NO843343L - PROCEDURE FOR GAS desulfurization. - Google Patents

Description

In power plants where carbon-containing fuels, particularly bituminous coal, are burned as an energy source, the gases must be cleaned from the burning fuel to remove sulfur oxides to avoid pollution of the atmosphere. The use of limestone (CaCO^) as an aqueous slurry has long been used for the purpose of cleaning the sulfur oxides from the flue gas. It is usually used in towers where the gas is in contact with the calcium carbonate slurry in a counter-current fashion. The reaction between the sulfur oxides in the gas and the aqueous slurry of calcium carbonate forms a precipitate of calcium sulphite and sulphate, whereby sulfur is removed from the gas stream.

One of the problems with removing the sulfur oxides by reaction with the calcium carbonate or other soluble alkaline earth metal salts is that when the metal sulphate is precipitated, it forms scale on the contact surfaces. It is thus desirable that the scale is prevented from being deposited during the contact process, but that it is then allowed to precipitate and separate from the solution. Thus, the solution can be reconstituted and recycled to the process for removing the sulfur oxides from the flue gas without recycling a major portion of the scale-forming substance.

Various methods of inhibiting scale or counteracting its effect have previously been tried. Intermittent washing of the contact surfaces was thus attempted, but this was only usable with a low gas volume or low sulfur content in the gas. When chelating agents or threshold agents were used, scale was prevented from forming during the contact step, but it could not then easily precipitate in the sedimentation tanks unless the threshold or chelation limits were exceeded. In this way, the boiler scale would form both in the contact stage and in the sedimentation tanks.

It would be desirable to find an aid by which the metal ions could be kept in solution during the contact phase while preventing boiler scale formation, but that they could then be allowed to precipitate so that their sulphate salts could be removed.

The present invention provides just such a method. Special methylene phosphonates have been discovered which will keep the metal ions in solution during the contact phase and give rise to precipitation in a subsequent step.

Polymethylenephosphonic acid derivatives of polymers of N-aminoethylpiperazine have now been found to be particularly suitable in a flue gas desulphurisation process in which the aforementioned polymers act as threshold agents by regulating metal ions in the gas contact step in the process, but give rise to subsequent precipitation of these ions such as sulfates in a later step.

The polymeric methylenephosphonated amines, mainly dimers and trimers of N-aminoethylpiperazine (AEP), which are suitable for the present invention, are prepared by reacting N-aminoethylpiperazine (AEP) with a dihalogen or epoxyhalogen compound and then reacting the polymer which is formed thereby, with phosphoric acid and an alkanal (aldehyde) at low pH, usually provided by the presence of a mineral acid, for example hydrochloric acid. N-aminoethylpiperazine has the structure:

and can be referred to by the name 1-(2-aminoethyl)-piperazine.

In what follows, it will be abbreviated "AEP".

AEP can be reacted with any number of dihalogen or epoxyhalogen compounds to form a dimer or polymer. Any suitable epoxy-haloalkane (epihalohydrin) can be reacted, with 1,2-epoxy-3-chloropropane (epichlorohydrin) being preferred. In general, the epoxy halogen compound will have 3-10 carbon atoms. Other epichlorohydrin-type compounds include: 1,2-epoxy-4-chlorobutane, 2,3-epoxy-4-chlorobutane, 1,2-epoxy-5-chloropentane, 2,3-epoxy-5-chloropentane, etc. Usually the chlorine derivatives are preferred, although the corresponding bromine or iodine compounds can be used. Mixtures of epoxyhaloalkanes can also be used.

Dihalides with 1-20 carbon atoms can be used.

Saturated dihalides with the formula X(CH2)nX, where X can be chlorine, bromine, iodine or combinations thereof and where n is an integer from 1 to approx. 10, but preferably 2-6, can be used. Thus, for example, dichloromethane (methylene chloride), 1,2-dichloroethane (ethylene dichloride), 1,2- or 1,3-dichloropropane, 1,4- or 1,2-dibromobutane and the like can be used.

Aralkylene dihalides can also be used with the formula X-H2C-Ar-CH2-X where Ar is

where R can be hydrogen, halogen, alkyl with 1-4 carbon atoms, hydroxy and hydroxyalkyl with 1-4 carbon atoms and X is a halogen atom.

Dihaloalkylene ethers can also be used, for example bis(chloromethyl)ether or bis(chloroethyl)ether. Formulas for such ethers also include

X-CH2CH2(OCH2CH2)nX, where n is 1-3 and

where X is a halogen atom.

The dihalides may also be unsaturated. Thus, 1,2-dichloroethene, 1,4-dichloro-2-butene and the like can be used.

The conditions for making the polymer are to use the reactants in an amount of from approx. 0.2 to approx. 1 mol of the chain-extending compound, preferably from approx. 0.25 to approx. 0.6, i.e. the diepoxy, dihalogen or epoxyhalogen compound, per moles of AEP. The reaction temperature is from approx. 50 to approx. 100°C, preferably 70-80°C at a pressure sufficient to keep the reactants in liquid phase.

The phosphonomethylation (Mannich reaction) is then performed on the product in the presence of a strong acid to maintain a pH of less than 1.

While the reaction will proceed at temperatures over a wide range, i.e. 85-150°C, it is preferred that the reaction medium is maintained at reflux. The reaction is preferably carried out at atmospheric pressure, although sub-atmospheric and super-atmospheric pressure can be used if desired. The reaction times will vary, depending on a number of variables, but the preferred reaction time is 1-5 hours and the most preferred reaction time is 2-4 hours.

Although the phosphoric acid and the alkane can be added together or separately in any order to the reaction mixture, it is preferred to add the phosphoric acid to the polyamine and then slowly add the alkane under reflux conditions.

Approximately equimolar amounts of alkanal and phosphoric acid are used for the phosphonomethylation of the amine. An excess of either the alkane or the acid can be used, although large excesses of one or the other will be uneconomical. In the preferred method, an amount of alkanal which is equal to the amount of available amine hydrogen atoms and a small stoichiometric excess of the phosphoric acid will be used.

Although methanal (formaldehyde) is preferred, other alkanals (aldehydes) can be used instead of methanal, such as ethanal (acetaldehyde), propanal (propionaldehyde) and the like, where the alkanal can contain a straight or branched chain containing up to 10 carbon atoms.

Thus, the compounds according to the present invention can be represented by the formula

where A is an organic radical of the formula where Z is hydrogen, hydroxyethyl, hydroxypropyl, or BA where M is hydrogen, an alkali metal or ammonium and where B is a divalent radical originating from an organic dihalogen, diepoxy or haloepoxy compounds with one of the following structures where n is 1-10, n<1> is 1-3 and where R is hydrogen or methyl and R' is hydrogen, an alkyl radical or a hydroxyalkyl radical with 1-4 carbon atoms, a hydroxy -radical or a halogen atom, m is 1-10 and at least 50% of the Z groups are groups. Preferred compounds which are suitable for the invention are those where the B group originates from 1,2-epoxy-3-chloropropane (epichlorohydrin) or 1,2-dichloride, where m is 1 or 2, the Z group is

and M is H, Na or NH 2 .

The threshold-active compounds used in the invention are effective in preventing the precipitation of metal ions,

for example, Ca<++>as a boiler stone, when water-soluble compounds containing these metals are used in aqueous cleaning solutions used for cleaning sulfur oxides from flue gas from the burning of sulphurous hydrocarbon fuels.

The compounds which are suitable for the invention are effective in preventing metal ion precipitation at temperatures up to approx. 70°C. At higher temperatures, they are less effective and will give rise to the precipitation of CaSO. in solutions

+<+>

which contains SO^ and Ca ions.

This unexpected special property allows for their use in the above-described flue gas desulfurization process where the flue gases are brought into contact with an aqueous solution or slurry of calcium compounds, for example CaC03 or Ca(OH)2, or other metal compounds that will remove S02 and SO^from streams of gas brought into contact with them. After such a contact, and according to the present invention, where these ions are prevented from precipitating as the metal sulfates during the contact time by the presence of the above-mentioned threshold agents, part of the aqueous solution is circulated to a quench sump where it is heated to a temperature which is sufficient to reduce the threshold agent's activity (usually to above approx. 70°C) and cause precipitation of the metal ions as sulphates. A side stream containing the precipitated sulfates is withdrawn and sent to a thickener and then discarded.

The amount of threshold agent, i.e. the phosphonomethylated polymeric AEP products, which are usually used, is from approx. 0.5 to approx. 500 ppm depending on the particular agent and the amount of metal ions present in solution, which in turn depends on the gas volume and the concentration of sulfur oxides therein.

The figure is a schematic diagram of a typical absorption tower and associated equipment used in a gas desulphurisation process. The figure shows an absorption tower 10 and a supply tank 11. In the supply tank 11 there is a stirring device 2 3 which is driven by motor m. In the absorption tower

10 is a packed section 21 for bringing gas and liquid together and a second packed section 22 for removing fog from the gas before it leaves outlet pipe 27. The gas to be desulphurised enters the tower 10 through inlet pipe 26. Conduit pipe 31 and 33 for circulation of liquid leads from supply tank 11 to absorption tower 10, and conduit 35 for recirculation of liquid from quench sump 51 leads to tower 10; liquid distribution devices, for example spray nozzles 41, 42, 43 and 44, are connected to conduits 31, 37, 33 and 35 respectively. Distribution devices 41 and 43 direct absorbent solution (slurry) to packed section 21 and into collector 52 to bring it in contact with the gas entering tower 10 via pipe 26. Distribution device 42 directs water into packed section 22 while removing particulate moisture from the upward-flowing gas before exiting via pipe 27. Distribution device 44 sprays a slurry from quench sump 51 which comes into contact with and cools gas entering the bottom of the column through pipe 26. Collector 52 carries essentially all the absorbent liquid that enters tower 10 via conduits 31 and 33 and returns it to feed tank 11 via conduit 34. A overflow line pipe 38 gives the opportunity to refill quench sump 51 from supply tank 11 as needed when parts of quench sump bottom materials send s to a thickening device (not shown) via conduit 36. Pumps in the system are indicated by the letter p.

When operating the flue gas desulfurization process in the apparatus described above, the threshold agent according to the present invention is added to the aqueous feed material in tank 11 or to the feed material in conduit pipes 31 and 33 at a point just before their entry into tower 10. The aqueous solution and/or slurry of CaCO^ is led into the upper part of tower 10 from feed tank 11 via conduits 31 and 33 and through distributors 41 and 43. The gas stream to be desulphurised enters tower 10 at its lower end through tube 26 at a temperature of approx. 85°C, where it first comes into contact with quench liquid from quench sump 51, which is pumped via conduit 35 to distributor 44. From there, the gas flows upwards and comes into contact with atomized liquid that flows out of distributors (spray nozzles) 4 3 and 41, the latter spraying the liquid onto packed section 21. The flue gas, from which essentially all sulfur oxides have been removed, is then passed through a second packed section 22, where it comes into contact with water from conduit pipe 37 which is sprayed through distributors 42. This removes effectively all particulate moisture and slurry remaining in the gas stream, which then exits the top of the tower through pipe 27 and is discharged into the atmosphere. The absorption liquid that has been in contact with the gas flow falls down towards the bottom of the tower, but a significant part is collected by collector 52 and carried back to supply tank 11 via conduit 34. The level in sump 51 is maintained by adding absorption liquid from the supply tank 11 via conduit 38. This is necessary because part of the sump recycle flow in conduit 35 is withdrawn either continuously or intermittently and sent to a sedimentation tank or a thickener (not shown) where the calcium sulfate is removed and the water removed and recycled to the process via conduit 37.

In the gas desulphurisation process described above, the temperature achieved during the bringing together step in the upper part of the tower is normally 55-60°C, while the temperature in the quench sump is typically 70-80°C. An ideal inhibitor should prevent deposition in the packed (contact) section of the tower but allow precipitation to occur in the sump.

The following examples illustrate the preparation and use of the phosphonomethylated amines as threshold agents as well as comparative examples where the products known in the field for such use are used.

EXAMPLE 1 (Manufacturing threshold product)

An N-aminoethylpiperazine (AEP)-based amine was prepared by reaction between 22.7 g of N-aminoethylpiperazine (0.176 mol), 9.8 g of 1,2-dichloroethane (DCE) (0.099 mol) and 17.5 g of deionized water (molar ratio between DCE and AEP =0.56) in a 500 ml round-necked reaction flask equipped with a water-cooled reflux condenser, mechanical stirrer, thermometer with temperature controller and an addition funnel. The reaction product was then phosphonomethylated by adding approx. 75 g of concentrated hydrochloric acid and 32.6 g (0.40 mol) of phosphoric acid to the aqueous amine solution, and the reaction mixture was heated to reflux and kept at this for 1 hour. Aqueous 37% methanal (formaldehyde) solution, 28.1 g (0.35 mol) was added through the addition funnel over a period of

1.5 hours. The reaction mixture was heated at reflux for an additional 3 hours and then cooled. The product was assessed by being used in the laboratory circulation apparatus described below.

EXAMPLE A (Comparison example)

Conditions found in the desulfurization process described above were simulated in the laboratory using the following apparatus: a packed column was connected by means of piping at its upper and lower ends to a container containing a supersaturated solution of calcium sulfate (starting pH 3.5-4). This solution was circulated through the column by said solution being introduced at its upper end and returned through the pipe system which connected its lower end with said container. The column was packed with pieces of polyethylene tubing, 0.95 x 0.95 cm. Precipitation of calcium sulfate scale on the packed material was noted with time, and calcium remaining in solution was determined by filtering a sample of the solution through a Millipore filter (4-5.5 µm diameter openings) and titrated with standard disodium ethylenediaminetetraacetate. When circulating at a temperature of 54°C without any scale inhibitor, nearly half of the calcium sulfate present was precipitated to form scale within 30 minutes to 1 hour. The amount of CaSO^ present in solution at the beginning of the test was 3985 ppm, after 30 minutes of circulation only 2162 ppm remained, and 2067 ppm (this is approximately the amount in a saturated solution of CaSO4 at 54°C) was left after 70 minutes.

EXAMPLE 2

The product according to example 1 was assessed in the test equipment according to example A at 10 ppm (active acid) at temperatures of 54°C, 75°C and 90°C. The data are summarized in Table I. The data indicate the efficacy of the DCE/AEP phosphonate inhibitor. It prevents the deposition of scale at 54°C (as in the contact section) but then allows the calcium sulphate to precipitate when heated (as in the sump section). Dashed lines indicate that no measurements were taken.

EXAMPLE B (Comparison example)

A supersaturated solution of calcium sulfate was prepared containing 10 ppm aminotrimethylenephosphonic acid (ATMPA), a commercially available organophosphonic acid inhibitor. The solution was circulated at 54°C and titrated periodically with respect to soluble calcium. The same was done, but using a commercially available aminomethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid (DETA-MPA), again at 10 ppm based on the weight of solution. The results are listed in Table II.

A significant amount of calcium sulfate had accumulated

on the filling material of ATMPA after 1-2 hours. This datum indicates that the inhibitor is not effective in keeping the calcium in solution, i.e. in preventing scale formation, in the packed contact section in the desulfurization process described. Very little calcium sulfate scale was observed on the filling material when using DETA-MPA at both 54°C and 75°C. This shows that diethylenetriaminepentamethylenephosphon-

the acid was effective in controlling boiler scale formation at 54°C in the same packed contact section but would not allow precipitation to occur when heated at 75°C as in the sump section in the described process. A repeated run at 90°C did not result in deposition of calcium sulphate.

EXAMPLE C (Comparison example)

Aminoethylpiperazine, the compound from which the methylenephosphonate according to the present invention is prepared, was reacted with formaldehyde and phosphoric acid during the preparation of its trimethylenephosphonic acid derivative. This was used at 10 ppm (active acid) as in Comparative Example B. Little, if any, calcium sulfate scale was observed on the filler material at both 54 and 75°C. As in Comparative Example B, the aminoethylpiperazine trimethylene phosphonic acid inhibitor is not suitable for use in the desulfurization unit since it will not allow the calcium sulfate to precipitate at 75°C, but remain in solution at both temperatures.

EXAMPLE 3

An amine derived from aminoethylpiperazine was prepared by reaction between aminoethylpiperazine and 1,2-dichloroethane with a molar ratio of DCE to AEP of 0.80. The product was phosphonomethylated with phosphoric acid and methanal (formaldehyde) in the presence of hydrochloric acid to obtain the methylenephosphonic acid derivative. The product was rated at 10 ppm (active acid)

at temperatures of 54°C and 75°C as in example 2. The results are shown in Table III.

The compounds which have been shown to be effective in the process are methylenephosphonic acids derived from phosphonomethylation of N-aminoethylpiperazine-1,2-dichloroethane reaction products. In addition to the acid derivatives themselves, various metal and alkali metal salts, ammonium and amine salts, partial salts of the methylenephosphonic acids and mixtures thereof can be used.

The preferred products are those in which the amino hydroxyl atoms have been completely replaced by methylenephosphonic acid groups. However, some amino hydrogen atoms can be left unreacted if desired. Preferably, at least 50% of the amino hydrogen atoms have been converted. Moreover, functionality can be incorporated into the compounds in addition to the methylenephosphonic acid group. Typical groups are hydroxyalkyl, methylene sulfonate, hydroxypropyl sulfonate, carboxymethyl, etc.

Thus, the present invention is an improvement of the method for gas desulphurisation in which the gas is brought into contact with an aqueous solution or slurry of a calcium salt or hydroxide while removing the sulfur oxides contained therein. This contact causes the formation of calcium sulphate which is precipitated as scale on the surfaces where the contact occurs. The invention is to use a specific threshold agent which will inhibit scale formation on the contact surfaces, but allow it to occur in another part of the system.

Since the contact occurs at a lower temperature than in the quench section of the system described herein, it was discovered that a particular threshold agent that was effective at these contact temperatures became less effective at the higher temperatures found in the quench section the ion. It was thus advantageous to use this agent for inhibiting boiler scale formation on the contact surfaces since it would give rise to precipitation of the boiler scale in a later step so that it could be removed and it could be disposed of in a satisfactory manner.

Claims (6)

1. Process for gas desulphurisation, where a hot gas containing sulfur oxides is led into a vapor-liquid contact column where (1) said gas is quenched by bringing the gas into contact with an aqueous solution or slurry containing a calcium compound which will react with said sulfur oxides, and (2) the reaction product of the sulfur oxides and the calcium compound is recovered, characterized in that (a) the•phosphonomethylated product from the reaction between (1) aminoethylpiperazine and (2) an organic dihalogen or halogenepoxy compound, where the molar ratio of dihalogen or the halogen epoxy compound and the amine compound are from approx. 0.2 0 to approx. 0.80, is included in said aqueous solution or slurry and (b) said recovery step (2) comprises heating the aqueous solution or slurry during precipitation of the sulphur-calcium reaction product. 2. Method according to claim 1, where the calcium compound is calcium carbonate. 3. A product according to claim 1, wherein the dihalogen compound, if present, is a 3-C^Q compound and the halogen-epoxy compound, if present, is a C-3-C^Q compound. 4. Method according to claim 1, where the threshold agent has the following formula where A is an organic radical with the formula where Z is hydrogen, hydroxyethyl, hydroxypropyl, or BA where M is hydrogen, an alkali metal or ammonium and where B is a divalent radical originating from an organic dihalogen or halogenepoxy compound having one of the following structures: where n is 1-10, n<1> is 1-3 and where R is hydrogen or methyl and R" is hydrogen, an alkyl radical or a hydroxyalkyl radical with 1-4 carbon atoms, a hydroxy radical or a halogen atom , m is 1-10 and at least 50% of the Z groups are 5. Method according to claim 6, where Z is and m is 1-3. 6. Method according to claim 3, where B is a divalent radical originating from 1,2-dichloroethane and m is 1-4.