Proper Design and Operation of NHT CFE Equipment Ensures Economy

Related Vendors

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

TURCK GmbH

Frewitt fabrique de machines SA

UOP LLC

Next step in the metallurgy selection

The next step in the metallurgy selection process is to determine corrosion rates. The corrosion rate on the effluent side will always be governed by H2/H2S corrosion. The corrosion rate on the feed side of the exchanger must consider high temperature sulphidic corrosion. However, the recycle gas, which contains H2S, is sent back to the reactor through the CFE.

In addition to high temperature sulphidic corrosion, the feed side may also get affected by H2/H2S corrosion, and whichever is higher will govern. To make these determinations UOP uses proprietary corrosion rate curves. Note that for the tubes, corrosion occurs from both sides. Therefore, the corrosion rates are added to determine a total corrosion rate for the tubes.

Practices followed in UOP

UOP designs the CFEs to a 20 year design life. In order to meet this design life, UOP will add corrosion allowance or upgrade metallurgy as necessary. UOP will add corrosion allowance only up to ¼ in. (6 mm). As mentioned previously, this frequently results in upgrading metallurgy from what was indicated by the API 941 curves. If a ¼ in. (6 mm) corrosion allowance will not result in meeting or exceeding the design life for the shell, channel and floating head, a stainless steel lining will be specified.

For the tubesheet, if the corrosion rate on the tube side is high – such that the tubesheet will not meet the design life criteria, we will specify lining on the tube side with stainless steel. We also specify tube side lining with stainless steel if the tubes are stainless steel and they are to be welded to the tubesheet. If the corrosion rate on the shell side is too high, we will specify a solid stainless steel tubesheet.

Since corrosion allowance is not added to heat exchanger tubes, UOP limits the total corrosion rate (adding the corrosion rates from both sides) to 5 mils per year (0.127 mm/yr). This frequently results in upgrading the tube bundles to stainless steel. Another consideration is the potential for ammonium chloride deposition. Ammonium chloride can be present in units that have higher than desired chloride levels in the feed or in the make-up hydrogen. For this reason, UOP prefers a feed chloride level not to exceed 0.5 ppm, and a maximum chloride level in the make-up hydrogen of 1 ppm. If the make-up hydrogen is coming from a UOP platforming Unit, we desire that the hydrogen goes through a double chloride treater before coming into the NHT.

Ammonium chloride has a higher sublimation temperature than ammonium bisulphide (which precipitates in the air cooler), so the ammonium chloride, if present, will precipitate somewhere in the CFE train. If this is a possibility, the designer or the design engineer will indicate to the metallurgist on the MSD in which exchanger the precipitation will occur. Wash water injection nozzles will be specified for this exchanger to prevent fouling of the exchanger in service.

Because wet ammonium chloride is extremely corrosive, the metallurgist will upgrade the tubes and lining in this exchanger to Alloy 625. A final consideration in the metallurgy selection for CFEs is wet H2S at the cold end of the exchanger train. UOP has adopted the definitions of wet H2S service given in NACE Standard MR0103.

The concern with wet H2S is that it cannot only cause general corrosion, it can also cause sulphide stress cracking, as well as blistering, hydrogen-induced cracking, and stress-oriented hydrogen-induced cracking.

To mitigate these concerns, when a piece of equipment made of killed carbon steel is designated as wet H2S service, UOP will add a 3/16 in. (5 mm) corrosion allowance (1/8 in. -- 3 mm for piping), and specify postweld heat treatment. If the hydrogen sulphide concentration is equal to or greater than 25 per cent, we will line the vessel with Type 316L stainless steel, and upgrade all associated piping to solid Type 316L.

* The authors are from UOP LLC, a Honeywell Company

* Des Plaines, Illinois, USA

(ID:28641820)