Process for the recovery of sulfuric acid

Abstract

Process for the recovery of sulfuric acid comprising: (a) contacting a stream containing sulfuric acid vapor and/or sulfur trioxide in an absorption stage with a ionic liquid absorbent,(b) withdrawing a gas substantially free of sulfuric acid vapor and/or sulfur trioxide from said absorption stage,(c) withdrawing a stream comprising the ionic liquid absorbent from said absorption stage, in which said ionic liquid absorbent contains sulfuric acid,(d) recovering sulfuric acid from said ionic liquid absorbent by passing the stream of step (c) through a separation stage,(e) withdrawing from the separation stage (d) a stream rich in sulfuric acid,(f) withdrawing from the separating step (d) a ionic liquid absorbent stream in which said ionic liquid absorbent contains sulfuric acid and returning said stream to the absorption step (a).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram of the process according to the invention.

FIG. 2 shows a schematic of the apparatus for recovery of sulfuric acid from the absorbent after the absorption stage.

DETAILED DESCRIPTION

The process is illustrated in FIG. 1. A feed stream 1 is introduced at the bottom of a packed counter flow tower in which the acid is absorbed by the ionic liquid at close to atmospheric pressure. The stream of ionic liquid with the absorbed sulfuric acid 3 is sent to an adiabatic flash vessel 11 in which the acid is subsequently flashed out of the ionic liquid mixture and condensed as stream 4 under reduced pressure (vacuum) 12. The hot stream of ionic liquid 5 from the flash is subsequently cooled into stream 6 in cooler 13 before being returned to the absorption tower 10. The transfer line carrying stream 3 operates under the pressure of the absorption. The process has two product streams: the cleaned gas 2 which contains less than 5 and preferably less than 1 ppmv sulfuric acid and the sulfuric acid stream 4 with 97% preferably above 99 wt % acid concentration.

Referring now to FIG. 2 a vacuum chamber 11 for the recovery of sulfuric acid from the absorbent is shown, in which a stream 3 containing the absorbent is introduced via inlet 15 into a bed of inert contact material 16 confined within a basket 17 and spaced apart from the cooled surface (wall) 18 of the vacuum chamber so as to leave a free annular space 19 through which evaporated acid from the bed travels, as depicted by the arrows to the wall of the vacuum chamber. The wall is cooled by jacket 20 having corresponding inlet 21 and outlet 22 for the passage of the cooling medium for instance water. The chamber is subject to reduced pressure through vacuum arrangement 23. Below the bed of inert contact material 16 a collection zone 24 having outlet 25 is provided for the collection and withdrawal of a liquid mixture containing the bisulfate salt together with the acid that has not evaporated, and which is returned to the absorption stage. The sulfuric acid condensed in the wall of the chamber falls as a film and is collected at the bottom of the chamber where it is withdrawn through outlet 26. The vacuum chamber may also be provided with an inverse-V shaped shield 27 at the upper part of the chamber (above the bed of inert contact material) and acid demister 28 adapted to the vacuum arrangement 23 in order to prevent any slip of acid mist during operation of the apparatus.

EXAMPLE

The stream numbers correspond to those of FIG. 1.

Two cases are considered both of which assumes 100,000 Nm³/h feed stream to the absorption stage at 350° C. In case 1 the gas contains 6 vol. % SO₃ and 10 vol. % water and in case 2, 15 vol. % SO₃ and 20 vol. % water. Table 1 summarizes the temperatures and compositions of the streams in the two cases.

	TABLE 1
	Case 1
	100.000 Nm3/h containing the
	equivalent of 6 mole % SO3 and
	10 mole % H2O (6 mole %
	H2SO4 and 4 mole % H2O)
	is received at 350° C. and the
	cleaned gas is delivered at 150° C.
	Pressure in stream 4: 0.1-0.5 mbar
	Stream
	2	3	4	5	6
Temperature	150	250	<250	210	150
(° C.)
Mass flow	—	326	26	300	300
(ton/hr)
H2SO4 (wt %)	—	40	>99	34	34
H2SO4 (ppmv)	1	—	—	—	—
Water content	—	<0.1	<1	—	—
(wt %)
Water content	4	—	—	—	—
(vol. %)
NH4HSO4 (wt %)	—	60	<0.1	66	66
	Case 2
	100.000 Nm3/h containing the
	equivalent of 15 mole % SO3 and
	20 mole % H2O (15 mole %
	H2SO4 and 5 mole % H2O) is
	received at 350° C. and the cleaned
	gas is delivered at 150° C. Pressure in
	stream 4: 0.1-0.5 mbar
	Stream
	2	3	4	5	6
Temperature	150	250	<250	210	150
(° C.)
Mass flow	—	665	65	600	600
(ton/hr)
H2SO4 (wt %)	—	40	>99	34	34
H2SO4 (ppmv)	1	—	—	—	—
Water content	—	<0.1	<1	—	—
(wt %)
Water content	5	—	—	—	—
(vol. %)
NH4HSO4 (wt %)	—	60	<0.1	66	66

Claims

1. Process for the recovery of sulfuric acid comprising: (a) contacting a stream containing sulfuric acid vapor and/or sulfur trioxide in an absorption stage with a ionic liquid absorbent, wherein the ionic liquid absorbent comprises a mixture of bisulfate and sulfuric acid, said bisulfate comprising at least one of the salts from the group R3NH HSO4, LiHSO4, NaHSO4, KHSO4, RbHSO4, CsHSO4, where R═H—, alkyl, aryl and wherein the content of sulfuric acid in the mixture is 5 to 90 wt %,(b) withdrawing a gas substantially free of sulfuric acid vapor and/or sulfur trioxide from said absorption stage,(c) withdrawing a stream comprising the ionic liquid absorbent from said absorption stage, in which said ionic liquid absorbent contains sulfuric acid,(d) recovering sulfuric acid from said ionic liquid absorbent by passing the stream of step (c) through a separation stage,(e) withdrawing from the separation stage (d) a stream rich in sulfuric acid,(f) withdrawing from the separating step (d) a ionic liquid absorbent stream in which said ionic liquid absorbent contains sulfuric acid and returning said stream to the absorption step (a).

2. Process according to claim 1, wherein the ionic liquid absorbent further comprises pyrosulfate in amounts not exceeding 5 wt %.

3. Process according to claim 1, wherein R═H.

4. Process according to claim 1, wherein the temperature of the absorbent in the absorption step (a) is in the range 100° C. to 350° C.

5. Process according to claim 1, wherein the stream containing the ionic liquid absorbent in step (a) is introduced in a column packed with inert bodies or in a sprayed tower.

6. Process according to claim 1, wherein the separation stage (d) is conducted by distillation under reduced pressure.

7. Process according to claim 6, wherein the distillation is conducted by passing the stream containing the ionic liquid absorbent through one or more vacuum chambers containing a bed of inert contact material and which are externally cooled.

8. Process according to claim 1 wherein prior to step (a) a feed gas containing SO2 passes through at least one catalytic bed in which SO2 is converted to SO3 to form said stream of sulfuric acid vapor and/or sulfur trioxide, and the stream rich in sulfuric acid of step (e) is passed to a subsequent catalytic bed in which SO2 is converted to SO3.

9. Apparatus for the recovery of sulfuric acid from a stream containing a mixture of sulfuric acid and additives in which said apparatus is a vacuum chamber, wherein said mixture containing the acid passes via an inlet in the vacuum chamber through a bed of inert contact material arranged within the chamber and from which at least part of the sulfuric acid in the mixture evaporates, wherein the evaporated acid leaves the bed towards a cooled surface of the chamber and is condensed as a concentrated sulfuric acid stream and is withdrawn from the chamber through a first outlet, and wherein a mixture of sulfuric acid and the additives in substantially liquid form is collected from the bed and withdrawn from the chamber through a second outlet.

10. Apparatus according to claim 9 , wherein the mixture of sulfuric acid and additives is a ionic liquid absorbent which comprises a mixture of bisulfate and sulfuric acid, said bisulfate comprising at least one of the salts from the group R3NH HSO4, LiHSO4, NaHSO4, KHSO4, RbHSO4, CsHSO4, where R═H—, alkyl, aryl, in which the content of sulfuric acid in the mixture is 5 wt % to 90 wt %.

11. Use as a ionic liquid absorbent of a mixture of bisulfate and sulfuric acid, said bisulfate comprising at least one of the salts from the group R3NH HSO4, LiHSO4, NaHSO4, KHSO4, RbHSO4, CsHSO4, where R═H—, alkyl, aryl, in which the content of sulfuric acid in the mixture is 5 wt % to 90 wt %.