GB2038648A - Purifying waste water - Google Patents

Abstract

A process for purifying contaminated water comprises contacting the contaminated water with air such that the contaminants are stripped into the air. The air may then be utilized as air of combustion in an incinerator where the contaminants are rendered harmless.

Description

SPECIFICATION Method of purifying contaminated water The present invention relates to a process for purifying contaminated water and more particularly the present invention relates to a process for purify ing water by stripping the contaminants from water with air.

Water is the universal media in chemical proces ses. It is used alternately as a reactant, as a transpor tation media, as a separation media, as a washing media and for many other purposes. In processes in chemical plants, it is not unexpected that the water becomes contaminated with impurities. Accord ingly, as the result of the passage of environmental laws it is necessary at the present time to purify the water before it can be released back to the environ ment. The above comments apply with respect to a Silicone Plant. In silicone plants, benzene and chlorobenzene are utilized or produced in the process for producing phenyl-containing polymers.

Methyl chloride is used in the basic process for producing the methyl chlorosilanes. Xylene and toluene are used as solvents in the preparation of silicone resins. Another undesirable chemical vinyl chloride is utilized in the preparation of vinyl containing silanes and polysiloxanes which silicone compounds find wide usage both in heat vulcaniz able silicone rubber composition and room temperature vulcanizable silicone rubber compositions.

It is noted that these chemicals are present in a silicone plant and find their way into the wastewater that is utilized in a silicone plant.

Most amounts of such materials are separated out from the wastewater by decantation, however, small amounts of such chemicals up to 1 to 2% are still present in the wastewater even though most of the undesirable chemicals have been removed by decantation. In some cases such amounts of chemical impurities are not tolerable in wastewaterthat is to be released to the environment.

As an example in the past, it was common to treat the wastewater from the chemical plant and specifically a chemical silicone plant in accordance with the following process for processing the water. The wastewater was passed into a flotation tank where all floating impurities were removed by skimming.

Then the wastewater was passed into a series of tanks in which a base was added to the water so as to make it alkaline. The water was then subsequently passed through a precipitation tank where any pre cipitate that was formed as a result of the alkalinization procedure was removed. The clear water was then taken and allowed to settle for a certain period of time and then passed to a second neutralizer area where it was neutralized with acid. The sediment that formed was allowed to precipitate and the clear water was allowed to pass into the environment. The procedure was acceptable in the past for the release of purified wastewater into the environment.

Accordingly, it was common to release such purified wastewater to the environment with a noticeable abount of impurities in them. However, as a result of recent Environmental Laws and Regulations, it has become necessary to further purify the wastewater such that the amount of impurities in the wastewater are reduced to parts per million or parts per billion.

Accordingly, as the result of the Environmental Regulations in this area it has become more and more necessary to revise and to change the pu rification processes so that most, if not all, of the impurities are removed from wastewater before it is released to the environment. One method that has been suggested for the purification of wastewater is a distillation procedure. However, the implementation of a distillation procedure where a number of distillation columns are utilized just for the purification of wastewater is unduly expensive. This is espe ciallytrue in view of the fact that in orderto handle and purify all the wastewater that is present in a chemical plant an extensive amount of distillation columns would have to be utilized.

Another method that has been proposed is a biodegradable treating procedure or process for the treating of the wastewater. The only difficulty with such a process is that it only works on chemicals that are biodegradable. Toluene and xylene are not biodegradable chemicals. In addition, the biodegradable procedure requires the presence of a large amount of settling and treating tanks.

Accordingly, it would be highly desirable to find an economic process for purifying wastewater emanating from a chemical plant.

Stripping procedures are well-known. The use of steam to strip or separate ingredients in liquid mixtures has been known for some time. Accordingly, for instance, in a silicone plant there may be utilized a steam stripper which passes steam or inert gas through a solution of a silicone resin so as to strip off or remove the solvent leaving behind the solid silicone resin. The steam mixed with a solvent may then be taken and the steam condensed so as to allow the two phase separation of the solvent from the water. Example of a steam stripper for utilization in a chemical plant is for instance to be found in the article entitled "Choosing the Process for Chloride Removal" by M. F. Nathan appearing in the January 30, 1978 issue of Chemical Engineering.

It should be noted that such steam stripping is of limited utility and certainly could not be utilized to purify wastewater, although it could be utilized to remove solvents or mixtures of solvents from other ingredients. Accordingly, it was highly unexpected that air could be utilized to remove certain contaminants from industrial wastewater. Another problem of utilizing steam for stripping materials from a solution was the separation of the steam or liquid water when it was condensed from the solvent, that is the liquid water might still have a certain amount of solvent after the decantation separation procedure, unless there were utilizied distillation separation procedures.

Accordingly, it was highly desirable to find a stripping procedure which could be utilized to remove impurities from water.

There is provided by the present invention a process for removing contaminants from water comprising (1) passing a first stream of a gas into the contact with the second stream of wastewater containing contaminants where the contaminants are immiscible or only slightly soluble in wastewater and said contaminants are volatile. By volatile compounds it is understood those compounds which have a substantial vapor pressure at the operating temperature of the system. As a way of quantifying the property of volatility, included in the category of volatile compounds are those boiling at 1500C or below, at atmospheric pressure.However, the value of 1 500C should not be understood as an inflexible limit beyond which the stripping process described herein ceases to operate, and (2) separating the third stream of said wastewater from said gas which has a smaller amount of contaminants than said second stream and removing a fourth stream of a gas from said wastewater wherein said gas has said contaminants therein. The process can be carried out where the wastewater is at a temperature anywhere from 0 to 95 C.

This process can be carried out by contacting the air with the wastewater for instance by passing the air through tanks in which the wastewater is rapidly stirred. The best type of contact is envisioned to be in a packed column. The volume of the packed column is determined from the equation, N = (k,a) V (AC)m where N is equal to pounds of contaminant to be removed per hour; V is equal to the volume of packing in ft.3; (k,a) is equal to the mass transfer coefficient in Ibs. of contaminant removed per hour, per ft.3, per unit of concentration gradient and (AC)m is the log mean concentration gradient.

The log means concentration gradient may be obtjined by simply plotting on graph paper the equilibrium curve which is obtained from experimental data which shows at various concentrations the equilibrium values of the precent contaminant in the water versus that in the air with which it is in equilibrium. To that curve there is added the operating curve, that is, the curve at which it is desired that the column operate with 0% of the contaminant in the air, that is the concentration of the contaminant in the water and air going into the column and coming out of the column. The percent contaminant in the air forms the vertical axis of the graph and the percent contaminant in the water forms the horizontal axis of the graph. From these two curves then the vertical line of the graph is the concentration gradient and the log means can be easily obtained from such a graph.The theoretical stages of the packed column may also be obtained from such a graph by procedures that are well-known and will be explained below.

As is evident there is necessary to have actual experimental data in orderto calculate the volume of the packed column and the experimental data must be obtained with respect to the contaminant that is to be removed and if there are a number of contaminants to be removed then the dimensions of the packed column are to be calculated from the equilibrium data of each contaminant. Then the volume is chosen for the packed column which results in the removal of all the contaminants to the desired deg- roe.

There are two aspects to the invention of the instant case. One aspect is the appreciation that impurities could be removed from water and especially minor amounts of impurities could be removed from water by stripping the wastewater with air or with an inert gas.

It was not until the present invention that it was appreciated that minor amounts of impurities in water could be removed from the water by stripping the water with air or an inert gas. Prior work in the art either with steam stripping or other type of stripping had involved the removal of solvents or other ingredients from a liquid mixture by the use of a stripping technique. The invention of the instant case lies in the discovery that impurities could be removed from water utilizing a technique but resulting in an effectiveness of the removal of impurities from the wastewater such that the wastewaterwould be essentially purified.

The second aspect of the present invention was in the conception and reduction to practice of carrying out the removal of the impurities by the stripping of the water with air or an inert gas in a packed column, and the calculation of the volume of such packed column, both the height and the diameter utilizing the techniques that will be disclosed hereinbelow.

This method of calculation differs from that disclosed in the application of R. W. Shade, 60Sl-192 entitled "Process for Removing Contaminants from Wastewaterfrom Silicone Processes" filed as a Patent Application in U.S.A. on the same date as the instant case. It is in the method and technique in deciding what impurities are removable from wastewater and the method by which the volume of the column, that is the height, as well as the cross section area is determined, that the present case differs from that of the disclosure of the Shade Patent Application.

To go into the present application, there are two conditions that have to be met in order for air strip ping in accordance with the present invention to function. The first is that the impurity must be immiscible or only slightly miscible, i.e., soluble, in the wastewater. Preferably it is totally immiscible in the wastewater. However, as long as the impurities are only partially miscible or soluble in the wastewater, the present process will work. It should be noted that where there is total immiscibility, the material can be decanted. However, still small amounts of the material would be present in the wastewater above that set by environmental requirements. Accordingly, it is preferred thatforthe present process to work the impurity be immiscible or only partially miscible in water. By partially miscible, it is meant that in a given amount of water no more than 0.5-2% by weight of the impurity can be dissolved.

The terms "immiscibility" or "partial miscibility" are explained in standard treatises on Physical Chemistry like: Physical Chemistry by Gilbert W.

Castellan -- Addison-Wesley Publishing Company.

The other requirement for the impurity to be removed from wastewater is that the impurity have a certain volatility, that is the impurity must have a sufficiently high vapor pressure such that it will be entrained and removed by the air passing through the wastewater. Accordingly, it is preferred that the impurity have a boiling point of 1 50 C or less.

In accordance with the present invention and in order for the air stripping of the present invention to work, that is, in order for an inert gas or airto be utilized to purify wastewater by air stripping, it is necessary that the impurity be immiscible or only partially miscible in water in accordance with the above requirements and also have a boiling point of 150"C or less. If it is more soluble than the above indicated extent, then the material will not be removed as effectively as would be desired. If it has a boiling point above 1500C,then of course, it will not be removed also as effectively as desired.By being able to be removed as effectively as would be desired, it is meant that 99.99% by weight of the impurity can be removed in a packed column by stripping the wastewater with air by utilizing a packed column that does not exceed 150 feet in height. It is, of course, obvious that a higher packed column could be utilized, however, such a higher packed column gets to be very expensive in terms of the construction of the column.

Accordingly, to utilize a reasonable size column and to obtain a large proportion of the impurities removed such as to be able to obtain 99.99% removal of the impurities with air as a stripping gas, it is important that the immiscibility and the boiling point factors mentioned above be met. However, it is noted that the present invention would operate to remove impurities with a boiling point above 1 50 C, however, such impurities could not be removed to the extent of 99.99% by weight from the wastewater.

It should be noted that the present invention contemplates the removal of at least 40% by weight of the impurities from the wastewaterto as much as 99.99% or more of the impurities from the wastewater. To obtain such removal, it is important in practical procedures for calculating the volume of the column, that the water rate and the air rate be set at practical levels.

Accordingly, for practical levels in the operation and the construction of a column, it is important that the water rate vary from 2000 to 60,000 Ibs. per hour per ft.2 of cross section of column into the column and the air rate vary from 100 to 3000 Ibs. per hour per ft.2 of cross section of column.

It should be noted that the water and air rate will vary depending on the percent removal of the impurities that is desired in the column and with respect to the dimensions of the column. However, nevertheless, to get at least 40% removal of the impurities from the wastewater and to utilize a column with practical diameter, it is desirable although not critical, that the above flow rates for water and air be maintained in the column.

It should be noted that an inert gas can be utilized to entrain the impurities from the water such as for instance nitrogen.

The nitrogen gas with the impurities entrained in it may be passed into a condenser where the nitrogen is cooled considerably so that the entrained gases or impurities are changed into liquids or solids which may then be separated from the cooled gas and dis posed of properly. However, in a plant with an incinerator, the use of air as in the process of the instant case is of immeasurable advantage. Thus, the air which is readily available and specifically clean air which is readily available can be passed through the column to remove the entrained impurities in the air. The air may then be passed as air of combustion into the incinerator where the impurities may be combusted to harmless by products, such as carbon dioxide and water, which are then vented into the atmosphere.All or most of the impurities that can be removed by the process of the instant case are hydrocarbons.

Accordingly, the byproducts of combustion of such materials would be water and carbon dioxide.

Accordingly, if the plant has an incinerator, the purification of the wastewater becomes immeasurably simplified in that air can be utilized to strip the water and then the air can be simply used as air of combustion to remove and combust the impurities that are entrained in the air. Where an incinerator is not available or cannot be installed, then an inert gas may be utilizied for the entrainment procedure.

However, the use of an inert gas, which had to be recycled increases the cost of the process. It should be noted that the process of the instant case can take place at any temperature between the freezing point of water and the boiling point of water. As long as water is in the form of liquid, it can be stripped in accordance with the process of the instant case. It should also be noted that there is another limitation of the temperature of the process and that is the boiling point of the impurities, if the water temperature is above the boiling point of the impurities, then some of those impurities may be vented to the atmosphere which depending on the locality may be undesirable.

Accordingly, the process of the instant case can be carried out at a temperature anywhere from 0 to 950C and more preferably from 0 to 600C. It is most preferably carried out at room temperature. However, it should be noted that if the wastewater is heated already as the result of the process that has been utilized such that its temperature is 40-50 C, it can be utilized in the process of the instant case without any further cooling. Preferably, as stated previously, the stripping is carried out at room temperature since this does not require any special heating or cooling of the wastewater. The impurities with low boiling points can be removed easily from the wastewater without being vented into the atmosphere. An important advantage or ramification of the invention as it is reduced to practice in the instant case, is the fact that the stripping of the wastewatertakes place in a packed column.

It should be noted that a packed column is the preferred embodiment of the instant case and is not necessarily the only means by which the invention of the instant case could be carried out. For instance, it is only necessary that the air come in contact with waterfora sufficient time sothatthe impurity can be stripped from the wastewater into the air or inert gas.

Accordingly, such stripping may be carried out in any device or apparatus which allows sufficient con tact between the wastewater and the inert gas or air.

From here on there shall be only reference to air in the specification, however, it should be understood that an inert gas may be utilized also as a stripping medium in accordance with the disclosure hereinbefore.

Accordingly, specifically, the contact between the air and the wastewater may take place in tanks in which there is sufficient agitation and there may be a number of agitated tanks in series in which the wastewater is passed from tank to tank and the air may be passed countercurrently to the wastewater in the tanks and as this method may be utilized to strip impurities from the wastewater. It can be appreciated that this method is not as effective as a packed column.

Accordingly, any method which agitates or brings air into contact with the water can be utilizied to strip the impurities from the wastewater, bearing in mind the criteria set forth previously with respect to the boiling point and immiscibility. I However, it has been found that the most efficient means for doing this is a packed column. The reason for this is that a packed column allows the maximum amount of contact with the air and the wastewater and thus is the most efficient means of contacting and stripping the impurities from the wastewater.

In addition, a packed column takes up the least amount of space since it is normally a vertical structure like a distillation column and does not take up horizontal space in the plant.

It should be noted that the boiling point that was given for the impurity to be removed within the criterion set forth previously from wastewater of up to 1 500C or less is at 760 millimeters mercury pressure and, of course, the boiling point would vary if it was determined at lesser or higher pressures. The volume of a packed column may then be determined by the mass transfer equation.The mass transfer equation is as follows: (1) N=(k#a)V(AC)rn where N is equal to pounds of contaminant or impurity to be removed per hour; V is equal to the volume of packing in the column in cu. ft.; (k,a) is equal to the mass transfer coefficient in Ibs. of contaminant removed per hour per cu. ft. per unit of concentration gradient and (AC)m is log mean concentration gradient. To utilize the above equation to determine the volume of the packed column there is needed certain empirical data.

Accordingly, the empirical data that is needed to determine the quantity (AC)m, the concentration gradient is determined as shown in Figure I. Figure I is a graph of the equilibrium curve and operating curve of a particular contaminant or impurity in water and air. It should be noted that if the gas was other than air then the equilibrium curve would have to be determined for the gas which was other than air. The equilibrium curve 200 is of the per cent of contamin ant in the air versus the per cent of contaminant in the water for a certain temperature and is plotted on graph paper.This equilibrium curve is determined empirically by simply taking samples and determin ing the per cent contaminant in water and in the air over the water which is an equilibrium with the water and determining the concentration of the impurity in the air for that temperature. Once the values are determined for various concentrations of the contaminant in the water and air, then the equilibrium curve for per cent contaminant in the air versus per cent contaminant of water can be drawn.

Then there is drawn the operating curve 210. The operating curve is a straight line and is determined at one point 220, the fact that there will be 0% contaminant in the air and there will be the per cent contaminant left in the wastewater as it leaves the column so as to determine one end of the operating curve 210. The other point of the operating curve 225 will be determined by the per cent of contaminant in the wastewater entering the packed column versus the percent contaminant is desired to have in the air coming out of the packed column. Between those two points, 225 and 220, there is drawn a straight line which is the operating curve 210.

It should be noted that with equilibrium curve 200, one point will go where the vertical axis will meet the horizontal axis 230 since if there is 0% contaminant in the water there will be O% contaminant in the air.

The other points on the equilibrium curve 200 will be determined by empirical testing as pointed out pre viously, or estimated otherwise. The vertical distance between the equilibrium curve and the operating curve such as 240 is AC or the concentration gradient. The mean vertical distances between the operating curve and the equilibrium curve is (AC)m for utilization in equation (1) above.

Then it is only necessary to determine the value for (k,a) to make the determination as to the volume of the packing of the column. The value for the mass transfer coefficient is also determined empirically and in one aspect it can be obtained from a packed column manufacturer for a particular type of packing. However, a better way of determining the mass transfer coefficient is to take a particular type of packing and particular contaminant and put it in a pilot plant packed column. In such a pilot plant packed column, all of the factors in the equation will be known except for (k,a). Accordingly, the (k,a) mass transfer coefficient can be easily determined for a particular type of contaminant or impurity for a certain type of packing, and a certain type of gas and air flow rate. It should be noted that the mass transfer coefficient is a function of the velocity of the air, the velocity of the water and the type of packing and for a particular type of packing, it has to be determined empiricallyfora particulartypeofcontamin- ant. It should be noted that even though the mass transfer coefficient would be determined for lower through puts of air and of water in a pilot plant column, nevertheless the value can be scaled up by well-known techniques for the actual column that will be built. The mass transfer coefficient has been determined for a particularcontaminantfora particulartype of packing in a pilot plant column and can be scaled up to the actual flow conditions of air and water in the column to be built. The volume of the packing can then be determined from equation 1.

Once this has been done, then the diameter of the packed column may be determined. This is done by procedures well known in the art as shown for ins tance in the following publications: (1) Tower Packings and Packed Tower Design, by M. Leva, published by the United States Stoneware Company, Akron, Ohio (2) Perry's Chemical Engineers' Handbook~ Perry, Chilton and Kirkpatrick- Editors, published by McGraw-Hill Book Company, Fourth Edition, Chapter 18.

(3) Technical Data Related to Tower Packing, Data Series Published by the U.S. Stoneware Co., Akron, Ohio.

The diameter of a column will be also a function of airflow rate and the wastewater flow rate. Thus, in accordance with the procedure set forth in the above publications once the gas and liquid flow rate have been determined and for a certain type of packing the diameter will be easily determined. Once the diameter of the column is obtained, then the height of the column can be determined.

It should be noted that for each contaminant that is to be removed an evaluation of AC has to be made in accordance with Figure 1 so as to determine the contaminant that will give the smallest AC. Once the smallest AC is determined for a series of contaminants, then the minimum volume of the column can be determined and accordingly, the minimum height of the column can be determined so as to give the desired separation in accordance with the disclosure set forth hereinbefore.

It should be noted that a series of equilibrium curves can be plotted for a series of contaminants that are present in the wastewater and the AC determined for each one or an eye evaluation basis and the curve that appears to give the smallest mean AC can then be evaluated to be utilized in Equation 1.

Once the minimum mean AC has been determined this can be used to calculate the minimum volume of the packed column and from a knowledge of gas and liquid flow rate desired in a packed column, then the minimum height of the packed column can be determined for a particular type of packing.

There is another way in which the height of the column may be determined or the volume of the column may be determined in accordance with the instant invention and that is taking Figure 1 there maybe drawn a series of horizontal and vertical lines in the form of a triangle such as lines 245 and 255, in which each triangle constituting a theoretical stage that is necessary in the packed column.Then beginning with such set of triangles drawn from the beginning point in the operating curve, that is point 225, which represents the contaminant concentrations in the water coming in and in the air coming out of the packed column, and drawing these sets of horizontal and vertical lines down through the area between the operating and the equilibrium curve until one arrives at point 220, that is the point in the operating curve where the uncontaminated air enters the packed column such as shown by dotted line 265 there is obtained a number of triangles from point 225 to point 265 on the operating curve in Fig ure 1.

By counting these triangles one arrives at the number of theoretical stages in the packed columns that are necessary to carry out the desired separation. Once the number of theoretical stages are known, then the height of the packed column can be determined. A worker skilled in the art can refer to Perry & Chilton's Chemical Engineer's Handbook~ Fifth Edition - Published by McGraw-Hill Company ~to determine the height of one theoretical stage once the above empirical data is known, that is the data set forth in Figure 1.

It should be noted if a number of contaminants or impurities are to be removed in a packed column, then the equilibrium curve for such materials will have to be plotted as in Figure 1, then the equilibrium curve closest to the operating curve will be taken to determine the number of theoretical stages and once the theoretical stages were known, then the height of the packed column could be determined.

Then knowing the air flow rate and liquid flow that is to take place in the packed column, the diameter of the packed column may be determined by the method set forth previously, that was utilized in the case where the volume of the packed column was determined by the mass transfer equation of Formula (1). As stated previously, where a number of contaminants are to be removed from the packed column then the equilibrium curves are to be drawn as shown for one contaminant in Figure 1 and then curve closest to the operating curve would be utilized to determine the number of theoretical stages.

From the empirical data and using Perry's Handbook, the height of the column can be determined to give the least desired amount separation of all the impurities concerned in the graph. Although both methods of calculation can be utilized to determine the volume of the column as well as its height, it is preferred that both methods be utilized and the calculation that gives the most conservative estimate be utilized to calculate the actual height and the diameter of the column.

It should be noted that utilizing the above method, the diameter and the height of the packed column can be determined for a group of contaminants or impurities such that the column will separate and will carry out the desired separation of the impurities from the wastewater. As it was pointed out previously, the main two factors in determining the type of impurities that could be separated from wastewater was the immiscibility of the impurity in wastewater and the volatility of the impurity. Utilizing such a determinant and as an example some of the impurities that can be stripped to generally at least 40% and more preferably to at least 99.99% by weight from the wastewater are for instance benzone, chlorobenzene, xylene, toluene, methylchloride, chloroform, carbon tetrachloride, trich loroethylene, 1 ,2-dichloroethane, perchloro ethylene, 1 ,2-dichloropropane, hexachloroethane, ally chloride, isopropyl chloride, 1,2,3trichloropropene, 2,3-dichloropropene, vinyl chloride, ethylene dichloride, 1,1,1 -trichloroethane, dichlorobenzene, etc.

Utilizing this method, there is obtained the purified wastewater in which the above impurities are removed by at least 40% and more preferably by at least 99.99% by weight. It should be noted that the above list is not all inclusive since other impurities can be removed coming within the broad limits of volatility and immiscibility which was discussed previously, but in which to save space are not listed above. The packed column can be utilized in accordance with the instant invention as a packed column in any convenient water purification process to strip and remove minor amounts of impurities from the wastewater or major amount of impurities for that matter from wastewater.

It should be noted that the major amount of the indicated impurities are not necessary to be removed by the present methods since they can be removed by decantation and then the minor amount of impurities that remain in the wastewater may be removed by the method of the present invention utilizing the air stripping technique with a packed column.

A convenient and effective process for utilizing the packed column of the instant case in the wastewater purification process shown in Figure II. Figure II is the schematic diagram of an overall process for purifying wastewater utilizing the air stripping column to remove minor amounts of impurities from the wastewater.

After a major amount of the impurities have been removed from the waste by decantation, the wastewater is passed through Line 10 into flotation tank 12 where skimming arm 16 on skimming apparatus 14 sweeps the overhead floating impurities from the surface of the wastewater into weir opening 18 from which it passes out of the flotation tank 12 through Line 22 to be disposed of as desired. The purified wastewater from which the floating impurities have been removed,;then passes through opening 24, Line 28 into a series of pH adjustment tanks 32,40 and 48, which tanks 32 and 40 are connected by Line 36 and tanks 40 and 48 are connected by Line 46. pH adjustment tanks 32,40 and 48 have agitators 50, 54 and 60 agitating the wastewater in said tanks respectively.

It should be noted that the flotation tank is for the purpose of removing floating impurities from the wastewater. Then the wastewater is passed into the pH adjustment tanks 32,40 and 48 so that wastewater can be neutralized or made alkaline with a base. It should be noted that this wastewater is slightly acidic, since it is wastewaterfrom a silicone plant and the process of Figure II is an exemplary overall wastewater treatment process for a silicone plant. To pH adjustment tanks 32, 40 and 48, there passes lime slurry through Line 68,72 and 76, respectively, from lime slurry tank 66. The purpose in the passing of lime slurry to the wastewater in the pH adjustment tanks is to slowly neutralize or make alkaline the wastewater as it passes from one tank to another which tanks are connected in series.The neutralized water is then passed out of tank48 through Line 80 into Clarifier Tank 82. It is noted that there is another reason for neutralizing the wastewater in tanks com ing from flotation tank 12 and that is by neutralizing the wastewater impurities which are soluble in an acidic state are precipitated out from the wastewater.

Accordingly, the neutralized wastewater passes into clarifiertank 82 where there is allowed to precipitate from the wastewaterthe impurities that are not soluble in neutral or alkaline water. The precipitated impurities deposit themselves on conical side 84 of the Clarifier Tank 82 and pass out to Line 86 to a disposal area. The overflow from the Clarified Tank is passed through weir opening 90 to Line 92 to sump tank 96. The wastewater from sump tank 96 is then piped through Line 102 through pump 104, then again through Line 106 into the top of packed column 110.Packed column 110 in which the air stripping is carried out in accordance with the pres- ent invention comprises a packed column containing packing 114, an air opening at the top of the Column 112 and the bottom part of Column 118, which is filled with purified water as a reservoir. Clean air is pumped through Line 120 to pump 124 through Line 128 to the bottom of Column 110, through the packed column and outthrough Line 132 at the top of the packed column. The air containing the contaminants coming out of the top of Column 110 passes through Line 132 into an incineratorwherethe impurities in the air are combusted to harmless ingredients.The purified water in stripping tower on packed Column 110 then passes from reservoir 118 at the bottom of the stripping tower on packed Column through Line 136 into a series of reservoir tanks 138, 142 and 146. A slight amount of acid such as hydrogen chloride is pumped from a tank through Line 150 into the first of the back neutralizer tanks and specifically Tank 138 so as to neutralize any remaining alkalinity in the water. The water passes through the series of Tanks 138, 142 and 146 from the highest level tanks to the lowest level tank by overflow as shown in Figure II. The neutralization is carried out here since the pH adjustment step may leave the water alkaline.

Accordingly, the water will be neutralized to pH 7 before it can be discharged to the environment. The neutralized water can be pumped from back neutral izertankl46thrnugh Line l54toapondforaddi- tional settling or into the environment as may be desired.

It should be noted that sump tank 96 is connected to overflow pond 100 through Line 98. The reason for this is that for any reason the overflow wastewater from Clarifier Tank 82 is decreased such there is not sufficient waterto reach sump tank 96 to be pumped to packed column or stripping Column 11Q, then water from overflow pond 100 will flow into sump tank 96 through Line 98 maintaining a sufficient volume of water in sump tank 96 so that there can be a continuous flow of wastewaterthrough Lines 102, pump 104 and Line 106 to a stripping tower 110 so as to keep the stripping tower in continuous operation. Such continuous operation of the stripping tower 110 is preferred for maximum effectiveness and efficiency of the process of the instant case.

It should be noted that the overall water pu rification process of Figure II is exemplary only. It is a preferred process for a silicone plant, however, the invention of the instant case does not lie in the overall process of Figure II. The invention of the instant case lies in the determination of what impurities can be removed from wastewater utilizing the principles of immiscibility and volatility and utilizing the principle of stripping wastewater with air to remove the impurities which is preferably carried out in a packed column.

It should also be noted that even though the present case was presented with the preferred embodiment of overall wastewater treatment process of Figure II which is specifically for silicone plants, that the process of the instant case is not limited to the treatment of wastewater only coming from silicone plants.

The process of the instant case can be utilized to purify wastewater coming from any and all types of plants in which the impurities in the wastewater meet the criteria set forth previously for the removal of impurities from wastewater.

Accordingly, the principles and the process of the instant case, that is the stripping of wastewater with a gas to remove impurities utilizing the criterion set forth previously can be utilized in any overall process to purify wastewater for any type of chemical or other type of plant, including a silicone plant so as to purify wastewater.

The examples below are given for the purpose of illustrating the present invention. They are not given for any purpose of setting limits and bounds for the instant invention. All parts are by weight.

Example I 1. Pilot Plant Data There was proposed a packed column of 1 ft.

diameter containing 4.5 ft.3 of 1/2" plastic Pall rings (Registered Trade Mark) with a total height of packed sections of 69"; a water flow rate of 9508 Ibs. H2O/ hr.

or Ibs. H20 12106 and an airflow rate of 672 Ibs.

hr. X ft.2 Ibs. air air/hr. or 856 hr. x ft.2 Toluene concentration in H2O coming in: 42 ppm Toluene concentration in H2O coming out: 20 ppm Toluene concentration in air coming in: 0 ppm Toluene concentration in air coming out: 316 ppm From Fig. No.3, which shows graphically the relationships of above concentrations and the equilibrium curve (estimated from physical-chemical data of the components) it can be calculated that the degree of toluene stripping from the water is equivalent to: 0.564 theoretical stages. Therefore, height of one theoretical stage is: 69" H ---10.2ft.

0.564 /with the above data, the estimate equipment size to treat 2000 GPM of water containing 200 ppm toluene was determined in which the treated water should contain no more than 5 ppm toluene and the maximum amount of air that can be used, so that it can succeedingly be used at an incinerator, is 10,000 Ibs. air/hr.

Fig. No.4 describes the above conditions graphically. It shows also that 6 theoretical stages are required.

Column sizing (approximately only): Example 2 In a packed column of 4.5 ft.3 of 1 1/2" plastic Pall rings packing with a Water flow of L = 6371 Ibs.

H2O/hr. x ft.2 and an Air flow of G -- 814 Ibs. air/hr. x ft.2 the mean concentration gradient of toluene (AC)m, between water and air: (AC)m = 196 ppm toluene (water based).

Amount of toluene, N, removed from the water: N = 0.624 Ibs. toluene/hr.

Mass transfer coefficient: N 0.624 (kLa) = ~~~~~~~~~ = 0.708 X 10-3 V (AC)m 4.5 x 196 Ibs. toluene hr. x ft.3 x ppm The above data was scaled up in which Water-in was 200 ppm toluene; Water-out was 5 ppm; Water flow was 2000 GPM = 1,000,000 lbs./hr. and Air flow was 40,000 lbs./hr., amount which can be used in an incinerator.

From material balance considerations, toluene concentration in exiting air, y, was: y1 = 4875 ppm toluene in air Toluene concentration in water x,* in equilibrium with y,: x1* = 30 ppm toluene in water (This value can be obtained from Figure No.3) Height of packed section: H = (height of one theoretical stage) x (number of stages) = 10.2 x 6 = 61.2 ft.

Diameter: To treat 2000 GPM 1,000,000 Ibs./hr. at the same flow rate as in the pilot plant, the cross section A required is: 1,000,000 A = = 82.6 ft.2 12106 which results in a diameter D, of: D = 10.25ft.

At this point the airflow rate in the full scale unit will not match that of the pilot plant. The above pro cedure would have to be repeated with other combi nations of water and airflow rates, in lbs./hr. x ft.2, until some are found such that the scaled up column will match those of the pilot plant.

Concentration gradient for design: (AC)m = 47 ppm toluene Packing Volume: N 195 V = = = 5,860 ft.3 of packing (k,a) (ssC)m 0.708 x 10-3 x 47 Column sizing (approximately): 1,000,000 Cross section, A = 157ft.2 6371 Diameter: 14.1 ft.

V 5.860 Packing height, H = - = = ----- 37.3 ft.

A 157 Final sizing of the column would be subject to same considerations explained at the end of Exam ple 1.

Claims (19)

1. A process for removing contaminants from water comprising (1) passing a first stream of air into contact with a second stream of wastewater containing contaminants wherein the contaminants are immiscible or only partially miscible with the wastewater and said contaminants have relatively high volatilities, as indicated by a boiling point at atmospheric pressure of 150 C or below and (2) separating a third stream of said wastewater from said air which has a smaller amount of contaminants than said second stream and removing a fourth stream of said air from said wastewater having said contaminants therein.

2. A process as claimed in claim 1 wherein said air is contacted with said wastewater at a temperature in the range of 0 to 95 C.

3. A process as claimed in claim 1 wherein the wastewater is at a temperature of 0 to 60 C.

4. A process as claimed in any one of the preceding claims wherein the air is contacted with said wastewater in a packed column.

5. A process as claimed in claim 4 wherein the volume of packing of the packed column is determined from the equation N=(k#a)V(AC)rn wherein N is equal to pounds of contaminant to be removed per hour. V is equal to the volume of packing in ft.3 (kit) is equal to the mass transfer coefficient in Ibs. of contaminant removed per hour per ft.3 per unit of concentration gradient. (AC)m is the log mean concentration gradient.

6. A process as claimed in claim 5 wherein (AC)m is determined by plotting an empirical equilibrium curve of percent of contaminants in air versus percent of contaminant in water and then plotting the desired operating curve for the packed column, the average distance between the curves being (AC)m.

7. A process as claimed in claim 5 or claim 6 wherein (k,a), the mass transfer coefficient is determined for a certain packing by empirical experiments.

8. A process as claimed in claim 4 wherein the volume of the packed column is determined by first determining the number of theoretical stages in the column which is done by plotting an empirical equilibrium curve of percent contaminant in airversus percent contaminant in water and then plotting the desired operating curveforthe packed column and then drawing succeeding triangles between the curves by drawing vertical and horizontal lines between the curves beginning with the upper point in the curve, at which it is desired to have the percent contaminant in the air going out from the column and ending at the lower point where the percent contaminant in the wastewater leaving the column is at or below the desired or acceptance concentration with a single triangle constituting a theoretical stage.

9. A process as claimed in any one of the preceding claims wherein at least 40% of the contaminants in the wastewater are removed.

10. A process as claimed in any of the preceding claims wherein the contaminants are benzene, chlorobenzene, xylene, toluene, methyl chloride, chloroform ortrichloro ethylene.

11. A process as claimed in any one of the pre- ceding claims wherein the removal of contaminants from said wastewater is 99.99% by weight

12. A process as claimed in any one of the preceding claims wherein said second stream of wastewater is first passed into a flotation tank to remove impurities and is then passed into pH adjustment tanks where a base is added to neutralize any acid in the wastewater.

13. A process as claimed in claim 12 wherein after said pH adjustment tanks said second stream of wastewater is passed into a precipitation tank wherein any precipitation that is formed from the addition of the base is deposited and then the second stream of wastewater is passed into said packed column.

14. A process as claimed in any one of claims 12 or 13 wherein after said third stream of wastewater is removed from said packed column, it is passed into a neutralizer tank wherein it is neutralized with an acid and then can be passed back into the environment.

15. A process as claimed in any one of the preceding claims wherein said fourth stream of air is passed into an incinerator as air of combustion.

16. A process for removing contaminants from wastewater comprising (a) passing a first stream of wastewater into a flotation tank to remove floating impurities and then transferring said first stream into a pH adjustment tank where a base is added to neutralize any acid in the wastewater; (b) then transferring said first stream of wastewater into a precipitation tank wherein any precipitate that is formed from the addition of the base is deposited; (c) transmitting said first stream of wastewaterto a packed column and passing a second stream of air into said packed column in contact with said wastewater wherein the volume of the packing of the packed column is determined from the equation, N = (k,a) V (AC)m where N is equal to pounds of contaminant to be removed per hour, V is equal to the volume of packing in ft.3 (kla) is equal to the mass transfer coefficient in Ibs. of contaminant removed per hour per ft.3 per unit of concentration gradient, (AC)m is the log mean concentration gradient, and wherein the contaminants are immiscible or only partially miscible with the wastewater and said contaminants have a boiling point at atmospheric pressure of 150 C or below, and further wherein the column is maintained at a temperature of 0 to 95 C, (d) removing a third stream of said wastewater from said packed column wherein from which at least 40% of the contaminants have been removed; (e) removing a fourth stream of air from said column, said air having said contaminants therein and passing said air as air of combustion into an incinerator; and (f) transferring said third stream of wastewater into a neutralizer tank and neutralizing the wastewater with an acid.

17. A process for removing contaminants from water as claimed in claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

18. A process for removing contaminants from water as claimed in claim 1 substantially as hereinbefore described in any one of the Examples.

19. Water when purified by a process as claimed in any one of the preceding claims.