WO2025224022A1

WO2025224022A1 - Process for production of urea with a total recycle process

Info

Publication number: WO2025224022A1
Application number: PCT/EP2025/060779
Authority: WO
Inventors: Leonardo MARRONE; Alberto Benedetti
Original assignee: Casale SA
Current assignee: Casale SA
Priority date: 2024-04-22
Filing date: 2025-04-18
Publication date: 2025-10-30
Anticipated expiration: 2026-10-22

Abstract

A total recycle process for the production of urea wherein ammonia and carbon dioxide are reacted in a urea reactor at a high pressure and a urea-containing reactor effluent is withdrawn from said urea reactor to be processed for recovery of unconverted reagents at a medium pressure and at a low pressure; the high pressure at which the urea reactor is operated is not greater than 170 bar, the urea reactor is operated without addition of a passivating agent; a plant arranged to produce urea with total recycle technology, wherein the urea reactor is realized in a material suitable to prevent corrosion by ammonium carbamate.

Description

Process for production of urea with a total recycle process

DESCRIPTION

Field of application

The invention is in the field industrial production of urea with total recycle technology.

Prior art

Urea is produced industrially by reacting ammonia and carbon dioxide at high temperature and high pressure, well above 100 bar. The reaction involves basically the formation of ammonium carbamate and its dehydration to form urea.

Various processes for the synthesis of urea are described in the literature, for example in Meessen, “Urea”, in the Ullmann’s Encyclopaedia of industrial chemistry 2012. In all urea processes, the chemical equilibrium dictates a maximum conversion achievable in the urea reactor, usually about 60%, resulting in a reaction effluent which is a mixture of urea, carbamate, water, CO2 and NH3. Different technologies have been developed to recover the significant amount of reagents still contained in the reactor effluent.

The so-called stripping urea process includes a decomposition of carbamate contained in the reaction effluent performed substantially at reaction pressure, in a high-pressure stripper; the vapours removed in the stripper are sent to a high- pressure condenser; the urea solution effluent from the stripper is processed in one or more recovery sections at lower temperature and pressure where the urea is purified and the reactants are progressively recovered and recycled to the high- pressure synthesis section.

The stripping process is the dominant technology for modern urea plants, particularly for large plants. The so-called total-recycle process however is still of interest, particularly for urea plant of a small capacity. For example, most urea plants in the range 100 MTD to 400 MTD (metric tons of urea per day) are based on the total recycle technology. In a total-recycle process, the reactor effluent is not subject to high-pressure stripping and is directly sent to processing at a lower pressure. Typically, in a total-recycle process the reactor effluent is sent to a medium-pressure decomposer of a medium-pressure (MP) section, and a urea solution from the medium-pressure section is further treated at low pressure (LP). The MP section is operated at a pressure of around 18-25 bar; the LP section is operated around 2 to 5 bar. More details can be found in the Ullmann’s Encyclopaedia of industrial chemistry 2012, pages 664 and 665.

A passivating agent, typically air, is introduced into the urea reactor to protect from corrosion. Normally, said passivating agent is introduced with the CO2 feed. There is therefore continuous fluxing of inert into the urea reactor.

The decomposition of carbamate in the recovery sections requires a heat input which is provided by steam and represents a relevant part of the energy consumption of the process, together with the consumption of compressors and pumps, particularly CO2 compressor and NH3 pump to feed the reactor, and carbamate pumps to recycle carbamate solution from the recovery sections.

The energy consumption of a total recycle process is generally higher than the consumption of a stripping process of the same capacity, however the total recycle process is still of considerable interest for its simplicity and low investment cost. There is therefore an effort to further develop the total recycle process and to reduce its investment cost and energy consumption in terms of steam and electric power.

US 3,981 ,684 discloses an apparatus for controlling a urea system. US 3,531 ,521 discloses a method for synthesis of urea.

Summary of the invention

The invention addresses the problem of how to reduce investment cost and energy consumption of a total recycle urea process. The problem is solved with a total recycle urea process according to claim 1 .

Another aspect of the invention is a urea plant according to the claims.

In the process of the invention, the urea reactor is operated without feeding a passivating agent. The pressure in the reactor is not greater than 170 bar, preferably not greater than 165 bar and more preferably not greater than 160 bar; preferably the temperature at the outlet of the reactor is not greater than 195 °C and more preferably not greater than 192°C or not greater than 190 °C. Said temperature at the outlet of the reactor is preferably at least 180 °C or at least 185 °C, so preferred ranges include 180 °C to 195 °C, 185 °C to 195 °C or 185 to 190 °C. A particularly preferred range is 180 °C to 192 °C or 185 °C to 192 °C. The above conditions are noticeably different from the usual conditions in the urea reactors of total recycle plants. In the prior art of said urea plants, the pressure is 175 bar or more, and the temperature is around 200 °C or more. The pressure is given in bar gauge.

The applicant has observed that the energy efficiency of the total recycle urea process depends on the overall conversion of CO2 at the urea reactor, which is increased when the degree of vaporization in the reactor is decreased. Said degree of vaporization is defined as the ratio of vapour molar flow over the sum of vapour and liquid molar flows. The removal of the feeding of the passivating agent, typically air, results in a significant reduction of the degree of vaporization in the urea reactor. By the same token, removing the passivation agent allows the degree of vaporization and overall conversion to remain constant or substantially constant while reducing the operating pressure and temperature of the urea reactor.

The lower operating pressure of the reactor reduces the electrical consumption of the machines serving said reactor. The lower temperature at the outlet of the reactor may increase the subsequent heat input required by the process, with respect to a conventional total recycle urea process. However, the applicant has further observed that the above-mentioned reduction of the electrical consumption offsets the possible increase of heat input, leading to a positive balance.

Description of the invention

The term total recycle process identifies a urea process wherein the effluent of the urea reactor is expanded and sent to one or more further recovery sections at a medium pressure (MP) and/or low pressure (LP), without a high-pressure stripping step. In a common embodiment, the reactor effluent is expanded and sent to MP recovery section followed by a LP recovery section.

Due to absence of a high-pressure stripping step, the reactor effluent after expansion to medium pressure typically still contains the large majority of the unconverted ammonium carbamate at the reactor outlet. Generally, the reactor effluent expanded and sent to the MP recovery section comprises at least 90 %wt. of the ammonium carbamate contained in the same effluent at the outlet of the urea reactor. In preferred embodiments the effluent after expansion contains nearly 100% of the carbamate.

In the MP recovery section and LP recovery section, the reactants are progressively recovered and recycled to the urea reactor.

A key parameter of this process for urea production is the overall conversion of CO2 at the urea reactor, which is computed as the molar ratio of urea effluent of the reactor to carbon dioxide fed to the reactor, according to the following equation: wherein: L and V are respectively the molar flows of the liquid (L) and of the vapour (V) effluent of the reactor; Xi and yi are respectively the molar fractions of a species / in the liquid (Xi) and in the vapour (yi).

Defining the degree of vaporization a as the ratio between V and the sum of L and V, the overall conversion can be expressed as: (1 cz) ■ x_urea

XcO2 overall (2)

(1 - a) ■ (x_urea + x_C02) + a ■ y_C02

In a total recycle urea process, the overall conversion at the urea reactor has a significant impact on the energy efficiency of the entire process: the greater the overall conversion, the lower the energy demand of the process. The equation (2) shows that the overall conversion is greater for lower degree of vaporization a, thus high pressure in the reactor. On the other hand, the endothermic dehydration of ammonium carbamate in the liquid phase inside the reactor is favoured by high temperature. For the above reasons, the prior art has always followed the approach of running the urea reactor at high pressure and high temperature.

The present invention, in contrast, takes the approach of reducing the pressure and the outlet temperature of the reactor. The applicant has found that, unexpectedly, operating of the reactor without a passivating agent allows to reduce the pressure and the temperature of the reactor without a substantial loss of efficiency. Reducing the pressure of the reactor has the noticeable advantage of reducing the cost of the equipment and the energy consumption of pumps and compressors to feed the reactor. The energy saving compensates the increase of thermal energy need associated to a reduction of the temperature at the outlet of the urea reactor.

The absence of injection of a passivating agent reduces the degree of vaporization a in equation (2). Accordingly, the overall conversion and energetical efficiency of the process at a given pressure and temperature are increased, or the pressure and temperature can be reduced without affecting the overall conversion and the efficiency of the process.

The invention goes also against the technical prejudice that a passivating agent such as air is required in the urea reactor to prevent corrosion when the parts in contact with ammonium carbamate are made of stainless steel.

In a preferred embodiment of the invention, the urea reactor is operated at a pressure preferably between 145 and 155 bar, more preferably at 150 bar, and at a reactor outlet temperature preferably not greater than 192 °C, more preferably in the range 180 °C to 195 °C and even more preferably in the range 185 °C to 192 °C. A preferred embodiment has pressure in the range 145 bar to 155 bar and outlet temperature in the range 185 °C to 192 °C.

In the description and claims, the pressure is given in bar gauge.

Another aspect of the invention is a total recycle plant for the production of urea according to the claims. Said plant is configured to operate as a total-recycle process, including a urea reactor suitable to operate at a pressure lower than 160 bar and without an injection of a passivating agent. The plant includes a line arranged to expand a urea-containing reactor effluent withdrawn from the urea reactor and to send the so obtained expanded stream directly to a medium pressure recovery section without passage through a high-pressure stripper.

To minimise corrosion by carbamate, the parts of the urea reactor which are in contact with ammonium carbamate are realized in a material suitable for operation in presence of carbamate and in absence of a passivation agent injection. A material suitable for operation in the presence of carbamate is understood as showing a corrosion rate < 0.15 mm/y, preferably < 0.12 mm/y. Said parts of the urea reactor may include an internal protective liner and/or perforated trays arranged in the reactor. Examples of suitable materials are listed below.

The medium pressure recovery section includes an apparatus, named MP decomposer, which includes a bundle of tubes wherein ammonium carbamate is decomposed to ammonia and carbon dioxide through a heat exchange across said tubes. Preferably, at least the parts of the tubes exposed to ammonium carbamate are also realized in a material suitable for operation in presence of ammonium carbamate and in absence of a passivating agent.

The above-mentioned materials suitable for operation in presence of carbamate and without a passivating agent include titanium, zirconium, super duplex materials, super ferritic steels. A superduplex material and/or a super ferritic steel are the preferred materials for achieving the above-mentioned goal of reducing the investment cost.

A super duplex material is an austenitic-ferritic iron and chromium-nickel alloy with molybdenum addition, characterized by an enhanced pitting and crevice corrosion resistance with respect to a more conventional duplex steel. Preferably the superduplex material is a stainless steel according to the designation UNS32906 (ASTM).

A super ferritic stainless steel (SFSS) is a steel characterized by structure and properties similar to those of more common ferritic alloys, with the feature of low carbon/nitrogen content and higher chromium and molybdenum levels, aimed to increase high temperature resistance and corrosion behaviour in aggressive environments, such as carbamate solutions. Preferably the super ferritic material is a stainless steel according to EN 1 .4613 (European designation) also identified as 470LI (AST designation).

Preferably, the urea reactor comprises a carbon steel body internally coated with said protective liner. Said protective liner has a thickness preferably in the range 4 mm to 10 mm, more preferably 5 mm to 6 mm.

Another and preferred aspect of the invention is: a total recycle process for the production of urea including: ammonia and carbon dioxide are reacted in a urea reactor at a high pressure, to form urea; a urea-containing reactor effluent is withdrawn from said urea reactor at a reactor outlet temperature; said reactor effluent is expanded to a medium pressure and processed for recovery of unconverted reagents in a medium-pressure recovery section; and a urea solution obtained in said medium-pressure recovery section is expanded to a low pressure and further processed in a low-pressure recovery section, obtaining one or more recycle streams which are recycled to said urea reactor; wherein said high pressure is not greater than 170 bar and the urea reactor is operated without addition of a passivating agent; wherein said process is performed in a plant comprising a high-pressure urea reactor, a medium pressure recovery section and a low-pressure recovery section, wherein: the plant does not include means for injection of a passivating agent in the urea reactor; said urea reactor is arranged to operate at a pressure lower than 170 bar and without an injection of a passivating agent, wherein at least the parts of the reactor in contact with ammonium carbamate are made of a material suitable for operation in presence of ammonium carbamate and in absence of a passivating agent, said material being preferably a super duplex steel or a super ferritic steel, more preferably in accordance with ASTM UNS32906 as superduplex steel, or in accordance with EN 1 .4613 as ferritic steel; the plant includes a line arranged to expand a urea-containing reactor effluent withdrawn from the urea reactor and to send the so obtained expanded stream directly to said medium pressure recovery section without passage through a high-pressure stripper.

Parts made of the above-mentioned material (e.g. the above-mentioned superduplex or super-ferritic steel) may include: a bundle of tubes of a decomposer, or at least the portions of said tubes exposed to ammonium carbamate; a liner of the urea reactor, internals of the urea reactor such as perforated trays.

Preferably the above high pressure is not greater than 165 bar and more preferably not greater than 160 bar. A preferred range is 100 to 170 bar, preferably 145 to 155 bar, more preferably 150 bar or around 150 bar (pressure in bar gauge).

Example

In this example, the operation of a total-recycle urea plant with a capacity of 200 MTD (metric tons per day) of urea solution was simulated. The example compares a process of the prior art wherein the urea reactor operates with injection of a passivating agent at temperature and pressure typical of the prior art, and a process according to an embodiment of the present invention, wherein the urea reactor operates without said passivating agent and at reduced temperature and pressure. The urea reactor is assumed to be internally coated with a protective liner of a super duplex steel.

Results are expressed in the following Table 1 :

In the table, the reactor temperature is the temperature at the outlet of the urea reactor, and the pressure is the operating pressure of the urea reactor. The degree of vaporization is the ratio a between vapour flow over the sum of vapor and liquid flows effluent of the urea reactor.

MP decomposer consumption is the consumption [Gcal/h] of the decomposer included in the medium-pressure section. LP decomposer consumption is the consumption [Gcal/h] of the decomposer included in the low-pressure section.

Total spec. cons. (HPS) is the consumption of the LP and MP decomposers expressed as kg of high-pressure saturated steam per metric ton of urea solution produced [kg/MT],

The symbol “Delta” (A) denotes a difference. A Spec. Electric Cons, is the difference in term of electric consumption between the plant of the inventive embodiment and a comparative plant of the prior art having the same capacity, expressed as the ratio between kWh of electrical energy consumed per metric ton of urea solution produced [kWh/MT],

A [Thermal + Electric Consumption] is the difference in term of sum of thermal and electric consumption (Gcal/MT) between the plant of the inventive embodiment and the similar plant of the prior art. This parameter includes both electric consumption and steam consumption and is expressed as Gcal of energy, both steam and electrical energy, required to produce 1 metric ton of urea solution. The thermal energy and electric energy are computed together assuming 1 kW electric equal to 2150 kcal of thermal energy, i.e. assuming 40% efficiency of thermal to electric conversion.

ACAPEX is the difference in term of capital expenditures between the plant according to the inventive embodiment and the similar plant of the prior art, expressed as millions of Euro [M€].

The table illustrates that the plant according to the invention allows a significant reduction of capital expenditures (ACAPEX) with respect to the comparative plant, while keeping substantially constant the energy consumption (A [Thermal + Electric Consumption]), thanks to a reduction of the operating pressure of the urea reactor from 216 bar to 150 bar and the absence of an injection of a passivating agent.

Thus, the plant according to the invention represents an important improvement of the prior art under an economic point of view.

Detailed description of the figures

Figure 1 represents an embodiment of a total recycle process for the production of urea according to the invention.

A fresh ammonia stream 1 is fed into an ammonia receiver tank 2. An ammonia stream 3 from said tank 2 is pumped with a NH3 pump 10, resulting in a pumped ammonia feed 11 which is heated by a heat exchanger 12 with heat removed from a LP stream 16 effluent of a LP decomposer 42, resulting in a heated ammonia feed 13.

Said ammonia feed 13 is fed into the urea reactor 14, together with a carbamate recycle stream 25 delivered by a pump 27 and a fresh CO2 stream 23 fed by a CO2 compressor 17, without injection of any passivating agent. In the urea reactor 14, ammonia and carbon dioxide react at a pressure lower than 160 bar, obtaining a gaseous effluent 15 and a liquid effluent 24 containing mainly urea, water and carbamate.

The liquid effluent 24 is expanded to a medium pressure (18-25 bar) in a valve 26 and is fed to a MP decomposer 31. Said decomposer 31 receives also the gaseous effluent 15 taken from top of the reactor 14.

Inside the MP decomposer 31 , thermal decomposition of the carbamate contained in the liquid effluent 24 occurs, producing a urea solution 30 and a gaseous stream 28 containing NH3 and CO2.

Said gaseous stream 28 is mixed with a LP recycle stream 33 and the resulting stream 18 is cooled in a two-stage heat exchange 34 and 35, to condensate most of the carbamate contained therein. The cooled stream 9, comprising ammonia and carbamate, is treated in a MP absorber 8 to separate a carbamate liquid stream 29 from an ammonia rich vapour 7. The carbamate liquid stream 29 feeds the carbamate pump 27 to obtain the recycle stream 25, whereas the ammonia rich vapour 7 is condensed in a condenser 6 and sent to the NH3 receiver tank 2.

The urea solution 30 effluent from the MP decomposer 31 is expanded in a valve 32 from the medium pressure (18-25 bar) to a low pressure (2-5 bar), resulting in a LP urea solution 40 that is sent to the LP decomposer 42, wherein carbamate contained in the urea solution 40 is further decomposed and a purified urea solution 49 is formed.

A LP gaseous stream 21 containing CO2 and NH3 is withdrawn from the LP decomposer 42. Said LP gaseous stream 21 is joined with a recycle stream 41 , coming from a WWT section 58 (via line 47 and pump 43), to obtain the LP stream 16.

As mentioned above, said LP stream 16 is cooled in the heat exchanger 12 by heating the pumped ammonia feed 11 .

The resulting LP cooled stream 44 is condensed in an evaporator 38, wherein the heat removed from the stream 44 is used to produce low pressure steam, resulting in a condensate effluent 45. The condensate effluent 45 is partly pumped by a pump 36 to obtain the LP recycle stream 33, and partly joined with an ammonia-rich stream 4 withdrawn from the NH3 receiver tank 2, to obtain a LP liquid stream 46. Said LP liquid stream 46 is fed into a LP absorber 59 wherein the stream 46 is washed with water to remove mainly ammonia and carbamate obtaining an offgas vented in atmosphere and a wastewater stream 60.

The purified urea solution 49 is further expanded to a pressure lower than the atmospheric pressure in a valve 48, resulting in an expanded solution 52 which is flashed in a flash vessel 51 to separate a water-rich vapour 50 and a concentrated urea solution 22. The concentrated urea solution 22 is the main product of the process.

Said urea solution 22 is stored in a buffer tank 53 and can be sent, if necessary, to evaporation steps to further remove water from urea. The water rich vapour 50 is condensed in a vacuum condensation unit 57, resulting in a wastewater condensate 56 which is treated in the wastewater treatment section 58, together with the wastewater stream 60 effluent of the LP absorber 59.

Claims

1 ) A total recycle process for the production of urea including: ammonia and carbon dioxide are reacted in a urea reactor (14) at a high pressure, to form urea; a urea-containing reactor effluent (24) is withdrawn from said urea reactor at a reactor outlet temperature; said reactor effluent is expanded (26) to a medium pressure and processed for recovery of unconverted reagents in a medium-pressure recovery section; and a urea solution (30) obtained in said mediumpressure recovery section is expanded (32) to a low pressure and further processed in a low-pressure recovery section, obtaining one or more recycle streams (7, 25) which are recycled to said urea reactor; wherein said high pressure is not greater than 170 bar, preferably not greater than 160 bar, and the urea reactor (14) is operated without addition of a passivating agent.

2) A process according to claim 1 wherein said high pressure is in the range 100 to 170 bar, preferably 145 to 155 bar, more preferably 150 bar.

3) A process according to claim 1 or 2, wherein said reactor outlet temperature is not greater than 195 °C.

4) A process according to claim 3 wherein said reactor outlet temperature is not greater than 192 °C.

5) A process according to claim 3 wherein said reactor outlet temperature is in the range 180 °C to 195 °C, preferably in the range 185 °C to 192 °C.

6) A process according to any of the previous claims, wherein the reactor effluent, after said expansion to medium pressure, contains at least 90% by weight of the ammonium carbamate contained in the high-pressure effluent at the outlet of the urea reactor.

7) A plant for the production of urea from ammonia and carbon dioxide according to a total-recycle process, the plant comprising a high-pressure urea reactor, a medium pressure recovery section and a low-pressure recovery section, wherein: the plant does not include means for injection of a passivating agent in the urea reactor; said urea reactor is arranged to operate at a pressure lower than 160 bar and without an injection of a passivating agent, wherein at least the parts of the reactor in contact with ammonium carbamate are made of a material suitable for operation in presence of ammonium carbamate and in absence of a passivating agent; the plant includes a line arranged to expand a urea-containing reactor effluent withdrawn from the urea reactor and to send the so obtained expanded stream directly to said medium pressure recovery section without passage through a high-pressure stripper.

8) A plant according to claim 7 wherein said material suitable for operation in presence of ammonium carbamate and in absence of a passivating agent includes at least one of: titanium, zirconium, a super duplex material, a super ferritic steel.

9) A plant according to claim 7 or 8, wherein at least the following items are made of a material suitable for operation in presence of ammonium carbamate and in absence of a passivating agent: a) an internal protective liner of the urea reactor; b) reactor effluent line c) perforated trays mounted internally in the urea reactor; d) a bundle of tubes of a decomposer of said medium-pressure recovery section, wherein at least the portions of said tubes exposed to ammonium carbamate are made of said material.

10) A plant according to claim 9 wherein the material of at least one of items a), b) and c) is a super duplex steel.

11 ) A plant according to claim 9 or 10 wherein the material of at least one of items a), b) and c) is a super-ferritic steel.

12) A plant according to claim 10, wherein the super duplex steel is in accordance with the designation ASTM UNS32906. 13) A plant according to claim 11 , wherein the super ferritic steel is in accordance with the designation EN 1.4613.

14) A plant according to any of claims 9 to 13, wherein the protective liner of point a) has a thickness of 4 mm to 10 mm, preferably 5 mm to 6 mm.

15) A process according to any of claims 1 to 6, wherein the process is performed in a plant according to any of claims 7 to 14.