CA1206323A - Process for sulfuric acid regeneration - Google Patents

Abstract

TITLE
IMPROVED PROCESS FOR SULFURIC ACID REGENERATION
ABSTRACT OF THE DISCLOSURE
Process for regenerating spent acid is improved by controlling oxygen content in the pyrolyzing furnace.

Description

3~3 TITLE
IMPROVED PR~:ESS FOR SULFURIC A~ID RE~EME:RATIC)~
FIEI.D O~ THE I2~VENTION
This invention relates ~o an improvement in 5 the process or regener~ting spent sulfuric acid after oleum has been u~ed in reaction processes.
~ACKGROUND OF THE INVEN ION
S~lfuric acid con~aining sulfur trioxide is useful an ~ number of commercial reactions. For 10 example, it is used in alkylation of hydrocarbons, in nitration processes for dehydration, and in the preparation of methyl methacryla~e, All of these proce~ses employ sulfuric acid corltaining sulfur trioxide (called oleum hereinaf~er~ and in all of 15 these proce~ses the oleum b~comes d~pleted or "spent"
and need~ ~o be reyenerated~ Th-ls in the prepara~ion of methyl methacrylate, oleum, acetone cyanohydrin (ACN~ and methanol are reacted in a two-step procedure to form a mixture of methyl methacrylate, 20 ammonium bisulfate and excess ~ilute sulfuric acid.
The methyl metha~rylate is removed, and the mi~ture of ~mmonium bisulfate and excess dilute sulfuric acid (the mixture is called ~pent acid~ is regenerated to produce more oleum. The spent acid is pyrolyzed to 25 form a mixture o~ g~seous oxides, includi~g ~ulfur dioxide; the ulfur dioxide is then oxidized to sulur tribxide which is absorbed in concentrated ~ulfuric acid o form oleum. The oleum thus regenerated is recycled ~or use in the alkylation, 80 nitration or methacrylate preparations referred ~o previously~
This application pertains to an improvement in the regenera~ion of oleum in the above processes, as opposed to the use o oleum in the primary ~' ~2~2~

alkylation~ nitra~ion, or methacrylate preparati ons xeferred to above. In thi s Ire~e}~eration of oleum, considerable amounts of fuel and oxygen, added in the form of air, are fed to a furnace in which the 5 ammonium bi~ul~ate and dilute sulfuric acid are pyrolyzedO Inert gases (predominantly nitrogen) in the air entering the furnace are detrimen.al in that: (1) they add to the heat load because they must be heated to ~he pyrolysis temperature along with the oxygen present; ~2~ nitrogen forms nitrogen oxides in the hot oxidizing environment of the furnace, ~hereby creating ni~rogen oxide pollutants that must ultimately be discharged as part of the stack gas and generating ni ~er. a product contaminant which reduce-c tne yield of the resulting oleum produc ; ~3) th~y dilute the concen~ration of S02 in the converter (whexe SO~ is converted to S03), thereby limiting conversion of SO~ to the desired S03 and increasing S02 discharge rate to the atmosphere as a pollutant in the ~tac~ gas; ~4~ they limit the strength of the oleum which can directly be pr~duced; (53 they reduce ~he holdup time of the reactants in the converter~ for a given throughput ~rate, making it necessary o use larger ~olumes of 25 catalyst for the desired reac~ion; and ~6 j they cause a pressure drop in ~he equipment used in the regeneration proGess.
An object of the process improvement of this invention is to increase the capacity of a spent acid regeneration facility by reducin~ ~he mass and volumetric flow in the system.
SUMMARY OF THE I~VENTION
This object is achieved by operating the furnace much closer to its stoic~iometric balan~e 0~3 than is normally used in sul~uric acid regeneration (~R) ~urnace~0 ~y accurately measuring the oxygen concentration in the gases exiting from the urnace during pyrolysi~ of the spent acid with durable, fast-response oxygen analyzers and using a closed computer control system with a fast respon~e loop microprocessor the oxygen concentration can be controlled in the range of 0.1 to 1.0~ by total volume of gases leaving the furnace in~tead o~ the

2-5% range conventionally pres~nt. Since the spent acid varies both in quantity and in composition, the ; fast response control system also controls the temperature of the furnace by adju3tment of the fuel and "air" 10ws. If the oxy~en concentration in the lS gases leaving the furnace is below the stoichiometric balance, ~ulfur and carbon particles are formed which plug downstream equipment. On the other hand, hi~h concentrations of oxygen in gases e~iting the furnace reduce the ef iciency o~ the process and produce more 20 ~x and wast~ S03.
BRIEF DESCRIPTIQN OF rHE ~ANIIIOS
Figure 1 is a diag:ramatic flow shee~ of a .
prior art spent acid r~gen~ration (SAR) system;
Figure 2 is a schematic diagram of the 25 furnace control system and d~spict~ the system used to control oxygen ~o O ,1~1. 0~ by total volume of gases leaving the furnace;
I)ETAILED DESCRI PTION OF THE I~VENTION

As shown in Figure 1, the main elements of a known art sulfuric acid re~enera~ion process comprise a furnace 10, a waste heat boiler 12, a scrubber and dryer 14, a main blow~r 16 dr:Lven by ~urbine 18, a converter 20 followed by an oleum tower and absorber 22 and stack 24.

Spent acid is fed into furnace 10 through pipe 2~ while auxiliary fuel such as natural ga~ i5 injected through pipe 28 and air (sometimes referred to hereinafter as primary oxidation ga.s) is supplied 5 through pipe 30. The ~pent acid is usually sprayed in through a number of nozzles surrounding the f lame created by the burning fuel. Combustion takes plac~
in furnace 10 and the furnace gases, which consi~t primarily of C0~, H~0, S02, S03, oxides of 10 nitrogen (N~3x3 ~ 2 and N;2, at el~vated temp0rature, exit ~hrough pipe 32 to waste heat boiler 12, ~here the furnace gases are cooled~ The ~urnace gase~ then pa~s through pipe 34 to scrubber-dryer 14 which has secondary air ~sometimes referred to hereinafter as secondaxy oxidation ~as~ supplied through pipe 35. In ~he scrubb~r-d~yer, particula~e matter and wa~er are removed, Cooling water (C.W.) circulates through pipe 36. The dry gas product exits the ~crubber-dryer ~hrough pipe 37. The gas is driven by main blower 16, and forced b~ way of pipe 38 through heat interchangers into converter 20. In converter 20, the S02 in the gas is oxidized to S03 in the presence of a catalyst. ~03 from converter 20 is conveyed thr~ugh pipe 40 and more heat exchangers (not shown~ to oleum tower and : ab~orber 22. Cooling water ~C.W~) is ~upplied through pipe 41~ In the tower and absorber, S03 i~
removed by absorption, first with concentrated H2S04 to form oleum, and then with lean acid~
i.eO, sulfuric acid of le~s than 98% concentration, for polishing (i.e., for removal of residual S03~.
Concentrated acid is partly recycled to ~crubber-dryer 14 via pipe 94 and returned (slightly diluted) via pipe 92 to tower and absorber 22.

Highly diluted acid generated in ~crubber 14 i~
discharged via pipe 96 carrying with it ~he water removed from the furnace gas and the secondary oxidation air, the S03 produ~ed in the furnace 10, and ash from corrosion product30 Fresh wa~er for absorbing the convertor produc~ S03 if needed to adjust acid/oleum ratio is added to the towex and absorber via line 98. Product sulfuric acid and/or oleum i5 removed through pipe 42 and stack gases are removed through pipe 44. The wa~e heat boiler 12 generates 6team which exits through pipe 46 with a portion supplied to turbine 18 and excsss steam, which can be usea or sther parts of the process or be exported, exi~s through pipe 4B.

to ~ 1,0~
Heretofore, conventional spent acid recovery (SAR) furnaces ordinarily did nok have ~ophisticated controls. Gas temperature.s within and leaving the ~ furnace were measured with thermocouples and the flow of auxiliary fuel (CUch as natural gas) was modulated by an automatic valve ~o conkrol the furnace outlet temperatuxe. Flow of ai~ ~primary oxidation gas3 and furnace pressure were contEolled manually by adjusting air inlet throttle valve and thP speed of the main blower. Flow of sp nt acid per spray nozzle was essentially fixed for optimum atomization, and overall spent acid flow was changed by manual addition o:r removal of spray lances. Re~idual oxygen in gas leaving the furnace (from excess primary oxidation gas) should be maintained high enough to compensate for variations in the composition o spent acid.

~ If the oxygen content o the air fed to the -: ~urnace drops below the stoichiometric balance, carbon particles and sulfur vapors are ormed which ; plug down~tream equipment. Since the spent acid entering the furnace varies both in qliantity and concentration, conventional pxactice has been to maintain the furnace exit gas oxygen concen~ration in the range of 2 to 5% by volume to insuse maintaining sufficient oxygen levels in the furnace. This corresponds to about a 20-50~ excess o~ air above that neede~ for stoichiometric balance of oxygen in the r~action. It has now been found that ~he efici~ncy of the proces~ can be improved by maintaining the oxygen COnCen~iatlOn in the furnace exit gas to a range of 0~1 to 1~0% ~y volume. This correspond~ to about a 1-10~ excess of air above that needed for stoichiometric balance of oxygen in the reaction, The temperature of the furnace should be in h~ range of 850C to 1150aC.
In the improved embodiment shown in Figure 2, gas feed entering the furnace is automatically and continualiy adjusted to compensate for variation~ in the composition of spent acidr thereby making it possible to operate with muC~ lower amounts of residual oxyg~n in the gas leaving the furnace than heretofore used, without risking plugging the system with soot and sulfur. Such careful control cannot be based on temperature alone but is directly dependent on gas COmpGsition as well.
Figure 2 shows the furnace control system of this invention. 0n pipe 2~ is mount~d control valve 50 and spent acid flow measuring device 52 with flow i~dicator 54. Correspondingly moun,ed on pipe 30 is a control valve 56, oxidation gas flow measuring &

~2~3;23 device 58 and ~low indicatt~r 60. A190 on the fuel ~upply pipe 28 i3 c:orltrol valve 62, a ~low measurirlg d~vice 64 and a flow indicator 66~ On pipe 32, through which the furnace gas exits, is a temperatllre 5 measuring device 68, and an oxygen analyzer 70 with associated measurirlg devices 72 and 74~ Temperature indicator 76 indicate~ the temperature in pip~ 32 and i5 a~sociated with temperature controller 78. All of th~se unit~ are connecte~ ~o microproc~ssor 80.
The contxol system shown in Figure 2 can accura~ely a~d with fast response control the temperature in furnace 10 as well as the concentration of oxygen leaving the furnas:eO In operation, temperature mea~uring device 68 and temperature controller 78 con~rol the valve 62 and thus determine the amount of fuel that is supplied through pipe 28. Temperature measuring device 68 is one of a pair of thermocouples. Temperature indicator 76 i~ a multipoint strip ~hart null balance 20 recorderO ~emperature con~roller 78 i a proportional controller with reset. Control valve 62 is a pneumatic air to~open valve with positioner.
Temperature controller 78 supplies data to mini eomput~r 80 with respect to the temperature of the fuxnace gases. In order to maintain the oxygen concentration in the exit gas at the desired low . percent range, two or more oxygen sampling devices, 72 and 74, feed information to the oxygen analyzer 70~ Backup oxygen samplinq devices are preferred.
Further, two different Xinds may be used. For example, oxygen sampling devic~ 72 can be a zirconium oxide semiconductor type and oxygen sampling device 74 can be a paramagnetic t~peO Information from the oxygen analyzers, temperature recorder and flow ;

~,.2~
meters are all fed to micro processor 800 Micro proces~or 80 analyzes the .in~ormation and responds by I resetting flows to maintain preprogrammed boundaries j o~ temperature and oxygen concentrations.
The furnace operate~ more efficiently as less oxygen is discharged with the furnace gas. This improves the overall efficiency of he spent acid recovery proce~s. Further, presence of excess oxygen re~ults in increa~ed formation of oxides o nitrogen which form impuri~ies in the acid and cause pollution when they are emitted ~rom the ~tacX. In addition, heating this excess o~ygen and i~s a~ociated nitrogen requi-es more fuel and combustion air.
Example and Com~arison 1~ The following example illustrates the improvement in furnace effectiYeness (~uel consumption) and reduction in the amount of inerts present in `the furnace exit gas, In the Example, the numerical values obtained were obtained by simulating the proc~ss in a computer, and makiny parameter calculations from the simulation results.
Comparative Example A exemplifies use of a co~ventional SAR furnace (Figure 1) with operating conditions and outputs as detailed in Table I.
Example 1 shows the improvement when the o~yg n concentration leaving the furnace i~ only 1%
by volume while u~ing atmospheric air as the primary oxidation gas in the spent acid urnaceO

g TABLE I
'~:
A
1. Temp. of Spent acid supplied to furnace, "C80 80 Temp. of primary oxidat:ion gas supplied to furrlace, C 677 677

3. % 2 (by volume) in the p~imary oxidation gas io ( air) supplied to Eurn,ace 21 ~1

4. StacX gas recycled, % ~
volume - -

5. Concentration of 2 i n 2 furnace exit ~a~; ~ % by Volume, wet basi.s)

6. Furnace Temp., C 1000 1000

7. Composition of ~urnace gas exit ( ~6 by volume dry basis) S2 11~1 12~3 so3 g~.2 0.1 2 3.1 1.7 ~2 75- g 75.4 C2 9.7 10.5 25 8~ R~lative Ratio of N2 to /SQX 1 . 0 0 . 9 9. Relative ratio of total inerts (N2 ~ C02) in furnac:e exit~gas to uni~ of H2SO4 produced 1. 0 0 . gl 10. Relative flow rate of gas , leavins furnace 1~0 - 0.74 11. Re lat i ve ~ uel consumpt i on1 . 0 0 0 4 9 3 12 ~ Re lati ve load of N0x prodused in furnace 1.0 0. 66

Claims

CLAIM

1. In the process for producing oleum from a spent acid mixture or ammonium bisulfate and dilute sulfuric acid which process comprises pyrolyzing the mixture by feeding spent acid, fuel and air into a furnace to obtain gaseous oxide products including SO2; oxidizing the SO2 to SO3 with oxygen; and absorbing the SO3 in concentrated sulfuric acid to form oleum: the improvement comprising:
maintaining the oxygen concentration leaving the furnace after pyrolysis of the spent acid at a level of 0.1 to 1.0 percent by total volume of gases exiting the furnace while maintaining the temperature in the furnace between 850 and 1150°C.