Minimising Butadiene Level in Liquefied Petroleum Gas (LPG) via Non-Stirred Blending with Numerical Approach
Keywords:CFD, LPG, off-specification, on-specification, jet mixing, homogeneous blending time
LPG has variable commercial grades due to its varying LPG composition will yield different properties. Because their properties depend on the composition, the LPG quality will differ based on the source of its production extracted either from refinery streams or natural gas. The composition of propane and butane, which are the two major components in LPG composition, play an important role in reflecting the properties of LPG, as the mixtures influence the boiling points and also meet product specification requirements. Butadiene is also present in LPG, albeit in trace amounts. Although butadiene is a minor component, it must be kept minimised of the total weight fraction due to regulatory limits. Butadiene is a hazardous chemical that, when inhaled, can cause cancer and genetic defects. LPG used for commercial purposes that contain of any carcinogenic substance such as butadiene must also be classified as carcinogenic. The LPG plant operator is facing the problem that imported LPG composition from outside sources contain levels of butadiene that exceed the regulatory limit of 0.5% of weight fraction. LPG composition, containing butadiene levels that exceed 0.5% of weight fraction is considered as off-specification, while butadiene levels less than 0.5% of weight fraction are considered as on-specification the LPG products. To reduce the levels of butadiene that exceed 0.5% of the weight fraction in off-specification LPG products, the blending of on-specification LPG products with off-specification was introduced and provided the most economical method inside the plant. The jet mixing approach was selected to predict the homogeneous blending time for each different off-specification LPG composition because it is the best approximation for natural mixing behaviour. Four empirical mixing time correlations of jet mixing were applied for the prediction of homogeneous blending time; the correlations were derived by Lane and Rice (1982), Maruyama, Ban and Mizushina (1982), Grenville and Tilton (1997) and Hiby and Modigell (1978). The homogeneous blending time predicted by these four mixing time correlations decreased as the quantity of on-specification LPG required increased, which is in good agreement with both simulation results of 95% and 99% mixing. Therefore, due to the development of individual jet mixer correlation, these four mixing time correlation results in different homogeneous blending times was going through the different measurement techniques and monitoring methods in jet mixed tanks.
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