Dry Out of an Extraction Unit

Preventing water damage in a residue supercritical extraction unit

Subject: Extraction Unit Dry-Out
Date: Wed, 31 Jan 2001 09:49:36

Andrew,

We are starting up a new supercritical extraction unit soon. The plant includes an asphaltene separator, asphaltene stripper, resin separator, resin stripper, de-asphalted oil separator, and de-asphalted oil stripper. This is not a name brand unit, we built it ourselves rather than using a licensed process.

Operations has some questions about the best way to dry this out. They're worried that putting hot oil into a wet system will cause a water explosion and damage the unit.

Do you have any recommendations about drying the unit out?

J., North American Plant

Subject: Extraction Unit Dry-Out
Date: Fri, 01 Feb 2001 08:33

J.,

Water damage resulting from putting water into hot oil systems (or oil into water pockets) is one of the major sources of tower damage. Your operations department's worries are justified. Water damage can happen in both vacuum and pressure towers. Figure 1 shows a typical configuration for the type of unit you are talking about. They typically have lots of places for water to accumulate during steam-out.

I'll respond to two sub-questions here (1) draining of bulk water from the system and (2) drying before hot oil in.

Draining bulk water

When steamed to remove air or using steam to heat the unit to operating conditions, condensate accumulates at low points. All low points required drains. I assume that you have put low point drains in and are planning to use them during startup. Just a couple of points here that should be checked:

1. Low point drains should not be too small. Mill scale, rust, and trash (even from initial construction) can easily block a low point drain. The smaller the drain, the easier it is to block. Avoid the temptation of putting too small a drain in. Some plants use drains as small as one-half inch. Don't do this. Use at least a one inch (and preferably a one and a half inch) drain on all low points.
2. Make sure that low point drains are in both sides of pump/standby pump piping configurations. Often, a drain gets put in on the main pump line, but not on the standby pump feed line. Both need them (Figure 2).
3. Pump casings include drains. Pump casing drains do not substitute for low point drains.
4. You must be able to rod out low point drains. Drains can plug up. Even putting steam to back pressure them often does not work. At initial water wash, the operators need to be able to rod out the drains. During one recent troubleshooting job, many low point drains with elbows on the end of the drains were found. These connections were fully welded because plant standards called for all small diameter fittings to be welded instead of threaded. This made the low points impossible to rod out (Figure 3).

Proper use of low points drains bulk condensate from the unit, however sufficient water to cause damage still remains.

Nevertheless, many units, including supercritical extraction and other hot oil systems start up by using low point drains coupled with oil circulation and gradual heat up. The gradual heat up restricts the temperature until the entire unit has warmed up past the water flash point. Gradual heat-in controls the quantity of heat in the system. This is supposed to prevent having enough duty available to flash a large quantity of water at one time.

Plant experience with gradual heat up varies. Some plants have very good experience. Other plants have continual problems. Success depends upon the exact details of unit construction, piping and procedures. Often, even small differences in plant piping can create difficult to drain spots. These are a major contributor to water damage events.

Drying before hot oil in

If more than oil circulation with low point draining is required, two very effective techniques to consider are methanol wash and vacuum drying. Methanol wash is most common in petrochemical plants and gas processing units. Vacuum drying is most common in refinery units.

Many petrochemical and gas plants use methanol washes to remove traces of water. A methanol wash flushes out bulk water then dry nitrogen or other gas purge dries out the methanol.

Using methanol to displace the water has three major benefits. First, the methanol that displaces the water has a much lower heat of vaporization than the water. This reduces the heat load required to clean the liquid out of the system. Second, the methanol vaporizes at a lower temperature. Even a low preheat temperature will suffice to dry out the system. Third, for trace amounts of residual methanol, the vaporization vapor-to-liquid expansion ratio for methanol is much lower. This reduces the pressure surge from vaporization.

The most common refinery drying method for systems susceptible to extreme pressure surges from water vaporization is vacuum drying. For your system, you would start with bulk water drying as already outlined. During bulk water draining you would turn your hot oil system for keeping the products at flowing conditions on. This would keep the unit hot.

Then you would stop steam purge to the unit and gradually pulling vacuum on the unit. As you pull vacuum, the vaporization pressure of the water drops. The hot oil system would provide enough heat to vaporize water at a controlled rate. Once the system pressure has stabilized (showing low water generation rate) you can start hot oil into the unit.

Vacuum drying can even be used at low temperatures as long as a deep enough vacuum can be pulled to allow the water to vaporize.

Some points to remember for vacuum drying include:

1. Your vacuum system needs to be stable down to very low load rates.
2. The equipment must be designed for full vacuum.
3. Heat load must be controllable.
4. The system must be dropped to a low enough pressure to vaporize water in all points where it might accumulate. Essentially, all low points must be heated or the pressure kept low enough for sufficient time to vaporize all low point water at ambient temperature.

Summary

The general approach for two different approaches to drying has been discussed. Dry out should start with bulk water draining. Methanol wash or vacuum drying is the second step. The third step is low temperature oil circulation to sweep any water pockets out (if possible). Details on procedures and equipment design are critical for success with both methods.

Andrew Sloley
DGI