Pretreat MVC system

[scoho83]

We have a MVC for producing Boiler Feed Water. It was designed for use with produced water from oil production. However, we use local brine for a make-up system, and current produced water production is not up to quality. This is causing some issues in that the brine has Ferrous contamination (7 ppm), TSS and H2S present. The MVC system has pre-heating prior to its deaerator,and the pH adjustment system is a atmospheric tank which allows air into it (casuign the Fe to turn into Iron Oxide) which is scaling up the pre-heaters rather quickly. (O2 levels are measured at 2 ppm). I don't want to "treat" the water (i.e. take the load off the MVC) but I was wondering whether fulling aerating then filtering the the make-up water might be a solution. (i.e. we haven't treated the water th BFW quality, but we have caused the iron to oxidize in a place we can control it. Or should we just try to filter the water prior to the pre-heaters since (hopefully) the iron is already percipitated in the pH adjustment tank. Thoughts?

 

[bimr]

I woild not think that you would have to aerate the brine, you should be able to filter the brine with a high rate filter to remove the iron.

As an alternative, have you considered the use of a condensate polisher in the sodium cycle? You could polish the distillate with a high rate softener. These are commonly used in the paper industry to improve the quality of condensate returns.

 

[bimr]

http://www.drifterprojects.ca/articles/article_world_oil_2007-10.pdf

 

[scoho83]

bimr,

THANKS! Our problem isn't with the distillate quality, but more with fouling caused by Fe+ contamination. It is generatibg a FeO scale and fouling of the pre-heating and system (and we are concerned about the vertical tubes too, but haven't taken it down to inspect yet). The Fe+ contamination is in the form of Ferrous Iron (i.e. it doesn't percipitate till we add air - water is clear). But once we add heat, oxidation is occurring at the exchanger surface. We are considering Chelating to keep the iron in solution, but I am thinking out of the box (i.e. should we just try to get rid of it? Is that a better approach).

Scott

 

[bimr]

Oxidation of iron occurs with aeration. However, it takes less time for oxidation to occur when the pH is elevated.

You probably should run this query by the manufacturer of the evaporator. Someone like Resources Conservation Company, or RCC, now part of GE.

 

[scoho83]

We are.. the mfg is proposing putting in acid cleaning of the heat exchangers to keep scaling down, but I am afraid that with the levels of iron we are seeing (7 ppm) we will have an usual high chemical costs, and we will still require allot of off-line cleaning and gasket replacement costs. So I am thinking "out-of-the-box" on alternatives, and bouncing it here to see what people think.

 

[sbush]

It is always better to remove iron and other contaminants as far upstream of the critical process as possible. Chemical conditioning does not remove the iron from the feedwater and, therefore, when operating conditions change, such as temperature elevation, the iron can become oxidized and form deposits. Chemical conditioning does not remove or alter suspended solids or gases.

There are ways to oxidize iron without traditional (i.e., packed tower, bubble tray, cascade trays, air sparger) aeration techniques; however, to remove large amounts of iron, oxidation followed by filtration is best. In some cases, sodium zeolite softening will remove soluble iron; however, in most cases, iron is present in soluble as well as insoluble form along with other contaminants such as suspended solids and hydrogen sulfide, elemental sulfur, etc. These all tend to foul ion exchange resins rapidly resulting in extensive maintenance and costs.

If sufficient dissolved oxygen is naturally present in the feedwater, catalytic oxidation and removal of iron and H2S can be accomplished using a pressure filter containing a manganese dioxide filter media. For your application I would recommend using a natural manganese dioxide ore (a.k.a. pyrolusite) as opposed to man-made coated-materials such as manganese Greensand, Greensand Plus, or Birm.

If dissolved oxygen needs to be introduced, I would recommend using a pressurized aeration device (a.k.a. air saturator or air contactor) that uses compressed air to introduce oxygen into water under pressure. The air saturator introduces oxygen from air into the feedwater almost instantaneously. Immediately after the air saturator, filtration can occur using the same manganese oxide media that I have describe above.

There is no need for chemical introduction or pH adjustment. Periodic backwashing of the pressure filter(s) using clean water will removes oxidized iron from the filter bed. Natural media will not breakdown overtime. With proper backwash and cleaning procedures and techniques, media should outlast the vessel containing it.

In the early 80s, I was involved with supplying pressure filters for iron removal from injection wells at the Union Oil Salton Sea project. But, that application sounds a bit different than yours. There are many experts on this forum (including the manufacturer(s) of your process equipment) that may have direct experience with your application and I would defer to them.

S. Bush

http://www.water-eg.com