 600 rpm with a 65.6 ºC inlet air temperature. MON (Motor Octane Number) is determined at engine speed of 900 rpm and 148.9 ºC inlet air temperature. Usually RON> MON RON-MON=sensitivity of the gasoline

RON=95.0 MON= 89.4