Paraffins mono-methyl

Examples of rate-selective adsorption are demonstrated using silicalite adsorbent for separation of Ciq-Cm n-paraffins from non- -paraffins [40, 41] and Ciq-Ch mono-methyl-paraffins from non-n-paraffins [42-45]. Silicalite is a ten-ringed zeolite with a pore opening of 5.4A x 5.7 A [22]. In the case of -paraffins/non-n-paraffins separation [40, 41], n-paraffins enter the pores of silicalite freely, but non-n-paraffins such as aromatics, naphthenes and iso-paraffins diffuse into the pores more slowly. However, the diffusion rates of both normal -paraffins and non-n-paraffins increase with temperature. So, one would expect to see minimal separation of n-paraffins from non-n-paraffins at high temperatures but high separation at lower temperature. 

For the rate-selective separation of Ciq-Ch mono-methyl-paraffins from non-n-paraffins [42-45], diffusion simulations were carried out using the Solids Diffusion module in the Accelrys Insight II molecular modeling package [44]. The modeling results from the diffusion simulations of four paraffins of varying carbon numbers in siUcalite are summarized in Table 6.9. 

This chapter reviews the adsorptive separations of various classes of non-aromatic hydrocarbons. It covers three different normal paraffin molecular weight separations from feedstocks that range from naphtha to kerosene, the separation of mono-methyl paraffins from kerosene and the separation of mono-olefins both from a mixed C4 stream and from a kerosene stream. In addition, we also review the separation of olefins from a C10-16 stream and review simple carbohydrate separations and various acid separations. 

In 1998, UOP announced the development of a new Sorbex process called the MMP Sorbex process [15-19] that was capable of simultaneously separating both Cio i6 mono-branched paraffins and Cio i6 normal paraffins from a corresponding kerosene stream or n-paraffin-depleted Molex raffinate stream. Previously, no commercial process existed to isolate significant quantities of mono-methyl paraffin derived from either kerosene or n-paraffin depleted kerosene. Mono-methyl paraffins are desirable because they are needed for a new type of anionic surfactant. 

Sohn, S.W., et al. (2003) Mono-methyl Paraffin Adsorptive Separation. U.S. Patent 5570519. 

The major role of ZSM-5 is to react with converted products and, in particular, certain gasoline components. The shape selectivity of the zeolite allows only linear and mono-methyl paraffins and olefins ready access to active sites whilst other structures, such as aromatics and multi-branched paraffins, will be restricted [2]. Octane enhancement with ZSM-5 arises from this shape selectivity as the most accessible hydrocarbons are also octane depressants, whilst those structures precluded from the zeolite structure e.g. aromatics have high octane ratings. 

MONO-METHYL PARAFFIN ADSORPTIVE SEPARATION PROCESS... 

For mono-methyl paraffm separation, two pulse test techniques, one with and one without iso-octane pre-pulse, were developed (2,3). In each test the feed was a mixture containing equal volumes of 3,3,S-trimethyl heptane, 2,6-dimethyl octane, 2-methyl nonane, n-decane, and I,3,S-trimethyl benzene. The pulse test column had a volume of 70 cc and was held at a temperature of 120 C in the experiments shown. The flow rate through the column was 1.2 ml/min. The adsorbent was silicalite and the desorbent was a 30/30 volume % mixture of n-hexane/cyclohexane. Test I was run without a pre-pulse and test 2 was run with a pre-pulse of 40 ml of iso-octane injected into the test loop immediately before the feed mixture was injected. Iso-octane pre-pulse diluted the n-hexane concentration at the adsorption zone and increased the adsorbent selectivity for mono-methyl paraffin. 

A graphical representation of the results of this comparison test run is shown in Figures 1 and 2. Figure 1 shows a plot of the relative concentrations of the components versus volume of effluent. The improved separation using the pre-pulse is also shows graphically in Figure 2. A comparison of the two plots shows that use of a pre-pulse resulted in a much better separation of mono-methyl paraffin (2-methyl nonane) from the balance of feed components. 

The process was demonstrated in a simulated continuous counter-current chromatographic separation pilot plant. Both the primary method of operation and the pre-pulse technique were demonstrated, with the pre-pulse technique showing improved recoveiy. Using commercial n-paraffin depleted kerosene (Molex Raffinate) feedstock we routinely demonstrated the ability to achieve better than 90% mono-methyl and normal paraffin purity with greater than 70% recovery of mono-methyl paraffins. 

We can t see the organic molecules as they wiggle in and out of these channels. But we can measure how quickly or slowly they diffuse into ZSM-5. From diffusion studies (Ref. 11), we know that only straight chain and mono-methyl paraffins and olefins, certain one-ring aromatic and naphthenic molecules diffuse at useful rates through ZSM-5. The less bulky the molecule, the faster the diffusion rate. Larger molecules either diffuse in slowly, and react at a lower rate, or they are completely excluded. We call this reactant shape selectivity. 

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