GB2190912A

GB2190912A - Process for operating a potassium carbonate/bicarbonate loop during stand-by with heat recovery

Info

Publication number: GB2190912A
Application number: GB08717230A
Authority: GB
Inventors: Amnon Yogev
Original assignee: Ormat Turbines Ltd
Current assignee: Ormat Industries Ltd
Priority date: 1984-04-27
Filing date: 1987-07-21
Publication date: 1987-12-02
Also published as: IL75022A; GB2158817A; FR2564138A1; GB2190912B; GB8510630D0; JPS6183622A; GB2158817B; IL75022A0; FR2564138B1; DE3515197A1; MX171199B; GB8717230D0

Abstract

A process for operating a potassium carbonate/potassium bicarbonate loop during stand-by comprises the steps of: a) contacting gaseous carbon dioxide and water vapour with a potassium carbonate solution to form potassium bicarbonate solution; b) heating said potassium bicarbonate solution to regenerate potassium carbonate solution while liberating heated carbon dioxide and water vapour; c) cooling said potassium carbonate solution of step b); d) recycling and contacting at least a portion of said carbon dioxide and water vapour of step b) with said potassium carbonate solution produced by step c) according to step a); and e) cooling said recycled carbon dioxide and water vapour prior to contact with potassium carbonate solution produced by step c) by passing said carbon dioxide and water vapour through a heat engine to recover at least a portion of the heat contained therein. <IMAGE>

Description

GB 2 190 912 A 1

SPECIFICATION

Manufacture of ammonia and related products and methods of and means for producing power and cooling This invention relates to a process and apparatus for producing power and cooling in the manufacture of 5 ammonia and related products, such as ammonium carbonate and ammonium sulphate.

The large-scale manufacture of ammonia and its derivative compounds has been performed sincethe earlytwentieth century according to a process developed by HABER and others.

The original process, substantially unmodified to the present day, begins with a fuel source, such as coke from bituminous coal or lignite. Thisfuel is blasted to incandescence and steam is passed through thefuel 10 bed, eventually yielding a mixture of carbon dioxide and hydrogen gases.

The carbon dioxide must be separated from the hydrogen beforethe synthesis of ammonia can take place.

This is accomplished by directing the gas mixture into a potassium carbonatelpotassium bicarbonate loop.

The loop must be kept in constant operation, even when the rest of the ammonia plant is shutdown. Inthe loop, the gaseous mixture of hydrogen and carbon dioxide istreated with cold potassium carbonate aqueous 15 solution. This results in the production of an aqueous potassium bicarbonate solution. Free hydrogen gas is removed and the potassium bicarbonate solution isthen heated to approximately 130'Cto regenerate pot assium carbonate by evolving carbon dioxide and watervapour. The carbon dioxide and watervapourare vented to the atmosphere. The hot potassium carbonate solution is cooled by means of a heatexchanger, and the cooled solution is recycled. The separated hydrogen gas is combined with a suitable source of 20 nitrogen such thatthe ratio of hydrogen to nitrogen is approximately 3: 1. The hydrogen and nitrogen entera synthesis loop where they pass over a catalyst in a high-pressure ammonia converter where the ammonia is formed.

Additional useful compounds can also be produced from the ammonia. For example, ammonium sulphate produced bytreating the ammonia with sulphuric acid. 25 Should the potassium carbonate loop in the ammonia manufacture process haveto be shut down because of trouble upstream or downstream of the loop, or because of lackof fuel or other material, considerable time and expense are involved in restoring the potassium carbonate cycleto normal operation.

Moreover,though ammonia and its chemical derivatives are very useful chemical compounds, a greatcleal of energy has to be expended in their manufacture. Itwould thus be desirableto provide a techniquefor 30 reducing the energy needed to manufacture a given quantity of ammonia, as well as minimizing the amount of energywasted when cycling the potassium carbonate loop when the rest of the system is shutdown.

Generally speaking,the process of the invention is unique in that ratherthan relying upon the normal contraction which occurs when a gas is cooled to condense it,the present invention relies instead on chemi cal reaction kineticsto provide a final produetwhich exerts a vacuum on itsfeedstreams. Thus, the invention 35 is applicableto reactions wherein thetemperature of thefinal product is not only lowerthan the reactants, as might be the case in a condensation phase change, but also in situations where the final product is atthe same or even higher temperatures than the reactants. This principleforms one important aspect of the invention which will be described with referenceto the accompanying drawing.

Although the drawing is a complete ammonia and ammonium sulphate production flow chart,the inven- 40 tive process and apparatus include a numberof distinct aspects which will be separately discussed below.

According to a first aspect of the invention, ammonium sulphate isformed byfirstreacting ammonia with carbon dioxide and watervapour in a reactortoform an ammonium carbonate solution (watervapour being provided in excess overthe stiochiometric amount, so asto ensure thatthe resulting ammonium carbonate solution is a free flowing slurry), and subsequently reacting the ammonium carbonate solution with a sul- 45 phuric acid to form ammonium sulphate. While it has been conventional to reactammonia directlywith sulphuric acid to form ammonium sulphate in the prior art, it has now been found that byforming the ammonium carbonate intermediate, the process of ammonia manufacture can be manipulated, so thaten ergy can be removed from the system.

Thus, if at least one of the reactants which form the ammonium carbonate is passed through a heat engine 50 upstream of the reactor priorto being exhausted into a closed reactor, the formation of the solid ammonium carbonate (in solution) in the reactor creates a vacuum in the reactor resulting f rom the decrease in volume between the reactants and the products. which results in a pressure drop across the heat engine. The reaction chamberthus acts as a condenserforthe exhaust of the heat engine which can be used to generate electrical power, for example. 55 The corresponding apparatus forthe manufacture of ammonium sulphate includes sources of ammonium, carbon dioxide and water vapour, as well as means forfeeding the ammonia, carbon dioxide and water vapour into a reaction chamberwherein the ammonium carbonate is ultimately formed. A source of sulphu ric acid is additionally provided and the apparatus includes means for reacting the ammonium carbonate with the sulphuric acid to form ammonium sulphate. Again. in the invented apparatus, a heat engine is 60 connected between at least one of the reactant sources and the reaction chamber so as to take advantage of the pressure drop which occurs in the reaction chamber as the result of theformation of the ammonium carbonate.

According to another aspect of the invention. the potassium carbonate/bicarbonate loop which is con ventionally used for separating the useful hydrogen out of flue gases in the manufacture of ammonia is 65 2 GB 2 190 912 A 2 modified so that the separated carbon dioxide and water vapour, which might otherwise be vented, are used to form ammonium carbonate. The advantage of this technique is that by saving and using the carbon dioxide in the system, the heat energy of the carbon dioxide, which is absorbed during heating of the pot assium bicarbonate, can beat least partially recovered bypassing the carbon dioxide through a heat engine priorto passage into the reaction chamber. Using this technique the pressure drop which occurs in the 5 pressure chamber as a result of the formation of the ammonium carbonate results in a pressure drop across the heat engine which can drive the engine to recover energy. The ammonium carbonate which is formed may then be reacted with sulphuric acid to form ammonium sulphate. Quite obviously, a second heat engine maybe positioned in the ammonia feed line to also take advantage of the reduced pressure in the reaction chamber. 10 In another aspect of the present invention, the reaction products of a process for the manufacture of ammonia and its by-products are thus used as working fluids in the production of power.

According to yet another aspect of the invention, when ammonium su I phate is not needed, such as when the material is in oversupply or when the available ammonia supply exceeds that needed for the manufacture of ammonium sulphate, the available ammonia supply maybe reacted with a water vapour-carbon dioxide 15 gaseous mixture to accumulate the ammonia in the form of ammonium carbonate, which may itself be stored. Again, energy maybe generated bypassing each of the streams through heat engines.

Alternatively, each of the gas streams maybe pressure-liquefied, and stored in vessels at room tem perature. When one desires to utilize the condensed liquids, the liquids can be simultaneously expanded through heat engines into an ammonium carbonate reaction chamber. In the process of gasification, the 20 storage vessels are cooled, and can be used to cool a refrigerant flowing through a heat-exchange system in physical contact with the walls of the vessels. The gasification process can be controlled to occur isotherm ally. The energy of expansion can thus be considered to be the difference in free energy between the free energies of the reactants (i.e., water vapour, carbon dioxide, and ammonia) and that of the product, ammonium carbonate. 25 In yet another aspect of the invention, the Applicant has developed a technique for continuously operating the potassium carbonatelbicarbonate loop, and more specif ical ly the retort heaterusedto heatthe pot assium bicarbonate solution continuously, even in the eventof shutdown ahead of, ordownstream, ofthe loop. This is important, since in theeventof system upset, e.g., in the eventthatthe heaters and contact chambers ofthe loop are shutdown, start-up isverycostly and very time- consuming. Thus, the Applicant has 30 found atechnique in which, during upset,the loop is operated continuously by recycling thecarbon dioxide and watervapour, which would normally have been vented upon leaving the potassium bicarbonate solution heater, and recovering the heatenergy in this recycled stream,thus reducing the costof operating the loop during this stand-by condition. Thus, according tothe invention,the carbon dioxide gas, hydrogen gasand water vapour are first treated with a potassium carbonate solution to form potassium bicarbonate solution. 35 Free hydrogen gas is separatedfrom the potassium bicarbonate solution andthe potassium bicarbonate solution is heatedto regenerate potassium carbonate solution while liberating carbon dioxide andwater vapour.The potassium carbonate solution is cooled in preparationfor reuse andthe liberated carbon dioxide and water vapour are cooled by passing thecarbon dioxide and watervapourthrough a heatengine.

Thewatervapour is provided in excess of the stiochiometric amount required in the reaction sothatafree 40 flowing slurry isformed andthe heatengine blades do not become encrustedwith material. Thecooled carbon dioxide and water vapour are then treated with the cooled potassium carbonate solution andthe process can be continuedforas long as desired while nevertheless recovering energywhich would other wise be lost if thecarbon dioxide stream wereto bevented to the atmosphere.

In one of its broader aspects, the invention can be characterized as setting forth a process of forming a 45 product in a reaction chamberfrom two or more reactantswith the product having a lower pressure inthe reaction chamberthan the pressure of each of the reactants in the reactantfeed lines. This resultsfromthe reduction in volume of the productsofthe reaction relativetothe reactants. Thus, upon entering the reaction chamberthe reactants reactand a vacuum is generated within the reaction chamberwhich results in a pressure drop across at leastone heatenginewhich is positioned in at leastone of the reactant lines. Itshould 50 be notedthat, depending on the reactants used andthe reaction conditions, it is possibleto pass all ofthe reactants through a single line,such aswhen the reaction requires a catalyst, orto segregate the reactants with heatengines being positioned in some orall of the segregated reactant lines. The reaction chambermay be cooled bya heat exchanger (not shown) forthe purpose of removing the heatwhich isformed in exo thermic reactionswithin the reaction chamber. 55 Accordingtothis broad aspectofthe invention,the reactants may be gaseswith the reaction products being gaseious, solid, liquid or mixtures thereof. As described specifically with referencetothe drawing,the reactants are ammonia, water vapour and carbon dioxide, withthe reaction product being ammonium car bonate in solution.

At leastfourtypes of reactions may be contemplated for purposes of achieving the process of the invention 60 relating to the generation of energy by using a chemical reaction as a means forcreating and/or increasing the pressure head of a system:

(1) Reactions involving a gas and a liquid from separate sources:

e.g., 2NH3 + H2S04--->(NI-14)2S04 65 3 GB 2 190 912 A 3 (2) Reactions involving different gases which come from the same source (a closed loop system):

e.g., H20 + C02 + K2C03;: 2KI-IC03; and (3) Reactions which involve different gases which come from different sources, i.e., an open loop:

5 e.g., 2NH3 + C02 + H20--->(NI-14)2C03 (4) Reactions which involve a gas and a solid:

e.g., C02 + 2NaOH --- > Na2C03 + H20 10 Byway of example, the technique of the invention may be used to generate a vacuum which may in turn be used to drive a turbine in connection with the following reactions in closed loop cycles:

1) 2NH3 (9) + H20(9) + S03(g):i: (N1-14)2S04 high pressure at 1000C at2000C low pressure 15 2) H20 (1) + Ca 0 (s).-,- Ca (0 H)2(1) 3) N H3 + H20 + C02::-- (N H4) H C03 4) 2 N H3 + H 2S 04 1- (N H4) 2S 04 20 5) 2NH2CH3 + H20 + C02R-- (NH3CH3)2C03 6) 2N H(CH3)2 + H20 + C02 <-- (N H2(CH3)2)2C03 25 7) 2N (CH36 + C02 + H20;:-- (N H (CH3)3)2C03 The various compounds are heated until the occurrence of vaporization and decomposition into their components which are then expanded through a turbine, and reacted in a reaction chamber so as to reform the original compound. 30 The invention extendsto the inventive apparatus which is used to form a product in a reaction chamber from at leasttwo reactants,the product having a lower pressure in the reaction chamberthan the inlet pressure of each of the reactants. In this embodiment, at least one line connects a source of each of the reactants to the reaction chamber and at least one heat engine is positioned in at least one of the lines whereby upon reaction of the reactants in the chambera pressure drop occurs acrossthe heat engine clueto 35 the reduced pressure in the reaction chamber.

Asthe product is formed in the reaction chamber, with the resultant lowering of pressure,the reactants are expanded through thefeed lines into the reaction chamber. This expansion can occur under eitheriso thermal or nonisothermal conditions. Any of the following three situations can exist:

40 1) Tin = T reaction chamber 2) Tin > T reaction chamber 3) Tin < T reaction chamber 45 where Tin is the inlet tem peratu re ofthe vapours (before expansion) and Tmaction chamber is the temperature of the reactor.

While continuous reference is madeto theterm "heatengine" throughoutthe application, it isto be under stood thattheterm is used to include all manner ofdeviceswhich can be used to extractthe energyfromthe 50 flowing reactants and may, for example, constitute a turbinewhich is connected to an electrical.generator.

An embodimentofthe invention is hereafter described in conjunction with the accompanying drawing which is a flow diagram ofthe process forthe manufacture ofammonia and related compounds.

Referring nowtothe drawing the conventional process steps are shown in dashed lines, while the steps of the invention are illustrated in solid lines. The drawing shows a combustion source 10, such as a coke 55 furnace, which provides a flue gas stream 12 ofgaseous hydrogen and carbon dioxide. This stream istreated with an aqueous potassium carbonate solution stream 13 at a temperature of about300C in a gas-liquid treatment column 14. The potassium carbonate solution reacts with carbon dioxide in the stream toform potassium bicarbonate. The potassium bicarbonate solution 16 is then directed into a retort 18, where it is heated to approximately 13OoC, thus regenerating the potassium carbonate solution 19, which isthen cooled 60 in a cooler 21 to a temperature ofabout 30'C. After being cooled, the potassium carbonate solution is re cycledtothetop ofthe treatment column 14, wherein it removes carbon dioxidefrom the input flue stream.

Along with the regeneration of potassium carbonate solution in the retort 18, watervapour and a carbon 4 GB 2 190 912 A 4 dioxide gas stream 23 at a temperature of about 130'C are formed. This stream is directed toward a valve 25, which can either vent the gases into the atmosphere (as in conventional techniques) through a vent 27, or direct the carbon dioxide and water vapourthroug h aline 29 to drive a heat engine including a turbine 31 connected to generator 33. Water vapour is used in excess over the stiochiometric amount required in the reaction so that a slurry is formed and the blades of the turbine do not become encrusted with material. The 5 outgoing carbon dioxide and water vapour stream 35 which has been cooled as a result of the work per formed can be returned to the top of the treatment column 14 to react with the potassium carbonate solution.

As a result of this configuration, a carbon dioxide stream can be continuously cycled through the system by adjusting the valve 25 to divert outgoing carbon dioxide through the heat engine 31. Using this technique, the retort 18 can be operated even when the carbon dioxide being generated is not used or when the input 10 flue stream 12 has been discontinued. Nevertheless, it is an advantage of the invention that the heat added to the carbon dioxide stream in the retort is at least partially recovered in the form of energy generated bythe heatengine31.

Free hydrogen 37 leaving the treatment col u m n 14 is fed into a reactor 39, which is also fed with a nitrogen stream 41 such that the ratio of hydrogen ton itrogen in the reactor 39 is approximately M, wherein gaseous 15 ammonia 43 is formed. A conventional ammonia reactor maybe used and operated conventionally forthis purpose.

Ammonia leaving the reactor 39 is directed by a valve 45 along one or both of two different streams. Thus, the formed ammonia can be used directly or be reacted, as in conventional techniques, with sulphuric acid to form ammonia sulphate. This reaction is known, and is shown in dashed lines. 20 However, according to the invention, ratherthan reacting the ammonia directly with sulphuric acid,the ammonia is first directed bythe valve 45 through a turbine 47 and into reaction chamber 49. In this embodi ment, carbon dioxide and watervapou r which might otherwise have been vented by vent stream 27 are diverted through the turbine 51 and into the reaction chamber49, where the carbon dioxide and watervapour are reacted with the ammonia to form aqueous ammonium carbonate. It is the ammonium carbonate sol- 25 ution stream 53 which is then fed into the contact chamber 55, where it is treated with sulphuric acid 57to form an ammonium sulphate stream 59.

Byfirstforming ammonium carbonate as a reaction intermediate priorto forming the ultimate ammonium sulphate stream,which is desired, it is possibleto achieve very desirable energysavings. Since the two gases reacting in the chamber49 form a solid (in solution) having a substantially reduced specificvolume as corn- 30 paredto thetwo reaction gases,there is a reduced pressure orvacuum exerted bythe reaction chamber relative to the iine pressures of the reactants, which results in a pressure drop across both turbines 47 and 51.

This pressure drop drives each of theturbines, which generate useful energy. Quite obviously, two turbines need not necessarily be used and it is possible, for example, to use only a single turbine positioned in one line. 35 The chamber49 is intended to be closed to the atmosphere so thatthe vacuum exerted upon reaction of the reactantsforms a pressure drop acrossthe heat engines. The solution of slurryformed in the reaction chamber may be removed by anyconventional means from the stream 61, while maintaining the reduced pressurewithin the reaction chamber. Sufficient water vapour (in excess of the stiochiometric amount) is used to ensurethat a freeflowing slurry isformed. 40 The solution orslurryformed in the reaction chamber may also betreated, by means known tothose skilled in the art, to regeneratethe NH3, C02 and watervapour, which can then be recycled throughthe chamber49to drive the turbines and form ammonium carbonate.

According to yetanother aspect of the invention, the system may be modified to allowforthe storage ofthe gaseswhich drive the turbines 47 and 51. To do this,vessels 42 and 50 are provided forstoring the gases at 45 ambient temperature under pressures sufficientto liquefythe gases. Pressurization means and lines are associated with each of the vessels for this purpose. The valves necessaryfor diverting the stream intothe vessels are schematically illustrated. Storage of the gases may become necessary, as where an oversupply& the products occurs. When the liquefied gases areto be re-gasified, the valves are opened, and thevessels are de-pressurized to permitthe gasification of the liquids. According to a preferred embodiment, thevessels 50 may have heat-exchange means associated with theirwalls. The heat- exchange means may contain a heat exchangefluid, such as a liquid, adapted to provide at least a portion of the heat necessary for gasification.

Thefluid isthus cooled, and may be used as a refrigerating fluid.

The principle of the invention is likewise applicable in producing an ammonium carbonate solution 61 as the desired end product. The ammonium carbonate may be stored and used, or subsequently converted to 55 ammonium sulphate bythe process of the invention, orany othertechnique.

It can thus be seen thatthe system of the invention provides a number of significant advantages over prior processes of forming ammonia, in thatthe potassium carbonatelpotassium bicarbonate loop can beoper ated continuously, so as to avoid shutting down the retort in the event of system upset. Furthermore, this loop can be operated continuously while notwasting the heat energy supplied to the retort, since the liberated carbon dioxide drives a turbine, which coolsthe carbon dioxide and recovers useful energy.

Additionally, the system of the invention improvesthe efficiency of producing ammonia sulphate byfirst forming a reaction intermediate which creates a pressure drop which can be used to drive at leastone turbine, and generate useful energy. The reaction products of the ammonia production process arethus used as working fluids in the generation of power. 65 GB 2 190 912 A 5 For purposes of simplicity, the process of the invention has been described with reference to a complete system, beginning with the initial reactants, and ultimately forming the desired end products (ammonia, ammonium carbonate, or ammonium sulphate). It is to be understood, however, that the invention is not limited to the process as a whole, and extends to the various individual inventive aspects when performed individually. 5 Furthermore, although described with reference to a particular production scheme, it is clearthatthe inventive process steps wi I I find application in connection with other flow schemes for providing a wide variety of compounds. To the extent that the inventive principles find other applications in other processes, the use of these principles is deemed to be included within the scope of the invention to the extent to which these principles fal I within the scope of the claims. 10

Claims

1. A process for operating a potassium carbonatelpotassium bicarbonate loop during stand-by oper ation, said process comprising the steps of: 15 a) contacting gaseous carbon dioxide and water vapour with a potassium carbonate solution to form potassium bicarbonate solution; b) heating said potassium bicarbonate solution to regenerate potassium carbonate solution while liberat ing heated carbon dioxide and watervapour; c) cooling said potassium carbonate solution of step b); 20 d) recycling and contacting at least a portion of said carbon dioxide and watervapour of step b) with said potassium carbonate solution produced bystep c) according to step a); and e) cooling said recycled carbon dioxide and watervapour priorto contactwith potassium carbonatesol ution produced by step c) by passing said carbon dioxide and water vapour through a heat engineto recover at least a portion of the heat contained therein. 25

2. A process as claimed in Claim 1 comprising recycling all of the heated carbon dioxide and watervapour liberated in step b).

3. A process as claimed in Claim 26 wherein said heat engine comprises a turbine.

4. A process for operating a potassium carbonate/potassium bicarbonate loop during stand-by operation substantially as hereinbefore described. 30 Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd, 10187, D8991685. Published by The Patent Office, 25 Southampton Buildings, London WC2A lAY, from which copies rnay be obtained.