TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Method: The streams (S1,S2) are blended by using signal to control the flow rate of the stream (S2) blended with stream (S1) to produce the targeted concentration of ethane in stream (S3). A signal representative of the concentration of ethane in the stream (S3) is generated, and is used to compare the concentration of ethane in the stream (S3) with the targeted concentration of ethane in the stream (S3). The signal representing flow rate of stream (S2) is generated by: V2=V1((TV3a-V1a)/(V2a-TV3a)) (V2 is volume of the stream (S2), V1 is volume of stream (S1), TV3a is the targeted ethane concentration in the stream (S3), V1a is the ethane concentration of the stream (S1), and V2a is the ethane concentration in the stream (S2)). Alternately, signal representing flow rate of stream (S2) is generated using signal representing flow volume of stream (S1), signal representing concentration of propane in stream (S1) and signal representing concentration of propane in stream (S2). The signal representing flow rate of stream (S2) is given by: V2=V1((Cax V1a)+(Cbx V1b)-TVP)/(TVP-(Cax V2a)-(Cbx V2b)) (Ca is vapor pressure of ethane, Cb is the vapor pressure of propane, V1b is concentration of propane in stream (S1), TVP is a targeted vapor pressure of stream (S3) and V2b is the concentration of propane in the stream (S2)). The streams (S1,S2) comprise propylene component and isobutane component, and signal representing flow rate of stream (S2) is given by: V2=V1((Cax V1a))+(Cbx V1b)+(Ccx V1c)+(Cdx V1d)-TVP)/(TVP-(Cax V2a)-(Cbx V2b)-(C-cx V2c)-(Cdx V2d)) (Cc is vapor pressure of propylene component, V1c is propylene component concentration in stream (S1), Cd is the vapor pressure of the isobutane component, V1d is the isobutane component concentration in the stream (S1), V2c is propylene component concentration in the stream (S2) and V2d is the isobutane component concentration in the stream (S2)). Alternately, the signal representing flow rate of stream (S2) is given by: V2=V1x ((TV3bx V1a)+(TV3bx V1c)+(TV3bx V1d)-((1-TV3b)x V1b))/(((1-TV3b)x V2b)-(-TV3bx V2a)-(TV3bx V2c)-(TV3bx V2d) (TV3b is a targeted minimum propane concentration in stream (S3)).    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The stream (S3) comprises less than 95 volume% of propane and 4 volume% or more of ethane. Preferred Property: The stream (S1) has vapor pressure of 170-186 psig at 100 degrees F, and stream (S2) has vapor pressure of 392-785 psig at 100 degrees F.