Vanquish Vacuum Distillation Difficulties

Readers recommend a variety of improvements.

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Second, higher concentrations of sulfur in crude will increase solids' carryover and cause erosion in the diffuser and on the external surface of the steam nozzle due to impingement. Perhaps a separator pot upstream of the first-stage ejector suction will help.

Lastly, if the steam nozzle split, it would leak steam not air. This would put an additional load on the ejector, increasing the suction pressure.

A few additional points are in order:

Wear in the diffuser and nozzle can also be an effect of wet steam. This wear is usually on the inside of the nozzle, causing "wire drawing." To operate efficiently, ejectors require 97% quality steam (minimum). The system may have a steam separator but the condensate, if not properly trapped, still will carry into the nozzle. No mention was made regarding the condition of the second-stage ejector. If its condition is reasonably good, then the corrosive gases are going out in the condenser. That being the case, what is the physical condition of the inter-condenser and tubes?
Louis Decker, manager
DecTecH Associates, Bridgewater, N.J.

A change in feedstock, such as a change to high sulfur Canadian crude, can impact a vacuum system in several ways. Ejector systems are sized to handle a specific load rate and composition. An increase in the overall flow or a change in a load's component makeup can have consequences, including performance and corrosion problems.

High cooling water temperature and low cooling water flow both impact a condenser's operating pressure. During the winter months, cooling water temperatures are colder and this allows the condensers to operate at much better pressures than would be seen during hotter summertime operation. Problems with the startup and operation of multistage ejector systems are more common during hot weather because of the warmer water temperatures. An ejector depends on the downstream equipment (condensers, ejectors and piping) to maintain a certain pressure at its discharge. If an ejector's downstream pressure deteriorates past its design point, unstable operation will likely occur. Hotter-than-design cooling water can cause poor condenser performance. This is why it is important that ejector systems be designed for the hottest cooling water temperature expected at the site.

Condenser fouling will have a similar impact on vacuum as hotter-than-design cooling water. As the heat transfer rate decreases, the operating pressure of the condenser becomes worse and the first stage ejector is then subject to a poorer-than-design discharge pressure. In these cases, the condenser bundles should be cleaned or replaced. Care needs to be taken when replacing condenser bundles as they are designed for vacuum service and have special construction features.

Vacuum control for a distillation tower is most commonly obtained by recycling load from the discharge of the first-stage ejector back to its suction. This allows one to manipulate the load to control the process vacuum. There are other control schemes for ejector systems but they have drawbacks that make recycle control the ideal choice for this application. Throttling the motive steam to the ejectors is not an acceptable way of controlling the tower pressure and can cause the vacuum system to be unstable.

Erosion damage is almost always attributed to wet motive steam. This type of damage is often seen inside an ejector's motive nozzle and in the converging section of an ejector's diffuser. Wet steam greatly reduces the service life of ejectors and has a negative impact on ejector performance.

Corrosion can also negatively impact an ejector. Certain crude slates, notably ones with higher sulfur levels, lead to the creation of acids. Inside a vacuum system, sulfur and other impurities mix with the motive steam, leading to the formation of corrosive compounds, like sulfuric acid.

Corrosion rates are accelerated by temperature. The outside of an ejector's motive nozzle, its diffuser throat and its diverging diffuser section are more prone to corrosion problems because they will be significantly hotter. Corrosion-controlling inhibitors can be considered as a means to combat corrosion. In other cases, the ejector bodies and condenser shells can be built in high-grade stainless steel or other alloys. The increased rate in the refining of more-sour crudes has led to a notable increase in the selection of duplex stainless steels.
Eric Michael Johnson, service engineer
Graham Corp., Batavia, N.Y.

Have you considered replacing the steam ejectors with vacuum pumps? However, the erosion issue you mentioned may be a problem for the vacuum pumps as well. An obvious question is have you considered reducing the "Canadian crude" content of the feedstock since you mentioned the problems started when more of the high sulfur feed was used? Upsizing your condensers or adding another in series may help you reduce your hydrocarbon vapors going to the thermal oxidizers.
Michael Dobrowolsky, controls engineer
SI Group, Schenectady, N.Y.

There appear to be two distinct problems: cooling water in the summer and, more importantly, steam quality.

If the tower cooling water is incapable of maintaining vacuum, you either need to increase the flow or the condenser temperature differential. Maybe you need chillers for the cooling water in the summer. One simple solution might be to adjust stage 2 so it receives fresh cold water instead of water from the first condenser. Also, you must look at the positions of the cooling-water control valves — if they're wide open in the summer, try putting in larger valves or larger water lines; on cooling lines I usually plan for butterfly valves because of their high Cvs.

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