Abstract
Process and plant for recovering LPG in a refinery process combining the use of sponge absorber, deethanizer and debutanizer. The process and plant enable high LPG recovery and removal of hydrogen sulphide in the LPG product to low levels.
Claims
1. A system for recovering LPG from a fractionation section of a refinery process, the system comprising: a sponge absorber comprising: an inlet for receiving a stream comprising a stripper overhead product; a top section for producing an off-gas stream, the top section comprising an outlet for withdrawing the off-gas stream; and a bottom section for producing a light oil stream, the bottom section comprising an outlet for withdrawing the light oil stream; a deethanizer column comprising: an inlet for receiving the light oil stream from the sponge absorber; a top section for producing an overhead vapour in the form of a C.sub.3 stripped stream, the top section being comprising an outlet for withdrawing the overhead vapour wherein the system is configured such that the entire C.sub.3 stripped stream flows from the outlet of the top section into the inlet of the sponge absorber without any portion of the C.sub.3 stripped stream being condensed and recycled directly back to the deethanizer column; and a bottom section for producing a bottoms stream, the bottom section comprising an outlet for withdrawing the bottoms stream; and a debutanizer column comprising: an inlet for receiving the bottoms stream from the deethanizer column; a top section for producing a LPG product stream, the top section comprising an outlet for withdrawing the LPG product stream; and a bottom section for producing a stabilized naphtha stream, the bottom section comprising an outlet for withdrawing the stabilized naphtha stream.
2. The system of claim 1, wherein the deethanizer column and debutanizer column are provided as a single column in the form of a dividing wall column.
3. The system of claim 2, wherein the dividing wall column comprises a dividing wall disposed at the upper section of the dividing wall column.
4. The system of claim 1 comprising an overhead condenser and overhead drum to receive the off-gas stream from the top section of the sponge absorber.
5. The system of claim 1 comprising means for supplying a portion of the stabilized naphtha stream to the off-gas stream.
6. The system of claim 1 comprising means for supplying the overhead vapour in the form of the C.sub.3 stripped stream to the sponge absorber inlet.
7. The system of claim 1 comprising means for supplying a portion of vapour from the debutanizer column as stripping vapour for the deethanizer column.
8. The system of claim 1 in which the refinery process comprises hydroprocessing.
9. The system of claim 8, wherein the hydroprocessing comprises hydrocracking, crude distillation, fluid catalytic cracking, delayed coking, or combinations thereof.
10. The system of claim 1, wherein the only off-gas from the system is the off-gas from the sponge absorber.
Description
BRIEF DESCRIPTION OF THE FIGURES
(1) The accompanying figures show specific embodiments of the invention.
(2) FIG. 1 shows a simplified process and plant arrangement for the recovery of LPG from the fractionation section of a hydrocracking unit according to the prior art comprising a debutanizer column upstream the deethanizer column.
(3) FIG. 2 shows a simplified process and plant arrangement according to a particular embodiment of the present invention comprising a deethanizer column upstream the debutanizer column.
(4) FIG. 3 shows a simplified process and plant arrangement according to another particular embodiment of the present invention comprising a deethanizer column upstream the debutanizer column with thermal coupling between these columns.
(5) FIG. 4 shows a simplified process and plant arrangement according to yet another particular embodiment of the invention in which the deethanizer and debutanizer are provided within a single column.
DETAILED DESCRIPTION
(6) In FIG. 1 (prior art) stripper overhead product 1 from a hydrocracking unit (not shown) is mixed with debutanizer overhead gas 2 to form a feed stream 3 that is passed through sponge absorber (LPG absorber) 4. At the top section of sponge absorber an off-gas 5 is withdrawn and mixed with stabilized naphtha stream 6. The mixture 7 is cooled and condensed in cooler 8, sent to overhead drum 9 and pumped to the top section of sponge absorber 4 as sponge oil 10. From overhead drum 9 a sour off-gas 11 and condensed water 12 are withdrawn. The condensed water 12 is mixed with bottoms stream 13 from the sponge absorber 4 to form sour water stream 14. From the bottom section of the sponge absorber 4 a light oil stream 15 rich in LPG (rich in propane and butane) is withdrawn and pumped to a fractionation section comprising a debutanizer column 16, LPG amine absorber 25, and deethanizer column 31. The light oil stream 15 enters first into the debutanizer column 16. In the bottom section of the debutanizer 16 a reboiler 17 is provided, and bottoms stream in the form of stabilized naphtha 6 is cooled in heat exchanger 18 and then trim cooler 19 before being mixed with off-gas 5 from sponge absorber 4. Heat exchanger 18 may suitably be used as means to heat the light oil stream 15 instead. From the top section of debutanizer 16 an overhead gas 20 is cooled and condensed (condenser not shown) and sent to overhead drum 21. From this overhead drum 21 condensed sour water stream 22 is withdrawn as so is a debutanizer overhead gas 2 which is mixed with the stripper overhead product stream 1 as described above. A reflux stream 23 is separated from overhead drum 21 and a portion is pumped as debutanizer reflux stream to top section of debutanizer column 16, while the other portion 24 is used as feed stream to LPG amine absorber 25 (amine scrubber). This unit 25 removes hydrogen sulphide and carbon dioxide by using a lean amine such as an alkanolamine (monoethanolamine, diethanolamine, etc) as is well known in the art. Overhead liquid stream 27 from LPG amine absorber 25 is washed with a circulating water stream 29 and passed through LPG coalescer unit 28 from which water 29 is withdrawn. The washed LPG stream 30 is fed to the deethanizer column 31. From the LPG amine absorber 25 a bottoms rich amine stream 32 is withdrawn, mixed with water 29 from coalescer 28 and sent as stream 33 to an amine regenerator. Deethanizer overhead gas 34 is cooled and condensed (condenser not shown) and sent to deethanizer overhead drum 35 where off-gas stream 36 and water stream 37 are withdrawn. A condensed HC-stream 38 is used as reflux to the deethanizer column 31. The bottom section of the deethanizer 31 is provided with reboiler 39. Finally, an LPG product stream 41 is cooled in air cooler (not shown) and a trim cooler.
(7) FIG. 2 shows an embodiment according to the present invention. Stripper overhead product 100 from a hydrocracking unit (not shown) is mixed with deethanizer overhead gas in the form of a C.sub.3 stripped stream 101 to form a feed stream 102 that is passed through sponge absorber (LPG absorber) 103. At the top section of sponge absorber 103 off-gas 104 is withdrawn and mixed with stabilized naphtha stream 105. The mixture 106 is cooled and condensed in cooler 107, sent to the overhead drum 108 and pumped to the top section of sponge absorber 103 as sponge oil 109. From overhead drum 108 a sour off-gas 110 and condensed water 111 are withdrawn. The condensed water 111 is mixed with bottoms water stream 112 from the sponge absorber 103 to form sour water stream 113. From the bottom section of the sponge absorber 103 a light oil stream 114 rich in LPG, i.e. rich in propane and butane, is withdrawn and pumped to a fractionation section comprising deethanizer column 115 and debutanizer column 118. The light oil stream 114 enters first into the deethanizer column 115. In the bottom section of the deethanizer 115 a reboiler 116 is provided, and bottoms stream 117 is then used as feed for debutanizer column 118. The reboiler duty is adjusted to reduce H.sub.2S content in bottoms stream to below say 2 wppm or lower. The top section of deethanizer column 115 is purposely not provided with overhead condenser and thereby there is no recycling of overhead liquid as reflux into this fractionating step. This overhead vapour stream 101 (C.sub.3 stripped stream) is mixed with the stripper overhead stripper product 100 as described above. Bottoms stream 117 from bottom section of deethanizer 115 is fed to debutanizer column 118 comprising a top and bottom section. In the bottom section a reboiler 119 is provided and bottoms stream in the form of stabilized naphtha stream 127 is cooled in heat exchanger 120 and then in trim cooler 121. The cooled stabilized naphtha stream is split to a naphtha product stream 128 and a recycle stream 105, which is mixed with off-gas 104 from sponge absorber 103 as described above. From the top section of debutanizer 118 an overhead gas 122 is cooled and condensed (condenser not shown) and sent to the overhead drum 123. From this overhead drum 123 a portion is pumped as debutanizer reflux stream 124 to top section of debutanizer column 118, while the other portion 125 is cooled in heat exchanger 126 and recovered as LPG product.
(8) In FIG. 3 the thermal coupling of deethanizer and debutanizer columns is illustrated. Same ref. numerals as FIG. 2 apply with the modification that reboiler 116 of deethanizer 115 becomes obsolete. In FIG. 3 means (e.g. a conduit) 127 is provided for supplying a portion of vapour from the debutanizer column 118 as stripping vapour for the deethanizer column 115.
(9) In FIG. 4 the deethanizer and debutanizer columns are provided as a single column in the form of a dividing wall column 215. Stripper overhead product 200 from a hydrocracking unit (not shown) is mixed with overhead gas in the form of a C.sub.3 stripped stream 201 to form a feed stream 202 that is passed through sponge absorber (LPG absorber) 203. At the top section of sponge absorber 203 off-gas 204 is withdrawn and mixed with stabilized naphtha stream 205. The mixture 206 is cooled and condensed in cooler 207, sent to overhead drum 208 and pumped to the top section of sponge absorber 203 as sponge oil 209. From overhead drum 208 a sour off-gas 210 and condensed water 211 are withdrawn. The condensed water 211 is mixed with bottoms stream 212 from the sponge absorber 203 to form sour water stream 213. From the bottom section of the sponge absorber 203 a light oil stream 214 rich in LPG is withdrawn and pumped to a fractionation section comprising now a single column in the form of a dividing wall column 215. In a dividing wall column, three product streams are formed: a C.sub.3 stripped stream 201, a stabilized naphtha stream and an LPG product stream. A dividing wall 219 extends from the top of the column down to a relevant tray within the column, suitable down to 70% from the top, for instance down to the seventh plate assuming a column having 10 trays. From the top section on one side of dividing wall, the C.sub.3 stripped stream 201 is withdrawn and mixed with the stripper overhead product 200. In the bottom section a reboiler 216 is provided and bottoms stream in the form of stabilized naphtha stream 225 is cooled in heat exchanger 217 and then in trim cooler 218 before being split to a recycle stream 205 and a stabilized naphtha product stream 226. Stream 205 is mixed with off-gas 204 from sponge absorber 203. From the top section on other side of the dividing wall column 215, an overhead gas 220 is cooled and condensed (condenser not shown) and sent to the overhead drum 221. From this overhead drum 221 a portion is pumped as reflux stream 222 to top section of dividing wall column 219, while the other portion 223 is cooled in heat exchanger 224 and recovered as LPG product.
EXAMPLE
(10) The performance of the different processes/plants as shown in FIG. 1-4 was calculated. The performance is shown in terms of LPG recovery as wt % LPG in product stream (e.g stream 41 in FIG. 1 or stream 125 in FIG. 2, with respect to stripper overhead product stream 1 in FIG. 1 or stream 100 in FIG. 2), H.sub.2S content in LPG product stream, electrical consumption of pumps used for circulating different streams in the processes, total reboiler duty, lean amine consumption in LPG amine absorber, and consumption of boiler feed water (BFW) as wash water. The results are presented in Table 1. Notably also, as one moves from the embodiment of FIG. 1 to FIG. 4, plot size area is significantly reduced.
(11) TABLE-US-00001 TABLE 1 Performance FIG. 1 FIG. 2 FIG. 3 FIG. 4 parameter prior art invention invention invention LPG recovery % 95.6 94.8 93.7 93.7 H.sub.2S in LPG, wppm 35 9 9 9 Relative electric 97 43 48 48 consumption Total reboiler 5.9 6.9 5.7 5.7 duty, MW Lean amine 10000 none none none consumption in LPG absorber, kg/h BFW wash water, 191 none none none kg/h
