Electrically heated reactor, a furnace comprising said reactor and a method for gas conversions using said reactor

Abstract

An electrically heated reactor is a tube surrounded by electrical heating means having radiative sheeting placed coaxially with regard to the reactor tube. The surface area of the sheeting facing the outer surface area of the reactor tube defines an inner surface area covering at least 60% of the reactor tube outer surface area. The distance between the reactor tube and the heating means is selected such that the ratio between the inner surface area of the electrical heating means to the reactor tube outer surface area is in the range of 0.7 to 3.0. The reactor is useful in many industrial scale high temperature gas conversion and heating technologies.

Claims

1. An electrically heated reactor having an outer surface area, an inlet and an outlet, wherein (a) the reactor is a tube surrounded by electrical heating means at a certain distance; (b) the electrical heating means comprises radiative sheeting placed coaxially around the reactor tube, the surface area of the sheeting facing the outer surface area of the reactor tube defining an inner surface area of the electrical heating means; (c) the inner surface area of the heating means covers at least 60% of the reactor tube outer surface area; and (d) the distance between the reactor tube and the heating means is selected such that the ratio between the inner surface area of the electrical heating means to the reactor tube outer surface area is in the range of 0.7 to 3.0.

2. The process according to claim 1, wherein the radiative sheeting is divided into at least two segments which are placed lengthwise along the reactor tube, each of which segments being connected to a separate power control.

3. The process according to claim 1, wherein the radiative sheeting comprises NiCr or FeCrAl based resistance heating materials.

4. The process according to claim 1, wherein the heating means is a radiative sheeting placed coaxially around the reactor tube, while leaving an opening along the length of the reactor tube with a size that matches the diameter of the reactor tube.

5. The process according to claim 1, wherein the heating means is a radiative sheeting consisting of panels of the radiative heating material.

6. A furnace, comprising within the furnace one or more reactor tubes according to claim 1, said one or more reactor tubes having an entrance and exit outside of the furnace; and one or more inspection ports in the furnace wall, each of which inspection ports being placed opposite to a reactor tube.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A. Schematic overview of a reactor tube according to this disclosure, fully surrounded with radiative sheeting in two segments. The power supply arrangement is shown in the top drawing.

(2) FIG. 1B. Schematic overview of a reactor tube according to this disclosure, partly surrounded with radiative sheeting in two segments, leaving an opening for sideways inserting and removing the radiative sheeting. The power supply arrangement is shown in the top drawing.

(3) FIG. 2. Schematic representation for a conventional gas-fired heated Steam Methane Reforming & Hydrogen Manufacturing unit. NG is Natural Gas; BFW is Boiler Feed Water; HTS is High Temperature Shift; PSA is Pressure Swing Adsorption.

(4) FIG. 3. Schematic representation of the power control for a reactor according to the present disclosure with four segments of radiative sheeting, each connected with a separate power control unit. The reactor is represented here by a narrow vertical rectangular tube depicted on the left of the drawing, which in reality may also be for example a U-bent tube, or a horizontal tube. Arrows indicate the reactant feed and product exit streams, respectively. TC001 is the reactor outlet temperature control, XY-099 converts the TC output to desired power, in the formula z=g.Math.k, g is the percentage output of the temperature control (i.e. TC-001), k represents the constant to convert from controller output to desired furnace duty (for example 100 MW/100%.fwdarw.1 MW/%). In dividing the requested duty over the reactor, each segment has a hand controller (HC-001 to HC-004). From the output of these hand controllers, the fraction is multiplied with the afore mentioned total requested duty z in calculation blocks XY-001 to XY-004. This required power is subsequently sent to the power control unit of the specific segment.

(5) Hereinafter the invention will be further illustrated by the following non-limiting examples.

EXAMPLES

(6) GeneralTemperature Control

(7) Temperature control in a reactor according to the invention takes place as shown in FIG. 3. A heat flux/temperature profile is set by means of (hand) controllers over the length of the reactor. The highest heat flux occurs at the top of the reactor tube where both the further heating to required reaction conditions of the reaction mixture occurs and reactions start to consume heat energy. A peak is reached in heat flux after which this declines while the temperature increases. The highest temperature combined with lowest heat flux occurs at the outlet. Here chemical equilibrium is virtually achieved at the desired final temperature. To fit this profile, four segments have been designed. Each segment delivers a pre-defined fraction of the total demanded duty. This will consequently lead to a segmentreactor tube temperature equilibrium according to radiative heat transfer principles as described before (vide supra).

(8) GeneralElectrical Infrastructure

(9) The design electrical power consumption of a 100 MW furnace, including 10% design margin=117 MWe. The design premise is to start with a 132 kV AC bus and, through transformers, reduce the voltage level to the desired 690 V. The concept is to use 6132/11 kV Transformers and 4711/0.72 kV Transformers. From a design perspective, the large grid transformers would likely be located remote from the electrical furnace since the incoming power may be via overhead lines to an outdoor substation.

(10) To achieve the CO.sub.2 emission reductions, the power is expected to come from renewable generation capacity, but waste stream power sources may also be used in an integrated process set-up.

Example 1

(11) Furnace with Reactor According to the Invention.

(12) A conceptual electrical furnace design for a 100 MWe powered SMR comprises of 260 reactor tubes. Each reactor tube is equipped with 12 segments of co-axial heater tubes (i.e. radiative sheeting) along the vertical distance of the reactor tube. Each segment is 0.9 m. Each segment is able to exchange a design heat-flux of up to 120 kW.Math.m-2 on reactor tube outer surface having a temperature of up to 870 C. The segments at each specific elevation are interconnected in series as to obtain reasonable electrical resistance, translating in voltage level needed to control the duty at said elevation (zone). The co-annular segments are placed such as to obtain an area ratio larger than or approximating 1 (For each specific segment: Area Radiant Heater/Area Process coil>1).

(13) Furnace viewports (inspection ports) are designed to inspect the condition of the heater tubes.

Example 2

(14) A Furnace According to Example 1 in Operation.

(15) Start-Up

(16) In comparison to a conventional SMR, electrical furnaces can be started gradually. The turndown ratio for electrical heating is virtually unlimited and consequently start-up is well controllable. Moreover, the heat distribution is uniform across all tubes. This is contrary to conventional hydrocarbon-fired SMRs where a few burners may be lit resulting in a temporary unbalance. To prevent damage to the electrical heating elements the heat-up rate should be limited.

(17) Shutdown

(18) To prevent damage to the reactor tubes a maximum cool down rate of 50 C..Math.hr.sup.1 must be adhered to. Considering that the turndown capability is very high and provided that the electrical heating system is functioning normally, this cooldown rate limitation can be adhered to. Moreover, in trip scenarios (i.e. unexpected stopping of the process, for example, when a fire occurs) the settle-out temperature, considering all heat capacity in the heating elements and refractory must be calculated. It is expected that this temperature is sufficiently low to prevent a reactor tube bursting. Moreover, steam purge, and reactor depressurization is part of normal shut-down procedures.

(19) Turndown

(20) Conventional SMR furnaces have a turndown ratio of 5 (turndown=design throughput/minimum throughput). This is predominantly governed by the ability of the furnace burners and fuel characteristics. Instead, electrically powered furnaces have a virtually unlimited turndown ratio. New limitations for the turndown are caused by the limitations on the process side, such as flow distributions over the reactor tubes.

(21) Trip

(22) To prevent power grid instability in the event of the load rejection associated with tripping for example a 100 MWe duty not associated with an electrical fault, a delay may be implemented to allow the electrical grid to adjust to the power rejection, so that the load is not all rejected in one step. Such a delay is in the order of seconds to a few minutes. Future development should identify the exact strategy by grid stability assessment. From a process point of view, such delays can be accommodated. When a trip occurs, steam is injected into the reactor tube and the process is depressurized.

(23) Trouble Shooting

(24) For various reasons, the reactor tubes can become overheated. For example, localized catalyst activity loss can occur, carbon formation resulting in a plugged reactor tube or voids can be present due to wrong catalyst loading. According to the present disclosure, it may be possible to monitor the reactor tubes during operation. Inspection ports can be designed in an electrical furnace to be able to inspect the reactor tubes during operation. Normally this is assessed using infrared radiant measurement techniques (e.g. pyrometer).

Example 3

(25) Comparative data for a 3 MW electrical capacity SMR hydrogen manufacturing unit using reactors according to the invention, when compared to a conventional hydrocarbon-fired unit:

(26) TABLE-US-00001 Electrically Conventional heated hydrocarbon (invention) fired Total hydrogen production kmol/h 118.27 118.27 Total hydrogen production ton/day 5.72 5.72 Steam/Carbon SMR Feed 3.20 3.20 Natural gas intake ton/day 11.60 19.33 CO.sub.2 emissions ton/day 31.55 52.84 Overall efficiency (incl. 88% 82% steam export) Overall efficiency (excl. 88% 74% steam export) SMR furnace (electrical) MW 3.00 2.44 heating duty SMR furnace process C. 860 860 temperature Steam production ton/day 63.12 92.84