EP3294846A1

EP3294846A1 - Process and apparatus for the production of calcined petroleum coke

Info

Publication number: EP3294846A1
Application number: EP16720822.2A
Authority: EP
Inventors: Edgar Gasafi; Peter Sturm; Günter Schneider
Original assignee: Outotec Finland Oy
Current assignee: Outotec Finland Oy
Priority date: 2015-05-12
Filing date: 2016-05-03
Publication date: 2018-03-21
Also published as: CN107636128B; CN107636128A; US20180112143A1; DE102015107433A1; WO2016180683A1

Abstract

The invention describes a process for the production of calcined petroleum coke. In a reactor green petroleum coke is combusted as a fuel with a substoichiometric amount of oxygen at a temperature of between 900 and 1400 °C to provide the required energy to evaporate volatile matter included in the green petroleum coke. Calcined petroleum coke and flue gas are obtained, whereby the flue gas comprises at least 80 wt-% of the volatile matter contained in the green petroleum coke. The flue gas is at least partially recycled to the reactor as an additional fuel.

Description

Process and apparatus for the production of calcined petroleum coke

The present invention relates to a process and an apparatus for the production of calcined green petroleum coke, wherein in a reactor green petroleum coke is combusted as a fuel with a substoichiometric amount of oxygen at a temperature of between 900 and 1400 °C to provide the required energy to evaporate volatile matter included in the green petroleum coke, wherein calcined petroleum coke and flue gas are obtained, and wherein the flue gas contains at least 80 wt-% of the volatile matter contained in the green petroleum coke.

Green petroleum coke (often abbreviated pet coke or petcoke) is a solid carbonization product from high-boiling hydrocarbon fractions obtained at temperatures below 630°C, e.g. in an oil refinery. Coking processes that can be employed for making petcoke include contact coking, fluid coking, flexicoking and delayed coking. Other coke has traditionally been delivered from coal.

This coke can either be fuel grade (high in sulfur and metals) or anode grade (low in sulfur and metals). The raw coke directly out of the coker is often referred to as green coke. In this context, "green" means unprocessed. Further, the petroleum coke is calcined to remove volatiles and then further processed at a temperature of at least 2500 °C to produce high-purity graphite petroleum coke, which is used e.g. for making anodes used in the steel and titanium smelter industry. Green petroleum coke is mainly composed of elemental carbon (between 80 and 95 wt-%) in a polycrystallinet carbon matrix which is filled with volatile components such as hydrocarbons, sulfur-compounds and hydrogen. The volatile content is generally between 9 and 21 wt % (see e.g. lUPAC (International Union of Pure and Applied Chemistry). 1995. Recommended Terminology for the Description of Carbon as a Solid. Pure Appl. Chem. 67(3):473-506. The process and apparatus of the invention exclusively relate to the calcining step and do not refer to the subsequent production of high-purity graphite petro- leum coke.

According to the state of the art, green petroleum coke is calcined in rotary kilns or rotary hearth plants by heating up to 1400 °C (see e.g. Predel, "Petroleum Coke", Ullmann's Encyclopedia of Industrial Chemistry, 2006). The necessary energy is usually provided exclusively by the incineration of a fuel/air mixture inside the reactor. This process has several drawbacks:

- High maintenance costs due to the rotating equipment

- Necessity of high temperatures (up to 1400°C), resulting in considerable ther- mal strain of the equipment as well as high energy costs

- A long retention time to achieve removal of volatile components and a sufficient amount of calcination

- Decreased carbon yield due to oxidation reactions of the green petroleum coke The problem to be solved is, therefore, to provide a process and an apparatus for calcining green petroleum coke that overcomes the disadvantages of the prior art, especially regarding energy requirements and carbon yield.

Said problem is solved by a process as defined in claim 1 .

Specifically, the process of the invention comprises the combustion of green petroleum coke as a fuel inside of a reactor with a substoichiometric amount of oxygen at a temperature of between 900 and 1400 °C to provide the required energy to evaporate volatile matter included in the green petroleum coke. Thereby, calcined petroleum coke and flue gas are obtained, wherein the flue gas contains at least 80 wt-% of the volatile matter contained in the green petroleum coke. According to the invention, the flue gas is at least partially recycled to the reactor as an additional fuel. The process of the invention is superior to the prior art processes due to various reasons.

The volatile matter results in carbon monoxide (CO) and hydrogen (H₂) as major components. The content depends largely on the composition of the feedstock and correspondents to the proximate analysis data. The incineration of said compounds in the reactor protects the carbon in the petroleum coke from incineration/oxidation, since the volatile matter are easier combustible than the petroleum coke. Concluding, the recycled flue gas provides part of the energy necessary for calcining the petroleum coke, thereby increasing the carbon yield since less carbon of the green petroleum coke is burned.

Another advantage of the process according to the invention is the reduction of thermal strain on the calcining equipment due to the reduced temperature inside the reactor compared to prior art processes. This has multiple benefits: a re- duced temperature means reduced energy consumption for processing. It also allows to use lower grade refractory material inside the furnace, which reduces the capital investment for the plant.

The term "at least partially recycled" preferably refers to an amount of at least 10 vol-% of the flue gas being fed back into the reactor, more preferably to an amount of 20 -30 vol-% of the flue gas being fed back into the reactor.

The process of the invention employs a Lambda (λ-value) of less than 1 , preferably 0.25 -0.7. The Lambda indicates the air-to-fuel ratio in the furnace. If lamb- da equals 1 , the air-fuel mixture is at the Stoichiometric ratio. The process of the invention achieves a reduction of volatile matter to below 1 wt-% from a starting concentration of usually about 10 wt-% in untreated green petroleum coke. Due to the recycling of at least part of the flue gas and the reducing conditions in the reactor the process of the invention is superior to the state of the art both with regards to minimizing the loss of carbon from the green petroleum coke as well as the required amount of energy.

Furthermore, retention times of less than 40 minutes are reduced compared to prior art processes due to the efficient removal of volatile components from the green petroleum coke.

Preferably, the green petroleum coke particles have a maximum diameter of 5 mm. Thereby, a complete calcining with a maximum carbon yield can be obtained.

In one embodiment of the invention, the reactor is defined as a rotary kiln. This reactor type features the advantage of providing a continuous process with very good mass transfer. Further, the circulated gas stream is relatively small. In another embodiment of the invention, the reactor is defined as a fluidized bed reactor. A fluidized bed reactor is characterized by a very good mass and heat transfer. The overall heat loss in a process according to the invention is very small. Even more preferred is an annular fluidized bed reactor. By using a fluidized bed reactor the reactor temperature can be lowered to temperatures between 950 and 1200 °C due to the small heat loss. Thereby, the carbon yield can be increased even further since less fuel is required for providing the needed energy. Preferably, the recirculated flue gas is used as a fluidizing gas for the fluidizing gas reactor. So, the overall gas stream is kept constant in comparison to a conventional process. Further, the fuel can be introduced in the region of the reactor, namely into the fluidized bed, where the calcining of the green petrole- urn coke takes place and where the energy and therefore fuel burning is required.

Preferably, the removed flue gas is fed into a Venturi pre-heater prior to recycling into the reactor for removal of solid particles. Removed solid particles can be recycled to the reactor, too for increasing the carbon yield. In another embodiment, the Venturi is replaced by a filter.

Even more preferably, the flue gas is fed from the Venturi pre-heater into a cyclone to achieve an additional purification and subsequently into at least one heat exchanger prior to recycling. Thereby, the energy of the flue gas can be used somewhere else in the process.

Prior to recycling the flue gas from the at least one heat exchanger into the reactor, the flue gas may preferably be fed into a filter to remove any residual solid particles.

As the oxygen source for the substoichiometric combustion, pure oxygen, air or any other gas with at least 5 vol-% of oxygen is introduced in the reactor. Using pure oxygen, a very sensitive controlling of the introduced oxygen amount is possible. Also, the introduced gas stream is very small. On the opposite, air is the cheapest oxygen source. Using another gas stream with at least 5 vol-% of oxygen offers the possibility of using a gas stream incurred somewhere else in the process. It may be preferred that the gas stream which is introduced into the reactor in substoichiometric conditions is preheated, thereby reducing the energy requirements of the process of the invention even further. Preferably, the gas stream is preheated to a temperature of more than 400 °C.

Preferably, the preheating is achieved by passing the gas stream through at least one heat exchanger. Even more preferably, said at least one heat exchanger is fed with the flue gas from the reactor. According to the process of the invention, the green petroleum coke can be fed directly into the reactor. Alternatively, the green petroleum coke is preheated to a temperature of 400 °C in a heat exchanger before entering the reactor, preferably using at least part of the flue gas as a heat transfer medium. This preheating is preferably effected in a Venturi pre-heater. Even more preferably, the preheated green petroleum coke is subsequently fed into a cyclone to concentrate the solid particles before guiding them into the reactor. Most preferably, the Venturi pre-heater and a cyclone, used to preheat and concentrate the solid green petroleum coke particles, are supplied with the flue gas from the reactor.

Preferably, the recycled flue gas and air are fed separately into the reactor in order to avoid mixing and ignition prior to injection into the reactor. After the green petroleum coke is sufficiently calcined, it is preferably removed from the reactor and fed into a product cooler. When the cooling process is concluded, the calcined petroleum coke can be removed for further processing.

The present invention is also directed to an apparatus for the production of calcined petroleum coke. Such an apparatus comprises a reactor for the com- bustion of green petroleum coke with a substoichiometric amount of oxygen at a temperature of between 900 and 1400 °C for obtaining calcined petroleum coke and flue gas. This reactor features an inlet for feeding green petroleum coke into the reactor, an outlet for removing calcined petroleum coke from the reactor, a supply conduit for a gaseous oxygen source into the reactor and a gas outlet for the flue gas containing at least 80 wt-% of the volatile matter contained in the green petroleum coke. Based on the idea of the invention, a recycling conduit from the gas outlet into the reactor is foreseen to allow at least partial recycling of the flue gas.

Various embodiments of the reactor type are possible, whereby a rotary kiln or even more a fluidized bed reactor are preferred.

Preferably, a Venturi pre-heater is connected to the reactor. Even more prefera- bly, a cyclone and at least one heat exchanger are sequentially connected to the Venturi pre-heater.

It may be preferred that the Venturi pre-heater has an inlet for feeding green petroleum coke into the pre-heater and an outlet for preheated green petroleum coke which is connected to the reactor. This advantageous embodiment reduces the energy that is required in the reactor to remove volatile components from the green petroleum coke.

Furthermore, the at least one heat exchanger has an inlet for air as well as outlets for flue gas and air which are connected to the reactor to enable recycled flue gas and air to be fed into the reactor.

Preferably, the flue gas inlet into the reactor is located centrally to emulate an annular fluidized bed for ideal flue gas incineration conditions. The reactor is preferably equipped with an outlet for the removal of calcined green petroleum coke. Said outlet is preferably connected to a product cooler where the sufficiently cooled final product is removed for further processing. The recycling of the flue gas can be effected by various means which are known to the skilled person. For instance, a gas compressor may be used to transport the flue gas from the heat exchanger(s) back into the reactor. Preferably, the flue gas is de-dusted prior to compression. De-dusting can be effected by e.g. a filter or a Venturi.

Alternatively, the flue gas may be transported from the heat exchanger(s) to the reactor by employing a thermo compressor with steam injection.

Additional features, advantages and applications of the invention will be appar- ent from the following example as well as the figure. All disclosed and/or depicted features, alone or in combination, form the subject of the present invention, independent of their arrangement in the claims or their dependencies.

Figure 1 shows schematically the process according to the invention.

The green petroleum coke with a solid matter content of about 90 wt-% carbon and 10 wt-% volatiles is calcined at a temperature of 1 100 °C for 30 min in a fluidized bed reactor 10 before being removed via outlet conduit 1 1 into cooler 12 before being discharged through conduit 13.

The produced flue gas is continuously removed from the reactor through an outlet and fed via outlet conduit 14 into a Venturi pre-heater 20 together with fresh green petroleum coke fed from conduit 21 . This enables the fresh green petroleum coke to be preheated to a temperature of between 350 and 500 °C, reducing the required energy for calcining the green petroleum coke in the reactor 10. The green petroleum coke particles have a diameter of less than 5 mm.

The flue gas/green petroleum coke mixture is subsequently moved through conduit 22 into cyclone 23 to separate the solid material from the flue gas. The solid material is fed via conduit 24 and inlet conduit 25 into the fluidized bed reactor 10, while the flue gas is directed through conduit 26 into a first heat exchanger 30 and subsequently through conduit 31 into a second heat exchanger 32. In both heat exchangers, the flue gas is used as a heat transfer medium to heat a gas stream. Further, the flue gas is directed via conduit 33 through a filter 34 to remove any residual solid particles. Solid particles can be recycled via conduit 38 into the inlet conduit 25 for the solid inlet or directly into the reactor 10. Part of the flue gas (10 - 30 %) is branched off from conduit 35 coming out of the filter 34 and is recycled via recycling conduits 40 and 41 into reactor 10. By passing the heat exchanger 30, it is further heated up again to 500 °C. The remaining amount of flue gas removed from the process via conduit 36. Air is fed into the second heat exchanger 32 via conduit 42 and heated to a temperature of about 400 °C.

The flue gas and the air are separately injected into the reactor 10 through recycling conduit 41 and supply conduit 43 to avoid premature mixing and igni- tion. In order to achieve substoichiometric conditions inside the reactor, the ratio by weight of air and recycled flue gas is between 0,3 to 0,6 preferably 0.4 - 0,5.

The flue gas and air are fed into the reactor 1 through a central nozzle 50 for optimal incineration conditions. A typical composition of the flue gas upon entering the reactor 1 is for example:

H₂O: 17 vol-%

CO₂: 2,2 vol-%

N₂: 49.2 vol-%

O₂: 0 vol-%

CO: 6,5 vol-%

H₂: 24,1 vol-% Due to the absence of any oxidizing agent, the calcination process is comparably mild and avoids significant oxidation of carbon in the green petroleum coke, thus increasing carbon yield.

This process results in a calcined petroleum coke having a residual content of volatiles of less than 1 wt-%.

List of reference numerals

10 fluidized bed reactor

1 1 outlet conduit

12 cooler

13 conduit

14 outlet conduit

20 Venturi pre-heater 21 ,22 conduit

23 cyclone

24 conduit

25 inlet conduit

26 conduit

30 heat exchanger

31 conduit

32 heat exchanger

33 conduit

34 filter

35-38 conduit

40,41 recycling conduit

42 conduit

43 supply conduit 50 central nozzle

Claims

1 . Process for the production of calcined petroleum coke, wherein in a reactor green petroleum coke is combusted as a fuel with a substoichiometric amount of oxygen at a temperature of between 900 and 1400 °C to provide the required energy to evaporate volatile matter included in the green petroleum coke, wherein calcined petroleum coke and flue gas are obtained, and wherein the flue gas comprises at least 80 wt-% of the volatile matter contained in the green petroleum coke, characterized in that the flue gas is at least partially recycled to the reactor as an additional fuel.

2. Process according to claim 1 , characterized in that the reactor is a rotary kiln.

3. Process according to claim 1 , characterized in that the reactor is a fluid- ized bed reactor.

4. Process according to claim 3, characterized in that the temperature in the reactor is between 900 and 1250 °C.

5. Process according to any of claim 3 or 4, characterized in that the recycled flue gas is used as fluidizing gas in the reactor.

6. Process according to any of the preceding claims, characterized in that at least a part of the flue gas is passed through a Venturi pre-heater.

7. Process according to claim 6, characterized in that at least a part of the flue gas is fed from the Venturi pre-heater into a cyclone and/or at least one heat exchanger prior to recycling.

8. Process according to any of the preceding claims, characterized in that pure oxygen, air or another gas containing at least 5 vol-% oxygen is fed into the reactor as a gasous oxygen source for the substoichiometric combustion.

9. Process according to any of the preceding claims, characterized in that prior to entering the reactor the gaseous oxygen source is preheated to a temperature of 400 - 500 °C and/or the green petroleum coke is preheated to a temperature of 350 - 500 °C.

10. Process according to claim 9, characterized in that the preheating of the gaseous oxygen source and/or the green petroleum coke is performed in a heat exchanger which is fed with at least a part of the flue gas.

1 1 . Process according to any of claims 8 to 10, characterized in that the gaseous oxygen source and the flue gas are fed separately into the reactor.

12. Apparatus for the production of calcined petroleum coke, comprising a reactor (10) for the combustion of green petroleum coke with a substoichiometric amount of oxygen at a temperature of between 900 and 1400 °C for obtaining calcined petroleum coke and flue gas, an inlet conduit (25) for feeding green petroleum coke into the reactor (10), an outlet conduit (1 1 ) for removing calcined petroleum coke from the reactor, a supply conduit (43) for a gaseous oxygen source into the reactor (10) and an outlet conduit (14) for the flue gas comprising at least 80 wt-% of the volatile matter contained in the green petrole- urn coke, characterized by a recycling conduit (40,41 ) into the reactor (10) to allow at least partial recycling of the flue gas.

13. Apparatus according to claim 12, characterized in that the reactor (10) is a rotary kiln.

14. Apparatus according to claim 12, characterized in that the reactor (10) is a fluidized bed reactor.

15. Apparatus according to claim 14, characterized in that the fluidized gas is introduced via a central nozzle.