Stage and system for compressing cracked gas

Abstract

A compression stage for the compression of cracked gas, the compression stage comprising a liquid separating means for separating liquid components from gaseous components of a cracked gas, a compressor connected to the liquid separating means, a gas cooling means connected to the compressor for cooling the compressed gas from the compressor, wherein the gas cooling means are cooled by a first cooling fluid from the cooling fluid source. The stage further comprises gas precooling means connected to the liquid separating means cracked gas, having an inlet for receiving the cracked gas.

Claims

1. A compression stage for the compression of cracked gas, the compression stage comprising: a liquid separating means for separating liquid components from gaseous components of a cracked gas; a compressor connected to the liquid separating means for compressing the gaseous components from the liquid separating means; a gas cooling means connected to the compressor for cooling a compressed gas from the compressor, wherein the gas cooling means are cooled by a first cooling fluid from a cooling fluid source; the stage further comprising gas precooling means connected to the liquid separating means for cooling the cracked gas before separation, the gas precooling means having an inlet for receiving the cracked gas, wherein the gas precooling means comprises a heat exchanging means and an absorption cooling means; wherein the heat exchanging means is cooled by a second cooling fluid from the absorption cooling means, wherein the absorption cooling means is arranged for generating the second cooling fluid using first cooling fluid from the cooling fluid source and heating fluid from a heating fluid source; wherein the first cooling fluid for the absorption cooling means is arranged to be supplied from a primary gas cooler of the gas cooling means of the stage, and wherein the first cooling fluid from the absorption cooling means is arranged to be supplied to a secondary gas cooler of the gas cooling means.

2. The stage according to claim 1, wherein the heat exchanging means of the gas precooling means comprises a chiller.

3. The stage according to claim 1, wherein the heating fluid from the heating fluid source for the absorption cooling means, has a temperature in a range of 70-110° C.

4. A system for cracked gas compression, comprising a plurality of cascaded compression stages in accordance with claim 1, wherein a first compression stage has its gas precooling means connected to a cracked gas source, and wherein a subsequent compression stage has its gas precooling means connected to the gas cooling means of its preceding compression stage.

5. A system according to claim 4, wherein the compressor in each stage is driven by a common compressor drive.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1a shows a schematic diagram of a compressor system having compressor stages according to the state of the art.

(2) FIG. 1b shows a schematic diagram of a compressor system having compressor stages according to the state of the art.

(3) FIG. 2a shows a compressor stage according to an embodiment of the invention.

(4) FIG. 2b shows a compressor stage according to an embodiment of the invention.

(5) FIG. 2c shows a schematic diagram of a compressor system having compressor stages according to an embodiment of the invention.

(6) FIG. 3 shows a schematic diagram of an absorption chiller according to the state of the art.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) As described in the prior art, many configurations are possible from three to five compressor stages with inter stage coolers in between. A configuration according to the prior art having five stages is shown in FIG. 1a. Each compressor stage comprises a quench column V-101, V-111, V-121, V-131, V-141 for liquid-gas separation, a compressor K-111, K-121, K-131, K-141, K-151 and an inter stage cooler H-111/112, H-121/122, H-131/132, H-141/142, H-151/151. After the inter stage coolers the water and hydrocarbon condensates can be routed 102, 116, 126, 136, 146, 156 for further processing for example to a fractionating tower, or to another compressor stage, to the quench column preceding the compressor. In liquid cracking most of the gasoline fraction containing the C6 to C8 aromatics is condensed in the inter stage coolers of the compressor also many of the C4 and C5 components. The compressors K-111-K-151 in this example are driven by a common turbine engine X-101 via a common shaft 208.

(8) In FIG. 1b another configuration known in the state of the art is shown similar to the configuration of FIG. 1b with rerouting of condensed water and hydrocarbons to a preceding stage, wherein the quench columns V-111-V-141 have an inlet for receiving the liquid components from the succeeding stage.

(9) Two computer simulations have been performed for a cracked gas compressor of five stages in accordance with FIGS. 1a and 1b respectively of compressing a cracked gas containing 100 t/h of ethylene originating from naphtha cracked with a mild cracking severity (propylene/ethylene ratio of 0.58). Inter stage cooler outlets 114-154 temperatures are kept at 30° C. Each compressor K-111-K-151 has an isentropic efficiency of 80%.

(10) Cracked gas (101) from the quench tower is sent to suction drum vessel (V-101), any condensates (102) are pumped back to the quench tower, while the cracked gas (1×1) is sent to the inlet of the (1.sup.st, 2.sup.nd, 3.sup.rd, 4.sup.th, 5.sup.th) compressor stage (K-1×1) at an absolute pressure of 0.15 MPa.sub.a. The outlet from compressors K-1×1 is cooled against cooling water from cooling water source 104 by heat exchangers (H-101 and H-1×2) to a temperature of 30° C., in suction drum vessel V-111, V-121, V-131, V-141 and V-151 the condensed hydrocarbons and water 106, 116, 126, 136, 146, 156 are separated from the cracked gas 101, 111, 121, 131, 141, 151, respectively.

(11) Between the 3.sup.rd and 4.sup.th compressor stage a gas treatment in a gas treatment unit Z-100 is performed as described in the prior art. In the gas treatment unit Z-100 the gas is purified by removing gasses such as carbon dioxide, dihydrogen sulphide, acid components, etc. The cracked gas is compressed to 3.6 Mpa.sub.a (stream 161) and the calculated compressor power in 36.2 MW.sub.mech. In the scheme of FIG. 1a the condensed streams (116, 126, 136, 146, 156) are not recycled (directly) back to the compressor but are treated in a fractionating tower, flash separation and or decanter to recover the products and might be pumped, heated and or cooled to desired pressure and temperature for further treatment.

(12) In the scheme of FIG. 1b, the condensed streams 116, 126, 136, 146, 156 are not separately treated but recycled back to the previous stage: 116 back to the quench tower or first stage suction drum (V-101). Stream 126 to V-111, stream 136 to V-121, stream 146 to V-132 and stream 156 to V-141, as visualized in FIG. 1b. For this scheme the total compressor power was calculated to be 39.5 MW.sub.mech.

(13) In FIG. 2a a compressor stage S-201 is shown comprising a suction drum V-201, a compressor K-211 driven by turbine engine X-201 via shaft 271, an interstage cooler H-211/H-212, and a pre-cooler H-201. The outlet 214 of the inter stage cooler H-211/H-212 can be connected to a subsequent compressor stage (S-201, see for example FIG. 2c).

(14) Cracked gas in inlet 201 is precooled in pre-cooler H-201 to 15° C. (range could be 1-25° C.) and sent to suction drum V-201 for separating liquid and gas components. Cooler H-201 transfers the heat from the cracked gas in a chilled water loop (208), that is cooled by absorption chiller Y-201. The absorption chiller uses heat from quench water source 274 by cooling it down from 80° C. to 73° C. and returning it to the warm quench water header 275. The absorption chiller Y-201 transfers the heat from the quench water and the chilled water loop to cooling water, from cooling water source 273 at 25° C. to cooling water header 272 at 35° C.

(15) In suction drum V-201 condensed water and hydrocarbons are separated out 203, pumped and heated to separate it in a gas stream 204 to be sent back to the quench tower and a liquid stream 205 of which the hydrocarbons can be separated from the water in a decanter (not shown in FIG. 2a).

(16) Quench water from the quench tower in the quench water source 274 is typically around 85-75° C. in temperature, but the use of this in the steam cracker is limited to some reboilers in distillation columns, where a reboiler for the C3 olefins/paraffin separation is the major consumer. A large amount of the quench water cannot be used and heat has to be disposed of by cooling to the air and cooling water. Thus quench water can be advantageously used to operate the absorption chiller Y-201. The operation of an absorption chiller will be elucidated in FIG. 3 and the corresponding description.

(17) The cold water source in FIG. 2b for the chiller 201 is the absorption chiller Y-211 which is cooled by cooling water from the primary cooler H-212. The cooling water from the absorption chiller Y-211 is subsequently supplied to the secondary cooler H-211. Thus the cooling water from the cooling water source 273 is used more efficiently.

(18) In FIG. 2c both configurations for using cooling water from the chiller 201 in the first stage according to FIGS. 1a and 1b respectively are shown. Chiller 201 of stage S-201 receives cooling fluid from absorption chiller Y-201 which is cooled directly with cooling water from cooling water source 273, whereas chiller H-213 of stage S-202 receives its cooling fluid from absorption chiller Y-211, which it cooled by cooling fluid from the primary gas cooler H-212 of stage S-201.

(19) In FIG. 2c further compressor stages S-203-S-205 are shown in a cascade of compressor stages. Compressor stage S-203 is provided with a gas treatment unit Z-200 with the same function as in the configuration in FIGS. 1a and 1b.

(20) In FIG. 2c, the vapor from suction drum V-201 (211) is sent to the first compressor stage S-201 and compressed by the compressor K-211 to 0.3 MPa.sub.a (212) where it is cooled by H-211 and H-212 to a temperature in a range of 20-50° C., preferably around temperature of 30° C., and further cooled by H-213 to temperature in a range of range 1-25° C., preferably around 15° C., and supplied to suction drum V-211 of the second stage S-202 via chiller H-213 where it is pre-cooled for stage S-202.

(21) In suction drum V-211 of stage S-202 condensed water and hydrocarbons are separated out 216 of which the hydrocarbons can be separated from the water in a decanter. The vapor from suction drum V-211 is supplied to the compressor K-221, from where it is cooled by gas coolers H-221, H-222.

(22) Cooler H-213 transfers the heat from the cracked gas in a chilled water loop 218, that is cooled by absorption chiller Y-211. The absorption chiller Y-211 uses heat from quench water source 274 by cooling it down from 80° C. to 73° C. and returning it to the warm quench water header 275. Heat from H-211, H-212 and Y-211 is transferred to a cascade of cooling water 217 that is first heated by H-212, then by Y-211 and then by H-211.

(23) Above sequence is repeated in stages S-203 and S-205. The vapor from stage S-202 is precooled in chiller H-223, supplied to suction drum V-221 from where it is supplied to the compressor stage K-231 and compressed to 3 bara (232) where it is cooled by H-231 and H-232 to a temperature of 30° C. (range 20-50° C.). etc.

(24) In between stages S-203 and S-204 a gas treatment unit Z-200 as described is included. In this case H-233 is located downstream of this gas purification step. Additional hydrocarbon containing streams can be added to suction vessels V-202, V-201, V-211, V-221, V-232, V-241, V-251 but are not shown.

(25) Compared to the system as described in FIG. 1a, the compressor work is strongly reduced, the total compressor work is reduced from 36.2 MW.sub.mech to 33.5 MW.sub.mech. In Table 1 the results per compressor stage are provided.

(26) TABLE-US-00001 TABLE 1 Method 100 Method 200 (See FIG. 1a according (see FIG. 2c according to the prior art) to the invention) Compressor Work (MW.sub.mech) Compressor Work (MW.sub.mech) K-111 7.8 K-211 5.9 K-121 7.7 K-221 6.4 K-131 7.4 K-231 6.8 K-141 7.0 K-241 7.1 K-151 6.4 K-251 7.4 total 36.2 33.5

(27) From the comparative example in table 1 it is clear that using precooling of the cracked gas prior to compression saves approximately 3 MW mechanical power.

(28) Another important effect of stage inlet cooling with an absorption chiller are the compressor stage outlet temperatures, which in this example of FIG. 2c is in a range of 9 to 17° C. lower than in FIG. 1a. This reduces the fouling in the compressor and inter stage coolers. Moreover, this enables a less expensive compressor design with fewer compressor stages.

(29) In table 2 inlet and outlet temperatures are shown of the compressors K-111, K-121, K-131, K-141, K-151 in FIG. 1a and compressors K-211, K-221, K-231, K-241, K-251 in FIG. 2c.

(30) TABLE-US-00002 TABLE 2 Work Method 100 Work Method 200 (see FIG. 1a state (See FIG. 2c according of the art) to the invention) Inlet Outlet Inlet Outlet temper- temper- temper- temper- ature ature ature ature Compressor ° C. ° C. Compressor ° C. ° C. K-111 30 74 K-211 15 65 K-121 34 79 K-221 19 64 K-131 34 81 K-231 17 64 K-141 32 81 K-241 16 65 K-151 31 83 K-251 16 67

(31) The quench water usage of Y-241 amounts to: 672 t/h. Hot quench water from header 274 is fed to absorption chiller Y-241 at 80° C. and after taking 6.3 MW.sub.th of heat it is returned at 73° C. as warm quench water to header 275. This warm quench water, can still be used, for example as reboiler heat (H-261) in a C3 fractionating tower. This cascade of quench water allows optimal use of the available heat transferred from the quench tower. The same principles and amounts apply to absorption chillers Y-201, Y-211, Y-221, Y-231.

(32) Regarding the cascaded cooling water streams 217, 227, 237 and 247, an example is provided for 247 in accordance with FIG. 2c. Cooling water is supplied first to the secondary cracked gas cooler H-242, than to absorption chiller Y-241, then to first cracked gas cooler H-241. This saves cooling water. Example: 1400 t/h cooling water (247) at 25° C. from cooling water source 273 is fed to second 4.sup.th stage inter stage cooler H-242, where it absorbs 1.9 MW.sub.th of heat and exits at a temperature of 26.2° C. Next the cooling water is used in the absorption chiller to absorb 10.6 MW.sub.th of transferred heat from the chiller (heat from quench water+heat removed from the process) and as a result is heated to 32.8° C. Then the cooling water is routed to primary 4.sup.th stage inter stage cooler H-241, where it absorbs 3.5 MW.sub.th of heat and is heated to 35° C.

(33) The scheme as described in FIG. 1b with full recycles can also apply to FIG. 2c. The difference compared to FIG. 2c is that product streams 216, 226, 236, 246 and 256 are not separate products, but are recycled back to the respective previous compressor stage suction drum.

(34) In table 3 the compressor work is shown of the system according to FIG. 1b according to the state of the art, compared to the system of FIG. 2c wherein the product streams 216, 226, 236, 246 and 256 are recycled.

(35) TABLE-US-00003 TABLE 3 Method 100 Method 200 (See FIG. 1b according (see FIG. 2c according to the prior art) to the invention) Compressor Work (MW.sub.mech) Compressor Work (MW.sub.mech) K-111 7.8 K-211 5.9 K-121 7.7 K-221 6.4 K-131 7.4 K-231 6.8 K-141 7.0 K-241 7.1 K-151 6.4 K-251 7.4 total 36.2 33.5

(36) From the comparative example in table 3 it is clear that using precooling of the cracked gas prior to compression saves approximately 3 MW mechanical power, also when the intermediate product streams 216, 226, 236, 246 and 256 are recycled in a previous stage.

(37) FIG. 3 show a schematic diagram of an absorption chiller 300 as used in the compression stages S-201-S-205 is explained below.

(38) The absorption cooling cycle in an absorption chiller, like the mechanical vapor compression refrigeration cycle, utilizes the latent heat of evaporation of a refrigerant to remove heat from the entering chilled water. Vapor compression refrigeration systems use a refrigerant and a compressor to transport the refrigerant vapor to be condensed in the condenser. The absorption cycle, however, uses water as the refrigerant 305 and an absorbent lithium bromide solution as absorbent 306 to absorb the vaporized refrigerant. Heat is then applied to the solution to release the refrigerant vapor from the absorbent. The refrigerant vapor is then condensed in the condenser.

(39) The basic single-effect absorption chiller 300 of FIG. 3 includes a generator 303, condenser 301, evaporator 310 and absorber 311 with refrigerant 305 and absorbent 306 as the working solutions. The generator 303 utilizes a heat source, for example a fluid 307 such as hot gas from a burner, steam or hot water to evaporate the refrigerant 302 from the absorbent solution 306 in the generator 303. The refrigerant vapor 302 that is released travels to the condenser 301 where it is condensed back into liquid refrigerant 305, transferring the heat to the cooling tower water 304 in heat exchanger 315. Once condensed, the liquid refrigerant 305 is sent to evaporator 310 where it is distributed over the evaporator tubes 308, removing the heat from the chilled water 316 in the evaporator tubes 308 and vaporizing the liquid refrigerant 305 into refrigerant vapor 314.

(40) The absorbent solution 306 from the generator 303 passes into the absorber 311 via cooling tubes 309, while absorbing the refrigerant vapor 314 from the evaporator 310 and dilutes itself. The diluted absorbent solution 312 is then pumped back via pump 313 to the generator 305 where the cycle is started again.

(41) The absorption chiller 300 has the capability to consume the low temperature heat released from condensing the water/steam from the reactor effluent and provide cooling duty in the temperature range of 0-30° C.

(42) The absorption chiller process is not a very energy efficient process, typically only 70% of the thermal heat input is converted to thermal cooling duty. However, because of the process can utilize low temperature waste heat, this is not an issue when sufficient waste heat is available.

(43) Although lithium bromide is used as a working solution in the example other working media are also possible. Absorption chillers based on an ammonia water mixture are common as well and could be used instead of the lithium bromide absorption chiller.

(44) In the compressor stage 201 in FIG. 2a, the heating fluid 307 for heating the generator 303 is obtained from the quench water source 274. The cooling water 304 for the condenser 301 is obtained from the cooling water source 273, or from the cooling water outlet of gas cooler H-212. The evaporator tubes 308, provide for cooling the cooling liquid 316 for the precooling chiller H-201 via loop 208 in FIG. 2a.

(45) The embodiments described above are given by way of example only. Deviations and modifications to these embodiments are possible without departing from the scope of protection as set out in the claims below.

REFERENCE NUMERALS

(46) 101, 114, 124, 134, 144, 154 cracked gas inlet K-111, K-121, K-131, K-141, K-151 compressor 108 common drive axle 112, 122, 132, 142, 152 compressor outlet 111, 121, 131, 141, 151 compressor inlet 117, 127, 137, 147, 157 cooling water inlet 113, 123, 133, 143, 153 interconnection 103 cooling water source 104 cooling water header 139 gas purifier inlet H-111, H-121, H-131, H-141, H-151 primary gas cooler H-112, H-122, H-132, H-142, H-152 secondary gas cooler 116, 126, 136, 146, 156 intermediate product streams Z-100 gas purifier 201, 202, 215, 225, 235, 245, 254 cracked gas inlet K-211, K-221, K-231, K-241, K-251 compressor H-211, H-221, H-231, H-241, H-251 primary gas cooler H-212, H-222, H-232, H-242, H-252 secondary gas cooler Y-201, Y-211, Y-221, Y-232, Y-241 absorption chiller V-201, V-211, V-221, V-232, V-241 liquid separator V-202 liquid separator H-201, H-213, H-223, H-233, H-243 precooling chiller 216, 226, 236, 246, 256 intermediate product streams 273 cooling water source 272 cooling water header 274 quench water source 275 quench water header 300 absorption chiller 301 condenser 302 refrigerant vapor 303 generator 304 condenser cooling water 305 refrigerant 306 absorbent 307 heating fluid 308 evaporator tubes 309 cooling tubes 310 evaporator 311 absorber 312 diluted absorbent 315 heat exchanger 316 chilled water