Heavy paraffin conversion

Fig. 3. The Shell middle distillate synthesis (SMDS) process. HPS = heavy paraffin synthesis. HPC = heavy paraffin conversion.

It is evident that the above approach was followed to arrive at a high selectivity towards middle distillates, the prerequisite being a second stage which can convert the heavy wax fraction in the HPS effluent very selectively into middle distillates, the Heavy Paraffin Conversion (HPC) stage (see Fig. 2). In the HPC the waxy product of the HPS is hydro-isomerized and hydrocracked to give the maximum yield of middle distillates. 

Compared to SMDS, this simplified process employs a different catalyst in the synthesis stage and does not include a Heavy Paraffinic Conversion stage for producing the finished middle distillate fractions. The syncrude product is a broad boiling range of hydrocarbons (Table 4), and the relative amounts of individual products can be varied by adjusting the reaction conditions. Alternatively, syncrude products can be processed in existing refineries into finished transportation fuels. 

The plant includes hydrocracking of the LTFT products over a dual functional catalyst in the Heavy Paraffins Conversion (HPC) unit. The products of the SMDS Bintulu plant include naphtha, kerosene, diesel and some fuel gas. The HPC unit is operated t5q)ically at 30-50 bar total pressure and at a temperature of about 300-350°C, actually performing four functions ... 

HPS HEAVY PARAFFIN SYNTHESIS HPC HEAVY PARAFFIN CONVERSION... 

The Heavy Paraffin Conversion process has now been developed to convert the heavy paraffins selectively into the desired middle distillates, kerosine and gas oil. It is a mild hydrocracking process using a dual functional (Shell proprietary) catalyst. 

THE HEAVY PARAFFIN CONVERSION OR THE FEASIBILITY OF THE SELECTIVE PRODUCTION OF HYDROCARBONS OF A SPECIFIC CARBON NUMBER RANGE... 

This principle is applied with success in the HPC (Heavy Paraffin Conversion) step of the SMDS process. 

Shell has been involved in syngas chemistry for many years, giving special attention to the options for the conversion of natural gas into more easily transportable liquid hydrocarbons. The first result of this effort has been the Shell Middle Distillate Synthesis (SMDS) plant commissioned in Malaysia in 1993. This plant makes use of cobalt FT catalyst and tubular reactors in the Heavy Paraffin Synthesis unit (HPS). A simplified flow scheme of this plant is presented in Figure 7. 

The same behavior was found with all aromatics compounds. The concentrations of Nt and Nj and heavy paraffins (Pt-P/i, only Py is shown in Figure 1) decrease as they undergo conversion. A high rate of conversion of naphthenes was found in the st 30 percent of the catalyst bed. After 60 percent of the catalyst bed, n hthenes concentration iproaches a very low steady-state value. 

Catalytic hydrocracking is a flexible process for the conversion of heavy, hydrogen-deficient oils into lighter and more-valuable produets. Part of its flexibility is its ability to handle a wide range of feeds, from heavy aromatics to paraffinic crudes and cycle stocks. The ability to extend this flexibility to customized product slates depends on the ability to control and manipulate the process chemistry, which is, in turn, enhanced by a rigorous representation of fte process chemistry. This motivates the present interest in a heavy paraffin hydrocracking kinetics model. 

We cite isomerization of Cs-Ce paraffinic cuts, aliphatic alkylation making isoparaffinic gasoline from C3-C5 olefins and isobutane, and etherification of C4-C5 olefins with the C1-C2 alcohols. This type of refinery can need more hydrogen than is available from naphtha reforming. Flexibility is greatly improved over the simple conventional refinery. Nonetheless some products are not eliminated, for example, the heavy fuel of marginal quality, and the conversion product qualities may not be adequate, even after severe treatment, to meet certain specifications such as the gasoline octane number, diesel cetane number, and allowable levels of certain components. 

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

Additional uses for higher olefins include the production of epoxides for subsequent conversion into surface-active agents, alkylation of benzene to produce drag-flow reducers, alkylation of phenol to produce antioxidants, oligomerization to produce synthetic waxes (qv), and the production of linear mercaptans for use in agricultural chemicals and polymer stabilizers. Aluminum alkyls can be produced from heavy olefin streams and olefin or paraffin streams have been sulfaled or sulfonated and used in the leather (qv) industry. 

The high influence of cracking catalyst on PE conversion was confirmed by Aguado et al. [11] in a continuous screw kiln reactor. The application of a sophisticated laboratory Al-MCM-41 cracking catalyst and process temperature of 400-450°C led to 85-87% yield of gas and gasoline fractions (C1-C12). Besides olefins and n- and iso-paraffins some quantity of aromatics, 5 wt% was determined in the process products. In the same reactor system with a noncatalytic process the gas yield was halved while similarly as in case of the fluid reactor system yields of gas oil and heavy waxes fraction (C13-C55) attained values of 62% (compared with 4 wt% in catalytic process) [12]. 

Long-chain paraffins are both valuable and highly prone to cracking. Therefore, to maintain high selectivity and yield, it is necessary to operate at relatively mild conditions, typically below 500""C, and at relatively low per-pass conversions. While this is economical for the production of heavy linear olefins, it is not for the production of light olefins. 

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