June 2018

Process Optimization

Remove carbonyls from gaseous hydrocarbon streams for ethylene plant service

Within the hydrocarbon processing industry (HPI), particularly the petrochemical sector, there remains an irritating series of reactions associated with acid gas removal (AGR) using dilute caustic soda—namely, aldol condensation reactions and their respective products.

Gondolfe J., Kurukchi S., Janus Technology Solutions LLC

Within the hydrocarbon processing industry (HPI), particularly the petrochemical sector, there remains an irritating series of reactions associated with acid gas removal (AGR) using dilute caustic soda—namely, aldol condensation reactions and their respective products. These reaction products cause fouling effects within the AGR itself and, perhaps more importantly, subsequent treatment of the spent solution from the AGR. It is well understood that the speciation of these highly reactive compounds can be classified as carbonyls. Reactions of carbonyls are catalyzed in the presence of free caustic in an aqueous solution to produce aldol condensation products.

A method to pretreat these carbonyls prior to their entry to the AGR system is the subject of this treatise (US Patent 9834498),1 as applied to ethylene plant service in this instance. This method comprises contacting the cracked gas with a sodium bisulfite (NaHSO3) solution in an absorber to allow transfer of the carbonyl, particularly acetaldehyde, from the gas phase to the liquid phase, where the carbonyl reacts with the NaHSO3 to form a solid adduct that is solubilized within the NaHSO3 solution itself. Therefore, this method applies absorption with chemical reaction.

TECHNOLOGY OVERVIEW

For an AGR system within virtually any given ethylene plant service, the traditional acid gas removal application uses a caustic scrubber within a steam cracker to remove carbon dioxide (CO2) and hydrogen sulfide (H2S) from the cracked gas. This is optimally accomplished at a specific interstage location of a multi-stage gas compression system, wherein the cracked gas is at a pressure of 10 atmospheres–20 atmospheres (147 psi–294 psi). The CO2 and H2S are removed from the cracked gas stream by intimate contact within the caustic scrubber using a dilute caustic solution. The CO2 and H2S are principally converted into sodium carbonate (Na2CO3) sodium sulfide (Na2S) and, to a lesser extent, sodium bicarbonate (NaHCO3) and sodium hydrosulfide (NaHS), which are dissolved in the caustic solution. After such gas scrubbing contact, the depleted sodium hydroxide (NaOH) solution (spent caustic stream) would contain unreacted NaOH, primarily Na2S, Na2CO3 and, as stated previously, to a lesser extent NaHS and NaHCO3.

Oxygenates and fouling

Steam cracking of hydrocarbons produces small quantities of oxygenated compounds, such as organic acids, phenols and carbonyls, mainly aldehydes and ketones. Phenols and organic acids are removed in the quench water tower due to their high solubility in the aqueous phase.

FIG. 1. Aldol condensation mechanism.
FIG. 2. Red oil formation in caustic tower.

Within a caustic scrubber, carbonyls and particularly acetaldehyde react strongly and polymerize by aldol condensation reaction mechanisms in the presence of a strong base, such as an NaOH solution, as shown in FIG. 1. The aldehydes in the caustic solutions react, producing polyaldols by such aldol condensation reaction(s). These polymers—known in the industry as “red oils”—induce fouling within the caustic scrubber. Aldol condensation products result in the formation of red oils, which is a consequence of carbonyl monomer reactions at temperature in free NaOH, whereby further polymerization leads to the formation of high-molecular-weight red/brown solid polymer, as shown in FIG. 2. Within an AGR system, these polymers will settle on internal equipment surfaces, leading to fouling and eventual plugging (FIG. 3). This means that the unit must eventually  be shut down for cleaning as a consequence of the cause and effect of aldol condensation. Whenever the AGR system is shut down for cleaning, a significant operational cost is incurred.

FIG. 3. Red oil fouling in caustic tower.

The capacity of a caustic scrubber can be (and frequently is) adversely affected by polymer formation. Polymer formation in the caustic scrubber reduces its capacity by reducing its operating efficiency, often necessitating the shutdown of the caustic scrubber for cleaning and removing deposits of polymeric material. Chemical polymer inhibitors supplied by Betz,2,3 Nalco4 and Dorf Ketal5 are normally added on a continuous basis to caustic towers to avoid reduced fouling rate and to extend the caustic tower cycle time.

Spent caustic

In addition to sodium salts, the spent caustic solution leaving the caustic scrubber contains a significant content of dissolved mono- and di-olefinic hydrocarbons, as well as free-carbonyls, carbonyl polymers and other contaminants. In this condition, the spent caustic solution presents various problems with respect to either its environmental disposal or to its reconditioning for subsequent use. For example, polymers tend to form in the spent caustic solution as long as the solution contains dissolved polymer precursors at an elevated temperature. Aldol condensation of dissolved oxygenated hydrocarbons (carbonyls, such as aldehydes and ketones) produces red oil polymeric products, which remain partially soluble in a spent caustic solution that exits the caustic scrubber.

Certain highly unsaturated hydrocarbons in the cracked gas, such as acetylenes and dienes (diolefinic hydrocarbons), that pass into the spent caustic solution in the caustic scrubber may polymerize to various degrees. Liquid red oil formation, which is a product of aldol condensation of a few numbers of aldehyde monomer, and further polymerization lead to the formation of high-molecular-weight red solid polymer. The insoluble polymeric species in the spent caustic solution precipitate out of solution and may be removed in a deoiling drum. Even downstream of the deoiling drum, the spent caustic still causes fouling, as it contains dissolved carbonyl monomers that would further polymerize to higher-molecular-weight polymers, depending on the solution residence time in the equipment.

From a disposal standpoint, sodium sulfide (Na2S), NaHS and the dissolved hydrocarbon that comprise the spent caustic solution impart a high chemical oxygen demand (COD) and biological oxygen demand (BOD) to allow for environmentally acceptable disposal, if untreated. Furthermore, the alkaline value of the spent caustic stream is not usable for other purposes due to the presence of polymeric contaminants. From either perspective, the species contained within the spent caustic solution, other than NaOH and water, are contaminants that render it unusable or non-disposable absent any further treatment. Commercial technologies used in treating spent caustic include wet air oxidation at elevated pressure,6,7,8 incineration9 and deep neutralization.

CARBONYL PRETREAT ABSORBER

The carbonyl pretreat absorber removes the highly reactive polymer precursor as acetaldehyde from a sour cracked gas upstream of the caustic scrubber. The benefits of this process virtually eliminate the formation of red oil that is formed as a consequence of aldol condensation reactions of carbonyl species. Prior to admission of the cracked gas to the caustic scrubber, an aqueous solution of NaHSO3 is contacted counter-currently with the cracked gas in a packed or trayed column to remove the acetaldehyde by reacting it with the NaHSO3 to form an adduct, as shown in FIG. 4.

FIG. 4. Carbonyl adduct reactions and adduct solid sample.

Acetaldehyde is absorbed and chemically reacts with the NaHSO3 solution to form a heavy adduct that is soluble in the aqueous phase, thereby eliminating the presence of acetaldehyde in the cracked gas stream before it enters the AGR system.

Therefore, fouling of the caustic scrubber will be drastically reduced; the spent caustic will contain sodium salts (hydroxide, carbonate, bicarbonate, sulfide and bisulfide) and dissolved gases to the limit of their solubility. The absence of red oil polymer precursors and solid polymer make it possible to strip the formed spent caustic of its dissolved hydrocarbons, and to obtain pretreated spent caustic that is now essentially a solution of sodium salts only.

The depleted NaHSO3 solutions containing the acetaldehyde adduct may be regenerated by heating the solution to a temperature of 100°C (212°F) or higher, whereby the acetaldehyde adduct dissociates to acetaldehyde and NaHSO3. The solution is then stripped to remove the acetaldehyde, and the regenerated solution can be readily recycled for reuse.

Experimental data

The pilot plant absorption column (FIGS. 5 and 6) operated with a propane-propylene gas mixture containing 199-wppm acetaldehyde fed to the bottom of the column, and contacted counter-currently with

  1. Test 1: 10 wt% NaHSO3 solution
  2. Test 2: 5 wt% NaHSO3 solution
  3. Test 3: wt% NaHSO3 solution.
FIG. 5. Absorption rig.
FIG. 6. Conventional caustic tower in ethylene plants.

The NaHSO3 solution in each test run was fed to the top of the packing. The acetaldehyde reacted completely with the NaHSO3 solution, forming an adduct soluble in the liquid solution. Therefore, the acetaldehyde was depleted from the gas phase and at the column outlet (exit gas) stream, and the concentration of the acetaldehyde measured less than 1 wppm, which is the limit of detection of the GC for all runs in the Test 1 and Test 2 series.

Synopsis of results

The results of Tests 1, 2 and 3 (TABLE 1) show that removal of acetaldehyde is almost complete when using a 10 wt% NaHSO3 solution and a 5 wt% NaHSO3 solution, respectively. When the NaHSO3 solution reaches 1 wt% in the absorption column, the removal of acetaldehyde from the hydrocarbon gas stream reaches 98.5 wt%.

For the stated application of acetaldehyde removal, the power and simplicity of this innovative technology solution demonstrates a fundamental approach to solve a given problem, rather than treating the symptom.

The cracked gas stream composition of ethylene plants

The cracked gas stream typically comprises hydrogen, hydrocarbons, CO2, H2S or other sulfur compounds, acetaldehyde and acetone, as well as other contaminants. Hydrocarbons may be present in an amount of 60%–85% by weight of the hydrocarbon gas stream; sulfur compounds, such as H2S, may be present in an amount of 20 wppm–500 wppm; acetaldehyde may be present in an amount of 20 wppm–200 wppm; acetone may be present in an amount of 5 wppm–20 wppm; and other acid gas species, such as CO2, may be present in an amount of 50 wppm–500 wppm. The cracked gas stream is pressurized in the cracked gas compressor and cooled in an interstage aftercooler prior to acid gas removal in the caustic tower. A knockout drum, which is positioned upstream of the caustic tower, separates the cooled cracked gas into condensate liquid and a cracked gas stream.

Caustic tower in conventional ethylene plants

The caustic tower typically consists of four sections, shown in FIG. 6. The three lower sections are used for caustic scrubbing, and the top section is used for washing the treated gas with cooled high-pressure (HP) boiler feedwater (BFW) to avoid caustic carryover into the downstream equipment. The gas enters the caustic tower below the bottom tray. As the gas flows upward, it comes in contact with circulating weak, intermediate and strong caustic solutions. The treated gas is cooled and the caustic is scrubbed out by BFW that is injected into the top of the caustic tower. A three-stage caustic treatment system is provided to ensure that the acid gases CO2 and H2S are removed from the process gas to meet ethylene product specifications, and to protect the acetylene hydrogenation catalyst from H2S poisoning.

FIG. 7. Carbonyl absorber (revamp option).

Makeup caustic from offsite is diluted with cooled HP BFW before being injected into the tower. BFW is also added to the washwater loop to account for vaporization within the tower.

The spent caustic withdrawn from the bottom of the tower is degassed before being sent to battery limits for further spent caustic treatment.

Carbonyl absorber upstream of caustic tower (revamp option)

The carbonyl absorber (FIG. 7) is positioned between the knockout drum and the caustic tower. A liquid NaHSO3 stream is pumped to the top of the carbonyl absorber for intimate counter-contact with the cracked gas stream. The fresh NaHSO3 stream may be 10 wt%–12 wt% NaHSO3. The acetaldehyde content in the cracked gas stream reacts with the NaHSO3 in the liquid bisulfite stream to form an adduct product that is soluble in the liquid bisulfite stream. The carbonyl absorber overhead cracked gas stream leaving the column is essentially free of the acetaldehyde, and the bottoms liquid contains the formed adduct and any unreacted liquid bisulfite. Bottoms liquid is recirculated back to the top of the column via a recirculating pump. The acetaldehyde content in the overhead hydrocarbon gas stream is reduced to less than 1 wppm. When the NaHSO3 content in the recirculating solution reaches approximately 1 wt%, both the valve on the spent bisulfite line and the valve on the fresh bisulfite line are opened to allow the addition of fresh bisulfite solution stream and the depleted bisulfite solution stream to discharge from the absorber.

FIG. 8. Carbonyl absorber section in caustic tower (grassroots option).

The circulating liquid bisulfite stream in the carbonyl absorber may release small ppm level of sulfur dioxide (SO2) into overhead cracked gas stream. Any SO2 produced will react and be converted to NaHSO3 in the caustic tower and end up in the spent caustic stream.

Carbonyl absorber section within caustic tower (grassroots option). The carbonyl absorber section (FIG. 8) is positioned at the bottom of the caustic tower. A liquid NaHSO3 stream is pumped to the top of the carbonyl absorber section for intimate counter-contact with the cracked gas stream. The fresh NaHSO3 stream may be 10 wt%–12 wt% NaHSO3. The acetaldehyde content in the cracked gas stream reacts with the NaHSO3 in the liquid bisulfite stream to form an adduct product that is soluble in the liquid bisulfite stream. The cracked gas stream leaving the top of the carbonyl absorber section is essentially free of the acetaldehyde, and the bottoms liquid contains the formed adduct and any unreacted liquid bisulfite. Bottoms liquid is recirculated back to the top of the column via recirculating pump.

The acetaldehyde content of the hydrocarbon gas stream at the top of the carbonyl section in the caustic tower is reduced to less than 1 wppm. When the NaHSO3 content in the recirculating solution reaches about 1 wt%, both the valve on the spent bisulfite line and the valve on the fresh bisulfite line are opened to allow the addition of the fresh bisulfite solution stream and the depleted bisulfite solution stream to discharge from the absorber.

The circulating liquid bisulfite stream in the carbonyl absorber may release a small-ppm level of sulfur dioxide (SO2) into an overhead cracked gas stream. Any SO2 produced will react and be converted to NaHSO3 in the caustic tower, and end up in the spent caustic stream.

Key learnings

The description here demonstrates the benefits of carbonyl removal by absorption with chemical reaction using a dilute solution of NaHSO3. These benefits are qualitative, as the precise quantitative benefits are best determined by actual analysis of the present operation of the given ethylene plant service. It is obvious that these benefits can be assessed with rigor for any given ethylene plant service by proper analysis of the base operation of the unit, and by the known performance enhancements defined by the foregoing.

By applying this innovative technology solution to the cracked gas stream prior to entering the caustic scrubber, the following qualitative performance enhancements can be realized for any ethylene plant:

  1. As all reactions that produce aldol condensation products are eliminated, requirements for any fouling inhibitors or other such additives typically used to retard the formation of these products are no longer necessary.
  2. Subsequent treatment of spent caustic is now free of all fouling precursors, thereby allowing for fouling-free operation of any commercially available spent caustic treatment method.
  3. As the caustic scrubber also enjoys a fouling-free operation, no capacity degradation is experienced by the AGR within the ethylene plant. Therefore, the desired plant capacity is achieved throughout the turnaround cycle for the plant.
  4. No extensive maintenance is required during turnarounds to repair and/or clean the internals of the caustic scrubber.
  5. The quality of the propylene product is improved by the absence of any acetaldehyde for downstream polymer units or any other specialty chemical unit using propylene as its feedstock.
  6. Any back-end liquid or gas phase C3 hydrogenation unit is improved by the absence of this temporary poison (i.e., acetaldehyde) with the extended lifecycle between catalyst regenerations.
  7. Similarly, the quality of the raw C4 product from the ethylene plant debutanizer is improved by the absence of acetone for downstream butadiene extraction, or any other specialty chemical unit using raw C4s as feedstock.
  8. Any back-end liquid or gas phase C4 hydrogenation unit is improved by the absence of this temporary poison (i.e., acetone) with the extended lifecycle between catalyst regenerations. HP

Literature Cited

  1. Kurukchi, S. A. and J. Gondolfe, “Removal of carbonyls from gaseous hydrocarbon streams,” US Patent 9834498, December 2017.
  2. Roling, P., “Method for prevention of fouling in a basic solution by addition of specific nitrogen compounds,” US Patent 4673489, June 1987.
  3. McDaniel, C, et al, “Method for the prevention of fouling in a caustic solution,” US Patent 5220104, June 1993.
  4. Lewis, V., “Method of inhibiting formation of fouling materials during basic washing of hydrocarbons contaminated with oxygen compounds,” US 5160425, November 1992.
  5. Subramaniyam, M, “Method of removal of carbonyl compounds along with acid gases from cracked gas in ethylene process,” US 7575669, August 2009.
  6. Ellis, C., et al, “Wet air oxidation of ethylene plant spent caustic,” AIChE Ethylene Producers Conference (EPC), April 1994.
  7. Kurukchi, S. A. and J. Gondolfe, “Spent caustic pretreatment process,” US Patent 5885422, March 1999.
  8. Kurukchi, S. A. and J. Gondolfe, “Spent caustic pretreatment and enhanced oxidation Process,” US Patent 6210583, April 2001.
  9. Holderness, J., “Spent caustic incineration at Dow’s new ethylene plant in Alberta, Canada,” AIChE Ethylene Producers Conference, April 1996.

The Authors

Related Articles

From the Archive

Comments