Process and plant for ammonia-urea production

Abstract

A process for ammonia-urea production where: liquid ammonia produced in an ammonia section is fed to a urea section directly at the ammonia synthesis pressure, and where the liquid ammonia is purified at high pressure with the steps of: cooling the liquid ammonia (20) obtaining a cooled liquid ammonia stream (21), separating a gaseous fraction (22) comprising hydrogen and nitrogen from said cooled liquid ammonia, obtaining purified liquid ammonia (23) at a high pressure, and reheating said purified liquid ammonia (23) after separation of said gaseous fraction, obtaining a reheated purified ammonia (24) having a temperature suitable for feeding to the urea synthesis process. The application also deals with an ammonia-urea plant comprising an ammonia cooler, a liquid-gas separator and an ammonia re-heater and with a method for revamping existing ammonia-urea plants.

Claims

1. A process for treating liquid ammonia for use in a urea synthesis process, comprising: a) cooling the liquid ammonia to obtain a cooled liquid ammonia stream, wherein the liquid ammonia is produced by an ammonia synthesis process operated at an ammonia synthesis pressure, wherein the liquid ammonia contains minor amounts of hydrogen, nitrogen, methane and eventually other urea-inert gases, b) separating a gaseous fraction comprising hydrogen and nitrogen from the cooled liquid ammonia to obtain purified liquid ammonia, and c) reheating the purified liquid ammonia after separation of the gaseous fraction to obtain reheated purified ammonia having a temperature suitable for feeding to the urea synthesis process, wherein the reheating of the purified liquid ammonia is performed prior to the reheated purified ammonia being fed to the urea synthesis process, wherein the cooling of the liquid ammonia, the separating of the gaseous fraction from the cooled liquid ammonia, and the reheating of the purified liquid ammonia are performed at substantially the same pressure as the ammonia synthesis pressure.

2. The process according to claim 1, wherein the liquid ammonia is cooled to a temperature between 35 C. and 15 C.

3. The process according to claim 1, wherein the re-heating temperature of the purified ammonia is in the range of 10 C. to 120 C.

4. The process according to claim 1, wherein the reheated purified liquid ammonia is subjected to a dehydrogenation process prior to entering the urea synthesis process to produce a further purified ammonia feed with a low H.sub.2 content for the urea synthesis process.

5. The process according to claim 1, wherein the urea synthesis process is a CO.sub.2-stripping process with total condensation, comprising a high-pressure urea synthesis loop with at least a reaction space, a CO.sub.2 stripping section and a total condensation section, the ammonia input of the urea synthesis process being directed fully or in part to the total condensation section.

6. The process according to claim 5, wherein a major part of the ammonia input is directed to the total condensation section, and the remaining part of the ammonia input is directed to the reaction space.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a scheme of ammonia-urea plant according to an embodiment of the invention.

(2) FIG. 2 is a scheme of ammonia-urea plant according to another embodiment of the invention.

(3) FIG. 3 is a scheme of the urea synthesis loop of the urea section of an ammonia-urea plant according to a preferred embodiment.

(4) FIG. 4 is a simplified scheme of a prior-art ammonia-urea plant.

(5) FIG. 5 is an example of the plant of FIG. 4 revamped according to one of the embodiments of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

(6) Referring to FIG. 1, an ammonia-urea plant comprises an ammonia section 10 and a urea section 16. The ammonia section 10 comprises a front-end 11 for production of a suitable ammonia make-up gas, an ammonia synthesis loop 12, a heat exchanger or ammonia cooler 13 for cooling liquid ammonia delivered by said synthesis loop 11, a gas-liquid separator 14 and a further heat exchanger 15 for re-heating liquid ammonia separated by gas-liquid separator 14.

(7) The front-end 11 of the ammonia section 10 is fed with desulphurized natural gas or synthesis natural gas or another hydrocarbon source, a steam flow for steam reforming and a flow of air or enriched air. Reforming of the hydrocarbon source produce a raw synthesis gas, which is then treated to obtain a make-up gas 19. These steps are known in the art, see e.g. EP 2 065 337, and are not described further. The make-up gas 19 is reacted in the loop 12 producing a high-pressure liquid ammonia stream 20 containing minor amounts of H.sub.2, N.sub.2, CH.sub.4, Ar. Since said minor components are substantially inert to the synthesis reaction between ammonia and carbon dioxide for the production of urea, they are referred to as urea-inert. The liquid ammonia 20 has usually a temperature between 10 C. and 10 C. and a pressure around 150 bar.

(8) In the example of FIG. 1, the whole of ammonia stream 20 is used to produce urea in the urea section 16, although other embodiments of the invention provides that a part of said stream 20 is taken to produce ammonia as end product, and the remaining part is used to produce urea. In this example, the ammonia stream 20 is cooled through the ammonia cooler 13 to a temperature preferably in the range 35 C. to 15 C. and more preferably 33 C. to 20 C. in order to lower the solubility of hydrogen and methane. Due to lower solubility, a gaseous stream 22 comprising hydrogen and methane is easily separated in the gas/liquid separator 14; purified liquid ammonia 23, now with a reduced content of urea-inert gases and especially of hydrogen and methane, is reheated in the heat exchanger 15 to a suitable temperature for feeding the urea section 16, preferably to a temperature in the range 10 to 120 C.

(9) It shall be noted that the cooler 13, separator 14 and reheater 15 operates substantially at the same delivery pressure of the synthesis loop 12. In other words, the liquid ammonia 20 is sent directly at the delivery pressure through steps of cooling, separation and re-heating, so that the pressure of purified liquid ammonia 24 is the delivery pressure of the loop 12, minus the pressure losses through the items 13, 14 and 15. Hence, the purified liquid ammonia 24 retains a substantial amount of the energy pressure of the liquid effluent 20 of the synthesis loop 12, and is made available to the urea section 16 at a high pressure which is close to the ammonia synthesis pressure and will usually be also close to the urea synthesis pressure. Hence, the purified liquid ammonia 24 can be fed to the urea section 16 without extensive pressurization. An ammonia circulation pump may be provided when necessary. Eventually, the ammonia stream 24 could be further pressurized when necessary for the feeding to the urea section 16.

(10) The CO.sub.2 source of the same urea section 16 is represented by flow 25. Optionally, some of the CO.sub.2 feed 25 could be recovered as by-product of the ammonia section, in particular by CO.sub.2 removal from raw syngas in the front-end 11.

(11) FIG. 2 is a scheme of another embodiment where the purified liquid ammonia 24 is subjected to a dehydrogenation process prior to feeding to the urea section 16. Said dehydrogenation process is carried out in a dehydrogenation unit 30 which also operates at the high pressure of the ammonia synthesis. Dehydrogenation of the liquid ammonia separates a H.sub.2-rich stream 31 that may be recovered for further use. The dehydrogenated and hence further purified liquid ammonia 27 is directed to the urea section 16.

(12) The urea section 16 may operate according to any of the known techniques for producing urea, including: the ammonia-stripping process, self-stripping process, CO.sub.2 stripping process; non-stripping process including total-recycle process.

(13) The urea section 16 will usually comprise a high-pressure synthesis loop and a recovery section including a medium-pressure and/or low-pressure treatment section(s). FIG. 3 discloses a particularly preferred embodiment of a high-pressure (HP) loop in the urea section 16. Referring now to FIG. 3, the HP loop 100 comprises a reactor 101, a stripper 102, a carbamate condenser 103 and a scrubber 104. The condenser 103 is preferably adapted to total condensation and in this case is referred to as full condenser. Preferably the stripper 102 is a vertical steam-heated shell-and-tube heat exchanger; the full condenser 103 is preferably a falling-film tube condenser as disclosed e.g. in WO 01/96287.

(14) The inputs of the loop 100 are the ammonia source 24, or the further dehydrogenated ammonia stream 27 of FIG. 2, and the CO.sub.2 feed 25. The ammonia input is preferably split into two portions, one directed to the reactor and one directed to the condenser. The example shows the ammonia input 24 split into portions 24a and 24b. Preferably the portion 24b directed to the condenser is larger, e.g. about (two thirds) of the total.

(15) The mixture 110 produced in the reactor 101 and containing urea, carbamate and unconverted ammonia is stripped with the CO.sub.2 feed 25 obtaining concentrated urea solution 111 and vapours 112 comprising ammonia and carbon dioxide. Said vapours 112 are preferably split into a first stream 113 directed to the full condenser 103, and a second stream 114 directed to the reactor 101.

(16) The condensate 119 is fed to the reactor, together with the portion 24a of the ammonia feed, via an ejector 120. Overhead off-gases 114 from the condenser 103 are sent to the scrubber 104, after mixing with off-gases 116 from the reactor. The off-gases 117 are scrubbed with a carbamate solution 130 returned from the (not shown) recovery section, i.e. obtained from the decomposition of the carbamate contained in the concentrated solution 111. A non-condensable fraction 115 is vented from top of the scrubber 104; the remaining carbamate-containing liquid fraction 118 is returned to the full condenser 103 together with the remaining part 24b of the ammonia feed, via a second ejector 121.

(17) An advantageous feature of the layout of FIG. 3 is that the scrubber 104 is a further barrier against accumulation of urea-inert gases in the reaction space, namely the reactor 101 in the example. In fact, the majority of urea-inert gases dissolved in the portion 24b of the ammonia feed 24 are vented with stream 115, prior to reaching the reactor 101. It can be understood that the layout of FIG. 3 is tolerant to a relatively high content of urea-inert gases in the ammonia feed 24, especially in the preferred embodiments where the portion 24b is the major portion of the ammonia feed 24.

(18) Optional features of the invention include the further removal of hydrogen, in order to avoid any risk of explosive mixtures especially in the scrubber 104. According to one embodiment of the invention, the vapours 117 are subjected to a process of dehydrogenation prior to entering said scrubber 104, i.e. a suitable dehydrogenation unit is installed upstream the scrubber 104. Another optional feature is dehydrogenation of the CO.sub.2 source flow 25.

(19) Dehydrogenation of any of the off-gases 117 or CO.sub.2 feed 25 is preferably carried out with DeOxo catalysts which is available e.g. from BASF and are specifically designed for the removal of O.sub.2 and/or H.sub.2 from gas streams. The by-products generated are H.sub.2O and CO.sub.2.

(20) Another aspect of the invention is the revamping of a known ammonia-urea plant. An example is given in FIGS. 4 and 5.

(21) FIG. 4 shows a scheme of a conventional ammonia-urea plant where the ammonia section includes a front-end 211 and a HP synthesis loop 212. The liquid ammonia 220 is expanded through expander 230 and separation of urea-inert gases takes place in a low-pressure separator 231. The purified ammonia is then pressurized again with a pumping stage 232, to form the high-pressure ammonia feed of the urea section 216.

(22) According to one of the embodiments of the invention, this plant can be revamped as in FIG. 5, adding high-pressure ammonia cooler 213, separator 214 and reheater 215, thus obtaining a high-pressure purified ammonia stream 224 without the need of the pumps 232. The invention is useful especially when the capacity of the ammonia synthesis section is boosted (i.e. flow 220 is larger after revamping) because the delivery rate of pumps 232 is often a bottleneck of the whole plant and replacing the pumps with larger ones or installing additional pumps is quite expensive. It shall be noted that the low-pressure equipments including the valve 230, separator 231 and the pumps 232 (dotted line of FIG. 5) may be discontinued or may still operate in parallel with the newly-installed high-pressure separation section of items 213, 214 and 215.