Urea plant revamping method

Abstract

Disclosed is a method of increasing the capacity of an existing urea plant. With reference to the regular components of a urea plant, including a synthesis section comprising a high pressure carbamate condenser and a reactor, and a recovery section, the method comprises installing an additional reactor between the recovery section and the high pressure carbamate condenser. The additional reactor is preferably installed in connection with an ejector, so as to allow ground placement of the additional reactor.

Claims

1. A method of increasing the production capacity of an existing urea plant, the existing plant comprising a high pressure carbamate condenser comprising a condensation chamber, a first reactor, a stripper, and a recovery section, wherein: the condenser comprises a liquid outlet and a flow line to transfer liquid to a liquid inlet of the first reactor; the first reactor comprises a liquid outlet and a flow line to transfer liquid to a liquid inlet of the stripper; the stripper comprises a gas outlet and a flow line to transfer gas to a gas inlet of the condensation chamber of the condenser and a liquid outlet and a flow line to transfer liquid to the recovery section; the recovery section comprises a liquid outlet and a flow line to transfer liquid to a liquid inlet of the condenser; the method comprising installing a second reactor and flow lines, such that the second reactor has a liquid inlet for intake of liquid from a flow line from the liquid outlet of the recovery section, a liquid outlet and a liquid flow line to transfer urea-containing solution from said liquid outlet of the second reactor to a liquid inlet of said condensation chamber of the condenser.

2. A method according to claim 1, wherein a high pressure scrubber is positioned between the urea recovery section and the additional reactor, wherein said high pressure scrubber: comprises an outlet and flow line to transfer carbamate solution to an inlet of the second reactor; and has an inlet for recovering gas from a gas outlet of the first reactor.

3. A method according to claim 1, wherein the additional reactor comprises a vertical reactor.

4. A method according to claim 1, wherein an ejector is present between the additional reactor and the high pressure carbamate condenser.

5. A method according to claim 1, wherein the additional reactor is placed on ground level.

6. A method according to claim 1, wherein the additional reactor is designed for counter-current operation.

7. A method according to claim 1, wherein the second reactor comprises a gas inlet for intake of gas from the gas outlet of the stripper.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 and FIG. 2 are block diagrams representing in FIG. 1 a conventional plant and in FIG. 2 a corresponding plant resulting from the revamping method of the invention;

(2) FIG. 3 and FIG. 4 are schematic drawings representing in FIG. 3 a conventional plant and in FIG. 4 a corresponding plant resulting from the revamping method of the invention;

(3) FIG. 5 and FIG. 6 are schematic drawings representing in FIG. 5 a conventional plant and in FIG. 6 a corresponding plant resulting from the revamping method of the invention;

(4) FIG. 7 and FIG. 8 are schematic drawings representing in FIG. 7 a conventional plant and in FIG. 8 a corresponding plant resulting from the revamping method of the invention;

DETAILED DESCRIPTION OF THE INVENTION

(5) In a broad sense, the invention is based on the judicious insight that the production capacity of an existing urea plant can be increased in an entirely different way than by enlarging the synthesis section. The latter refers to the classical ways of dealing with an increase in production capacity. One is installing an additional reactor downstream of the already present reactor in a urea plant. This amounts to enlarging the production capacity of the urea synthesis section. Such an additional reactor forms the revamp alternative to just building a new plant having a larger reactor. Another such revamp alternative to just building a new reactor, is proving an extension to the existing reactor (thus, in fact, enlarging it). The present invention adds a reactor in a totally different section of the urea plant, viz. directly after the recovery section. Moreover, the present invention does not merely add a urea reactor, which operates on the basis of the classical feed streams in a urea plant, but a reactor that is specifically to be fed with carbamate solution (i.e. operating so as to conduct only the second stage of urea synthesis, as outlined above).

(6) The invention not only provides benefits in the sense of increasing the urea production capacity. It has surprisingly been found that the installation of an additional urea reactor in accordance with the invention increases the steam pressure in the high pressure carbamate condenser (HPCC). Without wishing to be bound by theory, the inventors believe that this is caused by the presence of significant amounts of urea in the HPCC, thereby increasing the operating temperature of the HPCC. As a result the pressure in the HPCC will be higher. The higher pressure steam may be exchanged for increasing capacity or used to save energy. Also a higher conversion into urea is seen.

(7) The foregoing is reflected in the requirement that the additional reactor comprises a liquid outlet in fluid communication with an inlet to a condensation chamber of the condenser.

(8) It will be understood that a condensation chamber is the space of a condenser wherein normally gas enters, and is then condensed to liquid, in said condensation chamber. The liquid outlet of the additional reactor is thus in fluid communication with an inlet to the condenser that, normally, would be a gas inlet. E.g., in the event of a shell and tube condenser, the condensation chamber typically is the shell-side thereof. One would normally recognize that liquid (viz. cooling liquid) is sent to the tube-side of the condenser, and gas (to be condensed) is sent to the shell-side. In the present invention, the specific liquid identified, viz. the urea solution produced in the additional reactor, is sent to the condensation chamber, such as the shell-side of a shell-and-tube condenser.

(9) Further, in the process of the invention preferably only a part of the stripper gas is sent to the condenser, the remainder is sent to the additional reactor. This serves to provide heat to the additional reactor and increase the urea conversion.

(10) The specific steam consumption of the urea process of the invention will be lower. A particular advantage of the invention is that the additional reactor may be positioned on the ground floor. This reduces the need for significant structural support. A further advantage is that the additional reactor may be of a simple design, for example a vertical tower with trays.

(11) The additional reactor is sometimes also described as a pre-reactor or a Urea Pre-Reactor (UPR).

(12) In a preferred embodiment of the invention, the additional reactor is operated in countercurrent mode; that is, the liquid is entered at the top and flows down, whereas the gas-feed is entered into the bottom and rises up in countercurrent to the descending liquid. It has surprisingly been found that this mode of operation allows for a higher degree of conversion in the additional reactor as compared to the more usual co-current mode of operation for vertical urea reactors. It is believed that the main reason for this higher conversion is given in the fact that in counter-current mode of operation the liquid is withdrawn from the bottom of the reactor, whereas in the bottom the gas phase contains the lowest amount of non-condensables and the lowest content of light components in the gas-phase is observed.

(13) In the revamped plant of the invention, the recovered carbamate solution is subjected to urea forming conditions. These conditions generally entail an operating pressure between 12 and 18 MPa, and a temperature of from 160 C. to 210 C., preferably of from 175 C. to 190 C. The pressure preferably is between 13 and 16 MPa.

(14) It is conceivable to build a new plant (sometimes referred to as grassroots plant) based on the design of the present invention. This would provide the aforementioned advantages in terms of steam efficiency and urea conversion. However, it will be understood that the advantage of increasing of the urea production capacity, does not play a role in the case of grassroots plants. This typically plays a role in revamping existing plants.

(15) The revamp method of the invention particularly serves to increase the production capacity of the urea plant, after the revamp, as compared to the production capacity of the same plant, before the revamp. The key to this, according to the invention, is installing the additional reactor as outlined above. I.e., an additional reactor positioned between the recovery section and the high pressure carbamate condenser present in the existing plant.

(16) In this description reference is made to the production capacity of a urea plant. This refers to the volume of urea that can be produced in a given time period. More specifically, the invention pertains to increasing the production capacity of a urea plant by increasing the available reactor volume for urea synthesis.

(17) In the definition of a plant, reference is sometimes made to the terms liquid outlet, gas outlet and the corresponding inlets. It will be understood that a liquid outlet is an outlet through which liquid can flow, and a gas outlet is an outlet through which gas can flow. The same holds for the respective inlets. The types of outlets and inlets available for either or both of these purposes are fully familiar to the skilled person.

(18) Flow lines, for gas and/or liquid, are generally provided in the form of suitable piping.

(19) Fluid communication refers to an arrangement connecting two parts of a plant via a gas or liquid flow line, in a such a way that a fluid (being a gas, a liquid, or a supercritical liquid) can be transported from one to the other.

(20) A particular advantage of the invention is that the additional reactor may be positioned on the ground floor. This reduces the need for significant structural support. To this end, an ejector is placed downstream of the additional reactor. The ejector (which, as known to the skilled person, serves to provide a pumping function without mechanical means) is placed downstream of the additional reactor. I.e. particularly between the additional reactor and the high pressure carbamate condenser.

(21) In another preferred embodiment of the plant resulting from the invention, a high pressure scrubber is positioned between the urea recovery section and the additional reactor. In this embodiment, the invention advantageously combines the use of the carbamate recycle stream as a scrubbing liquid, with the production of urea in the additional reactor. The carbamate recycle stream, upon acting as a scrubbing liquid will become enriched in carbon dioxide and ammonia by absorbing this from the reactor overhead gas stream. The carbamate recycle stream thereby effectively becomes more concentrated. As a result, said stream will in turn produce more urea in the additional reactor.

(22) The additional reactorsimilarly to the conventional reactorpreferably is a vertical reactor. This provides a space advantage, as such a reactor provides the smallest thinkable footprint. The additional urea reactor may be of a simple design, for example a vertical tower with trays.

(23) In a preferred embodiment of the invention, the additional reactor is operated in countercurrent mode; that is, the liquid is entered at the top and flows down, whereas the gas-feed is entered into the bottom and rises up countercurrent to the descending liquid. It has surprisingly been found that this mode of operation allows for a higher degree of conversion in the additional reactor as compared to the more usual co-current mode of operation for vertical urea reactors. It is believed that the main reason for this higher conversion is given in the fact that in counter-current mode of operation the liquid is withdrawn from the bottom of the reactor, whereas in the bottom the gas phase contains the lowest amount of non-condensables and the lowest content of light components in the gas-phase is observed.

(24) The urea plant of the invention, apart from the additional reactor, can be just any urea plant based on stripping with either ammonia or carbon dioxide. Also a thermal stripping plant may be used as a starting plant. An overview of commercial processes for producing urea is given, e.g., in Ullmann Encyclopedia, 2005 Wiley-VCH Verlag, Weinheim, Germany, chapter Urea.

(25) A frequently used process for the preparation of urea according to a stripping process is the carbon dioxide stripping process as for example described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In this process, the synthesis section is followed by one or more recovery sections. The synthesis section comprises a reactor, a stripper, a condenser and a scrubber in which the operating pressure is in between 12 and 18 MPa and preferably in between 13 and 16 MPa. In the synthesis section the urea solution leaving the urea reactor is fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution. Such a stripper can be a shell and tube heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed to the synthesis is added to the bottom part of the stripper. At the shell side, steam is added to heat the solution. The urea solution leaves the heat exchanger at the bottom part, while the vapor phase leaves the stripper at the top part. The vapor leaving said stripper contains ammonia, carbon dioxide and a small amount of water. Said vapor is condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The heat released by the exothermic carbamate condensation reaction in said condenser is usually used to produce steam that is used in a downstream urea processing section for heating and concentrating the urea solution.

(26) Since a certain liquid residence time is created in a submerged type condenser, a part of the urea reaction takes already place in said condenser. The formed solution, containing condensed ammonia, carbon dioxide, water and urea together with the non-condensed ammonia, carbon dioxide and inert vapor is sent to the reactor. In the reactor the above mentioned reaction from carbamate to urea approaches the equilibrium. The ammonia to carbon dioxide molar ratio in the urea solution leaving the reactor is generally in between 2.5 and 4 mol/mol. It is also possible that the condenser and the reactor are combined in one piece of equipment. An example of this piece of equipment as described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The formed urea solution leaving the urea reactor is supplied to the stripper and the inert vapor containing non-condensed ammonia and carbon dioxide is sent to a scrubbing section operating at a similar pressure as the reactor. In that scrubbing section the ammonia and carbon dioxide is scrubbed from the inert vapor. The formed carbamate solution from the downstream recovery system is used as absorbent in that scrubbing section. The urea solution leaving the stripper in this synthesis section requires a urea concentration of at least 45% by weight and preferably at least 50% by weight to be treated in one single recovery system downstream the stripper. The recovery section comprises a heater, a liquid/gas separator and a condenser. The pressure in this recovery section is between 200 to 600 kPa. In the heater of the recovery section the bulk of ammonia and carbon dioxide is separated from the urea and water phase by heating the urea solution. Usually steam is used as heating agent. The urea and water phase, contains a small amount of dissolved ammonia and carbon dioxide that leaves the recovery section and is sent to a downstream urea processing section where the urea solution is concentrated by evaporating the water from said solution.

(27) The invention is not limited to any particular urea production process. Other processes and plants include those that are based on technology such as the HEC process developed by Urea Casale, the ACES process developed by Toyo Engineering Corporation and the process developed by Saipem (formerly Snamprogetti).

(28) The additional reactor placed in accordance with the invention can be of a standard type. A reactor generally is a vessel provided with the appropriate inlets and outlets, and provisions for controlling temperature an pressure. Particularly, the additional reactor of the present invention is made so as to be operated under the aforementioned high pressure urea synthesis conditions. In view of the generally corrosive environment, the additional reactor is preferably made of a highly corrosion-resistant type of steel. The latter particularly refers to duplex steels, and more particularly to duplex ferritic-austenitic stainless steel having a high content of Cr and N, and a low content of Ni. A reference in this respect is WO 95/00674. In another preferred embodiment, the additional reactor (and particularly the inner parts thereof) is made of a duplex stainless steel consisting of, in percent by weight, C: 0.03% or less, Si: 0.5% or less, Mn: 2% or less, P: 0.04% or less, S: 0.003% or less, Cr: 26% or more, but less than 28%, Ni: 7.3-10%, Mo: 0.2-1.7%, W: more than 2%, but no more than 3%, N: more than 0.3%, but no more than 0.4%, with the balance being Fe and impurities, in which the content of Cu as an impurity is not more than 0.1%. This steel is described in U.S. Pat. No. 7,347,903.

(29) The preferred additional reactor is made from a duplex, stainless steel alloy, containing, in percent by weight: C: maximally 0.05%, preferably maximally 0.03%; Si maximally 0.8%, preferably maximally 0.5; Mn 0.3-4%, preferably 0.3-1%; Cr 28-35%, preferably 29-33%; Ni 3-10%; Mo 1.0-4.0%, preferably 1.0-1.3%; N 0.2-0.6%, preferably 0.36-0.55%; Cu maximally 1.0%; W maximally 2.0%; S maximally 0.01%; Ce 0-0.2%;
the remainder being Fe and normally occurring impurities and additives, the ferrite content being 30-70% by volume, preferably 33-35% by volume.

(30) The invention is hereinafter illustrated with reference to the drawings. The drawings are for illustration purposes, and are not intended to be limiting to the invention.

(31) Legend

(32) In the drawings, the capital letters (A-E) indicate components of a urea plant. The small letters (a-f) indicate streams. The legend is as follows:

(33) A=High pressure carbamate condenser

(34) B=High pressure reactor

(35) C=High pressure stripper

(36) D=Urea recovery section

(37) E=Urea pre-reactor (UPR)

(38) F=High pressure Scrubber

(39) a=Ammonia

(40) b=Carbon Dioxide

(41) c=Carbamate recycle

(42) d=Strip gas

(43) e=Urea

(44) f=Stripgas to UPR

(45) g=Concentrated carbamate recycle

(46) h=Urea stream from UPR

(47) FIG. 1 and FIG. 2 are block diagrams representing in FIG. 1 a conventional plant and in FIG. 2 a corresponding plant resulting from the revamping method of the invention. In FIG. 1 a conventional operation is shown, wherein a carbamate stream (c) from the recovery section (D) is recycled back to the synthesis section, viz. to the high pressure carbamate condenser (A). In comparison therewith, FIG. 2 shows the addition of a reactor (E) to which the carbamate recycle stream (c) is led, whereby the formed urea stream (h) is led to the high pressure carbamate condenser (A).

(48) FIG. 3 is a schematic drawing of a conventional urea plant (similar to the configuration depicted in the block diagram of FIG. 1). Equally, FIG. 4 is a schematic drawing of a urea plant modified according to the invention (similar to the configuration depicted in the block diagram of FIG. 2).

(49) FIG. 5 is a schematic drawing of a conventional urea plant of the type having two high pressure carbamate condensers (A) in parallel. FIG. 6 is a schematic drawing showing the urea plant of FIG. 5 modified in accordance with the invention, viz. having an additional reactor (E) from which a stream of formed urea (h) is led to the condensers (A).

(50) FIG. 7 is a schematic drawing of a conventional urea plant of the type having a high pressure scrubber (F) positioned downstream of the recovery section (D). FIG. 8 is a schematic drawing showing the urea plant of FIG. 7 modified in accordance with the invention, viz. having an additional reactor (E) positioned downstream of the scrubber (F). From the scrubber, a concentrated carbamate stream (g) is fed to the additional reactor (E).