HK1161168B - Apparatus for treating a waste stream - Google Patents

Abstract

An integrated unit operations for the treatment of a waste stream, such as spent caustic, is provided in a single vertical vessel (1) having at least three separate zones: a mixing (2), a settling (3), and a mass transfer (4) zone.

Description

Apparatus for treating waste streams

Technical Field

The present invention relates to a new equipment design for treating a spent waste stream, particularly a spent caustic stream recovered in a petroleum refining or petrochemical process. In contrast to previous treatment processes, the present invention is a deep neutralization or deep acidification process using a number of separate vessels, the present invention using a single vertical column divided into at least three distinct zones: a mixing zone, a settling zone, and a mass transfer zone.

Background

In petroleum refining and petrochemical processing, hydrocarbon conversion products are typically washed with a caustic solution. For example, in petrochemical processing, the scrubbing removes hydrogen sulfide and carbon dioxide primarily as sodium sulfide, sodium carbonate, and sodium bicarbonate, as well as removing some of the higher molecular weight hydrocarbon components. The caustic solution can be used to remove naphthenic acids and other organic acids, as well as other sulfur compounds from cracked petroleum products and petroleum distillates. However, since caustic solutions are quite harmful to organic tissues, great care must be taken in the use of caustic solutions and disposal of spent caustic solutions to protect waterways, rivers, groundwater formations, and the like. Due to factors such as high pH and unacceptably high levels of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic Carbon (TOC), the spent caustic solution is generally not suitable for direct treatment in a biological sewage treatment plant.

Many methods have been proposed to dispose of spent caustic. These methods are wet air oxidation, chemical oxidation and incineration. In each of these known processes, many pieces of equipment are required to complete the treatment process. The process is capital intensive because it involves many separate processing vessels. Also, the process requires a large amount of space to accommodate many pieces of equipment. The present invention has solved these problems by implementing the treatment process in a single vertical column divided into at least three process zones. Such an apparatus and associated process represents an extremely economical method of treating waste streams, especially spent caustic, and thus minimizes capital and operating costs. These and other advantages will become apparent from the following more detailed description of the invention.

Disclosure of Invention

The invention relates to integration for treating waste streamsA process vessel comprising, in combination, a vertical column, the interior of which is divided into at least a mixing zone, a settling zone, and a mass transfer zone. The waste stream inlet is connected to a column for introducing a waste stream (e.g., waste caustic) into a mixing zone where the waste stream is mixed with a neutralizing agent (e.g., a strong acid such as sulfuric acid). The neutralizing agent may be added directly in-line to the waste stream and upstream of the inlet, or separately to the mixing zone. The vessel has a vent outlet in fluid communication with the mixing zone for venting N₂、H₂S, RSH, light hydrocarbons. An organic effluent stream in liquid communication with the settling zone is required to remove separated acid oils (e.g., naphthenic acids, DSO, mycophenolic acid) and entrained hydrocarbons. A liquid transfer line connects the settling zone with the mass transfer zone, and a vapor line connects the mass transfer zone to the settling zone. An inert gas inlet is in fluid communication with the mass transfer zone to strip out most of the acid gases present. Finally, the treated waste liquid is removed via an outlet in liquid communication with the bottom of the mass transfer zone.

The invention is characterized in that it reduces capital costs by minimizing the number of piping components, individual vessels, instrumentation and unit transport time. These and other aspects of the invention will be apparent from the detailed description of the preferred embodiments contained below.

Drawings

The figure schematically illustrates one possible embodiment of a waste stream apparatus of the present invention.

Detailed Description

As illustrated, the present invention relates to a single apparatus for treating a waste stream, preferably neutralizing spent caustic. One embodiment of the apparatus is illustrated in the drawings.

The figure shows a vertical column or vessel 1 having at least three separate process zones. Zone 2 is a mixing zone in which the waste stream to be treated is thoroughly mixed with a neutralizing agent. As mentioned, the preferred process for use in the present invention is the treatment or neutralization of spent caustic from one or more refining or petrochemical processes. The following description of this embodiment will use such a process as an example. The spent caustic stream enters vessel 1 through inlet 5 and into mixing zone 2. The spent caustic typically contains 1 to 10 wt% NaOH, 0 to 4 wt% sulfides and disulfides, 0 to 4 wt% mercaptides (as sulfur), 0 to 4 wt% carbonates (as CO 3), 0 to 5 wt% mycophenolic acid. The neutralizing agent is also preferably added to mixing zone 2 by adding it directly to the waste stream upstream of inlet 5. Optionally, a mixing device (not shown), such as an in-line static mixer, may be used to premix the caustic and neutralizing agent. The amount of neutralizing agent added can be controlled by any means known in the art, however, as shown in the figure, a convenient method is to measure the pH of the solution in mixing zone 2 using pH monitor 12 and compare the pH to the pH of the solution in settling zone 3. If the pH is higher than the desired pH, more neutralizing agent is added to mixing zone 2.

Inside the mixing zone 2 there is at least one means for thoroughly mixing the waste stream and the neutralizing agent. When the spent caustic is the spent liquor stream to be treated, then the preferred neutralizing agent is a strong acid (e.g., sulfuric acid), although other acids, such as hydrochloric acid, may be used. The amount of neutralizing agent mixed with the waste stream is selected so as to significantly reduce the PH of the solution to facilitate removal of impurities by physical means. Preferably, when sulfuric acid is used, it is added to the spent caustic stream directly upstream of the inlet 5. As shown in the figures, a paddle type mixer is the preferred mixing means, however, other mixing means known in the art may be used, such as a gas sparge agitator. The residence time in mixing zone 2 is selected to properly mix the specified acids to achieve a uniform PH and 100% completion of the desired reaction. To the extent any volatile hydrocarbons or other gaseous compounds present in the waste stream, they will be removed via the exhaust outlet 21 using a purge gas, as set forth below.

Part of the mixture from zone 2Is continuously removed via a downcomer and passed into settling zone 3. This zone allows the organic component to be separated from the aqueous component using any known mechanical separation device, such as the baffles shown in the figure. Since the organic phase is less thick than the aqueous phase, it can be decanted and removed from vessel 1 via organic outlet 9. When the spent caustic is the waste stream being treated, the organic phase consists primarily of acid oils (e.g., naphthenic acids, DSO, mycophenolic acid) or entrained hydrocarbons. The aqueous phase in settling zone 3 consists essentially of the sodium salt-containing acidic brine and is removed via line 10 and introduced into the top of mass transfer zone 4 where the acidic gases are stripped from the brine with an inert gas, such as nitrogen. The mass transfer zone 4 can contain any number of known packing or tray designs to assist in stripping the acidic brine from the aqueous solution. An inert gas, such as nitrogen, is added to the bottom of mass transfer zone 4 via inlet 18, where it flows counter-currently to the water phase. Stripping H from aqueous phase using inert gas₂S、CO₂And other gases, and then removed via line 19, with it being used as a purge gas in the top portion of settling zone 3, and then removed via line 20. Purge gas in line 20 is introduced into the top of mixing zone 1 and finally removed from vessel 1 via vent outlet 21.

After stripping the acid gases, the aqueous phase is removed from vessel 1 via line 11 and ultimately from the process via line 14, where it can be disposed of using known methods. Alternatively, as shown in the figure, a portion of the aqueous phase in line 11 is recycled back to mass transfer zone 4 via line 15 by means of pump 13. When recycling a portion of the aqueous phase, it is necessary to add a second neutralizing agent (e.g., NaOH or KOH) via line 16, followed by contact in a mixing means (e.g., in-line static mixer 17), as shown in the figure. To control the addition of the second neutralizing agent, an in-line pH meter can be used to monitor the pH of the aqueous phase exiting mass transfer zone 4 via line 11 and then compare that pH to the pH in the recycle stream downstream of mixing means 17.

The operating temperature of vessel 1 is in the range of about 80 c to about 130 c, more preferably from about 90 c to about 110 c. The internal pressure of the vessel 1 may be in the range of about 3 atmospheres to about 10 atmospheres (gauge pressure).

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation.

The means, materials, and steps for performing the various disclosed functions may take a variety of alternative forms without departing from the invention. Thus, the expressions "means to a. And the broadest interpretation of these terms is intended in the terms of the following claims.

Claims (5)

1. An integrated treatment vessel for treating a spent caustic effluent stream comprising, in combination:

a. a vertical column having an interior divided into at least a mixing zone, a settling zone, and a mass transfer zone, wherein the mixing zone has means for mixing liquids, the settling zone has one or more baffles configured to separate an aqueous phase and an organic phase, and the mass transfer zone is configured as a stripping column;

b. a waste stream inlet in fluid communication with the mixing zone;

c. a vent outlet in communication with the mixing zone for removing vent gas from the vessel;

d. an organic phase outlet in liquid communication with the settling zone to remove an organic phase from the vessel;

e. a liquid transfer line connecting the settling zone and the mass transfer zone to transfer an aqueous phase from the settling zone to the mass transfer zone;

f. a vapor line connecting the mass transfer zone and the settling zone for transferring inert gas between the mass transfer zone and the settling zone;

g. an inert gas inlet in communication with the mass transfer zone, the inert gas inlet configured to introduce an inert gas into the mass transfer zone;

h. a purge gas outlet configured to remove inert gas from the mass transfer zone, wherein the purge gas outlet is in fluid communication with the vapor line and the vapor line is in fluid communication with the top of the mixing zone and the exhaust outlet;

i. a treated spent liquor flow outlet in liquid communication with the mass transfer zone, the treated spent liquor flow outlet configured to remove a treated spent liquor flow from the mass transfer zone using a pump having an inlet and an outlet;

j. a line connected to the pump outlet configured to recycle a portion of the treated waste stream to the mass transfer zone;

k. a first pH monitor in fluid communication with the mixing zone and settling zone and configured to control the addition of a first neutralizing liquid to the mixing zone;

a second pH monitor in fluid communication with the treated effluent outlet and upstream of the pump;

an inlet line in fluid communication with the line connected to the pump outlet, the inlet line configured to add a second neutralizing liquid to the circulating portion of the treated waste stream;

n. a mixing member located in the line connected to the pump outlet downstream of the inlet line for the second neutralizing liquid; and

a third pH monitor in fluid communication with the line connected to the pump outlet and positioned downstream of the mixing means, wherein the second and third pH monitors are configured to control the addition of the second neutralizing liquid.

2. The integrated processing vessel of claim 1, further characterized by the waste stream inlet being in liquid communication with a neutralization line to add a neutralizing agent directly to the waste stream.

3. The integrated processing vessel of claim 1 further characterized in that the mixing means is selected from the group consisting of powered paddles, gas sparge agitators (gas dispersers), static mixers, and combinations thereof.

4. The integrated processing vessel of claim 1, further characterized by the mass transfer zone comprising packing or one or more trays.

5. The integrated treatment vessel of claim 1, further characterized by the mass transfer zone having a recycle stream inlet to aid in slipstream return of treated spent liquor removed from the treated spent liquor stream outlet.