The production of normal paraffins provides the ability of upgrading products from straight runs of hydrocarbon feed streams derived from crude oil fractionation. For example, straight run kerosene can be further processed to separate out normal paraffins for higher valued products, such as used in the production of linear alkyl benzenes (LAB). Normal paraffins in a particular range (e.g., C10 to C13) are important precursors to LAB production, which is in turn used to produce linear alkyl benzene sulfonate (LAS). LAS is the predominant surfactant used in the production of detergents.
The large utility of detergents and other cleaners has led to extensive development in the areas of detergent production and formulation. While detergents can be formulated from a wide variety of different compounds, much of the world's supply is formulated from chemicals derived from alkylbenzenes. The compounds are produced in petrochemical complexes in which an aromatic hydrocarbon, typically benzene, is alkylated with an olefin of the desired structure and carbon number for the side chain. Typically the olefin is actually a mixture of different olefins forming a homologous series having a range of three to five carbon numbers. The olefin(s) can be derived from several alternative sources. For instance, they can be derived from the oligomerization of propylene or butenes from the polymerization of ethylene. Economics has led to the production of olefins, where the dehydrogenation of the corresponding paraffin is the preferred route to produce the olefin.
Recovery of the desired normal paraffins from a hydrocarbon such as kerosene is performed by adsorption separation, which is one process in overall production of LABs. In a typical adsorption separation process, selected paraffins are separated from branched-chain and cyclic hydrocarbons by adsorption. The paraffins are then passed through a catalytic dehydrogenation zone wherein some of the paraffins are converted to olefins. The resultant mixture of paraffins and olefins is then passed into an alkylation zone in which the olefins are reacted with the aromatic substrate. This overall flow is shown in U.S. Pat. No. 5,276,231, which is incorporated by reference in its entirety, directed to an improvement related to the adsorptive separation of byproduct aromatic hydrocarbons from the dehydrogenation zone effluent, PCT International Publication No. WO 99/07656 indicates that paraffins used in this overall process may be recovered through the use of two adsorptive separation zones in series, with one zone producing normal paraffins and another producing mono-methyl paraffins.
In an example adsorption separation process, a hydrocarbon feed stream (e.g., kerosene) is passed through an adsorption separation unit of an adsorption separation system. Nonlimiting examples of adsorption separation units use simulated moving bed technology in a fixed bed system. The adsorption separation unit uses a suitable solid adsorbent (stationary phase) and a desorbent (mobile phase) to separate normal paraffins from a raffinate. More particularly, the adsorption separation unit creates an extract stream including normal paraffins in a desired range and a combined desorbent, as well as a raffinate stream including non-normal paraffins (e.g., branched and cyclic hydrocarbons) and a combined desorbent.
A nonlimiting example combined desorbent in a process for extracting normal paraffins from a hydrocarbon feed stream includes a desorbent material, such as normal pentane, and typically also includes one or more flush materials. These flush materials are selected to move through the adsorption separation system to clear out material from the hydrocarbon feed stream that has not been adsorbed before a desorbent zone passes. The flush materials also keep the desorbent material from breaking through to the adsorption zone, preventing a resulting loss of the normal paraffin product. Nonlimiting example flush materials include iso-octane and a mixture of iso-octane and an aromatic, such as paraxylene (p-xylene). One of the flush materials (or the only flush material, if only one is used), e.g., iso-octane, combines with the desorbent material to provide a desorbent mixture, which is used as a desorbent during a desorbent zone of the adsorption separation process.
The extract stream is passed to an extract separation column, for instance a fractionation column. To provide a normal paraffin product, the extract separation column separates the normal paraffin product from the combined desorbent. The raffinate stream is passed to a raffinate separation column, for instance a fractionation column. The raffinate separation column separates the raffinate from the combined desorbent.
Combined desorbents from the extract separation column and the raffinate separation column are processed to separate the desorbent material from the flush materials. If the adsorption separation system employs a combined desorbent that includes a desorbent material (e.g., normal pentane) and a single flush material (e.g., iso-octane), a dual split desorbent system is provided. On the other hand, if the adsorption separation system employs a combined desorbent that includes a desorbent material and two flush materials (e.g., iso-octane and an aromatic, such as paraxylene), a triple split desorbent system is provided.
The triple split desorbent system provides lower aromatics in the normal paraffins product, e.g., <1500 wt ppm aromatics, compared to the dual split desorbent system. However, the triple split desorbent system has the disadvantage of higher utilities, e.g., roughly 10-15% higher fuel and power consumption. For many applications, higher aromatics than that produced by the triple split desorbent system are acceptable, e.g., <5000 wt ppm, and this range is easily achieved by the dual split desorbent system.
Thus, the dual split desorbent system may be preferable where higher aromatics are acceptable, and the triple split desorbent system may be preferable when lower aromatics are required. Currently, however, there is not a way to switch easily from a dual split desorbent system to a triple split desorbent system, or vice versa.