REFINERY OPERATING EXPERIENCES REVEAL COMMON STEAM-STRIPPER PROBLEMS

Stephen P. Frayne
Nalco Chemical Co.
Sugar Land, Tex.

Waste water organic steam strippers present a unique set of operating problems for the refiner. Because this unit, by nature, requires a virtual 100% on stream factor, reliability and operability of the system are of paramount importance.

Understanding potential operating problems and developing a chemical treatment strategy to address them will minimize their impacts.

The examples presented here are a compilation of various field experiences. Virtually all systems will have to contend with at least one of these problems.

Major problems include short run lengths, high maintenance costs for frequent cleaning, and decreased stripping efficiency. Chemical treatment programs have been formulated to address these issues.

In many cases, one of these potential concerns may appear so overwhelming, that it masks secondary, but perhaps more serious, problems. Therefore, it is imperative that the system be evaluated as a whole and treated accordingly, rather than focusing on only the most obvious and current crisis.

BACKGROUND

The promulgation of two U.S. Environmental Protection Agency regulations-benzene national emissions standards for hazardous air pollutants (Neshap) and pending hazardous air pollutant (HAP) rules-has led industry to examine alternative technologies for the removal of volatile, hazardous organic compounds from waste water streams.

Technologies utilized to meet these requirements include:

Biological degradation
Air strippers in conjunction with incinerators, flares, and fuel consumers
Fuel gas strippers in conjunction with fuel consumers.

The use of these technologies is expected to intensify as additional regulations target potential air pollutants.

Industry information indicates that steam stripping either under slight pressure or under vacuum, is the preferred technology. This is not surprising because this technology is conventional and has predictable operating results.

STRIPPER PROBLEMS

Waste water strippers have several operating components (Fig. 1). First, there is some type of collection system to gather the streams that require treatment. These streams are brought to a central pretreatment location - typically a tank or break tank - where oil can float and solids settle.

This tank usually supplies a surge capacity of several hours to several days. The selection of this surge capacity and operating holding volume will dictate the degree of separation that will be achieved.

Because the break tank cannot achieve sufficient oil and solids removal, it is typically followed by a dissolved gas or induced-gas flotation unit (DGF/IGF). The purpose of the DGF/IGF is to remove additional oil and solids, leaving relatively low levels in the waste stream.

From the DGF/IGF, the waste water proceeds to the stripper, which consists of feed/effluent preheat exchangers, a stripping tower with an overhead condensing system, and a stripping steam source (reboilers are rarely used).

In the refining industry, the sour water stripper is often considered similar in operation to the waste water stripper. But while the two are very similar mechanically (Fig. 1), the waste water stripper has a unique set of operating concerns because of the nature of its feedwater.

A sour water stripper is fed waste water streams that originate as condensed fractionator stripping steam. The major contaminants in sour water are hydrogen sulfide, ammonia, cyanides, and water-soluble organics such as phenols.

Because sour water is basically steam condensate, it is very low in calcium/magnesium hardness and alkalinity, which cause scale in water systems. And because a sour water stripper is a closed system, the water is devoid of oxygen-one source of corrosion.

The method of contacting and separating sour waters from overhead accumulators results in water that is very low in free oil and grease (O&G) and suspended solids.

On the other hand, waste waters that must be steam stripped to remove HAPs have more varied sources and qualities. In refineries, the most common sources of steam-stripper waste water feeds include: desalter effluent, spent caustic, and crude and product - tank water draws.

Contaminated groundwater sources, and a host of intermediate waste water streams that fall under the regulatory umbrella, may become feedwater sources. In some cases, the entire waste water effluent stream is fed to the steam stripper as an integral part of the waste water treatment plant.

These varied feed streams present some unusual problems in the operation of the steam stripper.

OIL AND OILY SOLIDS

Free oil (O&G) typically enters the system from "desalter undercarry" or tank water-draws.

Desalter undercarry is the undesirable transport of crude oil out the bottom of the desalter along with the desalter brine. Undercarry can have a number of causes and it is strongly dependent on the crude source and desalter operating parameters.

Virtually all other waste water streams that are incorporated into the feedwater also are capable of carrying free oil with them during abnormal operation.

Desalter brine is also a major source of oily solids entering the system. Table 1 shows some random, desalter-effluent brine analyses. Oil and oily solids in the system generate two major operating problems in waste water steam strippers.

First, oil tends to cling to metal surfaces and act as a binder for suspended solids. This can cause loss of heat transfer in the feed/effluent exchangers, increasing energy consumption and possibly decreasing unit throughput.

Second, oily solids will settle in low-velocity regions, plugging stripper-tower trays or packing. The importance of this pretreatment equipment cannot be overemphasized because of its impact on stripper operability. Plugged trays and packing will result in poor fractionation efficiency, causing high HAPs in the discharge.

These mechanical separations can be enhanced by chemical treatment. Emulsion-breaking chemicals added to the oil-laden streams prior to field collection enhances emulsion resolution once the stream reaches the break tank. This allows maximum oil recovery as slop oil, as opposed to producing hazardous waste.

A combination of coagulants and flocculants can be used in the gas flotation equipment to enhance performance. The addition of such agents produces the lowest possible O&G levels, given the mechanical characteristics of the equipment.

Special control systems have been devised to monitor and control the separations area, because of its importance to the overall operability of the system. The advantage of these control systems is the feed-forward/feedback control logic, which maximizes O&G removal at the optimum chemical dosage.

Even if the break tank and gas flotation units are operating at maximum capability enough O&G may enter the stripper to create problems. The precise level of O&G in the feedwater that can be tolerated is specific for each system and must be determined from field measurements and evaluations.

In most cases, it is advantageous to use oil dispersants to enable the dispersed oil to pass through the system.

OXYGEN CORROSION

Some streams collected for stripping may have relatively high levels of dissolved oxygen. This is particularly true where whole effluent waste water is being stripped.

Depending on the types of streams collected for treatment, the temperature profile in the stripper system can start at ambient and increase through the feed effluent exchangers and the stripper itself, until it reaches the water-saturation temperature dictated by the stripper pressure. This temperature is usually 230-270 F.

Dissolved oxygen becomes very corrosive at stripper operating temperatures (Fig. 2). Oxygen corrosion can be quite prevalent on the feed side of the feed/effluent exchangers, up to and including the inlet of the stripper.

The stripper acts as a deaerator, removing any remaining dissolved oxygen and producing a stream with very low oxygen levels. Thus the stripper itself should not experience accelerated oxygen corrosion.

Actual corrosion rates will be dictated by the level of oxygen in the feedwater and are roughly proportional to the system temperature. Oxygen levels should be anticipated prior to start-up, based on the nature of the collected streams.

Should oxygen corrosion become a concern, sodium sulfite can be added to remove residual oxygen and prevent corrosion. Oxygen scavengers need to be fed far enough upstream of the vulnerable areas to ensure good mixing and give the required reaction residence time (this is normally only a few seconds).

After start-up, oxygen levels should be tested to determine baseline conditions. Carrying a slight sulfite residual and monitoring dissolved oxygen levels can optimize control parameters.

Another problem created by oxygen is in the deposition of sulfur. Sulfur deposits have been found to be foulants in these systems. Such deposits have resulted from feeding spent caustic that had been used to scrub H,S from gas streams. Elemental sulfur is generated via a reaction with oxygen (Reaction 1).

Sulfur dispersants originally developed for oil field applications can be used to help mitigate this problem.

SCALE DEPOSITION

Another major concern in stripper systems is scale deposits from calcium carbonate, magnesium hydroxide, and magnesium silicate. The potential for scale deposits is a function of mineral-constituent concentrations in the feed, pH of the feed, temperature of the system, and water velocities in the system.

Table 1 shows that calcium and magnesium hardness, alkalinity, pH, and silica can vary widely. This variability is caused by the desalter wash-water source and the respective constituents in the crude. Feedwater variability makes the prediction of scale potential site-specific.

Scale occurs when the mineral constituents in the water exceed their solubility and precipitate (in other words, When the water becomes saturated). It is important to understand that scale deposits can occur by two mechanisms.

Scale can form in the stagnant, thin-film, water boundary layer at heat-transfer surfaces when the water solution in the film becomes supersaturated. Besides having a higher temperature, the film also has a higher pH because of a higher concentration of OH- anions. If water velocities are low, this boundary-layer film is large enough that the precipitated mineral adheres to the metal surface rather than migrating to the bulk Water. This results in a hard, baked-on scale layer on the metal surface. Such a layer might be found in the feed/effluent exchangers and the reboiler.

In the second, scale-producing mechanism occurs when the solubility of the mineral in the bulk water is reduced because of high pH and high bulk-water temperature. When this happens, the mineral will precipitate from the bulk solution and the crystal will grow in size until it becomes a foulant. This material can deposit on metal surfaces by settling, impingement, or adhesion.

These types of deposits frequently are found in tower packing and trays. Such deposits often are associated with oil and oily solids, which have acted as a glue to immobilize the precipitated particles.

Table 1 shows that the predominant alkalinity species in desalter brine is the bicarbonate anion (HCO3-). Bicarbonate exists in equilibrium with dissolved carbon dioxide at a pH as high as 8.3 (Fig. 3).

In a closed system like the stripper feed piping, the application of heat in the feed/effluent exchangers causes the HCO3- to break down to carbonate (CO3). This chemical reaction, in the instance of calcium carbonate, is represented by Reaction 2.

Fig. 4 illustrates the change in CaCO3 solubility at typical stripper feed and effluent conditions. This figure shows that CaCO3 solubility decreases as it passes through the stripper, where the precipitated CaCO3 becomes scale. Therefore, CaCO3 Will be the predominant scale in the preheat portion of the stripper.

Because this portion of the stripper is a closed system, the carbon dioxide evolved by this reaction has to redissolve in the water, reforming another HCO3- by Reaction 3.

Once the feedwater enters the stripper tower, the system becomes an open one, with CO, being stripped from water as it evolves. This stripping of CO, causes the pH to increase, converting most of the bicarbonate alkalinity to carbonate. Carbonate alkalinity also degrades, much like bicarbonate but to a much lesser extent. In the case of sodium carbonate, Reaction 4 occurs.

The formation of OH- in the stripper column generates an MgSi2 and Mg(OH)2 scale. Silica in the feedwater, as either SiO2 or colloidal silica, is driven to the SiO,2- anion by hydroxide alkalinity (Fig. 5).

MgSiO2 forms as a hard, glassy-type scale that is difficult to remove. the Mg(OH)2 formed is virtually insoluble and precipitates, forming a sticky scale (Fig. 6).

Quite often Mg(OH)2 coprecipitates with silica to form a scale that can be misinterpreted as MgSiO, scale when analyzed. Therefore, under the stripper-tower conditions, MgSiO, and Mg(OH)2 will be the predominant scale deposits.

PH

The variable with the greatest impact on scaling tendency is pH. Therefore, it is imperative that if high-pH streams such as spent caustics are to be fed to the stripper, pH adjustment be made to neutralize the feed stream.

High pH also tends to saponify oils, making them more difficult to remove in oil-removal equipment.

Adjustment of pH should be made at the high pH source or in the break tank, but must be done upstream of the gas flotation unit.

The formation of scale in the system is bad enough, but a related problem-underdeposit corrosion-must also be combatted.

Underdeposit corrosion typically occurs because of the formation of concentration cells. Because this type of corrosion is difficult to detect until it has caused a failure, it is important to eliminate the root cause of the corrosion, i.e., deposits.

Although the thermodynamics t at drive scale formation are present, problems induced by scale and fouling can be controlled using appropriate chemical treatments. Statistical analysis of the feedwater constituents, combined with knowledge of the operating system, can be used with proprietary mineral-solubility models to predict the scaling tendencies of individual waters.

Saturation indices such as Langelier's or Ryzner's can estimate scaling potential; however, these are not as easy to use with a dynamic system such as that of a steam stripper.

In addition, simulation equipment has been used on existing feedwaters under actual operating conditions to determine the scale-inhibition capacity of dispersants. This has led to the development of specific scale inhibitors that can be used to minimize scaling problems in these systems.

While scaling is a potential problem, it is relatively easy to quantify its impact by monitoring the feed/effluent exchanger fouling factor, feed/effluent exchanger hydraulics, stripper-tower pressure drop, and reboiler fouling. These methods will give a clear indication of scale-inhibitor performance over time and should be used to determine programs and dosage efficacy.

FOAMING

Foaming in the stripper tower can be caused by surfactant-type materials or by solids that stabilize foams. Foam can create problems meeting the effluent HAP criteria and needs to be evaluated based on observations during operation. Again, high pH can play a role in foaming tendencies because of saponification of organics, and should be controlled for this and previously stated reasons. The addition of an antifoam agent can be helpful if foaming continues to be a problem.

GENERAL CORROSION

Corrosive species, such as H,S and cyanides, in the water can create generalized corrosion problems in the entire system, from the break tank to the stripper effluent. Residual corrosive species may end up in the overhead along with stripped organics and water vapor. As these species condense with the water phase, they create corrosion problems that must be addressed.

The breakdown of bicarbonates and carbonates to carbon dioxide generates CO,-rich overhead vapors in the stripper. The CO2 condenses with the steam, causing low pH and carbonic acid attack in the overhead system.

For general system corrosion, inhibitors can minimize this problem. In the overhead system, neutralizing or filming amines can be used to mitigate corrosion at the water dew point. These parts of the system also must be monitored for corrosion continually.