Use of top dividing wall in isomerization unit

Abstract

The invention is directed to a combined naphtha hydrotreating (NHT) and isomerization process scheme, which includes dividing wall columns (DWC) that replace multiple distillation columns and allow optimized heat integration within the system. The disclosed design provides reductions in both capital and energy costs compared to conventional schemes.

Claims

1. An isomerization unit comprising: a first dividing wall column comprising: a line configured to receive a feed of naphtha; a first side configured as a stabilizer column; a first condenser configured to reflux a portion of a first overheads stream from the first side of the first dividing wall column to a first overheads section of the first side of the first dividing wall column; a second side configured as a naphtha splitter column; a second condenser configured to reflux a portion of a second overheads stream from the second side of the first dividing wall column to a second overheads section of the second side of the first dividing wall column; and a first top dividing wall that separates the first side of the first dividing wall column and the second side of the first dividing wall column; and a second dividing wall column comprising: a first side configured as a depentanizer column; a third condenser configured to reflux a portion of a third overheads stream from the first side of the second dividing wall column to a third overheads section of the first side of the second dividing wall column; a second side configured as a deisohexanizer column a fourth condenser configured to reflux a portion of a fourth overheads stream from the second side of the second dividing wall column to a fourth overheads section of the second side of the second dividing wall column; and a second top dividing wall that separates the first side of the second dividing wall column and the second side of the second dividing wall column.

2. The isomerization unit of claim 1, further comprising: a deisopentanizer column coupled to the first dividing wall column configured to receive a light naphtha overhead stream from the second side of the first dividing wall column; an isomerization reactor coupled to the deisopentanizer column and configured to receive a bottoms stream from the deisopentanizer column; and a stabilizer column coupled to the isomerization reactor and configured to receive a stream comprising unstable isomerate from the isomerization reactor and to feed stable isomerate to the second dividing wall column.

3. The isomerization unit of claim 1, wherein the first dividing wall column comprises a first bottoms reboiler configured to receive a first bottoms stream from the first dividing wall column and to feed a portion of the first bottoms stream back to the first dividing wall column.

4. The isomerization unit of claim 1, wherein the second dividing wall column comprises a second bottoms reboiler configured to receive a second bottoms stream from the second dividing wall column and to feed a portion of the second bottoms stream back to the second dividing wall column.

5. The isomerization unit of claim 1, wherein the second dividing wall column includes a side cut from a position near a bottom of the second dividing wall column that does not contain the second top dividing wall.

6. The isomerization unit of claim 1, further comprising a packed flash drum coupled to the first dividing wall column and configured to receive a third bottoms stream from the first dividing wall column and an off-gas stream from a stabilizer column to generate a lean solvent that is fed back to the first dividing wall column.

7. An isomerization method being performed in an isomerization unit of claim 1, wherein a naphtha stream is provided to the first dividing wall column, wherein the method comprises the following steps: processing the naphtha stream with the first dividing wall column to produce an off-gas stream of non-condensable components, a light naphtha overhead stream with the second side of the first dividing wall column and a heavy naphtha bottoms product; and producing, via the second dividing wall column, a first stream comprising C.sub.6 isomerate product and a second stream comprising a heavy isomerate product.

8. The method of claim 7, wherein the first dividing wall column comprises a bottoms reboiler configured to receive a first bottoms stream from the first dividing wall column and to feed a portion of the first bottoms stream back to the first dividing wall column.

9. The method of claim 7, wherein the second dividing wall column comprises a bottoms reboiler configured to receive a second bottoms stream from the second dividing wall column and to feed a portion of the second bottoms stream back to the second dividing wall column.

10. The method of claim 7, wherein the second dividing wall column includes a side cut from a position near a bottom of the second dividing wall column that does not contain the second top dividing wall.

11. The method of claim 7, further comprising feeding a third bottoms stream from the first dividing wall column and an off-gas stream from the stabilizer column to a packed flash drum to generate a lean solvent that is fed back to the first dividing wall column.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 represents a prior art system of a combined naphtha hydrotreating and isomerization unit;

(2) FIG. 2 represents a process scheme in accordance with an embodiment of the invention for using DWC technology in a naphtha hydrotreating and isomerization unit;

(3) FIG. 3 represents a process scheme in accordance with an embodiment of the invention for a top DWC stabilizer/naphtha splitter design; and

(4) FIG. 4 represents a process scheme in accordance with an embodiment of the invention for a top DWC depentanizer/deisohexanizer design.

(5) DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(6) Embodiments of the invention are directed to an isomerization process wherein individual columns are replaced and/or combined together using DWC technology with the objective of minimizing utility consumption.

(7) Referring now to FIG. 2, an isomerization process scheme 200 is shown. Scheme 200 includes a first divided wall column (DWC) 210 and a second DWC 240. First DWC 210 includes a top dividing wall 211 that divides a top portion 212 of first DWC 210 into a first side 213 and a second side 214. In the embodiment illustrated in FIG. 2, first side 213 is configured to operate as a stabilizer column and second side 214 is configured to operate as a naphtha splitter column. In some embodiments, first side 213 includes a first overheads section 215 and second side 214 includes a second overheads section 216. A first condenser 217 is coupled to first overheads section 215 and is configured to condense overheads received therefrom. Reflux from first condenser 217 can be fed back to first overheads section 215. A second condenser 218 is coupled to second overheads section 216 and is configured to condense overheads received therefrom. Reflux from second condenser 218 can be fed back to second overheads section 216. A bottoms reboiler 219 is coupled to first DWC 210 and is configured to receive a bottoms stream from first DWC 210 and to return a heated stream back to a bottoms section 220 of first DWC 210.

(8) Second DWC 240 includes a top dividing wall 241 that divides a top portion 242 of second DWC 240 into a first side 243 and a second side 244. In the embodiment illustrated in FIG. 2, first side 243 is configured to operate as a depentanizer column and second side 244 is configured to operate as a deisohexanizer column. In some embodiments, first side 243 includes a first overheads section 245 and second side 244 includes a second overheads section 246. A first condenser 247 is coupled to first overheads section 245 and is configured to condense overheads received therefrom. Reflux from first condenser 247 can be fed back to first overheads section 245. A second condenser 248 is coupled to second overheads section 246 and is configured to condense overheads received therefrom. Reflux from second condenser 248 can be fed back to second overheads section 246. A bottoms reboiler 249 is coupled to second DWC 240 and is configured to receive a bottoms stream from second DWC 240 and to return a heated stream back to a bottoms section 250 of second DWC 240.

(9) An exemplary process flow for scheme 200 begins by feeding a stream 230 to first side 213 of first DWC 210. In the embodiment of FIG. 2, first side 213 is a stabilizer and second side 214 is a naphtha splitter. In some embodiments, stream 230 is sourced from a naphtha hydrotreating reactor. First side 213 removes non-condensable components from stream 230 as off-gas stream 231. A stabilized bottoms product descends first side 213 and enters second side 214. Second side 214 separates the stabilized bottom product from first side 213 into a light naphtha overhead stream 232 and heavy naphtha bottoms stream 233.

(10) The light naphtha overhead stream 232 is comprised mainly of C.sub.5-C.sub.6 components. Light naphtha overhead stream 232 is fed to a deisopentanizer column 251, which concentrates i-C.sub.5 as an overhead stream 234. The remaining C.sub.5-C.sub.6 components are obtained as a bottoms stream 235 of deisopentanizer column 251 and are fed to an isomerization reactor 252 for octane upgrading via isomerization reactions. A stream 236 containing unstable isomerate from isomerization reactor 252 is further processed in a stabilizer column 254. Light hydrocarbons are removed in an overhead stream 237 as off-gas and a stream 238 containing stable isomerate is sent to first side 243 of second DWC 240 to remove a concentrated stream of C.sub.5 components.

(11) In the embodiment of FIG. 2, first side 243 is a depentanizer and second side 244 is a deisohexanizer An overheads stream 261 that is rich in C.sub.5 is recycled from the first overheads section 245 of second DWC 240 to deisopentanizer column 251 upstream to remove i-C.sub.5 product. A bottoms product stream descends first side 243 and enters second side 244. A C.sub.6 isomerate product stream 262 is removed from second DWC 240 as an overhead stream and a heavy isomerate product stream 263 (mainly C.sub.7+ cut) is removed from second DWC 240 as bottoms stream. An n-C.sub.6 rich stream 264 is removed as a side cut from second DWC 240 and is recycled to the isomerization reactor 252.

(12) FIG. 3 is a side-by-side comparison of columns 102 and 104 of FIG. 1 with first DWC 210 of FIG. 2. Top dividing wall 211 segregates the top portion 212 of first DWC 210 into first side 213 and second side 214, namely the pre-fractionation side and the product side for reference. The process scheme is designed to remove the non-condensable as off-gas stream 231. Additionally, the scheme concentrates middle boiling components (C.sub.5-C.sub.7) as light naphtha overhead stream 232 on the other side, while the heaviest boiling components (heavy naphtha) are recovered at the bottom of the column as heavy naphtha bottoms stream 233. On the feed side of the top dividing wall 211, a lean naphtha stream reduces the loss of valuable C.sub.5 components into the off-gas by means of absorption. On the product side of top dividing wall 211, the middle boiling C.sub.5-C.sub.7 components move to the top and the heavy boiling components move downwards. The scheme, hence, performs a combination of distillation and absorption within the same column. Moreover, first condenser 217 on the absorption side is a partial water-cooled condenser, while the second condenser 218 on the distillation side is a total condenser using an air-cooled exchanger.

(13) DWC 212 operates at a high operating pressure of 100 psig and utilizes MP steam as the heating medium in bottoms reboiler 219 (e.g., a thermosiphon reboiler). The high temperature of the column allows heat integration with the downstream deisopentanizer column 251 that operates at a significantly lower pressure.

(14) Deisopentanizer column 251 is a conventional distillation column which removes an isopentane concentrated stream at the top (overhead stream 234). A reboiler of deisopentanizer column 251 utilizes LP steam, while another reboiler is heat integrated with the hot overhead C.sub.5-C.sub.7 vapors from the upstream DWC 212.

(15) FIG. 4 is a side-by-side comparison of columns 112 and 114 of FIG. 1 with second DWC 240 of FIG. 2. Second DWC 240 separates four product streams: a C.sub.5 recycle stream 261, C.sub.6 isomerate stream 262, and C.sub.7+ stream 263 along with a n-C.sub.6 recycle stream 264. Two total condensers 247, 248 are available on both sides of top dividing wall 241. In some embodiments, condensers 247, 248 are air-cooled exchangers. Bottoms reboiler 249 at the bottom of second DWC 240 operates on LP steam.

(16) In embodiments of the invention, column overhead pressures are maintained via a pressure controller on the overhead vapor product line. The pre-fractionation side has reflux coming from the overhead condenser.

(17) Table 1 below highlights energy and cost savings of scheme 200 versus prior art system 100.

(18) TABLE-US-00001 TABLE 1 Parameters Units Conventional Design DWC Design No. of columns — 6 4 Energy Savings % Base 30% of Base Capital Cost % Base 70% of Base

(19) Table 2 below highlights operating parameteres of scheme 200 versus prior art system 100.

(20) TABLE-US-00002 TABLE 2 DWC Design Conventional Design Stabilizer/Naphtha NHT Stabilizer Splitter Operating pressure psig 100  100  Reboiler utility MP Steam MP steam Naphtha Splitter — Operating pressure psig 75 — Reboiler utility MP Steam — Isomerization Isomerization Stabilizer Stabilizer Operating pressure psig 150  75 Reboiler utility MP Steam LP Steam/Heat integration with reactor effluent Depentanizer Depentanizer/ Deisohexanizer Operating pressure psig 20 20 Reboiler utility LP Steam LP Steam Deisohexanizer — Operating pressure psig  7 — Reboiler utility LP Steam —