JPS59146914A - Improvement on sulfuric acid regeneration - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved method for regenerating spent sulfuric acid after using oleum in a reaction process.

Sulfuric acid containing sulfur trioxide is useful in many commercial reactions.

For example, it is used in the alkylation of hydrocarbons, in nitration processes (for dehydration purposes), and in the production of methyl methacrylate.

All of these processes use sulfuric acid containing sulfur trioxide (hereinafter referred to as "oleum"), and in all of these processes, the oleum becomes depleted or "spent" and must be regenerated. Specifically, in the production of methyl methacrylate, oleum, acetone cyanohydrin (AON), and methanol are reacted in a two-step process to form a mixture of methyl methacrylate, ammonium bisulfate, and excess dilute sulfuric acid. The methyl methacrylate is removed and the mixture of ammonium bisulfate and excess dilute sulfuric acid (this mixture is called waste acid) is regenerated to produce additional fuming sulfuric acid. The waste acid is thermally decomposed to produce a mixture of gaseous oxides including sulfur dioxide, which is then oxidized to sulfur trioxide, which is absorbed into concentrated sulfuric acid to produce fuming sulfuric acid. The thus regenerated fuming sulfuric acid is recycled for use in the aforementioned alkylation, nitration or production of methacrylic acid esters.

The present invention uses fuming sulfuric acid for the above-mentioned main alkylation, ditocolation or methacrylic acid ester production;
The present invention relates to improvements in the regeneration of fuming sulfuric acid in the process. In this oleum regeneration, a considerable amount of added oxygen and fuel in the form of air is fed to a furnace in which ammonium bisulfate and dilute sulfuric acid are pyrolyzed. Inert gases (mainly nitrogen) in the air entering the furnace: Harmful in the following ways: That is, (1) they are necessarily heated to the thermal decomposition temperature together with the coexisting oxygen, increasing the heat load. (
2) Because nitrogen forms nitrogen oxides in the high-temperature oxidizing environment of the furnace, it produces and produces nitrogen oxide contaminants that ultimately need to be exhausted as part of the stack gas. It produces niter (ni-tθr), an impurity in the product that reduces the yield of the fuming sulfuric acid product.

(3) They dilute the concentration of SO2 in the converter (where SO+ is converted to SO3) so that the desired 80
This limits SO2 conversion to 3 and increases the rate of SO2 emissions to the atmosphere as a pollutant in flue gas. (4) They limit the strength of the fuming sulfuric acid that can be produced directly. (5
) they have a given throughput)
t) Relative to speed, reducing the residence time of the reactants in the converter requires the use of a larger capacity of catalyst for the desired reaction.

and (6) they cause pressure drops in the equipment used in the regeneration process.

The purpose of the process improvement of the present invention is to increase the capacity of the waste acid production facility by lowering the mass and volume flow in the system.

The objects of the present invention are achieved by operating the furnace closer to its stoichiometric balance than is normally used in BAR furnaces. Accurately measure the oxygen concentration in the gas exiting the furnace during the pyrolysis of waste acid using a durable fast-response oxygen analyzer and using a closed computer control system with a fast-response loop microprocessor. By reducing the oxygen concentration to 0.1 of the total gas volume leaving the furnace, rather than the normally present range of 2 to 5
It can be controlled within the range of ~1.0%. Since the waste acid varies in both quantity and composition, the fast-response control system also controls furnace temperature by adjusting fuel and "air" flows. If the oxygen concentration in the gas leaving the furnace is lower than the stoichiometric balance, sulfur and carbon particles are formed that plug downstream equipment. On the other hand, if the oxygen concentration in the gas leaving the furnace is high, The process becomes less efficient and produces more NO□ and waste SO5.

As shown in FIG.
Converter 20. Subsequent fuming sulfate group and absorber 2
2 and a smoke exhaust pipe 24.

Waste acid is fed to the furnace 1o through line 26, while auxiliary fuel such as natural gas is injected through line 2 and air (hereinafter sometimes referred to as "primary oxidant gas") is fed through line 60. be done. The waste acid is typically introduced by atomization through a number of nozzles surrounding the flame created by the combustion proceeds. There is a smell of combustion in the furnace 1o.

and mainly CO2,)I20.802,
Elevated temperature furnace gas consisting of SO3, nitrogen oxides (Nox), o2 and N2 exits through line 32 and is sent to waste heat boiler 12 where the furnace gas is cooled.

Next, the furnace scum passes through a pipe 34, and a second air (hereinafter sometimes referred to as "second oxidizing gas") is supplied through a pipe 35. - sent to dryer 14; In this scaler bird dryer, particulates and water are removed. Cooling water (a, W,
) circulates through the piping filter 6. The dry gas product exits the scrubber dryer through line filter 7. The gas is advanced by main blower 16 and then enters converter 2o via piping 38, through a heat exchanger, and into converter 201g.
In the process, SO2 in the gas is oxidized to SO5 in the presence of a catalyst. SO3 from converter 2o is conveyed to fuming sulfuric acid tower and absorber 22 through piping 40 and more heat exchangers (not shown). Cooling water (C, W,) is pipe 4
1. In that oleum tower and absorber.

SO5 is removed by first absorbing in concentrated sulfuric acid to form fuming sulfuric acid and then in a poor acid, i.e., sulfuric acid at a concentration below 98%, for polishing (ie, removal of residual 803). The concentrated acid is partially recycled to the scrubber dryer via line 94 and returned (in a slightly diluted state) to the fuming sulfuric acid tower and absorber 22 via line 92. The highly diluted acid produced in the scrubber 14 is mixed with water removed from the furnace gas and secondary oxidation air.
The SO3 produced in the furnace 10 and ash from corrosion products are discharged through piping 96, entrained therein. It is added to the oleum tower and absorber through line 98 as needed for dewatering the sump absorbing converter product SO3 and adjusting the acid/oleum ratio. The waste heat boiler 12 generates steam, a portion of which is supplied to the turbine 18 and exits through piping 46, as well as excess steno steam that can be used in other parts of the process or removed from the system. exits through pipe 48.

In the past, conventional waste acid recovery (SAR) furnaces were usually not carefully controlled. The gas temperature in and leaving the furnace was measured with thermocouples, and the flow of auxiliary fuel (such as natural gas) was adjusted with automatic valves to control the furnace exit temperature. Air (primary oxidant gas) flow and furnace pressure were controlled manually by adjusting the air inlet throttle valve and main blower speed. The waste acid flow per spray nozzle was essentially fixed for optimal atomization, and the total waste acid flow was varied by manually adding or removing spray lances. Residual oxygen in the gas leaving the furnace (from excess primary oxidizing gas) should remain high enough to compensate for variations in spent acid composition.

When the oxygen species of the air supplied to the furnace falls below the stoichiometric balance, carbon particles and sulfur vapors are formed that plug downstream equipment. Since the waste acid entering the furnace is high in both the moon and the condensate, the furnace outlet gas oxygen concentration should be maintained in the range of 2 to 5 volumes to ensure sufficient oxygen levels are maintained within the furnace. This is what is normally practiced. This translates to approximately a 20 to 50 excess air over that required for the stoichiometric balance of oxygen in the reaction. Now the oxygen concentration in the furnace outlet gas is 0.
It has been found that the efficiency of the process can be improved by maintaining the volume in the range of 1 to 1.0. This corresponds to approximately a 1-10% excess of air over that required for the stoichiometric balance of oxygen in the reaction.

Furnace temperature should range from 850'C to 115C (111:1).

In the improved embodiment shown in FIG. 2, the gas feed entering the furnace is automatically and continuously adjusted to compensate for variations in the composition of the spent acid, thereby reducing the can be operated with much lower residual oxygen levels than previously used, without plugging the system with soot or sulfur. Such careful control cannot be based on temperature alone, but also directly depends on gas composition.

FIG. 2 shows the furnace control system of the present invention. Piping 2
6 is equipped with a control valve 5o and a waste acid flow measuring device 52 equipped with a flow rate indicator 54. Correspondingly, the piping filter 0 is equipped with a control valve 56°, an oxidizing gas flow rate constant device 58, and a flow rate indicator 6°. The fuel supply pipe 28 is also equipped with a control valve 62, a flow rate measuring device 64, and a flow rate indicator 66. A temperature measuring device 68 and an oxygen analyzer 70 with attached measuring devices 72 and 74 are attached to the pipe 62 through which the furnace gas exits. Temperature display 76
displays the temperature within the pipe 32 and is linked to the temperature controller 78. All these units are connected to microprocessor 80.

The control system shown in FIG. 2 is capable of controlling the temperature within the furnace 10 and the temperature of the oxygen leaving the furnace accurately and with fast response. In operation, temperature measuring device 68 and temperature controller 78 control valve 62 and thus determine the amount of fuel supplied through line 28 . The temperature measuring device 6 is one of a pair of thermocouples. Temperature indicator 76 is a multi-point strip chart zero balance recorder. Temperature controller 78 is a proportional controller with reset. The control valve 62 is a nitrogen pressure release valve with a positioner. Temperature controller 78 supplies data regarding the temperature of the furnace gas to minicomputer 8o. Two or more oxygen sampling devices 72 and 74 supply information to the oxygen decomposer 7o in order to maintain the oxygen fI% in the outlet gas in the desired low percentage range. A backup oxygen sampling device is preferred. Furthermore, two different types may be used. For example, the oxygen sampling device 72 can be of the zirconium oxide semiconductor type 1 and the oxygen sampling device 74 can be of the magnetic type. Information from the oxygen analyzer, temperature recorder and flow meter are all fed to the microprocessor 8o.

Microprocera f8oil''i' maintains preprogrammed boundary values for temperature and oxygen concentration by analyzing that information and resetting the flow.

Such furnaces work more efficiently when less oxygen is exhausted with the furnace gas. This increases the overall efficiency of the waste acid recovery process. Additionally, the presence of excess oxygen leads to increased formation of nitrogen oxides which form impurities in the acid and cause pollution when vented from the stack. Furthermore, heating this excess oxygen and associated nitrogen requires more fuel and combustion air.

The following example illustrates the increase in furnace efficiency (fuel droplets*) and the reduction in the amount of inerts present in the furnace exit gas.

The numerical values obtained in the examples were obtained by simulating the process on a computer 2 and calculating parameters from the simulation results.

Comparative Example A is a conventional SAR reactor (see Figure 1) with operating conditions and output as detailed in Table 1.
This is an example using .

Example 1 shows an improvement when atmospheric air is used as the primary oxidizing gas in a waste acid furnace, with the oxygen concentration leaving the furnace being only 1 volume.

Table 1 Comparative Example A Example 1 1. Temperature (u) of waste acid supplied to the furnace 80
804, Recirculated flue gas (volume - 111 quantity q6) 6. Furnace m degrees (℃) 1000 10DD
2 Composition of furnace gas outlet (volume f%, dry state standard) 802 11.1 12.3
so5 0.2 0.
1Q25,1 1.7 N275.9 75. , lI CO29,710,5 aN2/SOX relative ratio 1,0 0.9
10. Relative flow rate of gas leaving the furnace 1.0
0.7411, relative fuel consumption 100
0.9312, relative load of NOx generated in the furnace 1
．． 0 (1,66

[Brief explanation of the drawing]

Figure 1 is a process diagram of a conventional waste acid regeneration (SAR) system (
flow sheet). FIG. 2 is a schematic diagram of the furnace control system used in the present invention to control oxygen to α1-1.0% of the total volume of gas leaving the furnace. 10... Furnace, 70... Oxygen analyzer, 76... Temperature display, 78... Temperature controller, 8 Old... Microprocessor. FIG, 2 Waste acid

Claims (1)

[Claims] A waste acid mixture of ammonium bisulfate and dilute sulfuric acid is pyrolyzed by supplying the waste acid, fuel and air to a furnace to produce SO.
obtaining a gaseous oxide product containing 2, oxidizing the E302 to 803 with oxygen, and absorbing the BO5 in concentrated sulfuric acid to form fuming sulfuric acid; characterized in that the concentration of oxygen leaving the furnace is maintained at a level of 0.1 to 1.0% of the total volume of gas leaving the furnace, while the temperature inside the furnace is maintained at 850 to 1150 °C,
A method for producing fuming sulfuric acid from a waste acid mixture of ammonium bisulfate and dilute sulfuric acid.