METHOD FOR MINIMIZING HYDROGEN HALIDE CORROSION IN A PARTIAL OXIDATION PROCESS

METHOD FOR MINIMIZING HYDROGEN HALIDE CORROSION IN A PARTIAL OXIDATION PROCESS

This application claims the benefit of U.S. Provisional Application Nos.

60/021,880; 60/021,882; 60/021,886; 60/021,881 and 60/021,891 all filed July 17, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for minimizing hydrogen halide corrosion in a

partial oxidation, entrained flow, quench gasifier.

2. Description of the Prior Art Petroleum, coal and other organic natural resources are used as fuels, for

transportation, heating and power generation, and also as feedstocks to partial oxidation

gasifiers for the manufacture of various industrial chemicals. This includes gaseous, liquid

and solid hydrocarbons. Almost any combustible material having carbon and hydrogen can serve as a feedstock for the partial oxidation gasification process, such as natural gas,

methane, crude oil, shale oil, bitumen, heavy residual oil. coal, petroleum coke, sewage

sludge, light hydrocarbon fuel, various coals including anthracite, bituminous, sub-

bituminous and lignite, and various mixtures of the above. Also useful as feedstocks for the

partial oxidation gasification process are various mixtures and slurries of the above, using

among other things, hydrocarbons or other gaseous or liquid materials to form a pumpable

slurry.

Diminishing natural resources as well as economic considerations have led to the

increasing use of organic feedstocks from impure sources, such as scrap or waste plastic

materials which contain relatively high levels of contaminants. The scrap plastic is used as a hydrocarbonaceous feedstock in a partial oxidation

reaction to produce mixtures of hydrogen and carbon monoxide, referred to as synthesis gas,

or simply "syngas." Syngas can be used to make other useful organic compounds or as a fuel

to produce power.

Waste or scrap plastic materials often comprise at least one solid carbonaceous

thermoplastic and/or thermosetting material which may or may not contain associated

inorganic matter, such as fillers and reinforcement material. Such materials may be derived

from obsolete equipment, household containers, packaging, industrial sources, recycling centers and discarded automobiles. Scrap plastic comprises solid organic polymers derived

from sheets, films, extruded shapes, moldings, reinforced plastics, laminates and foamed

plastics. The mixture of scrap plastics varies with the source and with the presence of non-

combustible inorganic matter incorporated in the plastic as fillers, catalysts, pigments and

reinforcing agents.

Inorganic matter may also include dyes and pigments such as compounds of

cadmium, chromium, cobalt and copper; nonferrous metals such as aluminum and copper in

plastic coated wire cuttings; metal films; woven and nonwoven fiber glass, graphite, and

boron reinforcing agents; steel, brass, and nickel metal inserts; and lead compounds from

plastic automotive batteries. Heavy metals, for example cadmium, arsenic, barium,

chromium, selenium, and mercury may also be present. The inorganic constituents may be

present in the solid hydrocarbonaceous plastic-containing material in an amount ranging from

about a trace to about 30 weight percent of such plastic-containing material.

Solid hydrocarbonaceous plastic waste typically consists of polyethylene,

polyethylene terephthalate, polypropylene, polyesters, polyurethanes. polyamides,

polyvinylchloride, polystyrene, cellulose acetate and mixtures thereof. Also found are polyurea, polycarbonates, cellulose, acrylonitrile-butadiene-styrene (ABS), acrylics, al yds,

epoxy resins, nylon, phenolic plastics, polyacetals, polyphenylene based alloys, styrene, acrylonitrile, thermoplastic elastomers, fluoride polymers, rubber stocks, urea and melamine.

Because of the reducing conditions which exist during the partial oxidation reaction,

the halogen content of the halogenated organic material is converted to hydrogen halide or

ammonium halide. Free halogens which can be formed during the complete combustion of halogenated organic materials are not formed under the conditions in the partial oxidation gas

reactor. Other halogen compounds such as phosgene (COCI2), cyanogen chloride (CNC1),

volatile halide compounds such as AICI3, CI4 and POCI3 are not formed during the partial

oxidation reaction even though aluminum, vanadium or phosphorus may be present in the

feedstock.

A quench gasifier is used to conduct partial oxidation reactions and to capture most of

the acid component of the syngas in the quench water. The partial oxidation reaction is

carried out in a free-flow unpacked noncatalytic quench gasifier. The reaction temperature is

about 1800°F to about 3000°F and the reaction pressure is about 1 to about 100 atmospheres,

preferably about 25 to about 80 atmospheres. Under such high temperatures and pressures,

substantially all halogenated organic materials are rapidly converted into hydrogen halides, carbon dioxide, carbon monoxide, hydrogen cyanide,

ammonia, carbonyl sulfide, hydrogen and trace amounts of other gases and small quantities of

carbon.

The presence of halogens in scrap plastic may range from trace amounts to as much as

10 weight % or more, and can cause severe corrosion problems. During the non-catalytic

partial oxidation reactions, the halogen content of the scrap plastic hydrocarbonaceous feeds is primarily converted to hydrogen halides. At high temperatures, these acidic halides make

heat recovery from the resultant syngas impractical because of corrosion problems prior to

removal of the acidic component. This problem is compounded when the halogen content is

highly variable and cannot be easily measured, as is the case with any waste material,

particularly plastics, thereby making implementation of corrective countermeasures very

difficult.

The extent of corrosion in the quench portion of the reactor and in downstream

equipment can be reduced by adding a base to control the pH. A second problem arises from

the variable nature of the halide content of the feed. If the pH in the water system becomes

too high, depending on the base used, salts can precipitate in parts of the process water

system causing blockage and/or obstruction. It is difficult to control the addition rate of base to match the halide feed rate because pH control of hot, pressurized process water or

feedforward control based on halide analysis of the feed is unreliable.

In the gasification of well defined feedstocks containing low amounts of halides, on

the order of about 0.05 weight % to about 2 weight %, the addition of a base to neutralize the

acidic halide content of gasification water streams is based on stoichiometric amounts of the

base for the average amount of acid produced in the gasifier. Since the buffering capacity of

the process water is typically greater than variations in the acid generating components in the

feed, the pH of the process water can be measured offline and addition rates of base adjusted

as needed. However, when the amount of halides in the feed can far exceed the natural

buffering capacity of the process water, and where this amount can change rapidly, such a

system cannot prevent severe corrosion. This can occur when there is at least about 0.5

weight % halide in the feed, and normal or low amounts of nitrogen or alkali metals in the feed, where "normal" is defined as those amounts typically found in oil residuum, coal or

petroleum coke.

U.S. Patent No. 4,468,376 to Suggitt discloses a method for disposing of fixed

amounts of halogenated organic material produced during a partial oxidation reaction. For

example, the quench zone of the gasifier is maintained at a pH above 7 with ammonia to

ensure that there is a sufficient amount of ammonia to react with the hydrogen halide in the

synthesis gas stream. Thus, the Suggitt patent discloses how to prevent contamination of

synthesis gas with acid, but does not disclose how to prevent corrosion in the process water system. The Suggitt patent does not explain how to control ammonia addition when the

feedstock to the gasifier contains variable amounts of halide, which is the situation

encountered when using waste plastic materials as the feedstock.

If an excessive amount of ammonia or other equivalent base is added to the quench water so that the pH rises above about 10, solid ammonium carbonate salts will precipitate as

a result of the reaction of NH₃ or the cationic portion of another equivalent base with

dissolved carbon dioxide in any water contacted with or condensed from the syngas. This can

cause plugging problems in the water system used in the gasification operation, particularly

the heat exchangers used to cool the syngas from the quench and scrubber temperatures of about 350°F to about 600°F, to temperatures of about 80°F to about 300°F, for further cleanup

or processing. Alternatively, if the amount of ammonia or other equivalent base is insufficient to

react with all the hydrogen halide in the synthesis gas stream, rapid and catastrophic acid

corrosion of materials of construction, such as carbon steel, can occur. Furthermore, the presence of ammonia or any volatile base contaminates the synthesis

gas product. Therefore the process has to be adapted to remove the neutralizing base from the synthesis gas and to recover or dispose of any excess base.

If one skilled in the art followed the teachings of the Suggitt patent, it could lead to

large amounts of excess water or plugging and blockage in parts of the water system used in

the partial oxidation reaction system. The Suggitt patent also does not disclose specific

means to assure that ammonia is always in excess, particularly for those feeds where the

halide content varies widely. Thus, the Suggitt patent does not disclose how pH control with

ammonia can be implemented, or how to deal with feedstocks containing a variable halogen

content. In essence, the Suggitt patent does not disclose how to control pH or corrosion as

the halide content of the feed varies, nor does it address the problem of ammonia contamination in the synthesis gas.

When gasifying a feed with high and variable chloride content on the order of about 10 weight % to about 15 weight %, where ammonia addition is based on periodic off-line sample analyses, for example, every 30 minutes, a sudden increase in the chloride content of

the feed caused the quench water pH to drop below 1 in less than 15 minutes. Failure was

experienced in a carbon steel component of the process water piping due to corrosion which caused a hole in the line. The time required for the pH to decrease to

dangerous levels, if ammonia addition is interrupted or inadequate, can be less than 30

minutes in a gasifier with more than 2 weight % chloride in the feed. This can cause rapid

corrosion of any carbon steel component, and can result in the release of pressurized hot gas

or water.

In similar circumstances to those cited above, difficulties in adjusting the output of the

ammonia addition pump caused the ammonia addition rate to be roughly twice the required amount for neutralization. This caused rapid plugging of the synthesis coolers in less than

one hour, resulting in shutdown of the reactors.

SUMMARY OF THE INVENTION

The present invention relates to a method for minimizing hydrogen halide corrosion in

a quench gasifier during the non-catalytic partial oxidation reaction of a variable halide

content hydrocarbonaceous feed, to produce a hydrogen halide-containing synthesis gas or

"syngas." The hydrogen halide-containing synthesis gas is contacted with quench water in

the quench zone of the gasifier which contains a neutralizing agent that is in excess of the

stoichiometric amount necessary to neutralize hydrogen halide acid gases contained in the

synthesis gas. The neutralized hydrogen halide gases form condensed halide salts in the

quench water. The halide salts can be purified for further use. A portion of the quench water

containing the halide salts is removed and treated to recover the halide salts. The treated

quench water can be either be recycled to the process or discarded in conformity with

environmental regulations.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is a simplified diagrammatic representation of the

operative steps for minimizing hydrogen halide corrosion when a variable halide content

hydrocarbonaceous feedstock undergoes partial oxidation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, corrosion control is provided in the gasifier

quench water process system, and the synthesis gas scrubbing and cooling operations of the

gasification system by adding a stoichiometric excess of neutralizing agent, such as ammonia

to the water being circulated in the quench zone of the gasifier, so that there is always more

than enough neutralizing agent in the quench water to neutralize the hydrogen halide acids present in the synthesis gas. The amount added is chosen to ensure that the pH of the quench

water exiting the quench zone of the gasifier is always in the range of about 3.5 to about 9.0.

This is because at lower pH, rapid and catastrophic corrosion can occur, whereas at higher

pH, plugging of the cooling system by ammonium carbonate can occur.

The inventive process is designed to provide an ammonia content that is always in

stoichiometric excess of the maximum expected amount of halide, and low enough so that

ammonium carbonate does not precipitate in the gasification process system, more

specifically, the syngas coolers.

While acid gases such as carbon dioxide, hydrogen sulfide and hydrogen cyanide are

formed during the partial oxidation reaction, the quench cooling of the synthesis gas and the

subsequent water scrubbing causes the gaseous hydrogen halide to preferentially react with

the ammonia. Formate ions (HCOO-) are formed in the quench water, and since they are not

volatile, remain in the water. Some carbon dioxide and hydrogen sulfide gases solubilize in

the water, but most remain in the gaseous state. Since the hydrogen halides and ammonia are

very soluble, all of the halides will be absorbed in the water. If there is excess ammonia in

the water, ionization will occur to form chloride (CP) and ammonium (NH₄ ^*) ions in

solution.

The hydrogen halide containing synthesis gas is passed from the reaction zone of the

gasifier into the quench zone where it is contacted with quench water containing the

neutralizing agent that is always in stoichiometric excess of the minimum amount needed to

neutralize the hydrogen halide acids present in the synthesis gas, to thereby form

corresponding halide salts in the quench water. The substantially halogen-free synthesis gas

is then passed to a scrubbing operation where the synthesis gas is scrubbed with water to

remove any entrained finely divided parti culate solids. The neutralizing agent is preferably added in an amount such that the quench water

exiting the gasifier quench zone is at a pH of about 3.5 to about 9.0 preferably about 4.5 to

about 8.5. The scrubbing water exiting the scrubber is also maintained at a pH of about 3.5 to

about 9.0 preferably about 4.5 to about 8.5. Below a pH of 3.5 catastrophic and rapid

corrosion from halides can occur. Above a pH of 9.0, plugging of the cooling system by

ammonium carbonate can occur.

The use of a volatile base such as ammonia as the neutralizing agent is preferred

because the vapor spaces in the quench chamber, scrubber, and the heat exchangers used to

cool the synthesis gas cannot otherwise be adequately protected. This is due to the volatility

of halogen acids. When a non-volatile base is used, acidic condensates can occur. However,

when a volatile and highly soluble base such as ammonia is used, there will always be

ammonia present in both the liquid and vapor phase if it is present in either phase in an amount in excess of the halides.

Since the stoichiometric quantity of the neutralizing agent is always greater than the

amount of halide in the hydrocarbonaceous feedstock, there will always be sufficient base

present in the liquid phase to prevent excessive corrosion. This results in the formation of

ammonium chloride with an excess of ammonia, rather than an excess of hydrogen chloride,

and assures that there will be little or no acidic condensate, and a non-corrosive pH of about 6

to about 10.5 will exist in any condensates. Thus, any condensate from the synthesis gas

exiting the quench portion of the gasifier will consequently be non-corrosive.

To remove the halides from the gasification system, an aqueous halide-containing

stream is purged or removed so that the halide content of the hydrocarbonaceous feed can be

discharged from the overall process. This is done in a purification step wherein the halide

salts are recovered for further use or disposed of in accordance with environmental standards. All or a portion of the purified water can be returned to the process, as needed, or disposed.

As a secondary consideration, it is generally desirable to minimize the amount of water added

to or removed from the overall gasification process water system. The composition of the

hydrocarbonaceous feed controls whether water is made or consumed during the gasification

process.

Referring to the FIGURE, a halogen containing hydrocabonaceous feedstock 2, and

any water needed or associated with the feedstock, and a free oxygen containing gas stream 4

are fed to the reaction zone 12 of the quench gasifier 10. Therein, the feedstock undergoes

partial oxidation to form a raw syngas and a slag by-product which passes to the quench zone

14 of the gasifier 10. The syngas and slag are contacted in the quench zone 14 with quench water 32 containing a neutralizing agent such as ammonia in excess of the stoichiometric

amount necessary to neutralize the maximum expected halide content of stream 2. Nontoxic

slag 6 exits the quench zone 14 to disposal or for use as a building material.

The substantially halogen-free syngas stream 8 with excess ammonia exits the quench

zone 14 and enters the scrubber 16 where it is contacted with scrubbing water 18, which

removes finely divided entrained paniculate solid material. Clean syngas stream 20 saturated

with water vapor at the operating pressure of the system and containing the excess ammonia

in the gas stream that is at or near equilibrium with the amount of ammonia in the water, exits scrubber 16 and enters the cooler 22 wherein the water vapor and ammonia are condensed by

being cooled to a temperature below the dew point. A clean, dry syngas stream 24 containing

substantially no hydrogen halide and only trace amounts of ammonia exits the cooler 22 for

further use. Condensed water stream 26 with ammonia exits cooler 22 and is divided into

streams 18 and 28. Ammonia-containing water stream 18 is fed to the scrubber 16 to serve as the

scrubbing medium for the synthesis gas stream 8. Ammonia-containing water stream 28

enters ammonia stripper 30, and exits as ammonia-rich water stream 32 and ammonia-lean

water stream 36. The ammonia-rich water stream 32, also referred to as the "stripper

overhead" is combined with additional ammonia from stream 64, as needed, to provide

sufficient ammonia in excess of the stoichiometric amount needed to neutralize the maximum

expected halide content of the hydrocarbonaceous feedstock 2.

The ammonia-lean water stream 36 exiting the ammonia stripper 30 is divided into

ammonia-lean water streams 38 and 40. Ammonia-lean water stream 38 can be discharged if

local regulations permit or it can be subjected to additional waste water

purification treatment if needed prior to disposal. Ammonia-lean water stream 40 is fed to the cooler 22 to condense the water vapor and ammonia from the syngas. Alternately, all or a portion of water stream 38 can be recycled to the quench zone 14 or to the scrubber 16,

or a portion to both. Since carbon dioxide is present in the synthesis gas at much higher concentrations

than ammonia at the conditions in cooler 22, carbon dioxide is essentially equimolar to

ammonia and is also absorbed in stream 26. Thus, excess ammonia in stream 20 has to be

limited to the solubility limit of ammonium carbonate in stream 26, thus the maximum pH of

9.0 in stream 44.

Quench water 42 at a pH of about 3.5 to 9.0 exits the quench zone 14 of the gasifier

10 and contains the neutralized halide salts condensed from the syngas. Quench water stream

42 is combined with the scrubbing water stream 44 which exits the scrubber 16 at a pH of

about 3.5 to 9.0 to form the combined water stream 46 which is laden with the particulate

matter removed from the syngas and the halide salts which have been neutralized and condensed from the acid halide vapors that were present in the syngas. The halide salts are

generally primarily chloride salts.

The water stream 46 enters the separator 48 wherein particulate solids comprising

carbon, ash and some precipitated salts are separated and exit as stream 50 for further

treatment and disposal in accordance with environmental regulations. The particulate-free halide salt rich water stream containing dissolved halide salts exits the separator 48 as stream

52 and enters the purifier 54 wherein the halide salts 56 are separated and recovered for

further use, including commercialization.

A portion of the halide salt-free water 58 exits the purifier 54 and is combined with the particulate-free water stream 60 which exits from the separator 48 to form combined water stream 62. All or a part of the water stream 62 can be recycled to the quench zone 14

of the gasifier 10. Alternatively, all or a part of water stream 62 can be recycled to the

scrubber 16 to serve as an additional source of scrubbing water. Lastly, all or a part of water

stream 62 can be introduced into the raw syngas stream 8 to enhance the scrubbing operation

before the syngas enters the scrubber 16.

Water stream 68 exits the purifier 54 to contact the halogen-containing

hydrocarbonaceous feedstock 2 entering the gasifier 10 wherein it serves as a temperature

moderator in the reaction zone 12.

Ammonia in excess of the actual amount needed to neutralize the halide content

present in the hydrocarbonaceous feedstock 2 eventually accumulates in stream 68 which is

combined with feedstock 2 or introduced separately in reaction zone 12, where the ammonia

is substantially destroyed by decomposition. Thus, excess ammonia can be fed to the reaction

zone 12 of the gasifier 10 without accumulating excess ammonia in the product synthesis gas

and eliminating the need for unreliable pH control. It has been found that by controlling the amount of neutralizing agent added to the

quench water so that it is always in stoichiometric excess of the maximum halide content of

the hydrocarbonaceous feedstock 2, and by controlling the amount of water in stream 28

entering the ammonia stripper 30, and controlling the amount if water in stream 52 entering

the purifier 54, and determining whether enough water is removed in stream 38 exiting the

system to require the addition of water to the system , and if water is added, selecting where

the water is to be added, conditions can be found such that the pH of the quench water and the

scrubber water remains in the non-corrosive range of about 3.5 to about 9.0, and the solubility

limit of ammonium carbonate is not exceeded in the synthesis gas coolers or other parts of the

gasification water system, except where halide salts are intentionally precipitated in purifier

54.

This is done by taking into consideration the synthesis gas and water flows throughout the system, operating temperatures and pressures, the concentration of soluble species, such

as the neutralizer, halides, and carbon dioxide, and the known equilibrium behavior of these

species, that is, ionization constants, and vapor pressure versus liquid concentrations, and

conducting mathematical simulation of the entire system. Such process modeling is well

known to those skilled in the art and simulates process conditions for maximum and

minimum hydrogen halide protection from a variable halide hydrocarbonaceous feedstock.

The process conditions are derived from the aforementioned criteria through

commercially available simulation packages or a simulation program can be prepared directly

from the known equilibrium relationships. In this manner, the process can be controlled so

that at the maximum halide content expected from the hydrocarbonaceous feed, the pH of the quench water exiting the quench zone of the gasifier and exiting the scrubbing zone will be greater than 3.5, and at the minimum halide content the pH will be less than 9.0. This

eliminates the need for unreliable on-line pH control loops.

The halogen content of the total hydrocarbonaceous feedstock to the gasifier can vary from about 0.05% to about 15 weight %, preferably about 0.1 weight % to about 10

weight % and most preferably about 0.2 weight % to about 5 weight % of the total feed to the

reaction zone.

The synthesis gas is contacted with quench water, having ammonia dissolved therein to neutralize and condense the hydrogen halide into the quench water. The quantity of

ammonia in the quench water is in excess of the stoichiometric amount necessary to

completely neutralize the hydrogen halide and other acids which condense in the quench

water.

While ammonia or an ammonium salt are the preferred neutralizing agents, any suitable equivalent neutralizing agent can also be used. Such neutralizing agents include

alkali metal salts, alkaline earth metal salts, a natural or synthetic mineral or ore containing

the aforesaid neutralizing agents, and mixtures thereof.

The feedwater to the quench zone and the scrubber is generally at a pH of about 7 to

about 10. The pH in the quench zone 14 and quench water exit stream 42 are maintained at

the desired range by the addition of liquid or gaseous ammonia in stream 64 at the calculated

amount needed. In another embodiment, the quench and scrubbing water can be maintained

at the desired pH level by saturating the feed water with a solid ammonium salt such as

ammonium carbonate or ammonium bicarbonate. Saturated solutions of these neutralizing

agents are essential at a given pH to ensure that the ammonia concentration will be sufficient

to neutralize any amount of chloride in the feed without on-line pH measurements or

feedback control. Nitrogen can comprise part of the hydrocarbonaceous feed. Typically about 10

weight % to about 25 weight % of the nitrogen in the hydrocarbonaceous feed is converted

into ammonia, and about 0.1% to about 1.5% by weight is converted into hydrogen cyanide

(HCN), with the remainder forming molecular nitrogen (N2), which becomes part of the

synthesis gas. Additional treatment of the residual ammonia is not required and discharge to the atmosphere is avoided, because excess ammonia is recycled to the gasifier and it is

converted to nitrogen and hydrogen, thereby preventing plugging and precipitation of

ammonium carbonate in the system and contamination of the synthesis gas product with ammonia. The water blowdown stream 28 is concentrated in the ammonia stripper 30 to a high

concentration ammonia stream 32 which is fed to the gasifier quench zone 14, and a low

concentration ammonia-water stream 36 which can be recycled to the gas cooler 22. The low concentration ammonia-water stream recycled to the gas coolers helps prevent salt

precipitation and lowers the ammonia concentration of the syngas. A portion of bleed stream

38 can be recycled to scrubber 16, or discarded in accordance with environmental standards.

Preferably, the minimum ammonia addition rate to quench zone 14 is such that the pH

of the water stream 42 exiting the quench zone or quench chamber of the gasifier is greater

than 3.5 at the maximum halide concentration in the feed. Under these conditions, vapors

which condense after exiting the quench zone will also have a pH greater than 3.5. This

property dictates the preferred choice of ammonia as the hydrogen halide neutralizing agent.

If a nonvolatile base is used, there would be no base in the synthesis gas when it is cooled.

The condensate in the coolers can then be acidic due to any low concentration of volatile or

entrained acids, and the carbon dioxide in the synthesis gas. The benefit of this invention is that halide corrosion is substantially reduced or

eliminated and less expensive materials of construction can be used to conduct the process.

An environmentally "clean" water can be generated and plugging of the water system due to

carbonate precipitation is avoided.

Although this invention has been described in the context of using ammonia as the

preferred base, other equivalent or suitable bases can also be used.