DE2065941A1 - Ammonia recovering from ammonia synthesis - Google Patents

Abstract

Ammonia synthesis gas is treated to recover ammonia by (a) absorbing the ammonia contained in the effluent from the synthesis reactor by means of an absorption film to obtain a concentrated ammonia soln. (b) condensing the water in the recycle gas and in the fresh gas feed by heat exchange with dried gas, to give a dil. ammonia soln. (c) injecting ammonical liquid in the total gaseous stream to dehydrate the gas by adiabatic evaporation, and (d) passing this gas to the heat exchanger after passage through a separator, from which a concentrated ammonia soln. is withdrawn. Very concentrated ammonia solns. can be obtained economically from the ammonia synthesis gas. The concentrated ammonia soln, can be used for absorbing the CO2 contained in the crude gas utilised for the ammonia synthesis. The ammonium carbamate formed can the used in urea synthesis.

Description

Snaia Progetti S-.p. ^. “.Ailand ,, Italy

Technique for treating gases in the synthesis of ammonia

The invention relates to a method of treatment of gases in ammonia synthesis.

It is known that in the plant for the ammonia synthesis the separation of the ammonia from the unreacted, the Oases leaving the synthesis reactor usually by fractional condensation, initially in a water-cooled one Condenser, then carried out in one or more condensers that are cooled by evaporation of ammonia will.

The fresh Bynthesegemixture is fed in generally before the final condensation stage, which is done by evaporation is cooled by ammonia so that the water contained in this feed is removed. According to the invention is intended to provide a process for a complete treatment of the gaseous stream leaving the ammonia synthesis reactor both with regard to the ammonia separation and the dehydration of the recycle stream and the fresh use

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to be found, whereby, as will be shown, itself some economical and work-related parts of the gate already known plants.

In the method according to the invention, the gaseous, the The stream leaving the synthesis reactor is fed to a film absorber after a possible heat recovery and cooling.

In this direction of the gate, the gaseous stream continues to flow above in countercurrent au a liquid j? ilm, which either consists of water or a weak ammonia water solution., which removes the ammonia present in the gaseous F-stream absorbed ;;, this door direction is with cooling facilities provided in order to eliminate the absorption of ammonia.

The strong cooling of the liquid film and the countercurrent mass transfer result in very strong ammonia solutions. The scrubbed gas leaving the absorber has a very low ammonia content (0.2 - 0.5 LoI %) and is saturated with water.

After releasing the inert gases in such a way that their concentration is kept constant in the cycle _? the fresh feed is added to the gaseous stream before the entire stream is fed to the dehydration stage.

The dehydration is carried out by putting in the gaseous Stream of liquid ammonia is injected through adiabatic evaporation for strong cooling and ensures the condensation of the water it contains.

j ± e use of a suitable heat exchanger between the moist stream and the dried does it allow _? very

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xix-veined saturation temperatures '-25 to ⁷ O ⁰ C' with less small "vaporized ammonia" can be achieved "he injected -ii" ^{: i} fjsi5 is deducted from 3λ3Β. ümlavi "as a strong Amüionia'rlüBung - the im ^? 'surplus voi'L & adene ^ mmoniak and thus aie ^! = -onsentration of the outlet solution are regulated in such a way that; -nciit more than 2 - ^ ppm water ii ·. dried jas preachoa are.

Sfaijh dam finspritaen of ammonia v / ith the .Power a separator fed _ft Aie in which said strong ammonia solution and a dried gaseous ISTR om · separated; cie dried gas. After they have passed through a heat exchanger as a cooling fluid in a countercurrent to the moist, dry, either a compressor or other suitable device et) ευ gel cotet. and there is a free run back to the r-ynths

. '^; aa casist-e in the moist ^ a .sen existing what is' v / ird ¹ nnex'ha.lb of the V / arraesustauschers condenses and a; .s weak ammonia solution withdrawn :: the residual water vvizc'i with the strong, from the «Separator removed from coming aramonia dumpling.

L ^l ie in this way ammonia solution to a distillation zone is fed ,, where in a well known manner, the ammonia from the water with a high degree of purity ;, ie 99 "9%>" is disconnected.

Regarding the drying method, it should be noted that Water has a high rate of dissolution in ammonia and that the larger the contact area between the to treating gases and liquid ammonia, the higher this speed becomes.

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Since the evaporation of liquid ammonia requires a certain amount, both of these facts can be exploited be to an excellent jehydration of the gases without reaching their saturation with ammonia, by the Contact time is reduced.

For this purpose, liquid ammonia is transformed into gaseous Current injected in a device suitable to give a high contact area, so that it is possible will reduce the contact time with the above results.

For example, this step can only be carried out bej by d? e Gases accelerated through a venturi tube will? Liquid ammonia is being injected in this direction of the gate? the high turbulence generated in this way creates a high contact area.

Behind the venturi is a separator or another suitable execution provided that the contact area too is able to reduce »

The larger the contact area, the narrower this separator can be, resulting in excellent drainage without saturating the gases with ammonia, since the reduced contact time is not long enough to cope with large amounts Vaporize ammonia while covering a large area of contact good drainage.

Another important feature of the invention for the "'all, where part or all of the ammonia produced is used Manufacture of urea used is in the ability to see part or all of the tunmoniak water solution obtained in the film absorber,

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auszunut ζ s _t to the in the raw iiiamoxiiaksynthesegasen. The carbon dioxide contained in it is glued to absorbent, which comes from the carbon monoxide conversion stage; this absorption takes place under conditions such that ammonium carbamate is produced which then undergoes its conversion to urea, while the carbon dioxide-free 3-ases are added after possible further treatments of dehydration and ammonia synthesis known in the art.

The ammonia synthesis gases formed by N ^ OIU are in the generally generated by weak oxidation of hydrocarbons (i.e. by steam reforming, Γ.! rapid oxidation with C> 2 Etc.); the gases generated undergo further processes, for example carbon dioxide conversion or extraction of carbon dioxide and residual oxygen-containing carbon compounds fed.

With the kit according to the invention, it will be possible to produce two expensive heat stages in modern production facilities to avoid ammonia, that's the carbon dioxide 2nt ~ removal stage with solvent regeneration and the ammonia separation stage

Tn suitable devices, therefore, which make it possible to use a A part or all of the ammonia-water solution can be used to achieve a very high contact surface between gases and liquids. obtained according to the invention can be exploited to remove the carbon dioxide in the crude To remove ammonia synthesis gases, whereby an Ammoniuiacarbamatlösung is obtained in this way, which then it is converted into urea.

3The absorption of the carbon dioxide can expediently in the return

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area, which is usually carried out in the urea synthesis process is used. As mentioned, the carbon dioxide-free gases are fed to the ammonia synthesis reactor, after being subjected to other known operations.

The invention will now be based on the accompanying drawing are explained in more detail, in which a particular embodiment of the invention is shown without the GQp absorption stage is.

According to the drawing, the effluents from the synthesis

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Reactor 1 at a pressure between 80 and * i80 kg / cm _$, optionally after heat recovery and cooling, which is not shown in the drawing, via 6 to a film absorber 2, in which the gaseous flow upwards within vertical tubes in countercurrent to a Liquid film flows, which flows downwards on the pipe surface and absorbs the ammonia present in the gases «

This liquid can either be water or a weak one Ammonia solution and is fed into 2 via 10.

The heat of ammonia absorption is removed by adding water outside the pipes, that is on the side of the wall, flow is left, with the water over 11 and over * 2 exit.

Its strong ammonia solution passes through 7 at the bottom of the film absorber and a gaseous stream with a very low residual ammonia content emerges from its head via 8 from, that is 0.2 to 0.5 mol%, which is practically saturated with water and, after the inert gases have been released from 9,

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the fresh insert is added via ή 3.

The resulting gaseous stream is passed through 14 the heat exchanger 2 supplied, in which the largest rope of its heat is transferred to the cooled, dried gases coming out of the separator 4 and entering at 18.

This cooling effect ensures the condensation of the largest Rope of water present in gases; in this way becomes a weak one from the bottom of the exchanger 5 via 15 Ammonia solution withdrawn. The drying is completed, by exiting from the heat exchanger 5 via 17 Gases liquid ammonia is injected, which by adiabatic evaporation in the gas stream for its cooling and thus ensures the condensation of the water it contains. This liquid ammonia is in i? introduced by 16.

The resulting mixture enters the separator 4; a strong ammonia solution gets from the bottom of this separator discharged via -T9? a dried, gaseous stream leaves the separator over 18 and contains no more than 2 3 ppm water and is used as a cooling fluid for the heat exchanger 5 led§ the gases then flow via line 20 and 2f to the compressor 5 »which circulates it to the synthesis reactor returns.

The following example illustrates the invention without relating it limit.

example

Dds example relates to a synthesis plant for the Production of 1200 sun ammonia per sag *

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..in flow of 780.930 Nm ^ 'h leaves the synthesis reactor; this stream has the following composition at a pressure of 125 atmospheres absolute:

H ₂ 58.99 YoI. %

N ₂ 19.68 YoI. % ;

CH ₄ 7.58 YoI. %

Ar 3.25 % by volume:.

NH _x 10.50 % by volume ,.

After cooling this stream down to 40 ⁰ G, it was led to the bottom of the film absorber 2 *

^ 8j969 kg / h of a 10bigen ^ weight) ammonia solution led to the head of the absorber.

108.865 kg / h of an ammonia solution with 60 weight -? * Ammonia left absorber 2 through 7 and 701.190 Nm / h of scrubbed gases through 8.

like gases at a temperature of 4-5 ⁰ ^ had the following composition, based on the dry matter -

H ₂ 65.59 YoI. Yo N ₂ 21.90 YoI. % CH ₄ 8.40 YoI. % Ar 3.61 Vol. % NH-, 0.50 YoI. %

Saturation water 525 kg / h.

10,900 NmVh of these gases were discharged via 9.

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The fresh food mix that was introduced over 13 had the following characteristics:

■ z

Flow rate 142.170 Nm / h composition

. $ 74.17 H ₂ vol

N ₂ $ 24.92 vol.>

GH. 0.82 io Vol.

Ar $ 0.29 vol.>

Saturation water 60 kg / h.

The two gas streams were mixed before entering the heat exchanger 3; all the electricity had that following features:

Flow rate 832.460 Nm / h

Pressure 122 atmospheres absolute

Temperature 44 ⁰ O

Composition (dry basis) H ₂ 68.08 vol. Io N ₂ 22.35 vol. Io GH, 7.12 vol. $ Ar 3.04 vol. Io NH * 0.41 vol. $ Saturation water 607 kg / h.

At the outlet of the heat exchanger, the flow had the following characteristics:

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a) gas -18 • 5 ° 0 temperature 35 kg / h Water content D) Ammonia solution 818 kg / h Flow rate 40 Weight% Ammonia content 70 Weight% Water content

The liquid ammonia was injected with 10.665 kg / h and evaporated almost completely "it cooled the gaseous stream of -18.5 ⁰ G to -28 ⁰ G.

The separator 4 gave at equilibrium:

a) 350 kg / h of 90 weight percent ammonia solution

b) 846,000 only / h of gases with the following composition?

H ₂ 66.0 toi » 7o

N ₂ 22.0 % by volume

OH ₄ 7.0 vol.%

Ar 3.0 vol. %

NH ₅ 2.0% by volume

H ₂ O <3 ppm

From the foregoing, all economic and technical advantages of the method according to the Derive the invention without further ado

These parts can be distinguished from the excellent absorption derive the ammonia in the film absorber; absorption is favored by the cooling system provided in the device

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device that removes the heat generated during absorption.

The advantages of the new drying process for the recycle gases and for fresh use are obvious ·, this new procedure makes it possible with very simple Devices to achieve good drying and at the same time a very good thermal efficiency.

It is also worth pointing out that the procedure according to the invention partly in the known method of Ammonia synthesis can be used in the sense that either the absorption stage alone or with the film absorber only the dehydration stage for im. recirculated and freshly introduced gases with the quenching of the Ammonia and the heat exchanger can be used in systems that are known in other respects and take advantage of devices.

Claims ι

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Claims (5)

SKIM Case: 327 div. I P 20 55 997.1-41 Irennakte I 1. Process for treating the gases from ammonia synthesis according to a known method, characterized in that the strong dehydration of the fresh feed gases and / or the recirculated gases is carried out by adding liquid ammonia to the gaseous stream is injected. 2. The method according to claim 1, characterized in that the moist gaseous stream and the dried gaseous stream are fed to a heat exchanger, and that these flow in countercurrent to each other, such that a refrigeration recovery is brought about by the dried gases, such that on this Way it is possible to reach very low temperatures (preferably between -20 and -3O ⁰ G) with a small amount of vaporized ammonia. 3. The method according to any one of the preceding claims, characterized in that the weak in the heat exchanger generated ammonia solutions, in which moist and dried gases flow in countercurrent to each other, from the synthesis cycle must be withdrawn from liquid ammonia prior to injection. 709819/0351 4. The method according to any one of the preceding claims, characterized in that the proportion of the injected liquid ammonia is in excess with respect to the vaporized ammonia and the injected excess on a residual water content in the dried exhaust gases is controlled by a few ppm, in particular 2-3 ppnii. 5. The method according to any one of the preceding claims, characterized in that for injecting the liquid Ammonia increases the contact area between the liquid ammonia and the humid gases through turbulence Device, in particular a Venturi tube, is used. 709819/03 51