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

Pinch Analysis and Other Steps

Since the combined feed exchanger utilizes heat in a process stream that requires cooling, pinch analysis helps determine the optimum heat exchanger network. Practical operating procedures also need to be considered. For example, if the naphtha feed contains some olefins, a significant temperature increase results across the reactor. Pinch analysis shows only a small charge heater is required. Startup considerations, as well as potential alternate operation with a straight-run only feed leads to consideration of a larger charge heater. The analysis must also achieve an all-vapour stream to the charge heater.

Once the process boundaries are defined, an analysis is done to compare the economics of incremental exchanger surface area offset by reduced fuel firing and air cooling costs to arrive at the right approach temperature, taking into account the corrected Log Mean Temperature Difference (LMTD). Most mixed-phase shell and tube heat exchangers have Overall Heat Ttransfer Coefficients (OHTCs) between 50 and 70 and this number is used for preliminary sizing of exchangers for the various cases. The allowable pressure drop per shell is 5 psi (0.35 kg/cm2) shell side and 5 psi (0.35 kg/ cm2) tube side. Typical TEMA fouling factors are assumed to ensure adequate shell and tube velocities. If higher fouling factors are assumed than required, the resulting low velocities could cause increased fouling. Economic analysis shows that generally 4 to 8 shells in series can be expected.

To set the CFE design pressure, the reactor circuit hydraulics are analyzed, including any expected long-term operations. For example, over the life of the catalyst, the top of the reactor can collect scale and gum that plug the top of the bed. A plugging allowance for this scenario is included when setting the design pressures of the equipment in the circuit. Design to the 10/13th rule is not applicable for this service due to the presence of two-phase flow. A pressure surge due to a tube leak or rupture from the high pressure side to the low pressure side is minimized by the compression of the vapour on the low pressure side.

However, the 10/13th criteria is often exceeded as the ratio of operating and design pressures is greater than 10/13 as they differ by a relatively small pressure drop in the circuit compared to actual pressure. Design pressure of the CFE is not graded. During startup, the reactor circuit is evacuated with an ejector to air-free the system. The exchanger exchanger is designed for full vacuum at the expected startup design temperature.

The CFE design temperatures are graded for cost effectiveness. All expected operating scenarios must be considered. Alloy selection for each shell is selected based on expected operating temperatures. If the number of shells is altered during the detailed design phase, it is essential that the metallurgy selection be reviewed with the change.

The exchanger project specification for this service shows TEMA type BEU exchangers are typically selected for this service. The U-type tube bundle is cost effective and minimizes the joints, where leakage may occur. UOP specifies them when the exchanger is in hydrogen service, defined as having a hydrogen partial pressure of 50 psia [3.5 kg/ cm2(a)] or greater, or for a service containing 90 vol% or higher at any pressure level. Horizontal baffle cuts are not used for the NHT CFE, because the feed is two phase. A vertical cut is used to ensure more even distribution of both phases around each baffle and reduce the potential for slug flow. All exchangers in process service are designed to the ASME Section VIII Code and to the requirements of TEMA. Application of TEMA Class R, Refinery Standards is required. UOP requires the use of at least a ¾ inch OD tube.

The exchanger project specification may specify strength-welded tube to tubesheet joints to prevent or minimize the possibility of cross leakage. For a typical NHT operation, strength-welding is not required because the unit can operate if minor cross-leakage occurs.

Strength-welding is required if a sulphur-sensitive isomerization unit is downstream of the NHT unit. When strength-welding is required, the metallurgist needs to know upfront to ensure that the metallurgy selected allows for welding. For a typical size unit, the CFE consists of one parallel train of multiple horizontal shell and tube heat exchangers in series. Since the reactor circuit is designed to be open without valving, PRV protection for exchangers alone is not required. With large units, the shell diameter may be limited by bundle pulling capacity necessary to clean and maintain the exchangers. If parallel shells are required, care should be taken to ensure equal flow distribution between trains. If a known fouling service exists and parallel trains are desired to allow cleaning without shutting the unit down completely, separate PRVs would need to be provided. Placement and relief destination of the hot streams would need to be considered.

Over the years, NHT operating temperatures have decreased due to concerns with formation of recombination sulphur products. Some feeds and reactor circuit conditions resulted in trace olefins and H2S recombining after leaving the reactor. To prevent this phenomenon, reactor temperatures were lowered to those specified today. Neutralization requirements for austenitic steel is therefore not itemized in the exchanger project specification for modern units and does not affect metallurgy selection.

Shell and tube heat exchanger’s temperature measuring equipment used to determine the performance of each individual shell is not specified in the exchanger design specification data. Typically, each refiner has specific requirements, which are included on the Piping and Instrument Diagram (P&ID) legend for implementation by the contractor. For the exchanger arrangement selected, the need to determine exchanger over- or underperforming should be considered on the process and equipment, as well as monitoring turndown, start-up and shutdown conditions.

Metallurgy selection

Metallurgy selection for all UOP units begins with the generation of a Material Selection Diagram (MSD). An MSD shows all pieces of equipment in individual boxes. All major piping circuits are also shown; however, the MSD does not contain the level of detail of a P&ID. The MSD will give design temperatures and pressures, maximum operating temperatures, hydrogen partial pressures and concentrations of contaminants such as sulphur, H2S, ammonia, chlorides, etc. MSDs are generated in accordance with the guidelines of NACE SP0407 (1).

Specific to hydro-processing units, UOP adds to the MSD a Metallurgy Review Information Sheet (figure 2). The Design Engineer fills in the process details for the shell side and tube side for each exchanger in the train, along with maximum operating temperatures for the interconnecting piping (if any). The metallurgy selection process for the CFEs begins with a check of temperature and hydrogen partial pressure against the curves in API 941 (2), sometimes referred to as the ‘Nelson Curves’ after their originator.

This check is done for both the shell side and tube side conditions. UOP uses design temperature and design pressure for this check, not operating temperature and pressure. This adds two layers of conservatism to our design. The required metallurgy for each side will be recorded, and this sets the minimum required metallurgy. However, the metallurgy will frequently need to be upgraded due to corrosion considerations as will be discussed later.

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