US3173969A

US3173969A - Method and apparatus for determining the titratable acidity of spent sulfuric alkylation acid

Info

Publication number: US3173969A
Application number: US153159A
Authority: US
Inventors: Kapff Sixt Frederick
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1961-11-17
Filing date: 1961-11-17
Publication date: 1965-03-16
Anticipated expiration: 1982-03-16

Description

March 16, 1965 S. F. METHOD AND APPARATUS FOR Filed NOV. 17, 1961 KAPFF 3, DETERMINING THE TITRATABLE ACIDITY 0F SPENT SULFURIC ALKYLATION ACID 3 Sheets-Sheet 1 Sump 3g 39.. v V v 3.9 FILTER Wafer J] V 32 27 K L w 43 26' /INVNTOR.

.Sixr Frederick Kapff ZZZzW W ATTORNEY I March 16, 1965 KAPFF 3,173,969

S. F. METHOD AND APPARATUS FOR DETERMINING THE TITRATABLEI ACIDITY OF SPENT SULFURIC ALKYLATION ACID Filed Nov. 17, 1961 3 Sheets-Sheet 2 Fig. 2

INVENTOR. Six! Frederick Kapff A T TOR/V5 Y March 16, 1965 s. F. KAPFF 3, METHOD AND APPARATUS FOR DETERMINING THE TITRATABLE CONDUCT/V/TY Microm/ros/cm x 10 CONDUCT/WT) MAX/MUM Microm/ms/cm x ACIDI'IY OF SPENT SULFURIC ALKYLATION ACID Filed NOV. 17, 1961 T/TRATABLE Aclo/rr (W1. 2; H2804 1 3 Sheets-Sheet 5 Fig. 3

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ORIGINAL ACID sr/mvam (WI. z H2 .90

INVENTOR.

Sixf Frederick Kapff ATTORNEY United States Patent h/ ETHOD AND APPARATUS FOR DETERMINING THE TITRATABLE ACIDITY 0F SPENT SUL- FURIC ALKYLATION ACE) Sixt Frederick Kapif, Homewoed, Ill, assignor to Standard Oil Company, Chicago, Ill., a corporation of Indiana Filed Nov. 17, 1961, Ser. No. 153,159 27 Claims. (Ci. 260-68362) This invention relates to determining the concentration of spent sulfuric acid employed in sulfuric acid catalyzed alkylation processes. More particularly, this invention relates to a method and means for measuring the titratable acidity of spent acid employed in the sulfuric acid alkylation of isoparaflin hydrocarbons with olefin hydrocarbons. More specifically, this invention relates to a method and means for measuring the concentration of such spent sulfuric acid by deter-mining changes in the electrical conductivity of a progressively diluted spent sulfuric acid sample.

The method of alkyla-ting isoparafiins and olefins in the presence of a concentrated sulfuric acid catalyst is well known in the petroleum refining art. Sulfuric acid alkylation processes have been described in detail in a number of publications such as the article Alkylation What You Should Know" published in the Petroleum Refiner, volume 37, No. 9, pages 316-329, September 1958. Typically, in this process, isobutane and butylenes are alkylated using a concentrated sulfuric acid catalyst which may range upwards in strength from about 85 weight percent H 80 The acid strength tends to decrease during the operation of the process due to dilution with su-lfonated hydrocarbons, i.e., catalyst-hydrocarbon complexes and esters, and with water. As the acid is diluted its activity decreases, especially as a result of water dilution. In thetypical instance, the alkylation reaction is carried :out in a series of mixing zones without the addition of acid to the materials passed through the zones. For satisfactory operation the acid strength in the last reactor must be maintained above a certain minimum concentration. When the acid strength drops below an established minimum, the acid is discarded. It is usual to recycle a portion of the spent alkylation acid inasmuch as is possible to reduce the operating costs of the process, since the cost of acid used is important. Thus, it is desirable to minimize the amount of spent acid discarded because of the drop in acid strength. However, the determination of the acid strength, i.e., the titratable acidity, is time consuming and expensive when carried out in the laboratory, and laboratory methods do not provide suiiicient rapidity for close operational control of the process.

The general object of the present invention is a method and means for measuring the strength of spent sulfuric acid alkylati-on catalyst which provides rapid and accurate information on the spent acid strength and which enables the unit operator to reduce acid consumption.

The principle of measuring the strength of various acids by determining the electrical conductivity of the acid solution has been recognized for some time. For example, it fresh concentrated sulfuric acid is diluted with water the electrical conductivity increases rapidly between about 98 percent and 92 percent and then drops oif before rising again as the dilution progresses. Thus, the first-occurring maximum in the electrical conductivity is at about a 92 percent by weight acid concentration. However, the electrical conductivities of spent sulfuric acid alkylation catalysts are very similar, regardless of the true titrat-able acidity, and these approximately correspond in electrical conductivity to a fresh acid-Water mixture of about 98 percent acid. If a spent acid sample is progressively 3,173,959 Patented Mar. 16, 1965 diluted with water the conductivity will increase to a maximum peak and then drop off as in the case of the diluted fresh acid. Now, it has been found that each of the spent acids has a characteristic maximum conductivity which is related to the original titratable acidity of the spent acid before water dilution. Thus, the maximum electrical conductivity can be determined for a spent acid sample which has been progressively diluted with water and titrata-ble acidity of the undiluted sample can be readily ascertained. The method of obtaining the relationship between the acid strength and the electrical conductivity characteristics comprises progressively diluting a plurality of spent sulfuric alkylation acid samples of known acid-i ties with Water, the total dilution of each sample being at least great enough to cause a first maximum in electrical conductivity to occur therein, measuring the first maximum electrical conductivity occurring in said samples at a known temperature, and correlating the original titratable acidity of each sample with the first indicated maximum conductivity of each sample.

The present invention further provides a method for determining the concentration of a spent sulfuric acid alkylation catalyst which comprises diluting a sample of spent sulfuric alkylation acid with water sufiicient to cause a first maximum in electrical conductivity to occur in the diluted sample, measuring the electrical conductivity of the diluted sample at a known temperature to determine a value indicative of the first-occurring maximum in electrical conductivity for the diluted sample and determining the ti-tratable acidity of the original sample by reference to a previously established correlation between the known titratable aoidities of a plurality of spent sulfuric alkylation acids and their corresponding values indicative of the firstoccurring maximum electrical conductivities.

The present invention additionally provides apparatus for determining the concentration of spent sulfuric acid alkylation catalyst which comprises mixing chamber mean-s, means for introducing a spent sulfuric alkylat-ion acid into said chamber means, means for progressively diluting an acid sample in said chamber means with wafor sufficient to cause a first maximum to occur in'the electrical conductivity of the diluted sample, temperature control means for maintaining liquid in said chamber means at a substantially constant temperature, measuring means for measuring the electrical conductivity of liquid in said chamber means, indicator means connected to said measuring means for receiving a signal from said measuring means and for indicating changes in the electrical conductivity of the liquid in said chamber means, and means for correlating the electrical conductivity characteristics of the diluted sample with the titratable acidity of the original sample.

The present invention will be more fully understood by reference to the following description of a preferred embodiment of apparatus according to the present invention and to the accompanying drawings wherein:

FIGURE 1 is a schematic representation of a-preferredembodiment of apparatus for monitoring the strength of spent sulfuric acid alkylation catalyst;

FIGURE 2 which is a detailed view of a conductivity cell and cell holder employed in the apparatus of FIG-" URE 1;

FIGURE 3 wherein it is shown a plot of electrical con ductivity versus the known titratable acidity for a number of sulfuric acid alkylation catalysts; and

FIGURE 4 which is a plot of the first-occurring conductivity maximum versus the original undiluted acid strength for a number of spent sulfuric acid alkylation catalysts.

Turning now to FIGURE 3, it is seen from plot A that if a sample of fresh sulfuric acid of about 98 percent concentration is progressively diluted with increasing amounts of water in increments of about 2 percent, the electrical conductivity of the acid rises rapidly to a peak at about 92 percent and then decreases, If diluted further, the conductivity will begin to rise again. However, if a sample of spent sulfuric alkylation acid is diluted in a similar manner the electrical conductivity will exhibit a first maximum at a titratable acidity which is unique for each spent acid sample. In plots B, C and D, the conductivity characteristics are plotted for spent alkylation acid samples taken from the alkylation of isobutane and butylenes and having acid strengths of approximately 94 percent, 92 percent and 88 percent, respectively. From these latter plots it is seen that each spent acid sample exhibits a firstoccurring maximum in electrical conductivity which may readily be correlated with the initial titratable acidity of the sample. Generally speaking the addition of about 8 percent water, based on sample has been found sufficient to achieve the first-occurring conductivity maximum in the acid samples tested. However, greater or lesser degrees of dilution may be employed, dependent upon the particular samples run and system employed in testing.

Preferably, the above correlation is performed graphically as shown in FIGURE 4, wherein the values indicative of each conductivity maximum are plotted versus the known original titratable acidity for each of a number of spent alkylation acid samples. From FIGURE 4 it is seen that once the correlation has been made between the firstoccurring maximum in electrical conductivities and the initial titratable acidities for a series of acid samples, the original titratable acidity for a sample can be readily determined by reference to the previously established correlation,

The correlation shown graphically in FIGURE 4 was established for a series of acid samples from the alklation of isobutane and butylenes. Similar correlations can readily be established for spent alkylation acids from the sulfuric acid alkylation of other isoparaflins, such as isopentane, isohexane, etc., and olefins, such as propylene, amylene, etc.

Apparatus for monitoring the strength of spent sulfuric alkylation acid can be utilized to aid the plant operator to more closely control acid strength. The following description refers to a preferred automatic acid monitor. As shown in FIGURE 1, spent sulfuric alkylation acid is passed by way of line 11 through valve 12 to the inlet of acid pump 13. At a preselected pressure this pump discharges at a substantially constant flow rate, which typically is about 60 ccs. per minute. The discharge is passed by way of line 14 through a check valve 16 to the inlet of a mixing chamber 17 which is positioned in a constant temperature bath 18 employed to remove the heat of mixing and to maintain the diluted sample at substantially constant temperature, i.e. about 100 Water is passed by way of

lines

21 and 23 to the inlet of a dilution pump 24 which is a variable discharge pump, which typically has a discharge rate varying between about 0 and 16 ccs. per minute at about 30 p.s.i. The discharge from pump 24 is passed by way of line 26 through a check valve 27 and by way of line 28 to the inlet of the mixing chamber 17 wherein the water intimately contacts and dilutes the incoming acid.

The continuously flowing acid sample is progressively diluted in the mixing chamber 17 by the water admitted thereinto and the mixture passes from the chamber 17 by way of line 29 to the inlet end of an electrical conductivity cell 31, also positioned in the constant temperature bath. The conductivity cell is of a type commercially available and comprises two spaced-

apart electrodes

71 and 72, as shown in FIGURE 2. The conductivity cell is positioned within the cell holder 73 as shown. The liquid from line 29 enters the cell inlet 74 and passes through the conductivity cell to the cell outlet 75 whilie an electrical current is passed between the two electrodes. The conductivity cell is positioned within the cell holder by means of the insulating inserts 76 and 77, which, typically, are made of tetrafluoroethylene (Teflon) resin. A fluid tight seal is accomplished at the outlet end of the cell by means of 0 rings 79 and 80, also made of Teflon. The inlet chamber 82 in the cell holder is provided as an additional protection against bubbles entering the conductivity cell and openings 83 are provided in the insert 76 to bleed 01f any trapped gases and to permit them to bypass the measuring system. Further, a means of pressure control is connected to the cell outlet by line 4,5 to maintain a constant pressure in the system and to prevent gas bubbles from forming in the acid-water mixture, since bubbles result in erratic readings in the conductivity cell. The pressure controller 40 employs a vertical inlet extending upwardly into the controller chamber and a weighted mass is fitted over the discharge end of the inlet tube to maintain the system pressure at about 30 p.s.i. This pressure has been found suflicient to prevent the formation of gas bubbles.

As mentioned above, the rate of water flow to the mixing chamber is controlled by pump 24. For this purpose a pneumatic controller 32 is employed, and, upon a signal on a timer (not shown), solenoid valve 33 opens which bleeds the air chamber 34 down to about 3 p.s.i., as limited by pressure relief valve 36. The valve 33 next connects the chamber 34 to a source of pressured air, the pressure of which is regulated by means of pressure regulator 37 (about 15 p.s.i.) and air then flows into the chamber 34 at a rate which is determined by the setting of the valve 38 which acts as a variable orifice. The air flows into the air chamber for a predetermined time after which the solenoid valve 33 again opens and the cycle is repeated. As the pressure in the chamber 34 builds up, the pneumatic pressure is transmitted by way of line 39 to the pneumatic controller 32 and acts to open the discharge of the pump 24 to progressively increase the discharge rate from the dilution pump. The timing is such that the maximum water flow rate from the pump 24 is about 16 ccs. per minute and the total cycle time is about 20 minutes.

The circular chart recorder 41 is electrically connected by lead means 42 to the conductivity cell 31 to receive an electrical signal from the conductivity cell and to continuously provide a graphic record of the changes in the electrical conductivity of the sample-water mixtures as the sample is diluted with greater amounts of water. If de sired, the chart may be printed to read directly the initial titratable acidity of the acid sample.

Water is supplied by way of

lines

21 and 43 to a heat exchanger coil 44 in the constant temperature bath to remove heat from the bath and provide a means of achieving substantially constant temperature control as described above. The amount of water flowing through the coil 44 may be regulated by the valve 46 in the outlet 47 connected to the coil. Additionally, water is supplied by way of line 48, with the flow rate being controlled by valve 49, to the pressure control valve 40 to provide suflicient flow through the pressure control device for optimum operation of the apparatus. Further, flushing water may be supplied as desired by way of line 51 to the inlet of the acid pump 13 so that the system may be flushed out with water to remove acid trapped therein.

The above-described apparatus may be constructed of any suitable corrosion-resistant material such as Teflon or other resins and alloys such as Hastelloy B, Hastelloy D or Carpenter 20.

The foregoing description of the preferred embodiment of the present invention is given for illustrative purposes only. From the foregoing description various modifica tions in the construction and operation of apparatus for practicing the present invention will become apparent to the skilled artisan, and, as such, these fall within the spirit and scope of the present invention. For example, the water may be supplied to the mixing chamber through a series of timer-actuated solenoid valves arranged in parallel and operated by a timer program to be successively opened and change the water addition in approxiaivaeeb mately 2 percent increments of 0 to about percent water. Likewise, the above-described pumping system may be employed using a variable speed dilution pump :to achieve theincremental dilution of the sample. Similarly, various batch-type systems can be devised to accomplish the results obtainable with the above-described apparatus. 7

What I claim is:

1. The method of obtaining a relationship between the titratable acidity of spent sulfuric acid alkylation catalyst and the electrical conductivity thereof which method comprises progressively diluting a plurality of spent sulfuric alkylation acid samples of known acidities with water, the total dilution of each sample being at least great enough to cause a first maximum in electrical conductivity to occur therein, measuring the first maximum electrical conductivity occurring in said samples at a known temperature, whereby said first maximum electrical conductivity provides a measure of the original titratable acidity of each sample.

2 The method of claim 1 wherein said dilution is varied from about 0 weight percent to at least about 8 weight percent, based on sample.

3. The method of claim 1 wherein said acid sample is taken from a process for the alkylation of isobutane and butylenes.

4. The method of determining the titratable acidity of a spent sulfuric acid alkylation catalyst which comprises diluting a sample of spent sulfuric alkylation acid with Water sufiicient to cause a first maximum in electrical conductivity to occur in the diluted sample, measuring the electrical conductivity of the diluted sample at a known temperature to determine a value indicative of the first-occurring maximum in electrical conductivity for the diluted sample whereby said value provides a measure of the titratable acidity of said spent sulfuric acid alkylation cataylst.

5. The method of claim 4 wherein said spent acid sample is taken from a process for the alkylation of isobutane and butylenes.

6. The method of claim 4 wherein said spent acid sample is continuously passed through a mixing chamber at a substantially constant flow rate, said water is passed through said mixing chamber at a flow rate which increases substantially continuously from about 0 percent to at least about 8 percent of said sample flow rate to intimately contact said sample in said chamber, and wherein said conductivity measurement is made substantially continuously.

7. The method of regulating the acid strength in the sulfuric acid alkylation of isoparaffins with olefins which method comprises vvdthdrawing a sample of spent sulfuric acid from the alkylation operation, progressively dilut ing the withdrawn sample with water sufiicient to cause a first maximum in electrical conductivity to occur in the diluted sample, measuring the electrical conductivity of the diluted sample at a known temperature to determine the first-occurring maximum in electrical conductivity, whereby said first-occurring maximum provides a measure of the original titratable acidity of said spent sulfuric acid, and employing said original tn'tatable acidity to control the discard of spent acid from said alkylation process.

8. The method of claim 7 wherein said dilution progresses from about 0 percent to at least about 8 percent, based on sample.

9. The method of claim 7 wherein said process is the alkylation of isobutane with butylenes.

10. The method of claim 8 wherein said dilution is continuous and wherein said conductivity measurement is made continuously during said dilution.

11. Apparatus for determining the concentration of spent sulfuric acid allcylation acid which apparatus comprises a mixing chamber, means for introducing spent sulfuric alkylation acid into said chamber, means for progressively diluting an acid sample in said chamber with water, temperature control means for maintaining liquid in said chamber at a substantially constant temperature, measuring means for measuring the electrical conductivity of liquid in said chamber, indicator means connected to said measuring means for receiving a signal from said measuring means and for indicating changes in the electrical conductivity of the liquid in said chamber.

12. The apparatus of claim 11 wherein said dilution means supplies water in an amount increasing from about 0 percent to at least about 8 percent, based on sample, to said chamber.

13. The apparatus of claim 11 wherein said sample introduction means is a constant flow rate pump and said dilution means is a variable discharge pump having a flow rate variable between about 0 and at least about 8 percent of the flow rate of said constant flow rate pump.

14. The apparatus of claim 11 wherein pressure control means are provided to maintain a preselected pressure in said mixing chamber.

15. Alkylation acid monitor apparatus which comprises a mixing chamber provided with inlet means and outlet means, an acid pump connected to said inlet means for passing an acid sample through said chamber at a substantially constant rate of flow, a dilution pump having a variable discharge rate connected to said inlet means for progressively diluting liquid in said chamber, means for varying the discharge rate of said dilution pump from 0 to a least about 8 percent of the flow rate of said acid pump, an electrical conductivity cell connected to said outlet means to receive liquid from said chamber, a recorder electrically connected to said cell to provide a graphic record of the electrical conductivity characteristics of liquid passed through said cell, a constant temperature bath for maintaining the liquid in said chamber and in said cell at a substantially constant temperature and means for correlating the electrical conductivity characteristics of the diluted sample with the titratable acidity of the original sample.

16. The apparatus of claim 15 wherein there is provided a pneumatic valve on said dilution pump to control the discharge rate therefrom and means for varying the pneumatic pressure supplied to said valve to control said discharge rate according to a preselected timed schedule.

17. The apparatus of claim 15 wherein there is provided pressure control means connected to said chamber to maintain the pressure therein at a preselected level.

18. Apparatus which comprises a mixing chamber provided with inlet means and outlet means, a constant flow rate sample pump connected to said inlet means, a variable discharge rate dilution pump connected to said inlet means, means for varying the discharge rate of said dilution pump from 0 to a least about 8 percent of the flow rate of said sample pump according to a preselected timed schedule, an electrical conductivity cell provided with an inlet and an outlet and connected to said chamber outlet means to receive liquid from said chamber, a pressure controller connected to said cell outlet for maintaining the pressure therein at a preselected level, a constant temperature bath for maintaining liquid in said chamber and in said cell at a substantially constant temperature, and a recorder electrically connected to said cell to provide a graphic record of the electrical conductivity characteristics of liquid passed through said cell.

19. A method for determining the titratable acidity of spent sulfuric acid alkylation catalyst which comprises progressively diluting said spent alkylation catalyst with water, and determining the electrical conductivity of said catalyst during said diluting, whereby the change of said electrical conductivity during said diluting provides a measure of the titratable acidity of said spent sulfuric acid alkylation catalyst.

20. The method of claim 19 wherein said spent sulfuric acid alkylation catalyst contains more than about percent by Weight of sulfuric acid.

21. The method of claim 19 wherein the pressure of the catalyst is maintained sufiiciently high while determining the electrical conductivity to prevent bubble formation in said catalyst.

22. The method of claim 19 wherein the amount of water used for said diluting is within the range of -20 ,percent by weight based on said spent sulfuric acid alkylation catalyst.

23. The method of determining the titratable acidity of spent sulfuric acid alkylation catalyst which comprises progressively diluting a sample of said alkylation catalyst with Water suflicient to cause a first maximum in electrical .conductivity to occur in the diluted sample, and measuring the electrical conductivity of the sample during said dilutisoparaflins with olefins, which method comprises withdrawing a sample of said catalyst, progressively diluting said sample with water sufficient to cause a first-occurring maximum electrical conductivity of said sample, measuring the electrical conductivity of said sample during said diluting to determine said first-occurring maximum electrical conductivity, and controlling the fresh sulfuric acid addition rate to said process in response to said determined first-occurring maximum electrical conductivity to maintain said first-occurring maximum electrical conductivity at a value corresponding to the desired predetermined titratable acidity of said catalyst.

26. The method of claim wherein the diluting-water concentration in said sample is varied within the range of 0-2O percent, based on said sample.

27. The method of claim 25 wherein the measurement of electrical conductivity is carried out at a substantial- 1y constant temperature.

References Cited in the file of this patent UNITED STATES PATENTS 2,592,063 Persyn Apr. 8, 1952 2,765,218 Arnir Oct. 2, 1956 2,977,199 Quittner Mar. 28, 1961