FIELD OF THE INVENTION
The present invention relates to an apparatus for neutralizing alkaline waste water using carbon dioxide enriched stream having impurities like hydrocarbon, amine and H2S. The present invention also relates to a process for neutralizing alkaline waste water.
BACKGROUND AND PRIOR ART
It is well known in the industry that large amount of alkaline waste water is discharged to waste water treatment plant from gas processing complexes, petrochemicals and refineries. The neutralisation of the alkaline waste water has been made generally by injecting mineral acid. However, use of mineral acids lead to
i. excessive accumulation of salts such as chlorides, sulphates, etc,
ii. excessive acidification of waste water.
iii. corrosion in associated piping and equipment
Therefore, an alternative to the use of mineral acid as the neutralizing agent is to use carbon dioxide enriched stream, such as off gas from gas processing unit and flue gas has been utilized. The neutralization of alkaline waste water using carbon dioxide depends on various factors like dissolution efficiency, carbon dioxide purity in off gas/ flue gas etc. One of the drawbacks of the use of carbon dioxide gas stream that leads to inefficient neutralization of the alkaline waste water is poor dissolution efficiency and pressure drop due to foaming. This also leads to escalation of cost of the process.
For instance, document JP2003190976A discloses the method by which gaseous CO2 from flue gas is dissolved and diffused in a large volume of alkali waste water to neutralize the alkali waste water. The apparatus utilizes two or more tanks for neutralization wherein the neutralization to pH 9-8.0 is carried out in the first tank and neutralization to less than or equal to pH 8 is carried out in the second and subsequent tanks. Although, the use of multiple tanks leads to increased dissolution efficiency up to pH 8.0 even when the concentration of gaseous CO2 in flue gas is 10 vol.% and neutralization to less than or equal to pH 8 is carried out in the second and subsequent tanks i.e. the volume of flue gas required can be reduced as compared with the case of one neutralization tank. The use of increased number of tanks leads to high maintenance costs.
Another document, JP2009279465A discloses an apparatus for neutralizing alkaline waste water, in which a CO2-containing gas is used as a neutralizing agent. The apparatus discloses
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a self-priming type diffuser that sucks gas itself and injects into water to be treated, in a water storage tank. The disclosed apparatus reduces the equipment cost and running cost by increasing the dissolution efficiency of CO2 in waste water. However, the apparatus does not solve the efficiency issues of the carbon dioxide enriched streams having impurities like hydrocarbon, amine etc.
In yet another document, US20100230830A1 discloses an apparatus and process capable of removing vast quantities of carbon dioxide and other atmospheric pollutants from gaseous waste streams and sequestering them in storage-stable forms. The apparatus is useful for neutralizing the waste water by utilizing industrial gases comprising CO2 by utilizing a bubble column that is configured to produce bubbles of the industrial gas within the absorbing solution such that at least 10% by weight of the carbon dioxide in the industrial gas is transferred to the absorbing solution. However, the apparatus utilizes very high purity CO2 stream for the neutralization process.
The above discussed prior art suffer from major drawbacks such as: foaming while handling CO2 stream having impurities like heavier hydrocarbon with amine leading to reduced separation efficiency, pressure drop due to foaming and high input of liquid and vapour flow rates through the column etc. The present invention addresses one or more problems as discussed above and other problems associated with the art by providing an apparatus and a process which utilizes CO2 enriched stream having impurities like hydrocarbon and amine along with other impurities in an efficient cost-effective way to neutralize alkaline waste water received from water treatment plant.
OBJECTIVE OF THE INVENTION
The primary objective of the invention is to provide an apparatus for neutralization of alkaline waste water comprising a carbonation reactor which has an inlet for feeding the waste water stream (WWS) from top and an inlet for feed carbon dioxide enriched stream (CS) having impurities like hydrocarbon, amine etc. from bottom and a skimming facility installed across the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water to drain foam from the system.
Another objective of the invention is to provide a process for neutralization of alkaline waste water comprising the following steps:
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a) feeding the WWS from top and CS stream containing impurities like hydrocarbon and amine from bottom into the carbonation reactor;
b) allowing the two streams WWS and CS to mix in a first reactor section leading to the neutralization of WWS to a pH below 11.5 at near atmospheric pressure and ambient temperature;
c) allowing the two streams WWS and CS to mix in a second reactor section leading to the neutralization of WWS to a pH level of 7.5 to 8.5 at near atmospheric pressure and ambient temperature;
d) draining the foam formed during the neutralization of step (c) through the skimming facility installed across the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water;
e) allowing the treated water to come out of the carbonation reactor, wherein the treated water has the pH in the range of 7.5 to 8.5.
SUMMARY OF THE INVENTION
The present invention provides an apparatus for efficient and cost-effective neutralization of the alkaline waste water using carbon dioxide enriched stream having impurities like hydrocarbon, amine etc. The apparatus comprises a carbonation reactor having a first section consisting of packing or tray (2) and providing low pressure drop, a second section consisting of gas sparger (1) for bubbling carbon dioxide enriched stream through the pool of waste water and a skimming facility installed across the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water to remove foam from the system.
The present invention also provides a process for efficient and cost-effective neutralization of the alkaline waste water using carbon dioxide enriched stream having impurities like hydrocarbon, amine etc. The process provides the treated waste water at a pH of 7.5 to 8.5 which is required for further biological treatment of waste water.
BREIF DESCRIPTION OF ACCOMPANYING FIGURES
Figure 1. Shows a schematic flow diagram illustrating the apparatus for the neutralisation of alkaline waste water.
Figure 2. Shows a schematic layout of carbonation reactor having two sections. The upper first section where waste water is contacted with CO2 to reduce pH below 11.5 while the lower second section, which is the bulk reduction section of pH to the range 7.5-8.5.
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Figure 3. Shows a schematic layout of experimental set up for the neutralisation process.
DETAILED DESCRIPTION OF THE INVENTION
While the invention is susceptible to various modifications and alternative forms, specific aspect thereof has been shown by way of example and will be described in detail below. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention.
The terms “comprise”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such apparatus or process. In other words, one or more elements in a system or process proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the apparatus or process.
Accordingly, one aspect of the present invention is to provide an apparatus for neutralization of alkaline waste water comprising:
a carbonation reactor for neutralization of alkaline waste water comprising:
- an inlet for feeding the waste water stream (WWS) which leads to a liquid distributor (3) provided at the top of the carbonation reactor;
- a first section comprising packing or tray (2) and provides low pressure-drop;
- a second section comprising a gas sparger (1) for bubbling the enriched carbon dioxide stream (CS) through a pool of waste water;
- an inlet for feeding enriched carbon dioxide stream (CS) to the gas sparger (1) is provided at the bottom of the carbonation reactor;
- a mist eliminator (4) and an off-gas outlet for venting residual gas out into the atmosphere is provided above the liquid distributor (3);
- a treated-water outlet for releasing the processed water is provided below the gas sparger (1);
and
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a skimming facility (6) installed across the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water to remove foam from the system;
wherein the the packing or trays (2) are designed to minimize the pressure drop resulting from the contact of the two phases WWS and CS by including a random packing or trays and/or structured packing or trays.
In another embodiment, the carbonation reactor has a skimming facility the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water to remove foam from the system;
In another embodiment, the carbonation reactor of the disclosed apparatus has a gas sparger (1) for feeding the carbon dioxide stream (CS) and a liquid distributor (3) for feeding the waste water stream (WWS).
In another embodiment, the gas sparger (1) for feeding the carbon dioxide stream (CS) is provided at the bottom and the liquid distributor (3) is provided at the top of the carbonation reactor to enhance dissolution efficiency of the CS.
In yet another embodiment, the carbonation reactor is a two-section column. The first section comprising packing or tray (2) and provides low pressure-drop, while in the second section comprises a gas sparger (1) for bubbling the CS through a pool of waste water.
In yet another embodiment the first section comprising packing or tray (2) comprises structured and/or random packings or trays to increase the mass transfer area between liquid and gas stream.
In yet another embodiment, the packing or trays (2) are designed so as to minimize the pressure drop resulting from the contact of the two phases i.e. waste water stream (WWS) and carbon dioxide stream (CS) comprising impurities like hydrocarbon, amine etc. The packing or trays provide high contact surface area and results in enhancement of dissolution of CO2 in waste water.
In another embodiment, the first section of the carbonation reactor comprising packing or trays (2) reduces the pH below 11.5 while in the second section comprising the gas sparger (1), the reduction of pH level to permissible range of 7.5 to 8.5 is reached by increasing dissolution efficiency of CO2 to waste water through bubbling the CS.
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In another embodiment, the carbonation reactor includes a mist eliminator (4) which is a demister above the liquid distributor for eliminating the formation of mist.
In another embodiment, the carbonation reactor includes a skimming facility installed across the highest liquid level of the waste water in the carbonation reactor i.e. skimming facility is installed at liquid hydrocarbon (HC)-liquid water interface to drain foam from the carbonation reactor. Due to density difference hydrocarbon (HC) floats on water and the foaming issue arises due to the bubbling of CO2 stream through the pool of liquid waste water. This causes a very high-pressure drop-in system and leads to tripping of plant thereby severely affecting the efficiency of the apparatus and process. According to the embodiment of the invention a skimming facility is installed at liquid (HC)-liquid (water) interface to drain foam from system and thereby improving the efficiency of the system.
In yet another embodiment, the skimming facility includes at least two numbers of skimming nozzles. The skimming facility removes foam from the carbonation reactor for steady continuous operation while handling CO2 stream having impurities like heavier hydrocarbon and amine along with other impurities.
In yet another embodiment, the carbonation reactor has a diameter greater than 8 inches. It has been observed that the diameter of the carbonation reactor lower than 8 inches significantly reduces the dissolution efficiency of carbon dioxide gas to waste water due to formation of slugs.
In another embodiment, the carbonation reactor operates at atmospheric conditions or slightly higher-pressure values. The pressure drop in each section is also very low. It is to be understood that there is no preferred pressure range. Generally, dissolution efficiency increases with increase in system pressure and therefore the operating pressure range is optimised to make it cost effective. A typical range of the operating pressure is 1-6 kg/cm2g and that of temperature is ambient to 60 °C. In a preferable embodiment, the pressure conditions relate to 2-4 kg/cm2g.
In yet another embodiment, the waste water stream to be treated comprises alkaline waste water from refinery, petrochemical, gas processing complex and any other process plant. It is to be understood that the apparatus of the invention is useful for a wide range of alkaline waste water and there is no preferred range of pH for the waste water discharged from the process unit. Generally, this pH is in the range of 9-14. Therefore, a preferable embodiment of
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the present invention relates to the apparatus that can handle pH of complete alkaline range having pH of 9-14.
In another embodiment, the fed carbon dioxide stream (CS) into the apparatus of the present invention is a CO2 rich stream comprising of off gas or flue gas from gas processing unit.
In yet another embodiment, the carbon dioxide gas stream utilized in the present invention has purity in the range of 50 to more than 99% (volume), more preferably in the range of 90 to more than 99% (volume). The carbon dioxide gas stream is fed into the carbonation reactor with the flow rate in the range of 45 to 99 (cc/s), more preferably in the range of 80 to 95 (cc/s).
In another embodiment, the carbon dioxide gas stream utilized in the present invention further comprises impurities such as heavier hydrocarbon, amine, nitrogen, H2S (trace), carbon monoxide (trace), moisture etc. In another embodiment, the carbon dioxide gas stream utilized in the present invention further comprises nitrogen and moisture.
In another embodiment, the treated water has a pH in the range of 7.5 to 8.5. This pH range (7.5-8.5) is required for biological treatment of waste water.
Another aspect of the present invention is to provide a process for the neutralization of alkaline waste water comprising the following steps:
a) feeding the waste water stream (WWS) from top through a liquid distributor (3) and enriched carbon dioxide stream (CS) containing impurities like hydrocarbon and amine from bottom through the gas sparger (1) into the carbonation reactor;
b) allowing the two streams WWS and CS to mix in a first reactor section comprising packing or trays (2) which neutralizes the WWS to a pH below 11.5 at near atmospheric pressure and ambient temperature;
c) allowing the treated WWS from step (b) to mix by bubbling CS in a second reactor section comprising a gas sparger (1) which neutralizes the WWS to a pH level of 7.5 to 8.5;
d) draining the foam formed during the neutralization of step (c) through a skimming facility (6) installed across the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water; and
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e) allowing the treated water to come out of the carbonation reactor through the treated water outlet at a pH range of 7.5 to 8.5.
In another embodiment, the fed carbon dioxide stream (CS) in the process of the present invention is a CO2 rich stream comprising of off gas or flue gas from gas processing unit.
In another embodiment, the fed carbon dioxide stream (CS) in the process of the present invention contains impurities like hydrocarbon and amine along with other impurities.
In yet another embodiment, the process includes: a step of feeding WWS at a temperature range of ambient to 60°C and a step of feeding CS containing impurities like hydrocarbon and amine at a temperature range of ambient to about 80°C. It is to be understood that high temperature reduces dissolution efficiency of carbon dioxide in water. Further, very low temperature may lead to condensation of hydrocarbon while handling CO2 stream containing impurities like hydrocarbon.
The term “carbonation” and variations thereof as used herein refer to the neutralisation process using CO2.
The term “ambient conditions” and variations thereof as used herein refer to room conditions.
Further salient features of the apparatus and the process for neutralization of alkaline waste water and the enhancements achieved are discussed in the examples provided below. The applicants would like to mention that the examples are mentioned to show only those specific details that are pertinent to understanding the aspects of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
EXAMPLES
The following examples are given to illustrate the present invention and should not be construed to limit the scope of the present invention.
Accordingly, to achieve the efficient neutralisation process of waste water, the process was carried out in the apparatus according to the invention having diameter 8 inch and length 2 metre. Figure 3 illustrates the experimental set up of neutralisation process for alkaline waste water. The diameter of carbonation reactor is selected in such a way so that slug flow and wall effect in reactor can be avoided. The diameter of reactor lower than 8 inches significantly
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reduces the dissolution efficiency of CO2 to water due to formation of slugs. Model alkaline water is prepared by adding NaOH bead to water and used as feed for this experiment. Experimentation was carried out for different cases. Details of the experimental set up, feed specifications, operating window and results for different cases are tabulated below.
Table 1: Detailed description of experimental set up
Table 2: Feed specifications
Feed Specifications
Alkaline Water
1-9% of caustic solution.
pH =13
Carbon di-oxide
Purity : 93%
Table 3: Operating Parameters
Operating Parameters
Temperature
Ambient temperature
Pressure
1-6kg/cm2g
S. No.
Element
Description
1
Gas sparger
Gas distribution system.
Type : Perforated pipe
Pipe Size: 0.5 inch
2
Mass Transfer Medium
Low pressure drop packing
Bed length : 0.5 meter
3
Liquid Distributor
Liquid distribution system.
Type : Perforated pipe
Pipe Size: 0.5 inch
4
Liquid inlet
Size : 0.5 inch
5
Gas inlet
Size : 0.5 inch
6
Skimming nozzle
Size : 0.5 inch
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Table 4: Results
I. Reactor with packing vs without packing
Parameter
Reactor without packing
Reactor with packing
Flow rate of model alkaline water (liter/hr.)
6
6
Flow rate of CO2 (cc/s)
45
45
Temperature (oC)
Ambient
Ambient
Pressure (kg/cm2)
3
3
pH
9-10
8-8.5
II. Case Study: Study of dissolution efficiency vs CO2 Purity.
Parameter
Case – 1
Case-2
Case-3
Case-4
Components
(mol%)
CO2
>99
>96
>90
>50
Moisture
<1
<4
<4
<4
Impurity( Hydrocarbon & amine, H2S)
<100 ppmv
<100 ppmv
<100 ppmv
<100 ppmv
Nitrogen
<10 ppmv
<10 ppmv
<10 ppmv
<46 ppmv
Flow rate of model alkaline water (litre/hr)
6
6
6
6
Flow rate of CO2 (cc/s)
45
52
58
95
Temperature C
Ambient
Ambient
Ambient
Ambient
Pressure kg/cm2
3
3
3
3
pH
8-8.5
8-8.5
8-8.5
8-8.5
As evident from the Table 4 (I) above the carbonation reactor having packing or tray (2) resulted in the treated water having pH 8-8.5 in comparison to the carbonation reactor without packing or tray (2). Further, the Table 4 (II) above shows that the dissolution efficiency increases with increase in CO2 purity leading to an efficient neutralization of the alkaline waste water at a comparably low flow rate.
The advantages of the disclosed invention are thus attained in an economical, practical and facile manner. While preferred embodiments and example have been shown and described, it
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is to be understood that various further modifications and additional configurations will be apparent to those skilled in the art. It is intended that the specific embodiments herein disclosed are illustrative of the preferred and best modes for practicing the invention and should not be interpreted as limitations on the scope of the invention.

We Claim:
1. An apparatus for neutralization of alkaline waste water comprising:
a carbonation reactor comprising:
- an inlet for feeding the waste water stream (WWS) which leads to a liquid distributor (3) provided at the top of the carbonation reactor;
- a first section comprising packing or tray (2) and provides low pressure-drop;
- a second section comprising a gas sparger (1) for bubbling the enriched carbon dioxide stream (CS) through a pool of waste water;
- an inlet for feeding enriched carbon dioxide stream (CS) to the gas sparger (1) is provided at the bottom of the carbonation reactor;
- a mist eliminator (4) and an off-gas outlet for venting residual gas out into the atmosphere is provided above the liquid distributor (3);
- a treated-water outlet for releasing the processed water is provided below the gas sparger (1); and
- a skimming facility (6) installed across the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water to remove foam from the system;
wherein the the packing or trays (2) are designed to minimize the pressure drop resulting from the contact of the two phases WWS and CS by including a random packing or trays and/or structured packing or trays.
2. The apparatus as claimed in claim 1, wherein the packing or tray section of the carbonation reactor reduces pH of waste water stream below 11.5.
3. The apparatus as claimed in claim 1, wherein the packing or tray section of the carbonation reactor reduces pH of waste water stream in the range of 7.5 to 8.5.
4. The apparatus as claimed in claim 1, wherein the fed carbon dioxide stream (CS) is a CO2 rich stream comprising of off gas or flue gas from gas processing unit.
5. The apparatus as claimed in claim 1, wherein the fed carbon dioxide stream (CS) contains impurities hydrocarbon, amine and along with other impurities.
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6. The apparatus as claimed in claim 5, wherein the fed carbon dioxide stream (CS) is having impurities hydrocarbon, amine and H2S.
7. The apparatus as claimed in claim 1, wherein the waste water stream to be treated comprises alkaline waste water from refinery, petrochemical, gas processing complex and any other process plant.
8. The apparatus as claimed in claim 7, wherein alkaline waste water is received from water treatment plant.
9. The apparatus as claimed in claim 1, wherein the pressure is in the range of 1-6 kg/cm2g, more preferably 2-4 kg/cm2g .
10. The apparatus as claimed in claim 1, wherein the temperature is in the range of room temperature to 60 °C.
11. A process for the neutralization of alkaline waste water, wherein the process comprises the following steps:
f) feeding the waste water stream (WWS) from top through a liquid distributor (3) and enriched carbon dioxide stream (CS) containing impurities like hydrocarbon and amine from bottom through the gas sparger (1) into the carbonation reactor;
g) allowing the two streams WWS and CS to mix in a first reactor section comprising packing or trays (2) which neutralizes the WWS to a pH below 11.5;
h) allowing the treated WWS from step (b) to mix by bubbling CS in a second reactor section comprising a gas sparger (1) which neutralizes the WWS to a pH level of 7.5 to 8.5;
i) draining the foam formed during the neutralization of step (c) through a skimming facility (6) installed across the highest liquid level of pool of waste water at the interface of liquid hydrocarbon and liquid water; and
j) allowing the treated water to come out of the carbonation reactor through the treated water outlet at a pH range of 7.5 to 8.5.
12. The process as claimed in claim 11, wherein the fed carbon dioxide stream (CS) is a CO2 rich stream comprising of off gas or flue gas from gas processing unit.
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13. The process as claimed in claim 11, wherein the fed carbon dioxide stream (CS) contains impurities hydrocarbon, amine and along with other impurities.
14. The process as claimed in claim 13, wherein the fed carbon dioxide stream (CS) is having impurities hydrocarbon, amine and H2S.
15. The process as claimed in claim 11, wherein the waste water stream to be treated comprises alkaline waste water from refinery, petrochemical, gas processing complex and any other process plant.
16. The process as claimed in claim 15, wherein alkaline waste water is received from water treatment plant.
17. The process as claimed in claim 11, wherein the pressure is in the range of 1-6 kg/cm2g, more preferably 2-4 kg/cm2g.
18. The process as claimed in claim 11, wherein the temperature is in the range of room temperature to 60 °C.