CN112358327A - Energy-saving type large-particle urea granulation system and granulation process - Google Patents
Energy-saving type large-particle urea granulation system and granulation process Download PDFInfo
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- CN112358327A CN112358327A CN202011349891.3A CN202011349891A CN112358327A CN 112358327 A CN112358327 A CN 112358327A CN 202011349891 A CN202011349891 A CN 202011349891A CN 112358327 A CN112358327 A CN 112358327A
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Abstract
The invention belongs to the field of urea industrial production, and particularly relates to an energy-saving type large-particle urea granulation system and a granulation process, wherein a raw material storage unit conveys a urea solution to an evaporation unit through a power transmission unit, the evaporation unit is used for evaporating and concentrating the urea solution into a urea solution with a granulation standard concentration, the power transmission unit conveys the concentrated urea solution to a granulation unit, the granulation unit is used for producing large-particle urea, a washing unit is used for washing urea dust generated by the granulation unit, recovering a washing solution and conveying the washing solution to a concentration unit through the power transmission unit, the concentration unit is used for concentrating the low-concentration washing solution recovered by a washing unit into a urea solution with a storage standard concentration, and conveying the urea solution generated by the concentration unit to the raw material storage unit through the power transmission unit, and the system does not generate wastewater, and does not generate waste water, Waste residues and waste gases, only a small amount of urea dust is discharged through a chimney along with tail gas.
Description
Technical Field
The invention belongs to the field of industrial production of urea, and particularly relates to an energy-saving large-particle urea granulation system and a granulation process.
Background
While the gravity center of urea production in the world is gradually shifted from developed countries to developing countries and regions with rich air sources and low prices, the food price is continuously increased along with the promotion of the rapid increase of the yield of urea downstream products in China and the further implementation of the policy of benefiting agriculture, so that the purchasing power of farmers is enhanced, and the demand for urea is steadily increased. Under the urgent situation, as the biggest developing countries in the world, China has a large population and relatively scarce energy, and the energy industry in China faces huge pressure on economic growth, environmental protection and social development, so that the problem of how to reasonably utilize energy and use safety is more and more concerned by people. Therefore, the energy-saving and yield-increasing of urea in the prior art is still the main trend of future development. The large-particle urea evaporation system is the last process of urea production, and whether the large-particle urea evaporation system is stable or energy-saving directly influences the continuous, stable and economic operation of the whole urea production system. Therefore, the energy-saving modification work of the urea evaporation system is particularly important and should be paid attention. The industrial development of urea has undergone a lengthy process. Wuler, German chemist (Fvriedrich Wohler) in 1824 produced urea by reacting cyanic acid with ammonia, breaking the popular "life theory" at that time, and becoming the mark of the rise of modern organic chemistry. In 1932 Du Pont produces urea ammonia water by direct synthesis, solid urea is produced in 1935, and unreacted substance returns to a synthesis tower in the form of ammonium carbamate aqueous solution, thus forming a prototype of a modern aqueous solution full circulation method. In the 30 s, German, English and American countries built a series of continuous non-circulation urea plants with a considerable scale. Since then, semi-circulation and high-efficiency semi-circulation processes appeared, and the direction of process improvement focuses on how to recover the unreacted ammonia and carbon dioxide aqueous solution to the maximum extent in the urea production process by the full-circulation method, and the industrial development is achieved after the semi-circulation method in the 50 s. Although it is almost rejected in Europe, its place of birth, it is still exuberant in China, and the medium and small-sized urea production equipment in China mostly adopts the traditional aqueous solution complete circulation method production process, at present, it has about 190 sets of medium and small-sized urea production equipment in China, and its total production capacity is about 2000 ten thousand tons/year, and most of them are designed in China, and mainly according to 11 ten thousand tons/year general design, it adopts traditional aqueous solution complete circulation method urea production technology. The method can be developed rapidly, and besides the advantages of low investment per ton of urea, easily available materials and complete set manufacturing in China, the method is more important in that the energy consumption is greatly reduced while the production capacity is expanded by the transformation of an advanced process technology, and a new and improved urea production process by an aqueous solution full circulation method is endowed with a new connotation. The industrialization of urea in China starts in 1967, and in the 70 s of the 20 th century, 13 large-scale urea devices are introduced into the whole set of the urea production system in China, and the production capacity of each set of the urea production system is more than 48 million tons per year. After 80 s, more than 100 ammonium bicarbonate manufacturers changed to produce urea, and the production capacity of urea is rapidly increased due to the continuous expansion of a plurality of existing manufacturers. According to statistics, at present, 186 urea production enterprises exist in China, and 17 large-scale enterprise production enterprises with the capacity of more than 52 million tons per year exist. 1994.88 ten thousand tons of urea are co-produced in 2005.
With the increase of the urea production capacity, the problem of energy-saving modification of urea production is increasingly concerned. The energy can be saved, the cost can be saved, and the competitiveness of company products is increased, so that the method is distinguished from increasingly intense market competition. The difference of large granule urea and tiny particle urea mainly reflects on the specification of granule, and large granule urea particle size is 2 ~ 4.75mm, and at present most producer produces tiny particle urea at home, and its product granule is less, the homogeneous degree is poor, and has mutual adhesion phenomenon, has influenced the holistic appearance quality of urea product. The product is easy to absorb moisture and agglomerate in the processes of packaging, transporting and storing, and the sale of the product and the exertion of fertilizer efficiency in application are influenced. Under the condition that the current fertilizer market faces intense competition, the method improves the internal and external quality of urea products, and is an effective measure for enhancing the product competitiveness of enterprises. The large-particle urea has certain use experience at home and abroad and obvious effect, and is gradually accepted by the wide users. The development and production of large-particle urea are proved to be an important way for improving the quality of urea products by years of practice, and become the direction of the improvement of urea production technology in the future.
The large particle urea is comparable to normal urea in its nitrogen content. In terms of internal quality, it has a low biuret content and a low water content. In addition, the crushing strength of the large-particle urea is more than 2 times higher than that of the common urea, the large-particle urea is not easy to crush and absorb moisture to agglomerate, and the large-particle urea is suitable for long-distance bulk transportation and storage. Not only reduces the packaging, transportation and storage cost of the urea, but also brings great convenience to merchants and users. Research shows that the fertilizer efficiency of the large-particle urea is lasting, the nitrogen loss is small, the fertilizer efficiency of the large-particle urea with the particle size of 2.00-4.75 mm is improved by 10% when the large-particle urea is used in a paddy field compared with the common urea, and the yield of paddy is increased by about 10%. The deep application of the large-particle urea can improve the nitrogen utilization rate by more than 15 percent. The urea hydrolysis speed is slowed down, and the crop yield and income increasing effect is obvious. The system consumption of the large-particle urea in production is high mainly due to the fact that the design allowance of a fan is large, the yield is low, the capacity is not exerted (the large-particle urea is basically operated at 1300 tons of daily output and 98 percent of load), the operation period is short, the large-particle urea is basically operated for about 20 days, the fluidized bed cooling equipment with high power consumption is adopted, and the washing and discharging of urea dust are unreasonable.
Disclosure of Invention
The invention aims to provide an energy-saving large-particle urea granulation system which does not generate waste water, waste residues and waste gases, and only a small amount of urea dust is discharged through a chimney along with tail gas; the invention also provides an energy-saving granulation process of the large granular urea.
In order to achieve the purpose, the invention adopts the following technical scheme:
an energy-saving type large-particle urea granulation system comprises a raw material storage unit, an evaporation unit, a granulation unit, a washing unit and a concentration unit which are connected in a circulating manner through a power transmission unit, wherein the raw material storage unit is externally connected with a urea supply pipe network and used for storing urea solution generated by a synthesis system, the raw material storage unit conveys the urea solution to the evaporation unit through the power transmission unit, the evaporation unit is used for evaporating and concentrating the urea solution into urea solution with granulation standard concentration and conveying the concentrated urea solution to the granulation unit through the power transmission unit, the granulation unit is used for producing large-particle urea, the washing unit is used for washing urea dust generated by the granulation unit and recovering washing liquid and conveying the washing liquid to the concentration unit through the power transmission unit, and the concentration unit is used for concentrating low-concentration washing liquid recovered by a washer unit into urea solution with storage concentration standard, and the urea solution generated by the concentration unit is conveyed to the raw material storage unit through the power transmission unit.
Further, raw materials memory cell includes the urine groove, the evaporation unit includes evaporimeter, evaporative separator, first condenser, the import that the evaporimeter lower part was equipped with passes through the pipeline with the export in urine groove and links to each other, is equipped with the force pump on the pipeline, and the evaporimeter is linked together with evaporative separator, and evaporative separator's top is equipped with the gaseous phase export, and the lower part is equipped with the liquid phase export, and evaporative separator's gaseous phase export passes through the pipeline and links to each other with the import of first condenser, the export of first condenser bottom passes through the external ammonia groove of pipeline, and the export on first condenser upper portion passes through pipeline switch-on evaporation sprayer, evaporation sprayer passes through the external technology gas pipe network of aiutage.
Further, a static mixer is arranged on a pipeline connecting the evaporation unit and the granulation unit, the static mixer is used for mixing the formaldehyde solution and the urea solution, the granulation unit comprises a fluidized bed granulator, the fluidized bed granulator is used for atomizing the urea solution and then spraying the atomized urea solution on the seed crystals and growing the urea solution into the required large-particle urea, and a nozzle in the fluidized bed granulator is arranged in a fluidized layer of the fluidized bed granulator and is far away from the outer wall of the granulator.
Further, the washing unit comprises a washing tower, a washer circulating tank, a plurality of washing circulating pumps and a urea solution recovery tank, wherein the washing tower and the washer circulating tank are externally connected with a dilute urine pipe network, the input end pipeline of the washing circulating pump is communicated with the washer circulating tank, the output end pipeline of the washing circulating pump is respectively communicated with the urea solution recovery tank and a washing liquid inlet of the washing tower, the washer circulating tank is arranged at the bottom liquid phase outlet of the washing tower, the washing circulating pump is connected with the washer circulating tank through a pipeline, the washer circulating tank is communicated with the urea solution recovery tank through the washing circulating pump, and the washing circulating pump is used for circularly washing urea dust with dilute urine to remove urea dust.
Further, the concentration unit comprises a flash tank, a flash separator and a second condenser, wherein an inlet arranged at the lower part of the flash tank is connected with an outlet of a urea solution recovery tank through a pipeline, a recovery pump is arranged on the pipeline in the urea solution recovery tank and used for conveying urea solution to the flash tank when the concentration of urea in the urea solution recovery tank reaches a certain standard, the flash tank is communicated with the flash separator, a gas-phase outlet is arranged at the top of the flash separator, a liquid-phase outlet is arranged at the lower part of the flash separator, the gas-phase outlet of the flash separator is connected with an inlet of the second condenser through a pipeline, an outlet at the bottom of the second condenser is externally connected with an ammonia water tank through a pipeline, an outlet at the upper part of the condenser is communicated with an evaporation ejector through a pipeline, and the evaporation ejector is.
An energy-saving granulation process of large granular urea comprises the following steps:
s1, urea solution with the concentration of 72% produced by a synthesis system enters a urine tank, the urine tank conveys the urea solution with the concentration of 72% to an evaporation unit through a pressure pump, and the evaporation unit evaporates and concentrates the urea solution with the concentration of 72% into the urea solution with the concentration of 95% -96% through an evaporator and an evaporation separator;
s2, mixing the generated urea solution with the concentration of 95-96% with a formaldehyde solution with the concentration of 37% in a static mixer to form a urea-formaldehyde solution, and feeding the urea-formaldehyde solution into a granulation unit through a melting pump to produce large-particle urea;
s3, absorbing urea dust generated by a granulation unit into a washing tower, wherein the concentration of urea in a dilute urine pipe network is 35% -40%, washing liquid obtained after urea dust is washed by dilute urine is discharged to a washing device circulation tank, a low-concentration washing liquid in the washing device circulation tank is circularly conveyed into the washing tower by a washing liquid circulation pump to wash the urea dust, the concentration of urea in the washing device circulation tank is gradually increased, when the concentration of the washing liquid in the washing device circulation tank reaches 45%, the washing liquid is conveyed to a urea solution recovery tank by the washing device circulation pump and is conveyed to a concentration unit by a urea solution recovery pump, the concentration unit concentrates the urea solution into 60% -72% urea solution by a flash tank, the concentrated urea solution is separated by a flash separator and is recovered to a urine tank by the circulation pump.
Further, the evaporation temperature of the evaporation unit in S1 is 125-.
Further, the process for producing the large-particle urea by the granulation unit in the S2 comprises the following steps: the urea-formaldehyde solution with the concentration of 95-96% and the temperature of 132-.
Further, the urea dust extracted from the fluidized-bed cooler, the urea dust extracted from the granulator, and the dust extracted from the final product cooler in S3 are directly introduced into the washing tower; the urea dust generated by the safety screen, the vibrating screen, the crusher, the bucket elevator and the product conveyor is sucked into the washing tower by the dust removal fan, the urea dust entering the washing tower is circularly washed by dilute urine by a circulating pump of the washing device to remove the urea dust, and the washed air is discharged to the emptying cylinder by a draught fan of the washing device to be emptied.
Further, the urea solution recovery tank in S3 is further communicated with a fluidized bed granulator to receive and dissolve urea particles overflowing from the granulation unit, a steam ejector is disposed in the urea solution recovery tank, urea is dissolved by stirring of the low-pressure steam ejector, and urine in the urea solution recovery tank is sent to the concentration unit by a urea recovery pump.
Advantageous effects
The energy-saving large-particle urea granulation system provided by the invention is relatively simple, flexible to operate and high in production operation flexibility.
The product produced by the process has high mechanical strength, less water content and low content of biuret as a harmful component. The product is not easy to agglomerate, easy to store and transport, and has wide application and obvious comprehensive benefit.
The system does not produce waste water, waste residue and waste gas in the production operation, only a small amount of urea dust is discharged along with tail gas through a chimney, and the amount of the urea dust meets the national environmental protection standard of China. The air discharged to the atmosphere from the whole large particle urea plant contains no more than 30mg/Nm of urea after wet scrubbing3。
The invention adds the concentration unit, the original washing circulating liquid urea with the concentration of 40 percent directly enters the urine tank, and the concentration is improved to more than 60 percent after the concentration device is added and then enters the urine tank, so that the system can reduce the dust generation amount, improve the yield, increase the operation period and further reduce the power consumption. The yield is increased by 100 tons every day, and the power consumption of each ton of urea can be reduced by about 13 degrees.
Drawings
FIG. 1 is an overall system flow diagram of the present invention;
FIG. 2 is a schematic diagram of the connection of the overall system of the present invention;
FIG. 3 is a schematic view of the connection of the evaporation unit of the present invention;
FIG. 4 is a schematic view of the connection of a pelletizing unit and a washing unit of the invention;
fig. 5 is a schematic diagram of the connection of the concentration unit of the present invention.
Detailed Description
The present invention will be further described with reference to the following examples. The following examples are intended to illustrate the invention only and are not intended to limit the scope of the invention.
As shown in fig. 1-5, an energy-saving type granulation system for large granular urea comprises a raw material storage unit, an evaporation unit 1, a granulation unit 2, a washing unit 3 and a concentration unit 4, which are connected in a circulating manner by a power transmission unit, wherein the raw material storage unit is externally connected with a urea supply pipe network 5 for storing urea solution generated by a synthesis system, the raw material storage unit conveys the urea solution to the evaporation unit 1 by the power transmission unit, the evaporation unit 1 is used for evaporating and concentrating the urea solution into urea solution with standard granulation concentration, and conveys the concentrated urea solution to the granulation unit 2 by the power transmission unit, the granulation unit 2 is used for producing large granular urea, the washing unit 3 is used for washing urea dust generated by the granulation unit 2, and recovering washing liquid and conveying the washing liquid to the concentration unit 4 by the power transmission unit, the concentration unit 4 is used for concentrating the low-concentration washing liquid recovered by the scrubber unit into a urea solution with a standard storage concentration, and conveying the urea solution generated by the concentration unit 4 to the raw material storage unit through the power transmission unit.
As shown in fig. 2 and 3, the raw material storage unit includes a urine tank 6, the evaporation unit 1 includes an evaporator 11, an evaporation separator 12, and a first condenser 13, an inlet provided at a lower portion of the evaporator 11 is connected to an outlet of the urine tank 6 through a pipeline, a pressure pump is provided on the pipeline, the evaporator 11 is communicated with the evaporation separator 12, a gas phase outlet is provided at a top portion of the evaporation separator 12, a liquid phase outlet is provided at a lower portion of the evaporation separator 12, the gas phase outlet of the evaporation separator 12 is connected to an inlet of the first condenser 13 through a pipeline, an outlet at a bottom portion of the first condenser 13 is externally connected to an ammonia tank 15 through a pipeline, an outlet at an upper portion of the first condenser 13 is connected to an evaporation ejector 16 through a pipeline, and the evaporation ejector 16 is externally connected to a.
As shown in fig. 2 and 4, a static mixer 21 is arranged on a pipeline connecting the evaporation unit 1 and the granulation unit 2, the static mixer 21 is used for mixing the formaldehyde solution and the urea solution, the granulation unit 2 comprises a fluidized bed granulator 22, the fluidized bed granulator 22 is used for spraying the atomized urea solution on the seed crystals and growing the urea solution into the required large-particle urea, and a nozzle in the fluidized bed granulator 22 is arranged in a fluidized layer of the fluidized bed granulator 22 and is far away from the outer wall of the granulator.
The washing unit 3 comprises a washing tower 31, a washer circulating tank 32, a plurality of washing circulating pumps 33 and a urea solution recovery tank 34, wherein the washing tower 31 and the washer circulating tank 32 are externally connected with a dilute urine pipe network 8, the input end pipeline of the washing circulating pump 33 is communicated with the washer circulating tank 32, the output end pipeline of the washing circulating pump 33 is respectively communicated with the urea solution recovery tank 34 and a washing liquid inlet of the washing tower 31, the washer circulating tank 32 is arranged at the bottom liquid phase outlet of the washing tower 31, the washing circulating pump 33 is connected with the washer circulating tank 32 through a pipeline, the washer circulating tank 32 is communicated with the urea solution recovery tank 34 through the washing circulating pump 33, and the washing circulating pump 33 is used for circularly washing urea dust with dilute urine to remove urea dust.
As shown in fig. 2 and 5, the concentration unit 4 includes a flash drum 41, a flash separator 42, a second condenser 43, an inlet arranged at the lower part of the flash tank 41 is connected with an outlet of the urea solution recovery tank 34 through a pipeline, a recovery pump 35 is arranged on the pipeline in the urea solution recovery tank 34, the recovery pump 35 is used for delivering the urea solution to the flash tank 41 when the concentration of urea in the urea solution recovery tank 34 reaches a certain level, the flash tank 41 is communicated with the flash separator 42, the top of the flash separator 42 is provided with a gas phase outlet, the lower part of the flash separator 42 is provided with a liquid phase outlet, the gas phase outlet of the flash separator 42 is connected with the inlet of the second condenser 43 through a pipeline, the outlet at the bottom of the second condenser 43 is externally connected with an ammonia water tank 15 through a pipeline, the outlet at the upper part of the condenser is communicated with an evaporation ejector 16 through a pipeline, and the evaporation ejector 16 is externally connected with a process gas pipe network through an exhaust funnel.
An energy-saving granulation process of large granular urea comprises the following steps:
the urea solution with the concentration of 68 percent sent by low-pressure decomposition enters a flash tank with the pressure of 0.045MPa (absolute pressure), the evaporated NH3 and CO2 gas is led out from the top and enters a flash tank condenser, the condensed liquid enters an ammonia water tank 15, the uncondensed gas is merged with the gas from an evaporation separator 12 in an evaporation unit 1 and then goes to the evaporation condenser, the temperature of the flashed urine is reduced to about 90 ℃ from 135 ℃, the concentration is 72 percent, the flashed urine is conveyed to a urine tank 6 through a urea supply pipe network 5 and is conveyed to an evaporator 11 of the evaporation unit 1 by a urine pump, the evaporation temperature is adjusted to 125-130 ℃, the vapor-liquid mixture from the evaporator 11 enters the evaporation separator 12, the vapor phase enters a section of evaporation condensate for condensation, the condensed liquid enters the ammonia water tank 15, the uncondensed gas is pumped out by a section of evaporation ejector 16 and enters an exhaust funnel, the operation pressure of the evaporation unit 1 is 0.033MPa (absolute pressure), the vacuum degree is controlled by an air suction valve provided at an air outlet pipe of the evaporator-separator 12.
The 95% urine from the evaporation separator 12 is sent to the fluidized bed granulator 22 by the melting pump, the 37% formaldehyde solution is added before the pump, the mixed urine containing 0.45% formaldehyde is adjusted to 0.2MPa by the adjusting valve, atomized by the atomizing fan with 0.045MPa, contacted with the crystal seeds from the vibrating screen and the crusher, and gradually wrapped and grown in the fluidized bed layer. The granulated urea granules were cooled to 70 ℃ by a fluidized bed cooler. The atomized air is provided by an atomized air blower, preheated to 135 ℃ by an atomized air heater and then enters the peripheral annular space of the nozzle to atomize the urine, the fluidized air is provided by a fluidized fan, preheated to 40-50 ℃ by the fluidized air heater and then sent to the lower box body of the granulator and enters the upper box body through a porous plate distributor to form a fluidized state, the temperature is regulated to 110 ℃, and the atomized air and the urea dust carried by the fluidized air are pumped out from the upper box body of the granulator and sent to a tail gas scrubber for washing. Specifically, the raw material urine in the urine device is sent to the granulator by a pump at the upstream of the granulator. Formaldehyde at a concentration of 37% was added to the raw urine solution by a static mixer 21 installed on a conventional feed line to form a urea-formaldehyde solution. The total ratio of formaldehyde added to the process (expressed as pure formaldehyde) was about 4.5 kg per ton of final granulated product. The addition of formaldehyde ensures that the process stream is unobstructed without further treatment. The formaldehyde-containing starting material urine (96% concentration, temperature around 132 to 135 ℃) is fed to the main distributor header of the granulator under a pressure of 0.25 to 0.45mpa (g). Inside the granulator L63601, the raw material urine is sprayed to the seed material fluidized layer, and after turbulent mixing, urea particles of uniform size are formed.
Part of the low pressure (about 0.35mpa (g)) atomizing air was used for atomization of the raw material urine. Ambient air was used as atomizing air and was heated to 135 to 145 ℃ before entering the granulator. Ambient air is used directly as fluidizing air, which is slightly preheated and fed into the granulation chamber when operating at low loads or when producing large particles. After passing through the fluidized bed, the atomizing air or fluidizing air contains part of the urea dust, and the dust from the top of the granulator is carried out by the air and directly sent to a wet scrubber for further treatment. The air purified by the granulator scrubber exhaust fan is discharged to the atmosphere through an exhaust funnel. Urine is sprayed into the first three prilling chambers, the latter and the fluidized bed cooler of the priller acting as condition chambers where the fluidized particles are dedusted and cooled to about 70 ℃. Dust from the top of the fluidized bed cooler is carried off by air and sent directly to a wet scrubber for further treatment. The air purified by the granulator scrubber exhaust fan is discharged to the atmosphere through an exhaust funnel.
The urea particles from the granulator are taken out through a vibration extractor, fall into a bucket elevator through a belt conveyor and a safety sieve and are lifted to a sieving section. At the hopper elevator discharge, the product is dosed in two parts operating in parallel in a vibrating screen, the urea particles being divided into three layers: the upper layer is screened with large granular urea, granular urea with right size and the lower layer is screened with fine granular urea. The small urea particles are recycled directly back to the granulator and the urea particles larger than the desired size are crushed by a roll crusher before entering the urea particles cycle. The small urea particles and the caking urea are used as seed materials after being crushed and enter a fluidized bed for granulation according to requirements. The urea particles of the right size are further cooled in the final cooler to the desired temperature of the final product and finally transported to the bulk storage. This is because the product is less likely to agglomerate after cooling than when it is hot.
The content of urea dust contained in the exhaust air was about 4.5% of the total output and could be almost completely recovered in the wet scrubber producing 45% of the urine which was recycled to the evaporation section of the urine plant. Typically, the wet scrubbing section employs condensate from the urine plant as makeup scrubbing liquid and to make up for losses due to the venting of moisture-laden air. In the battery limits, a small dissolving tank (underground tank in the pelletizing plant) is provided, where all the overflowing solids and liquids are collected. The diluted urine is circulated by the invasive circulating pump to the urine collecting device.
The granulator is the core device of the plant and in the design the urea nozzles are placed inside the fluidization layer, far from the outer wall of the granulator. This configuration serves a dual purpose:
the liquid jet completely fluidizes the bed, preventing the entrainment of urine droplets from the air discharged from the granulator and the fouling of the product on the walls (which takes time to remove);
the special design of the nozzle enables the low-pressure urea solution to be atomized by low-pressure atomizing air, and power consumption is reduced to a great extent. Urea particles are formed by the slow growth of fine urea droplets on seed material (e.g., scale-forming urea is destroyed and small urea particles are recycled to the granulator) and no caking occurs. Through the slow growth process, the urine drops solidify one by one, and through the fluidizing air and evaporation process, the water in the raw material urine is continuously removed. In other granulation processes, however, it is generally desirable to keep the residual moisture as low as possible, using nearly dry molten urea as the starting material. And (4) biuret produced in urea circulating liquid (returned to a urine device) of the re-granulation process. Since the circulating liquid is only one tenth of the raw material urine and keeps the concentration low, the increase of the biuret component is not caused. The large particle device increased biuret by 0.05% in total, and could be as little as negligible.
An important indicator of a fluid bed granulation apparatus is the stability of the internal system. When too large or too small a granulation occurs, the recirculation rate can be automatically adjusted at very small varying levels. Under normal operation the (total circulation flow)/(final product) ratio can be kept at 0.5:1 and 2:1 or higher compared to other types of granulation apparatus. Thus, solids processing equipment (e.g., bucket elevators, belt conveyors, vibrating screens, and roller crushers) need not be overloaded for operation.
The urea particles are cooled to 70 ℃ in a fluidized bed cooler, cooling air is provided by a fluidized fan of a granulator, the cooled urea is sent to a product belt conveyor through a granulator discharging device and a safety screen, and then is lifted to a vibrating screen through a bucket elevator, and the urea is divided into three specifications through a vibrating screen feeder: (1) oversized granules (2), finished product urea (3) and fine granules of urea. After being crushed by the crusher, the oversized granules and the fine granular urea enter the granulator together to be used as seed crystals, and the material return ratio is 2:1 during normal operation.
And cooling the finished product urea in a final product cooler to reduce the temperature of the product to less than 50 ℃, and conveying the urea out of the final cooler to a package through a product conveyor. The basic principle of the fluid bed granulation process is: the urea granules are produced by spraying urine onto the seed particles, thereby ensuring stable fluidization, and the seed particles grow by accumulation, and a large amount of tiny urine drops continuously evaporate and solidify on the seed particles to obtain a completely consistent fine grain structure.
The urea raw material used in the process is a molten urea solution which is extracted by an evaporation unit 1 of an upstream urea device, and the concentration of the urea solution is 95-96% (urea and biuret). Mainly produces granular urea products with the diameter of 2-4 mm.
The urea dust generated by the granulation unit 2 is sucked into the washing tower 31, the urea concentration of the dilute urine pipe network 8 is 35% -40%, the washing liquid obtained by washing the urea dust with the dilute urine is discharged to the washer circulation tank 32, the low-concentration washing liquid in the washer circulation tank 32 is circularly sent into the washing tower 31 by the washing liquid circulation pump to wash the urea dust, the urea concentration in the washer circulation tank 32 is gradually increased, when the washing liquid concentration in the washer circulation tank 32 reaches 45%, the urea solution is conveyed to the urea solution recovery tank 34 by the washer circulation pump and is conveyed to the concentration unit 4 by the urea solution recovery pump 35, the urea solution is concentrated into 60% -72% urea solution by the concentration unit 4 through the flash tank 41, the concentrated urea solution is separated by the flash separator 42 and is recovered to the urine tank 6 by the flash pump. Urea dust extracted from the fluidized bed cooler, urea dust extracted from the granulator, dust extracted from the final product cooler are directly fed to the washing tower 31; and urea dust generated by the safety screen, the vibrating screen, the crusher, the bucket elevator and the product conveyor is sucked into the washing tower 31 through the dust removal fan, the urea dust entering the washing tower 31 is circularly washed by dilute urine through a circulating pump of the washer to remove the urea dust, the washed air is discharged to an emptying cylinder through a draught fan of the washer and emptied, and the content of the emptied urea dust is lower than 30mg/m3 dry basis. The urea solution recovery tank 34 receives and dissolves a small amount of massive urea in the discharge of the granulator and overflow urea particles of equipment such as a crusher and the like besides collecting the washing liquid, a steam ejector is arranged in the recovery tank and is stirred by a low-pressure steam ejector to be beneficial to urea dissolution, and the urea recovery pump 35 sends the urine in the recovery tank to the urine tank 6. The concentration of the original washing circulating liquid urea is 40 percent and is directly transferred to the urine tank 6, and the concentration is improved to more than 60 percent after the concentration device is added and then transferred to the urine tank 6, so that the system can reduce the dust generation amount, improve the yield, increase the operation period and further reduce the power consumption. The yield is increased by 100 tons every day, and the power consumption of each ton of urea can be reduced by about 13 degrees.
The granulation technology of the large granular urea has the advantages of relatively simple process, flexible operation and large production operation elasticity.
The product produced by the process has high mechanical strength, less water content and low content of biuret as a harmful component. The product is not easy to agglomerate, easy to store and transport, and has wide application and obvious comprehensive benefit.
The device does not produce waste water, waste residue and waste gas in operation, only a small amount of urea dust is discharged through a chimney along with tail gas, and the amount of the urea dust meets the national environmental protection standard of China. The air discharged to the atmosphere from the whole large particle urea plant contains no more than 30mg/Nm3 of urea after wet scrubbing.
There are two main sites full of urea dust: a granulator and a fluidized bed cooler.
In addition to these locations, there are several other dust points (e.g., hopper tops, roller crushers, vibrating screens, product belt conveyors to warehouses, etc.) where the air is extracted to maintain a safe, clean plant environment.
All the dust removal points are connected to a conventional dust removal fan, by means of which air laden with urea dust is continuously drawn off and blown to the wet scrubber. Urea dust from the exhaust gas and accumulated in the scrubbing liquid is collected in the scrubber underground scrubber tank and efficiently extracted by the wet scrubber. The washing liquid, which is continuously discharged from the washing unit 3, is finally recycled from the washer to the urea synthesis plant. The urea recycle solution contained about 45% urea. No liquid effluent oil plants discharge to the sewer. No solid matter is discharged and all the overflow is collected and recovered internally in the process.
Claims (10)
1. An energy-saving type large-particle urea granulation system is characterized by comprising a raw material storage unit, an evaporation unit, a granulation unit, a washing unit and a concentration unit which are circularly connected through a power transmission unit, wherein the raw material storage unit is externally connected with a urea supply pipe network and used for storing urea solution generated by a synthesis system, the raw material storage unit conveys the urea solution to the evaporation unit through the power transmission unit, the evaporation unit is used for evaporating and concentrating the urea solution into urea solution with granulation standard concentration and conveying the concentrated urea solution to the granulation unit through the power transmission unit, the granulation unit is used for producing large-particle urea, the washing unit is used for washing urea dust generated by the granulation unit and recovering washing liquid and conveying the washing liquid to the concentration unit through the power transmission unit, the concentration unit is used for concentrating the low-concentration washing liquid recovered by a washer unit into the urea solution with storage concentration standard, and the urea solution generated by the concentration unit is conveyed to the raw material storage unit through the power transmission unit.
2. The energy-saving type granulation system for large granular urea as claimed in claim 1, wherein said raw material storage unit comprises a urea tank, said evaporation unit comprises an evaporator, an evaporation separator and a first condenser, said evaporator has an inlet at its lower part connected to the outlet of the urea tank via a pipeline, said pipeline is provided with a pressure pump, said evaporator is connected to the evaporation separator, said evaporation separator has a gas outlet at its top and a liquid outlet at its lower part, said evaporation separator has a gas outlet connected to the inlet of the first condenser via a pipeline, said first condenser has an outlet at its bottom connected to an ammonia water tank via a pipeline, said first condenser has an outlet at its upper part connected to an evaporation ejector via a pipeline, said evaporation ejector is connected to a process air pipe network via an exhaust funnel.
3. An energy-saving granulating system for large granular urea as claimed in claim 2, wherein a static mixer is provided on the pipeline connecting the evaporating unit and the granulating unit, the static mixer is used for mixing the formaldehyde solution and the urea solution, the granulating unit comprises a fluidized bed granulator which is used for atomizing the urea solution, spraying the atomized urea solution on the seed crystal and growing the urea solution into the required large granular urea, and the nozzle in the fluidized bed granulator is arranged in the fluidized layer of the fluidized bed granulator and is far away from the outer wall of the granulator.
4. The energy-saving type granulation system for large granular urea as claimed in claim 3, wherein the washing unit comprises a washing tower, a scrubber circulation tank, a plurality of washing circulation pumps and a urea solution recovery tank, the washing tower and the scrubber circulation tank are externally connected with a dilute urine pipe network, an input end pipeline of the washing circulation pump is communicated with the scrubber circulation tank, an output end pipeline of the washing circulation pump is respectively communicated with the urea solution recovery tank and a washing solution inlet of the washing tower, the scrubber circulation tank is arranged at a bottom liquid phase outlet of the washing tower, the washing circulation pump is connected with the scrubber circulation tank through a pipeline, the scrubber circulation tank is communicated with the urea solution recovery tank through the washing circulation pump, and the washing circulation pump is used for washing urea dust with the dilute urine circulation to remove urea dust.
5. An energy-saving type large granule urea granulation system as claimed in claim 4, the concentration unit comprises a flash tank, a flash separator and a second condenser, an inlet arranged at the lower part of the flash tank is connected with an outlet of a urea solution recovery tank through a pipeline, a recovery pump is arranged on the pipeline in the urea solution recovery tank, the recovery pump is used for conveying the urea solution to the flash tank when the concentration of the urea in the urea solution recovery tank reaches a certain standard, the flash tank is communicated with the flash separator, the top of the flash separator is provided with a gas phase outlet, the lower part of the flash separator is provided with a liquid phase outlet, the gas phase outlet of the flash separator is connected with the inlet of the second condenser through a pipeline, an outlet at the bottom of the second condenser is externally connected with an ammonia water tank through a pipeline, an outlet at the upper part of the condenser is communicated with an evaporation ejector through a pipeline, and the evaporation ejector is externally connected with a process air pipe network through an exhaust funnel.
6. An energy-saving granulation process of large granular urea is characterized by comprising the following steps:
s1, urea solution with the concentration of 72% produced by a synthesis system enters a urine tank, the urine tank conveys the urea solution with the concentration of 72% to an evaporation unit through a pressure pump, and the evaporation unit evaporates and concentrates the urea solution with the concentration of 72% into the urea solution with the concentration of 95% -96% through an evaporator and an evaporation separator;
s2, mixing the generated urea solution with the concentration of 95-96% with a formaldehyde solution with the concentration of 37% in a static mixer to form a urea-formaldehyde solution, and feeding the urea-formaldehyde solution into a granulation unit through a melting pump to produce large-particle urea;
s3, absorbing urea dust generated by a granulation unit into a washing tower, wherein the concentration of urea in a dilute urine pipe network is 35% -40%, washing liquid obtained after urea dust is washed by dilute urine is discharged to a washing device circulation tank, a low-concentration washing liquid in the washing device circulation tank is circularly conveyed into the washing tower by a washing liquid circulation pump to wash the urea dust, the concentration of urea in the washing device circulation tank is gradually increased, when the concentration of the washing liquid in the washing device circulation tank reaches 45%, the washing liquid is conveyed to a urea solution recovery tank by the washing device circulation pump and is conveyed to a concentration unit by a urea solution recovery pump, the concentration unit concentrates the urea solution into 60% -72% urea solution by a flash tank, the concentrated urea solution is separated by a flash separator and is recovered to a urine tank by the circulation pump.
7. The energy-saving granulation process for large granular urea as claimed in claim 6, wherein the evaporation temperature of the evaporation unit in S1 is 125-.
8. The energy-saving granulation process of large granular urea as claimed in claim 6, wherein the granulation unit in S2 is used for producing large granular urea: the urea-formaldehyde solution with the concentration of 95-96% and the temperature of 132-.
9. An energy-saving granulation process of large granular urea as claimed in claim 6, wherein the urea dust extracted from the fluidized bed cooler, the urea dust extracted from the granulator, and the dust extracted from the final product cooler in S3 are directly fed into the washing tower; the urea dust generated by the safety screen, the vibrating screen, the crusher, the bucket elevator and the product conveyor is sucked into the washing tower by the dust removal fan, the urea dust entering the washing tower is circularly washed by dilute urine by a circulating pump of the washing device to remove the urea dust, and the washed air is discharged to the emptying cylinder by a draught fan of the washing device to be emptied.
10. An energy-saving granulation process for large granular urea as claimed in claim 6, wherein in S3, the urea solution recovery tank is further connected to the fluidized bed granulator for receiving and dissolving the urea granules overflowing from the granulation unit, a steam injector is arranged in the urea solution recovery tank, urea is dissolved by stirring with a low-pressure steam injector, and urine in the urea solution recovery tank is sent to the concentration unit by a urea recovery pump.
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