CA1258044A - Removal of corrodants from nmp solvent by contacting with sacrificial metal - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Corrosion of the process equipment in solvent extraction units utilizing N methyl-2-pyrrolidone as the selective extraction solvent is significantly reduced by means of a sacrificial metal contacting zone or bed containing a metal or metal alloy with a higher electro chemical potential than the metals used in the extraction unit vessels and lines. Preferred sacrificial metals are magnesium, zinc, calcium, barium, and strontium, most preferably magnesium, as metal strips, shavings, ribbons, sponge, filings, chips, blocks, bricks, etc.

Description

Selective N methyl-2-pyrrolidone (NMP) 3 _solvent extraction processes are plagued by process 4 equipment corrosion problems. It has been discovered that this corrosion can be significantly reduced by 6 contacting the NMP streams with a sacrificial metal in 7 a contacting zone or bed containing a metal or metal 8 alloy which possesses an electrochemical potential 9 higher than tnat of the metal used in the construction of the process equipment, including reaction/extraction 11 vessels, solvent recovexy vessels, solvent handling 12 lines, cooling vessels, etc. Preferred sacrificial 13 metals for use in those process plants wherein the 14 processing equipment is constructed of carbon steel or stainless steel are magnesium, zinc, calcium, barium, 16 strontium, preferably ma~nesium. These sacriEicial 17 metals may be employed in any convenient Eorm including 18 bars, rods, ribbons, strips, shavinys, sponge, ilings, 19 chips, donuts, beads, nodules, blocks, bricks, sheets, etc. The sacrificial metal can be inserted into -the NMP
21 recovery stream at any convenient location, preferably 22 at a point in the recovery stream wherein said stream 23 is predominately NMP. The sacrificial metal can even be 24 inserted as a large solid block or sheet in the flash zone of a tower at turnaround intervals. No special 26 precautions need be taken as to the conditions under 27 which the NMP stream is contacted with or passed over 28 or through the sacrif icial metal. It is pre~erred, 29 however, that the sacrificial metal be located at a poi~t in the solvent recovery stream wherein the 31 temperature of the NMP recovery stream is elevated, 32 about 250 to 600F, preferably about 400 to 600F.

33 Consequently, the preferred location for placement of 34 the sacrificial metal is in the NMP recovery overheads ,'. ~

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stream wherein the temperature of the stream is about 36 525F, and the NMP stream is preferably in the form of 37 a vapor just starting to condense.

39 Figures 1 and 2 constitute schematics of typical NMP solvent extractive plants showing the 41 various vessels and lines constituting such plants.
42 Those places where sacrificial metal contacting zones 43 or ~eds can be advantageously located are identified by 44 A-F and A-C respectively in the figures. One or more of such zones or beds can be used as required. Preferably, 46 the sacrificial metal will be located at the p.oint 47 designated A in Figure 1, the flash tower overhead 48 stream. Second choice would be locating a sheet oE the 49 sacrificial metal in the flash zone oE the drier tower or the high pressure Elash tower.

51 DETAI~ED DESCRIPTION OF THE FIGURES

52 Figure 1 presents a schematic of NMP recvvery 53 from extract in a steam stripped plant. Extract from 54 the extraction process is fed via line (1) to drier

(2). It has been preheated in heater (3) by indirect 56 heat exchange therein with dry solvent in line (4)~ In 57 drier (2) the extract is dewatered yielding an over-58 heads fraction, line (5), consisting primarily of water 59 (which is eventually recombined with the NMP for use in the extraction zone [not shown]) and an extract solvent 61 fraction, line (6). The extract from drier (2) in line 62 7 is heated by means of heat exchange, in unit (8) with 63 dry solvent overheads in line (4). Extract/ solvent 64 from the drier (2), via line (6), is passed through a heater (furnace 9) and sent to flash tower (lO) wherein 66 the solven~ is ~lashed off as overheads (line 4) and 67 the extract is recovered via line (ll) and sent to a 68 stripper ~121 wherein any residual solvent is stripped 69 off using steam (line 13). The residual solvent is recovered from the stripper (12) via line (14) for 71 recycle to the solvent recovery proc:ess while the 72 extract product is recovered via line (lS).

73 In this steam stripping scheme the 74 sacrificial metal contacting zone or bed can be located at a number of sites. In the experiment presented below 76 the sacrificial metal was located at site (A) on the 77 dry solvent overheads line (line 4 in the figure) from 78 the flash zone. At location (A) the steam is in the 79 vapor form at about 525F. Alternate locations are site ~ ~B) in the drier at the heated extract/solvent stream 81 inlet for line (7), wherein the streasn is in the 82 vapor/liquid form at about 450F. Site (C) is in the 83 flash tower at the solvent inlet wherein the stream is 84 in the vapor/liquid form at about 600F. Site ~D) is in the overheads line (14) from the stripper wherein the 86 stream is in the vapor form at ahout 400F. Site ~E) is 87 on the overheads line (5~ from the drier wherein the 88 stream is in the water rich vapor form at about 250F.
89 Site ~) is on the extract/solvent feed line ~l) ~leading to the drier) wherein the stream is in the 9l liquid form at about 390F.
.

92 Figure 2 is a schematic of NMP recovery from 93 extract in a gas stripped plant. Extract/solvent stream 94 in line ~lO) passes through exchanger (20) wherein it is heated by indirect contact with dry solvent in line ~30) 96 coming from the rec~ifier (40). The heated extract/
97 solvent from heater (20) is sent via line ~50) to ~urnace 98 heater (60) and thence via line(70) to rectifier (40).

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g9 From the rectifier dry solvent is recovered via line 100 ~30)and an ex~ract/solvent stream is recovered via line lOl (80) and sent to the stripper (90). In stripper (90) a 102 stripping gas stream (linellO) is used ~o s~rip off 103 residual solvent which is sent via linello back to the 104 rectifier. An extract product stream is recovered via 105 line (120) from the stripper. In a gas stripped plant 106 the sacrificial metal can be preferably located at site 107 (~) in the dry solvents overhead line (line ~ of the 108 figure) from the rectifier, wherein the stream is in 109 the vapor form at about 525F. Alternatively, the 110 sacrificial metal can be at site (~) in the rectifier 111 at the hot extract/solvent inlet wherein the stream is 112 in the vapor/liquid form at about 600~ or at site (I) 113 on the extract/solvent line (line 50) leading to the 114 furnace haater wherein the stream is in the liquid 115 state at about 480F.

116 ÆXAMPLE

117 At an NMP solvent extraction plant, all cool-118 ing water exchangers were repaired or replaced to elim-119 inate water in leakage. A test bed of magnesium chips 120 in a 6 inch diameter by 2 foot long vessel was 121 installed on a small slip stream of hot NMP vapors 122 (~525F) coming from the solvent flash tower overheads 123 (site A in Figure 1). The NMP vapor was permitted to 124 contact the magnesïum chips for a number of days such 125 that the total volume of NMP flowing over the magnesium 126 bed was at least twice the inventory of WMP in the 127 system, after which time the test bed was opened and 128 examined. It was observed that a large portion of the 129 magnesium had been consumed. Measurement of pH of the 130 circulating NMP before and after the magnesium bed was 131 installed revealed an increase of about 1 to 1.5 pH

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132 units, presumably resulting from removal of strong 133 acids which had built up over several months and were 134 recycling in the NMP stream. The magnesium salts 135 produced were presumably withdrawn from the system in 136 the extract product and not recycled. Corrosion of the 137 vessels and lines making up the plant ceased.

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A method for arresting the corrosion in selective N-methyl-2-pyrrolidone (NMP) solvent extraction process plants by use of a sacrificial metal of a higher electrochemical potential than that of the metals used in the construction of the plant, wherein the NMP solvent stream is contacted with said sacrificial metal.

2. The method of claim 1 wherein the sacri-ficial metal is selected from the group consisting of magnesium, zinc, calcium, barium, and strontium.

3. The method of claim 2 wherein the sacri-ficial metal is magnesium.

4. The method of claims 1, 2, or 3 wherein the NMP is contacted with the sacrificial metal at a temperature of about 250 to 600°F.