Hexane as a by-product of isomerization unit using a dividing wall column

Abstract

A dividing wall column system for producing hexane includes a dividing wall column including a dividing wall that divides the dividing wall column at least partially into a first side and a second side, with one side of the first and second sides configured to operate as a deisohexanizer column and the other side of the first and second side configured to operate as a hexane column to produce hexane.

Claims

1. A method of producing, as a by-product from a C.sub.5-C.sub.6-isomerization unit, hexane having a benzene content of <3 ppm wt., a sulfur content of <0.5 ppm wt. and an n-hexane content of >40 wt %, the method comprising: producing a stable isomerate feed in the C.sub.5-C.sub.6-isomerization unit, feeding the stable isomerate feed to a first side of a dividing wall column, the dividing wall column comprising a dividing wall that divides the dividing wall column at least partially into the first side and a second side, with one side of the first side and the second side configured to operate as a deisohexanizer column and the other side of the first side and the second side configured to operate as a hexane column to produce the hexane; producing a hexane feed from the second side of the dividing wall column; feeding the hexane feed and hydrogen to a mixer of a benzene hydrogenation unit connected with the dividing wall column to form a hexane-hydrogen mixture; preheating the hexane-hydrogen mixture; feeding the preheated hexane-hydrogen mixture to a polishing reactor of the benzene hydrogenation unit, wherein the polishing reactor hydrogenates at least a part of benzene included in the hexane-hydrogen mixture; and feeding an output stream from the polishing reactor to a stripper column for separating light components from hexane to form the hexane having the benzene content of <3 ppm wt., the sulfur content of <0.5 ppm wt. and the n-hexane content of >40 wt %, wherein the stripper column is arranged downstream of the polishing reactor.

2. The method of claim 1, wherein the preheating comprises exchanging heat between the hexane-hydrogen mixture and a feed from the stripper column in a first heat exchanger; and exchanging heat between the hexane-hydrogen mixture and the output stream from the polishing reactor in a second heat exchanger.

3. The method of claim 1, wherein one or more of the following are fulfilled: the hexane has an improved quality compared to a quality of hexane produced by a solvent extraction process, the hexane meets specifications for use in food, pharmaceutical, and polymer processes, and the method of producing hexane has a variable operating cost that is around 90% lower than hexane produced by a solvent extraction process.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail hereinafter with reference to the drawings.

(2) FIG. 1 represents a process flow scheme for production of hexane in accordance with the prior art;

(3) FIG. 2 represents the process flow scheme of an isomerization unit for the production of only isomerate as the desired product in accordance with the prior art;

(4) FIG. 3 represents the conventional process scheme of a high-purity hexane column in conjunction with a deisohexanizer in accordance with the prior art;

(5) FIG. 4 represents the concentration profile inside a conventional deisohexanizer column in accordance with the prior art;

(6) FIG. 5 is a graph illustrating a concentration profile inside a conventional hexane column in accordance with the prior art;

(7) FIG. 6 illustrates a process scheme using a DWC with top dividing wall in accordance with embodiments of the disclosure;

(8) FIG. 7 illustrates a process scheme using a DWC with a bottom dividing wall in accordance with embodiments of the disclosure;

(9) FIG. 8 illustrates a process scheme using a DWC with a middle dividing wall in accordance with embodiments of the disclosure; and

(10) FIG. 9 illustrates a process scheme for a hexane polishing unit in accordance with embodiments of the disclosure.

DETAILED DESCRIPTION

(11) It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

(12) In the instant disclosure, an isomerization unit deisohexanizer (“DIH”) column is a dividing wall column (DWC) and is used to produce hexane, as a byproduct, along with the main product of isomerate. An n-hexane rich product (about 32-45 wt % n-C.sub.6) is obtained from the isomerization unit DIH column. Other C.sub.6 components (e.g., 2-methylpentane, 3-methylpentane and methylcyclopentane) make up the rest of the product. Besides high-purity n-hexane, the other products from the isomerization unit column are light isomerate (mainly i-C.sub.5) and heavy isomerate (mainly i-C.sub.6).

(13) Conventionally, high-purity hexane can be obtained by distillation in a deisohexanizer column followed by a hexane column. FIG. 3 illustrates a prior art system 300 for producing high-purity hexane. System 300 includes a DIH column 302 and a hexane column 304. Bottoms from DIH column 302 are fed to hexane column 304. Hexane column 304 produces high-purity hexane and heavy isomerate. System 300 has certain disadvantages. For example, the boiling points of C.sub.6 components are very close to that of the non-desirable components (e.g., C.sub.5 paraffins and naphthenes). To obtain good separation, the process of system 300 requires a significant number of trays (leading to a bigger column) as well as high reboiling energy.

(14) Systems with two columns also have the problem of back-mixing of a concentrated hexane stream within the DIH column. Thus, the energy spent in concentrating the hexane stream to higher purity levels is lost due to the back-mixing of hexane with the heavy isomerate at the bottom of the column. The concentration profiles of light isomerate, hexane, and heavy isomerate fractions in the DIH column are shown in FIG. 4. Additional energy is spent in the hexane column (see FIG. 5) to separate the hexane from the heavy isomerate, thereby reducing an overall energy efficiency of the process.

(15) A solution to this thermodynamic problem is to separate the hexane from the heavy isomerate at the peak of its concentration within DIH column 302 to optimize an energy requirement of system 300. Furthermore, since two columns are required for the process, capital costs increase due to additional equipment and bigger plot space. For such applications, a DWC concept can be applied to provide an alternative solution.

(16) A DWC combines operations of the two columns (e.g., DIH column 302 and hexane column 304) into a single column thereby lowering both the capital and energy (operating) costs by approximately 20-30%. In general, dividing wall columns are broadly classified into three types based on the location of a wall disposed with the DWC. The wall can be located in top section, a middle section, or a bottom section. In a DWC scheme, three (or four) products are typically withdrawn from the DWC: a lightest cut and a heaviest cut are withdrawn at the top and bottom, respectively, of the DWC; and a middle cut is obtained from the DWC as a side cut. In a majority of DWCs in operation worldwide, the dividing wall is present in a middle section of the DWC. In DWCs, a location of the dividing wall primarily dictates the movement of vapor within the column and can affect a quality of the separation. The dividing wall in a DWC leads to the splitting of the top (or bottom or middle) half of the column into two separate columns, which produces two high-purity products at the top (or bottom or middle). Top, bottom, and middle dividing walls are shown in FIGS. 6, 7, and 8, respectively. A feed (e.g., stable isomerate) is introduced on one side of the dividing wall (pre-fractionation) and the side cut is withdrawn from the other side (main fractionation). The process scheme is similar to that of a direct or indirect sequence of a two-column conventional separation.

(17) The systems of FIGS. 6-8 have several advantages. For example, FIG. 6 illustrates a top DWC system 600 that includes a top DWC 602. A top dividing wall 604 divides a top section 606 of top DWC 602 into a first side 608 and a second side 610. Top dividing wall 604 extends from a top of top DWC 602 and terminates above a bottoms section of DWC 602. By incorporating top dividing wall 604 at the top of top DWC 602, first side 608 and second side 610 remain isolated from one another with no chance of contamination or back-mixing. Because first side 608 and second side 610 are two parallel sections created in a single column, a higher number of trays are available to achieve better fractionation within the same column. This tends to reduce the final height of the column by lowering the number of trays required. Lastly, first side 608 and second side 610 operate independent of each other. One side can operate as a rectification section while the other side can operate as the absorption (or rectification) section, with independent controls on each side. In this type of DWC, there are two separate overhead systems, a first overhead system 612 and a second overhead system 614. Each overhead system 612, 614 can include, for example, a heat exchanger (e.g., an air-cooled heat exchanger) and an overhead receiver.

(18) First side 608 of top DWC 602 receives a stable isomerate feed (e.g., from an upstream process, such as an isomerization unit). Top DWC 602 outputs light isomerate as a lights product from first side 608 and high-purity hexane (e.g., having a hexane purity of 40 to 45 wt.-%) as a lights product from second side 610. A portion of the light isomerate can be returned to first side 608 as reflux and the remainder can be collected as a portion of total isomerate produced by top DWC system 600. Top DWC 602 also outputs heavy isomerate as a bottoms product. A portion of the bottoms product can be returned to top DWC 602 after passing through a reboiler 616 and the remainder can be output with the remainder of light isomerate as the other portion of the total isomerate output by top DWC 602. The heating duty provided by the reboiler helps to move the middle boiling components up the other side of the top dividing wall.

(19) FIG. 7 illustrates a bottom DWC system 700 that includes a bottom DWC 702. Bottom DWC 702 works on a similar principle as that of top DWC 602 and includes a bottom dividing wall 704. Bottom dividing wall 704 extends from a bottom of bottom DWC 702 and divides a bottom section 703 of the bottom DWC 702 into a first side 706 and a second side 708. Compared to top DWC system 600, bottom DWC 702 includes two bottom reboilers, a first reboiler 710 and a second reboiler 712, and a common rectifying section 714. Both sides 706, 708 in bottom DWC 702, are controlled independent of each other.

(20) First side 706 of bottom DWC 702 receives a stable isomerate feed (e.g., from an upstream process, such as an isomerization unit). Bottom DWC 702 outputs light isomerate as a lights product from the top of bottom DWC 702 and high-purity hexane (e.g., having a hexane purity of 40 to 45 wt.-%) as a bottoms product from second side 708. A portion of the light isomerate can be returned to the top of bottom DWC 702 as reflux from common rectifying section 714 and the remainder can be collected as a portion of total isomerate produced by bottom DWC system 700. Bottom DWC 702 also outputs heavy isomerate as a bottoms product from first side 706. A portion of the heavy isomerate can be returned to first side 706 after passing through first reboiler 710 and the remainder can be output with the remainder of light isomerate as the other portion of the total isomerate output by bottom DWC 702.

(21) FIG. 8 illustrates a middle DWC system 800 that includes a middle DWC 802. Middle DWC 802 works on a similar principal as that of top DWC 602 and bottom DWC 702 and includes a middle dividing wall 804. Middle dividing wall 804 extends the length of a middle portion 803 of middle DWC 802 and divides middle DWC 802 into a first side 806 and a second side 808 to pre-fractionate the feed and concentrate the middle boiling components on the second side 808 to produce a high-purity product. Middle dividing wall 804 does not extend all the way to the top or bottom of middle DWC 802. Middle DWC system 800 also includes a reboiler 810 and a rectifying section 812. As in the top and bottom DWCs 602, 702, there is no back-mixing of the feed and the side cut. This results in an efficient separation with lesser consumption of reboiling duty.

(22) First side 806 of middle DWC 802 receives a stable isomerate feed (e.g., from an upstream process, such as an isomerization unit). Middle DWC 802 outputs light isomerate as a lights product from a top of middle DWC 802. A portion of the light isomerate can be returned to the top of middle DWC 802 as reflux and the remainder is output as a portion of the total isomerate produced by middle DWC 802. Middle DWC 802 outputs heavy isomerate as a bottoms product. A portion of the heavy isomerate can be returned to the bottom of middle DWC 802 after passing through reboiler 810 and the remainder is output as the other portion of the total isomerate produced by middle DWC 802. High-purity hexane (e.g., having a hexane purity of 40 to 45 wt.-%) is produced as a side cut from second side 808.

(23) FIG. 9 illustrates a system 900 for processing hexane in a polishing unit (polisher) comprising a polishing reactor 902, a mixer 904, three heat exchangers 906, 908, 910 and a hexane stripper column 912, to ensure that benzene content of the hexane is in a desired range. In some embodiments, the hexane is fed from an isomerization unit DWC. In the polishing unit, hydrogen and raw hexane are preheated before being routed through an adsorbent. The hydrogen and raw hexane are mixed in the mixer 904 prior to the preheating. The preheating can include heating from one or more heat exchangers. For example, as illustrated in FIG. 9, preheating can be accomplished via heat exchangers 906, 908, and 910. Heat exchanger 906 uses an output from the hexane stripper column 912 to heat the hydrogen/raw hexane feed. Heat exchanger 908 uses an output of the polishing reactor 902 to heat the hydrogen/raw hexane feed. Heat exchanger 910 uses an additional heat source (e.g., an upstream feed) to heat the hydrogen/raw hexane feed. In various embodiments, one or more of heat exchangers 906, 908, and 910 are optional. After adsorption, lights from polishing reactor 902 are separated from hexane in a hexane stripper column 912. The hexane from hexane stripper column 912 can be used as a heat source in heat exchanger 906, collected as an end product, or used as part of a downstream process.

(24) The advantage of producing hexane, as a byproduct from isomerization unit, is that its quality is much superior to that produced by the traditional solvent extraction process. Hexane produced from isomerization unit meets specifications for food, pharmaceutical, and polymer grade hexanes. Additionally, the cost of production of hexane, as a byproduct from isomerization unit, is much lower than the cost of hexane produced by the solvent extraction process.

Examples

(25) Tables 1-6 below demonstrate various operating parameters for conventional processes and systems and processes and systems of the instant disclosure that utilize DWCs.

(26) TABLE-US-00001 TABLE 1 Performance of Conventional Design versus Dividing Wall Column Design DWC Design Top Bottom Middle Conventional Dividing Dividing Dividing Items DIH Hexane Wall Wall Wall Columns Units Column Column Column Column Column Feed kg/hr 67,955 67,955 67,955 67,955 Light Isomerate kg/hr 43,262 42,474 43,303 43,237 Heavy Isomerate kg/hr 11,993 12,862 11,934 12,106 High-purity Hexane kg/hr 12,700 12,700 12,700 12,700 n-Hexane content wt % 39.4 35.2 38.2 37.9 Reboiler Duty MMkcal/hr 13.5 9.1 16.4 16.7 16.4 Condenser Duty MMkcal/hr 16.8 9.2 19.8 20.1 19.7 Energy Savings % — 27.4 26.1 27.4

(27) TABLE-US-00002 TABLE 2 Material Balance of Conventional DIH Colum + Hexane Column High- Stream Light purity Heavy Description Units Feed Isomerate Hexane Isomerate Flowrate kg/hr 67,955 43,262 12,700 11,993 Composition profile H2 wt. % 0.00 0.00 0.00 0.00 C3− wt. % 0.00 0.00 0.00 0.00 C4 wt. % 0.29 0.46 0.00 0.00 Paraffins i-Pentane wt. % 8.40 13.19 0.00 0.00 n-Pentane wt. % 2.68 4.21 0.00 0.00 C5 wt. % 0.60 0.94 0.00 0.00 Naphthenes C6 wt. % 58.82 80.44 40.66 0.06 i-Paraffins Hexane wt. % 8.24 0.69 39.36 2.50 C6 wt. % 16.08 0.07 19.97 69.69 Naphthenes Benzene wt. % 0.00 0.00 0.00 0.00 C7 wt. % 2.22 0.00 0.00 12.57 Paraffins C7 wt. % 2.68 0.00 0.01 15.17 Naphthenes Total wt. % 100.00 100.00 100.00 100.00 Reboiler MMkcal/ 13.5 + 9.1 Duty hr Condenser MMkcal/ 16.8 + 9.2 Duty hr

(28) TABLE-US-00003 TABLE 3 Material Balance of Top Dividing Wall Column High- Stream Light purity Heavy Description Units Feed Isomerate Hexane Isomerate Flowrate kg/hr 67,955 42,474 12,700 12,862 Composition profile H2 wt. % 0.00 0.00 0.00 0.00 C3− wt. % 0.00 0.00 0.00 0.00 C4 wt. % 0.29 0.46 0.00 0.00 Paraffins i-Pentane wt. % 8.40 13.44 0.00 0.00 n-Pentane wt. % 2.68 4.29 0.00 0.00 C5 wt. % 0.60 0.96 0.00 0.00 Naphthenes C6 wt. % 58.82 80.35 44.66 0.48 i-Paraffins Hexane wt. % 8.24 0.46 35.21 7.21 C6 wt. % 16.08 0.04 20.13 66.41 Naphthenes Benzene wt. % 0.00 0.00 0.00 0.00 C7 wt. % 2.22 0.00 0.00 11.74 Paraffins C7 wt. % 2.68 0.00 0.00 14.16 Naphthenes Total wt. % 100.00 100.00 100.00 100.00 Reboiler MMkcal/ 16.4 Duty hr Condenser MMkcal/ 17.0 + 2.8 Duty hr

(29) TABLE-US-00004 TABLE 4 Material Balance of Bottom Dividing Wall Column High- Stream Light purity Heavy Description Units Feed Isomerate Hexane Isomerate Flowrate kg/hr 67,955 43,303 12,700 11,934 Composition profile H2 wt. % 0.00 0.00 0.00 0.00 C3− wt. % 0.00 0.00 0.00 0.00 C4 wt. % 0.29 0.46 0.00 0.00 Paraffins i-Pentane wt. % 8.40 13.28 0.00 0.00 n-Pentane wt. % 2.68 4.25 0.00 0.00 C5 wt. % 0.60 0.95 0.00 0.00 Naphthenes C6 wt. % 58.82 80.33 40.70 0.07 i-Paraffins Hexane wt. % 8.24 0.69 38.22 2.51 C6 wt. % 16.08 0.04 21.08 69.53 Naphthenes Benzene wt. % 0.00 0.00 0.00 0.00 C7 wt. % 2.22 0.00 0.00 12.64 Paraffins C7 wt. % 2.68 0.00 0.00 15.25 Naphthenes Total wt. % 100.00 100.00 100.00 100.00 Reboiler MMkcal/ 12.0 + 4.7 Duty hr Condenser MMkcal/ 20.1 Duty hr

(30) TABLE-US-00005 TABLE 5 Material Balance of Middle Dividing Wall Column High- Stream Light purity Heavy Description Units Feed Isomerate Hexane Isomerate Flowrate kg/hr 67,955 43,237 12,700 12,106 Composition profile H2 wt. % 0.00 0.00 0.00 0.00 C3− wt. % 0.00 0.00 0.00 0.00 C4 wt. % 0.29 0.46 0.00 0.00 Paraffins i-Pentane wt. % 8.40 13.20 0.00 0.00 n-Pentane wt. % 2.68 4.21 0.00 0.00 C5 wt. % 0.60 0.94 0.00 0.00 Naphthenes C6 wt. % 58.82 80.40 40.87 0.07 i-Paraffins Hexane wt. % 8.24 0.69 37.94 4.22 C6 wt. % 16.08 0.09 20.68 68.69 Naphthenes Benzene wt. % 0.00 0.00 0.00 0.00 C7 wt. % 2.22 0.00 0.22 12.25 Paraffins C7 wt. % 2.68 0.00 0.28 14.76 Naphthenes Total wt. % 100.00 100.00 100.00 100.00 Reboiler MMkcal/ 16.4 Duty hr Condenser MMkcal/ 19.7 Duty hr

(31) TABLE-US-00006 TABLE 6 Comparison of Quality of Hexane produced by the Solvent Extraction Process vs from Isomerization Unit Solvent Extraction Process Isomerization Unit n-hexane % wt. >40 Sulfur mg/kg 1.0-5.0 <0.5 Benzene mg/kg 130-240 <3.0

(32) The term “substantially” is defined as largely but not necessarily wholly what is specified (and includes what is specified; e.g., substantially 90 degrees includes 90 degrees and substantially parallel includes parallel), as understood by a person of ordinary skill in the art. In any disclosed embodiment, the terms “substantially,” “approximately,” “generally,” “around,” and “about” can be substituted with “within [a percentage] of” what is specified, where the percentage includes 0.1, 1, 5, and 10 percent.

(33) The foregoing outlines features of several embodiments so that those skilled in the art can better understand the aspects of the disclosure. Those skilled in the art should appreciate that they can readily use the disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the disclosure, and that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the disclosure. The scope of the invention should be determined only by the language of the claims that follow. The term “comprising” within the claims is intended to mean “including at least” such that the recited listing of elements in a claim are an open group. The terms “a,” “an” and other singular terms are intended to include the plural forms thereof unless specifically excluded.