The foam layer refers to that in the delayed coking process, the raw materials are rapidly heated to nearly 500 ℃ in the heating furnace, and then quickly transferred to the coke tower, where severe thermal reaction occurs.The thermal reaction of heavy oil is divided and condensed. The cracking reaction generates oil and gas, and the condensation reaction generates coke.During coking, many intermediate products are high viscosity resin like substances, which will form a temporarily stable gas-liquid dispersion system under the stimulation of oil and gas.
With the deterioration of coking raw materials, there are many new problems in delayed coking process. For example, when processing heavy oil, a very high foam layer often appears in the coke tower, which brings great harm to the safety production of the unit;The coke powder and unreacted heavy oil in the foam layer are easily carried by oil and gas, resulting in coking of large oil and gas pipelines and the bottom of fractionator, even leading to blockage of feed pump and coking of heating furnace tube, which seriously threatens the safety and long-term operation of the device;The coke powder in coking gasoline and diesel oil causes harm to the downstream hydrogenation process and increases the economic cost of the refinery.In addition, in order to prevent entrainment of mist, a space height of 6~8m must be reserved in the coke tower, which greatly reduces the utilization rate of the device.Therefore, it is necessary to study the formation reasons and influencing factors of the foam layer and explore reasonable restraining measures to ensure the safe and efficient operation of the delayed coking unit[1]。
Formation process
Announce
edit
In the delayed coking process, the raw materials are rapidly heated to nearly 500 ℃ in the heating furnace, and then quickly transferred to the coke tower, where severe thermal reaction occurs.The thermal reaction of heavy oil is divided and condensed. The cracking reaction generates oil and gas, and the condensation reaction generates coke.During coking, many intermediate products are high viscosity resin like substances, which will form a temporarily stable gas-liquid dispersion system, namely foam layer, under the stimulation of oil and gas.With the continuous input of raw materials, the thermal reaction continues, and the newly generated oil and gas continue to enter the intermediate product for "bubbling", while "bubble breaking" continues to occur at the top of the foam layer.Under certain conditions, the two achieve dynamic balance, so that the foam layer can be maintained at a stable height.At the same time, the intermediate products continue to undergo deep condensation, and finally coke is generated and deposited at the lower part of the tower, so the foam layer will always be located above the coke layer.There will be a certain thickness of foam layer in the coke tower from the beginning of feeding to the end of feeding.
Formation mechanism
Announce
edit
Research shows that there are usually three ways to generate bubbles.The first is the dispersion method, that is, the gas enters the liquid through mechanical agitation;The second bubbling method is to input gas into the foaming solution through porous medium;The third is bubble nucleation, such as the liquid phase contains supersaturated gas, or the liquid phase undergoes in-situ reaction to generate gas.
The generation of foam layer in coke tower mainly belongs to the third way.The raw materials undergo cracking reaction in the coke tower, and the generated oil and gas enter the viscous intermediate product and form a large number of small bubbles.Due to the large difference in the density of gas and liquid phases, small bubbles will rise in white.In the process of rising, in order to reduce the surface energy of the system, small bubbles will continue to coalesce into large bubbles.At the same time, due to gravity, the liquid will flow down along the liquid film between bubbles, leading to the gradual thinning of the liquid film between bubbles. Therefore, the bubbles above the foam layer are no longer spherical, but polyhedral three-dimensional structure.In addition, under the action of capillary force, the liquid in the liquid film between bubbles tends to flow to the junction of the liquid film.In this way, as the height of the foam layer increases, the liquid film between bubbles in the foam will become thinner and thinner, and finally burst at a certain height.Therefore, the density of the foam layer decreases gradually from the bottom to the top[2]。
Cause analysis
Announce
edit
Foam is a gas-liquid dispersion system with a large ratiothe measure of areaThis is thermodynamically unstable.In order to form a stable and lasting foam, there must be surfactant to reduce the surface energy of the system.There are a large number of sulfur, nitrogen, oxygen and other heteroatoms in heavy oil, which mainly exist in the form of polar groups in the molecular structure of heavy oil. These polar substances are naturalsurface active agent 。
Short chain carboxylic acids andrelative molecular massPhenols ≤ 400 are especially helpful for foam stability.Some factors slow down the thinning and cracking of the liquid film and improve the stability of the foam. For the foam layer in the coke tower, these factors mainly refer to the liquid viscosity and the surface adsorption layer.The effect of liquid viscosity is mainly to provide resistance for the discharge of foam film, which affects the liquid discharge rate in the foam film.In general, the greater the viscosity, the smaller the liquid discharge rate, and the more stable the foam.When the delayed coking unit processes heavy oil, it produces foam layers up to several meters, which is not only related to the high viscosity of heavy oil itself, but also related to the increase of viscosity under reaction conditions.
The surface adsorption layer refers to the adsorption layer formed by surface active substances gathering on the gas-liquid surface to reduce the surface energy of foam.Due to the interaction between surfactant molecules, it is easy to form a cross-linked structure on the surface, thus increasing the surface viscosity of the foam film and improving the stability of the foam.In the model for studying the foaming heavy oil, it is pointed out that asphaltene and resin will be adsorbed on the bubble film when the heavy oil bubbles, forming a semi-rigid protective layer to prevent further coalescence of bubbles.Therefore, the foam layer in the coke tower has quite high stability when processing low-quality heavy oil, which may be related to the formation of highly viscous adsorption layer by adsorption of polar molecules in heavy oil on the liquid gas interface.
influence factor
Announce
edit
Raw material properties
The properties of raw materials are closely related to their foaming performance in the coke tower.Generally, the more foamy the raw material is, the higher the foam layer will be.State Key Laboratory of Heavy Oil, China University of PetroleumWhen studying the foaming performance of heavy oil coking, it is found that the heavy oil with higher foaming height under non reaction has stronger foaming performance under thermal reaction.In addition, the coking foam layer of naphthenic base and intermediate base residue is generally higher than that of paraffin base residue. For example, the foam layer produced by Shengli vacuum residue is 2-6m higher than that of Daqing vacuum residue under the same conditions.
From the perspective of raw material composition, asphaltene, as the most polar component in heavy oil, must have an important impact on the foaming performance of heavy oil.Crude oil with high asphaltene content often has strong foaming ability.State Key Laboratory of Heavy Oil, China University of PetroleumA similar rule was also found in the study. The residue with high asphaltene content produced a higher foam layer.This is also true in the actual coking process.On the one hand, the cross-linking structure between asphaltene molecules can increase the viscosity of heavy oil, which is conducive to improving the stability of foam.On the other hand, asphaltene molecule has a multinuclear aromatic ring structure, and the surrounding alkyl side chain is connected with heteroatoms, which has strong polarity and acts as asurface active agent Role of.Some foreign studies have also confirmed this view.
Operating parameters
The operating parameters of delayed coking process include treatment capacity, outlet temperature of heater, coke tower top pressure and circulation ratio, which have different effects on the height of foam layer in coke tower.
(1) Handling capacity
The treatment capacity of delayed coking unit has a direct impact on the height of foam layer in coke tower.With the increase of treatment capacity, the amount of oil and gas generated by cracking increases, and the number of high viscosity intermediate products generated by condensation reaction increases, leading to the increase of foam layer height.On the contrary, the height of foam layer decreases.
(2) Heating furnace outlet temperature
The outlet temperature of the heating furnace has an important influence on the height of the foam layer, because it directly determines the reaction depth in the coke tower.The foam layer itself is the intermediate product of incomplete reaction. If the outlet temperature of the heating furnace is increased, the foam layer can fully react at high temperature and generate coke, thus reducing the foam layer.However, there are also literature reports that the temperature rise at the heater outlet, on the one hand, reduces the foam layer according to the above mechanism, on the other hand, makes the coking reaction tend to crack and decompose, increasing the amount of oil and gas generated, and increasing the height of the foam layer.In fact, when the temperature rises by 1 ℃, the reaction depth will be greatly deepened, and the amount of oil and gas will also be increased while the intermediate products will be greatly reduced, but its impact on the foam layer is far less significant than the former.The overall effect is that the height of foam layer decreases significantly.
(3) Coke tower top pressure
The influence of coke tower top pressure on the foam layer in the coke tower is first a physical bubble breaking process.With the increase of tower top pressure, the difficulty of bubble formation increases. At the same time, the bubbles in the formed foam layer become easy to break due to the influence of external pressure, thus reducing the height of the foam layer.Secondly, increasing the tower top pressure will increase the heavy hydrocarbons retained in the coke in the tower, and the residence time of gas products in the tower will be prolonged, which will increase the chances of secondary reaction, thus increasing the yield of coke and slightly increasing the gas yield. The yield of above liquid products will decline, that is, the relative oil and gas volume will decline, which will also reduce the foam layer.In order to obtain distillate oil to the maximum extent, most coking units in use low-pressure operation, which is not conducive to reducing the foam layer, so more attention should be paid to the control of the foam layer during production.
(4) Circulation ratio
The influence of circulation ratio on foam layer is relatively significant.The increase of circulation ratio is equivalent to the dilution of coking raw materials, the reduction of viscosity of coking feed, and the reduction of foam layer height.In addition, when the treatment capacity of the heating furnace is fixed, the increase of the circulation ratio is equivalent to the reduction of the treatment capacity, which will also reduce the height of the foam layer.However, the development trend of delayed coking is low circulation ratio or even zero circulation ratio, which is not conducive to reducing the height of foam layer.
Other factors
In addition to the above factors, the water (or steam) injected into the furnace tube of the heating furnace, the mixing amount or quality of light and heavy waste oil and other factors also have a certain impact on the height of the foam layer in the coke tower.If the amount of water (or steam) injected into the furnace tube increases, the gas velocity in the coke tower will increase, which will directly affect the foaming situation in the coke tower.If the gas velocity is too high, it is easier to blow the coke powder in the foam layer into the large oil and gas line, causing entrainment of mist. Therefore, the gas velocity of domestic coke towers is generally not greater than 0.15m/s[3]。
Inhibition of foam layer
Announce
edit
Optimize process operation parameters
Optimizing the operating parameters of the delayed coking unit can reduce the height of the foam layer in the coke tower. For example, by reducing the input rate of raw materials or the target processing capacity of the coke tower, the top foam can be provided with a burst time before the foam layer reaches the top of the coke tower;Or increase the tower top pressure, increase the circulation ratio, and increase the outlet temperature of the heating furnace to reduce the height of the foam layer.These measures have to pay a certain economic cost, but not taking measures is not conducive to improving the production efficiency of the device.Previously, the safe and effective measure was the temperature change operation at the outlet of the heating furnace.The production practice shows that properly increasing the outlet temperature of the heating furnace by 1-3 ℃ 2~3h before switching the four-way valve is not only conducive to reducing the height of the foam layer, but also can improve the yield of liquid products and the quality of petroleum coke.In addition, reasonable quench oil injection mode can also play a certain role in inhibiting the foam layer.However, the inhibition effect of these measures on the foam layer in the coke tower is still very limited.
Inject defoamer
In order to effectively suppress the foam layer, the method of injecting defoamer has been widely used at home and abroad.Reasonable injection of defoamer can reduce the height of foam layer by 2~6m and greatly improve the volume utilization of coke tower.Defoamer is a substance with high surface activity, which is characterized by good dispersion, stable property and insoluble in foaming medium. Its mechanism is that after entering the foam film in the form of liquid drops, the defoamer reduces the local interfacial tension, causing uneven membrane surface tension, thus promoting the rupture of the foam layer.Defoamers used in delayed coking process can be divided into silicon containing and silicon free defoamers.Silicon containing defoamer is widely used, and its effective component is silicone oil（Polydimethylsiloxane）It has the advantages of rapid defoaming, long foam suppression time, low consumption, low cost, etc.
However, it has been found in recent years that the pyrolysis products of silicone oil will poison the downstream after entering the coking distillate oilHydrogenation catalystThis has become a bottleneck restricting the mass application of such defoamers.Silicon free defoamers are mainly polyethers or alcohols, but their defoaming effect and thermal stability are not as good as the former, so they are seldom used.The defoaming effect of defoamer is closely related not only to its own properties, but also to its injection mode.
Under the same conditions, the defoaming effect of injection from coke tower top is better than that of injection from four-way valve, but it is also reported that injection from four-way valve is better than that from tower top.In fact, the two injection methods have their own advantages.When the tower top is injected, the defoamer is easier to enter the surface of the foam layer, and the defoaming speed is fast. However, the coke tower has a large diameter, which is difficult to ensure that the defoamer can be evenly distributed, and some defoamers are easy to be taken away by oil and gas, which will lose some defoaming effect.In addition, the defoamer injection system must be switched when changing the tower, which is difficult to operate and prone to accidents.The advantage of the injection in front of the four-way valve is that the defoamer can be fully mixed with the reaction materials, and the defoaming effect is obvious, and the action time is long, and the foam suppression effect is good. The disadvantage is that the defoamer is easy to decompose after contacting with high-temperature materials, and the defoaming performance will decline.During actual operation, appropriate injection mode shall be selected according to the device conditions and the nature of defoamer.
The injection amount of defoamer also affects its defoaming effect.Generally speaking, the quality of defoamer has the greatest impact on its injection amount, and the injection amount of defoamer with good quality is low.In addition, the injection amount is also related to the injection method. The amount of the same defoamer is different with different injection methods.As for the injection time of defoamer, generally, when the neutron level gauge in the middle section of the coke tower detects that the foam density value is greater than 30%, it starts to inject defoamer until 0.5~1h after the tower is replaced[1]。
summary
Announce
edit
(1) Foaming substances in the coke tower are mainly the coke precursor of coking reaction of coking raw materials, of which natural surfactant and its intrinsic viscosity are the internal causes of forming foam layer.The raw materials with high asphaltene, high viscosity and high sulfur content have high foam layer height during coking.
(2) The operation parameters of coking process are the external factors affecting the foam layer.The height of foam layer can be reduced by reducing the processing capacity of the unit, increasing the outlet temperature of the heating furnace, increasing the tower top pressure and increasing the circulation ratio.In actual production, appropriate operating parameters shall be determined according to the properties of raw materials and other conditions.
(3) The injection of defoamer into the coke tower can effectively reduce the height of foam layer. In actual production, the injection amount, injection method and injection time of defoamer should be determined according to the specific situation.
(4) At home and abroad, the research on the composition and formation mechanism of coke tower foam layer, as well as the action mechanism of defoamer, is not deep enough, and there is still a lack of complete theoretical basis.It is suggested to start with the composition of foam layer, explore the foaming mechanism of different raw materials, the action mechanism of defoamers and the compatibility law of different defoamers with coking raw materials, so as to lay a theoretical foundation for the research of foam layer[3]。
