\begin{center}
    \begin{tikzpicture}
        \draw[step = 0.5cm, gray, very thin] (-7.25, -1.75) grid (7.25, 0.25);

        \node[column, /column= tray] (A) at (-6.0, -0.75) {};
        \node[anchor = south] at (-6.0, 0.25) {\textbf{tray}};

        \node[column, /column= fluidized] (A) at (-2.0, -0.75) {};
        \node[anchor = south] at (-2.0, 0.25) {\textbf{fluidized}};

        \node[column, /column= fixed] (A) at (2.0, -0.75) {};
        \node[anchor = south] at (2.0, 0.25) {\textbf{fixed}};
        
        \node[column, /column= staggered baffle] (A) at (6.0, -0.75) {};
        \node[anchor = south] at (6.0, 0.25) {\textbf{staggered baffle}};
    \end{tikzpicture}
\\[0.25cm]
    \begin{tikzpicture}
        \draw[step = 0.5cm, gray, very thin] (-7.25, -1.75) grid (7.25, 0.25);

        \node[column, /column= bubble caps] (A) at (-6.0, -0.75) {};
        \node[anchor = south] at (-6.0, 0.25) {\textbf{bubble caps}};

        \node[column, /column= valve] (A) at (-2.0, -0.75) {};
        \node[anchor = south] at (-2.0, 0.25) {\textbf{valve}};

        \node[column, /column= double fixed] (A) at (2.0, -0.75) {};
        \node[anchor = south] at (2.0, 0.25) {\textbf{double fixed}};
        
        \node[column, /column= spray] (A) at (6.0, -0.75) {};
        \node[anchor = south] at (6.0, 0.25) {\textbf{spray}};
    \end{tikzpicture}
\end{center}