Crude Unit Operation

Mr. Norman Lieberman – I have an old seminar manual from 1996. You have explained optimizing crude desalter operation – but do not explain optimizing the desalter temperature. Is there a single value? I’m trying to train a young process engineer assigned to our crude pipe still.

--Bob, Louisiana

            Bob – Anything I wrote in 1996 is outdated. I’m much smarter now. There is no single optimum desalter temperature. A desalter outlet temperature of 300°F may damage the desalter electrical components. I’m sure anything below 220°F is too cold.

            Raising the desalter temperature will cut viscosity and s.g., which will reduce height of emulsion and brine (BS & W) carry-over into deslated crude. However, the increased solubility of water in hotter crude will increase desalter amps and water content of desalted crude. The extra water in fractionator feed will promote corrosion in top few trays of the tower due to H₂O condensation.

            Another big problem is vaporization in the desalter. I have one client in Indiana who runs their desalter at 45 psig and thus cannot increase desalter temperature much over 220°F without stirring up the contents with vaporization of steam and light hydrocarbons. Keep the desalter inlet at least 10°F below the calculated bubble point of the crude taking into account that it is saturated with water.

            A typical desalter outlet temperature for heavy (18°API) Venezuelan crude is 285°F. A typical desalter temperature for a light, sweet crude (35°API) is 245°F. If you have excessive growth in the emulsion layer, a warmer desalter will be optimum. If BS & W is less than 0.1 wt%, a lower desalter temperature may be optimum. Hope this longish answer helps.

--Regards, Norm

Crude Unit Operation

Dr. Lieberman – Here is Nehi. Should we add NaOH to desalt effluent?

--Best Greetings – Nehi, India

            Nehi – Add caustic to crude effluent from desalter is normal. You should minimize caustic addition. I use two methods. One way is to add NaOH based on ⅓ MgCl₂ content of raw crude. That is, about 65% of MgCl₂ is extracted with brine and the remainder hydrolyzes to HCl. Second method is to reduce chloride content of crude tower overhead drum boot to 10 to 15 ppm. Too much caustic is bad. Regardless never use more than 3-5 ppm of NaOH of crude. Spent caustic from a mercaptan extraction unit can be used as well as fresh 10-15 Be NaHOH.

--I’m not a Doctor – just Norm

Crude Unit Operation

Hi Norm – For a crude desalter, what is optimum mix valve delta P? Our crude is 25°API, we use 6 wt% water and desalter temperature is 250-270°F.

--Carl, California

            Carl – I used to think 15-25 psi. But, now I know better. Let’s say you have NaOH in the wash water and your brine pH is nine, rather than an optimum 6½. Thus, you have the desalter interface between H₂O and crude filled with several feet of emulsion so that you’re getting water carrying over into your crude heater. You then need to back way off on your mix valve delta P to reduce emulsion carry-over. It’s senseless to have a 20 psi mix valve delta P if it promotes excessive emulsion carry-over. In my experience, the most important controllable variable in desalter operation is pH control of the brine. Temperature is also critical but not often controllable. You can call me at 1-504-887-7714 for an in-depth discussion.

--Norm

Crude Unit Operation

Dear Norman – I enjoyed working with you last month. A follow-up question. How come our crude tower floods with such a tiny increase in delta P?

--Sam, Washington State

            Sam – Fe(HS)₂ deposits accumulate on the upper few trays of the crude tower, between the jet draw and the second to top tray. If fouling causes flooding starting at the top few trays, jet will entrain overhead. However, flooding progresses up a tower and not down. Thus, in your 40 tray tower, only the few trays flood, but delta P remains low. I recall explaining all this to you before, but I guess you were distracted by a text message from your girlfriend. Incidentally, iron sulfide deposits are insoluble in water and hence cannot be water off on-stream.

--Norm

            P.S. You’re running your tower top temperature 20°F too cold.

Crude Unit Operation

Norman Lieberman – What is a reasonable salt content in the desalter effluent? Our chemical vendor says 90% salt removal in our single stage desalter is good. Are desalting chemicals really helpful?

--Pranab, India

            Pranab – Desalting dispersant chemical is truly helpful. However, it is best added not to the desalter but to the crude charge tank to remove brine upstream of the desalter. I’m referring only to the oil soluble chemicals, not water soluble dispersants.

            The 90% salt removal is not the best measure of desalter efficiency. Most of your salts (90%) are NaCl which is not subject to hydrolysis to HCl. Best just to measure the MgCl₂ cone in desalter effluent which is the only salt in crude that will mostly hydrolyze at crude unit heater outlets of minus 700°F. CaCl₂ also hydrolyzes, but typically in the hotter vacuum tower heater outlet.

--Norm Lieberman