Baffle for heat exchanger

Abstract

This disclosure provides a baffle plate for a shell-and-tube heat exchanger. The baffle plate includes a flat plate having a region comprising a plurality of annular elements designed to receive tubes from the shell-and-tube heat exchanger. The annular elements are arranged in at least two rows, wherein the rows are staggered relative to adjacent rows. The outer diameter of each annular element is less than 130% of its inner diameter, and each annular element engages with all its adjacent annular elements via a bridging structure oriented along a line connecting the centers of two adjacent annular elements in the plane of the plate, thereby defining a plurality of openings in the plate. This disclosure also provides: a shell-and-tube heat exchanger; a method for heating a liquid composition; a method for stripping a liquid composition comprising urea, carbamate, ammonia, and water; and a method for producing a solid particulate urea-based composition.

Description

Technical Field

[0001] This disclosure relates to the field of heat exchangers. In particular, this disclosure discloses a novel design for a baffle plate for a shell-and-tube heat exchanger. Background Technology

[0002] Heat exchangers are widely used in chemical plants. These devices are used to transfer heat between a product and a fluid, which can be a liquid, solid, liquid / vapor, or slurry composition. The fluid can be a cooling or heating fluid. A common heating fluid used is steam, as steam is generally available in chemical plants and can be used to heat compositions.

[0003] Several designs exist for heat exchangers, one of which is called a shell-and-tube heat exchanger: this type comprises a body consisting of multiple tubes enclosed within a shell, specifically straight tubes, and particularly straight tubes having the same outer diameter. The composition to be heated or cooled is distributed within these tubes, and the cooling or heating fluid is guided into the shell of the heat exchanger to cool or heat the tubes. These multiple tubes must be held within the shell to ensure continuous operation and limit their vibration. Two main support systems can be used: a network of metal rods arranged in multiple directions within the shell, or a series of baffles comprising multiple orifices corresponding to the multiple tubes.

[0004] When using a heating fluid such as steam, it is important to ensure that the steam is evenly distributed throughout the heat exchanger shell to guarantee homogeneous heating of the tubes. Furthermore, it is crucial to remove the steam condensate (i.e., water) formed by the condensation of steam on the tubes to prevent buildup. Non-uniform distribution can lead to problems such as uneven heat release along the tubes, resulting in inadequate heat exchanger performance or uneven temperatures along the tubes (with some tubes receiving more heat than others), and corrosion that may occur when the composition being heated is corrosive. It is well known that increased temperature exacerbates corrosion.

[0005] CN203881201U discloses a baffle for a heat exchanger, wherein the baffle includes two series of holes: a first series of circular holes corresponding to the tubes of the heat exchanger, and a second series of rectangular holes to reduce the pressure drop across the shell and improve the overall heat transfer performance. Summary of the Invention

[0006] It has been found that the heating profile of multiple tubes in a shell-and-tube heat exchanger, especially the distribution of steam condensate within the heat exchanger shell, can be improved by providing new types of baffles.

[0007] In one aspect, this disclosure provides a baffle for a shell-and-tube heat exchanger. The baffle includes a planar assembly of a plurality of annular structures (also referred to herein as hollow cylinders) interconnected with bridging structures, wherein the annular structures are designed to receive a plurality of tubes from the shell-and-tube heat exchanger; wherein the outer diameter of the annular structure is less than 130% of the inner diameter of the annular structure; wherein the annular structures are positioned in at least two rows, wherein the rows are staggered relative to their adjacent rows; wherein each annular structure is connected to all its adjacent annular structures by bridging structures oriented along a line between the centers of two adjacent annular structures, thereby forming a plurality of openings in the baffle, wherein the annular structures are positioned at the corners of the openings. Thus, the baffle of the present invention includes: a plurality of circular openings (i.e., within the annular elements); and a plurality of openings defined by three bridging structures and three staggered annular elements. In a particular embodiment, the baffle is made of metal, more particularly of steel or stainless steel.

[0008] In another aspect, this disclosure provides a shell-and-tube heat exchanger comprising: a top end fluidly connected to a bottom end via a plurality of straight tubes (also referred to as "a plurality of tubes"), wherein each tube or straight tube has the same outer diameter, i.e., the plurality of straight tubes being disposed within a shell; the top end including an inlet for liquid and optionally an outlet for gas; the bottom end including an outlet for liquid and optionally an inlet for gas; a heating fluid inlet and a heating fluid outlet fluidly connected to a shell-side space, the plurality of straight tubes being disposed within the shell-side space; the shell-and-tube heat exchanger having a longitudinal direction and a transverse cross section, the longitudinal direction being parallel to the tubes and the transverse cross section being perpendicular to the longitudinal direction; a plurality of baffles according to this disclosure arranged in the shell-side space, wherein the baffles are substantially parallel to the transverse cross section; wherein the tubes are received within circular openings of annular elements of the baffles; and wherein the inner diameter of the annular elements in the baffles is equal to or greater than the outer diameter of the straight tubes, i.e., wherein the inner diameter of each annular element is equal to or greater than the outer diameter of each of the plurality of straight tubes.

[0009] In another aspect, this disclosure provides a method for heating a liquid composition, comprising the steps of: providing a shell-and-tube heat exchanger according to this disclosure; providing a liquid composition to an inlet; providing a heating fluid, particularly steam, to a shell-side space through a heating fluid inlet; heating the plurality of straight tubes by the heating fluid; and extracting the heated liquid composition at an outlet.

[0010] In another aspect, this disclosure provides a method for stripping a liquid composition comprising urea, carbamate, ammonia, and water, comprising the steps of: providing a shell-and-tube heat exchanger according to this disclosure; providing a liquid composition comprising urea, carbamate, ammonia, and water to an inlet; providing steam, particularly saturated steam at a temperature between 195°C and 225°C, to a shell-side space through a heated fluid inlet; providing stripping gas comprising carbon dioxide to the inlet; contacting the liquid composition with the stripping gas in a tube-side space disposed within a plurality of straight tubes; heating the plurality of straight tubes with steam; extracting at an outlet a liquid composition comprising urea and water, as well as reduced amounts of carbamate and ammonia and water; extracting at an outlet a gas mixture comprising ammonia, carbon dioxide, and water; and extracting steam and steam condensate at a heated fluid outlet.

[0011] In another aspect, this disclosure provides a method for producing urea, comprising the step of guiding a liquid composition comprising urea, carbamate, ammonia and water into a shell-and-tube heat exchanger according to this disclosure. Attached Figure Description

[0012] The following description of the accompanying drawings, which illustrate specific embodiments of the system according to this disclosure, is given by way of example only and is not intended to limit this description, its application, or use. In the drawings, the same reference numerals denote the same or similar parts and features.

[0013] Figure 1 This represents a portion of an embodiment of the baffle according to the present disclosure.

[0014] Figure 2 This indicates an embodiment of a shell-and-tube heat exchanger according to the present disclosure.

[0015] Figure 3 This indicates another embodiment of the shell-and-tube heat exchanger according to the present disclosure. Detailed Implementation

[0016] Unless otherwise defined, all terms (including technical and scientific terms) used in disclosing this invention have the meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Further guidance includes the inclusion of terminology definitions to better understand the teachings of this invention.

[0017] All references cited in this specification are hereby deemed to be incorporated herein by reference in their entirety.

[0018] As used herein, the following terms have the following meanings:

[0019] Unless the context clearly specifies otherwise, as used herein, “a,” “an,” and “the” refer to both the singular and plural referents. For example, “section” refers to one or more sections.

[0020] As used herein, the term "about" to refer to measurable values ​​such as parameters, quantities, and durations means a variation that covers or deviates from the specified value by + / -20% or less, particularly + / -10% or less, more particularly + / -5% or less, even more particularly + / -1% or less, and still more particularly + / -0.1% or less, such variations being suitable for implementation in the disclosed invention. However, it should be understood that the values ​​referred to by the modifier "about" are themselves specifically disclosed.

[0021] As used herein, “comprise,” “comprising,” “comprises,” and “comprised of” are synonymous with “include,” “including,” “includes,” or “contain,” “containing,” and are inclusive or open-ended terms that specify the presence of the following thing (e.g., a component) and do not exclude or preclude the presence of additional, unlisted components, features, elements, components, or steps known in the art or disclosed herein.

[0022] The enumeration of a range of values ​​by endpoints includes all numbers and fractions contained within that range, as well as the endpoints of the enumeration.

[0023] The term “tube” (particularly in the expression “multiple tubes”) is generally used interchangeably with the term “straight tube” (particularly in the term “multiple straight tubes”) when it pertains to heat exchangers as disclosed herein.

[0024] In one aspect, this disclosure provides a baffle for a shell-and-tube heat exchanger. The baffle includes a planar assembly of a plurality of annular structures (also referred to herein as hollow cylinders) interconnected with bridging structures, wherein the annular structures are designed to receive the plurality of tubes of the shell-and-tube heat exchanger; wherein the outer diameter of the annular structure is less than 130% of the inner diameter of the annular structure; wherein the annular structures are positioned in at least two rows, wherein the rows are staggered relative to their adjacent rows; wherein each annular structure is connected to all its adjacent annular structures by bridging structures oriented along a line between the centers of two adjacent annular structures, thereby forming a plurality of openings in the baffle, wherein the annular structures are positioned at the corners of the openings.

[0025] Baffles in shell-and-tube heat exchangers are devices that serve different purposes: first, to hold the multiple straight tubes of the heat exchanger in their positions and limit their vibration; and second, to increase the shell-side heat transfer coefficient by increasing the fluid velocity on the shell side.

[0026] The baffle is positioned within the shell-side space defined by the shell of the heat exchanger. Since each of the multiple straight tubes in a shell-and-tube heat exchanger typically has a circular cross-section, the baffle needs to include a circular opening (also referred to herein as a cylindrical through-hole) to accommodate these tubes. Since the tubes in a shell-and-tube heat exchanger typically have a cylindrical cross-section, it is preferable that the baffle includes a circular opening or an annular element.

[0027] These circular openings or annular elements are distributed in at least two rows in the area of ​​at least the baffle, wherein the two rows are staggered.

[0028] Within the shell-side space, it is crucial that the heating fluid (which can be a heating fluid containing or composed of steam) be distributed throughout the entire shell-side space to ensure uniform heating of all tubes along the entire length of the heat exchanger. To achieve this, various options are available in the design of the heat exchanger and its baffles: for example, the baffles can be segmented, meaning they do not cover the entire cross-section of the shell; alternatively, the baffles can include circular openings with a diameter larger than that of the heat exchanger tubes; or, in addition to cylindrical through-holes designed to receive the heat exchanger tubes, the baffles can include another series of orifices.

[0029] The inventors of this disclosure have recently discovered a problem. When the liquid solution to be heated contains a corrosive material (such as ammonium carbamate (which will be referred to as carbamate for the remainder of this document)), some tubes in the carbon dioxide stripper have been observed to exhibit abnormal levels of corrosion after only a few years of operation.

[0030] A stripper is a heat exchanger in which some of the components of a liquid injected into the heat exchanger are stripped during its residence in the heat exchanger. In urea production, strippers are used to concentrate and purify aqueous solutions containing urea, carbamates, ammonia, and water. The solution exiting a urea reactor always contains a mixture of urea, carbamates, ammonia, and water because the conversion of carbamates to urea is not quantitative, and excess ammonia is added to the reactor. The decomposition of carbamates from aqueous solution back to carbon dioxide and ammonia, and the removal of ammonia from the aqueous solution, can be accomplished simply by heating the solution in the heat exchanger, but this can be improved by adding stripping gas containing ammonia and / or carbon dioxide to the bottom of the heat exchanger or stripper.

[0031] Carbamates are known corrosive compounds (especially to steel), and while new materials have been developed to limit this degradation, there is a strong desire to limit corrosion through other means. Unbound by theory, it is thought that the accumulation of condensate between the tubes of a stripper on standard baffles can cause a reduction in heat transfer between the tubes and the steam present in the shell-side space due to condensate buildup. Condensate falls very slowly and accumulates easily on baffles. The accumulation of steam condensate on baffles can vary depending on several factors, such as the geometry of the baffles (e.g., disk and ring configuration) and the size of the stripper (larger strippers require larger baffles, and larger baffles may accumulate more condensate). If some regions face higher steam condensate accumulation than others, the heat transfer coefficient in the tubes in these regions will be lower. To compensate for the performance deficiencies of such tubes, other tubes less affected by steam condensate accumulation will have to exchange more heat, which can only be achieved by increasing the steam temperature. Therefore, the surface temperature on the process side of these tubes will be higher than that of other tubes, leading to a higher corrosion rate. For example, in large CO2 high-pressure strippers equipped with disc baffles and annular baffles, the corrosion rate in the outer tubes has been found to be higher than that in the inner tubes (due to the increased accumulation of steam condensate on the inner tubes compared to the outer tubes in the stripper). In the context of this disclosure, a stripper comprising more than 2000 tubes in its shell can be considered a large stripper. A stripper comprising fewer than 2000 tubes can be considered a small stripper.

[0032] This buildup can be prevented by using rod-shaped baffles instead of plate baffles. In fact, the rods allow steam condensate to flow freely to the bottom of the stripper and maintain the tube temperature profile within the desired range. However, the entire stripper geometry must be modified to accommodate the rod-shaped baffles, typically resulting in a larger casing and higher production costs.

[0033] A new type of baffle plate has been discovered in which the accumulation of steam condensate is minimized. As mentioned above, a baffle plate must include circular openings or orifices to receive the plurality of tubes of the heat exchanger. It has been found that a reduction in condensate accumulation is achieved by designing a baffle plate in which circular openings are obtained and defined by annular elements, wherein the outer diameter of the annular elements is less than 130% of the inner diameter of the annular elements, and wherein each annular element engages with all its adjacent annular elements in the plane of the baffle plate by a bridging structure oriented along a line connecting the centers of two adjacent annular elements, thereby defining a plurality of openings in the baffle plate, wherein the annular elements are positioned at the corners of the openings. The annular elements ensure that the baffle plate can hold the tubes in place during operation and limit the amount of condensate that can accumulate at each baffle plate before they fall to the next baffle plate. In particular, the inner diameter of the annular elements of the baffle plate is equal to or greater than the outer diameter of the straight tubes among the plurality of straight tubes of the heat exchanger. To provide structural strength to the baffle plate, adjacent annular elements are connected to each other by bridging structures. It was found that the bridging structure oriented along the center of the two adjacent annular elements is suitable for limiting condensate buildup and providing sufficient strength to the baffles so that they perform as they should during operation.

[0034] In addition to the circular opening defined by the annular elements, staggered rows of annular elements or hollow cylinders, along with bridging structures connecting adjacent annular elements in each row and adjacent staggered rows, define multiple openings in the baffle. The difference between the outer and inner diameters of the hollow cylinders directly affects the amount of water that can accumulate on the baffle. The larger the outer diameter compared to the inner diameter, the more water can be accumulated. However, if the difference is too small, the baffle may be too weak to hold the pipes in place during operation. The minimum difference between the outer and inner diameters is affected by various factors, such as the inner diameter of the annular elements and the material used for the baffle. It has been found that the outer diameter of the annular elements can be up to 130% of the inner diameter and still provide baffles with suitable performance.

[0035] In one embodiment, the outer diameter of the annular element is less than 125% of its inner diameter. In another embodiment, the outer diameter of the annular element is less than 120% of its inner diameter.

[0036] In one embodiment, the outer diameter of the annular element is less than 115% of its inner diameter. In some cases, it has been found that the outer diameter of the annular element can be at most equal to 115% of its inner diameter. This allows for less condensate buildup while still providing sufficient strength for the baffle.

[0037] In one embodiment, the outer diameter of the annular element is at least 101%, 102%, 103%, 104%, or 105% of its inner diameter. The minimum wall thickness of the annular element may depend on the actual value of its inner diameter. For example, if the inner diameter is 20 mm, the outer diameter may have to be at least 21.0 mm or 22.0 mm, i.e., 105% or 110% of the inner diameter. However, if the inner diameter is 50 mm, an outer diameter of at least 52 mm (i.e., 104% of the inner diameter) may be sufficient.

[0038] In one embodiment, the outer diameter of the annular element is 2.0 mm to 4.0 mm larger than the inner diameter of the annular element. For baffles comprising annular elements with an inner diameter of 20 mm to 40 mm, it has been found that the outer diameter can be 2.0 mm to 4.0 mm larger than the inner diameter.

[0039] In one embodiment, the width of the bridging structure between two adjacent annular elements is at least 2.0 mm, particularly between 2.5 mm and 6.0 mm, more particularly between 2.5 mm and 5.0 mm, and even more particularly between 2.5 mm and 4.0 mm. In another embodiment, the width of the bridging structure between two adjacent annular elements can be 3.0 mm. The bridging structure connects all the annular elements to each other and provides structural strength to the baffle, enabling it to withstand the vibration and temperature of the tube during operation. However, the wider the structure, the more water can accumulate on the baffle, so a trade-off must be made between these two aspects. In some instances, a bridging structure of at least 3.0 mm between two adjacent annular elements has been found to be satisfactory.

[0040] In one embodiment, the height or thickness of the baffle is between 4.0 mm and 10.0 mm, particularly between 4.5 mm and 7.0 mm, and particularly 5.0 mm. The strength of the baffle increases with its height. However, the production cost also increases with the height. In many fields, a baffle height between 4.0 mm and 10.0 mm, particularly 5.0 mm, is considered standard.

[0041] In one embodiment, the baffle includes between 1,000 and 8,000 circular holes. The number of holes in the baffle depends on several factors, such as the size of the heat exchanger, the flow rate and temperature of the fluid to be heated or cooled, and the expected production rate. Typically, for industrial urea plants, the baffle may include between 1,000 and 8,000 circular holes.

[0042] In one embodiment, the baffle includes an outer ring enclosing the annular element, and the outer ring has an outer diameter that is 1 mm to 10 mm smaller than the diameter of the heat exchanger housing. The outer ring can provide additional structural strength to the baffle and facilitate its installation in the heat exchanger.

[0043] In another aspect, this disclosure provides a shell-and-tube heat exchanger comprising: a top end fluidly connected to a bottom end via a plurality of straight tubes disposed within a shell, wherein each straight tube has the same outer diameter; the top end including an inlet for liquid and optionally an outlet for gas; the bottom end including an outlet for liquid and optionally an inlet for gas; a heating fluid inlet and a heating fluid outlet fluidly connected to a shell-side space, the plurality of straight tubes being disposed in the shell-side space; the shell-and-tube heat exchanger having a longitudinal direction and a transverse cross section, the longitudinal direction being parallel to the tubes and the transverse cross section being perpendicular to the longitudinal direction; a plurality of baffles according to this disclosure arranged in the shell-side space, wherein the baffles are substantially parallel to the transverse cross section; wherein the tubes are received within circular openings in the baffles; and wherein the diameter of the circular openings in the baffles (i.e., the inner diameter of the annular element) is equal to or greater than the outer diameter of the straight tubes.

[0044] The baffles are substantially parallel to the transverse cross-section, as this configuration simplifies their production and installation. The baffles can also be installed such that the angle between the baffle and the transverse cross-section is not equal to 0°, particularly where the angle is less than 20°. This configuration can offer other advantages over parallel baffles, such as improved flow of the heating fluid within the shell-side space. In this case, the baffles must be carefully designed so that the orifices correspond to the tubes of the heat exchanger.

[0045] A baffle in which the inner diameter of the annular element is equal to the outer diameter of the straight tubes in the plurality of tubes will provide excellent tube stability because these tubes will have no space for vibration. However, it may be advantageous to make the inner diameter of the annular element larger than the outer diameter of the straight tubes to facilitate the installation of the baffle. In one embodiment, the inner diameter of each annular element is 0.5 mm to 4.0 mm, particularly 0.5 mm to 3.0 mm, larger than the outer diameter of each of the plurality of straight tubes.

[0046] In one embodiment, the heat exchanger includes a multi-stage baffle. The heat exchanger may include baffles having only a fraction of the total number of orifices for the straight tubes included in the heat exchanger; specifically, each baffle may have a number of orifices equal to or less than 50% of the total number of straight tubes housed in the heat exchanger. In one embodiment, the baffle may have a number of orifices equal to or less than 50% of the total number of straight tubes housed in the heat exchanger. In one embodiment, the baffle may have a number of orifices equal to or less than 33% of the total number of straight tubes housed in the heat exchanger. In one embodiment, the baffle may have a number of orifices equal to or less than 25% of the total number of straight tubes housed in the heat exchanger. Such baffles have a smaller area than a single baffle and therefore allow less condensate to accumulate on their surface. All tubes in the plurality of tubes housed in the heat exchanger must be supported by at least one baffle. For example, if each baffle includes enough orifices to accommodate 25% of the tubes housed in the heat exchanger, at least four baffles will be required in the heat exchanger. Multi-segment baffles can have various designs: for example, baffles can be circular sectors in the cross-section of a heat exchanger, and in particular, they can be quarter circles, i.e., circular sectors with a 90° angle.

[0047] In one embodiment, the baffles are disc-ring baffles comprising disc-shaped baffles and annular baffles, each disc-shaped baffle being formed as a disc and each annular baffle being formed as a ring. The diameter of the disc-shaped baffles is equal to or greater than the inner diameter of the annular baffles by a margin of less than 10%. The disc-shaped and annular baffles are positioned continuously and alternately along the longitudinal direction of the shell-and-tube heat exchanger. The use of disc-shaped and annular baffles ensures that all tubes are supported by several baffles while ensuring that each baffle is not too large. Baffles with a diameter smaller than the diameter of the heat exchanger shell allow for better flow of steam and condensate within the shell. In one embodiment, the shell-and-tube heat exchanger comprises 6 to 12 baffles. For heat exchangers in industrial plants (e.g., carbon dioxide-driven urea strippers), 8 to 10 baffles are found to be satisfactory. The number of baffles can be determined by the intensity of tube vibration. The greater the tube vibration, the more baffles are required. The number of baffles can also be affected by the tube length, as longer tubes require more baffles. However, increasing the number of baffles also increases the pressure drop in the stripper. Depending on the plant's parameters, it may be recommended to limit the pressure drop below a certain value.

[0048] In one embodiment, the shell-and-tube heat exchanger includes a first baffle placed directly on top of a second baffle. To increase the strength of the baffle, its thickness can be increased. However, producing baffles with non-standard thicknesses can be too difficult or too costly. Instead of incurring their high procurement costs, a baffle can be placed directly on top of another baffle. The two stacked baffles will have properties similar to a baffle twice their thickness.

[0049] In one embodiment, the diameter of the circular opening or hole in the baffle (or in other words, the inner diameter of the annular element) is 1.0 mm to 4.0 mm larger than the outer diameter of the straight tube of the heat exchanger. It is preferable that the inner diameter of the annular element in the baffle is larger than the outer diameter of the straight tube, thereby facilitating installation. However, if the gap between the tube and the baffle is too large, the baffle will not sufficiently reduce vibration. A gap between 0.5 mm and 2.0 mm (i.e., a diameter difference of 1.0 mm to 4.0 mm) between the tube and the annular element has been found to be suitable. The gap also allows steam condensate to drain from the baffle.

[0050] In another aspect, this disclosure provides a method for heating a liquid composition, comprising the steps of: providing a shell-and-tube heat exchanger according to this disclosure, the shell-and-tube heat exchanger comprising: a top end connected to a bottom end via a plurality of straight tubes enclosed in a shell to define a shell-side space; an inlet for a liquid contained in the top end; an inlet for a heating fluid contained in the shell and connected to the shell-side space; and an outlet for a liquid contained in the bottom end and a heating fluid contained in the shell; providing a liquid composition to the inlet; providing a heating fluid (particularly steam) to the shell-side space through the heating fluid inlet; heating the plurality of straight tubes by the heating fluid; extracting the heated liquid composition at the outlet; and extracting the heating fluid at the heating fluid outlet.

[0051] The shell-and-tube heat exchanger described above can be used to heat any liquid composition. The liquid composition is guided to an inlet included at the top of the heat exchanger by means of a device such as pipes. As the liquid enters the top, it is distributed as evenly as possible into the plurality of straight tubes of the heat exchanger and forms a film on the inner wall of the tubes.

[0052] Devices or features can be added to the top of a heat exchanger to improve the distribution of the liquid composition within the tubes. Such devices or features are well known in the art.

[0053] A heating fluid (which may contain or consist of steam) is introduced into the shell-side space through a heating fluid inlet. The heating fluid fills the shell-side space and heats the multiple straight tubes. The tubes are made of a conductive material (e.g., a metal, particularly steel such as stainless steel). The material chosen for the tubes can depend on several factors of the heat exchanger's operation, such as the nature of the heating fluid, the temperature of the heating fluid, and the composition of the liquid to be heated.

[0054] The heating fluid contacts and heats the multiple straight tubes, and the tubes transfer heat to the liquid composition that forms a film on the inner wall of the tubes. The liquid composition falls to the bottom of the heat exchanger by gravity, where it can be extracted.

[0055] The heating fluid is extracted from the shell-side space via the heating fluid outlet.

[0056] The baffles described in this disclosure ensure that steam condensate accumulates as little as possible on the baffles, and that the temperature profile of the plurality of tubes is as constant as possible throughout the entire cross-section of the heat exchanger and across its longitudinal direction, and that the highest temperature reached inside the tubes is lower than the highest temperature reached in heat exchangers including baffles with other designs for the same heat exchanger performance.

[0057] In another aspect, this disclosure provides a method for stripping a liquid composition comprising urea, carbamate, ammonia, and water, comprising the steps of: providing a shell-and-tube heat exchanger according to this disclosure, the shell-and-tube heat exchanger comprising: a top end fluidly connected to a bottom end by a plurality of straight tubes enclosed in a shell to define a shell-side space; an inlet for a liquid contained in the top end; an outlet for a gas contained in the top end; an inlet for a heating fluid fluid contained in the shell and connected to the shell-side space; an inlet for stripping gas contained in the bottom end; an outlet for a liquid contained in the bottom end; and an outlet for a liquid contained in the shell. The process includes: a heated fluid outlet; supplying a liquid composition comprising urea, carbamate, ammonia, and water to the inlet; supplying steam, specifically saturated steam between 195°C and 225°C, to the shell-side space through the heated fluid inlet; supplying stripping gas containing carbon dioxide to the inlet; contacting the liquid composition with the stripping gas in the tube-side space provided within the plurality of straight tubes; heating the plurality of straight tubes with steam; extracting the liquid composition comprising urea and water, as well as reduced amounts of carbamate and ammonia, at the outlet; extracting a gaseous mixture comprising ammonia, carbon dioxide, and water at the outlet; and extracting steam and steam condensate at the heated fluid outlet.

[0058] The baffles described in this disclosure can also be used in urea strippers to decompose urethane from a liquid composition containing urea, urethane, ammonia, and water. Adding a stripping gas (such as carbon dioxide) to the urea stripper improves its efficiency by reducing the partial pressure of ammonia in the aqueous solution and accelerating its evaporation. The liquid composition from which the urethane is stripped typically also contains ammonia; since excess ammonia is used in the urea reactor, it is important to improve the removal of ammonia from the liquid composition.

[0059] Compared to heat exchangers that include baffles with other designs, using a urea stripper that includes baffles according to this disclosure can reduce the maximum temperature reached within the plurality of tubes. It is well known that the corrosion rate depends on the temperature of the urethane solution, therefore a lower temperature results in a lower corrosion rate.

[0060] Strippers can achieve a reduction of 80% to 99% in the amount of carbamates and / or ammonia present in a liquid composition, particularly 80%, 85%, 90%, or 95%. The reduction of carbamates and ammonia can depend on many factors specific to each operation.

[0061] The gas mixture containing ammonia, carbon dioxide, and water collected at the outlet can be reused during the production process to increase the conversion rate of the feedstock ammonia and carbon dioxide. In particular, the gas mixture can be partially condensed (e.g., in a high-pressure urethane condenser) to produce low-pressure steam (e.g., 2 to 8 bar) after the heat released from the urethane by the condensation of ammonia and carbon dioxide is partially condensed.

[0062] In another aspect, this disclosure provides a method for producing urea, comprising the steps of: a) reacting a mixture of ammonia and carbon dioxide in a urea reactor to produce an aqueous composition comprising urea, carbamate, and ammonia; b) directing the liquid composition comprising urea, carbamate, and ammonia obtained in step a) into a shell-and-tube heat exchanger according to this disclosure, the shell-and-tube heat exchanger including an inlet for heating a fluid, an inlet for a gas, and an outlet for a gas; c) directing a saturated vapor stream at a temperature between 195°C and 215°C into the inlet of the shell-and-tube heat exchanger; d) directing stripping gas comprising carbon dioxide into the inlet; e) extracting a liquid composition comprising urea and a reduced amount of ammonia and carbamate from the shell-and-tube heat exchanger; f) concentrating the liquid composition comprising urea and a reduced amount of ammonia and carbamate obtained in step e) to obtain a concentrated urea aqueous solution; g) converting the concentrated urea aqueous solution obtained in step f) into a solid particulate urea-based composition.

[0063] The industrial production of urea can be accomplished using various technologies and processes; however, they all begin with the mixing of carbon dioxide and ammonia in a reactor under high pressure and temperature. The resulting composition is an aqueous solution containing urea, carbamates, ammonia, and water. This aqueous solution needs to be purified and concentrated by removing ammonia and carbamates. This is typically done in a shell-and-tube heat exchanger known as a urea stripper. Producing urea using a process that includes a stripper containing baffles according to this disclosure ensures an extended service life for the stripper compared to a standard stripper due to the lower corrosion rate in its tubes. This reduces plant operating costs and lowers the final cost of urea produced by this process. The aqueous solution obtained from the stripper has a significantly lower carbamate and ammonia content compared to the solution obtained from the urea reactor. The urea aqueous solution is then concentrated to obtain a concentrated urea solution or melted via methods known in the art, and the concentrated urea solution can be converted into a solid particulate urea-based composition in a granulation unit. Several devices are known for producing solid particulate urea-based compositions from concentrated urea solutions: for example, granulation towers, granulators (such as fluidized bed granulators), spherodizers, and pasteurizers.

[0064] It is well known in the field of urea production that solid particulate urea-based compositions can contain additional additives or elements that have agronomical benefits. For example, the composition may contain one or more sources of one or more elements selected from the group consisting of: nitrogen, phosphorus, potassium, magnesium, calcium, sulfur, iron, boron, manganese, molybdenum, zinc, and copper.

[0065] Figure 1 This is a 2D view showing a cross-section of the baffle plate according to the present disclosure, viewed from above. The baffle plate 1 includes a plurality of hollow cylinders defining a plurality of circular openings or holes, referred to herein as annular elements or rings 2. The annular elements are distributed in a plurality of staggered rows. Each cylinder (or annular element / ring) is connected to all its adjacent cylinders by a bridging structure or segment 3. Each segment is oriented along a line connecting the centers of the two adjacent annular elements it is connected to. The annular elements 2 and segments 3 define a plurality of hexagonal openings, wherein each such opening consists of three straight walls defined by the segment 3 and three curved walls defined by a portion of the annular element 2. In other words, the annular elements 2 and segments 3 define a plurality of openings, wherein the segment 3 defines triangles, with the annular elements 2 positioned at or superimposed on their corners. The openings 4 can therefore be defined as triangular-like structures defined by the bridging structure 3, wherein the corners of the triangles are replaced by inwardly pointing curves defined by the annular elements 2. Figure 1 In this case, the outer diameter of the hollow cylinder is equal to 120% of the inner diameter.

[0066] Figure 2This refers to a shell-and-tube heat exchanger or stripper according to the present disclosure. Stripper 100 is a carbon dioxide stripper and includes a shell 160 and a plurality of tubes 150 disposed within the shell 160. Furthermore, stripper 100 includes a top end 110 and a bottom end 120. The plurality of tubes 150 are disposed between the top end 110 and the bottom end 120. A tube-side space 140 is disposed within each of the tubes 150. A shell-side space 130 is disposed between the tubes 150 and the shell 160. The shell-side space 130 is separate from the top end 110 and the bottom end 120. The top end 110 and the bottom end 120 are in fluid communication with the tube-side space 140. The bottom end 120 includes an outlet 122 for a liquid composition concentrated in urea and an inlet 121 for stripping gas (particularly carbon dioxide). The top end 110 includes an inlet 111 for a liquid composition comprising urea and carbamates and an outlet 112 for a gas mixture comprising stripping gas and one or more stripped compounds. The stripper 100 further includes a heating fluid inlet 131 and a heating fluid outlet 133. The heating fluid inlet 131 and the heating fluid outlet 133 are in fluid connection with the shell-side space 130. Furthermore, the heating fluid inlet 131 is adjacent to the top end 110 of the stripper 100. The heating fluid outlet 133 is adjacent to the bottom end 120 of the stripper 100. Each of the plurality of tubes 150 has a length of 6.0 m and an outer diameter of 31.0 mm. The stripper has a cylindrical shell with a diameter of 3.1 m. According to this disclosure, the stripper includes a plurality of baffles 170. Each baffle includes a circular opening or orifice for each tube 150 included in the stripper.

[0067] Alternatively, the stripper may include multiple disc-shaped baffles 171 and annular baffles 172, such as Figure 3 As shown in the diagram, the diameter of the disc-shaped baffle 171 can be approximately 75% of the diameter of the housing 160. The inner diameter of the annular baffle 172 can be approximately 70% of the diameter of the housing 160, while the outer diameter of the annular baffle 172 can be 95% to 99% of the diameter of the housing 160.

Claims

1. A baffle (1, 170) for a shell-and-tube heat exchanger (100), the baffle comprising a planar assembly of a plurality of annular structures (2) interconnected with a bridging structure (3), The annular structure (2) is designed to receive a plurality of straight tubes (150) of the shell-and-tube heat exchanger (100). The outer diameter of the ring structure (2) is less than 130% of the inner diameter of the ring structure (2). The ring structure (2) is positioned in at least two rows, wherein each row is staggered relative to its adjacent row. Each of the ring structures (2) is connected to all its adjacent ring structures (2) by a bridging structure (3) oriented along the line between the centers of two adjacent ring structures (2), thereby forming a plurality of openings (4), wherein the ring structures (2) are positioned at the corners of the openings (4).

2. The baffle according to claim 1, wherein the outer diameter of the annular element (2) is less than 115% of the inner diameter of the annular element (2).

3. The baffle according to claim 1, wherein the outer diameter of the annular element (2) is 2.0 mm to 4.0 mm larger than the inner diameter of the annular element (2).

4. The baffle according to any one of claims 1 to 3, wherein the width of the bridging structure (3) between two adjacent annular elements (2) is at least 2.0 mm.

5. The baffle plate according to any one of claims 1 to 3, wherein the height of the baffle plate (1, 170) is between 4.0 mm and 10.0 mm.

6. The baffle according to any one of claims 1 to 3, wherein the baffle (1, 170) comprises between 1,000 and 8,000 annular elements (2).

7. A shell-and-tube heat exchanger (100), comprising: The multiple straight tubes (150) having the same outer diameter are fluidly connected to the top end (120) via the top end (110), which are disposed within the housing (160); The top end (110) includes an inlet (111) for liquid. The bottom end (120) includes an outlet (122) for liquid. A heating fluid inlet (131) and a heating fluid outlet (133) are fluidly connected to the shell-side space (130), and the plurality of straight pipes (150) are disposed in the shell-side space; The shell-and-tube heat exchanger (100) has a longitudinal direction and a transverse cross section, the longitudinal direction being parallel to the plurality of straight tubes (150) and the transverse cross section being perpendicular to the longitudinal direction; According to any one of claims 1 to 6, the plurality of baffles (170) are arranged in the shell-side space (130), wherein the baffles (170) are substantially parallel to the transverse cross section; Each of the plurality of straight tubes (150) is received within the annular element (2) of the baffle (170); and The inner diameter of the annular element (2) in the baffle (170) is equal to or greater than the outer diameter of the plurality of straight pipes (150).

8. The shell-and-tube heat exchanger (100) according to claim 7, wherein the top end (110) further includes an outlet (112) for gas, and / or the bottom end (120) further includes an inlet (121) for gas.

9. The shell-and-tube heat exchanger (100) according to claim 7, wherein the baffle (170) is a multi-segment baffle.

10. The shell-and-tube heat exchanger (100) according to claim 9, wherein the baffle (170) is a disc-and-ring baffle (171, 172) comprising a disc baffle (171) and an annular baffle (172), each disc baffle (171) being formed as a disc and each annular baffle (172) being formed as an annular baffle, the diameter of the disc baffle (171) being equal to or greater than the inner diameter of the annular baffle (172) by a margin of less than 10%, the disc baffle (171) and the annular baffle (172) being positioned continuously and alternately along the longitudinal direction of the shell-and-tube heat exchanger (100).

11. The shell-and-tube heat exchanger (100) according to any one of claims 7 to 10, wherein the inner diameter of the annular element (2) in the baffle (170) is 0.5 mm to 4.0 mm larger than the outer diameter of the straight tube (150).

12. The shell-and-tube heat exchanger (100) according to any one of claims 7 to 10, wherein the shell-and-tube heat exchanger comprises 6 to 12 baffles (1, 170).

13. The shell-and-tube heat exchanger (100) according to any one of claims 7 to 10, wherein the shell-and-tube heat exchanger (100) includes a first baffle (1, 170) directly stacked on top of the second baffle (1, 170).

14. The shell-and-tube heat exchanger (100) according to any one of claims 7 to 10 is used for stripping liquid compositions comprising urea and carbamates.

15. A method for heating a liquid composition, comprising the steps of: - A shell-and-tube heat exchanger (100) according to any one of claims 7 to 13 is provided, the shell-and-tube heat exchanger comprising: a plurality of straight tubes (150) fluidly connected to a top end (110) of a bottom end (120) by means of a plurality of straight tubes (150) enclosed in a shell (160) thereby defining a shell-side space (130); an inlet (111) for a heating fluid fluid contained in the shell (160) and connected to the shell-side space; and an outlet (122) for a liquid contained in the bottom end (120) and a heating fluid contained in the shell (160). - Provide the liquid composition to the inlet (111); - A heating fluid containing steam is supplied to the shell-side space (130) through the heating fluid inlet (131); - The plurality of straight pipes (150) are heated by the heating fluid; - Extract the liquid composition at the outlet (122); - The heating fluid is extracted at the heating fluid outlet (133).

16. A method for stripping a liquid composition comprising urea, carbamate, ammonia, and water, comprising the following steps: - Provides a shell-and-tube heat exchanger (100) according to any one of claims 7 to 13, the shell-and-tube heat exchanger comprising: a plurality of straight tubes (150) fluidly connected to a top end (110) of a bottom end (120) by means of a plurality of straight tubes (150) enclosed in a shell (160) thereby defining a shell-side space (130); an inlet (111) for a liquid contained in the top end (110); an outlet (112) for a gas contained in the top end (110); an inlet (131) for a heating fluid contained in the shell (160) and connected to the shell-side space; an inlet (121) for stripping gas contained in the bottom end (120); an outlet (122) for a liquid contained in the bottom end (120); and a heating fluid outlet (133) contained in the shell (160). - A liquid composition comprising urea and carbamate is provided to the inlet (111); - Steam is supplied to the shell-side space (130) through the heating fluid inlet (131); -Provide stripping gas containing carbon dioxide to the inlet (121); -The liquid composition is brought into contact with the stripping gas in the tube space (140) provided within the plurality of straight tubes (150); -The plurality of straight pipes (150) are heated by the steam; - Extract a liquid composition containing urea and water, as well as reduced amounts of carbamate and ammonia, at the outlet (122); - Extract a gas mixture containing ammonia, carbon dioxide and water at the outlet (112); - Steam and steam condensate are extracted at the heating fluid outlet (133).

17. The method of claim 16, wherein the steam supplied to the shell-side space (130) through the heating fluid inlet (131) is saturated steam at a temperature between 195°C and 225°C.

18. A method for producing a solid particulate urea-based composition, comprising the following steps: a) Reacting a mixture of ammonia and carbon dioxide in a urea reactor to produce an aqueous composition containing urea, carbamate, and ammonia; b) The liquid composition containing urea, carbamate and ammonia obtained in step a) is directed to a shell-and-tube heat exchanger (100) according to any one of claims 7 to 13, the shell-and-tube heat exchanger comprising an inlet (131) for heating fluid, an inlet (121) for gas and an outlet (112) for gas. c) A saturated steam stream at a temperature between 195°C and 215°C is directed to the inlet (131) of the shell-and-tube heat exchanger (100). d) Guide stripping gas containing carbon dioxide to the inlet (121); e) Extract a liquid composition containing urea and reduced amounts of ammonia and carbamate from the shell-and-tube heat exchanger (100); f) Concentrate the liquid composition obtained in step e) containing urea and a reduced amount of ammonia and carbamate to obtain a concentrated aqueous urea solution; g) The concentrated urea aqueous solution obtained in step f) is converted into a solid particulate urea-based composition.

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