New graphical procedure to design reactor supports used in industry

Two reactor models used in gas recovery demonstrate a new design strategy that can lead to significant savings in material cost and engineering design time.

Osama Bedair

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Article Summary

Chemical reactors are used extensively in hydrocarbon extractions and gas recovery processes. Rapid expansion in the petrochemical industry has accelerated the development of various reactor models. In gas cooling (or condensing) operations, for example, vertical vessels are often used to facilitate the contact of hot gas with cooling liquid. Water is used for cooling or condensing processes.

The structural design of the support system requires knowledge of reactor operation and maintenance procedures to define the applied loading. Engineering planning that includes pipe routing and tie-in details must be developed for detailed installation of the reactor on the supporting system. A design interface between process, mechanical, electrical, instrumentation, and civil engineering disciplines is required to examine all engineering aspects prior to execution.

Much of the published work1-9 has focused on the process design aspects of chemical reactors. Very few structural guidelines are available in engineering design codes of practice and industry standards dealing with structural aspects. Over the past few years, industry leaders have made extensive efforts to develop economic procedures in mega projects that overlooked critical structural design issues.

Against this backdrop, a new strategy to design the concrete supports of reactors can be used in the refinery and petrochemical processing industry. The procedure is effective and leads to significant savings in material cost and engineering design time. The finite element method is used to idealise the load transfer from the reactor to the attached supports and to simulate structure-soil interaction. The procedure is demonstrated in the design of two reactor models used in gas recovery units.

Design space concept
Chemical reactors vary significantly in size and weight in the hydrocarbon industry. It helps to examine an effective design spectrum that can be utilised by various engineering disciplines to design industrial reactors of multiple sizes.

Consider a typical reactor shown in Figure 1 with a diameter (DR) and height (Hz) measured from the top of the base plate. The reactor is connected to a circular skirt with an internal diameter (DSk) to reduce heat transmission to the neighbouring structures. The skirt is welded to a base plate of thickness (tP).

External heat exchangers are connected in some cases to the reactor valves to regulate feedstock inlet temperatures. Anchor rods are installed on the octagonal concrete pedestal around the skirt perimeter. Void form (or cushions) is used on the lower surface of the pile cap to resist the up-heave pressure. The concrete pedestal is projected by distance (DP) measured from the top surface of the pile cap. The exposed length (Dp1) is determined by piping requirements.

Figure 2 shows Section (B-B) of the reactor support system to identify the parameters used in the graphical concept. The reactor footprint is identified by the long dashed central circle in light blue. Pile-cap dimension is denoted by (LF)x(BF)x(tF). The octagonal pedestal geometry is defined by the parameters (β) and (α). The long side is denoted by (β) and the short side by (α). The pedestal cross-sectional area is denoted by (AP). The small blue circles identify pile locations. The number of rows and columns are denoted by (m) and (n), as shown in the top left corner. Nominal pile diameter is denoted by (ΦP), embedded length (LP), and interior spacing (SX) and (SY). Note that the pile longitudinal reinforcements must be projected inside the pile cap.

Figure 3 illustrates the design space concept. The reactor height (HZ) is plotted versus the maximum pile load (PP, QP). Vertical lines (L1) and (L2) denote pile capacities that are a function of the corresponding pile embedment lengths (LP1, LP2). The bottom blue arrow shows the direction of increasing (LP). It can be observed that by increasing (LP), the maximum load on each pile is decreased. The rectangular area  enclosed by green lines represents the feasible design domain of (L1). Line segments enclosed by the feasible domain are shown in an identical colour and legend as (L1). Lines outside the feasible domain are shown in a different colour and legend.

The yellow arrow shows the direction of increasing the reactor height (Hz). It can be observed that the pile load increases by increasing the reactor height (Hz). Therefore, increasing (Hz) requires either a larger number of piles (m,n) or a longer embedment length (LP). Piling configurations are denoted by radial lines (A), (B) and (C). These curves represent various combinations of (m) and (n). Note that the slope of each curve increases by increasing the number of piles, as shown by the arrow rotating counterclockwise.

Star symbols represent the points of intersection of line (L1) with piling configurations (A), (B), (C). The x-coordinate denotes a fixed value (LP1), and the y-coordinate is variable, depending on the reactor height (HZ) of configurations (A)-(C). Similarly, solid triangles identify the points of intersections with (L2) and (Hz) of configurations (A)-(C). The difference in reactor heights between two configurations is denoted by    (LP)[δHZ]B-C. The subscript denotes the configuration tag being compared and the LHS superscript reference (LP).

The engineer can use two options to select the reactor support configuration. In option (I), the pile embedment length (Lp) is fixed while modifying the pile-cap configuration to achieve the desired reactor capacity. This option is suitable to use on Greenfield projects or when the soil profile varies significantly, which may limit the depth of embedment length (Lp) to a specific length. In option (II), the engineer may fix the pile-cap size (or the number of piles) and modifies the pile embedment length (Lp). This scenario is suitable when the construction area of the pile cap is limited to a specific size. The engineer, in this case, may modify the pile length to achieve the target reactor capacity. Space limitations are frequently encountered in expansion or Brownfield projects.

Analysis model
Finite element models were developed to populate the design space using various reactor layouts. Computationally efficient models can be used in practice to analyse this class of reactor. Shell elements are used to model the support system. The skirt reactions were transferred to piles using displacements and rotational compatibility constraints along the connection nodes.

Flexural beams were used to transfer the skirt loading to the pile cap. Auxiliary nodes were generated at the bolt locations to simulate out of plane bending and model the load transfer of the reactor loads into the pile cap. Nodes around bolts were connected to their adjacent node on the pile cap support using extensional and rotational spring elements.

Compatibility of displacement in z-direction and rotation in y-direction were imposed in this model. A non-linear elastic-plastic material model is used for the support system. Shear deformation of concrete support was ignored. Furthermore, the reactor diameter was increased to compensate for attached steel platforms, pipes, and cable trays.

Piles were restrained bi-laterally throughout the embedded length using horizontal springs. The pile head rotations and in-plane translations were restrained in the x and y directions (θx=θy=ux=uy=0). This assumption is justified by casting the pile cap with the pile head concurrently. The stiffness of piles was variable to reflect the variation in the pile casings. The sub-grade lateral stiffness was determined using borehole test data. The lateral spring stiffness was variable through the embedment length, and the spring constant was averaged using 4m intervals. Flexible supports were used at the bottom of the piles to stimulate the end bearing in the soil bed.

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