Urea dressing and exhaust gas treatment plant and method
阅读说明：本技术 脲修整和废气处理工厂以及方法 (Urea dressing and exhaust gas treatment plant and method ) 是由 J·科洛马冈萨雷斯 J·H·门嫩 于 2018-07-13 设计创作，主要内容包括：本发明涉及用于含脲的材料的修整方法、用于修整含脲的材料的工厂、修改现有工厂的方法、以及用途。本发明公开了用于防止管道堵塞的方法,所述管道用于修整部分与处理部分之间的废气。(The present invention relates to a finishing method for urea-containing materials, a plant for finishing urea-containing materials, a method of modifying an existing plant, and uses. A method for preventing plugging of a conduit for use in conditioning exhaust gas between sections and treating sections is disclosed.)
1. A finishing method for a urea-containing material, comprising:
subjecting the urea-containing liquid stream to solidification in a urea finishing section, producing a urea-containing solid product and an exhaust gas stream comprising air, urea dust and ammonia,
-conveying the exhaust gas flow from the outlet of the urea finishing section to an exhaust gas treatment section through a duct having a wall, wherein the exhaust gas has a temperature T at the outlet1，
-subjecting the exhaust gas stream to a treatment to remove at least a portion of the urea dust and/or ammonia from the air in the treatment section, and
-maintaining the temperature of the wall of the pipe above T in at least one portion in at least one bend of the pipew,minIn which the diameter of the pipe changes and/or over at least 10%, at least 50% or at least 90% of the length of the pipe, wherein Tw,min＝T1–50℃。
2. A finishing method according to claim 1, wherein the wall of the pipe is provided with a thermally insulating material and with a heating element.
3. Finishing method according to claim 1 or 2, in which the temperature of the wall is maintained at or above T1The temperature of (2).
4. The finishing process according to any one of claims 1 to 3, wherein the temperature of the exhaust gas stream is higher than 60 ℃, preferably higher than 65 ℃, even more preferably higher than 70 ℃ in an area less than 2cm from the wall.
5. Finishing method according to any of claims 1-4, wherein the temperature difference between the gas flow near the wall of the duct and the gas flow in the center in the cross-section of the duct is less than 10 ℃, preferably less than 5 ℃ at the same position of the length of the duct.
6. The finishing method according to any one of claims 1 to 5, comprising maintaining the wall at a temperature of at least 60 ℃.
7. The finishing process of any of claims 1-6, wherein the urea-containing solid product comprises urea particles, Urea Ammonium Nitrate (UAN) particles, or Urea Ammonium Sulfate (UAS) particles.
8. The finishing method according to any one of claims 1 to 7, wherein the solidifying comprises granulating a urea-containing melt to obtain urea particles.
9. A finishing method according to claim 1, comprising
-prilling urea in a forced-draft urea prilling tower using cooling air and using a blower and/or a fan, wherein the prilling tower has an outlet for exhaust gas at the top of the tower, wherein the exhaust gas has a temperature T at the outlet1，
-subjecting the exhaust gas to dust scrubbing, and optionally acid scrubbing in an exhaust gas treatment section having an inlet for exhaust gas at a height of 0m to 20m above ground level,
-supplying off-gas from the outlet at the top of the urea prilling tower to the inlet of the off-gas treatment section, and
-maintaining the temperature of the wall of the pipe above Tw,min＝T1–10℃。
10. A plant for finishing urea-containing materials, wherein the plant comprises:
-a trim part for solidifying the urea-containing liquid stream,
-an exhaust gas treatment section, and
-a conduit for discharging exhaust gases from an outlet of the conditioning section to an inlet of the treatment section, wherein the conduit comprises a wall,
and wherein at least a part or all of the pipe is provided with an insulating material and/or with one or more heating elements for maintaining a minimum temperature of the wall, wherein the insulating material and the one or more heating elements are configured for maintaining the temperature of the wall of the pipe above T in at least one bend, in at least one portion, of the pipe in at least one bendw,minIn which the diameter of the pipe changes and/or over at least 10%, at least 50% or at least 90% of the length of the pipe, wherein Tw,min＝T1-50 ℃ of which T1Is the temperature of the exhaust gas at the outlet.
11. The plant defined in claim 10 wherein the conduit is provided with a thermally insulating material and with a heating element, wherein the heating element comprises an electrogram and/or a vapogram.
12. The plant of any one of claims 10-11, wherein the finishing section is a urea prilling tower, and wherein the inlet for the off-gas of the treatment section is at a height of 0m to 20m above ground level.
13. The plant defined in any one of claims 10 to 12 wherein the pipeline is provided with an insulating material having a thermal conductivity of less than 1.0W/(m.k) and a thickness of at least 10mm, preferably wherein the pipeline is provided with an insulating material comprising one or more materials selected from the group consisting of: polymeric materials, fiber-based materials, and inorganic non-metallic materials.
14. Use of a thermogram for preventing the clogging of a conduit for a gas stream comprising urea-containing dust, ammonia and isocyanic acid.
15. A method of modifying an existing trim portion for curing a urea-containing material to avoid plugging of a pipe having a wall for exhaust gas from the trim portion, wherein the pipe is disposed between the trim portion and a treatment portion for treating exhaust gas from the trim portion, the method comprising providing a pipe having an insulating material and/or having one or more heating elements for avoiding cold spots of the wall of the pipe, and/or wherein the insulating material and the one or more heating elements are configured for maintaining the temperature of the wall of the pipe above T in at least one bend of the pipe in at least one portionw,minIn which the diameter of the pipe changes and/or over at least 10%, at least 50% or at least 90% of the length of the pipe, wherein Tw,min＝T1-50 ℃ of which T1Is the temperature of the exhaust gas at the outlet.
Technical Field
The present invention relates to the production of urea and urea-containing fertilizers, and in particular to urea dressing. In particular, the present invention relates to conveying exhaust gas (i.e., exhaust gas containing urea and ammonia) from a trim section to an exhaust gas treatment section. The treatment section is for example used for dust washing and/or acid washing to remove urea and/or ammonia from the exhaust gas. In the trim section, the urea-containing melt is typically solidified to produce, for example, urea granules, Urea Ammonium Nitrate (UAN) granules, or Urea Ammonium Sulfate (UAS) granules.
Background
Urea plants typically include a trim section for solidifying a urea-containing liquid stream (e.g., a urea melt) into a urea-containing solid product. Common finishing sections are prilling towers and granulators. The prilling tower and granulator use cooling air, thus producing as exhaust air a stream of air contaminated with urea-containing dust and ammonia. Urea dust typically comprises submicron particles containing, for example, urea, UAS or UAN. Such an exhaust gas needs to be treated to remove a major portion (or even substantially all) of the solids and ammonia before the treated exhaust gas stream (clean air stream) is discharged to the atmosphere. Exhaust gas treatment is often necessary to comply with environmental regulations that limit the allowable urea and ammonia emissions. Recovery of components such as urea and ammonia from gas streams is also economically desirable. This improves the efficiency of the plant.
The removal of urea dust is inherently challenging because the amount of off-gas (mainly air) is large and the concentration of urea dust is low. An exemplary indicated air flow for a relatively small urea prilling tower of about 1500 metric tons/day is 500000Nm3And/hr. A larger urea prilling tower can, for example, have a size of 1.0X 106Nm3An air flow per hr, for example with a urea capacity of about 2500 metric tons per day or even higher. A typical concentration of urea dust therein is about 0.02% by weight. Furthermore, a portion of the urea dust has submicron dimensions. Meeting current standards means that a large portion of this submicron urea dust needs to be removed.
A prilling tower may for example have a height of 60m to 80 m. Smaller plants may have a free fall path of 50m or less. Some of the largest plants have prilling towers 125m high. Some prior art urea prilling towers are reported to have urea dust emissions in excess of 200mg/Nm3。
Older prilling towers often discharge the off-gas directly into the atmosphere without any dust abatement or ammonia abatement treatment. The tower configuration typically provides the maximum available space on top of it and the maximum additional weight that can be supported by its structure, and therefore the design of any abatement system that is installed on top as part of a retrofit is limited. Existing emission abatement technologies typically require large and heavy fans or pumps to overcome the additional pressure drop they require. The greater the efficiency of dust capture, the greater the pressure drop required, especially when dealing with submicron particulate removal. Many exhaust gas treatment systems are not suitable for mounting on top of existing prilling towers due to their weight, but can be mounted at or near ground level (e.g., inlets at heights of 0m to 20 m). This may involve first venting the off-gas to a lower level through a pipe (e.g., a conduit), and thus also the construction of a conduit from the top of the urea prilling tower to about ground level. The duct is generally arranged outside the prilling tower.
The exhaust gas treatment is typically carried out in a separate treatment section (emission abatement section) having an inlet for the exhaust gas connected by a conduit from the urea finishing section. Generally, the connecting duct may have a wall for a major portion exposed to the external environment. The environment may have a low temperature, such as below 0 ℃ or below-10 ℃, for example overnight or in the winter.
The off-gas treatment typically comprises washing with an aqueous solution to remove dust (scrubbing), or washing the gas stream with an acid solution to remove ammonia by conversion to ammonium salts (acid scrubbing), or both in series or simultaneously. If both dust washing and acid washing are used in series, the dust washing is usually performed first. If they are carried out simultaneously, urea is mixed with ammonium salts such as UAS and UAN. In the scrubber, a scrubbing liquid (e.g., a solution) is sprayed into the gas stream, e.g., co-currently and/or counter-currently. The wash solution is typically circulated in order to have the desired urea concentration. The purge stream is removed from the scrubber and, for example, in the case of dust scrubbing with water, treated by recycling to the urea plant to recover urea.
Typically, the remaining scrubbing droplets are removed from the gas stream in the treatment section using, for example, a demister. Sometimes it is necessary to combine devices of different operating principles to achieve sufficient cleaning of the exhaust treatment section to comply with emission regulations. Examples of other types of abatement devices are wet electrostatic precipitators and venturi scrubbers. The necessary equipment will therefore have a certain weight and size.
In some off-gas treatment sections, a quenching step is applied prior to scrubbing, for example spraying an aqueous solution in a manner that results in evaporative cooling, in order to reduce the size of the gas stream, in order to allow smaller equipment in downstream processing steps such as scrubbing. Furthermore, WO 2015/002535 and US 2016/0184758 describe a process for removing urea dust from off-gas of a trim section of a urea production plant, the process comprising subjecting the off-gas to quenching with water so as to produce a quenched off-gas having a temperature below about 45 ℃, and subjecting the quenched off-gas to scrubbing using at least one venturi scrubber.
The present invention relates to the transfer of exhaust gases from a urea finishing section to an exhaust gas treatment section. The exhaust gas is typically carried through a conduit between the urea finishing section and the exhaust gas treatment section.
Generally, and also in the present invention, the conduit is, for example, a pipe, a tube or a conduit, or any other type of gas delivery system. The conduit typically includes a wall and a gas flow path. The conduit may have a length of, for example, at least 2m, at least 5m, at least 10m, at least 20m, or at least 40 m.
For example, the prilling tower may have an exhaust gas outlet at a height of at least 10m, at least 20m, at least 40m or at least 60m above ground level (i.e. at the top of the tower). Although some small-sized off-gas treatment sections may sometimes be placed on top of the urea prilling tower, due to the weight and/or size of the equipment, many larger off-gas treatment sections need to be placed on ground level and have inlets, for example, 0 to 5m above ground level. This requires a corresponding length of the pipe, for example at least 10m, at least 20m, at least 30m, at least 40m or at least 60 m. And for other types of urea finishing sections and off-gas treatment sections, a certain minimum length of piping may be given by the design constraints of the plant. This may also be the case if, for example, an existing urea plant is modified in a so-called retrofit.
Furthermore, pure solid urea is known to have a tendency to cake, i.e. agglomerate and form lumps, at higher temperatures, e.g. above 60 ℃. Thus, any urea dust that precipitates in the duct runs the risk of caking at higher temperatures, especially when exposed to moisture such as air humidity.
A problem in the piping used to convey the exhaust gases from the urea trim section to the treatment section is that after some operating time of the exhaust gas treatment section, an increase in pressure drop tends to occur. This may result in higher operating costs and/or reduced throughput of the exhaust treatment section. This increase in pressure drop is typically caused by plugging of the piping (e.g., conduits) between the trim portion and the exhaust treatment portion. This blockage is caused by solid deposits in the pipe. The solid deposits comprise urea, such as solid urea species formed on at least a portion of the wall of the pipe. The blockage results in an increase in pressure drop that may prevent operation or require additional energy consumption of the exhaust fan or pump (in the case of a venturi or venturi ejector) for conveying the exhaust gas to the treatment section.
Some of the prior art relates to plugging scrubbers used to treat urea trim exhaust. EP 0084669 mentions that the nozzle for spraying the liquid into the gas stream is blocked. To solve this problem, formaldehyde is added to the washing solution. However, formaldehyde is toxic and expensive and is therefore highly undesirable in the exhaust gas treatment section which is intended to produce a clean air stream to be discharged into the atmosphere. US 4104041 describes a urea prilling exhaust gas treatment process wherein a liquid film of scrubbing solution is formed transversely across the entire channel to remove urea dust having a particle size of about 1 μm. This is said to solve the pore plugging problem of prior art bag filters. US 4153431 also relates to the problem of clogging of prior art filters, which results in increased pressure drop, which is considered to be a major disadvantage of natural draft and forced draft urea prilling towers. A method is described that includes directing a scrubbing liquid co-current with a filter with a gas flow. EP 0084669, US 4104041 and US 4153431 do not relate to urea deposits in the pipes between the finishing section and the exhaust gas treatment section, but to urea deposits inside the respective treatment sections.
Disclosure of Invention
The present invention relates in a first aspect to a finishing method for a urea-containing material, the method comprising:
subjecting the urea-containing liquid stream to solidification in a urea finishing section, producing a urea-containing solid product and an exhaust gas stream comprising air, urea dust and ammonia,
-conveying the exhaust gas flow from the outlet of the urea finishing section to an exhaust gas treatment section through a duct having a wall, wherein the exhaust gas has a temperature T at the outlet1，
-subjecting the exhaust gas stream to a treatment to remove at least a portion of the urea dust and/or ammonia from the air in the treatment section, and
-maintaining the temperature of the wall of the pipe above T in at least one portion in at least one bend of the pipe1-50 ℃, the diameter of the conduit in the at least one section is changed and/or over at least 10%, at least 50% or at least 90% of the length of the conduit.
The invention also relates to a plant for reconditioning urea-containing material, wherein the plant comprises a reconditioning part for solidifying a urea-containing liquid stream, an exhaust gas treatment part, and a conduit for discharging exhaust gas from an outlet of the reconditioning part to an inlet of the treatment part, wherein the conduit comprises a wall, and wherein at least a part or all of the conduit is provided with an insulating material and/or with one or more heating elements for maintaining a minimum temperature of the wall.
The invention also relates to the use of a thermograph for preventing the clogging of a pipe for a gas stream comprising urea-containing dust, ammonia and isocyanic acid.
The invention also relates to a method of modifying an existing trim part for curing urea-containing material to avoid clogging of a pipe having a wall for exhaust gases from the trim part, wherein the pipe is arranged between the trim part and a treatment part for treating exhaust gases from the trim part, the method comprising providing a pipe having an insulating material and/or having one or more heating elements for avoiding cold spots of the wall of the pipe.
Drawings
Fig. 1 is a photograph of solid material adhered to the inner wall in a comparative pipe between the finishing section and the treating section.
FIG. 2 schematically illustrates a process and urea finishing plant according to one embodiment of the invention.
Fig. 3 shows a photograph of a conduit between a prilling tower and a scrubber according to the present invention, wherein the conduit is insulated.
Detailed Description
In some embodiments, the present invention is based on the judicious recognition that urea deposition in the duct between the trim portion and the exhaust gas treatment portion can be avoided by preventing cold spots in the duct walls. Thus, urea deposits can be addressed, for example, by heating and/or thermally insulating the pipe wall or at least a portion thereof.
Without wishing to be bound by theory, it is believed that the initial accumulation of solid urea in the conduit (e.g., conduit), particularly on the interior wall surfaces of the conduit exposed to the exhaust gas stream, is due to condensation and adhesion of at least some of the isocyanic acid present in the exhaust gas stream and by reaction with NH3The reaction is caused by the reverse conversion of isocyanic acid to urea.
Isocyanic acid (HNCO) is derived from ammonium cyanate (NH)4NCO) by thermal decomposition. Ammonium cyanate and urea (NH)2CONH2) In chemical equilibrium:
the decomposition of ammonium cyanate with isocyanic acid and ammonia is promoted by low pressure and high temperature, such as when the urea solution is concentrated for solidification, e.g. during a pelletizing operation. The reaction products are volatilized into the exhaust gas. Thus, the exhaust gas contains, for example, at least 10mg NH3/Nm3Or at least 50mg NH3/Nm3Or at least 100mg NH3/Nm3. The exhaust gases containing, for example, at least 10mg of dust/Nm3Or at least 50mg dust/Nm3Or at least 100mg dust/Nm3And preferably this amount of urea-containing dust. Preferably, the off-gas comprises at least 10mgNH3/Nm3Or at least 50mg NH3/Nm3Or at least 100mg NH3/Nm3Urea. Waste materialThe gas containing, for example, at least 5.0mg of isocyanic acid/Nm3Or at least 10mg isocyanic acid/Nm3Or at least 50mg isocyanic acid/Nm3。
The reverse reaction may occur in the pipe when isocyanic acid condenses at the cold spots of the walls of the pipe and reacts with ammonia also present in the exhaust gas stream to form urea. The reaction of the isocyanic acid with ammonia may take place before, simultaneously with or after condensation.
The urea particles formed can be dragged by the air flow and are expected to be small in size (e.g. below 1 μm) due to the chemistry of their formation mechanism. However, the reaction with ammonia can also occur on the walls and lead to adhesion of solids. Eventually, this creates urea buildup on the walls of the pipe, particularly when large chunks or lumps of solid material are built up, as shown, for example, in fig. 1.
By avoiding cold spots on the wall of the conduit, e.g. by insulating and/or heating the conduit or at least a part of its wall, e.g. using a trace as a heating element, the problem of urea deposits is solved, thereby avoiding cold spots on the inner wall of the conduit. Advantageously, the operating time of the finishing plant (finishing section and treatment section) is increased due to the reduced down time for maintenance and cleaning. Advantageously, the operation is simple and no additional flushing system is required, since no water or steam has to be used to flush the pipe. Another advantage is that additives such as formaldehyde do not have to be added to the process stream to prevent plugging.
Furthermore, advantageously, the amount of submicron urea particles at the inlet of the treatment section can be reduced.
In general, the present invention may be applied, for example, in a finishing section and/or an exhaust gas treatment section of the type described above.
Accordingly, in one aspect, the present invention relates to a urea dressing process. As used herein, urea dressing refers to a process for solidifying urea-containing solutions, especially urea-containing melts. Examples include melts of urea, Urea Ammonium Nitrate (UAN) and Urea Ammonium Sulfate (UAS). Examples of urea finishing processes include granulation and prilling of urea, UAN and UAS. The melt typically contains less than 5 wt.% water, and typically greater than 50 wt.% urea.
At one isIn embodiments, the method includes subjecting a liquid stream containing urea to curing in a urea finishing section. This produces a solid product containing urea and an exhaust gas stream comprising ammonia and dust particles comprising urea. The offgas stream also contains air and (small amounts) diurea and isocyanic acid, and, in the case of pelletization, formaldehyde. The method also includes conveying the exhaust gas stream from the outlet of the urea finishing section to an exhaust gas treatment section through a duct having walls (e.g., through a conduit). Preferably, the conveying involves forced ventilation, for example using a blower or fan. The exhaust gas having a temperature T at said outlet1. Temperature T1Is the average temperature over the cross section of the outlet. The method further comprises subjecting the exhaust gas stream to a treatment for at least partially cleaning the exhaust gas, for example removing at least a portion of the urea-containing dust and/or ammonia in the treatment section.
In some embodiments, the finishing section, curing step, treatment step, and/or treatment section are, for example, as described in the introductory portion of this patent application.
The finishing section is, for example, a prilling tower or a granulator. The invention is particularly advantageous for prilling towers. The prilling tower is for example of forced draft or natural draft type. The granulator is, for example, a fluid bed granulator, a spinneret bed granulator, a pan granulator, or a drum granulator. Fluid bed granulators and spin-bed granulators are preferred, and air flow is used. The solidification step includes, for example, granulation in a granulation tower or granulation in a granulator. Solidification involves the removal of the heat of crystallization and typically also involves re-cooling of the solidified urea product. Solidification involves cooling the droplets of urea-containing liquid, for example, using cooling air. Typically, most of the heat of crystallization/cooling is removed by air cooling. For example, 3 to 30kg of air per kg of the final cured product is used for cooling, preferably 5 to 15 kg. Optionally, the solidification involves granulation and a portion of the heat is removed by evaporating the water. Depending on the nature of the cooling process, the cooling air leaves the trim part at an elevated temperature in the form of exhaust gases. In the trim section, air is in direct contact with the urea melt and with the solidified urea particles. This results in some contamination of the air with some urea dust and ammonia. Depending on the type of conditioning section and the operating conditions, the amount of dust present in the gas stream at the outlet of the conditioning section before any scrubbing is, for example, 0.01 wt.% to 1.0 wt.% (based on the mass of the gas stream). Typical temperatures of the off-gas leaving the finishing section (i.e. the outlet) of the urea plant are, for example, at least 30 ℃, at least 50 ℃, at least 70 ℃, such as at least 80 ℃, at least 90 ℃, at least 100 ℃, and usually less than 150 ℃, less than 140 ℃, or less than 120 ℃. For granulation, especially fluid bed granulation, the temperature is, for example, 70-150 ℃, or 80-140 ℃, such as about 105 ℃.
The urea-containing liquid is, for example, a solution or melt comprising urea, having, for example, at least 80 wt.%, at least 90 wt.%, or at least 95 wt.% urea. The liquid may also be, for example, a solution and/or melt of, for example, Urea Ammonium Nitrate (UAN) or Urea Ammonium Sulfate (UAS). Fluid bed granulation of UAS is mentioned, for example, in WO 2017/007315.
In another embodiment, the liquid may also for example comprise small amounts of ammonium salts, such as ammonium nitrate and/or ammonium sulphate, such as up to 1 wt% or up to 5 wt%, for example for treating salts from acid washing of off-gases. The liquid may also contain additives such as formaldehyde. The liquid typically contains less than 5 wt% water, for example less than 2.0 wt% water for granulation, and typically less than 0.50 wt% water for granulation.
The treatment section and the off-gas treatment step typically involve dust washing and/or acid washing. Scrubbing generally involves contacting the gas stream with an aqueous solution, such as an aqueous solution comprising urea, for example, by spraying the solution into the exhaust gas stream. The solution has, for example, a neutral pH (pH 6-8, e.g. pH 7), a low pH (pH below 4 or below 3) for dust washing to remove urea dust, or for acid washing. If dust washing and acid washing are combined, the acid washing is applied, for example, simultaneously with or downstream of the dust washing. Dust scrubbing typically involves recycling a scrubbing solution from which a purge stream containing, for example, 10 wt% to 60 wt% urea is withdrawn. The purge stream is treated, concentrated, for example by water evaporation, and returned to the trim section.
The treatment section may have many designs, but typically includes droplet removal devices, such as a mesh and a chevrons mist eliminator. The treatment section may also include a venturi scrubber. For example, as described in WO 2015/002535, a venturi scrubber comprises one or more tubes having a converging section, a narrow or "throat" section, and typically a diverging section. As the air moves through the throat, it accelerates to a high velocity. A scrubbing fluid (typically an aqueous solution) in the form of droplets is added to the venturi, typically at the throat, and enters the gas stream. The water droplets used are typically many orders of magnitude larger than the contaminant particles (urea dust) to be collected and are therefore accelerated through the venturi at different velocities. The differential acceleration causes an interaction between the water droplets and the contaminant particles such that the contaminant particles are collected by the water droplets.
In an exemplary embodiment, the dust removal system includes a plurality of venturi ejectors operating in parallel. For example, the MMV section (micro mist venturi) can be used, in particular for granulation. The MMV section includes a plurality of parallel venturi tubes. In the MMV section, the liquid is sprayed co-currently with the gas flow through a single phase nozzle in the throat of each venturi, e.g. arranged directly upstream of the converter tube components, to produce consistent and adjustable droplet sizes, typically in the range of 50 μm to 700 μm. Optionally, a throat spray counter, currently with gas flow, is used to control the pressure drop across the venturi section, e.g. with a nozzle in the throat.
In one embodiment, the treating step comprises subjecting the off-gas to quenching with water to produce a quenched off-gas, e.g., having a temperature of less than about 45 ℃, and subjecting the quenched off-gas to scrubbing. The washing step uses, for example, at least one venturi scrubber.
The treatment step comprises, for example, subjecting the waste gas stream to a cooling step such as a quenching step, for example to a temperature of less than 45 ℃ or less than 40 ℃, and/or cooling, for example at least 50 ℃ or at least 60 ℃. For example, cooling is effected by spraying and evaporating an aqueous stream (such as a urea solution), for example by evaporating at least 1.0g, at least 10g or at least 20g water/Nm3And (4) exhaust gas. The cooled gas stream has a Relative Humidity (RH) of, for example, at least 70% or at least 80%. Spraying results, for example, in a spray having a particle size of less than 100 μm, less than 40 μm or less than 20 μmDroplets of average droplet size. Preferably, co-current spraying is used for quenching.
The piping between the finishing section and the exhaust gas treatment section includes, for example, pipes, tubes, and/or conduits. More generally, any gas flow delivery line may be used. The duct has walls that act as gas impermeable boundaries with the outside atmosphere. The conduit has a length of, for example, at least 2m, at least 5m, at least 10m, at least 20m, at least 40m or at least 60 m. For example, the pipe extends from the top of the urea prilling tower to a lower level, for example to a treatment section placed at a lower height, such as an inlet placed at ground level or having a height of 0m-20m, and/or a treatment section having a height of: the height is at least 5m, at least 10m, at least 20m, or at least 40m below the exhaust outlet.
The present invention is generally directed to preventing cold spots on the wall.
Features A to F
Thus, the following measurements a to F may be applied individually and/or in combination with each other.
A) The method may comprise maintaining the wall at a temperature T greater than the gas flow at the outlet of the conditioning apparatus1At a temperature not exceeding 60 ℃. Thus, the method may include maintaining the wall temperature TwIs equal to or higher than Tw,minWherein T isw,min＝(T1-60 ℃), or wherein Tw,min＝(T1-50 ℃), or wherein Tw,min＝(T1-30 ℃), or wherein Tw,min＝(T1-10 ℃), or even wherein T isw,min＝T1. In the latter case, Tw≥T1. Such wall temperature TwTypically in at least one bend of the pipe, in at least one section where the diameter of the pipe varies and/or varies over at least 10%, at least 50% or at least 90% of the length of the pipe. Wall temperature (T)w) Refers to the temperature at the inside of the wall (i.e. the temperature at the inner wall, i.e. the surface of the wall that is in contact with the exhaust gas).
B) The method may comprise maintaining the wall at a temperature of at least 30 ℃, at least 40 ℃, at least 60 ℃, at least 80 ℃, at least 100 ℃, or at least 120 ℃. Such wall temperatures are typically maintained in at least one bend of the conduit, in at least one section in which the diameter of the conduit varies and/or varies over at least 10%, at least 50% or at least 90% of the length of the conduit. The wall temperature refers to the temperature at the inside of the wall (at the surface of the wall that is in contact with the exhaust gas).
C) The temperature of the exhaust gas flow in the vicinity of the wall, e.g. in an area less than 2cm or less than 1cm from the wall (inner surface, i.e. the surface exposed to the exhaust gas), is preferably higher than 60 ℃, preferably higher than 65 ℃, even more preferably higher than 70 ℃. Thus, the temperature of the gas stream near the wall has such a preferred temperature for at least one location in the length of the conduit (e.g. along the gas stream), typically in at least one bend of the conduit, in at least one section where the diameter of the conduit varies and/or varies over at least 10%, at least 50% or at least 90% of the length of the conduit.
D) The temperature difference between the gas flow near the wall of the conduit (e.g. in an area less than 2cm or less than 1cm from the inner surface of said wall) and the gas flow in the centre of the cross-section of the duct is preferably less than 10 ℃, more preferably less than 5 ℃, even more preferably less than 3 ℃ at the same location over the length of the duct. Therefore, the temperature difference of the gas flow in the transverse cross section (perpendicular to the gas flow) is preferably small.
E) Preferably, the method comprises heating at least a portion of the wall of the conduit. The heating may for example be applied over at least 10%, at least 20%, at least 50% or at least 90% of the length of the conduit (e.g. along the gas flow) and/or over at least 10%, at least 20%, at least 50% or at least 90% of the surface of the wall (e.g. the outer surface of the wall). Heating may also be applied to the entire wall. The heating may be applied, for example, by electrical heating and/or by direct or indirect heat exchange with a heating fluid. The heating fluid is preferably steam or condensate. For example, in some embodiments, the steam may be low pressure steam supplied by a high pressure urethane condenser of a urea plant. Preferably, a thermographic wire is applied to the wall of the conduit, for example an electrographic wire and/or a heated fluid wire such as a steam jacket.
F) Insulation may be applied as described below. These features a-F may be used alone or in any combination. As are preferred sub-features. Some exemplary combinations are features a and B; A. b and E and/or F; c and E; d and E; c and D and optionally E; and B and E.
The above measurement is particularly advantageous for a process comprising pelletizing urea-containing solid particles in a prilling tower having an outlet at the top (e.g. above the spraying equipment of the prilling tower) for cooling air as off-gas, wherein the off-gas has a temperature T at the outlet1And wherein the method comprises subjecting the exhaust gas to treatment, preferably dust scrubbing and optionally acid scrubbing in an exhaust gas treatment section, for example placed at ground level (such as a height of less than 20 m), wherein the treatment section has an inlet for exhaust gas at ground level (e.g. a height of 0m to 20m or 0m to 10 m). The method further comprises passing a waste gas stream from the outlet at the top of the urea prilling tower through a conduit positioned outside the prilling tower to the inlet of the treatment section. The treatment section preferably comprises a venturi scrubber, such as the quench spray described.
Preferred urea tailoring methods include:
-prilling urea in a urea prilling tower using cooling air, preferably forced ventilation using a blower or fan, wherein the prilling tower has an outlet for exhaust gas at the top of the tower, wherein the exhaust gas has a temperature T at the outlet1，
-subjecting the exhaust gas to dust scrubbing, and optionally acid scrubbing in an exhaust gas treatment section having an inlet for exhaust gas at a height of 0m to 20m above ground level,
-supplying off-gas from the outlet at the top of the urea prilling tower to the inlet of the off-gas treatment section, and
-maintaining the temperature of the wall of the pipe above T1–10℃。
The invention also relates to a plant for finishing urea-containing materials. A plant for conditioning urea-containing materials includes a conditioning section (such as a granulator or prilling tower) for solidifying a urea-containing liquid stream and an off-gas treatment section. The plant for conditioning urea-containing material also comprises a duct for discharging exhaust gases from the outlet for exhaust gases of the conditioning section to the inlet for exhaust gases of said treatment section. In a particular embodiment, the finishing section is a urea prilling tower, and/or the treatment section is placed at a height of 0m to 20m, for example at ground level.
At least a portion of the pipeline is provided with an insulating material and/or with one or more heating elements, preferably both. Preferably, the plant is configured for carrying out the method. Preferably, the insulating material and, if used, the heating element(s) are configured (each separately or together) for maintaining the temperature of the wall of the pipe above T in at least one portion in at least one bend of the pipew,minIn which the diameter of the pipe changes and/or over at least 10%, at least 50% or at least 90% of the length of the pipe, wherein Tw, min ═ T1-50 ℃ of which T1Is the temperature of the exhaust gas at the outlet, even more preferably Tw, min is as described above.
The heating element is for example used as an external heat source or provides heat from an external heat source. The insulating element and/or the one or more heating elements are configured to maintain a minimum temperature of the wall of the conduit. The heating element is, for example, an electrical heating element or a heat exchanger for exchanging heat with a heating fluid, such as steam or condensate. The heating element is provided, for example, as a pipe heating insulation. The electrical trace heating elements are provided, for example, as electrical heating elements that are in physical contact along the length of the tubing (e.g., as a thermal trace strip). The heating fluid trace heating element is provided, for example, as a conduit for heating a fluid in physical contact with the conduit over a length of the conduit.
The thermographic strip comprises, for example, electrical wires encapsulated in a polymeric tape. The conduit is for example provided with a self-regulating thermographic strip, the resistance of which varies with the temperature. Such tapes include, for example, cables comprising two parallel bus bars encapsulated in a semiconductive polymer loaded with carbon.
For example, the conduit is provided with a thermograph, such as an electrogram and/or a heated fluid trace, in at least one bend of the conduit and/or at least a portion of the conduit where the diameter of the conduit is reduced, over at least a portion or the entire length of the conduit (in the direction of gas flow), such as over at least a 1m length, at least a 2m length, or at least a 5m length, or the entire length. Preferably, the duct has a circumference and the insulating element and/or the one or more heating elements are provided, for example in such a length of the component, over at least 10%, at least 20%, at least 50%, or even at least 90% of the circumference of the duct.
Preferably, the insulating material has a thickness of at least 1.0mm, at least 5mm, at least 10mm, at least 5cm or at least 10cm, for example 5cm to 15 cm. Preferably, the insulating material is porous and/or comprises fibres. For example, the material comprises a foam material. Preferably, the insulating material comprises voids filled with air and has a void volume fraction of at least 10%, at least 20% or at least 50%. Preferably, the insulating material comprises one or more materials selected from polymeric materials, fibre based materials and inorganic non-metallic materials. Preferably, the material is a glass-based material, such as glass fibers. For example, the insulation material has less than 1.0 or less than 0.20 or less than 0.10W m at 1 bar and 293K-1K-1(W/(m.k)) thermal conductivity; and preferably has a thickness as described above. Preferably, the material is applied at least 0.50m2At least 1.0m2At least 5m2Or at least 10m2The above.
Preferably, the plant comprises a forced air prilling tower, wherein for example fans or blowers are used. In such plants, the reduced pressure caused by the blower can cause more formation of isocyanic acid, resulting in a greater potential blockage. In particular, this may be the case: if the treatment section is added to an existing finishing section, a blower or fan needs to be installed in or downstream of the finishing section.
The present invention also provides a method of modifying an existing trim part (e.g. a urea trim part) for curing a urea-containing material, wherein the modification is for the purpose of avoiding or at least reducing clogging in the pipe. Accordingly, modifications are made to at least partially avoid (such as to prevent or reduce) clogging of a conduit disposed between the conditioning section and the treatment section for treating the exhaust gas from the conditioning section. Thus, the conduit is part of, or added to, an existing urea finishing section. For example, the method may include installing a processing portion, such as a replacement processing portion, an additional processing portion, and/or a new processing portion for a finishing portion that does not already have a processing portion. In the case of such an installation of the treatment section, the inlet of the exhaust gas thereof and the additional piping are connected to the outlet of the finishing section. In the method of the invention, the existing or new pipeline is provided with an insulating material and/or with a heating element. Preferably, an insulating material and/or one or more heating elements are provided to avoid cold spots in the wall of the pipe. Thus, the insulating material and/or the one or more heating elements are configured to avoid cold spots. The cold spot is for example located at the inner surface of the wall, i.e. the part of the wall surface that is in contact with the exhaust gas flow. Cold spots include spots having a temperature such that the isocyanate condenses. In the method of the invention, cold spots of the inner wall are avoided. The cold spot typically has a temperature lower than the temperature of the exhaust gas. There is no need for a cold spot to be present at all times without the presence of insulation and/or heating elements. For example, cold spots may be formed during the night or winter, rather than during the day or summer. Typically, the cold spot is the point during at least 10% of the operating time where the walls (inner surfaces) are cooler than the exhaust gas. The insulating material is preferably as described for the plant. The added insulation and/or the heating element or elements (if used) are adapted to reduce clogging of the pipe by avoiding such cold spots.
Preferably, the added insulation material and/or the added heating element or elements are configured (if used) for keeping the temperature of the wall of the pipe above T in at least one portion in at least one bend of the pipew,minIn which the diameter of the pipe changes and/or over at least 10%, at least 50% or at least 90% of the length of the pipe, wherein Tw,min＝T1-50 ℃ of which T1Is the temperature of the exhaust gas at the outlet, even more preferably Tw, min is as described above.
Alternatively and/or in combination, a method of modifying an existing urea trim section to avoid plugging in a duct having a wall for exhaust gases from the trim section, the method comprising: providing a conduit between the conditioning section and a treatment section for treating exhaust gas from the conditioning section, wherein the length of the conduit is less than 10m, less than 5m, or even less than 2.0 m. This length serves the purpose of reducing the risk of pipe blockage and in particular for reducing the risk of isocyanate condensation and urea back reaction on the inner surface of the wall of the pipe. Preferably, the inlets of the treatment section are positioned at about the same height (e.g., below or above at most 5m, or below or above at most 2m) as the outlets for the off-gas of the trim section. For example, the treated section is placed on top of a urea prilling tower. Preferably, a pipe having such a short pipe length is provided with an insulating material and/or one or more heating elements, such as the thermal marker band described above.
The process is preferably carried out in a plant as described above. The plant is preferably adapted for use in the process as described above. The method of modifying the existing urea finishing section preferably results in a plant as described above. The plant may be constructed as a grassroots plant (i.e., newly constructed), or by modifying or retrofitting an existing urea finishing plant.
The invention also relates to the use of a thermograph, such as an electrograph and/or a thermothermograph, for preventing clogging of a conduit for a gas stream comprising urea-containing dust, ammonia and isocyanic acid.
The invention also relates to a method of reducing clogging of a pipe for a gas stream comprising urea-containing dust, ammonia and isocyanic acid, wherein the pipe has a wall, wherein the method comprises applying one or more of said features a-F, and providing said gas stream to an inlet of the pipe and withdrawing the gas stream at an outlet of the pipe.
The source of the gas stream may be any source, typically the source in a plant in which urea is produced. The conduits may, for example, discharge, stack, or treat portions, for example, as described above.
The gas stream contains, for example, at least 10mg isocyanic acid/Nm at the beginning of the pipeline and at the end of the pipeline3Or at least 20mg, or at least 50mg, or at least 100mg of isocyanic acid. Preferably, the concentration of isocyanic acid at the outlet of the conduit is at least 80%, or at least 90%, or even at least 99% of the concentration at the inlet of the conduit.
The gas stream contains, for example, at least 10mg NH at the inlet and usually also at the outlet of the duct3/Nm3Or at least 20mg, or at least 50mg, or even at least 100mg NH3/Nm3. The conduit is, for example, a pipe, a tube or a conduit.
Embodiments of the invention will now be further illustrated in the following figures and examples, which do not limit the invention or the claims.
Figure 1 shows the deposits observed in the comparative piping between the conditioning section and the treatment section after 10 consecutive days of operation of the pilot plant. The pipe is not insulated or heated. Solids form in all the perimeters of the conduit and are due to condensation and crystal growth.
Figure 2 schematically shows an exemplary embodiment of the invention comprising a urea prilling tower a and a treatment section B (for dust and/or acid washing) placed at ground level. The
Figure 3 shows two photographs of a conduit between a prilling tower and a scrubber according to the present invention, wherein the conduit is insulated. These photographs were taken after 2 weeks of discontinuous operation of the pilot plant. The settled solids are located only at the bottom part of the conduit and are due to settling of urea particles (urea dust) by gravity. In contrast to fig. 1, no large solid deposit plug is formed. The settled particles do not adhere to the wall and are easily removed as shown in the right panel of figure 3.
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