Jim Archuletta Bruce Bitler
Conoco Inc.
Commerce City, Colo.
Mark Binford
Betz Process Chemicals Inc.
The Woodlands, Tex.
John Modi
Betz Process Chemicals Inc.
Denver, Colo.
Conoco Inc. and Betz Process Chemicals Inc. (Betz ProChem-a specialty chemical supplier) worked together to identify and address corrosion problems at a crude unit through a series of mechanical changes, operating adjustments, and unique chemical treatments.
As a result of the work done at the sweet crude unit, Conoco Inc. and Betz ProChem developed a "partner" relationship (not a legal partnership) that encompassed the entire refinery.
This allowed both parties to commit resources and share information in order to solve problems and consolidate business to the benefit of both companies.
For more than 5 years, the sweet crude unit at Conoco's Commerce City, Colo., refinery had experienced severe corrosion in the crude prefractionator column and overhead system as well as crude furnace and prefractionator fouling.
These corrosion and fouling problems reduced refinery profitability by decreasing run lengths, limiting throughput, and reducing quality.
In 1987, Conoco Inc. began a concerted effort at both the local and corporate levels to eliminate these problems and improve overall crude unit operation.
This article describes the problems encountered and solutions applied at the sweet crude unit, and also discusses the benefits of the refiner/supplier relationship developed at the Commerce City refinery.
CORROSION HISTORY
Conoco Inc.'s Commerce City, Colo., refinery has a 35,000-b/d crude unit designed to process sweet crude from production fields in central Wyoming and northeastern Colorado. Other operating units at the refinery include an FCC, reformer, VRU, cat poly, rerun, and asphalt crude unit.
From 1981 to 1987, Conoco Inc. was plagued by problems in the crude unit, stemming primarily from poor crude quality. Crude price differentials encouraged the unauthorized blending of incompatible asphalt-based crudes into the sweet crude stream.
Surfactant floods and tertiary recovery methods in the production fields led to numerous downstream problems in the form of poor desalting; excessive fouling in the crude preheat train, crude furnace, and prefractionator bottom grid; and severe corrosion activity in the prefractionator tower and its overhead condensing system (Fig. 1).
These problems were addressed by several, largely unsuccessful, measures. Desalting severity was reduced by cutbacks on mix valve pressure differential (_P) and washwater rates in order to maintain system control.
Crude tower and overhead system metallurgies were upgraded in an attempt to increase unit reliability. A combination of filming and neutralizing amine was injected into the prefractionator tower overhead vapor line.
This chemical treatment reduced corrosion in the condensing system somewhat, but provided no protection to the tower where much of the corrosion was occurring.
An antifoulant program was initiated in the crude preheat, providing substantial energy savings by greatly reducing fouling in the hot train exchangers. However, fouling was still a problem in the prefractionator bottom grid, the crude furnace, and the product side of several preheat exchangers.
These fouling areas had not been treated because of insufficient justification due to frequent corrosion-related shutdowns. In spite of these steps, the crude unit continued to suffer from run lengths as short as 6 months and excessively high maintenance costs.
It eventually became necessary to replace the top of the prefractionator tower as well as the overhead condensing bundles, due to wall thinning.
TECHNICAL PROBLEMS IDENTIFIED
In 1987, the major operational problems facing the Commerce City refinery were poor desalting, the continued corrosion in the prefractionator system, and fouling in the crude furnace and prefractionator bottom grid. With overall refinery throughput and profitability suffering as a result of these problems, Conoco Inc. launched a major investigation aimed at finding solutions.
Conoco solicited input from local plant resources and Conoco Inc. corporate engineering, as well as several specialty process chemical suppliers, to help identify the causes of these problems. The operating conditions, chemical treatments, and design of all major pieces of the crude unit and related equipment were analyzed.
This review indicated that the variable most influencing prefractionator overhead corrosion was the chloride loading in the tower. As a result, attention was focused on the desalter.
This equipment is designed to remove inorganic chloride salts as well as inorganic solids from the crude. It was believed that inorganic solids were contributing to fouling experienced in sections of the hot exchanger train and the prefractionator tower.
Some analyses indicated that corrosion products from the top trays of the crude tower were migrating to the bottom of the tower as well. These solids were also contributing to bottom tray and grid fouling.
Therefore, with the desalting operation targeted as the most appropriate starting point to address the wide range of operational problems, Conoco Inc. decided to evaluate the effectiveness of the chemical emulsion breaker used at the desalter.
PROBLEM SOLUTIONS
In the desalting process, raw crude and fresh washwater are mixed and then separated through electrical coalescence of the diluted brine dispersed in the crude. The key to effective salt removal is intimate contact between the washwater and crude.
This contact, however, can also lead to the formation of stable emulsions. In Conoco's case, crude contamination with high solids and production chemicals stabilized these emulsions and limited the amount of mixing energy that could be used.
Better emulsion resolution in the desalter would allow more mixing between the crude and washwater. Chemical emulsion breakers are commonly used to aid in the emulsion resolution process. After evaluation of several programs, Conoco Inc. determined that Betz ProChem's Embreak emulsion breaker treatment program provided the most improvement in desalter performance, significantly reducing prefractionator overhead chlorides.
This, in turn, reduced corrosion as measured by corrosion-probe readings, water analyses, and acid loadings. Along with the new chemical treatment program, several significant changes were implemented in 1987 that had a positive impact on desalter performance.
DESALTER EMULSION BREAKER CHANGES
The Embreak emulsion breaker was selected based on product testing conducted at the Conoco Inc. plant and at Betz ProChem's corporate research center in The Woodlands, Tex. Because of the tendency of Conoco Inc.'s crude to form reverse (oil in water) emulsions at high shear conditions, new product screening techniques were developed specifically for this crude.
These new techniques involved mixing the treated crude and washwater at varying degrees of emulsification to determine which product prevented the formation of reverse emulsions.
A proprietary desalter simulator was also used which allowed testing to be conducted at process temperatures and in the presence of a high energy electrical field, thus providing a closer simulation to actual field conditions.
These more realistic test conditions led to better product selection (Table 1).
MECHANICAL VARIABLES
A review of desalter performance data showed desalter salt removal to be erratic. High levels of salt in the desalted crude were common.
Some of these salts hydrolyzed in the hottest preheat exchangers and migrated to the prefractionator tower overhead in the form of HCI. After careful analysis of both stages of the desalter system, Conoco and Betz ProChem attributed this erratic performance to inadequate mixing of the raw crude and washwater ahead of the first-stage desalter.
The first-stage mix valve _P was typically low (5-10 psi) and was reduced further when problem crudes were processed through the desalters. These problem crudes formed reverse emulsions which led to oily effluent brine.
Due to limitations in the wastewater treatment system, this could not be tolerated. When confronted with this problem, the crude unit operators' only choice was to lower mix valve AP and reduce washwater rates, causing salt removal to deteriorate even further.
The introduction of the new desalter emulsion breaker improved emulsion resolution in the first stage. These changes, and a crude preheat revamp to increase desalter temperature, allowed Conoco to maintain system control while raising desalter operating severity.
In order to achieve more complete oil/water contact in the first stage, washwater rates and mix valve _Ps were raised to the mechanical limitations of the unit. Mix valve _P was run as high as possible without backing out crude charge due to excessive pressure drop.
Washwater rates were increased to 6.5-7%, the maximum washwater pump output. Analysis of the data showed that once the first-stage mix valve _Ps were increased, salt removal improved dramatically (Fig. 2).
As a result of these initial improvements, the first-stage desalter mix valve was replaced with a more efficient valve to further improve salt removal.
CRUDE-TOWER PROTECTION
Several changes in the prefractionator chemical-treatment program were implemented to further reduce corrosion. A series of on-site studies was performed in early 1987 to determine the corrosion mechanisms occurring in the tower and overhead system.
The pH was measured at several points in the system under various operating conditions in order to determine both the initial condensation point (ICP) temperature and the concentration of acids at each location. Betz ProChem provided its condensate online analyzer (COLA) in order to perform the studies.
From analysis of the condensed waters from the COLA, a pH profile of the system was constructed. Results indicated the ICP pH in the overhead vapor line was higher than that of the overhead accumulator drum.
This was due, in part, to the high concentration of soluble H2S in the accumulator relative to the concentration at the ICP (Table 2). As a result, the pH control range for the overhead accumulator water was lowered, which reduced neutralizer treatment costs and provided a more optimum pH at the overhead vapor line ICP.
The resultant pH allowed the filmer to form a more tenacious film on the metal surfaces, further reducing corrosion potential. The COLA studies also confirmed the location of another dew point in the upper section of the prefractionator tower.
The pH and condensing acids in the top trays were also quantified, which provided insight for designing a proper corrosion-treatment program. Filmer-injection points were modified to ensure that all metal surfaces were protected from direct acid attack.
In addition to filmer injection into the overhead vapor line, a supplemental point was added to the top pumparound in order to protect the upper section of the tower.
The filming amine was upgraded to a Betz ProChem product which was specifically developed to provide effective protection over the entire pH range of the system and in the wet H2S environment.
Fig. 3 shows the original corrosion inhibitor-injection points as well as the new injection point and monitoring equipment installed by Conoco to improve corrosion protection.
CRUDE UNIT ANTIFOULANT CHANGES
The improvements made in desalter performance and prefractionator tower corrosion protection allowed Conoco to double crude unit run lengths compared to the longest previous run. Run lengths had previously been limited due to either overhead corrosion (bundle failures, upper tray corrosion, and loss of fractionation) or plugging of the bottom trays and grid by both organic and inorganic material.
The shutdown, occurring at the end of this longest run, was due to organic fouling of the prefractionator tower bottom grid, as determined by analyses conducted on samples taken during the turnaround. This fouling of the bottom grid had not only shortened the unit run length, but had also limited throughput due to excessive pressure drop and poor fractionation in this section of the tower.
Fouling had also been identified during this run on the product side (atmospheric gas oil) of the final two preheat exchangers. The sweet crude stream has a very high organic-fouling potential.
The crude is gathered from north-central Colorado and southeast Wyoming in several independent pipeline systems.
As a result, the crude can be contaminated with sour asphaltic crudes produced in the same areas.
The sour crude exhibits incompatibility with the sweet crude as the combined stream is heated. This, left untreated, results in heavy organic fouling of the preheat train and increased fouling of the prefractionator tower bottom grid and crude furnace.
Historically, the fouling in the preheat exchangers had been treated using a Betz ProChem multifunctional antifoulant.
The cost of the program was justified on energy savings alone.
These savings were a result of lower preheat exchanger fouling rates which decreased downstream crude-furnace, fuel-gas usage over the length of the run.
Lower corrosion rates and longer run lengths had now increased the need to prevent the fouling in the tower-bottom grid and downstream furnace.
The antifoulant chosen to treat these two areas incorporates a high-temperature antifoulant that is thermally stable at the crude furnace-outlet temperature.
In order to realize the maximum benefit of the program, the antifoulant injection was split. The wash-oil spray for the bottom grid was treated with a portion of the antifoulant.
The remainder of the antifoulant was injected ahead of the last two preheat exchangers.
This injection point not only provided additional protection for these hot exchangers, but provided some protection in the downstream crude furnace as well.
A new injection point was also installed to add antifoulant into a hot atmospheric gas oil stream which is cooled by the last two crude preheat exchangers (Fig. 4).
Focus was placed on the last two exchangers for two reasons.
First, they are the hottest bundles and therefore are the most prone to fouling. Second, and equally as important, because there is no additional heat applied to the crude before it enters the furnace, the recovery factor is the highest of all exchangers.
Table 3 shows the clean heat-transfer coefficients and duty of these two exchangers as compared to the rest of the hot-train preheat. Any heat lost here has to be made up in the furnace, and this, of course, results in a fuel-gas penalty.
PREHEAT MONITORING/ANTIFOULANT OPTIMIZATION
To monitor the effectiveness of these changes, service and monitoring on the crude preheat was increased. In addition to weekly temperature surveys and heat-transfer coefficient calculations, periodic use of the Betz ProChem's Insite device was initiated.
Insite is a patented analytical tool which generates a quantitative value that is proportional to the fouling tendency of whatever crude is being tested. Also, as part of a rigorous monitoring program, multiple regression analyses (MRA) techniques were used to analyze crude-furnace data.
The MRA techniques normalize the scatter that is inherent in data of this type. Information such as flow rates, fuel-gas usage, and inlet temperature are all normalized back to a single starting point.
As a result, Conoco could more accurately monitor furnace fouling vs. time without risking misinterpretation due to fluctuations in other independent variables (Fig. 5).
MECHANICAL/OPERATIONAL MODIFICATIONS
In addition to the chemical changes, a significant mechanical change and one operational change also took place in early 1989. The grid in the bottom of the W-1 tower was redesigned and replaced.
The new design utilizes a tangential flow distribution into the flash zone to minimize overflash and channeling in order to reduce fouling. A new grid was also installed in the top of the tower, replacing the top two trays. The process change allowed an improvement in unit yields, and the temperature of the tower top was increased from 210 F. to 280 F.
This eliminated the existence of a bulk dew point in the tower itself and shifted it, instead, to the overhead condensing system.
Shifting the dew point resulted in a less corrosive environment in the top of the tower.
CRUDE QUALITY ISSUES
As previously mentioned, Conoco's sweet crude stream exhibited extremely high fouling tendencies. This was unexpected because it is a light sweet crude at 40 API and 0.5 wt % sulfur.
A review of local production indicated the presence of several different contaminants including: surfactant chemicals from tertiary recovery, organic chlorides from production chemicals, and asphalt-based crude. These materials were mixed with Conoco's production in the common carrier pipeline system.
More stringent crude testing at the receiving station dramatically reduced the chemical contamination. Specific testing for surfactants and organic chlorides is now being conducted on each incoming batch of crude.
The asphalt crude contamination is encouraged by the high price differential between sweet and asphalt crudes.
Most of the sweet production is actually around 0.2 wt % sulfur, so that 10-20% of the 2.5-3.5 wt % sulfur asphalt crude can be blended in and still meet the pipeline specification of 0.5 wt % sulfur.
This type of contamination can lead to asphaltene precipitation and extremely high fouling as the crude is heated and partially vaporized.
Part of Betz ProChem's proprietary treatment program is designed to prevent the agglomeration of these asphaltenes and reduce their growth via polymerization/condensation reactions throughout the entire crude preheat and furnace system.
RESULTS
Since 1987, when Conoco Inc. and Betz ProChem first began working together to solve problems at the sweet crude unit, a partner relationship has developed at the Commerce City refinery between the two companies. Betz ProChem continues to maintain a strong service presence in the refinery, and on-site program reviews by corporate support engineers from both companies are commonplace.
From a performance standpoint, Conoco continues to benefit from optimum desalter performance. Average desalting efficiency has risen from approximately 72% in 1985 to over 93% throughout 1989 (Fig. 6).
Chloride levels measured in the prefractionator tower overhead accumulator have dropped from an average of 175 ppm in 1985 to the recent levels of 30-35 ppm (Fig. 7).
The corresponding corrosion rates have also dropped dramatically.
Increased solids removal at the desalters and lower prefractionator corrosion rates reduced inorganic solids deposition throughout the system as confirmed by visual inspection of equipment at shutdowns and deposit analyses taken from key locations in the unit (Fig. 8).
A recent review of the crude preheat and prefractionator system showed that fouling rates are noticeably lower than the previous run and dramatically lower than historic rates. Besides the obvious throughput benefits and run length incentives, the increased high-temperature antifoulant addition has led to fuel-gas savings at the furnace which have more than paid for the chemical program (Table 4).
Field operating and analytical data have been collected for the past 2 years and incorporated into Betz ProChem's desalter computer data base. The desalter data base compiles operating and analytical data from large desalter systems across the country.
These data can then be manipulated to identify performance trends and find cause/effect relationships. Key process variables can also be plotted in statistical process control (SPC) format for analysis.
This treatment of data allows identification of variables that do or do not fall into reasonable control limits. Process variables can then be sorted into those that need improvement and those that do not.
Through this process, areas for improvement can be targeted and the end result or improvement quantified.
This technique was used to improve control of first-stage desalter salt removal (a key parameter in minimizing prefractionator tower overhead chlorides) and, most recently, to identify a relationship between prefractionator tower overhead chlorides and desalter temperature (Fig. 9). Use of the desalter data base in this manner has allowed Conoco to improve operations from year to year and to continue to target projects for further improvement.
As a result of the partner relationship, Conoco has not only made significant progress in extending crude unit run lengths and increasing total throughput, but has also effectively addressed fouling and corrosion problems in other operating units.
For instance, a fouling problem in the slurry circuit off the bottoms of the FCC main fractionator led to reduced efficiency and output from a slurry heated steam generator.
Use of a high-temperature dispersant, combined with close monitoring, has minimized the problem and extended run lengths from 6 months to more than 3 years. In addition to the corrosion rate reductions achieved at the sweet crude unit, other joint projects targeted at specific problem areas have resulted in corrosion rate reductions in the FCC and rerun units as well (Fig. 10).
After more than 3 years, Conoco and Betz Process Chemicals have come to recognize the benefits inherent with this type of partner relationship.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
