WO2002090252A1

WO2002090252A1 - Method of increasing the efficiency of a gasification process for halogenated materials

Info

Publication number: WO2002090252A1
Application number: PCT/US2001/014499
Authority: WO
Inventors: Dennis W. Jewell; John P. Henley; Connie M. Galloway
Original assignee: Dow Global Technologies Inc.
Priority date: 2001-05-04
Filing date: 2001-05-04
Publication date: 2002-11-14
Also published as: DE10197239T5

Abstract

Methods for improving a gasification process for halogenated materials and in particular for producing useful end products such as anhydrous or highly concentrated hydrogen halides and/or synthesis gas, the methods including recycling water/hydrogen halide vapors and/or carbon dioxide to a gasification reactor.

Description

METHOD FOR INCREASING THE EFFICIENCY OF A GASIFICATION PROCESS FOR HALOGENATED MATERIALS

The invention relates to apparatus and methods to be utilized for the gasification of halogenated materials, and in particular to apparatus and methods that efficiently produce useful end products such as anhydrous or highly concentrated hydrogen halide and/or synthesis gas.

Related inventions include a prior patent application for a Method and Apparatus for the Production of One or More Useful Products from Lesser Value Halogenated Materials, PCT international application PCT/US/98/26298, published 1 July 1999, international publication number WO 99/32937. The PCT application discloses processes and apparatus for converting a feed that is substantially comprised of halogenated materials, especially byproduct and waste chlorinated hydrocarbons as they are produced from a variety of chemical manufacturing processes, to one or more "higher value products" via a partial oxidation reforming step in a gasification reactor.

Liquid thermal oxidation processes represent the current industry standard for treating such waste and byproduct streams. Gasification offers several advantages over thermal oxidation including reduced costs, reduced emissions and the capture of maximal chemical value from the feed stream constituents. Gasification is also more flexible than competing known alternatives to thermal oxidation, in that a significantly broader range of acceptable feedstock compositions can be processed.

In an exemplary gasification process for halogenated materials as described in the above-referenced application, a source of oxygen (in gaseous form) is mixed with a source of one or more, typically liquid halogenated material feeds, the mixture taking place in at least one gasification reactor to produce syngas. The syngas typically comprises a hydrogen halide, CO and H₂ with residual CO₂, H₂O and trace elements including carbon ("soot").

Such gasification in a reactor occurs at partial oxidation conditions, that is, at oxygen to fuel ratios that are substoichiometric with reference to complete combustion. Under such conditions small amounts of carbon particles or soot can, as mentioned, be formed as a side product. This soot requires additional capture and treatment steps downstream in the process, thereby decreasing the economic efficiency of the process as a whole. One goal of the instant invention is accordingly to minimize the production of C (carbon particles or soot), CO₂ and H₂O. Higher oxygen to fuel ratios can reduce the formation of soot. However, oxygen to fuel ratios are limited by permissible flame temperatures.

In various processes for gasifying essentially hydrocarbonaceous fuels or waste products, steam is known to be used as a gasifying agent. Under suitable conditions steam is known to react with carbon (or carbonaceous waste products or soot) to convert the carbon to carbon monoxide and the steam to hydrogen, both carbon monoxide and hydrogen being desirable products. Steam is also known to be used as a "moderator" in regard to several functions in the environment of gasifying hydrocarbonaceous materials. The addition of steam "moderates" flame temperatures, allowing higher oxygen to fuel ratios to be utilized. Higher oxygen to fuel ratios, as mentioned above, can reduce the formation of soot due to a higher partial pressure of oxygen.

Steam is also known to be used, in gasification processes for essentially hydrocabonaceous materials, for adjusting the hydrogen to carbon monoxide ratio of a product synthesis gas to meet the requirements of downstream customers. In the process of the gasification of hydrocarbonaceous materials, however, unlike in the instant gasification process, excess water created by used steam can be purged as waste water from downstream unit operations with a near negligible loss of valued products. In the gasification of halogenated organic materials as contemplated in our prior application, however, the situation is otherwise. While the addition of steam to the gasification reactor can have the same beneficial effects mentioned above (of reducing soot and allowing higher oxygen to fuel operating ratios and supplying additional hydrogen), the addition of steam can have negative consequences as well. If the halogenated organic gasification process includes the production of a hydrogen halide in an anhydrous form, or even as a highly concentrated aqueous solution, the purge of the excess water required as a consequence of steam addition can result in the loss of valuable hydrogen halide product. In both processes, excess steam or water must be purged from the system downstream to maintain a water balance. In the case of the production of anhydrous or concentrated hydrogen halides, the purge will inevitably contain a significant concentration of the hydrogen halide. This loss is in proportion to the amount of steam moderator furnished to the gasifier. The present invention teaches a method to close the water balance in the halogenated organic gasification process while minimizing the loss of valuable hydrogen halide product in an aqueous purge. More particularly, in a gasification process for halogenated materials as contemplated according to our prior application, if separated hydrogen halide is anticipated to be sold as an anhydrous product or in the form of a highly concentrated solution, the process includes a distillation step to separate hydrogen halide product from water (in particular from water absorbed when hydrogen halide gas passes through an absorber stage).

The present invention teaches the use of a vapor side-draw as part of this distillation step, wherein water/hydrogen halide vapor is extracted and recycled to the gasifier as a mixed steam "moderator" stream. The distillation system can be run at a pressure higher than the gasifier, thereby providing pressure to straightforwardly feed the extracted water/hydrogen halide vapor into the gasifier. Optionally, to help avoid liquid carryover in the "moderator" stream to the gasifier, the water/hydrogen halide vapor stream can be superheated with an appropriate heat source, such as steam, a heat transfer fluid or the like.

The recycled vapor from the distillation step is principally water vapor but contains significant amounts of hydrogen halide. The hydrogen halide recycles through the gasifier to be subject to recapture again in the hydrogen halide recovery stage. The water vapor or steam is primarily consumed via gas shift reactions and carbon consuming reactions. In such manner, the water balance of the process is maintained or completed while also achieving the desired objective of soot reduction. A combination of steam as well as recycled vapor can be utilized in whatever ratio needed in order to match and achieve the process water balance, as necessary. Recycled vapor with or without additional steam feed to the reactor can also be used to adjust H₂ to CO ratio of the product syngas.

Moderator streams are typically supplied to a gasification reactor through a suitably designed burner for intimate and appropriate mixing of all reactants. Various burner or feed nozzle designs are known to those skilled in the art. An alternate methodology of the present invention teaches the use of another moderator, either together with or in lieu of the water/hydrogen halide vapor moderator, for helping to drive and to maintain the water balance of the gasification reactor process. As discussed above, synthesis gas created from the gasification of halogenated organics contains carbon dioxide. Methods for the removal and capture of carbon dioxide from synthesis gas are known. Carbon dioxide has some of the same reforming tendencies as steam. That is, carbon dioxide reacts with carbon and soot particles to produce carbon monoxide under gasification conditions. It is another aspect of the present invention that the carbon dioxide produced in the synthesis gas reaction is captured and recycled as an alternate or further moderator, augmenting or displacing steam. Some water vapors are produced due to the gas shift reaction, for example, CO + H₂O < - - > CO₂ + H₂. The use of carbon dioxide as a moderator and/or a combination of steam and carbon dioxide thus further allows the process water balance to be managed without purging or losing hydrogen halide in commercially significant degrees in an aqueous discharge. It can also be used to adjust H₂ to CO ratio in the product syngas. Depending on the operating pressure of the carbon dioxide recovery system, carbon dioxide can be pressured back to a gasifier reactor or a compression operation can be included for pressurizing the CO₂ stream to suitable pressures for feed to the gasifier. Alternately, carbon dioxide can be purchased and stored as a commodity. Carbon dioxide thus stored can be supplied at appropriate pressure to the gasifier.

As discussed above, while it is known in current gasification practice for conventional hydrocarbonaceous materials to use steam, and to a lesser extent carbon dioxide, to minimize soot formation and to adjust hydrogen to carbon monoxide ratios in the product syngas for intended consumers, the instant invention improves upon the above in that the "moderator" is or can be a recycled process fluid. Using a recycled water/hydrogen halide vapor as a moderator provides a means for controlling the water balance of the process, with the additional advantage of minimizing the aqueous waste volume discharged from the plant and minimizing the loss of hydrogen halide product mixed in with the aqueous purge stream. As a further advantage, by providing a method for managing water balance, using a recycled water/hydrogen halide vapor as a moderator permits the use of higher water addition rates to a hydrogen halide absorption column. Use of higher water addition rates to a hydrogen halide absorption column for the synthesis gas creates a higher recovery efficiency of hydrogen halide.

Recycling the purged water/hydrogen halide vapor as a moderator has the further advantage, in relation to the gasification of chlorinated organic materials, of also permitting efficient utilization of a wider array of feed stock compositions. That is, feed stocks with a lower halide content can be processed while still producing anhydrous or highly concentrated hydrogen halide product since the loss of hydrogen halide from the process has been reduced. Said otherwise, without use of the water/hydrogen halide vapor as a recycled moderator in the gasification reactor, recovered aqueous hydrogen halide from low halide feed concentration materials might be unsuitable for anhydrous recovery because of the otherwise excessive halide loss through aqueous discharge.

The instant invention has a further advantage of requiring no additional significant equipment, except perhaps a vapor superheater. Generation of the water/hydrogen halide vapor and its recycling can be easily integrated into the distilling system. Whether anhydrous or aqueous hydrogen halide product is desired, recycling water/hydrogen halide vapor from a distillation stage allows the production of more concentrated solutions by managing water balance without loss of product. Further, for feed stocks lean on hydrogen, the recycled water/hydrogen halide vapor serves as an additional source of hydrogen for converting all halide from gasifying these feeds to a hydrogen halide component.

The present invention offers improved methods for a gasification process for halogenated materials. The improvements include one or more of the following goals: increasing the efficiency of the process; increasing and/or maximizing the anhydrous hydrogen halide recovery; minimizing the aqueous discharge; and adjusting the H to CO ratio.

The present invention includes apparatus and methods for increasing the efficiency of a gasification process for halogenated materials. The invention in one embodiment includes removing water/hydrogen halide vapors from a distillation stage of a gasification process and recycling the vapor as a reactant and/or moderator feed to a gasification reactor stage of the process.

The method includes managing the pressure, temperature and flow rate of the water/hydrogen halide vapor to control the water balance, to reduce soot output and to moderate flame temperature in the gasification reactor. The method and apparatus include alternately or additionally capturing carbon dioxide from synthesis gas produced by a gasification of halogenated materials, or otherwise securing carbon dioxide, and feeding the carbon dioxide as a reactant and/or moderator gas to a gasification reactor stage of the process. The carbon dioxide may be added in addition to or in lieu of the water/hydrogen halide vapor moderator.

A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which: Figures 1 A and IB illustrate block flow diagrams for a gasification process for halogenated materials; Figure 1 A illustrates recycling water/hydrogen halide vapor while Figure IB illustrates recycling captured CO₂.

Figure 2 illustrates in more detail a gasifier stage for a gasification process of Figures 1.

Figure 3 illustrates a quench and solids removal stage of a gasification process of Figure 1.

Figures 4A and 4B illustrate an absorber and an aqueous acid cleanup stage of a gasification process of Figures 1. Figure 5 illustrates an anhydrous distillation stage of a gasification process of

Figures 1.

Table 1 illustrates a numerical simulation of a run of a gasification reactor for halogenated materials demonstrating sensitivity of the outlet gas composition to varying the moderator flow rate. An embodiment for a gasification process for halogenated materials is indicated in block diagram form in Figures 1A and IB. Figure 1A illustrates an embodiment of the process in which water/hydrogen halide vapors 530 (assumed for the purpose of the embodiment to be H₂O/HCl) are recycled from a distillation unit 500 back to a gasifier 200, in a first aspect of the present invention. Figure IB illustrates an embodiment of the invention wherein syngas produced from a gasifier 200 is finished in a gas finishing stage 700 and is further processed in a CO₂ recovery stage 700^' with the carbon dioxide stream 730 being recycled back to gasifier 200. Of course, a preferred embodiment could provide for both recycling of carbon dioxide and water/hydrogen halide vapors. Further, carbon dioxide could be purchased and/or stored as opposed to, or in addition to, being captured in a recovery stage.

More particularly, Figures 2 - 5 illustrate in more detail aspects of an embodiment of a gasification process for halogenated materials than are indicated in block diagram form in Figures 1 A and IB. Elements of Figures 2 and 5, in particular, will be discussed in detail to illustrate preferred embodiments of the instant invention and to place the instant invention in perspective.

Figure 2 illustrates a gasifier 200 in accordance with a preferred embodiment. The particular gasifier design of Figure 2 has two stages, primary gasifier R-200 and secondary gasifier R-210 for converting a fuel comprised substantially of halogenated materials to reaction products including hydrogen halide and synthesis gas components. For the purpose of this discussion the halogenated material will be assumed to be comprised of chlorinated hydrocarbons (RCl's). In Figure 2, RC1 liquid stream 144 is atomized in primary reactor R- 200 with a pure oxygen stream 290 and a steam stream 293, both injected through a main burner or nozzle BL-200. In the harsh gasification environment inside gasification reactor R-200, the RC1 components are partially oxidized and converted to carbon monoxide, hydrogen and hydrogen chloride, with the product gas stream including lesser amounts of water vapor and carbon dioxide as well as generally trace amounts of other materials such as soot (essentially being carbon). The product gas stream from R-200 flows into secondary reactor R-210 to allow all reactions to proceed to completion, thus yielding very high destruction efficiencies of all species and minimizing the undesirable side products such as soot. The output from R-210 is stream 210.

The primary gasifier R-200 of the instant embodiment functions to atomize the liquid fuel, evaporate the liquid fuel, and thoroughly mix the fuel with oxygen, moderator, and hot reaction products. The gasifier R-200 of the preferred embodiment operates at approximately 1450°C and 5 bars, gauge (75 psig). These harsh conditions are selected to be appropriate to provide near complete conversion of all feed components in R-200.

The reactions that take place in gasifier R-200 are many and complex. The reaction pathways and kinetics are not completely defined nor understood. Indeed, for the numerous species that comprise the commercially significant chlorinated organic by-product and waste materials, the multiple reactions and their kinetics for each will be somewhat different. However, because of the extreme operating conditions in the gasifier, the reactions can be fairly represented by the overall reactions as defined below, in a close approach to equilibrium for most species. RC1 partial oxidation:

Chlorinated organics are partially oxidized to CO, H₂ and HC1:

C_vH_wCl_y+ (v/2)O₂ → (v)CO + [(w-y)/2]H₂ = (y)HCl However, since the gasifier operates with a slight excess of oxygen above this stoichiometry, further oxidation occurs. Water vapor and carbon dioxide can also participate as oxidizers at gasification conditions:

C_vH_wCl_y+ CO₂ → (v+l)CO + [(w-y)/2)]H₂ + (y)HCl C_vH_wCl_y+ H₂O → (v)CO + [l+(w-y)/2]H₂ + (y)HCl Further oxidation reactions: CO + J4O₂ → CO₂ H₂ + &O₂ → H₂O The oxidation reactions with oxygen, including the reaction C_vH_wCl_y + (v/2)O₂ →

(v)CO + [(w-y)/2]H₂ = (y)HCl, are highly exothermic, and thus provide the energy for driving the other reactions, maintaining the gasifier temperature as desired. Thermal decomposition reactions:

In local fuel rich zones resulting from the less than perfect mixing inherent to any burner, thermal decomposition occurs in the absence of oxygen or oxidizing species:

C_VH_WC1_X → C_r + (x)HCl + (v-r)CH₄ + [w-x-4*(v-r)/2]H₂ Gas shift reaction:

CO + H O <→ CO₂ + H , the classic gas shift reaction, driven primarily by gas composition, pressure and temperature also occurs within the gasifier. CH₄ + H₂O <→ CO + 3H₂, the steam - methane reforming reaction, is driven almost completely to the product (right) side at gasifier conditions.

Soot is also subject to to similar partial oxidation reactions as for RCl's as described above, but excluding the chlorine atom. Other reactions: Due to the low partial pressure of oxygen in the gasifier, essentially all halogens, including chlorine as shown above, equilibrate to the hydrogen halide.

Operating temperature in the gasifiers R-200 and R-210 should not be allowed to drop below approximately 1350°C. Conversion efficiency is reduced at lower temperatures. Because of accelerated corrosion attack to the refractory system, the gasifier temperature should not be allowed to exceed 1500°C. Conversion efficiency is very high at 1450°C and only limited gains are made at higher temperatures, not justifying the accelerated refractory corrosion. Preferably, no RC1 or liquid fluid is introduced to the gasifier until it is preheated to an acceptable operating temperature (approx. 130 C), although operation without preheating is also acceptable. As described above, the oxidation reactions provide the heat to drive reactor temperature. The 0₂/fuel ratio will therefore be increased or decreased as necessary to adjust reactor temperature to the targeted value. This ratio must be carefully controlled because of the sensitivity in using pure oxygen where small increments can cause significant temperature changes. The control band must also be limited to approximately one-half of the stoichiometric oxygen/fuel ratio to insure that the flammable mixture (syngas) environment in the gasifier is always maintained in a reducing state. Hazardous deflagrations can occur if excess oxygen is introduced to the fuel rich reactor chamber. Target oxygen to fuel ratio for the base feedstock is 0.489 lb of oxygen per 1.0 lb of liquid fuel. This will of course vary as the feed composition changes and if moderator flow is varied.

Not only steam stream 293 but also, or alternately, an HCl/water vapor mixture stream 530 from a desorber T-510 (Figure 5) can be used as moderator flow. The moderator flow can be used to temper the flame temperature of the pure oxygen/fuel burner. This moderator can also serve as a coolant flow for the burner. Depending on the heating value of the liquid fuel, pure oxygen and the fuel can operate at the target gasifier temperature with insufficient oxygen to complete the partial oxidation reactions. This results in decreased conversion efficiency and increased soot. To correct this deficiency, moderator flow can be increased, thus permitting additional oxygen while maintaining the target gasifier temperature within limits. Moderator flow can be increased until sufficient oxidant is present to complete the desired reactions. In practice this can be defined by the concentration of fully oxidized species in the exit gas. For example, CO₂ and H₂O may be targeted to be no less than 1.0 volume percent each in the exit gas, and values as high as 10 — 15 volume percent may be acceptable for heavy sooting or poorly converting feedstocks. Steam as a moderator flow should be limited as possible because it does put additional load on the plant water balance and decreases the concentration of aqueous HC1 absorbed downstream.

The burner BL-200 is an integral and vital component of a primary gasifier. The discharge jet from the burner provides a momentum source for mixing in a primary gasifier. The main burner should atomize the liquid into this mixing jet. Target atomization performance might be defined as where 99 percent of the liquid volume is of a droplet size of 500 microns or smaller. In preferred embodiments of the burner or feed nozzle BL-200, liquid is injected through an annular arrangement of orifices centered around a central oxygen discharge. Pressure drop through these orifices initiates coarse atomization of the discrete liquid jets. The orifices, and thus the liquid jets, are directed to intersect out in front of the face of the burner, or more specifically, along the axis of the oxygen discharge, and so intersect with the oxygen discharge jet. The oxygen discharge jet provides a primary energy source for atomization. Static pressure of the oxygen is converted to kinetic energy through the burner nozzle, and preferably near sonic or supersonic velocities are achieved. The velocity differential between gas and liquid provides an atomization energy which reduces the liquid jet to fine, discrete droplets. Moderator steam may also be mixed with the oxygen upstream of the burner in this particular operating mode. Oxygen to the gasifier is preferably preheated (for example, to about 120 degrees Celsius) where the oxygen is expanded as it is discharged, to offset a temperature drop associated with the expansion. To avoid induction of hot gasification reactor products into a near pure oxygen jet immediately at a burner face, and to avoid the extreme temperature conditions which would result, moderator or some portion thereof can be jetted into the gasifier as an annular film surrounding the oxygen/fuel jet. This "inert" layer tends to move the hot oxidizing zone out away from the face of the burner, thus reducing the heat flux and resulting temperatures on the burner face. Figures 3, 4 A and 4B illustrate a quench and solids removal stage 300 of a preferred embodiment of a gasification process and an absorber 400 and aqueous acid 450 cleanup stage of a preferred embodiment of a gasification process. The quench, solids removal absorber and cleanup stages of the preferred embodiment lead to an anhydrous distillation stage 500 of Figure 5, which is of particular significance to the instant invention. Figures 3, 4A and 4B are included for background purposes and clarification.

Figure 5 illustrates features of a preferred embodiment for an anhydrous distillation process. The anhydrous distillation area 500 in general consists of a distillation system including desorber T-510, with auxiliary equipment to desorb a hydrogen halide stream (treated herein as an HCl stream) from an aqueous hydrogen halide (HCl) stream. A desorber overheads stream 503 in the preferred embodiment of Figure 5 should essentially be a saturated HCl stream (greater than 99 vol. percent of HCl). This HCl stream 503 can be further processed in one or more condensors, E-515 and E-520, and in an anhydrous HCl drying and compression area 600, including an HCl drying tower T-620. Desorber bottoms from desorber T-510, in the form of stream 501, should comprise an azeotropic (approximately 22 wt. percent HCl, for example) aqueous HCl stream which can be recycled to an HCl recovery absorber as stream 554, where it can be reconcentrated to target aqueous acid strength. A hydrogen chloride - water system is a highly non-ideal mixture. It forms an azeotrope at approximately 20.0 wt. percent HCl at atmospheric pressure. Water has a higher activity coefficient above this concentration. The azeotrope shifts with pressure, decreasing (HCl concentration reference) as pressure increases. The azeotrope is approximately 16.6 wt. percent at 59 psig. When an absorber bottoms stream 500' enters a desorber T-510 above the azeotropic concentration in the desorber, HCl is a volatile species and is fractionated overhead.

In the preferred embodiment of Figure 5, aqueous acid from storage illustrated as stream 483 and referenced in Figure 4, can be cross exchanged with the bottoms stream 510 and fed to the HCl desorber T-510 as bottoms stream 500'. The stream 500' is preferably introduced between an upper and lower packed section of desorber T-510. The HCl desorber can fractionate HCl overhead while discharging a weak aqueous HCl stream from the bottoms. At preferred base design conditions (6.9 bars gauge (100 psig), 45°C from the secondary condenser E-520) the overheads gas should be about 96 vol. percent HCl, 0.12 vol. percent H₂O, with small amounts of noncondensibles -primarily CO₂ and to a lesser extent N₂. Essentially all of the noncondensibles should be driven overhead in the desorber. Column bottoms may operate at approximately 175°C, and an acid concentration of about 22 wt. percent HCl could be expected. Condensed liquid from both a primary E-515 and a secondary E-520 condenser can be collected in a reflux drum D-515 and pumped back as column reflux. A knock-out drum D-520 after the secondary condenser can also remove free liquid to help prevent its carryover into the anhydrous HCl drying system. The column reboiler E-510 can be driven by 235 lb. steam. Condensate level on the stream (shell) side of the reboiler can be controlled to manipulate heat transfer surface area, and thus reboiler duty for the column. When producing anhydrous HCl, as per the present invention, the water balance is preferably closed by using a sidedraw vapor 514 from a desorber as a moderator for the gasifier. This vapor may be, for instance, about 59 wt percent H₂0 and 41 wt. percent HCl. When operating in this mode, the delivery pressure to a gasifier dictates the operating pressure of the desorber, which is about 6.9 bars gauge (100 psig). If no sidedraw vapor is required for the gasifier, operating column pressure can be reduced to 4.5 to 5 bars gauge (65 - 75 psig). The advantage of a lower operating pressure is a cooler bottoms temperature, which results in lower corrosion and permeation rates for the equipment. Boiling HCl as may exist at the bottoms of the desorber can be very aggressive, and milder operating conditions are more favorable to equipment reliability. Bottoms temperature is preferably not allowed to exceed 185°C due to limitations of the typical impregnated graphite materials of reboiler tubes and the typical Teflon linings for towers and piping. The water balance can also be closed by extracting a portion of the aqueous HCl produced in the Absorber system 484 or 485.

The bottoms liquid stream 510, which is cross exchanged with a desorber feed, can be further cooled to approximately 40°C (or by using cooling tower E-550, which may include use of even sea water) and directed on to a middle section of an HCl absorber where it absorbs additional HCl. A small blowdown to an associated wastewater treatment facility can be used to control contaminant concentrations if these materials (for example salts, metals and the like) build up to unacceptable levels.

The following example, produced by computer model, illustrates typical parameters of a gasification reactor process for halogenated materials. Example 1

The following feeds streams were fed to a gasifier through an appropriate mixing nozzle:

Chlorinated Organic Material 9037 kg/hr

Oxygen (99.5 percent purity): 4419 kg/hr Recycle Vapor or moderator: 4540 kg/hr

[58.8 wt percent water vapor, 41.2 wt percent hydrogen chloride] The resulting gasification reactions resulted in a gas stream rich in hydrogen chloride.

In a preferred embodiment of the present invention, referencing the above example, this stream would be cooled or quenched and passed through an absorption step where the hydrogen chloride is recovered in an aqueous solution. This aqueous solution would be forwarded to a distillation system whose principal purpose is to distill nearly water free hydrogen chloride as an overhead product. The distillation tower is preferably operated at a pressure sufficient to flow side-draw vapor through a superheater, through a control valve, and through a gasifier mixing nozzle. A vapor side-draw is preferably extracted from a "reboiler section" of a distillation tower at a flowrate to complete the plant water balance. For the above example this would be per the flowrate and composition described for a gasifier feed. The vapor is preferably passed through a superheating exchanger imparting typically 10 - 20°C superheat to the vapor, to insure that no liquid droplets remain. This vapor would then be fed to a gasifier mixing nozzle as a moderator stream

Alternatively and/or in addition to the above system, a synthesis gas which has been absorbed free of bulk hydrogen chloride, as described above and illustrated as stream 418 in Figure 4, passes through a finishing system 700, Figure IB, where essentially all hydrogen chloride and other contaminants are recovered. This clean synthesis gas can then be fed to a conventionally known and commercially available carbon dioxide removal system, illustrated as unit 700' in Figure IB. Carbon dioxide can be absorbed, as is known, from the syngas, liberated from any solvent or sorbent, compressed if necessary, and fed back to a gasifier feed nozzle as stream 730 in Figure IB, also as a moderator.

Figure IB, discussed initially, illustrates in block flow diagram form the addition of a carbon dioxide recovery unit 700' after syngas finishing unit 700. Suitable methods for recovering carbon dioxide from a synthesis gas stream are well known in the art and need not be described herein, see, for example, United States Patents No. 6,165,432 and

6,207,121, also Kohl and Nielsen, Gas Purification. 5^th ed., Gulf Publishing (1997), pp. 1369 — 1373. Figure IB also illustrates a CO₂ recycle stream 730 recycled back and fed to a gasifier 200. The CO₂ would preferably be fed through a nozzle or burner in a passageway provided for an inert gas moderator, such as steam. Table 1 illustrates the mole fractions of exit gas from the secondary reactor of Figure

2 in a model run upon varying the moderator flow rate. The tables chart the breakdown of stream 210 when using a hydrogen halide/steam recycle moderator. The flow rate in lbs/hr of the moderator stream was varied from 2,000 lbs/hr to 20,000 lbs/hr. Results by mathematical model were computed with and without a nitrogen purge. Note the increased oxygen content as evidenced by increasing CO₂ and H₂O concentrations as recycled vapor moderator flow is increased. The higher concentrations allow for increased destruction of soot. Another key factor to note for the operation is the decreasing fraction of HCN (hydrogen cyanide), and MCBZ ( monochlorobenzene), for the various moderator flows, indicating more complete destruction of undesirable species as the moderator flow increased. HCN and MCBZ have been selected because they are representative of organics which can be present in the gasifier outlet. The math model used a thermodynamic equilibrium calculational method to determine these compositions. Actually measuring concentrations of 10-28 mol fraction is, of course, only a theoretical exercise.

The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape, and materials, as well as in the details of the illustrated system may be made without departing from the spirit of the invention. The invention is claimed using terminology that depends upon a historic presumption that recitation of a single element covers one or more such elements, and recitation of two elements covers two or more of the element.

Table 1

Claims

WHAT IS CLAIMED IS:

1. An improved method for a gasification process for halogenated materials, comprising: drawing a water/hydrogen halide vapor stream from a distillation stage of the gasification process; and recycling the vapor stream as a reactant and/or moderator feed to a gasification reactor stage of the process.

2. The method of claim 1 that includes managing the pressure, temperature and flow rate of the water/hydrogen halide vapor stream to control the process water balance, lower soot output from the reactor stage and moderate flame temperature in the gasification reactor stage.

3. The method of claim 1 wherein the gasification process includes at least a gasification reactor stage in fluid communication with a quench stage, in fluid communication with an absorber stage, in fluid communication with a distillation stage.

4. The method of claim 1 that includes adding carbon dioxide as an additional reactor and/or moderator gas to the gasification reactor stage.

5. The method of claim 1 that includes heating a drawn vapor stream prior to recycling the vapor stream.

6. An improved method for a gasification process for halogenated materials, comprising capturing carbon dioxide from a product gas stream produced by the gasification of halogenated materials; and feeding the carbon dioxide as a reactant and/or moderator gas to a gasification reactor stage of the process.

7. The method of claim 6 wherein the gasification process includes at least a gasification reactor stage in fluid communication with a quench stage, in fluid communication with a quench stage, in fluid communication with an absorber stage, in fluid communication with a distillation stage.

8. Apparatus for improving a gasification process for halogenating materials, comprising: a gasifier, the gasifier in fluid communication with a source of halogenated materials; an absorber in fluid communication with the gasifier; a distillation unit in fluid communication with the absorber; and a conduit providing fluid communication from a vapor-draw from the distillation unit to the gasifier.

9. The apparatus of claim 8 wherein the gasifier is in fluid communication with a source of oxygen.

10. The gasifier of claim 8 wherein the absorber is in fluid communication with a source of liquid hydrogen halide.

11. The apparatus of claim 8 that includes a conduit attached to a heater for heating the vapor.

12. The apparatus of claim 8 that includes a quench in fluid communication with, and between, the gasifier and the absorber.