ACID REGENERATION – ALKYLATION SPENT ACID

FEBRUARY 2002

FROM: SULPHURIC ACID TECHNOLOGY

1. Introduction

Alkylation of isobutane with C3-C5 olefins is generally considered to be the best method of producing high octane number gasolines for motor vehicles.  The classic reaction involves isobutylene and isobutane to produce trimethyl pentane (isooctane), which has a defined octane number of 100.

      CH3                 CH3                      CH3     CH3       |                      |                           |          |CH3-C=CH2 + CH3-CH-CH3    -->  CH3-C-CH2-CH-CH3                                                       |                                                       CH3

The above reaction is a catalyst based reaction that has been used throughout the world since the phaseout of lead based octane gasoline additives.  Concentrated sulphuric acid in the range of 88% to 92.5% H2SO4 is used as a catalyst for the reaction with approximately an equal amount of water and organics making up the remaining portion.

In theory, a catalyst promotes a chemical reaction and is not consumed or modified by the reaction.  In the alkylation reaction, the acid is diluted as a result of side reactions and feed contaminants.   The alkylation process ‘consumes’ approximately 0.18 to 0.27 kgs (0.4 to 0.6 lbs) per gallon of alkylate.  The loss of sulphuric acid is made up by fresh concentrated (98.5 wt%) sulphuric acid and a bleed stream of spent acid is removed from the process.  The spent acid stream undergoes a series of process steps to remove entrained hydrocarbons which are returned to the process with the spent acid being sent to storage.

The acid concentration in the alkylation process must be strictly controlled to prevent runaway reactions from occurring.  A runaway reaction will occur when the acid strength is between 85 to 87% H2SO4.   At the acid concentration, the reaction conditions do not favour the alkylation reaction between olefins and isobutane.  Instead, olefins will react with each other to form conjunct polymers which are also known as acid sludge, acid soluble oil and read oil.

The conjunct polymers are soluble in sulphuric acid which has the effect of decreasing the concentration of the sulphuric acid.   As the acid concentration decreases, the reaction to form additional conjunct polymers is favoured and the reaction accelerates.

Another reaction that occurs is the oxidation of polymers by sulphuric acid.  The polymer is oxidized to a tar like substance and sulphuric acid is reduced to water and SO2.  

These reactions can occur within the alkylation process unit or in the spent acid storage tanks.

The acid regeneration or spent acid recovery process is designed to regenerate or recover sulphuric acid from a variety of spent acids containing a varying degree of contaminants. 

A typical sulphuric acid regeneration plant consists of three major components:

1.                   Regeneration furnace for the decomposition of spent acid

2.                   Gas cleaning system

3.                   Contact sulphuric acid plant

The basic process is the decomposition of H2SO2 in a furnace operating at 1000 to 1200ºC.  The decomposition reaction is an endothermic reaction so heat must be provided for the reaction to occur. Hydrocarbons that are present in the spent acid provide part of the fuel to maintain furnace operating temperature with the remaining portion being supplied by burning fuel such as natural gas, No. 2 fuel oil, etc. 

2. Regeneration Furnace

A regeneration furnace is generally a horizontal refractory lined furnace designed to decompose H2SO2 into SO2 and H2O.  The key design parameter is the residence time which should be sufficient to allow the decomposition reaction to go to completion.  Typically, a three (3) second residence time is provided.

Spent acid is typically spray into the furnace at one end using a two-fluid atomizing type spray nozzle with compressed air as the atomizing fluid.  At the same end of the furnace, fuel burners provide the additional heat input to maintain the decomposition reaction and furnace operating temperature.

Sulphur can also be burned in the regeneration furnace to supplement acid production.  Sulphur burned in the furnace will reduce the quantity of fuel required since it is an exothermic reaction.

Combustion air required for the process can be enriched with oxygen to varying degrees all the way to 100% oxygen.  The use of oxygen enrichment reduces the amount of gaseous inerts (i.e. nitrogen) that must be carried through the downstream equipment.  In an existing plant this has the effect of increasing the throughput of spent acid without the need to increase the size of downstream equipment.  In a new plant the overall size of equipment can be reduced.   The use of oxygen enrichment also increases the SO2 concentration providing for a smaller downstream sulphuric acid plant.

Regeneration furnaces are typically operated with a 2% oxygen content in the gas exit the furnace.  The hot gases leaving the furnace enter a waste heat boiler where high pressure steam is generated for process use or power generation.   The gases are cooled to about 350-375ºC before entering the gas cleaning section of the plant.

3. Preconcentration

Preconcentration of the waste acid feed reduces the amount of water that must be handle in the furnace.  The result is a reduction in the size of furnace and the amount of fuel burned.  The degree to which the feed can be concentrated is dependent on the concentration of solubles in the waste acid.   The waste acid can be concentrated to the point where these solubles begin to precipitate out of solution.

4. NOx

NOx is a general term used to describe nitrogen oxide (NO) and/or nitrogen dioxide (NO2).  NOx is generally classified as either fuel NOx or thermal NOx depending on how it is formed.  Fuel NOx results from the oxidation of fuel bound nitrogen while thermal NOx is created by the fixation of nitrogen in the combustion air at high temperatures.

The formation of fuel NOx is dependent on the nitrogen content of the fuel, total excess air/oxygen and relative distribution of primary and secondary combustion air/oxygen.  Formation of thermal NOx is affected by the oxygen concentration, temperature, pressure and residence time.

Some general parameters regarding the formation of thermal NOx are:

Thermal NOx production rate doubles for every 90 K temperature increase beyond 2200 K (1927°C, 3500°F)

Above 1538°C (2800°F) significant quantities of thermal NOx is generated Thermal NOx increases exponentially (~ 4th power) with absolute temperature Thermal NOx increases roughly with the square of the available oxygen

CorrosionJune 6, 2005

Introduction

The moment process gases or liquids are introduced to the plant, corrosion begins and the plant starts its slow but sometimes fast decline.   Selecting the correct materials of construction is the best method of slowing the progress of corrosion.  In order to select the correct material the designer and operator of an acid plant must know how a particular material will withstand exposure to the process conditions.

Back to top

Carbon Steel

Although the overall corrosion rate of carbon steel in concentrated sulfuric acid service is low, some corrosion still occurs.  The reaction products from the corrosion process are iron sulfate and hydrogen gas.

Fe + H2SO4 -> FeSO4 + H2

This reaction can be considered as the sum of two different reactions occurring at different sites on the metal surface:

Fe -> Fe++ + 2e- (oxidation)

2H+ + 2e- -> H2 (reduction)

The iron sulphate film is relatively weak and can be disturbed by high acid velocities or other disturbances in the liquid.

Hydrogen gas can be a problem with carbon steel in sulphuric acid environments because it can literally scrub off the mechanically weak iron sulphate film, which is the only thing protecting the steel from attack.

During periods of normal flow, the gas bubbles are very small and evenly distributed, and they are quickly carried downstream along with the acid, generally without causing a problem to the pipe wall.  However, when acid flow stops, the gas bubbles accumulated along the pipe wall in the upper half of the pipe. When the flow resumed, the hydrogen gas bubbles are dislodged and rise to the very top of the pipe, scrubbing off the protective film along the way. With the film gone along the top of the pipe, the surface corroded very quickly until the iron sulphate film reforms.

Repeating this process over and over again results in a pattern of curved grooves in the top half of the pipe, all radiating toward a central longitudinal groove at the very top.

Although we're usually not privileged to see such an excellent example of hydrogen grooving, it's not an uncommon occurrence in sulfuric acid service, particularly under upset conditions. To guard against it, the usual recommendation is to use very thick steel, in recognition of the possibility of localized corrosion, and to keep the average velocity in pipes below 3 ft/sec to reduce the scrubbing effect of any hydrogen bubbles that do form.

Back to top

Corrosion Charts

Corrosion charts are one source of information that will help identify the correct material of construction for a particular application.

Material Description Source

254 SMo

Isocorrosion CurvesCorrosion Rate = 0.1 mm/y (4 mpy)0-50% H2SO4, 2000 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)Naturally Aerated, Corrosion Rate = 0.1 mm/y (4 mpy)0-100% H2SO4, 2000 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)0-100% H2SO4, 200 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)

654 SMo UNS S32654, Avesta Bulletin 9603

Avesta Sheffield Corrosion Handbook, Avesta Sheffield, 1994

300 Series Stainless Steels

Isocorrosion Curves0-100% H2SO4

?

310M Stainless Steel Type 310M Corrosion Rates in Strong Sulphuric Acid ?

654 SMoUNS S32654

Isocorrosion CurvesCorrosion Rate = 0.1 mm/y (4 mpy)0-50% H2SO4, 2000 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)Naturally Aerated, Corrosion Rate = 0.1 mm/y (4 mpy)0-100% H2SO4, 2000 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)

654 SMo UNS S32654, Avesta Bulletin 9603

Avesta Sheffield Corrosion Handbook, Avesta Sheffield, 1994

904L

Isocorrosion CurvesCorrosion Rate = 0.1 mm/y (4 mpy)0-50% H2SO4, 2000 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)Naturally Aerated, Corrosion Rate = 0.1 mm/y (4 mpy)0-100% H2SO4, 2000 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)0-100% H2SO4, 200 ppm Chloride Ions, Corrosion Rate = 0.1 mm/y (4 mpy)

654 SMo UNS S32654, Avesta Bulletin 9603

Avesta Sheffield Corrosion Handbook, Avesta Sheffield, 1994

Alloy C-22 Corrosion Rate ?

Alloy G-30 Corrosion Rate ?

Carbon Steel Isocorrosion Curves ?

Carpenter 20Cb-3

Corrosion Rate in Non-Aerated H2SO4 at 80°C (176°F)Corrosion Rate versus Concentration in Boiling H2SO4, 0-50% H2SO4

Isocorrosion Curves 0-100% H2SO4

Corrosion of Annealed 20Cb-3 to Boiling H2SO4 at Various Concentrations (5%, 10% and 25% H2SO4)

20Cb-3 Stainless Steel, Carpenter Technology Brochure

Chlorimet 2 Isocorrosion CurvesThe Duriron Company, Inc., Bulletin A/3g, April 1987

Chlorimet 3 Isocorrosion CurvesThe Duriron Company, Inc., Bulletin A/3g, April 1987

Durimet 100Isocorrosion CurvesCorrosion Rate = 20 mpy  

Durimet 20Isocorrosion CurvesCorrosion in Sulphuric AcidCorrosion Rate = 20 mpy

Durimet 20, Bulletin A/1h, The Duriron Company, Inc., January 1985Durimet 100, Bulletin A/7g, The Duriron Company, Inc., July 1987

Duriron Isocorrosion CurvesDuriron, Duchlor 51, Superchlor, Bulletin A/2g, The Duriron Company, Inc., March 1986

Hastelloy B2Isocorrosion CurvesSulphuric AcidSulphuric Acid, 200 ppm Chlorides

Hastelloy Alloy B-2, Cabot Stellite Division, Brochure, 1977

Hastelloy C-276Isocorrosion CurvesSulphuric AcidSulphuric Acid, 2000 ppm Chlorides

Hastelloy Alloy C-276, Haynes International, Bulletin H-2002B

Hastelloy D-205 Corrosion RatesReagent Grade Sulphuric Acid at 93°C - Corrosion Rates 20-60% H2SO4

Commercial Grade Sulphuric Acid at 130°C -

Alfa Laval Brochure?

?

Corrosion Rates 96-99% H2SO4

Hastelloy GCorrosion Resistance in Non-Aerated Sulphuric Acid at 80°C (176°F)

Carpenter Alloys for Controlling Severe Corrosives, Carpenter Technology Corporation, 9/89-7.5M

Inconel Alloy 686General Corrosion ResistanceIsocorrosion Curve, 20 mpy (0.51 mm/y)Isocorrosion Curves

Magazine advertisementSpecial Metals Technical Brochure

LeadIsocorrosion CurveCorrosion Rate = 5 mpy  

NiobiumIsocorrosion CurveCorrosion Rate = 5 mpy  

Noridur 9.4460Isocorrosion CurvesAerated, Velocity = 10 m/s, Corrosion Rate = 0.5 mm/y

KSB, Klein, Schanzlin & Becker Aktiengesellschaft, I/1980

SAF 2205UNS S31803

Isocorrosion CurvesDeaerated H2SO4, Corrosion Rate = 0.1 mm/y (4 mpy)Corrosion Rate = 0.1 mm/y (4 mpy)Static Conditions, Corrosion Rate = 0.1 mm/y (4 mpy)

Avesta Sheffield Corrosion Handbook, Avesta Sheffield, 1994Avesta 2205, UNS 31803, Avesta Bulletin 9060Sandvik Steel, Bulletin S-1875-ENG, July 1992

SAF 2204UNS S32304

Isocorrosion CurvesDeaerated H2SO4, Corrosion Rate = 0.1 mm/y (4 mpy)0-20% , Static Conditions, Corrosion Rate = 0.1 mm/y (4 mpy)

Avesta Sheffield Corrosion Handbook, Avesta Sheffield, 1994Avesta SAF 2304, UNS S32304, Avesta Bulletin 9137

Sandvik HT 9067Isocorrosion CurvesCorrosion Rate = 0.1 mm/y (4 mpy)

Edmeston HT Alloy Brochure

Sandvik SX

Corrosion Rates - Static ConditionsIsocorrosion Curves80-100% , Static Conditions, Corrosion Rate = 0.1 mm/y (4 mpy)Corrosion Rate = 0.1 mm/y (4 mpy)

Monsanto Enviro-Chem System BrochureEdmeston Materials System Engineering

Sanicro 28

Isocorrosion CurvesNaturally aerated, Corrosion Rate = 0.1 mm/y (4 mpy)Stagnant, Corrosion Rate = 0.1 mm/y (4 mpy)

Avesta Sheffield Corrosion Handbook, Avesta Sheffield, 1994Sandvik Steel, Bulletin S-1875-ENG, July 1992

Titanium

Isocorrosion CurvesDeaerated H2SO4, Corrosion Rate = 0.1 mm/y (4 mpy)H2SO4 with Copper Sulphate, Corrosion Rate = 0.1 mm/y (4 mpy)

Avesta Sheffield Corrosion Handbook, Avesta Sheffield, 1994

ZeCorIsocorrosion Curve1 mpy, 90-100% H2SO4

?

Zeron 100

Isocorrosion Curves Figure 1: Corrosion Rate = 0.1 mm/y (4 mpy)Figure 2: 0-40% H2SO4, Corrosion Rate = 0.1 mm/y (4 mpy)Figure 3: 0-60% H2SO4, 2000 mg/L Cl, Corrosion Rate = 0.1 mm/y (4 mpy)Figure 4: 30-70% H2SO4, Corrosion Rate = 0.1 mm/y (4 mpy)Figure 5: 70-100% H2SO4, Corrosion Rate = 0.1 mm/y (4 mpy)Figure 6: 80-100% H2SO4, Ferrous Ions, Corrosion Rate = 0.1 mm/y (4 mpy)Figure 7: Oleum, 30% SO3, 70-130°C

The Performance of Zeron 100 Super Duplex Stainless Steels in Sulphuric Acid, Weir Materials & Foundries

Zirconium Isocorrosion Curve, Corrosion Rate = 5 mpyWah Chang Magazine Advertisement

Properties – Sulphuric Acid

Introduction

Sulphuric acid is a colourless to amber, slightly cloudy, oily liquid with a specific gravity almost twice that of water.  At normal temperatures and a concentration of 98% or lower, it has little odour.  At higher temperatures and above a concentration of 98% irritating sulphur trioxide (SO3) fumes may be liberated.  Oleum (fuming sulphuric acid) is a colourless to white, heavy, oily liquid containing sulphur trioxide dissolved in sulphuric acid.  On exposure to air, oleum releases irritating white fumes of sulphur trioxide.

All grades of sulphuric acid have an affinity for water resulting in evolution of heat when diluted.  In the case of oleum, the reaction occurs with explosive violence.  Sulphuric acid and oleum will absorb moisture from the air resulting in dilution of the acid.  All grades are strongly corrosive and personal safety precautions should be carefully followed.

Many organic substances, wood, starch, sugar, paper, etc. are charred on contact with sulfuric acid, forming carbon.  Very small quantities of organic matter will blacken sulphuric acid as a result of suspended carbon.

ColourClear, colourless liquidContaminants in the acid may impart a colour to the acid.

ReactivitySulphuric acid will attack many metals and in its concentrated form is a strong oxidizing agent and may cause ignition on contact with organic materials, nitrates, carbides, chlorates, etc.

Flash Point None

Structure

Specific Gravity Varies with concentration and temperature.   See Table  See Chart

Freezing Point See Table  See Chart

Boiling Point See Table  See Chart

Viscosity See Chart

Conductivity See Chart

Enthalpy See Chart

 

PropertiesApril 12, 2003

Degrees Baumé

Specific Gravity

% H2SO4 Freezing Point Boiling Point

(15.6°C /15.6°C)

°C °F °C °F

0 1.0000 0.00 0.0 32 100 212

1 1.0069 1.02 -0.5 31.2

2 1.0140 2.08 -1.0 30.5

3 1.0211 3.13 -1.2 29.8

4 1.0284 4.21 -1.8 28.9

5 1.0357 5.28 -2.1 28.1

6 1.0432 6.37 -2.7 27.2

7 1.0507 7.45 -3.2 26.3

8 1.0584 8.55 -4.0 25.1

9 1.0662 9.66 -4.4 24.0

10 1.0741 10.77 -5.0 22.8

11 1.0821 11.89 -5.6 21.5

12 1.0902 13.01 -6.6 20.0

13 1.0985 14.13 -7.9 18.1

14 1.1069 15.25 -8.7 16.3

15 1.1154 16.38 -9.7 14.6

16 1.1240 17.53 -10.8 12.6

17 1.1328 18.71 -12.2 10.0

18 1.1417 19.89 -13.7 7.3

19 1.1508 21.07 -15.2 4.7

20 1.1600 22.25 -17.0 18.8

21 1.1694 23.43 -18.8 -1.8

22 1.1789 24.61 -21.2 -6.2

23 1.1885 25.81 -24.0 -11.0

24 1.1983 27.03 -27.1 -15.4

25 1.2083 28.28 -29.5 -21.0

26 1.2185 29.53 -33.5 -28.2 136.1 227

27 1.2288 30.79 -37.4 -35.3

28 1.2393 32.05 -42.7 -44.7 109.4 229

29 1.2500 33.33 -47.8 -54.0

30 1.2609 34.63 -54.8 -66.6 111.1 232

31 1.2719 35.93 -61.7 -79.1

32 1.2832 37.26 -59.0 -74.3 113.3 236

33 1.2946 38.58 -57.1 -70.8

34 1.3063 39.92 -55.2 -67.3 115.6 240

35 1.3182 41.27 -54.1 -65.3

36 1.3303 42.63 -53.0 -63.3 117.8 244

37 1.3426 43.99 -49.7 -57.4

38 1.3551 45.35 -45.7 -50.2 120.0 248

39 1.3679 46.72 -42.5 -44.5

40 1.3810 48.10 -38.9 -38.0 122.8 253

41 1.3942 49.47 -36.4 -33.5

42 1.4078 50.87 -34.1 -29.3 126.7 260

43 1.4216 52.26 -32.0 -25.4

44 1.4356 53.66 -30.3 -22.6 130.6 267

45 1.4500 55.07 -29.2 -20.6

46 1.4646 56.48 -28.6 -19.4 135.0 275

47 1.4796 57.90 -28.4 -19.1

48 1.4948 59.32 -28.8 -19.9 139.4 283

49 1.5104 60.75 -29.9 -21.8

50 1.5263 62.18 -31.7 -25.0 143.9 291

51 1.5426 63.66 -33.1 -27.5

52 1.5591 65.13 -36.8 -34.2 151.1 304

53 1.5761 66.63 -37.8 -36.0

54 1.5934 68.13 -38.8 -38.0 158.9 318

55 1.6111 69.65 -42.8 -45.0

56 1.6292 71.17 -40.4 -40.8 167.8 334

57 1.6477 72.75 -39.6 -39.2

58 1.6667 74.36 -34.0 -29.0 177.8 352

59 1.6860 75.99 -22.0 -7.5 185.0 365

60 1.7059 77.67 -11.2 11.6 192.2 378

61 1.7262 79.43 -2.5 27.5 197.8 388

62 1.7470 81.30 3.9 39.0 207.8 406

63 1.7683 83.34 8.0 46.2 217.2 423

64 1.7901 85.66 7.3 45.2 227.8 442

65 1.8125 88.65 0 31.8 247.2 477

65.5 1.8239 90.60 -9 16.1 260.0 500

65.75 1.8297 91.80 -16.5 2.3

65.9 1.8331 92.60 -23.0 -9.5

65.92 1.8335 92.70 -24.0 -11.1

65.93 1.8339 92.80 -24.8 -12.6

65.95 1.8343 92.90 -25.8 -14.5

65.97 1.8347 93.00 -27.0 -16.6

65.98 1.8350 93.10 -28.2 -18.8

66.0 1.8354 93.19 -29.5 -21.0 276.1 529

66.01 1.8357 93.30 -29.8 -21.7

66.02 1.8360 93.40 -31.0 -23.8

66.04 1.8364 93.50 -32.1 -25.8

66.1 1.8381 94.00 -32.0 -25.0

1.8407 95.00 -20.7 -5.3

1.8427 96.00 -14.7 5.5

1.8437 97.00 -6.3 20.7

1.8437 98.00 -1.0 30.0

1.8424 99.00 4.5 40.1 309.4 589

1.8391 100.00 10.0 50.7

Boiling PointSeptember 27, 2003

Associated Links

Freezing PointSeptember 27, 2003

 

ViscosityApril 30, 2003