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    "slug": "continuous-stirred-tank-reactor-cstr-explained",
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        "rendered": "Kontinuierlicher R\u00fchrkesselreaktor (CSTR) erkl\u00e4rt"
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        "rendered": "<p>A continuous stirred tank reactor is a vessel that takes feed in and pulls product out at the same time while an impeller keeps the contents uniform. Composition inside matches the outlet, temperature holds steady through a jacket, and the reaction runs at one fixed operating point instead of climbing a batch curve.<\/p>\n<nav class=\"unionclay-toc\" aria-label=\"Table of contents\">\n<ul>\n<li><a href=\"#what-it-is\">What a continuous stirred tank reactor actually is<\/a><\/li>\n<li><a href=\"#how-it-works\">How does a CSTR work, step by step?<\/a><\/li>\n<li><a href=\"#design-equation\">What does the design equation actually tell you?<\/a><\/li>\n<li><a href=\"#vs-batch\">CSTR or batch reactor: which behaviour do you want?<\/a><\/li>\n<li><a href=\"#other-reactors\">Where do packed bed and fluidized bed reactors fit?<\/a><\/li>\n<li><a href=\"#hardware\">What hardware does a CSTR need?<\/a><\/li>\n<li><a href=\"#glass-or-steel\">Glass or stainless steel?<\/a><\/li>\n<li><a href=\"#faq\">Frequently asked questions<\/a><\/li>\n<\/ul>\n<\/nav>\n<h2 id=\"what-it-is\">What a continuous stirred tank reactor actually is<\/h2>\n<p>The idea is old and simple: keep a tank full, keep it stirred, keep feeding it, keep taking product off. Nothing accumulates. After startup the tank settles into steady state \u2014 concentration and temperature stop changing with time, and the stream leaving the discharge valve looks exactly like the bulk inside the vessel.<\/p>\n<p>That last clause is the whole model. Textbooks call it the perfect-mixing assumption, and it is what makes a CSTR mathematically friendly: you solve one balance at one operating point instead of integrating along a length or a batch clock.<\/p>\n<p>It is also the thing that breaks first in real hardware. Whether your vessel earns the assumption depends on the impeller, the speed you can hold, and how viscous or solids-laden your mixture is. A two-blade anchor impeller turning at 60 to 600 rpm sweeps the wall and keeps a viscous batch moving; it does not turn a settling slurry into a uniform suspension by wishful thinking. If your mixture fights the stirrer, perfect mixing stops being true and conversion drops below what the equation promised.<\/p>\n<h2 id=\"how-it-works\">How does a CSTR work, step by step?<\/h2>\n<p>Four things happen at once, which is exactly why the concept confuses people the first time. Here is the loop in the order the fluid meets it.<\/p>\n<figure class=\"unionclay-figure\">\n<img decoding=\"async\" src=\"https:\/\/www.unionclay.com\/wp-content\/uploads\/2026\/07\/continuous-stirred-tank-reactor-cstr-explained-cover.png\" alt=\"Four-step process diagram of a continuous stirred tank reactor: continuous feed, mixing to uniformity, jacketed steady-state reaction, continuous withdrawal\" width=\"1160\" \/><figcaption>The CSTR loop on a double-layer stainless vessel. Port sizes and stirring range are from our BSF-50L and BSF-100L parameter sheets.<\/figcaption><\/figure>\n<p><strong>Feed enters continuously.<\/strong> On our double-layer stainless vessels that means a KF25 feeding valve, or the KF25 constant-pressure funnel port if you are dosing liquid against the vessel&#8217;s own headspace pressure. Solids go in the KF40 charging port.<\/p>\n<p><strong>The impeller destroys the gradient.<\/strong> The instant feed hits the bulk, mixing dilutes it to the vessel&#8217;s own concentration. This is the counterintuitive part of a CSTR and the reason it behaves the way it does: fresh feed never sees a high concentration of itself. It sees the outlet composition, immediately.<\/p>\n<p><strong>The jacket holds temperature.<\/strong> Circulating fluid runs through the outer layer via four male-thread ports on our stainless units, fed by a chiller or heating circulator you size separately. Because a CSTR sits at steady state, the jacket is not chasing a moving exotherm the way it is in a batch run; it holds a constant duty. What that duty is depends on your heat of reaction and feed rate, which we cannot guess for you.<\/p>\n<p><strong>Product leaves continuously.<\/strong> Out the bottom discharge, roughly 421 mm off the frame on both our 50 L and 100 L stainless vessels, at the same volumetric rate as the feed. Level stays put, volume stays put. That constant volume is what the design equation depends on.<\/p>\n<h2 id=\"design-equation\">What does the design equation actually tell you?<\/h2>\n<p>This is where most explanations stop and hand you algebra. Mean residence time is tau = V \/ v0, where V is the reacting volume and v0 the volumetric feed rate. The steady-state mole balance, V = FA0 X \/ (-rA), rearranges to the same message: bigger tank, longer average stay, more conversion for a given rate law.<\/p>\n<p>Fine. But V is not a symbol you choose freely. V is a vessel you buy, in a size someone manufactures. So here is that equation in the form that matters when you specify equipment \u2014 the two nominal volumes we build in double-layer stainless, against feed rates you pick yourself.<\/p>\n<table class=\"unionclay-spec-table\">\n<thead>\n<tr>\n<th>Volumetric feed rate v0 (you set this)<\/th>\n<th>tau in a 50 L vessel<\/th>\n<th>tau in a 100 L vessel<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>0.25 L\/min<\/td>\n<td>200 min<\/td>\n<td>400 min<\/td>\n<\/tr>\n<tr>\n<td>0.5 L\/min<\/td>\n<td>100 min<\/td>\n<td>200 min<\/td>\n<\/tr>\n<tr>\n<td>1 L\/min<\/td>\n<td>50 min<\/td>\n<td>100 min<\/td>\n<\/tr>\n<tr>\n<td>2 L\/min<\/td>\n<td>25 min<\/td>\n<td>50 min<\/td>\n<\/tr>\n<tr>\n<td>5 L\/min<\/td>\n<td>10 min<\/td>\n<td>20 min<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><em>tau = V \/ v0 computed against the nominal 50 L and 100 L capacities on our BSF-50L and BSF-100L parameter sheets. Feed rates are illustrative values you choose, not equipment specifications.<\/em><\/p>\n<p>Read the table backwards and it becomes a purchasing decision. If your kinetics need 90 minutes of residence time and the process must swallow 1 L\/min of feed, a 50 L vessel gives 50 minutes and will not get there; the <a href=\"https:\/\/www.unionclay.com\/product\/bsf-100l-double-layer-stainless-steel-reactor\/\">100 L double-layer stainless vessel gets you to 100 minutes<\/a> at the same flow. That is the scale-up argument in one line, and why we would rather show you the volumes we build than another rearranged equation.<\/p>\n<p>Two honest corrections before you trust those numbers. First, V is the reacting volume, not the nameplate. Nominal capacity is 50 L or 100 L; usable volume is lower once you leave headspace for the anchor impeller&#8217;s vortex and a clear path to the KF40 reflux port. How much lower depends on how hard your mixture foams \u2014 only your own trial will tell you. Second, tau is a mean, not a guarantee. Some fluid short-circuits to the outlet in seconds; some lingers far past tau. That spread is intrinsic to the CSTR, and it is why one stirred tank gives lower conversion than a plug flow reactor of the same volume for most positive-order kinetics.<\/p>\n<h2 id=\"vs-batch\">CSTR or batch reactor: which behaviour do you want?<\/h2>\n<p>A batch reactor is the same vessel with the valves shut. Charge it, run it, empty it, and concentration falls along a curve the whole time. A CSTR is the same vessel with the valves open, sitting at one point on that curve forever. Same steel, different operating philosophy.<\/p>\n<p>Batch wins when you are still learning the chemistry, when one vessel makes many products, or when the reaction needs a temperature ramp or a staged addition. Our stainless lids carry a constant-pressure funnel port, a thermometer sleeve, an illuminated sight glass and a solid charging port because batch chemists watch, dose and adjust mid-run. Make one product all day at a fixed recipe and none of that flexibility earns its keep.<\/p>\n<p>Continuous wins when the recipe is frozen and the volume is real. You stop paying for charge and discharge time, quality stops drifting between batches, jacket duty stops swinging. The catch: a CSTR runs at the outlet concentration, for most reactions the lowest concentration in the process, and low concentration means low rate. That is the tax on perfect mixing, and it is why continuous plants run stirred tanks in series rather than one large tank.<\/p>\n<p>Our practical read: if the recipe is still moving, buy the vessel and run it batch. The hardware overlaps almost completely. A <a href=\"https:\/\/www.unionclay.com\/product\/bsf-50l-double-layer-stainless-steel-reactor\/\">50 L double-layer stainless reactor with a 120W gear-reduction drive and a jacket<\/a> runs either mode, and continuous feed wants the same KF25 and KF40 fittings a batch chemist already uses.<\/p>\n<h2 id=\"other-reactors\">Where do packed bed and fluidized bed reactors fit?<\/h2>\n<p>Three other names show up in the same lecture, so place them properly rather than pretend everything is a stirred tank.<\/p>\n<p>A plug flow reactor is a tube, not a tank. Composition changes along its length rather than sitting at one value, so nothing mixes backwards, and for the same volume and positive-order kinetics it out-converts a single CSTR. That advantage costs you the CSTR&#8217;s easiest feature: one steady operating point you can hold and measure.<\/p>\n<p>A packed bed reactor is a tube filled with solid catalyst. Fluid flows through the void space and reacts at the catalyst surface. No impeller, so nothing to keep uniform; the design problem shifts to pressure drop, catalyst deactivation and heat removal through a bed with poor thermal conductivity.<\/p>\n<p>A fluidized bed reactor suspends fine catalyst particles in an upward flow until the bed behaves like a boiling liquid. That buys temperature uniformity and easy catalyst replacement, at the cost of particle attrition and carryover. Notice what it is really doing: getting the mixing benefit of a CSTR without an impeller, using the fluid itself as the stirrer.<\/p>\n<p>We build stirred tanks, not packed or fluidized beds. If your chemistry lives on a solid catalyst in a flowing stream, none of our vessels is the right answer and we will say so. The two worlds meet at bench scale, where catalyst screening often runs in a stirred tank with the catalyst in suspension \u2014 one temperature, one composition, a rate you can read cleanly.<\/p>\n<h2 id=\"hardware\">What hardware does a CSTR need?<\/h2>\n<p>A CSTR needs four things a plain vessel does not: a way in, a way out, a way to mix, and a way to hold temperature. Concretely \u2014 feed ports, a bottom discharge, a driven impeller with a speed you can set and read, and a jacket with circulation connections. Everything else is configuration. Here is what that looks like on the two double-layer stainless vessels we build.<\/p>\n<table class=\"unionclay-spec-table\">\n<thead>\n<tr>\n<th>Parameter<\/th>\n<th>BSF-50L<\/th>\n<th>BSF-100L<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<th>Vessel capacity<\/th>\n<td>50 L<\/td>\n<td>100 L<\/td>\n<\/tr>\n<tr>\n<th>Vessel material<\/th>\n<td>Stainless steel<\/td>\n<td>Stainless steel<\/td>\n<\/tr>\n<tr>\n<th>Motor<\/th>\n<td>120W 1\/3 gear reduction<\/td>\n<td>250W 1\/3 gear reduction<\/td>\n<\/tr>\n<tr>\n<th>Stirring speed<\/th>\n<td>60-600 rpm<\/td>\n<td>60-600 rpm<\/td>\n<\/tr>\n<tr>\n<th>Impeller type<\/th>\n<td>Two-blade anchor type<\/td>\n<td>Two-blade anchor type<\/td>\n<\/tr>\n<tr>\n<th>Controller<\/th>\n<td>LCD digital display for temperature and speed<\/td>\n<td>LCD digital display for temperature and speed<\/td>\n<\/tr>\n<tr>\n<th>Jacket inlet\/outlet<\/th>\n<td>4 male thread circulation ports<\/td>\n<td>4 male thread circulation ports<\/td>\n<\/tr>\n<tr>\n<th>Stirring port (A)<\/th>\n<td>&#934;50 mm flange port<\/td>\n<td>&#934;50 mm flange port<\/td>\n<\/tr>\n<tr>\n<th>Feeding valve port (E)<\/th>\n<td>KF25<\/td>\n<td>KF25<\/td>\n<\/tr>\n<tr>\n<th>Reflux + condensation port (C)<\/th>\n<td>KF40<\/td>\n<td>KF40<\/td>\n<\/tr>\n<tr>\n<th>Solid charging port (G)<\/th>\n<td>KF40<\/td>\n<td>KF40<\/td>\n<\/tr>\n<tr>\n<th>Thermometer sleeve (D)<\/th>\n<td>KF25, sleeve length 400 mm<\/td>\n<td>KF25, sleeve length 400 mm<\/td>\n<\/tr>\n<tr>\n<th>Condenser<\/th>\n<td>&#934;102 &times; L300 coil, 12 mm barbed ports<\/td>\n<td>&#934;102 &times; L300 coil, 12 mm barbed ports<\/td>\n<\/tr>\n<tr>\n<th>Bottom discharge height<\/th>\n<td>approx. 421 mm<\/td>\n<td>approx. 421 mm<\/td>\n<\/tr>\n<tr>\n<th>Vacuum<\/th>\n<td>Reaches -0.095 MPa<\/td>\n<td>Reaches -0.095 MPa<\/td>\n<\/tr>\n<tr>\n<th>Working dimensions<\/th>\n<td>790 &times; 680 &times; 1680 mm<\/td>\n<td>790 &times; 680 &times; 1680 mm<\/td>\n<\/tr>\n<tr>\n<th>Power supply<\/th>\n<td>220 V\/50 Hz, customizable 110 V\/60 Hz<\/td>\n<td>220 V\/50 Hz, customizable 110 V\/60 Hz<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><em>Values from the UnionClay BSF-50L and BSF-100L double-layer stainless steel reactor parameter sheets.<\/em><\/p>\n<p>Two lines deserve a comment. The motor doubles from 120W to 250W while the stirring range stays at 60-600 rpm, because doubling the volume doubles the torque the impeller must deliver at the same speed. Run something thick and the real question is not &#8220;how many rpm&#8221; but &#8220;can the drive hold my speed in my fluid&#8221; \u2014 which depends on your viscosity, not on a datasheet.<\/p>\n<p>The other is the vacuum figure. Our stainless sheets state the vessel reaches -0.095 MPa. They do not state a positive design pressure. If your process must operate above atmospheric, treat that as a spec to confirm with our engineers, not something to infer. Guessing at a pressure rating is how people get hurt. The vessels and configurations are listed in our <a href=\"https:\/\/www.unionclay.com\/product-category\/chemical-reactor\/stainless-reactor\/\">stainless steel reactor category<\/a>.<\/p>\n<h2 id=\"glass-or-steel\">Glass or stainless steel?<\/h2>\n<p>For CSTR work this comes down to whether you need to see the reaction or need the vessel to take pressure and abuse.<\/p>\n<p>Glass wins on visibility and on resistance to the acids that eat steel. Our <a href=\"https:\/\/www.unionclay.com\/product\/double-layer-jacketed-glass-reactor-10l-100l\/\">double-layer jacketed glass reactors run 10 L through 100 L in GG3.3 borosilicate<\/a>, with the same 60-600 r\/min stirring range as the stainless units and motor power scaling 90W on the 10 L to 30 L models, 120W at 50 L and 250W at 100 L. Note the parallel: at 50 L and 100 L, glass and stainless use the same drive power. Not a coincidence \u2014 same mixing job.<\/p>\n<p>Here is the line that decides most cases. Our glass reactor sheets state normal or negative pressure, no positive pressure, with vacuum up to -0.095 MPa. If your process ever sits above atmospheric, glass is out before you compare anything else. If it never does, and you want to watch a continuous run develop, glass is often the better instrument. We work through the trade in our comparison of <a href=\"https:\/\/www.unionclay.com\/product-category\/chemical-reactor\/\">stainless steel reactors versus glass reactors<\/a>, and the volume-and-circulation questions in the <a href=\"https:\/\/www.unionclay.com\/product-category\/chemical-reactor\/glass-reactor\/\">jacketed glass reactor selection guide<\/a>.<\/p>\n<p>Both share one limit: a jacket is half a temperature-control system. The vessel gives you circulation ports; the circulator pushing fluid through them is sized against your heat load, and that number comes from your chemistry, not our catalogue.<\/p>\n<h2 id=\"faq\">Frequently asked questions<\/h2>\n<h3>What does CSTR stand for?<\/h3>\n<p>Continuous stirred tank reactor. You will also see continuous-flow stirred-tank reactor, backmix reactor, or mixed flow reactor. All describe the same thing: a stirred vessel with feed entering and product leaving continuously at steady state.<\/p>\n<h3>Is a CSTR the same as a batch reactor?<\/h3>\n<p>No, though it is often the same hardware. A batch reactor is charged, run, and emptied, so composition changes with time. A CSTR runs with feed and discharge open and holds one composition indefinitely. Our double-layer stainless vessels run either way, which is why the lid carries both a KF25 feeding valve and a KF40 solid charging port.<\/p>\n<h3>How do you calculate residence time in a CSTR?<\/h3>\n<p>Divide the reacting volume by the volumetric feed rate: tau = V \/ v0. Feeding 1 L\/min into a 50 L vessel gives a mean residence time of 50 minutes; the same flow into a 100 L vessel gives 100 minutes. Use reacting volume, not nameplate capacity \u2014 headspace does no chemistry.<\/p>\n<h3>Why does a CSTR give lower conversion than a plug flow reactor?<\/h3>\n<p>Perfect mixing drops the feed straight to the outlet concentration, and for positive-order kinetics lower concentration means a lower rate everywhere in the tank. A plug flow reactor keeps concentration high at the inlet and lets it fall along the length, so its average rate is higher for the same volume. Stirred tanks in series recover part of that gap.<\/p>\n<h3>What stirring speed should a continuous stirred tank reactor run at?<\/h3>\n<p>Fast enough that the tank is uniform, which is a property of your fluid rather than a number we can print. Our 50 L and 100 L stainless vessels give 60 to 600 rpm on a two-blade anchor impeller with an LCD readout, so you can find the setting and repeat it. Thin, low-solids mixtures reach uniformity low in that band; viscous or settling systems need testing, not assumptions.<\/p>\n<h2 id=\"selection\">Picking the vessel, not the equation<\/h2>\n<p>Work backwards from residence time. Get tau from your kinetics or your bench trial, multiply by the feed rate the process must sustain, and you have the reacting volume you need. Add headspace, then buy the nearest vessel above that number, not below it. A 50 L vessel asked to behave like a 60 L vessel will disappoint you every run.<\/p>\n<p>Skip the 100 L unit if your feed never asks for more than 50 minutes of residence time at 1 L\/min \u2014 the 50 L covers it with a lighter drive and the same port set. Go to 100 L the moment tau and throughput multiply past roughly 50 litres of reacting volume. If the recipe is not frozen, buy for your eventual continuous duty and run it batch until it is.<\/p>\n<p>If you need positive pressure, say so before anything else gets specified: our published stainless data covers vacuum to -0.095 MPa, and our glass reactors are limited to normal or negative pressure. The configuration questions we get asked most sit on our <a href=\"https:\/\/www.unionclay.com\/faq\/\">reactor FAQ page<\/a>, and our engineers confirm working volume, port layout, stirring configuration and jacket connections against your process before anything is built.<\/p>",
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        "rendered": "<p>Ein kontinuierlicher R\u00fchrkesselreaktor flie\u00dft und entl\u00e4dt sich w\u00e4hrend des Mischens. Wie ein CSTR funktioniert, was tau = v \/ v0 bedeutet und die 50-l- und 100-l-Schiffe dahinter.<\/p>",
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