Waste gas washing section of melamine device

Abstract

A melamine off-gas scrubbing section (100) in which melamine off-gas (3) is scrubbed in a scrubber (101) with a urea melt and, after scrubbing, the urea melt (9) collected from the scrubber is partially recirculated to the scrubber, in which the recirculated urea melt is cooled in a urea melt cooler (104) external to the scrubber, in which the urea melt (9) is cooled in the urea melt cooler (104). The urea melt cooler is floating head shell-and-tube equipment.

Description

Technical Field

[0001] The present invention belongs to the field of industrial production of melamine. In particular, the present invention relates to an offgas scrubbing section configured to scrub melamine offgas with urea melt. Background Art

[0002] Technologies for producing melamine on an industrial scale include a non-catalytic high pressure (HP) process and a low pressure (LP) catalytic process. The non-catalytic high pressure process is considered to be the most advantageous and is becoming dominant.

[0003] In the high pressure process, the urea melt is reacted at a pressure usually above 70 bar, typically 75 bar to 200 bar. The reaction temperature is generally about 375°C.

[0004] The melamine-containing product stream is sent to further treatment, generally at a lower pressure. The treatment may include quenching, purification, crystallization, solid-liquid separation and drying to obtain solid melamine of the desired purity. Generally, the purification and crystallization of melamine are carried out in an alkaline environment, and ammonia or sodium hydroxide are the most commonly used alkaline agents.

[0005] The conversion of urea to melamine produces a gaseous stream ("melamine off-gas") which contains primarily ammonia and carbon dioxide, some melamine and other minor components.

[0006] It is often desirable to remove melamine from the off-gases in order to recover the melamine and to purify the off-gases for further use, such as recycling the off-gases as feed material for an associated urea plant. A known technique for purifying melamine off-gases is to scrub them with urea melt in a suitable scrubber. A portion of the available urea melt feed can be used for this purpose, and the scrubbed urea melt can be sent to the melamine synthesis section.

[0007] US 7311759 discloses a method for scrubbing melamine waste gas, in which part of the urea melt removed from the scrubber is cooled in an external heat exchanger ("urea melt cooler") and then recycled to the same scrubber. The heat exchanger is generally a shell and tube heat exchanger in which the urea melt passes through the tube side. The shell side is generally supplied with water to produce steam. In particular, when the cooling medium is a boiling medium (most commonly water converted into steam), boiling is highly preferred to occur in the shell side for reasons of stability of the boiling process. In addition, according to common design practices, fluids at higher pressures or dirtier (dirtier) are preferably directed to the tube side of the heat exchanger. Due to the nature of the fluid to be cooled (urea melt), the cooling process has some disadvantages even when the cooling process is carried out in the tube side.

[0008] Due to the contact with the exhaust gases during the scrubbing process, the urea melt removed from the scrubber contains melamine precursors and ammonia. Due to its composition, the urea melt can leave deposits of unwanted solid material on the inner surface of the tubes, forming a layer of fouling or causing tube blockages. Fouling leads to a reduction in the heat exchange coefficient and an increase in the flow resistance (pressure drop). The performance of the heat exchanger is seriously affected. In addition, if the melamine melt is overcooled, for example due to deviations from normal operation, precipitation of melamine cyanurate / salts can occur in the tubes. Summary of the invention

[0009] The problem faced by the present invention is how to increase the reliability of the above-mentioned heat exchanger (urea melt cooler) of the melamine waste gas scrubbing section and how to maintain its constant performance.

[0010] This problem is solved by means of an exhaust gas scrubbing section according to the claims. The invention judiciously uses a floating head heat exchanger as a urea melt cooler.

[0011] The floating shell and tube heat exchanger has a bundle of straight tubes connected to two tube sheets at opposite ends. One tube sheet is fixed to the shell, while the other tube sheet (the "floating tube sheet") is movable relative to the shell.

[0012] The present invention originates from the understanding that, in the case of concern, the tubes through which the urea melt passes can only be properly cleaned by mechanical cleaning (also called drilling), which requires access to the tubes. Therefore, in the current case, a U-shaped tube bundle is not advantageous. U-shaped tubes, especially U-shaped bends, cannot be mechanically cleaned by drilling. This is contrary to the common design practice, especially in kettle heat exchangers, which recommends using U-shaped tubes as a measure to avoid expansion joints on the shell side.

[0013] The invention also stems from the understanding that when the tube cannot be drilled, the alternative cleaning methods available are ineffective against deposits caused by urea melt after exhaust gas scrubbing. These methods include introducing chemicals, spraying water at high pressure or increasing the shell side temperature to liquefy the deposits.

[0014] Melamine cyanurate deposits cannot be removed by heating. Experience has shown that heating can lead to the formation of polycondensates which are even more difficult to remove than the cyanurate itself. Washing with known chemical agents is ineffective; alkaline or acidic solutions or organic solvents have proven ineffective for removing cyanurates. Washing with water is also unsatisfactory; cyanurates are hard and insoluble in water. In particular, chemical cleaning or water washing will be completely ineffective if the pipes are blocked and do not allow the washing liquid to flow freely.

[0015] The advantage of the present invention is that the floating head heat exchanger allows access to the tubes, and direct removal of deposits and possible blockages of the tubes by mechanical action (such as drilling into the tubes). Any melamine cyanurate / salt deposits can be separated from the metal surface by drilling and then removed by washing with water.

[0016] Another significant advantage of the floating head heat exchanger is that it does not require expansion joints in the shell. Expansion joints are a structural weakness and are expensive. The art of designing heat exchangers suggests that expansion joints can be avoided by using U-shaped tubes, but as mentioned above, U-shaped tubes have been found to be of no advantage in this application (urea melt cooler for an exhaust gas scrubber) because deposits may form that are difficult to clean or remove.

[0017] Another aspect of the invention is a melamine plant comprising a melamine synthesis section in which urea melt is converted into melamine and off-gases comprising ammonia and carbon dioxide are formed; the melamine plant further comprising an off-gas scrubbing section according to the claims, the off-gas scrubbing section being arranged to receive off-gases from the synthesis section. Still another aspect of the invention is a method for synthesizing melamine according to the appended claims. DETAILED DESCRIPTION

[0018] The invention relates to a melamine off-gas scrubbing section arranged to treat off-gases containing ammonia and carbon dioxide discharged from a melamine synthesis section in which urea reacts to form melamine.

[0019] The exhaust gas scrubbing section comprises a scrubber arranged to provide scrubbing of the exhaust gas with urea melt. The scrubber is connected to a feed line of fresh urea melt. The scrubbing section comprises: a line arranged to collect a flow of urea melt effluent from the scrubber after the scrubbing process; a recirculation line, which is located outside the scrubber and is arranged to reintroduce a portion of the urea melt effluent into the scrubber; a heat exchanger, which is arranged in the recirculation line to cool the urea melt by transferring heat to a cooling medium. Thus, the scrubber receives a urea melt flow comprising fresh urea melt and recycled urea melt.

[0020] The heat exchanger is a floating head shell and tube device. Generally speaking, the heat exchanger includes a tube bundle contained in a shell. The heat exchanger has a first inlet / outlet connection for the tube bundle and a second inlet / outlet connection for the shell. Therefore, the heat exchanger has a tube side and a shell side that can be passed through by the first medium and the second medium, respectively. In the present invention, the heat exchanger is connected so that the recycled urea melt passes through the tube side and the cooling medium passes through the shell side.

[0021] The tubes are arranged straight from the first tube sheet to the second tube sheet. According to the floating head principle, the heat exchanger has a fixed tube sheet and a floating tube sheet. The fixed tube sheet is fixed to the flange, while the floating tube sheet is free to move, for example after thermal expansion of the tubes. Traditionally, the fixed tube sheet is considered to be the front end of the heat exchanger and the floating tube sheet is considered to be its rear end.

[0022] Preferred embodiments are described using TEMA (Tube Exchanger Manufacturers Association) nomenclature. The TEMA notation is familiar to those skilled in the art and is summarized here.

[0023] Stationary head type

[0024] A Channel box and removable cover

[0025] B Bonnet (integrated cover)

[0026] C Tube box integrated with tube sheet and removable cover; removable tube bundle

[0027] N Tube box integral with tube sheet and removable cover; non-removable tube bundle

[0028] D Special high pressure sealed pipe box (closure)

[0029] Shell Type

[0030] E one-way housing

[0031] F Two-way shell with longitudinal baffles

[0032] G split flow

[0033] H double shunt

[0034] JDivided flow

[0035] K kettle

[0036] X-Flow

[0037] Back-end head type

[0038] L fixed tube sheet, similar to "A" fixed head

[0039] M fixed tube sheet, similar to "B" fixed head

[0040] N fixed tube sheet, similar to "N" fixed head

[0041] Outside packed floating head

[0042] SFloating head with backing device

[0043] T pull-through floating head

[0044] UU type tube bundle

[0045] W External Seal Floating Tube Sheet

[0046] In an embodiment of the invention, according to TEMA, the shell can be type E, F, G, H, J, K or X. The front end can be type A, B, C, N, D. The rear end head is a floating head, so it can be type P, S, T or W.

[0047] These symbols may be combined. The three-letter code identifies, in order, the type of front head, the type of kettle, and the type of rear head. For example, "CKT" indicates a heat exchanger in which the front head is a fixed head according to the "C" type; the shell is according to the "K" type; and the rear head is a floating head according to the "T" type. Various embodiments of the present invention include combinations of the above-mentioned types of front heads, shells, and rear ends.

[0048] The construction details of heat exchangers and the associated nomenclature need no explanation as they can be found in the literature, for example, Perry's Chemical Engineers' Handbook, 7th edition, Chapter 11, "Heat Transfer Equipment".

[0049] In a preferred embodiment, the heat exchanger has a plurality of channels in the tube pass. Preferably, the number of channels in the tube pass is an even number, and particularly preferably, the number of channels is two, four or six.

[0050] Preferably, the tubes of the heat exchanger are seamless tubes. Seamless tubes increase reliability and corrosion resistance. Preferred materials for the tubes include nickel alloys or titanium.

[0051] According to TEMA's designation, the front end of the heat exchanger is preferably a C-type or N-type fixed head.

[0052] In one embodiment, the tube bundle is completely removable from the shell of the heat exchanger to allow cleaning in a separate location.

[0053] The heat exchanger may be a kettle type heat exchanger, preferably a CKT exchanger specified according to TEMA.

[0054] The heat exchanger may include a manhole adapted to provide access to the tube side for cleaning the tubes. When combined with a kettle heat exchanger, it is particularly preferred to provide such a manhole.

[0055] The cooling medium in the shell side can be any suitable medium. In some embodiments, the cooling medium is at least partially evaporated in the shell side. The preferred cooling medium is water, and in a preferred embodiment, water is evaporated in the shell side to produce steam. Therefore, the shell side has an inlet connected to a water supply line and an outlet connected to a steam pipe.

[0056] In some embodiments, a condensate stream is purged from the shell side of the heat exchanger to control the salt concentration in the water. The amount of condensate purged is preferably not more than 100 kg / h.

[0057] The urea melt is preferably cooled to a temperature in the range of 165° C. to 245° C. A temperature of the urea melt above 165° C. avoids precipitation of cyanurate / salt, while a temperature of no more than 245° C. is beneficial to avoid corrosion, i.e., control the corrosive effect of the urea melt flowing through the urea melt cooler.

[0058] The non-recycled part of the urea melt can be sent to the melamine synthesis section.

[0059] The scrubber may be single-stage, or may comprise two stages. The contact between the melamine offgases and the urea melt is preferably counter-current. Preferably, the scrubber is arranged vertically.

[0060] In a two-stage embodiment, the scrubber comprises a first stage and a second stage. In the first stage, the melamine off-gases are contacted in a countercurrent direction with a urea melt comprising a recycled urea melt loaded with ammonia and a melamine precursor; in the second stage, the off-gases discharged from the first stage are contacted in a countercurrent manner with fresh urea melt, which is introduced into the second stage and passes through the second stage and the first stage in sequence. The two stages are operated in sequence so that the partially purified off-gas effluent from the first stage is treated in the second stage. The first stage may be operated at a higher temperature than the second purification step, or the two steps may be performed at substantially the same temperature.

[0061] A preferred embodiment of a two-stage scrubber is as follows. The second purification stage is placed above the first purification stage; the melamine offgas flows upward in the first stage and then flows upward in the second stage; urea melt is sprayed on the offgas from the top of the second stage and flows downward through the second stage and the first stage; the urea melt loaded with ammonia and melamine precursors is collected at the bottom of the first stage and a part of it is recycled to the same first stage after cooling. The melamine offgas is firstly washed in countercurrent with the urea melt from the second stage and the urea melt is recycled in the first stage; then fresh urea melt is introduced in the second stage. Preferably, the first stage is operated at a temperature in the range of 170°C to 250°C, while the second stage is operated at 135°C to 230°C.

[0062] In some embodiments, the off-gas scrubbing includes the addition of carbon dioxide. The carbon dioxide stream may be added to the melamine off-gas before it enters the scrubber, or the carbon dioxide stream may be introduced separately into the scrubber. In an embodiment with two stages, the carbon dioxide is preferably added to the first stage.

[0063] The washing of melamine waste gas with urea melt is carried out under high pressure, preferably at a pressure of at least 50 bar, and more preferably at a pressure equal to or substantially equal to the melamine synthesis pressure. The waste gas washing can be carried out at a pressure slightly lower than the melamine synthesis pressure, wherein the difference does not exceed 20 bar or does not exceed 5 bar. The cleaned waste gas after washing can be recycled to the supporting urea device.

[0064] According to one embodiment, the exhaust gas is introduced into the scrubber through an exhaust gas distributor, which is above or below the level of the urea melt containing ammonia and melamine precursor. A preferred embodiment of the exhaust gas distributor is disclosed in US7311759. The urea melt can be introduced into the scrubber as a single stream, or can be divided into multiple streams and introduced at multiple locations of the scrubber, for example, at different vertical heights.

[0065] Freshly made urea melt is preferably a urea melt obtained from a urea plant after recovery of unreacted substances and evaporation of water. Generally, the urea melt contains at least 96% urea, the remainder being residual water and unavoidable impurities.

[0066] In the melamine synthesis section, the urea melt is reacted under non-catalytic high pressure melamine synthesis conditions to produce a crude melamine product and a melamine waste gas stream comprising ammonia, carbon dioxide, melamine and minor components. The melamine synthesis pressure is preferably 70 bar or above, for example, 70 bar to 200 bar.

[0067] According to a preferred embodiment, the synthesis of melamine comprises a conversion step and a stripping step, wherein the conversion step comprises reacting the urea melt feed stream under suitable melamine synthesis conditions to produce a crude melamine product, and the stripping step comprises stripping the crude melamine product in the presence of gaseous ammonia to remove carbon dioxide contained in the crude melamine.

[0068] In some embodiments, the melamine synthesis section comprises a single reactor from which crude melamine and melamine off-gas are withdrawn. In other embodiments, the melamine synthesis section comprises a primary reactor, where a urea melt is reacted, followed by a secondary reactor, where the effluent of the primary reactor containing melamine is stripped with gaseous ammonia. In these embodiments, each of the primary reactor and the secondary reactor produces its own melamine off-gas stream. Although their compositions may differ, both melamine off-gas streams consist primarily of ammonia and carbon dioxide.

[0069] According to an embodiment of the present invention, the melamine offgas subjected to washing with urea melt may comprise only the melamine offgas stream from the primary reactor or comprise two melamine offgas streams from the primary reactor and the secondary reactor (possibly combined into a single stream). In a further embodiment, the combined reactor performs the functions of the primary reactor and the secondary reactor; for this purpose, the combined reactor comprises a primary reaction stage and a secondary reaction stage.

[0070] The melamine-containing effluent of the synthesis stage is treated in a melamine purification stage according to known techniques.The capacity of a melamine synthesis process, which is the amount of solid melamine obtained from the process, is generally between 5 and 15 tons of melamine per hour.

[0071] In a combined urea-melamine embodiment, ammonia and carbon dioxide react in a urea synthesis section to form a urea solution, the urea solution is treated in at least one recovery section to obtain a purified urea solution, and water is removed from the solution to form a urea melt. The urea melt is used in the above-mentioned process for synthesizing melamine. Melamine waste gases generated during the melamine synthesis process are recovered to produce urea. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 It is a schematic diagram of the melamine waste gas scrubbing section.

[0073] Figure 2 is a schematic diagram of a urea melt cooler that can be used in a washing section according to one embodiment of the present invention.

[0074] Figure 3 is a schematic diagram of a urea melt cooler according to another embodiment.

[0075] Figure 1 The main projects in are:

[0076] 1Melamine synthesis section

[0077] 2. Crude melamine melt outflow

[0078] 3 Waste gas extracted from the melamine synthesis section ("melamine waste gas")

[0079] 4 Low pressure (LP) treatment section of crude melamine melt

[0080] 5Solid melamine

[0081] 100 Melamine waste gas scrubbing section

[0082] 101 Exhaust gas scrubber

[0083] 102 Urea melt recycling line

[0084] 103 Urea melt recirculation pump

[0085] 104 Urea melt cooler

[0086] 6 Fresh urea melt from urea plant

[0087] 7 Freshly made urea melt input to scrubber 101

[0088] 8 Freshly prepared urea melt directed to melamine synthesis section 1

[0089] 9 Urea melt after washing process

[0090] 10 Urea melt delivered by pump 103

[0091] 11 Urea melt recycled to scrubber 101

[0092] 12 Urea melt sent to synthesis section 1

[0093] 13 Cooling of urea melt

[0094] 14 Water

[0095] 15 Steam

[0096] 16 Urea melt fed to synthesis section 1

[0097] 17 CO fed to scrubber 101 2

[0098] 18 Cleaned exhaust gas removed from scrubber 101

[0099] Reference numbers are intended to indicate process flows and corresponding connecting lines.

[0100] Urea melt feed 16 reacts in melamine synthesis stage 1 to produce melamine-containing product 2 and melamine offgas 3 .

[0101] The melamine waste gas 3 is treated in a washing section 101 to obtain a purified waste gas 18, which can be recycled to the associated urea plant. In particular, the melamine waste gas 3 is washed with urea melt in the scrubber 101. The waste gas washing is carried out with fresh urea melt 7 and recycled urea melt 13 previously cooled in a urea melt cooler 104. The scrubber 101 can be single-stage or double-stage. Washing is carried out in the scrubber 101 in countercurrent, possibly with the aid of carbon dioxide 17.

[0102] The recirculation line 102 comprises a urea melt pump 103 and a urea melt cooler 104. The recycled urea melt 11 is a portion of the urea melt 9 withdrawn from the scrubber 101. The remaining portion 12 is added to the portion 8 of fresh urea melt to form the feed 16 of the melamine synthesis section 1.

[0103] In the urea melt cooler 104, heat is removed from the recycled urea melt 11 and transferred to water 14, which is converted into steam 15. The urea melt cooler 104 is a shell and tube device where the urea melt 11 is delivered to the tube side and evaporation of the water 14 occurs in the shell side.

[0104] One embodiment of the urea melt cooler 104 is Figure 2 It shows:

[0105] 201 stationary-head channel

[0106] 202 splitter plate

[0107] 203 Fixed tube sheet

[0108] 204 Discipline

[0109] 205 Shell

[0110] 206 Floating tube sheet

[0111] 207 Shell cover

[0112] 208 floating head cover.

[0113] Figure 3 Shows an alternative kettle implementation: 301 fixed head tube box

[0114] 302 splitter plate

[0115] 303 Fixed tube sheet

[0116] 304 Discipline

[0117] 305 Housing

[0118] 306 Floating Tube Sheet

[0119] 307 Shell cover

[0120] 308 Floating head cover.

Claims

1. A melamine off-gas scrubbing section (100) arranged to treat off-gas (3) containing ammonia and carbon dioxide discharged from a melamine synthesis section (1), wherein: The exhaust gas washing section comprises: a scrubber (101) connected to the urea melt feed line, said scrubber being arranged to provide scrubbing of said exhaust gas with urea melt; a line (9) arranged to collect a flow of urea melt effluent from the scrubber after the scrubbing process; a recirculation line (102) located outside the scrubber and arranged to reintroduce a portion of the urea melt effluent (11) into the scrubber; a heat exchanger (104) arranged in the recirculation line to cool the recirculated urea melt (11) by transferring heat to a cooling medium; The heat exchanger (104) is a shell and tube device having a tube bundle contained in a shell, the tube bundle and the shell having inlet and outlet connecting pipes arranged so that the recycled urea melt (11) passes through the tube side and the cooling medium (14) passes through the shell side; The heat exchanger (104) is a floating head heat exchanger.

2. The melamine waste gas scrubbing section according to claim 1, wherein: The heat exchanger has a plurality of channels of the tube side.

3. The melamine waste gas scrubbing section according to claim 2, wherein: The number of channels in the tube pass is an even number, preferably two, four or six channels.

4. Melamine off-gas scrubbing stage according to any one of the preceding claims, wherein The tubes of the heat exchanger are seamless tubes.

5. Melamine off-gas scrubbing stage according to any one of the preceding claims, wherein The heat exchanger has a front end with a C-type or N-type fixing head according to TEMA designation.

6. The melamine waste gas scrubbing section according to claim 5, wherein: The tube bundle is completely removable from the housing of the heat exchanger to allow cleaning in a separate location.

7. Melamine off-gas scrubbing stage according to any one of the preceding claims, wherein The heat exchanger is a kettle heat exchanger, preferably a CKT exchanger according to TEMA designation.

8. The melamine waste gas scrubbing section according to claim 7, wherein: The heat exchanger includes an access port adapted to provide access to the tube side for cleaning the tubes.

9. Melamine off-gas scrubbing stage according to any one of the preceding claims, wherein The heat exchanger is configured to generate steam (15) in the shell side, which has an inlet connected to a water supply line and an outlet connected to a steam pipe.

10. A melamine plant comprising a melamine synthesis section, in which urea melt is converted into melamine and off-gases comprising ammonia and carbon dioxide are formed, and an off-gas scrubbing section according to any of the preceding claims, arranged to receive off-gases from the synthesis section.

11. A method for synthesizing melamine, comprising the following steps: reacting a feed stream of urea melt under non-catalytic high pressure melamine synthesis conditions to produce a crude melamine product (2) and an offgas (3) comprising ammonia and carbon dioxide, scrubbing the waste gas (3) with urea melt in a scrubber (101), wherein a stream of urea melt (9) containing ammonia and melamine precursors resulting from the scrubbing process is withdrawn from the scrubber; wherein a portion (11) of the urea melt (9) taken out of the scrubber is recycled to the same scrubber via a recycling line (102) outside the scrubber; The recirculation circuit comprises a heat exchanger (104) arranged to cool the recirculated urea melt (11) by transferring heat to a cooling medium; The heat exchanger (104) is a shell-and-tube device having a tube side and a shell side, wherein the urea melt passes through the tube side and the cooling medium is sent to the shell side; The heat exchanger (104) is a floating head heat exchanger.

12. The method according to claim 11, wherein the floating head heat exchanger is a heat exchanger according to any one of claims 2 to 9.

13. The method according to claim 11 or 12, wherein: The cooling medium is water, which is converted into steam in the shell side of the heat exchanger.

14. The method according to claim 13, wherein: A condensate flow is introduced from the shell side of the heat exchanger to control the salt concentration in the water, and the amount of the introduced condensate is not more than 100 kg / h.

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