INDUSTRIAL A N D ENGINEERING CHEMISTRY PUBLISHED BY T H E AMERICAN CHEMICAL SOCIETY W A L T E R J. M U R P H Y , EDITOR
ANALYTICAL CONTROL FOR AMMONIA SYNTHESIS r d ( h i t rol for -\nimonia Syn t Iiesis“, prepared by Harl H. Brown and co-workers at Tennessee Valley Authority, JVilson Dam, Ala. I n these we have an excellent representation of development and application in this type of analysis for control of an important industrial process. Further, to illustrate tlie co11)t,iitional approach of Ilit> analyst in the determination of a specific compound a i i d t lie compromises needed to develop a procedure for ( wtinuous recording, two articles on determining r i i r t l i ) ] bromide in air follow irntncdiately aftrr the group of W \ * ( ’ I l . 1,. T, €1 \I.I.I I I . 1 wiciutr Editor
Control Requirements in the
TVA
EARL H. BROWN A N D MAURICE M. FELGER’ The analytical control requirements for the TVA synthetic ammonia plant are given. A description of the control laboratory and a flow sheet of the process showing the sample points are also included, with a table describing the samples analyzed, constituents determined, methods used, and frequency of determinations.
Plant
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of three Orsat analyzers is located along one wall near the outlets of the sample lines. A bench with glass-blowing equipment is provided for the emergency repair of glass apparatus during periods when this regular service is not available. A small storeroom in the basement below the laboratory serves the immediate needs for chemicals and apparatus. PLANT ANALYTICAL REQUIREMENTS
In the TVA ammonia plant the synthesis mixture is derived from semi-water gas by catalytic oxidation of most of the carbon monoxide with steam, and subsequent purification of the hydrogennitrogen mixture. In the purification process the concentration of carbon dioxide is reduced to about 0.7% by scrubbing with water a t 17 atmospheres' pressure. The residual carbon monoxide and carbon dioxide are removed by scrubbing with cold ammoniacal copper solution a t 121 atmospheres. The ammoniacal copper solution contains cupric ammino, cuprous ammino, ammonium, formate, and carbonate ions, and uncombined ammonia. The gas next passes through the caustic scrubbers where any remaining traces of carbon dioxide can be removed by scrubbing with sodium hydroxide solution. Actually, owing to the high efficiency of carbon dioxide removal in the water and copper scrubbers, the caustic scrubbers have not been operated. The purified hydrogen-nitrogen mixture is fed into the circulating gases of the synthesis system at a rate sufficient to replace the gas removed as ammonia and that discarded to control the concentration of inert gases. The laboratory supplies analytical data for the control of: (1) the quality of the raw gas and of the finished product, (2) the steam-gas and hydrogen-nitrogen ratios, (3) the conversion in the water gas and synthesis converters, (4) the purification of the hydrogen-nitrogen mixture, and (5) the regeneration of the solution used for absorbing hydrogen sulfide from the semi-water gas and of the copper solution used in the final removal of oxides of carbon from the synthesis mixture. A summary of the plant analytical requirements for the control of these factors follows. The numbers given in parentheses refer t o the sample point designations shown in Figure 1. The sample points on train 1 have odd numbers while those on train 2 have even numbers, but this system does not apply where the sample point is common to the two trains comprising the TVA plant. Sample point designations, as well as the constituents determined and the methods used, are given in Table I. The determinations marked by an asterisk are made by special methods deP ii veloped in the TVA 4 research l a b o r a $ tories; these new developments are
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ANALYTICAL EDITION
May, 1945
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Table I. Sample Points and Analyses for Control of the Ammonia-Synthesis Process Sample Point No. Gas Plant GS-1, 2, 3, 4
Raw gas from generators Gas from blow run
GS-5 GS-6 GS-7
Raw gas to soda ash scrubber Raw gas from soda ash scrubber Soda ash solution to actifiers
GS-8
Soda ash solution from actifiers
Raw gas in holder Raw gas from holder, before blow gas addition Blow gas in holder Blow gas from holder Boiler feed water Blast gas-From stack Raw gas to coke filter Raw gas from coke filter Raw gas, exit Roots blowers Raw gas to primary converters Converted gas from secondary converters
HS-9, 10 Raw gas from saturator towers HS-11,12,13,14 Gas from primary converters HS-16 Raw gas to saturator, after blow gas addition HS-16 HS-17
Constituents Determined
Sample
Converted gas in holder Converted gas t o compressors
Purification Section PS-11 12 Converted gas t o water scrubbers PS-13: 14 Gas from water scrubbers Gas from copper scrubbers Gas from caustic scrubbers Vent gas from breakdown tank Make-up gas to synthesis SS-21, 22
Gas to ammonia converters
SS-23, 24
Gas from ammonia converters
SS-25, 26
Gas a t circulators
Nz, Hz, C o t , CO, CHI,
Method 0 2
0 2
Tutweiler titration HIS Tutweiler titration HzS HS-, SCN-, & O s - - , total Titration alkalinity, HzS Titration Total alkalinity HS-, SCN-, HCOa-, SzOs;-, Titration cos--, HIS, total alkalinity Hydrometer Specific ravity Orsat N z Hz 60, CO CHI 0 2 Orsat Nt' H i Cot' CO' CHc) Oz Orsat NZ'Hz' COz' CO' CHI' Oz Orsat N z : Hz: COz: CO: CHI: 0 2 Iuodified Winkler Orsat Gravimetric Gravimetric Gravimetric Steam-gas ratio
cot, co, 0 2
Hz N,, Hz, COz, G O , CHI, 0 Steam-gas ratio c o z , co, 02
2
H* --. Nz Hz, COz
CO CH4 Os Nz: Hz, COz: CO: CH,: 01
Hz
Nz Hz COz CO CH4 Oz Nz: HI: COz: CO: CHI: Oz
N t , Hz, COz, CO, CHI, Nz
co + coz
SS-31
Purge gas from degasifier
8s-32, 33 99-34, 35
Anhydrous ammonia, water-cooled condenser Anhydrous ammonia, ammonia-cooled condensex Anhydrous ammonia from weigh tanks Vent gas from absorption tower Aqua ammonia
Exit copper scrubber Base of re%uxtower 1st preheater, reflux tower 2nd preheater, reflux tower 1st section, regenerator 2nd section, regenerator CS-8s 3rd section, regenerator 4th section, regenerator CS-9s CS-10 Low-pressure pump suction From oxidizing tower CS-11 * Special methods developed in TVA research laboratories.
Every 2 hours Daily Daily As required Daily (composite) As required Daily (composite) Everv 8 hours As required Daily Daily Daily Hourly Every 8 hours Continuous Daily Hourly Dailv Coihous Every 3 days As required Continuous Every 3 days Daily (composite)
Hz,CHI NHs A, H2, CHI Nz A NHs Nz, Hz NHa Oil Oil Water Oil, water NHa NHa
Electrical conductivity* Tuermal conductivity Orsat Thermal conductivity Orsat Absorption and titration Thermal conductivity Absorption and titration Therma! conductivity* Absorption and titration Orsat Absorption and titration Orsat Thermal conductivity* Absorption and titration Orsat Absorption and titration Gravimetric Gravimetric Volumetric Same as SS-34, 35 Absorption and titration Specific gravity
Continuous Continuous Evety 3 days Continuous Every 3 days Daily Continuous Daily Continuous Daily Daily As required As required As required As required As required As required As required Daily Daily As required As required As required
NHa activity Bivalent copper Cu +,Cu +,COX,"a, HCOOH Bivalent copper Bivalent copper Bivalent comer Bivalent copper Bivalent copper Bivalent copper Bivalent copper Bivalent cop er C u + + C u + 80, NHs HCOOH Cu Cu COz: KHs: HCOOH
Electrical conductivity* Photometric* Gravimetric and volumetric Volumetric Photometric* Volumetric Photometric* Volumetric Photometric* Volumetric Volumetric Gravimetric and volumetric Gravimetric and volumetric
Continuous Continuous Daily As required Continuous As required Continuous As required Continuous As required As required As required As required
Hi Nz, Hz, CHI Hz, NHa, A Nz -I-A, Hz, CHI N Ha NHa NHs, A
Nz f 4 ,
+
+
CS-2w CS-3s cs-4s cs-5s CS-65 cs-7s
Daily Every 8 hours or as required Every 2 hours Every 2 hours Every week
As required Continuous Daily Continuous Continuous Daily (composite)
N t , HI, COz, CO, CH4, 01
"8
Gas to ammonia-cooled condenser Bleeder gas
Volumetric Orsat Thermal conductivity Orsat Volumetric Orsat Thermal conductivity Orsat Orsat Thermal conductivity Orsat Orsat
co, con co, cot, 0 2 co coz
"8
SS-27, 28 69-29, 30
Orsat Orsat
Frequency of Determination
Gas PLANT.Coke is analyzed for carbon, sulfur, and ash; the fusion point of the ash also is determined. Boiler water (GS-13) is analyzed in connection with the treatment to minimize boiler corrosion. Raw gas is produced by blowing steam first up and then down through a hot bed of coke until the bed cools; air is then blown through the bed until the coke reaches the proper temperature for the repetition of the steam cycle. (A portion of the gas produced on the air blast is used to adjust the nitrogen content of the raw gas.) Gas samples from the various phases of the cycle, up-steam, back-steam, and blow runs, are analyzed (GS-1, 2, 3, and 4) for hydrogen, nitrogen, carbon dioxide, carbon monoxide, methane, and oxygen in order to check the quality of the gas produced. The raw gas is passed through a water scrubber, where it is cooled and part of the dust and coke breeze is removed, and then
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through a soda ash scrubber to remove the hydrogen sulfide produced as a by-product in the as production cycle. The gas entering (GS-5) and leaving (Gb-6) the hydrogen sulfide absorber is analyzed for hydrogen sulfide to determine the completeness of removal. After absorption of the hydrogen sulfide, the soda ash solution is regenerated by blowing air through it in the two actifier towers. Adjustment of the composition of the solution is controlled by the analysis of samples taken a t the entrance (GS-7) and the exit (GS-8) of the actifier towers. The raw gas from the hydrogen sulfide absorbers contains considerable dust, most of which must be removed to prevent excessive clogging of the hydrogen plant catalyst beds. This dust is removed by passage through a coke filter. Samples taken a t the entrance (D-5) and the exit (D-6) of the coke filter and a t the entrance to the hydrogen system (D-8AB) are analyzed for dust as a check on the operation of the filter and of the gas generators.
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IIYUROGEN PL&NT.
:n ratio in the raw #peroperation of tho final adjustment t,o a ratio of 3 to 1 is maae after m e gas nas passed through the riiw gas holder on ita way to the hydrogen plant by adding the required %mount of blow gas (GS-12), which is high in nitrogen. This blow gas is obtained from tho blow gas holder in which part, of the gas produced on thc blow runs at the gas generators is storod. The amount of blow gas to be added is determined by continuous analyses for hydrogen a t several points in the hydrogen and synthesis systems (HS.5, 6, 7, 8, 15, and 17; SS-19, 20, 21, and 22). After adjustment of the hydrogen-nitrogen ratio, the raw g:w i8 passed through hot, watcr in t,hr saturator tower whore it is diluted by about one vdume of steam per volume of gas (HS-!? and 10). Tho ratio of stcam to gas i s then further increased ti, about 3 to 1 by the uddiiian oi the proper amount of live stc;un. Tho steam-gas ratio is determined frequently (HS-1, 2, 3,and 4 ) , since rapid fluctuations may result from changes in gas flow I.IIIU or steam prossuvc. Tho stoam-gas mixture then passcs succcsvively through Lwo estslyst chambers in which the stea.m and carbon monoxide react to form hydrogen and carbon dioxide. The convcrted gases ieaving the primary (HS-11, 12, 13, and 14) and tho sccondssy (HS-5, 6, 7, and 8 ) convert carbon monoxide and carbon dioxide to determine the operating lrffieioncy of the catalyst. P U R I F I C ASrsmnr. T ~ ~ N The converted gas leaving the hydrogen plant contains large amounts of carbon dioxide and some nnconvertcd curhon monoxide, both of which must bo romoved. Must of tho carhon dioxidc is removd by passing t,he gas under prcnsure through a'water sornbber. The earhon dioxidc content of the effluent gas (PS-13 and 14) is detonnincd continuously, and the results arc used in control oi' ilio amount of water pumped to the serubbcrs. The watw from ihe sorubhers is ideased t o ntmospherie pressure in a breakdown chambar, and tho gas released (1's-19)isanalyzed for carhon dioxide, hydrogen, and nitrogen to determine the amount of hydrogen and nitrogen last by solution in the scrubbing vatcr. After passing through tho xmter scrubber, the gas is further compressed and is passed through the copper scruhher, where the unconverted carbon monoxide and residual carbon dioxide are removed by absorption in ammoniacal copper solution. Since it is very important that no appreciable amount of tlic oxides of carbon enter the synthesis system, tho gas from tho copper scrubbcr is analyzod continuously far these oxides a t several points (PS15, 16,17, and 18; SS-19 a,nd 20). The gas cntcriug thc srruhher gas is maintained sligl gas-,.uuu,u.vL. -fcle :
SYNTHESIR S Y h m . Gases in t.hc synthesis system cirnll:itr bhrnngh a eonvertcr, ~!-hcrepart of tho hydrogen-nitrogen misto ammonia.; through two condensers, ~ h o r c onia produccd is liqncfied and romoved Irom through n filter, here the make-up gns in,m the purification systcm i s nddod. The circulating gas is annlysed eontinuowly (SS-21, 22, 23, 24, 25, and 2fi) to check f l i c cflicieney ai tho convcrtcr and eondcnsors. Argon hiid nwthmc, minor c of the raw xiis, I ~ C iiot romovod in any of Lhc plan and their concent,ruas. Thcreforc. aifitjon builds up in the cirenlat,ing rient gas in hlcd to thc atmosph thc eoncentrathi o r tlicse inert gases bclow n fixed valuc. :Inalyses for arnorr mal mothano arc mado (99.21, 22, 25, 26. and 30) to dct.mminc fhc amount of gas to ho hlcd. Thc amnionia liqueficd in tho condcnscrs collects i i i sopar:i~ors and ~ O C Rinto storam tanks. halyscs for oil and wntor are I I I R ~ C (SS-32, 33, 34, m d 35) to dct,crminr thc quality of thc produel. COPPERllmmmn.
