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