Patent Application
20080063593
More particularly the invention relates to a process wherein the sulfuric acid is regenerated by first decomposition to SO2 and water in the presence of a hydrocarbon reducing agent and the SO2 subsequently converted to SO3 which is converted to concentrated sulfuric acid. The process of the present invention is the regeneration of spent sulfuric acid contaminated with water to produce pure concentrated acid comprising the steps of:
(a) dispersing the spent sulfuric acid onto a solid surface as a thin layer in a decomposition zone and
(b) heating the resultant mixture in the presence of a reducing agent to decompose the spent sulfuric acid to sulfur dioxide and water. Preferred embodiments include decomposition temperature in the range of 300 to 600° F., more preferably about or less than 500° F.; a reducing agent comprising hydrocarbon contaminant which is decomposed to carbon and precipitated out of the mixture as solid particulates; the solid surface comprising a portion of precipitated solid carbon particulates; converting the sulfur dioxide to sulfur trioxide in a converter by reacting the sulfur dioxide with oxygen in the presence of water; condensing sulfuric acid of about one hundred percent sulfuric acid; and heat being recovered from the conversion of sulfur dioxide to sulfur trioxide and used to provide heat to the decomposition zone.
It was discovered that strong sulfuric acid (90+%) can be reduced to sulfur dioxide and water by reaction with a suitable reducing agent at elevated temperatures. High boiling hydrocarbons such as those present in gas oils or already present in the spent acid are suitable reducing agents. It was further discovered that some of the carbon and hydrogen values in the hydrocarbon can report as a solid phase or as carbon dioxide and water depending on the reaction time and temperature. The stoichiometry is shown below for reactions in which the hydrocarbon is converted only to carbon (1) or to carbon dioxide (2) using eicosane (C20H42) as model compound:
21H2SO4+C20H42==21SO2+42H2O+20C ΔH500F=−8292 btu/lb mol H2SO4 (1)
61H2SO4+C20H42==61SO2+82H2O+20CO2 ΔH500F=+7291 (2)
Those skilled in the art recognize that spent acid furnace combustion technology suffers from the fact that ash components in the spent acid accumulate on the tube surfaces of the waste heat boiler. Frequent shutdowns of the system are required to clean out the tubes so as to maintain an acceptable heat transfer rate. This is expensive with regard to maintenance cost and to operating efficiency (on-stream time). Clearly it is desirable to have an acid decomposition system that does not have this maintenance problem. An intractable solid phase is not formed if the acid is first dispersed as a relatively thin layer on a solid surface and then heated to reaction temperature by conductive, convective or radiant heat transfer.
Any type of flat or curved surface can be used. For example the surface can be a flat plate as in a continuous belt tunnel dryer or the curved surface of a rotating vacuum drum dryer. The carbonaceous residue can be readily removed from such surfaces in powder form by scraping with a “doctor” blade at the discharge end of the dryer while the gas phase is removed from the dryer enclosure by a blower.
The reaction type (1) can be the predominating reaction at an appropriate temperature. In the case of spent acid from a gasoline alkylate plant the temperature is preferably in the range of about 500° F. or less. Any ash present in the feed acid ends up residing in the resulting solid phase avoiding the production of a high temperature ash phase and the related tube fouling problem described above.
Alternatively, the spent acid can be deposited unto the surface of small particulates which can be transported through a heated zone, for example as in a hollow paddle/screw type processor (heat transfer fluid circulated through the paddles/screw) or as in a fluidized bed (heat transfer fluid circulated through tubes immersed in the bed; bed fluidized by recycling gas phase). In this case heat is transferred from the equipment heat transfer surface to the inventory of particulates which in turn functions as the heat source for the decomposition reaction. The carbonaceous residue can be removed from the particles by shaking and screening devices after which the residue free particulates are recycled. The weight ratio of the spent acid feed to the recycled solids determines the initial acid loading on the solids. This ratio can be varied over a wide range, from about 0.01 to 0.5, depending on the flow characteristics of the acid/solid mixture.
In a preferred mode of the invention the particulates can be the recycled carbonaceous residue powder itself. The net product is then simply withdrawn as a slip stream from the mechanically mixed or fluidized bed of solids at the rate needed to maintain a constant solids inventory in the system.
In another preferred mode the decomposition reactor is maintained under slight vacuum.
In another preferred mode, if the hydrocarbon impurity in the spent acid has insufficient reducing capacity to convert all the sulfuric acid to SO2, external hydrocarbon is added to the spent acid feed in an amount sufficient to ensure the complete conversion of the acid to SO2.
In another embodiment a novel energy efficient process is used for producing concentrated sulfuric acid from the decomposition step vapor product comprising:
(a) maintaining the H2O/SO2 mole ratio in the feed to the SO2/SO3 converter at one or slightly greater than one and using oxygen or oxygen enriched air as the oxidant. This contrasts with the prior conventional dry (Monsanto type) or wet (Haldor Topsoe type) technologies: the former resorting to the removal of all water from the feed generated in the fuel/air/acid combustion zone or the latter which uses the wet feed and subsequent water removal in downstream processing.
(b) using as the converter a single shell and multi-tube configuration fixed bed reactor system. Catalyst, such as the vanadia used in the prior art, is in the tubes; molten salt (HiTec®) coolant is in the shell. The temperature profile in the tubes is controlled by adjusting coolant temperature and flow rate. Prior conventional technology uses a series of adiabatic fixed bed reactors with inter-stage blowers and coolers.
(c) linking converter and decomposer heat exchange systems so that the heat generated in the former at higher temperature (˜700° F.) provides most of the endothermic heat requirement for the latter operating at lower temperature (˜500° F.). The heat exchange can be done directly (HiTec® is heated in the converter exchanger system and cooled in the decomposer exchanger system) or indirectly (e.g., via a Dowtherm® system which links the converter and decomposer heat exchanger systems). Prior conventional technology requires the addition of external fuel for decomposition.
(d) producing essentially 100% H2SO4 from the converter by direct condensation (at ˜500° F.) in a boiler. High pressure (600 psi) steam is generated. The large amount of heat generated results from the reaction between SO3 and H2O to form liquid H2SO4. In contrast the prior conventional dry system absorbs the SO3 in dilute H2SO4 to generate the concentrated H2SO4 while the wet system uses a countercurrent condenser/stripper system to generate the product acid. In both cases high temperature heat is not recovered.
(e) scrubbing the low volume of tail gas with cold spent acid feed to essentially eliminate SO2 emission in the process tail gas. The FIGURES illustrate regenerating spent sulfuric acid from a gasoline alkylate plant generating 12,500 lb/h of spent acid. Table 1 provides the stream flows. Composition of the spent acid (stream 4) is 92.0 wt % H2SO4, 5.0 wt % C20H42, and 3.0 wt % H2O.
Referring now to
The decomposition reaction is carried out in paddle/screw decomposer system (37/13). Component 13 comprises a conveying system for recycling the carbonaceous particulates to the feed end of the decomposer. A hot heat transfer fluid at ˜700° F. (not shown) is circulated through the paddle/screw and jacket (not shown) of the vessel providing the heat of reaction (˜5 MM btu/h). Spent acid feed (stream 4) and a requisite amount of gas oil (stream 9 modeled as C20H42) to ensure complete conversion of the acid is sprayed at ambient temperature onto the agitated solids. Net carbonaceous product is removed via stream 32.
Stream 11 is the vapor effluent from the decomposer comprising SO2, water, a small amount of gas oil, and possibly a small amount of particulates. Stream 11 is cooled and partially condensed in an appropriate system (simply shown as condenser and drum 29) to knock out the contained hydrocarbon and to remove particulates. Overheads 43 comprise mainly SO2 and some water vapor, Condensate stream 50 containing mainly water, some entrained SO2 and hydrocarbon is fed to water stripper 34 which separates stream 50 into bottoms fraction stream 49 (containing the hydrocarbon vaporized in the decomposer, the water from the hydrocarbon oxidation reaction, and the water in the spent acid feed) and overhead vapor stream 48 contain water and SO2. The reboiler duty for column 34 is −4MM btu/h which can be provided by low pressure stream (reboiler temperature is 206° F.). Streams 43 and 48 are combined to form stream 19 which contains essentially only SO2 and H2O in 1:1 mole ratio.
Stream 19 is suction fed to converter feed compressor 17 and exits as stream 30 which is at a pressure sufficient to overcome the pressure drop through the converter and downstream acid recovery system described in
Stream 49 goes to oil/water decanter 36 where excess water is removed as stream 54 and the oil phase 52 recycled to paddle/screw decomposer system (37/13).
Stream 30 is mixed with oxygen in stream 77 to form stream 53 which becomes stream 74 feed to the converter 40 after preheat to reaction temperature by exchange in exchanger 18 with the molten salt heat transfer fluid 31 circulating through the shell (not shown) of the paddle/screw decomposer system (37/13). The oxygen level in the feed 74 is in approximate 10 mole % excess. The stream 73 is the return from the paddle/screw decomposer system (37/13) for indirect heat exchange reheating in the converter 40 by the reaction. The reheated stream 73 passes as stream 76 to the heat exchanger 18. SO2 conversion across the converter 40 is 99% (equilibrium conversion at the converter exit temperature is 99.5%). Converter 40 exit stream 65 is condensed at 500° F. in primary acid condenser/boiler 39 generating high pressure steam (600 psi) and producing stream 5 which is separated in condensate drum 14 into condensate stream 3, comprising 99+% H2SO4, and vapor stream 2 containing O2, SO3, SO2, and H2SO4. The vapor stream is further cooled in a vent condenser (not shown) generating stream 6 which is separated in drum 27 into tail gas stream 7 and additional acid condensate stream 24 which combines with stream 3 to form stream 13.
Hot acid product stream 13 contains a small amount of SO2 (˜500 ppm). A stabilizer column (not shown) can be added to the flow scheme to remove the SO2 which would then be joined with the tail gas stream. The SO2 in the combined streams can then be recovered by absorption (not shown) in the cold spent acid feed and recycled to the decomposer.
This example illustrates low temperature decomposition of spent sulfuric acid by heating 10 cc's spent sulfuric acid from an alky pilot plant in 50 cc fresh sulfuric acid. Gas evolved at 160-190° C. and was steady at 200° C. The first drop of overhead condensate was observed at 288° C. Sulfuric acid was being recovered overhead at 315° C. Black solids were formed in the pot
A similar test was made with only fresh H2SO4 that yielded little or no SO2 evolution. The test was repeated with iron oxide catalyst, but still no significant decomposition occurred up to the boiling point of sulfuric acid (about 315° C.).
A 500 cc, 3 neck flask with heating mantle, insulated top and Teflon® coated magnetic stirring bar. The flask was fitted with an acid feed inlet, thermometer and overhead vapor takeoff (atm press). The overhead takeoff included a condenser, liquid receiver and vent gas takeoff port to a gas sampling bag
Spent acid was decomposed in various mediums:
The gas oil results were superior to the other mediums evaluated. The specifics of the gas oil run are set out below.
