Crack Gas Generator, method for crack gas generation

The invention relates to a crack-gas-generator comprising: at least one cracker,

at least one first crack-gas-compressor downstream of said cracker for compressing a gas flow containing hydrocar bons from a cracking process of said cracker.

Further the invention relates to a method for crack gas gen eration comprising the following steps:

a. a feed stream is fed into a cracker

b. said feed stream is increased in temperature during a residence time in said cracker and transformed into a crack gas stream,

c. transferring heat energy leaving said cracker direct ly or indirectly to a working fluid of a Brayton-cycle by means of a first heat exchanger,

d. said Brayton-cycle converting energy of the working fluid into mechanical energy by expanding the working fluid .

A method for cracking hydrocarbon is known from US2904502A.

A cogeneration process using augmented Brayton-cycle is known from US 4,392,346.

From documents US 2015/089949 Al, WO 2018/044558 A1 steam me thane reforming with power generation from waste heat is known. Document US 7 622 033 B1 shows a crack gas generation process. The Brayton cycle is known to be operated with waste heat from US 9 624 793 Bl, US 2017/081980 Al .

The production of olefins is of major importance for the pet rochemical industries. Due to their basic importance for many chemical products - in particular, the importance of light olefins like ethylene, propylene, butane etc. - the demand and generation of these products by crack gas generators re spectively corresponding processes for crack gas generation is still increasing. In particular Naphtha (may be produced from natural gas con densates, petroleum distillates, and the distillation of coal tar and peat) but also gas oil and natural gas are preferably fed into a steam cracker to obtain the desired products re spectively olefins. The terminology of the invention consid ers the compression of the crack gas as one step of the crack gas generation. A typical crack gas generator respectively a typical crack gas generation process comprises a hot section and a cold section. The hot section comprises a convection zone, a radiation zone, a quench section, a fractionation column and a compression of the crack gas.

The cold section downstream of the hot section comprises sev eral stages of fractionating and separating the different hy drocarbons from each other and from other components or impu rities of the crack gas by stepwise cooling and separating the crack gas. For this purpose

Thermal Cracking of hydrocarbons takes place at temperatures between 800°C up to 1200°C in a cracker comprising a furnace.

While the cracking reaction itself is an endothermic reaction considering the bond dissociation energy a large positive en tropy change resulting from the fragmentation of large mole cules into respectively several smaller pieces results in an exothermal reaction. Ethylene, Propylene is primarily pro duced by thermal cracking of hydrocarbons in the presence of steam at approximately 830°C.

Intense efforts are undertaken to recover the heat energy from the crack gas leaving the furnace to reduce the overall energy consumption respectively to reduce the amount of Crack gas generation can be done with or without dilution of the crack gas with steam. The addition of steam to the feed en hances the ethylene yield and reduces the coking tendency in the coils of the furnace.

It is one object of the invention to enhance the energy re covery in the process improving the overall efficiency. In order to improve the efficiency the invention proposes a crack gas generator of the incipiently defined type compris ing the further features of the characterizing portion of claim 1.

Further the invention proposes a method for crack gas genera tion according to the incipiently defined method comprising the further features of the independent method claim. The re spective dependent claims relate to preferred embodiments of the invention.

Compared to a conventional steam based cycle system the car bon-dioxide Brayton-cycle can achieve high efficiencies over a wide temperature range - heat flux range or power range - of the heat source respectively the crack gas leaving the cracker. This is in particular true for supercritical carbon dioxide as a working fluid of the Brayton-cycle. In addition, the compactness of the components of the carbon dioxide Bray ton-cycle - in particular, the heat exchangers - result in a significantly smaller system foot print and therefore lower investment costs and operating costs.

One preferred embodiment of the invention provides that the working fluid is always kept at a pressure above 73.75 bar and above a temperature of 70.98°C. Keeping carbon dioxide above these thermodynamic parameters respectively - above the critical parameters - in a supercritical state - avoids a phase change between gaseous and liquid state of the working fluid. Consequently, all disadvantages of a phase change are avoided - in particular, specific design features necessary to cope with a phase change - in particular, in heat exchang ers. In addition, the supercritical state of the working flu id of the Brayton-cycle makes the operating range more flexi ble with regard to pressure and temperature as long as the operation takes place above the critical point.

Another preferred embodiment provides that said working fluid is heated in said heat exchanger up to 600°C, preferably above 600°C. Another preferred embodiment provides that said at least one expander of the Brayton-cycle is mechanically coupled to said first crack gas compressor. It is further proposed that said crack gas generator comprises at least one first refrigerant compressor, preferably more than one refrigerant compressor being driven by an expander of the Brayton-cycle preferably the respective expander of the Brayton-cycle being directly mechanically coupled to the respective refrigerant compres sor .

Another preferred feature of the method provides that said working fluid of carbon dioxide is permanently kept in a su percritical state in the Brayton-cycle.

The above-mentioned attributes and other features and ad vantages of the invention and the manner of attaining them will become more apparent and the invention itself will be better understood by reference to the following description of the currently best mode of carrying out the invention tak en in conjunction with the drawings, wherein:

Figure 1: shows a schematic flow diagram of a crack gas generator according to the invention and showing the features of the method of crack gas generation according to the invention.

Figure 1 is a schematic depiction of a crack gas generator CGG according to the invention showing the features of the method of crack gas generation according to the invention.

The crack gas generator CGG according to the invention com prises a hot section HTS and a cold section CLS . The essen tial features of the invention are incorporated in the hot section HTS. Said hot section HTS comprises a furnace-and- heat-exchanger-arrangement FHA, a closed loop Brayton-cycle BCY, a steam drum STR, a quencher QNC, several crack gas com pressors CGI, CG2, CG3, being driven by a first expander EX1 of the Brayton-cycle BCY, several refrigerant compressors CR1, CR2, CR3, CR4, being driven by a second expander EX2 and a third expander EX3 of said Brayton-cycle BCY. Said cold section CLS comprises several fractionation columns CL1, CL2, CL3 for fractioning different hydrocarbons and sep arating these streams from each other based on their boiling points .

Impurities are removed before the feed is fed to a first heat exchanger HE1. A water content in vapor phase is removed by a non-depicted drying process in the compression process of said crack gas stream CGS preferably between a third and fourth compressor CR3, CR4.

The crack gas generator CGG comprises a crack gas CRC in the hot section HTS located in said furnace-and-heat-exchange- arrangement FHA in a radiant furnace RGF section. This crack er CRC is the core of the crack gas process being heated up to a temperature of 1100 °C to crack down large hydrocarbon molecules into smaller ones. The radiant furnace RDF is heat ed by fuel being supplied to the furnace-and-heat-exchange- arrangement FHA. For the purpose of heat recovery the fur- nace-and-heat-exchange-arrangement FHA comprises said first heat exchanger HE1 of said Brayton-cycle BCY . The Brayton- cycle BCY circulates a working fluid WRF in a closed loop, wherein said working fluid is carbon-dioxide C02 in a super critical state. In first heat exchanger HE1 a first cooling fluid CF1 - here a flue gas from said radiant furnace RGF section - provides heat to crack gas stream CGS (for preheat ing) as well as to said working fluid WRF of said Brayton- cycle BCY.

According to the method of the invention said crack gas gen eration takes place with said crack gas generator CGG. A feed stream FDS - here Naphta NPF - is fed into said cracker CRC. In cracker CRC said feed stream FDS is increased in tempera ture during a residence time RDT and transformed into a crack gas stream CGS. Said first cooling fluid CF1 - here the flue gas from said radiant furnace RGF section - provides heat to said working fluid WRF by means of said first heat exchanger HE1 of said Brayton-cycle BCY. In this example this heat transfer takes place indirectly by said first cooling fluid CF1 passing said cracker CRC and said first heat exchanger HE1. Other heat transfer options are possible as well - for example direct heat transfer.

Direct heat transfer - in this context - means that the heat is not transferred from said cracker to said first cooling fluid CF1 but that said working fluid WRF of said Brayton- cycle BCY passes said cracker CRC and is cooling said crack er CRC directly.

Said cooling fluid CF1 is preferably an open cycle circulated air stream. Said first cooling fluid CF1 takes heat energy from said radiant furnace RDF and carries this heat energy to a convection zone CNV of said furnace-and-heat-exchange- arrangement FHA. Said first cooling fluid CF1 passes several heat exchangers HE3, HE5, HE1, HE4, HE2 before leaving the furnace-and-heat-exchange-arrangement FHA. Said Brayton-cycle BCY converts the energy of the working fluid WRF respectively the supercritical carbon dioxide CCg into mechanical energy TWO by means of a first expander EX1, a second expander EX2 and a third expander EX3. After the working fluid WRF has been expanded in the three expanders EX1, EX2, EX3 it enters downstream a high temperature recuperator HTR and a low tem perature recuperator LTR to exchange heat with a stream of said working fluid WRF on its way to the downstream located first heat exchanger HE1. After passing the high temperature recuperator HTR and the low temperature recuperator LTR the working fluid WRF enters a cooler COL and downstream of the cooler a supercritical carbon-dioxide compressor SCC to de liver the working fluid WRF at a higher pressure level for the next loop of expansion after temperature increase. Down stream of said supercritical carbon-dioxide compressor SCC the working fluid WRF enters the energy receiving side of said low temperature recuperator LTR and further downstream said high temperature recuperator HTR before entering said downstream located first heat exchanger HE1.

Said feed stream FDS is diluted with dilution steam DST after temperature increase in said second heat exchanger HE2. Down stream of the dilution said feed stream enters a third heat exchanger HE3 (being split in two heat exchangers HE3 in the example of figure 1) taking heat energy from said first cool ing fluid CF1. The diluted and heated feed stream FDS enters said cracker CRC to be cracked during a resident time T=RDT . Downstream of said cracker the cracked gas CGS enters a steam drum STR to be quenched generating a steam STM entering down stream of said steam drum STR a fifth heat exchanger HE5 to be superheated in the furnace-and-heat-exchange-arrangement FHA by said first cooling fluid CF1. Downstream of said steam drum STR the crack gas stream CGS enters a quencher QNC to be cooled and stop any crack reaction and to be separated from pyrolysis gas oil PGO. The remaining crack gas stream CGS further enters the first crack gas compressors CGI, the sec ond crack gas compressor CG2 and after passing a caustic wash CWS the third crack gas compressor CG3 in a serial order. The three crack gas compressors CGI, CG2, CG3 are driven by said first expander EX1, wherein the driving first expander EX1 is mechanically coupled to the three crack gas compressors CGI, CG2, CG3. Downstream of the crack gas compression a first fractionation column CL1 divides said crack gas stream CGS in a stream of heavier crack gas HCG and lighter crack gas LCG. The lighter crack gas LCG enters downstream a second frac tionation column CL2 dividing the lighter crack gas LCG into ethylene C ₂ H ₄ and ethane C2H6. The heavier crack gas HCG en ters a downstream third fractionation column CL3 dividing the heavier crack gas HCG into propylene C3H6 and liquid pressure gas LPG. Upstream of said second fractionation column CL2 and said third fractionation column CL3 a heat exchanging cooler is provided to prepare the streams of hydro carbon for being split in the downstream columns. Said coolers are operated with an ethylene refrigerant train ERT and a propylene re frigerant train PRT respectively. Said ethylene refrigerant train ERT operates said cooler COE at a temperature of -108°C and said propylene refrigerant train PRT operates said cooler COP at a temperature of -50°C. Said ethylene refrigerant train ERT is operated with a first refrigerant compressor CR1 and a second refrigerant compressor CR2 both being driven by the second expander EX2 of said Brayton-cycle BOY. Said pro pylene refrigerant train PRT is operated with a third refrig erant compressor CR3 and a fourth refrigerant compressor CR4 both being driven by said third expander EX3 of said Brayton- cycle BCY.

Said furnace-and-heat-exchange-arrangement FHA is further used to heat up a boiler feed water BFW by means of a fourth heat exchanger HE4. Said crack gas stream CGS has a tempera ture of 830°C when leaving said furnace-and-heat-exchange- arrangement FHA entering said steam drum STR. Said convection section CNV is operated at a medium temperature of approxi- mately 605°C of said first cooling fluid CF1.